1st Colloquium - Embryology.pdf

1st Embryology colloquium 1. Teratology and embryology: main terms, historical aspects and actuality of investigations. Ontogenesis. Experimental embryology Teratology (Gr. “Miracle”) is the scientific branch of embryology dealing with malformations occurring through embryonic development....

1st Embryology colloquium 1. Teratology and embryology: main terms, historical aspects and actuality of investigations. Ontogenesis. Experimental embryology Teratology (Gr. “Miracle”) is the scientific branch of embryology dealing with malformations occurring through embryonic development. This science dates back to ancient times where the term “monster” was used describing certain birth defects. The view on malformations in pre-medival times was a supernatural one where a malformed child was a positive sign indicating happiness. This was however changed in the medival era when the view became an infranatural one indicating possession of the devil and death. The incidence of birth defects greatly differs in literature but have been described to be from 2,0-5,6% of all births at birth but this number increases within the first few years of postnatal life when motor deficiency becomes evident. Most of the incidence accounts for lighter malformations as nature tend to auto-abort fetuses with the heaviest chromosomal errors. The word “stigma” in embryology denotes light marks of peculiar development. These stigmas include abnormal ears, philtrum, mouth etc. and may be a sign of beauty (Julia Roberts). “Terratogen” – inducer of congenital malformation There are several causes of congenital malformations but they are often categorized into two groups: 1. Genetic factors – mutagenic response to ionizing radiation, chemicals (nicotin) and other mutagenic compounds. Chromosomal aberrations, abnormalities are also included which are induced by translocations or non-disjunctions. 2. Environmental factors – account for all factors outside the genome causing malformations. a. Drugs: a.i. Thalamicoide – malformations of ears a.ii. Warfarin – hypoplasia of nose a.iii. Tetracyclin – enamel dysplasia b. Maternal conditions b.i. Diabetes – congenital heart defects b.ii. Alcoholism – fetal alcohol syndrome, retarded growth and mental development c. Intrauterine infection c.i. Toxoplasmosis – oligohydramnios c.ii. CMW - same c.iii. Rubella – same d. Physical factors d.i. Noise, virbration, radiation e. Hypoxia --> fetal stress --> malformations Embryology is the scientific branch dealing with the part of ontogenesis from zygote until birth. The field of embryology includes; comperative, descriptive and experimental embryology The founder of embryology is credited the german embryologist Reichert which described the first two week old human blastocyst in 1873 but society have shown interest in the improper development of mammals and is thus related to teratology. Experimental embryology is a field of embryology dealing the following issues: 1. Prevention of body malformations including detection of sensitive periods for each organ 2. Reserarch into artifical insemination including in vitro fertilization 3. Intrauterine surgery improvement 4. Stem cell research Terms in embryology Agenesia – total absence of an organ or structure Aplasia – failure to develop a functioning organ Atresia – absence of opening Dyschromia – disorders in developmental time Ectopia – atypical localization of organs Heterotrophia – presence of organ in atypical position Hypoplasia – underdeveloped organ or structure in size and mass Hypotrophia – underdeveloped fetal mass Macrosomia – increase in body length Persistence – presence of embryonic structures in postnatal life Stenosis – narrow canal or opening Synsythium or sinsin – fused together 2. Ethological factors for developmental malformations and their classification The development of malformations is named teratology and the principle ethiology behind these malformations is divided into two groups: 1. Genetic factors which include factors changing the healthy genome whether it is chromosome, single genes or meiotic factors. A mutation that occurs in a germ cell may have terratogen effect on the developing embryo in case it fertilizes or becomes fertilized. Chromosomal abnormalities are based on numerical or structural aberrations usually caused by non-disjunction in meiosis or translocations (stray from the normal 46XX/Y) Trisomy – usually lead to spontaneous abortions but can occur: Trisomy 21 – down syndrome (1/1000 births – risk increases with maternal age) characterized by mental retardation, broad/flat face, poor vision, heart anamolies etc. Trisomy 18 – Edwards syndrome Trisomy 13 – Patau disease 2. Environmental factors account for all factors outside the chromosomes including intra-/extracellular spaces, placenta, intramaternal space and the atmosphereic surroundings. These factors are divided into biological, chemical and physical factors. Chemical substances: AB can cross the placental barrier and cause terratogen effect on the embryo. Other drugs may also have this effect: Thalimoide – anomalies of ears Warfarin – hypoplasia of nose Tetracyclins – enamel dysplasia Biological factors include maternal conditions and intrauterine infections Maternal diabetes – may cause congenital heart defects Maternal alcoholism – may induce mental/physical retardation, fetal- alochol syndrome Toxoplasmosis, CMV and rubella infections – severe oligohydramnios Physical factors – vibration, noise, temperature, radiation and hypoxia may influence development 3. Sensitive phases of intrauterine life. Periods of intrauterine life. Sensitive and critical phase of body development A sensitive period is the time when terratogenic effect is most significant for each independent organ development when there is an excessive mitotic division of the blastema (primary formation). Determination of sensitive periods is one of the aims of experimental embryology. It is important to clarify the periods for each organ because they all develop at different times at different durations. Periods of intrauterine life: divided into three periods however, gametogenesis – the development of the paternal reproduvtive cells occur before fertilization but may affect further development of the zygote because chromosomal anomalies happens in this stage. The three periods of actual intrauterine life is: 1. Blastogenesis – from fertilization to day 16. Development of blastula (early ball of cells) and blastocyst (a blastula where differentiation has started) develop. Characterized by development of bilaminar embryo. Teratogens usually inflict abortions but if the developing conceptus survive it may cause heavy anomalies such as two headed baby. 2. Embryogenesis – from day 16 to day 60. Characterized by development of a trilaminar embryo and development of primordial for main organs, foetal membranes and placentation. Teratogens induce embryopathies affecting embryo, anion and chorion 3. Foetogenesis – day 60 to birth – maturation of organs. Foetopathies are related to metabolic disorders, retarded mental and physical growth and some malformations 4. Gametogenesis. Embryonal gonads. Gametogenesis is the production of cells capable of fertilization and the primary event is reduction to haploid chromosome number by meiosis. The first primordial germ cells are known as gonocytes and appear at day 24 from the endoderm in the yolk sac. They are large cells rich with alkaline-phosphatase, glycogen and can perform ameboid movements which is why they start to migrate from the endoderm to the posterior body wall at around TV10 where they are incorporated within the primitive gonads. (In case they do not complete this migration they become teratomas --> embryonic tumors, very aggressive) The cells increase in number to millions of cells through mitosis and at this point the difference between male and female gametes are distinguished: Female germ cells increase to about 7 million at the 5th embryonic month followed by ateria which decreases the number to about 1-2million at birth. The number keeps decreasing to about 10-40.000 at puberty. Only about 400 undergo ovulation and when at menopause the ovaries are infiltrated by CT. Spermatogonia maintain mitosis throughout life but it decreases at old age 65- 70years when male [testosterone] decreases. At puberty the spermatogonia start periodic mitotic division in the tubuli semineferi contrortii and undergo meiosis in synchronous groups. The SRY gene determine the male sexual characteristics and at 7th week this gene start production of testosterone. If this gene lacks, female phenotype will develop. Gonads = organ developing gametes which is the primitive gonads at TV10. 5. Ovogenesis. Characterization of ovocyte I. The oogenesis consist of three periods: division, growth and maturation. Division occurs under the embryonic period of intrauterine life where the female gonads produce about 1-2million oogonia (44+XX) at birth (7million at 5th embryonic month decreasing to 1-2million at birth.) At birth only around 1-2million remain and the number is reduced to about 40.000 at puberty. Growth start in the foetal period when the oognonia enter prophase I of the first meiotic division where they are known as primary oocytes (4n). Right after birth they enter diplotene of meiosis I and remain in this stage until just before ovulation. During the diplotene stage the 5000 cortical granuli is produced. Maturation start from puberty when 10-30 primary oocytes complete the first meitotic division with each menstrual cycle based on hormonal stimulation. The oocyte has 4n chromosome number and is the largest cell of the human body up to 0,2mm in diameter. The oocyte has a large eccentric nucleus and have cortical granules around the cell border. The cell is surrounded by a columnar layer of cells called corona radiata which produces the ununiformily thick zona pellucida. The cell border have antigens related to the fertilization process and in experiments altering these AG showed that oocytes missing theses AG would not be fertilized. After completing the first meiosis under hormonal stimulation the cell is called a secondary oocyte and it consist of a large secondary oocyte and a small polar body. The follicular cells have theca interna producing estrogen and theca externa which works as a CT. The secondary oocyte starts meiosis II but is halted in metphase II until it is fertilized by a spermatozoa. 6. Spermatogenesis The process where spermatogonia develops into sperm. Spermatogenesis is divided into four stages: division phase,Growth phase, Maturation and spermiohistogenesis. 1. Division phase – when spermatogonial stem cells (2n) produces three types of cells starts allready in early embryogenesis. a. Spermatognonia type Ad (dark) are believed to be stem cells for seminferious epithelium which can divide into more dark spermatogonia or to the Ap (pale spermatogonia) b. Spermatogonia type Ap – spermatogonic stem cells which after mitotic division differentiate into type B spermatognonia c. Spermatogonia type B --> primary spermatogonia 2. Growth phase – the cells transform into primary spermatocytes 3. Maturation includes two divisions: Meiosis I and meosis II which is more like a regular mitosis. The primary spermatocytes (with two pairs of every chromosome) enter the first meiosis and undergo Prophase: Lepotene, zygotene, pachytene, diplotene, diakenesis, Metaphase I, Anaphase I and ends with the production of two secondary spermatocytes with only one pair of every chromosome. This process takes about 22 days frequently observed in histological slides. Meiosis II is actually very similar to a regular mitosis and takes about 6-8 hours and it starts right after meiosis I, which is why it is very rarly seen in slides. In anaphase II the sister chromatids are seperated and when meiosis II ends with 4 spermatids (from the original primary spermatocyte) which are connected with cytoplasmatic bridges (due to the APAK 82 gene which is located only on the X chromosome). Spermiohistogenesis The spermiogenesis occurs while the cells are attached to sertoli cells by gap junctions. This process is divided into four phases: golgi phase, cap phase, acromsome phase and maturation phase. Golgi stage The spermatids start to express polarity. The head starts to form in one end and the Golgi apparatus start to form enzymes that will develop into the acrosome (hydrolytic enzymes; acrosine, trypsinase). At the other end a mid piece forms where mitochondria gather and the distal centriole start to form an axoneme with the classic 9x2+2 configuration (a ring of 9 doublet microtubules around a single pair of microtubles in the middle) The DNA becomes highly packed and regular nuclear protein is replaced with protamines making it very densly packed. Cap stage The acrosomal vesicle spreads out across the nuclear envelope making the acrosomal cap. Acrosomal stage The spermatid orients itself so that the tail extend into the lumen of TSC. One of the centriols elongate and form the tail of the spermia. A temporary structure named the manchette aid this elongation. Maturation stage Excess cytoplasm forms residual bodies and is phagocytosed by sertoli cells. The spermatozoa is at the point of spermiation immobile and unable to penetrate the ovum however if inserted into the ovum it can fertilize it. The movement towards the epidydmis is mediate by cilliary movement of the TSC. In the epidydmis the spermatozoa increase mobility, decrease the cell volume, mature the acrosome, stabilizes the membrane, aquires receptors for protein on zona pellucida. 7. Differences between oogenesis and spermatogenesis Female oogenesis Male spermatogenesis Meiosis initiated once in a finite population of cells Meiosis initiated continuously in a mitotically dividing stem cell population One gamete produced per meiosis (+2 polar, unequal Four gametes produced per meiosis bodies) Completion of meiosis delayed for months or years Meiosis completed in days or weeks Meiosis arrested at first meiotic prophase and Meiosis and differentiation proceed continuous reinitiated in a smaller population of cells without cell cycle arrest Differentiation of gamete occurs while diploid, in first Differentiation of gamete occurs while haploid, meiotic prophase after meiosis ends All chromosomes exhibit equivalent transcription and Sex chromosomes excluded from recombinatio recombination during meiotic prophase and transcription during first meiotic prophase Oogenesis occurs at normal body temperature (36,6) Spermatogenesis occurs at 33,6 degrees The meiotic divisions in oogenesis give 2 cells of In spermatogenesis, the divisions are equal. unequal size; the divisions are unequal. Oocytes are only mature after fertilisation (they are Spermatozoa are mature cells. arrested in metaphase after the first meiotic division) Oogenesis begins before birth Spermatogenesis begins at puberty All ova are present at birth Spermatozoa develop at puberty and onwards Oocytes are immotile Spermatogenesis involves the development of flagella 8. Spermiohistogenesis and characterization of spermatozoa Spermiohistogenesis The spermiogenesis occurs while the cells are attached to sertoli cells by gap junctions. This process is divided into four phases: golgi phase, cap phase, acromsome phase and maturation phase. Golgi stage The spermatids start to express polarity. The head starts to form in one end and the Golgi apparatus start to form enzymes that will develop into the acrosome (hydrolytic enzymes; acrosine, trypsinase). At the other end a mid piece forms where mitochondria gather and the distal centriole start to form an axoneme with the classic 9x2+2 configuration (a ring of 9 doublet microtubules around a single pair of microtubles in the middle) The DNA becomes highly packed and regular nuclear protein is replaced with protamines making it very densly packed. Cap stage The acrosomal vesicle spreads out across the nuclear envelope making the acrosomal cap. Acrosomal stage The spermatid orients itself so that the tail extend into the lumen of TSC. One of the centriols elongate and form the tail of the spermia. A temporary structure named the manchette aid this elongation. Acromsome covers 2/3 of the nucleus. The tail consist of three parts: The neck part with many mitochondria --> motoric center The main part with two folded lines Tip part tip Maturation stage Excess cytoplasm forms residual bodies and is phagocytosed by sertoli cells. The spermatozoa is at the point of spermiation immobile and unable to penetrate the ovum however if inserted into the ovum it can fertilize it. The movement towards the epidydmis is mediate by cilliary movement of the TSC. In the epidydmis the spermatozoa increase mobility, decrease the cell volume, mature the acrosome, stabilizes the membrane, aquires receptors for protein on zona pellucida.  The spermatazoa is the final gamete cell developed from the male and it’s function is to fertilize the oocyte in order to create a zygote. It has a haploid cell amount and it’s movement is mediated by the tail part with its characteristic 9x2+2 axoneme moving at 2-3mm/min in the female reproductive tract. The normal amount should be 100- 400million/mL/ejaculate. 9. Fertilization. Stages and biological sense. Abnormal fertilization Preconditions: There should be between 100 and 400 million spermatozoa per mL of ejaculate. The border for infertility is 30million/mL. The oocyte only survives about 1-2 days after ovulation and the spermatozoa also survive for a couple of days after coitus unless they are in the mucous of the reproductive tract where they survive for about 7 days. The spermatozoa should move at about 2-3mm/min and reach the serosal cavity within 5-6hours. Stages of fertilization: 1. Capacitation – is the process of eliminating inhibiting factors (glycoprotein) from the spermatozoa head which make the cell hyperactive and dissataches from the tubal epithelium. This occurs in the female reproductive tract 2. Penetration of corona radiata by release of hyaluronidase and movement of the spermia 3. Penetration of zona pellucida by realease of acrosine which dissolves the ZP 4. Fusion between the cytoplasm of the oocyte and the spermia due to binding of fertilin of the spermia to intregrin molecules on the oocyte surface. The spermatozoa including its tail (protein source and germ-like axial) enter the oocyte. 5. The cortical reaction by release of cortical granule which contain hydrolytic enzymes. 6. The second meiotic division occurs and a new diploid cell is formed --> the zygote Result (biological sense) is restoration of diploid chromosome number, determination of the embryo gender, new variations due to recombinations of chromsomes and the initiation of mitotic division of zygote into blastomeres. Abnormal fertilization: Dispermia – when two sperm enter the oocyte and the result is a zygote with 69 chromosomes which is almost never viable. Superfetatio – when a second fertilization occurs during pregnancy (not seen in humans but often in animals) Superfecundatio – when two oocytes are fertilized by different males Parthogenesis – when the oocyte develops without the presence of spermia due to physical and chemical factors. Hermaphroditen → reproduktive pathology ( testislovary ) at least alt sterile | of them → 10. Clevage. Blastocyst formation. Embryoblast. Trophoblast Is a series of asynchronus mitotic divisions occurring within 24 hours after fertilization. Since the oocyte is still within it’s ZP it does not grow and the result of the mitotic division is new smaller cells named blastomeres. After several divisions the conceptus starts compactation where the blastomeres adhere by expressing intercellular junctions. When there is 16-32 blastomeres at the 4th day after fertilization the conseptus (all structures that develops from the zygote) is reffered to as a morula. There are two types of blastomeres --> dark and light ones. The dark cells undergo slower division The light blastomeres and ZP will develop a cellular ring called a trophoblast around the developing dark cells which wich form an embryoblast which will eventually develop into the embryo. The morula moves towards the uteus for implantation and when it enters the uterus (day 4) fluid passes into the extracellular spaces of the conceptus forming a blastocyst. The trophoblasts will express tight intercellular junctions. They also express sodium/potassium ATPases which will start influx of Na+ into the blastocyst and water follows eventually forming the blastocoelic fluid. The embryoblasts is localized now in the embryonic pole and the opposite side of the blastocyst is called the aembryonic pole. The blastocyst consists of throphoblasts and embryoblasts: Trophoblats surround the blastocyst and will develop into placenta and membranes. The embryoblast will form the embryo. As fluid gather inside the blastocyst the surrounding ZP will rupture. 11. Implantation. Conceptus, different decidua. Ectopic implantation The corona radiata remain around the conceptus for the start of cleavage. The ZP remain until the 5th day when the blastocyst produce trypsinase and gather fluid. The implantation should take place in the anterior or posterior wall of the uterus inbetween glands of the hypertrophic endometrium. (Placenta brevia – implantation close to cervix uteri --> may induce heavy bleeding.) The implantation should take place at 5th-6th day after fertilization with the growh of a syncytiotrophoblast layer from the trophoblasts which express digestive enzymes (e.g. trypsinase) “digging a hole” into the endometrium. This hole is usually filled with maternal tissue and blood used by the conceptus for histotrophic feeding. The implantation should end at the 13,5 day. The syncytiotrophoblast also secrete HCG with the properties of LH The functional layer of the endometrium during pregnancy is called decidua. There are three decidua during pregnancy: 1. Decidua capsularis – closest to the uterine cavity 2. Decidua basalis – at the placental site 3. Decidua parietalis – the rest of the endometrium Ectopic implantation: - In the peritoneal cavity - On the surface of the ovary - Within the oviduct The epithelium lining respond by increasing vasculature, however these may frequently rupture and threathen the life of both mother and embryo. Surgery performed in short time after implantation may allow the progeny to be moved to the uterus. 12. Bilaminar embryo. Extraembryonic mesoderm Day 7,5 – the embryoblast form two types of cells: Cuboidal hypoblasts closest to the aembryonic pole Columnar epiblasts The remaining cells form amnioblasts associated with the forming amniotic cavity. (day 8) The primitive yolk sac is lined by Heuser membrane formed by cytotrophoblasts and the hypoblasts. The yolk sac will soon be replaced by a secondary yolk sac. Function of the yolk sac: Hoste gonoblasts till 3rd week before their migration. Intravascular hemopoesis. Development of extraembryonic mesoderm (13,5 days) The Heuser membrane shrinks away from the cytotrophoblasts and forms the secondary yolk sac and the heuser membrane cells become replaced by hypoblast cells. Extraembryonic mesoderm cells form between the yolk sac and the cytotrophoblasts in the place where the chorionic cavity (extraembryonic coelom) The amniotic and yolk sac cavity becomes progressily smaller as the chorionic cavity increases in size. The chorionic cavity is lined by extraembryonic mesoderm also covering the yolk sac and amnion. The chorion is the three layered membrane covering the chorionc cavity consisting of extraembryonic mesoderm, cytotrophoblasts and cyncitotrophoblasts. The extraembryonic mesoderm covering the trophoblasts and the amnion is called the extraembryonic external lamina (somatopleuric mesoderm) and the covering of the yolk sac is called extraembryonic internal lamina (sphlancnopleuric mesoderm). 13. Trilaminar germ disc. Gastrulation. Development of chord The process of germ layer formation is called gastrulation and it happens at the beginning of embryogenesis at day 15 or 16. This trilaminar embryonic structure consist of the three germ layers: ectoderm, mesoderm and endoderm. Cell proliferation/migration occurs at the dorsal surface of the epiblast recognized as a primitive streak under the influence of choridin, nodal and Vg1. By this cell proliferation a primitive streak and a primitive node is formed. The primitive node is an organizer that express molecular markers: chordin and some transcriptional factors: goosecoid. Chordin regulate left/right symetry w/other molecules. During cell migration some cells from the epiblast invaginate into the hypoblast (now endoderm) to form embryonic mesoderm. - Remaining epiblast cells = ectoderm - Hypoblast = endoderm - Mesoderm comes from invaginating epiblast cells Cells migrating from the primitive node form the notchcordal process between ectoderm and endoderm. At the caudal end of the primitive streak the embryonic discs remain bilaminar in the location where the future anus will be formed known as the cloacal membrane. The notchcord define the primitive axis of the embryo. The vertebral column forms around this structure and become the remnant nuclei pulposi. 14. Neurolation. Neural plate, neural crest, neural tube Day 18 neurolation: The notchord induces the above ectoderm to transfer into neuroectoderm which starts folding. Angles are called neural crest. The neural groove forms the neural plate which separate from surface ectoderm and forms the neural tube. The neural tube is lined by neural epithelium and both ends of this tube is open and close at different times: Cranial end: neuroporus rostralis at day 23-24 Caudal end: neuroporus caudalis at day 26 No neuropores = anecephaly The neural plate is the structure that was induced by the notchocord to start folding forming the neural groove. The neural angles are known as the neural crest and during neurolation these neural crest cells detach and migrate beside the neural tube and they will form a variety of structures: Pupillary muscle, cilliary body, autonomic ganglia, pigment cells, meninges, muscular components of the head (head mesectoderm) 15. Development of paraxial intermediates and lateral mesoderm Mesoderm mainly give the material for construction of body walls and extremities. The mesoderm which migrated during the gastrulation is found as an intermediate between the ectoderm and endoderm and it forms three distinct masses called: Paraxial mesoderm – somites Intermediate cell mass – genitourinary system --> nephrotomes Lateral mesoderm – give rise to body wall structures and is continous with the extraembryonic mesoderm The paraxial mesoderm become segemented --> somites These somites first appear at day 20 starting from the cranial end towards the caudal part. In total there are 42-44 pairs. Each somite give rise to three parts: - Sclerotomes --> migrate around notchcord and form vertebra - Dermatomes --> develop more externally into skin - Myotomes --> more externally and become muscle Except for the first 5 cervical the somites respond to vertebra. The lateral mesoderm is associated with the intraembryonic coelom (which forms due to apoptosis). The coelom split the lateral mesoderm into somatopleura and splanchnopleura. 16. Tissue iduction, differentiation and determination Embryological processes should follow a well defined coordinated sequence. Such events is controlled by induction which is the stimulation of group of cells to undergo differentiation by another closly related group of cells. E.g. The notchcord which induces the overlying ectoderm to form the neural plate which folds into the neural groove --> neural tube. There are three big groups of inductors: I. The largest is the paracrine group which influence from a distance (hormones/GF) 1. FGF’s --> development of mesoderm and CNS development 2. TGF-ß --> many subgroups such as BMP which influence mesoderm 3. Hedgehog gene class --> specific structures e.g. cell junctions, symmetry of body 4. WNT-genes --> development of dermamyotomes 5. Ephrines --> relativily unkonwn but blood vessel typing II. ECM proteins --> require receptors III. Cell-surface proteins working via receptors Determination is the process of inducing the development of distinct tissue/organs from blastemae. The cells will loose their omnipotence and development become dependent on epigenesis. Determination usually takes place during gastrulation by blockage of certain genes. Differentiation depend upon cell migration, interaction and proliferation/apoptosis. The differentiation is the development of specific structures with specific function. Wihout differentiation all cells would express the same phenotype and there would not be any multicellular organisms. 17. Tissue growth and factors influencing the growth process Growth is the increase in cell number, size and ECM. Cell of a large individual is equal in size to those of a smaller individual but there are more cells in the larger individual. Growth is stimulated by growth factors and hormones, substances with growth properties. The main growth factors during embryogenesis is: - insulin - Epidermal growth factor - TGF-ß - Parathyroid hormone related protein (PTHRP) - Oncogenes Growth is inhibited by chalones. Chalones are inhbited by anti-chalons. These substances may be used in theraphy, especially haematology. 18. Derivates of the germ layers Karl von Baer coined the term germ layer. Most organs develop under the influence of all three germ layers. Thus we usually point to the origin of the epithelium in the organ to state the germ-layer origin. Ectoderm: Two types Surface ectoderm --> epidermis, hair, hair follicles, nails, skin glands, adenohypophysis, teeth, enamelum and inner ear. Epithlium of anal canal Neuroectoderm: Neural tube – Brain hemispheres, spinal cord, retina, pigemented epithelium of retina, cilliary muscle, epiphysis and neurohypophysis Neural crest – autonomic ganglia, adrenal medulla, pigmented cells, schwann cells Headmes-ectoderm – head mesenchyme, neurocranium, meninges, head muscles, dentinum/cementum, CT Endoderm – epithlium og GI-tract, pharynx, larynx, trachea, cavum-tympani, tonsils, thyroid gland, parathyroid gland, thymis, bronchi, lungs, liver, pancreas, urinary bladder Mesoderm: Three types Paraxial msoderm --> skeletal/muscles of trunk, dermis and hypodermis Intermediate mesoderm --> nephrotomes --> kidneys, gonads, some reproductive cells and excretory duct of reproductive system Lateral mesoderm: Sphlancnopleura – SM, heart, hemapoetic cells, mesothelium of inner organs, BV, adrenal cortex, spleen Somatopleura – pericardial mesothelium, omentum 19. Cephalocaudal flexion and lateral folding. Development of intraembryonic coelom Embryonic folding starts in the 4th week (day 21 - ) due to differential growth of the tissues. The embryo grow faster in the length then in width so the flexion is more pronounced in the cranial and caudal region. At the same time the notchcord, neural tube and somites stiffen the dorsal axis. Cranial and lateral folding day 22 Caudal region at day 23 Formation of the the cranial and caudal fold result in the formation of the foregut, midgut with its vitelline duct connected to the yolk sac from the endoderm. The caudal fold result in the formation of hindgut connected to the allantoise. The lateral folding result in the formation of the intraembryonic coelom which is horseshoe shaped cavity that will form the pericardium, pleura and peritoneal cavity by the 2nd month. The place for the intraembryonic coelom is formed by apoptosis. The pericardium and the pleura will later be seperated by pleuropericardial membranes. The thoracic and abdominal cavity will be seperated with the formation of the diaphragm from four sources: septum transversum, mesophageum dorsale, pleuroperitoneal folds and somatopleura. The pleura and peritoeneum by the pleuraperitoneal membrane. If the lateral folds fail to fuse we get anterior body wall herniation. In case parts of the thoracic body wall fails to form due to fusion of lateral folds we may get ectopia cordis. If the different parts of the diaphragm fuse diaphragmatic herniation may occur 20. Chorion leave and chorion frondosum. Primary, secondary and tertiary villi. Anchoring villi Until week 8 villi covers the whole surface of the chorion but on the side towards the lumen of the uterus they will become atrophic and form the chorion leave (“-smooth”) and on the end facing decidua basalis the villi increase rapidly in size and form chorion frondosum or villious chorion. Villi starts appearing allready by the start of the 2nd week. Both chorions are connected at the chorionic plate consisting of: Amiotic columnar cells, extraembryonic mesoderm, BV’s, cyto- and syncitio- trophoblasts. After the 4th month --> fibrinoid. Placentation starts by day 13,5 while villi starts to develop from beginning of 2 nd week. There are four types of villi: 1. Primary chorionic villi – cytotrophoblasts and synsitiotrophoblasts will migrate into the endometrium and form villious extensions. Maternal blood flow in spaces between these extensions. 2. Secondary villi – when the villi receives extraembryonic mesoderm from chorion 3. Tertiary villi – When mesenchymal cells differentiate into BV’s which are connected to the intraembryonic vascular network. 4. When tertiary villi anchor to decidua basalis they become anchor villi 21. Allantochorion. Development of the umbilical cord When the allantois and the chorion fuse they form a membrane called the allantochorion. The umilical cord develops from the connecting stalk (with allantois and vessels) and the yolk sac (w/vessels) merge and become enveloped by the amnion. - The extraembryonic mesenchyme of these structures become the Wharton’s jelly which protects the BV’s of the mature umilical cord - The yolk sac and connecting stalk degenerates - Allantoise vessels become the umilical vessels but one vein (the right) degenerates leaving 1 vein and 2 arteries. - In approximatly 0,5% of cases the umilical cord contains only one artery. This leads to cardiovascular anamolies in around 20% of cases. - The umbilical cord is not innervated by nerves and the umbilical vein can be used for blood transfusion. The length of the final umbilical cord is about 50cm. In seldom cases hyperactivity of the child leads to the umbilical cord enveloping a limb or even the neck causing strangulation grooves or even death of the fetus. 22. Amnion. Amnion liqour, its content. Fetal liqour The amnion encloses the amniotic fluid surrounding the developing embryo. It is formed by columnar amnioblasts from the epiblasts of the bilaminar embryo and a thin layer of extraembryonic mesoderm. It is replaced every 3 hours by maternal circulation. Functions: Allows foetal movements Protective cushion – against mechanical stress, dryness, temperature and adhesion This fluid is swallowed by the foetus due to unkonwn reasons. Amniotic liqour: Increases to 1000mL at week 35 but decreases to 800mL at birth Content: - 99% water - Detached epithelial cells --> used for karyotyping after amniocentesis - Protein - Fat - Carbohydrates - Hormones - Pigment - Foetal urine - Estradiol – aliveness of the fetus - Urea – increase indicate intrauterine hypoxia - Alpha-fetoprotein – important dectector for neural tube defects + Down syndrome 23. Paraplacental pathway The paraplacental pathway is an alternative nutrition pathway between chorion leave and decidua capsularis. This pathways is of no significance unless the mother dies and the developing foetus can be held alive for about 2 hours during which time we may perform surgery in order to save the developing foetus. 24. Cotyledonis. Significance of different trophoblasts With development the decidua basalis is eroded by syncitotrophoblasts forming intervillous spaces. Some of the decidua remain and it forms compartments which contain a villious tree with many branches called a cotyledon. At the start of placentation (13,5 day) there are about 200 cotyledons which decrease to about 50- 60 before birth. These can be recognized in the placenta as lobules. The trophoblasts have two main functions: 1. Developing into hormonal active cells of peripheral trophoblasts which secrete: a. HCG – used as a pregnancy detector as it appears in maternal blood 9 days after fertilization. It’s function is to prevent disintegration of corpus luteum --> corpus luteum graviditionis which maintain progersterone production b. Placental lactogen – stimulate general growth c. Chorionic corticotropin (similar effect ot ACTH by stimulating metabolism and cardivascular function in the mother) d. Later it produces progesterone e. Estrogens in coorporation with fetal liver and adrenal gland f. Prostaglandins and IL 2. The trophoblasts replace the endothelium of the materanl spiralic arteries in decidua basalis in order to stabilize the maternal blood supply to the placenta. If this does not occur pre-eclampsia may develop (maternal hypertension). Cytotrophoblasts of the villi may comglumerate and form proliferation buds which aid growth of the villi. Some of these proliferation buds may detach and enter the maternal blood flow where they may reach the lungs possibly causing tumors. 25. Fetal part of the placenta The fetal part of placenta consist of the chorionic plate and tertiary villi. The chorionic plate consist of amnioblasts (columnar epithelium from the epiblasts), extraembrynoic mesoderm, vessels and after the 4th month fibrinoid. The anchor villi builds a brigde between the fetal and the maternal part of placenta. 26. Maternal part of the placenta The maternal portion is formed by decidua basalis and tertiary villi. The decidua contains decidual cells which dissepear in the 1st trimester and hormonal active cells of peripheral trophoblasts which secrete: (see above) 27. Placental circulation. Intervillous space During placentation (starts day 13,5) the syncitotrophoblasts secrete hydrolytic enzymes such as trypsinase which will degrade the endometrium/decidua basalis and form spaces called lacunae where the branching villi grow into. Some parts of the decidua remains forming the intervillous septs. The lacunae is filled with blood (about 150mL) from about 100spiralic arteries which provide the tertiary villi with oxygenated blood. Pressure from the blood distribute blood throughout the intervillous spaces where the exchange of substances occur. When the pressure is decreased blood flows back into the endometrial veins. The placenta contains three main vessel: The umbilical vein, two umilical arteries. These vessels are surrounded by whartons jelly (from extraembryonic mesoderm of the yolk sac/allantois) which protect the vessels. The placental barrier consists of six structures: 1. Epithelial cells of syncitotrophoblasts 2. Cytotrophoblasts (dissepaer at 4th month) 3. Basal membrane of the epithelium 4. CT of the tertiary villi 5. Basal membrane of endothelium 6. Endothelium 28. Functions of the placenta. Adaption (ageing) of placenta. Fibrinoid Functions: - Respiration - Nutrition - Excretion - Protection - Storage - Hormonal production --> progesterone The placenta transfers oxygen, water, electrolytes, nutrients, hormones, ABs, iron, trace-elements aswell as drugs, viruses, toxic substances from the mother to the fetus. From the conceptus to the mother it transfers: carbonic acid, water, electrolytes, urea, creatinine, bilrubin, hormones, erythrocyte AGs. Adaption of placenta: Fibrinoid forms from maternal fibrinogen and embryonic cytorophoblats. This transformation occur allready at the start of placentation but increase after the 4 th month. Increased fibrinogen formation may be seen in cases of immuneconflict: E.g. erythroblastosis fetalis (Rh- mother/ Rh+ fetus) or blood group conflict. Surface volume increases to 15m2 due to growth of microvilli. Hofbauer cells from villous mesenchyme become embryonic macrophages. Calcium despositions in the tertiary villi CT The syncitotrophoblats form an unhomogenous thickness of the villi surface where the thicker region has increased hormone production and the thinner regions have higher diffusion rate. 29. The formation and the role of the fibrinoid in full-term placenta Fibrinoid is a desposition of maternal fibrinogen and derivates of trophoblast cells from the tertiary villi. This is a normal process and it starts allready right after placentation but production of fibrinoid spikes at the 4th month.  Excessive amounts of fibrinoid can cause infarction of the fetal blood supply. This can lead to cardivascular anamolies. A placenta after birth should be elastic but will appear rough if excessive fibrinoid is accumulated. Fibrinoid accumulation increases with immune conflics: - Erythroblastosis fetalis - Blood group conflict - HIV positive mothers or mother with hepatitis 30. Umbilical cord Develops from the connecting stalk with allantois and vessels and the yolk sac with vessels. These two structures merge and become surrounded by the expanding amnion. The extraembryonic mesoderm from the yolk sac/allantois forms Wharton’s jelly which works as a protective layer around the umbilical vessels. The allantois and yolk sac will degenerate and the allantois vessels (two veins and two arteries) will form the umbilical vessels. One vein degenerates leaving two arteries and one vein. If only one artery is present (0,5% of cases) the chance of intravascular anamolies is about 20%. The umbilical cord becomes about 50cm long and in cases of excessive movement of the fetus the umilical cord may sling around it’s neck causing anoxia or death. If the umbilical cord encircle a limb it causes a strangulation groove leading to local hypoplasia. The umbilical cord is not innervated by nerves and the umbilical vein may be used for blood transfusions. The umbilical cord is cut after partuition and the insertion point – umbo – remains as one of the weakest points of the body frequently causing herniation which is very fixable. 31. Morphofunctionl basis of delivery Parturition is a complicated process --> especially the first time and it involves several hormonal pathways with effect on the uterus and the reproductive tract. It starts with decrease in placental progesterone and estrogen secretion which causes the first uteral contractions. The foetal sac ruptures and the amniotic fluid is expelled. The foetus should move head first downward irritating mechanoreceptors of the pelvis which stimulates nucleus paraventricularis of hypothalamus which stimulate the release of oxytocin from the hypophysis which increases the uterine contractions. Within 30min the placenta should also parturate. Child birth is not a risk free process. Even in well developed countries such as Sweden the rate of maternal mortality is still 10 deaths/100.000 births. Complications: Placenta previa Poor uterine strength Large babies --> foreceps and clamps may be used Cessarian sections Factors such as coitus, spicy food, hot baths etc. have been said to induce childbirth but in the clinic oxytocin is given per injection to induce labour. 32. Prenatal diagnostic methods In modern age even the foetus have become a patient. We screen and perform diagnostic exercises on the foetus on a daily basis and such diagnosis is based on: Aminocentesis – by transabdominal insertion of a needle and taking a sample of the amniotic fluid in order to detect presence of protein, hormones, enzymes but also to acquire detached epithelial cells which can be studied in order to find chromosomal defects. Maternal serum diagnosis – detects the folic acid levels etc. A decrease in folic acid levels indicate defects related to the neural tube, heart defects and Down syndrome. Ultrasound screening – the most important and widly used in the clinic especially in early stages of development. Used for detecting morphological details about the fetus e.g. the cervical fold can give indications of down syndrome. Foetoscopia – insertion of an intrauterine endoscope which observes the fetus Amnio-foetography – used in addition to ultrasound screening and is based on the x- ray scan of the fetus. Preimplantation diagnostis – after IVF we can obtain one blastomere from the conceptus in order to detect informatino about its status 33. Development of body external shape. Morphofunctional principles of inductors for common body development The human body follows certain principles of body development: - Division of the body into caudal and cranial poles - Metametric division of certain organs. E.g. two lungs, two eyes, two kidneys etc. - Regional development is usually provided by dermatomes and myotomes developing together In the head however these principles are controlled by induction centers which are specific for this region: Proencephalon – induction to eyes, nose and anterior base of skull Rhombencephalon – inducts ears, occiptal skull and brachial apparatus Hindbrain – inducts medulla oblongata, spinal cord, vertebral column and GI tract The existence of these centers are proved by developmental conditions such as cyclopia. Molecular control of skeleton formation: Hox genes have general control over skeletal formation Sox-9 activate transcription of collagen IIa in mesenchymal cells --> precartilage cells Core binding protein – influence mesenchymal cells --> osteoblasts BMP – especiially BMP-2,4,7 --> concerned with bone development 34. Development of verebral column, ribs and sternum The vertebra develops from somites through three stages: mesenchymal (starting at the 4th week), carilaginous period (at the end of embryonic development) and bone period in postnatal life. Mesenchymal stage: During the 4th embyonic week the sclerotomes from the somites surround the notchcord (persist as nucleus pulposi) and the caudal part of one sclerotome fuses with loose mesenchymal cells of the inferior sclerotome. The seperation of the vertebra is regulated by the Pax-1 gene. Processus spinosus, transversus and costarius arise and the vertebral arch is formed by fusion between the growth centers in the ventral and dorsal part. Myotomes migrate to the level of intercostal discs and attach to the vertebra above and below aswell as the the next vertebra in order for the vertebral column to hang in the muscle to provide free movement. (The exception is atlas and axis where the dense axis develops protruding upwards. Cartilaginous stage: Vertebra undergo endochondral ossification from 3 primary ossification centers (1 ventral 2 dorsal) where the dorsal ones fuse to form the vertebral body. Failure to do so results in spina bifida. The bone stage: After birth secondary ossification centers appear at the superior and inferior surfaces and they act as growing centers. Ossification ends first in the lumbar region around the age of 14-16 and in the os sacrum all ossification centers fuse in order to create one bone. Fusion of all ossification centers mark end of grwoth at age 21-25. Before this period we can repair “computer child syndrome.” Curvatures (lordosis and kyphosis) appear in the first year of postnatal life. The development is under control of Hox a/b/c/d Ribs & sternum: Ribs develop from condensed mesenchymal cells lateral to centrum. Proximal part of ribs develop from the venteromedial sclerotome while the distal part develops from venterolateral part of adjecent somites. Untill ossification they are seperated from the vertebra untill ossification at 20th postanatal year. Sternum starts formation as two lateral mesodermal condensations of ventral body wall and one interclavicular blastema. Sternum ossifies between age 21-25. 35. Development of cranium: chondrocranium and desmocranium The cranium develops from four sources: 1. Unsegmented head-mesectoderm 2. Prechordal mesoderm 3. 4 cranial somites 4. mesoderm of the first two brachial arches The bones of the cranium form through two processes. The bones of the viscerocranium form by intermembraneous ossification and they are known as the desmocranium forming the bones of the face. The neurocranium forms the bones surrounding the brain, oral cavity, pharynx and upper respiratory ways. They are formed by endochondral ossification. The neurocranium starts development at the 2nd embryonic month by independent development of three cartilaginous laminae: - At the base of the skull - In the nasal cavity enrolling the olfactory cells - In the ear region enrolling the labyrinths of the inner ear These laminae will fuse and by endochondral ossification they will form os occipitale, os temporale (pars petrosa and pars mastoidea), os sphenoidale. Some of the neurocranial bones incorporate membraneous elements in their formation and develop: os parietalis, os temporalis (pars squamosa), os frontale and parts of os occipitalis. The viscerocranium forms at 3rd month of embryonic development. The viscerocranium forms by intermembraneous ossification and the results in os zygomaticus, nasale, vomer, maxialla, mandibula, lacrimale, palatinum and tympanic ring. At birth these bones are seperated by sutures and synchondroses which acts as growth centers. In the cranium there are a series of unossified membranes, fontanellae: The anterior fontanelle is located between two frontal and two parietal bones and closes at 1,5 years of age. (It’s bulging is an indicator of increased intracranial pressure and can be palpated) The posterior fontanelle between the occipitalbone and the two parietal bone ossifies at age of 1. The lateral fontaneallae are located between frontal, parietal and temporal bone and ossifies at 3 years of age. 36. Ossification of cranium and limb bones Ossification of cranium: Chondral ossification is characteristic for os ethmoidale, concha nasalis inferior, os sphenoidale, os temporale (pars petrosa), os occipitale (pars basilaris) and ear ossicles (malleous, incus, stapes). Desmosification is characteristic for maxilla, os zygomaticum, os palatinum, vomer, os nasale, os lacrimale, os frontale, os parietale, os temporale (pars squamosa et tympanica), os sphenoidale (pars lamina medialis), processes pterygoideus, ala major, os occipitale (pars squamosa) and mandibula Ossification of limb bones: Limb skeleton develops via endochondral ossification under influence of BMP. The first ossification centers appear and the ossification starts at week 6-7 and by the 3 rd month the diaphyses will be ossified. The epiphyses will ossifiy after birth with the growth zone marking the growth of the limbs. 37. Development of limb skeleton. Main skeleton malformations Limbs develops as outgrowths of the venterolateral body wall by expression of FGF- 10 and Hox-8 gene which stimulate signaling centers in the limb bud. The upper limbs develop first followed by the lower limbs. The differentiation of limb buds occurs between 5th and 8th week. The upper limb buds appear on day 24 The lower limb buds appear at day 28 Limb bud development goes along three axes: The proximo-distal axis from the base of limb to tip of fingers – growth of length – Hox genes The antero-posterior axis – 1st to 5th digits – Shh and Hox genes Dorso-ventral axis – back of hand/palm = dorsal and palm/sole = ventral – WNT gene With the rapid growth of the limb buds the distal part of the limbs flatten and form flipper-like limbs. The digits will be seperated by apoptosis based on expression of BMP-2 and MSX-1 and 2 expressed in the interdigital space. During the limb bud differentiation (5th – 8th week) there are some especially significant days/events: Day 33: The upper limb presents shoulder, arm, forearm and hand plate The lower limb presents a rounded distal tip in the caudal end of the embryo Day 38: Upper limb – digital rays are clearly represented and apoptosis occur in interdigital spaces. Lower limb show clear foot plate Day 47: Upper limbs start horizontal flexion. Lower limb shows digital rays and apoptosis and start horizontal flexion. Day 52: Upper limbs show elbow and tactile pad bends. Lower limb becomes longer and digital plates are visible Day 56: The fingers of the upper limbs should cross in the midline Day 60: Lower limb development should be complete This: The limb skeleton develops from mesodermal condensations at the long axis of the limbs at the 5th week. The condensations express BMP-2 and 4 and when they mature BMP-6. The osteogenesis occurs through endochondral ossification with appearance of primary ossification centers in the diaphysis followed by secondary ossification centers in the epiphysises. Ossification of the primary center starts at 6 th-7th week and is done by 3rd month while the secondary centers are the basis for growth and they ossifiy in postnatal life. Main skeletal malformation: Hereditary achondroplasia – AD manifested condition where the body proportions are normal except for long bones in extremities which are shorted resulting in dwarfism. Osteogenesis imperfecta – mutation in the collagen I gene resulting in brittle bone Spina bifida – incomplete formation of the vertebral arch that may result in herniation of the spinal cord. Amelia – absence of limbs Phocomelia – shortening of proximal limbs Syndactylia – fusion of fingers 38. Skeletal striated and smooth muscle development The skeletal muscle develop from somites --> myotome cells which differentiate into myogenic cells and start mitotic division and become postmitotic myoblasts. These postmitotic myoblasts will differentiate into multi-nucleated muscle fibers with cross striations. General SM growth is under influence of FGF and TGF-ß. The formation of actin and myosin is regulated by insulin related growth factor. - Satelite cells are belived to differentiate from another separate cell line The phenotype of skeletal muscle is depdendent on the expression of light and heavy myosin chains. These parameters are not fixed and can be changed by hypertrophy, atrophy and deinveration. In general fast neurons innervate fast muscle and slow neurons innervate slow muscle so the detection of origin can be detected by the innervation. Muscles of the trunk develop from myotomes which divide into epimers --> extensors of the vertebral column innervated by dorsal branches of spinal nerves. Hypomers --> flexors of the trunk innervated by the ventral branches of spinal neurons. Head muscle develops from cranial somites (tounge muscles) Prechordal mesoderm together with neural crest cells (external eyeball muscles) Brachial arches (facial, masticatory and pharyngeal muscle) Smooth muscle forms from lateral mesoderm except: m. sphincter and dilatator pupillae --> neural crest Intestinal smooth muscle --> sphlancnopleura mesoderm Smooth muscle of BV’s --> local mesoderm 39. Development of muscles in inner organs, heart, body, head, viscera, limbs, diaphragm and muscle developmental abnormalities Muscles of inner organs: develop from splanchnopleura mesoderm Cardiac muscle: form from splanchnic mesoderm that evnelopes the epithelial heart. The myogenic cells fuse and form intercalated discs at their junctions. The conduction system is formed from special cardiomyocytes with irregular myofibrils. Body muscle: ? Head muscle: from four different sources Cranial somites --> muscles of tounge Prechordal mesoderm under influence of neural crest cells --> external eye ball muscles Branchial arches --> masticatory, facial, pharyngeal, laryngeal muscles Visceral muscle: apperently from the branchial arches Limbs: form condensations of somitic mesoderm at week 5. These masses develop 50-100 migrating dermatomyotomes which are divided into ventral and dorsal group. The muscle development is under the influence of hepatic growth factor (HGF) and FGF. Muscles are fixed in locations where muscle molecules Myo-D are located. The muscles cells undergo myoblast --> mature cells with centrally located nucleus in the 4th month. Regeneration is possible from myosatelite cells. Ventral group forms flexors of upper/lower limb Dorsal group forms extensors of upper/lower limb Diaphragm: develops from four sources 1. Septum transversum --> central tendon 2. Mesentery of oesophagus --> crura of diaphragm 3. Pleuroperitoneal membranes --> lateral part of diaphragm 4. Somatopleura Abnormalities of muscular development - Aplasia of muscles --> lack of a muscle - Abnormal muslces --> additional insertions/length etc. - Body wall deformation --> herniation - Muscular dystrophy --> DMD, Beckers MD, Myopathy 40. Periods of embryonal haemopoesis The embryonic haemapoesis is divided into three period: 1. Megaloblastic period – day 13-14-18 Starts with the appearance of bloodislets in the extraembryonic mesenchyme of the yolk sac, chorion and the connecting stalk. Hemangioblasts appear and they have bi- potential meaning they can develop into both endothelial cells and hemapoetic cells. This hemapoesis occurs intravascularily with developmen of blood cells and blood vessels simoutaniously. 2. Intraembryonic hemapoesis – start 7th week Really starts right before day 22 with the first heart beat. At day 28 hemapoesis occurs in para-aortic hemapoetic islands. During 5-6 week the hemapoesis changes to the liver and a small amount in skin, spleen and omentum. The cells produced in the liver have special fetal hemoglobin. The liver hemapoesis decline after the 6th month 3. After the 6th month the hemapoesis start in the red bone marrow. Both red and white cells are produced. This change is regulated by cortisol and if absent it will continou in the liver. Hox genes are active in control of hemapoesis. The fetal erythrocytes have shorter life span then adult cells (50-70days) and they posses a fetal type of hemoglobin with increased affinity to oxygen. The change to adult ertythrocytes occur at week 30. 41. First blood vessels and circulatory system During somite formation primordia of blood vessels appear. At the same time vessels form in the extraembryonic mesoderm from mesenchyme. The seperated vessels will fuse and form the primitive embryonic vascular network. =vasculogenesis The angioblasts form the basis for the vasculogenesis during the embryonic period and later vessels will be formed from sprouting (angiogenesis) from these vessels, a process which continous in postnatal life. = angiogenesis Both vasculogenesis and angiogenesis depend on several growth factors: VEGF-2 and VEGF-a --> formation of angioblasts from mesenchyme Angiopoietin-1 --> sprouting PDGF (from endothelial cells) --> migration of mesenchymal cells to vessels TGF-ß --> stimulation of mesenchyme into pericytes Arteries --> Ephrin-B2 Veins --> Ephrin-B4 The first main blood vessels to appear are the precardinal and postcardinal veins that return blood to the heart. The vitteline veins that return blood from the yolk sac, the umbilical veins that return blood from placenta to heart and finally the two dorsal aortas that fuse and form one in the caudal half of the embryo. The intraembryonic circulation starts at day 22 with the first heart beat. 42. Heart development: endocardial tube, seperation of atrium and primitive ventricles, conductive system, coronary arteries The sphlancnic mesoderm proliferate and form angiogenetic clusters in a horseshoe manner. The ventral cells of these clusters will form the cardiogenic area at day 17- 18-19 while the two dorsal parts form the dorsal aortas. - Cords spread out from the cardiogenic area and canalizes into two endocardial tubes - Due to lateral folding these tubes will form one single endothelial tube - The cephalocaudal flexion pushes the tube into the pericardial cavity The myoepicardial mantle form around the endothelial tube from splanchnic mesenchyme. The mantle consist of: outer cells --> epicardial cells, middle cells --> myocardium, internal cells --> endocardium. The cardiac jelly (in the endocardium) contains adherons --> proteoglycans, fibronectin and matrix protein. They induce together with TGF-ß endothelial cells to transform into mesenchyme. These mesenchymal cells secrete protases which destroy adherons and form endocardial cushions responsible for development of main part of heart valves. Disturbance of events lead to heart anamolies. In the 3rd week the heart undergoes dextral looping under the influence of HAND-1 and HAND-2 transcriptional factors which are responsible for asymmetry. MEF-2 and Nkx2-5 are also involved. The result of this looping is an S-shaped heart with characteristic regions: 1. The venous sinus (2x) contain the common cardinal veins, the vitteline veins and the umbilical veins which receive heart input 2. The primitive atrium 3. The atrioventricular channel 4. The primitive ventricle 5. Bulbus cordis with conus arteriosus 6. Truncus arteriosus --> form the aortic arches The development of the cardiac valves occur from the mesenchymal proliferations that is caused by the adherons of the cardiac jelly. They will form the tricuspid and the bicuspid valve in the attrioventricular canal seperating the primitive atrium and ventricles. Septation occur when a primitive sept called septum primum grows from the superior wall down towards the endocardial cushions dividing the atrium into right and left. The sept does not reach the cushion but stops to form the primary opening. A second sept, septum secundum forms to the right of septum primum and it is more muscular covering the primary opening developing the second opening --> foramen ovale which is an embryonic shunt between the right and left heart. The pressure that occurs when the baby is born with the first respiration should close this valve. Simoultaniously there is a sept growing from the apex of the heart seperating the two cardiac ventricles. The bulbus cordis consist of the proximal conus arteriosus and the distal truncus arteriosus which will fold when the aortic-pulmonary sept appears and divide the conus arteriosus into the pulmonary trunk and the ascending aorta. The conduction system develops from mesenchyme of the splanchnopleura in the venous sinus forming the SA-node. Following the AV-node will develop in the atrial septal region. Endothelin-1 produced by coronary capillaries induce the change from cardiomyocytes into conduction cardiomyocytes. Coronary arteries develop from the superficial blood vessel network of the heart. 43. Development of the main arteries The first arteries appear in early stages of somite formation as continuations of the endocardial tubes. When brachial arches form they receive their own arteries called aortic arch arteries starting from the heart outflow tract. These arteries never exist at the same time. They empty into the paired dorsal aortas and form into new vessels as the vascular network matures: The 1st aaa mostly degenerate and form a.maxillaris and a.carotis externa. The 2nd aaa --> a.hyoideus and a.stapedius The 3rd aaa --> a.carotis communis and proximal a.carotis interna The 4th aaa --> arcus aorta and proximal a.subclavia The 5th and 6th may not even exist but the 6th form ductus arteriosus Blood vessels of aorta: Dorsal intersegmental --> aa.intercostales and aa.lumbales Lateral segmental --> aa.renales, aa.suprarenales, a.testicularis/ovaricus Ventral segmental --> truncus coeliacus a.mesenterica superior/inferior The umbilical arteries --> form plica umbilicalis medialis, a.iliaca interna, a.vesicalis superor The right vitelline artery --> parts of a.mesenterica superior 44. Development of main veins In week 5 the venous network consist of the vitelline veins, the umbilical vein and cardinal veins. The pre- and post-cardinal veins join and form ductus venosus which enter the venous sinus of the heart together with the vitelline veins and the umbilical veins. The right common cardinal vein --> v.cava superior The vitelline veins form the capillary network of the liver: The left one degenerate The right one form v.portae + venous sinuses The left umbilical vein will form the round ligament of the liver 45. Fetal circulation. Newborn circulation The blood is oxygenized in the placenta and flows towards the heart through the umbilical vein. One part bypass the kidneys through ductus venosus (bloof flow is regulated by the muscular spincter of ductus venosus) to v.cava inferior and the other part flows through the liver and into the v.cava inferior. The blood is returned to the right atrium and the major portion goes from right to left atrium via foramen ovale and from left atrium --> left ventricle --> aorta The minor portion returns to the major circulation through ductus arteriosus between truncus pulmonalis and arcus aorta (become lig. Arteriosus) The blood from the upper body is returned from v. cava superior --> right atrium --> right ventricle --> truncus pulmonalis --> ductus arteriosus --> aorta. The blood in the aorta is thus mixed oxygenated and deoxygenated. The potency of ductus venosus and ductus arteriosus is maintained by prostaglandin E2 and prostaglandin I2 under the influence of NO. Blood is then returned to the placenta by the umbilical arteries. Changes to newborn circulation should occur by the first inspiration after birth and changes should be complete by the first 2-4 weeks. These changes are: - closing of foramen ovale - closing of ductus arteriosus --> lig. arteriosus - Umbilical arteris form plica - Umbilical vein --> lig.teres uteri 46. Development of lymphatic system and lymph nodes The lymphatic system develops in the 5th week of embryonic development. The lymphatic capillaries develop from anastomizing perivascular spaces under the influence of VEGF-3. At the 6th week the six lymphatic sacs develop: The jugular lymph sacs The retroperitoneal sac Cisternae chyli --> do not change in postnatal life The two posterior lymph sacs --> transform into sinus lymphaticus iliacus/inguinalis Collecting lymph vessels connect these sacs by the end of week 9 Finally ductus thoracicus develop connecting cisternae chyli with the junction of v.subclavia and v.jugularis interna. The right 1/3 of the body is drained from the right jugular sac. The lymph nodes deveop in the places where the lymph sacs fold and infiltrated by a capillary network. This structure develop the vas afferentes, subcapsular sinus and vas efferens at the opposite side. The capsule is formed by surrounding mesenchyme. The first germinative centers appear after first sensitization to AG. 47. Inductive factors for heart development. Risk periods in heart development. Main morphofunctional anomalies in heart and circulatory system Critical periods: - Fusion of the endocardial tubes - Folding of the embryo - Septation of atria and ventricles - Development of valves - Development of truncoconal region Factors influencing heart devlopment: Dynamic blood pressure Apoptosis Migration of neural crest cells Congenital anomalies: 1. Open ventricular sept (foramina interventricularis) 2. Arterial sept defects such as open foramen ovale – this is a very common occurrence and the degree of opening determine the functional effect 3. Truncus arteriosus persistance --> incomplete septation of truncus arteriosus 4. Tetralogy of fallot --> hypertrophy of left ventricle, stenosis of pulmonary arteries, ventricular sept defect and right formed aorta. 5. Artresia (a passage is closed or absent) of the aorta/pulmonary trunk 20% of all anomalies affect heart and major blood vessels (5-8/1000) and factors include: lithium, trisomy, rubella, diabetes, alochol abuse 48. Pronephros and mesonephros The kidneys develops from several structures where the first two structures regress and form male excretory ducts and supplements to the kidneys while the final structure form the definite kidneys. These structures are the pronephros, mesonephros and metanephros. The pronephros is rudimentary and will degenerate without any function. It forms from the intermediate cell mass in the cervical region called the cervical nephrotomes. These structures differentiate under the influence of: Lim-1 --> aggregation of mesenchymal cells Pax-2 --> transformation of mesenchyme into tubules. The pronephros will dissepear by the 24/25 day. At the end of day 24 a pair of mesonephric ducts appear from nephrotomes in the thoracic and lumbal region. Associated with these are about 20 mesonephric units with: Glomerulus – connected to the dorsal aortas Bowman’s capsule Mesonephric tubuli These partial kidneys are functional between the 6th and the 10th week and they produce a small amount of urine excreted into the amniotic fluid in this period. Afterwards they start to regress and in males they form the reproductive tubuli system (ductuli efferentis testis) while in females they dissepear. The ducts will fuse with the cloaca in the 4th week and in the 5th week the ureteric bud starts to form from the mesonephros. 49. Metanephros. Inductors for metanephros development. Main kidney development anomalies At the 5th week the definite kidneys start to form from two sources influencing each other: The metanephros – the blastema of the nephrons appear from sacral nephrotomes which start to secrete inductor molecules GDNF (glial dervived neurotrophic factor). The ureteric bud – from the mesonephric ducts start to migrate towards the metanephros. They enter the metanephros and they will form the ureters, the renal pelvis, the calyxes and the collecting tubuli. They also secrete inductors directed to the mesenchyme of the metanephros; WNT and BMP which induces the metanephros to form the nephrons consisting of proximal tubuli, loop of henele and distal straight and convoluted tubuli. At the same time vascular endothelial cells form from metanepric mesoderm to form the glomeruli. The kidneys are formed in the pelvis of the embryo but during week 6 and 9 they ascend to the abdomen while they are progressivly revascularized by the dorsal aorta. The urine production of the metanephric kidneys start at the 11th week and they secrete fluid into the amnion creating the major portion of this fluid. The placenta does the work of removing wastes. Bilateral agenesis thus result in very small amniotic space, this is called oligohydramnios. Congenital malformation - Horseshoe kidneys - Renal agenesis - Polycystic kidneys – no fusion between ureteric bud and metaneprhos - Pelvic kidneys – the kidneys fail to migrate to the abdomen between week 6 and 9 - Hypoplasia of kidneys 50. Development of reproductive system until 6th embryonic week Allthough the genetic sex is determined by the spermatozoa allready at fertilization the development of gender specific genitalia takes more time. The development of the reproductive system occurs in three stage: 1. The indifferent sex stage (week 4-6) 2. The development of gonads into testis and ovaries (week 8) 3. The development of external genitalia and ducts of the reproductive system The indifferent stage starts between week 4 and 6 when the urorectal septum divides the cloaca into two parts: the anterior urogenital sinus and the posterior rectum. The urogenital sinus is again divided into three parts: the upper part forms the urinary bladder, the middle part forms the membranous part of urethra in females and prostate in males and finally the lower part forms the vestibulum of the vagina and the urethra of penis in men. At the same time the ureteric bud is intercalated into the posterior wall of the urogenital sinus The mesonephros and the coelomic epithelium forms the primitive sex cords. The primitive sex cords consist of cortex and medulla. The development of both early kidneys and gonads is under the influence of Lim-1 and without this factor neither kidneys or gonads will develop. The paramesonephric ducts develop from coelomic epithelium and form the paramesonephric ducts (müllerian ducts) producing the duct system of the female reproductive system. They are identical to the mesonephric ducts which forms the tubule system of the male reproductive system. The determination of gender is based on several candidate genes located on the Y chromosome. These genes are H-V antigen which code for a minor-histobility complex. Second is the ZFY (zinc finger Y) and finally, the most conclusive (used for gender determination) the SRY gene. 51. Development of male reproductive system Cells from the coelomic epithelium differentiate into the genital ridges in the posterior body wall in the 5th week and they develop into the sex cords under the influence of Sox-9. The sex cords will develop into testicular tubules after incorporation of germ cells. Testis develop earlier and faster then the ovaries and by the 8th week sertoli cells start to secrete antimüllerian factor which makes the paramesonephric ducts regress. Sertoli cells also induce the migration of future leydig cells which secrete androgen binding protein and meiosis inhibiting factor (responsible for starting spermatogenesis at puberty). The testis are cut off by a thick dense CT named tunica albuginae which forms septs into the testis tissue and divide it into compartments. The outer portion of the sex cords form the seminefrious tubuli and the inner part form rete testis. The parts of the paramesonephric ducts which remain after regression form utriculus prostaticus and appendix testis. The leydig cells appear around the 8th week and start to produce androgens; testosterone and adrostenione. Especially dyhydrotestosterone which is secreted stimulate the formation of epidydmis, ductus deferentes, seminal vesicles and ejaculatory ducts from the mesonephric ducts. The prostate and bulbourethral glands develop from the urogenital sinus. Other factors involved is Hoxa-13, BMP-4, TGF-ß and FGF-10. The leydig cells secrete hormones between 9th and 14th week but then transform into fibroblast like cells and reappear in puberty. As the mesonephric kidneys regress the gonads descend towards the pelvis and processus vaginalis form canalis inguinalis through the peritoneum in which the gubernacle ligaments should extend and shorten so that the testis enter the scrotum before birth. (If they do not enter the scrotum = cryptorschidism which should be surgically dealt with in case they do not descend). 52. Development of female reproductive system When the candidate genes determining male gender is not present the gene regulating development of female characteristics is the Dax-1 gene produced at the same time instead of SRY. It inhibits growth of testis and stimulate the formation of primary female gonads which are of unknown origin: Either ceolomic epithelium, mesonephros or a combination of both. The primary sex cords are replaced by secondary sex cords which develop from the surface of the developing ovary. During the 4th month the oogonia enters these and start meiosis under the influence of meiosis-stimulating factor. The oogonia that enters meiosis is now known as oocytes and they are halted in the diplotene stage of first meiosis untill they are activated again at first menarche. The oocytes are surrounded by follicular cell and the formed structure is known as a primordial follicle. The ovary is surrounded by tunica albuginea and the cortex keeps the primordial follicles. The medulla containing CT and blood vessels develop from mesonephros. Ductus mesonephricus degenerate while ductus paramesonephricus form the tuba uterina. The origin of the vagina and the uterus is still unclear. 53. Development of external genitalia. Main anomalies of reproductive system The first formation of external is bisexual in the indifferent period of development. It starts with the apperance of the genital tubercle at the cloacal membrane. Labioscrotal folds and urogenital folds appear next to the genital tubercle. The clocal membrane rupture at the 8th week and the urethral plate secrete Shh, FGF- 8 and the genital tubercle secrete FGF-10 which influence formation of external genitalia. The tubercle forms phallus which form penis or clitoris. The urogenital folds form spongy urethra in male and labia minora in females The labioscrotal folds form scrotum in males and labia majora in females External genitalia can be recognized by the 12th week. Anomalies: Cryptorscidism – no testis in scrotum True hemaphrodites – both testicles and ovaries Pseudohemaphrodoites: 46XX – external genitalia shaped like a male  46XY – external genitalia with variable development of penis Hypospadia – abnormal opening of the urethra on the ventral aspect of the penis Epispadia – abnormal opening of dorsal side of the penis Gonadal dysgenesis 54. Development of skin The development of skin occurs in three period: The formation of epidermis, the formation of dermis and hypodermis, the ingrowth of neuronal structures into the skin. The skin initially consist of a single layer but by the end of the first month it has differentiated into two layers consisting of a peridermal layer and a basal (germinal layer). The peridermal layer cells exchange water, sodium and glucose between the amniotic fluid and the epidermis. By the 4th month the epidermis consist of all 5 characteristic layer (only in thick skin). During the 6th month the cells undergo massive apoptosis and the peridermal layer detach as a response to increased urine concentration in the amniotic fluid. Now the skin works as a protective barrier. Melanocytes of neural crest origin inflitrate the skin together with langerhans cells and merkel cells of unkonwn origin. Factors include EDGF, FGF, TGF-ß, insulin and insulin like growth factor. Dermis development: Dermis of the dorsal trunk --> dermatomes Dermis of limbs, anterior/lateral trunk --> lateral mesoderm Dermis of face/neck --> mixed with neural crest cells. Mesenchyme develops into skin CT (fibroblasts) and the interaction between epithelium and underlying mesoderm is important for development of skin appendices and if mesoderm and epithelium don’t interact then the skin does not differentiate at all. The sensory part of the skin comes from neural crest cells wich migrate at week 8. The embryonic skin is 10 times more innervated then adult skin but as it is stretched out it becomes like the adult at age of seven. 55. Development of skin derivates Hair The hair papilla develops when the proliferation of skin cells become invaginated by mesoderm. Centally located cells become keratinzed to form the hair. Peripheral cells become root sheath and the external mesoderm become the erector pili muscle. It normally takes about 2 months from the apperance of hair primordia to the first hair starts to show. The first hairs can be observed in cavum nasi and in the eyebrows but after 3rd month hair is seen everywhere as lanugo. Sebaceous glands Develop as invaginating epithelium into the mesoderm and the central cells start production of sebum followed by degeneration. They secrete a cheese-like smear which covers the surface of the body. The sebacous glands are prominent in the facial and anal region. Sweat glands develop as epithelial cords from the surface ectoderm and they start production of sweat in the first weeks of postnatal life depending on the temperature. Nails develop as proliferation of the epithelium which become keritanized and cover the digits. They are complete before birth. By week 32 the nail should extend to the tip of the finger. Mammary glands develop from epidermal milk lines formed in the 1st week. They form across the whole anterior body but most but the thoracic ones degenerate. In the thoracic region the milk line cells starts invaginating the underlying mesoderm and starts sprouting in a manner similar to that of the main salivary glands. These ducts become the lactiferous ducts and the final development of mammary glands is finalized only after first pregnancy with lactation. 56. Development of taste and olfactory organs The tounge starts developing as a mesenchymal proliferatoin of the 1st branchial arch which forms two lateral swellings and a medial tuberculum impar. These fuse and form the body of the tounge. A second swelling, copula form from the 2nd,3rd and 4th branchial arch forms which makes up the root of the tounge. The body and the root fuse marked by sulcus terminalis. Papilla develop under influence of BMP, FGF-8 and Shh signaling molecules. The foetus is able to taste. The musculature of the tounge comes from cranial somites. The nasal cavity is formed by the anterolateral neuroectoderm under the influence of Pax-6. Nasal placodes form nasal pits whos epithelium induces the surrounding mesenchyme to form cartilaginous capsules around it. The epithelium of the most dorsal pits form pseudostratified olfactory epithelium. The smelling sense does not develop completely untill after birth. 57. Development of endocrine organs: thyroid gland, epiphysis, hypophysis, parathyroid glands, paraganglions, adrenal gland. Main malformation in the endocrine system The hypophysis develops from two sources which is the origin of it’s dual functionalism. The adenohypophysis develops from oral ectoderm which forms the Rathke’s pouch that loses its oral connection and move towards the neural process. The neurohypophysis develops from neural ectoderm as a tubular outgrowth from the floor of the third ventricle.  The anterior part Rathke’s pouch form pars distalis, the posterior part – pars intermedia and the neural ectoderm forms the neurohypophysis or the posterior lobe. In some cases parts of the rathke’s pouch may remain in the pharynx and even secrete hormones without it having any severe effect on the body. Adrenal glands form from two sources also. The adrenal cortex comes from clusters of mesodermal cells which form 80% of the gland. They secrete glucocorticoids such as cortisol during embryogenesis (inductor for bone marrow hemapotoesis) but the development of the gland is not complete until puberty when the reticular zone develops. The medualla comes from neural crest cells which produces norepinephrin and epinephrin. The thyroid gland develops as an epithelial downgrowth from the first and second branchial arch which bilobulates and descend to the trachea. There is a communication between the oral cavity and the thyroid gland for a short while called ductus thyreoglossus which degenerate by apoptosis. The parathyroids The inferior parathyroids --> 3rd pharyngeal pouch The superior parathyroids --> 4th pharyngeal pouch Pineal gland Develops as an outpocketing from diencephalon Malformations Adenohypophysis remain in pharynx Several lobuli of thyroid gland Open ductus thryoglosseus 58. Development of thymus and lien The thymus develops from the 3rd branchial pouch and cells migrate down to the mediastinum as a cylinder and the epithelium becomes infiltrated by the neural crest cells which become surrounded by a CT capsule. The thymus starts secreting colonizing factors which stimulate t-lymphoblast migration into the thymus. The thymus develops hassels-corpuscles and produces t-lymphocytes which spread throughout the lymphoid organs during embryogenesis. Spleen develops from mesenchymal condensations and as the stomach rotates it is pushed to the left of the body and the lienal ligaments form. If there are more condensations additional spleens may form. It develops through 3 stages: 1. Immature stage – when the spleen can perform hemapoesis during the intraembryonic stage of hemapoesis 2. Transitional stage – the spleen becomes lobulated 3. Lymphoid organ stage – when the spleen becomes occupied by T- lymphocytes