FHB 101 General Embryology Summary (PDF)

FHB 101 GENERAL Embryology (SUMMARY) By Prof.Dr:Nahed H. Koura - Dr.Waleed Badawy A: Reproductive systems: 1- The female reproductive system consists of two ovaries, two uterine tubes, a uterus and a vagina. 2- The wall...

FHB 101 GENERAL Embryology (SUMMARY) By Prof.Dr:Nahed H. Koura - Dr.Waleed Badawy A: Reproductive systems: 1- The female reproductive system consists of two ovaries, two uterine tubes, a uterus and a vagina. 2- The wall of the body of the uterus consists of three layers: endometrium, myometrium and perimetrium. 3- The uterine tubes are the site of fertilization between the oocytes and sperms. 4- The male reproductive system includes two testes, two epididymis, two seminal vesicles, two ejaculatory ducts, a prostate and a penis. 5- The primary sex organs (gonads) that produce gametes are testis in male and ovary in female. B: Gametogenesis: 1- Gametes are formed from the germ cells of unborn baby that migrate from the endodermal lining the roof of the umbilical vesicle to the developing testis or ovaries. 2- Gametogenesis is the conversion of the germ cells (46 chromosomes termed diploid number) into male and female gametes (23 chromosomes termed haploid number). 3- The female gamete is called secondary oocyte (X). 4- The male gamete is called sperm (X or Y). 5- The gamete contains 22 autosomes and one sex chromosome. 6- Disturbances of meiosis during gametogenesis, result in formation of an abnormal gamete as Down’s syndrome. C: Female reproductive cycles: 1- The ovarian cycle is the cyclic changes occurring in the ovary every lunar month. 2- The menstrual cycle is the cyclic changes occurring in the endometrium every lunar month. 3- The phases of the ovarian cycle are follicular, ovulation and luteal phases. 4- The phases of the menstrual cycle are ischemic, menstrual, proliferative and luteal phases. 5- Some breast- feeding women may cease menstruation till the end of weaning. D: Fertilization : 1- Fertilization is the process by which male and female gametes fuse to form the zygote. 2- The common site of fertilization is the ampulla of the uterine tube. 3- Fertilization restores the diploid number of chromosomes (46 chromosomes). 4- Fertilization determines chromosomal sex of the embryo (XX or XY). 5- The embryonic age begins at fertilization two weeks after the LNMP (last normal menstrual period). E: Cleavage (Segmentation): 1- The cleavage is due to the repeated mitotic division of the zygote into embryonic cells called blastomeres. 2- The cleavage occurs along the uterine tube during the first week of pregnancy and a morula is formed. 3- The inner cells of the morula form an embryoblast and the outer cells form the trophoblast.  by the end of 1st week 4- At the beginning of the 2nd week, a cavity develops inside the morula to form a blastocyst and the process is called blastogenesis F: IMPLANTATION: 1- The implantation is the embedding of the blastocyst into the endometrium of the uterus. 2- The common site of the implantation lies in the upper posterior wall of the fundus of the uterus. 3- By the 10th day, the blastocyst is completely embedded leaving a defect in the endometrial epithelium filled by a closing plug. 4- At the end of the second week of pregnancy the endometrial epithelium is completely regenerated and covers the closing plug. 6- By the end of the 2nd week, the endometrium is filled with edematous cells loaded with the glycogen and lipids hence the endometrium is called the decidua. 7- In the early stages of the 2nd week, the embryoblast splits into two layers the epiblast and hypoblast while the trophoblast also splits into two layers, an inner cytotrophoblast and outer syncitiotrophoblast. 8- In the early stages of the 2nd week, the amniotic cavity is enclosed by the amnioblasts and its floor is formed by the epiblast, while the wall of the umbilical vesicle develops from the hypoblast which becomes a roof for the vesicle. 9- The cells of the wall of the umbilical vesicle produce the extraembryonic mesoderm. The extra embryonic mesoderm splits into two layers. 10-The trophoblast and the layer of the extraembryonic mesoderm beneath it, form the chorionic membrane. The space which is formed between the two layers of the extraembryonic mesoderm termed the chorionic cavity. 11-At the end of the second week the chorion membrane produces villi projecting all-round the surface of the chorionic sac called primary and secondary chorionic villi sequentially. 12-The connecting stalk suspends the amnion and the umbilical vesicle to the chorionic membrane. G: The gastrulation: 1- The gastrulation is the beginning of morphogenesis (development of body form). The three germ layers (ectoderm, mesoderm and endoderm) are formed during the third week. 2- The first sign of gastrulation is the formation of the primitive streak. 3- Early in the 3rd week, the primitive streak is formed as a thickened linear band of epiblast in the in the median plane in the caudal aspect of the embryo. 4- The primitive node is the cranial end of the primitive streak. 5- The growth of the primitive streak leads to the development of the notochord.  H: The 3rd week 1- During the 3rd week, the notochordal process develops cranially as a median cellular cord from the primitive node. 2- The notochordal process forms a plate which folds inwards to form the notochord. 3- The notochord is the primordial axis of the embryo. 4- The notochord provides signals that are necessary for the development of the central nervous system and axial musculoskeletal system. 5- The remnants of the notochord form the nucleus pulposus of the intervertebral disc. 6- The chorionic villi at the end of the third week contain blood vessels to be tertiary chorionic villi. The embryonic disc is also split into three germ layers. 7- The embryonic disc is changed from a flat round to a flat elongated disc with a broad cephalic and narrow caudal end in the third week of pregnancy. *** FOLDING OF THE EMBRYO: 1- During the fourth week, the folding changes the flat embryonic disc into a cylindrical embryo. 2- The folding consists of a head, tail and the two lateral embryonic folds which occur simultaneously. 3- As a result of folding, the umbilical ring is formed. It is the region where the umbilical cord develops. As a result of folding, the chorionic villi at the site of embryonic pole enlarged and produce more branches, these form the chorion frundosum. The chorion frundosum forms the chorionic plate of the placenta. The chorionic plate gives about 20 stem villi which project into the villous space that contains the maternal blood. The villous space is formed in the decidual plate of the placenta. Regardless the chorionic villi of the chorionic plate; the remaining villi degenerate. J: Derivatives of the germ layers: 1- The epiblast is the source of all the three germ layers. Cells of the epiblast migrate towards the primitive streak and slip beneath the epiblast to displace the hypoblast and form the embryonic endoderm. Other cells of the epiblast slip between it and the endoderm to form the mesoderm. The remaining cells of the epiblast form the ectoderm. The three germ layers are ectoderm, mesoderm and endoderm. 2- The ectoderm is divided into neuroectoderm and dermoectoderm.The neuroectoderm gives the neural tube and the neural crest. The dermoectoderm gives the epidermis of the skin The neural tube gives rise to the brain and spinal cord. The suprarenal medulla, nerve ganglia and pigment cells are derived from the neural crest cells. 3- The mesoderm gives three columns; paraxial, intermediate and lateral plate. I. The paraxial mesoderm: a- The paraxial mesoderm is segmented into somites.  b- At the end of the fifth week, the number of somites is about 44 pairs. c- The somites are paired cuboidal bodies of paraxial mesoderm on each side of the developing neural tube. d- Each somite has its own nerve supply. e- Each somite is divided into sclerotome, myotome and dermatome. f- The sclerotome differentiates into vertebrae, ribs, tendons and cartilage g- The myotome and the dermatome form the dermomyotome. h- The dermomyotome gives the dermis of the skin, muscles of the back and intercostal muscles. N.B. During the 4th and 5th weeks the number of somites are used as criteria determining the age of the embryo. II- The intermediate mesoderm: The intermediate mesoderm differentiates into the urogenital structures. II.The lateral plate mesoderm: a- It has a parietal and a visceral layer. b- The cavity of the lateral plate mesoderm is the intra embryonic celom. The intraembryonic celom gives the pericardial, pleural and peritoneal cavities. 4- The endoderm gives the epithelial lining of the gastrointestinal and respiratory tracts. K: Placenta: 1- The placenta is the vital organ of nutrient and gas exchange between the mother and baby. The full-term placenta is discoid and weighs about 500 g. At birth, the placenta is expelled from the uterus within 30 minutes after child birth. 2- The fetal side of the placenta is smooth because it is covered by the amniotic membrane. The fetal part of the placenta develops from the chorion frundosum 3- The maternal side of the placenta is rough and develops from the decidua basalis. The maternal side of the placenta is irregular and divided into about 20 cotyledons. 4- The umbilical cord is attached to the fetal side of the placenta and covered by the amnion.  5- The placental membrane consists of four layers and becomes two layers in the last trimester of pregnancy. 6- The placenta has three main functions: a- Metabolism, the placenta synthesizes glycogen and fatty acids which serve as a source of nutrient and energy. b- Transfer of oxygen and nutrients from mother. c- Endocrine secretion, human chorionic gonadotrophic hormone is secreted to maintain the corpus luteum and prevent menstruation. 7- Abnormal implantation of the placenta inside the endometrium leads to placenta previa. 8- Abnormal implantation in the uterine tube gives ectopic pregnancy. 9- Placenta accreta is the abnormal invasion of the placenta onto the uterine wall. L- Umbilical cord: 1- At full term the umbilical cord is about 50 cm. 2- The umbilical cord has two arteries and one vein surrounded by Wharton’ jelly. -THE Amnion: 1- The amnion forms a fluid filled sac that surrounds the embryo. 2- The amnion permits symmetric growth of the embryo and maintains homeostasis of fluid and electrolytes. 3- The content of the amniotic fluid changes every 3 hours 4- The amount of the fluid is normally 800- 1000 ml. Excess of the fluid more than 1500- 2000 ml is called hydramnios while decrease less than 400 ml is oligohydramnios. Umbilical vesicle (old name is yolk sac): By the end of the 10th week, the umbilical vesicle has shrunk to a pear-shaped remnant and is connected to the mid gut by a vitelline duct. The umbilical vesicle is the source of the germ cells. M: The Twins: 1- Twins originating from two zygotes are dizygotic and they are not alike. 2- Twins originating from one zygote are identical twins. 3- Conjoined twins in which the fetuses are not entirely split.  Triplet Quadruplet Conjoint twins The expected date of delivery of a fetus: It is 38 weeks after fertilization ± 2 weeks. The intrauterine period may be divided into three trimesters; each of which is three months. The parturition: It is the child birth. It starts by uterine contractions, followed by dilatation of the cervix, then expulsion of the baby and the last stage is the expulsion of the placenta. N: 1- CRL (crown rump length) is the measure from the vertex of the skull to the midpoint between the apices of the buttocks. 2- CHL (crown heel length) is the standing height from the vertex to the heel of the embryo. 3- The embryo is the developing human during the first 8 weeks of pregnancy. 4- The fetus is the developing human from the ninth week of pregnancy and until birth. 5- The congenital anomalies are abnormalities of development that are present at birth. Clinically oriented problems: Case 1 A pregnant mother had polyhydramnios. What is the meaning of this clinical condition? Explain why this occurs? Find it in the book of general embryology (amnion). Case 2 A medical student examined a section of the umbilical cord and observed that there was only one umbilical artery. What kind of fetal abnormalities might be associated with this condition? Find it in the book of Langman’s medical embryology (anomalies of the umbilical cord) Case 3 A pregnant woman in the last trimester, comes to the clinic complaining of bleeding. What is the explanation of this clinical condition? What is the advice given by her doctor? Find it in the book of the developing human clinically oriented embryology by Moore Persaud (anomalies of the placenta). Case 4 A young woman consulted her physician about severe right lower abdominal pain. She had two missed periods. A diagnosis of ectopic pregnancy was made. What is the most likely site of the extrauterine implantation? What do you expect is the most important clinical symptom? Find in the book of the general embryology (anomalies of the placenta).