Embryology PDF

INTRODUCTION TO BODY STRUCTURE 85 GENERAL EMBRYOLOGY INTRODUCTION TO BODY STRUCTURE 86 Meiotic divisions and crossing over Spermatogenesis Pinterest /...

INTRODUCTION TO BODY STRUCTURE 85 GENERAL EMBRYOLOGY INTRODUCTION TO BODY STRUCTURE 86 Meiotic divisions and crossing over Spermatogenesis Pinterest / Career power INTRODUCTION TO BODY STRUCTURE 87 GAMETOGENESIS Definitions Primary sexual organ: it is the organ containing the cells capable of forming a gamete (half of the future human being). They are the testis in males and the ovaries in females. Primary sex cells: The cells which are capable of forming a gamete. They are the spermatogonium in males and oogonium in females. Gametogenesis: it is formation of a gamete (sperms in males and ova in females). Mitotic division: is the division of a cell leading to two daughter cells similar to each other and to the mother cell. Meiotic division: is the division of a cell leading to two daughter cells not similar to each other or the mother cell. It shows a decrease in the number or the amount of chromosomes and DNA. In gametogenesis there are two meiotic divisions: First meiotic division: The 46 chromosomes in the nucleus of a cell arranges into two columns (each contains 23 chromosomes). Each chromosome incompletely splits into two chromatids. Crossing over: each chromosome exchanges a segment with its neighboring chromosome (this leads to uncounted number of chromosomal variations). The cell divides into 2 cells (each containing 23 chromosomes). Second Meiotic division: Each of the 23 chromosomes splits completely into two chromatids. Each chromatid develops into complete chromosome (note that the two chromatids are not identical, So, the newly formed chromosomes are not similar). The cell divides into 2 cells (each containing 23 chromosomes). SPERMATOGENESIS Definition: formation of a sperm from spermatogonium. Site: in seminiferous tubules of the testis. Process: Spermatogonia lies on the basement membrane of the seminiferous tubules. It contains 46 chromosomes (44+X+Y). Each spermatogonium divides by mitosis forming 2 daughter spermatogonia (each contains 46 chromosomes (44+X+Y). One of the daughter spermatogonia (spermatogonia A) remains in contact with basement membrane while the other (spermatogonia B) starts to mature. Spermatogonia B enlarges in size and transforms into primary spermatocyte (44+X+Y chromosomes). Primary spermatocyte is the largest germ cell. Primary spermatocyte divides by 1st meiotic division forming two secondary spermatocytes (22+X and 22+Y chromosomes). This division decreases the number of chromosomes. Each secondary spermatocyte divides by 2nd meiotic division forming two spermatids (22+X or 22+Y). This division decreases the amount of DNA in each chromosome. INTRODUCTION TO BODY STRUCTURE 88 Sperm Scanning EM of sperms Abnormal forms of sperms Vector stock / Quara / ResearchGate INTRODUCTION TO BODY STRUCTURE 89 Spermiogenesis: is the transformation of spermatid into sperm: The nucleus of the spermatid becomes the head of the sperm. The Golgi apparatus of the spermatid becomes the head (acrosome) cap of the sperm (containing enzymes to facilitate the penetration of ova). One of the centrioles of the spermatid elongates forming the axial filament of the sperm (middle piece and tail). The mitochondria of the spermatid surrounds the upper part of the axial filament forming the middle piece. The rest of the axial filament forms the tail of the sperm. The neck is a narrow junction between the head and the middle piece. The sperm is about 55 μm (0.055 mm) long where the head is 5 μm and the tail is 50 μm (1/10). Characteristics of spermatogenesis: The process from the mitotic division of spermatogonia to the formation of the sperm lasts for about 2 months. The spermatogenesis results in formation of 4 sperms from each spermatogonium with a preservation of a copy for further spermatogenesis (that’s why spermatogenesis does not stop by age). The sperms passing from testis are immotile, they acquire motility during passage in epididymis (2 weeks). Each testis is divided into 200 compartments, each contains 2 seminiferous tubules, each of them is 2 feet long. A large number of sperms are continuously formed. SEMINAL FLUID (SEMEN) ❖ It consists of sperms (formed in testis and transported through epididymis, vas deference and ejaculatory duct) and secretions from male genital glands (mainly seminal vesicles and prostate). Normal parameters of semen: Amount: 2-6 ml/ ejaculate. Lower volume is called hypospermia (hypo = below), the absence of semen with ejaculation is known as aspermia (a = no). Sperms count: 20-200 million sperm/ ml (40-1200 million sperms/ ejaculate). Lower values are called oligozoospermia (oligo = few). Vital sperms: ≥ 60%. Lower values are called necrozoospermia (necro = dead). Normal sperm forms: (≥ 4%), Some abnormal forms are capable of fertilization. Lower values are called teratozoospermia (terato = malformed). Sperm motility: ≥ 40% and ≥ 30% with aggressive motility. Lower values are called asthenozoospermia (astheno = weak). ❖ Out of the ejaculated sperms, only 500 sperms reach the ampulla of Fallopian tubes (site of fertilization). This process lasts 2 hours after ejaculation. ❖ Sperms survives for 2 days in female genital tract. INTRODUCTION TO BODY STRUCTURE 90 Oogenesis Pinterest INTRODUCTION TO BODY STRUCTURE 91 OOGENESIS Definition: formation of a ova from oogonium. Site: in the cortex of ovaries. Process: Oogonia contains 46 chromosomes (44 + X + X). Oogonium enlarges in size and transform into primary spermatocyte (44+X+X chromosomes). Primary oocyte divides by 1st meiotic division forming two cells; one secondary oocytes (22+X) and the other degenerates forming 1st polar body (22+X). Secondary oocyte divides by 2nd meiotic division forming two cells; mature ova (22+X) and the other degenerate forming 2nd polar body (22+X). Characteristics of Oogenesis: Oogenesis timing: ⎯ During female intrauterine life, all oogonia are transformed to primary oocytes, which starts 1st meiotic division but does not complete it. ⎯ With each ovarian cycle, primary oocyte completes the first meiotic division forming secondary oocyte and 1st polar body, the secondary oocyte starts the 2nd meiotic division but does not complete it, this is the cell which is ovulated. ⎯ If fertilization occurs, secondary oocyte completes the 2nd meiotic division forming mature ova and 2nd polar body. Oogenesis population ⎯ During intrauterine life, the oogonia undergoes several mitotic divisions reaching 5 million, most of them degenerate and only 1 million are transformed into primary oocytes (at birth), only 40.000 primary oocytes survive to the age of puberty. ⎯ Starting from puberty, each ovarian cycle 20-100 primary oocytes start the process of ova formation, only one of them succeeds. The large consumed number are necessary for hormonal production to complete the process. ⎯ The oogenesis results in formation of 1 ovum from 20-100 oocyte (that’s why oogenesis stops by age leading to menopause). ⎯ Only one ovum is produced each month. It lasts for 12-24 hours only. INTRODUCTION TO BODY STRUCTURE 92 Follicle development Pinterest INTRODUCTION TO BODY STRUCTURE 93 OVARIAN FOLLICLES AND OVULATION Primary follicle: during intrauterine development, each primary oocytes is surrounded by a single layer of flat follicular cells. Secondary follicle: at puberty, and parallel to the development of primary oocyte into secondary oocyte, the follicular cells become granulosa cells (multilayers of cubical cells containing granules (hence the name). Tertiary follicle: cavities start to appear between granulosa cells. Mature Graafian follicle: The cavities enlarge and unite forming an antrum (= chamber). The outer layer of the follicle is called stratum granulosum (= granular layer). The secondary oocyte with its surrounding cells is called cumulus oophorous (= ovarian elevation) and is formed of: ⎯ Secondary oocyte. ⎯ Zona pellucida (= transparent zone): a membrane surrounding the secondary oocyte. ⎯ Corona radiata (= radiating crown): the granulosa cells around the zona pellucida. The ovarian tissue due to the compression of enlarging follicle becomes compact and form a capsule called theca folliculi which is further divided into: ⎯ Theca interna: formed of connective tissue cells which secrete estrogen. ⎯ Theca externa: formed of fibrous tissue. Ovulation: due to the enlargement of the follicle and pressure on the ovarian cortex, the overlying cortex becomes ischemic and rupture leading to: The secondary oocyte with the surrounding zona pellucida and corona radiata leaves the ovary and enters the Fallopian tube. The rest of the granulosa cells and surrounding theca interna cells start to accumulate yellow pigments forming corpus luteum (= yellow body) and secrete progesterone and small amount of estrogen. ⎯ If fertilization occurs, the dividing cells secrete HCG which stimulates the corpus luteum to enlarge and continue secreting progesterone during the first half of pregnancy (corpus luteum of pregnancy) till it is replaced by placenta. ⎯ If no fertilization occurs the corpus luteum gradually degenerates within ten days (corpus luteum of menstruation → corpus albicans). INTRODUCTION TO BODY STRUCTURE 94 Ovarian cycle Pinterest INTRODUCTION TO BODY STRUCTURE 95 OVARIAN CYCLE ❖ It is cyclic changes which occur in the ovarian follicles under the effects of the pituitary hormones. ❖ This cycle occurs monthly from puberty to menopause. Phases of ovarian cycle: Follicular phase: the pituitary gland starts to secrete FSH which leads to: Primary oocyte completes the 1st meiotic division forming secondary oocyte and 1st polar body. Maturation of primary follicle to mature Graafian follicle. The granulosa cells secrete estrogen which at certain level has negative feedback on FSH (decreases its secretion) and stimulates LH secretion (also from pituitary gland). The high estrogen level of this phase is responsible for the proliferative phase of uterine cycle. Ovulation: caused by LH surge leading to liberation of secondary oocyte with its surroundings from the ovary. Luteal phase: LH secreted from pituitary gland causes corpus luteum to secrete progesterone and small amount of estrogen (both suppress FSH stopping crossing cycles). If fertilization occurs, the corpus luteum enlarges and continues secreting progesterone leading to inhibition of FSH → stopping new cycles. If fertilization does not occur, corpus luteum gradually degenerates, and progesterone level decreases leading to an increase in FSH hormone causing initiation of a new cycle. The high progesterone level of this phase is responsible for the secretory phase of uterine cycle. INTRODUCTION TO BODY STRUCTURE 96 Ovarian and uterine cycles Pinterest INTRODUCTION TO BODY STRUCTURE 97 UTERINE (MENSTRUAL) CYCLE ❖ The uterus is a muscular organ lined with endometrium formed of basal, compact and spongy layers. ❖ The uterine cycle is a monthly changes which occurs in the uterine endometrium under the effect of the ovarian hormones. ❖ This cycle occurs monthly (21-35 days) from puberty to menopause. Phases of Uterine cycle: Proliferative (estrogenic – follicular) phase : Under the effect of estrogen (secreted by the granulosa cells during the follicular phase of ovarian cycle). The estrogen stimulates the basal layer to develop leading to construction of compact and spongy layers (the cells increase in number, the glands and the arteries increase in number and size). It lasts for 10 days. Secretory (progestational – luteal) phase : The endometrium proliferates under the effects of progesterone (mainly) and estrogen secreted by corpus luteum in the luteal phase of the ovarian cycle. The cells enlarge and accumulate nutrients, the glands dilate and are filled with glycogen and the arteries dilate and become spiral. It lasts for 14 days. If fertilization occurs, corpus luteum of pregnancy continues to secrete progesterone keeping the enlarged endometrium with the dividing cells implanted into it (the endometrium will be called decidua). If fertilization does not occur, the corpus luteum degenerates with decreased progesterone level leading to vasoconstriction of the uterine arteries causing ischemia followed by shedding and bleeding. Menstrual phase: It occurs due to decreased estrogen and progesterone levels due to degeneration of corpus luteum. Shedding of the compact and spongy layers of endometrium occurs leaving only the basal layer. It lasts for 3-7 days. MENOPAUSE ❖ It is cessation of ovarian cycle due to exhaustion of primary follicles during previous cycles → little number of follicular cells is present to be stimulated by FSH → decreased estrogen and progesterone levels → cessation of uterine cycle. INTRODUCTION TO BODY STRUCTURE 98 Fertilization Cleavage, morula and blastocyst Quora / Shutter stock INTRODUCTION TO BODY STRUCTURE 99 FERTILIZATION Definition: fusion between a single sperm and an ovum to form a fertilized ova (zygote). Steps: Capacitation: It occurs during the passage of the sperm into the female genital tract due to exposure to its enzymes. It is chemical changes affecting the acrosomal cap and increased activity. Acrosome reaction: It occurs in Fallopian tube due to exposure to secondary oocyte. It is perforations in the acrosomal cap leading to the release of its enzymes. Penetration of corona radiata: by many sperms. Penetration of zona pellucida: it occurs by a single sperm. However, many sperms are needed to pore their enzymes. Zonal reaction: chemical changes in zona pellucida so that it is not affected by acrosomal enzymes anymore. Fusion of cell membranes of the sperm and the secondary oocyte. Completion of 2nd meiotic division and formation of mature ova and 2nd polar body. Fusion of pronuclei: the head of the sperm (male pronucleus) and the nucleus of the ova (female pronucleus) fuse together, leading to restoration of diploid number of chromosomes (46) and detection of sex (44+XX is a female and 44+XY is a male). The zygote starts cleavage (division of cells). Hormonal effect: the dividing cells produce HCG which stimulates corpus luteum growth (corpus luteum of pregnancy) → high progesterone and estrogen levels → maintenance of secretory phase of uterine cycle and inhibition of FSH and LH leading to ovarian cycle pause. CLEAVAGE, MORULA AND BLASTOCYST Definition of cleavage (segmentation): is repeated mitotic division of the zygote. During cleavage the zygote moves by cilia of Fallopian tube towards the uterus. The cells divide into 2-4-8-16-32 cells. 16-32 cells mass is called Morula (= mulberry). The morula is still surrounded by zona pellucida and reaches the uterus on the 4th day after fertilization. During the 5th and 6th days, the uterine enzymes dissolve the zona pellucida and the dividing cells shows a cavity between them. The cell mass is called blastocyst, the cavity is called blastocoele and the cells are called blastomeres (nearly 64 cells) and divides into: ⎯ Outer cell mass (trophoblast): nutrient cells. ⎯ Inner cell mass (embryoblasts): embryo forming cells. On the 7th day of fertilization, the blastocyst implants inside the wall of uterus. INTRODUCTION TO BODY STRUCTURE 100 Implantation Normal and abnormal implantation Semantic scholar / Shutter stock INTRODUCTION TO BODY STRUCTURE 101 IMPLANTATION Definition: it is the invasion of the blastocyst to the endometrium of the uterus. Duration: between 7th and 14th day after fertilization. Site: usually in the endometrium of the upper 1/3 of the uterus (post wall). Mechanism: The trophoblast over the embryoblast (embryonic pole) projects forming finger like processes and produces enzyme to invade the endometrium (which is prepared forming nutrient filled cells, glycogen rich glands and spiral arteries). The last part of blastocyst sinks into the uterus (ab-embryonic pole), the site of implantation is closed by fibrinous tissue followed by regeneration of the epithelium. Accordingly, the whole trophoblasts are inside the endometrium (and not just attached to it). Changes of blastocyst during implantation (2nd week after fertilization): The trophoblasts divide into: Outer syncytiotrophoblasts: loose its cell walls for easier penetration Inner cytotrophoblasts. Both layers secrete HCG maintaining corpus luteum (of pregnancy). The embryoblasts arrange into a bilaminar disc with two layers of cells: Ectoderm: columnar cells. Endoderm: cubical cells. The blastocoele is divided by the disc into two cavities: Amniotic cavity: dorsal to ectoderm. Its roof is called amnion (amniotic membrane) and is formed from cells derived from trophoblasts. Primary yolk sac: ventral to endoderm. Endodermal cells migrate to line it forming (Heuser's membrane). Abnormal sites of implantation: 1) Placenta Previa: implantation in the lower part of the uterus. The developing placenta will be under the fetus (previa = exceeding). 2) Ectopic pregnancy: a) Tubal pregnancy: in the uterine tube. b) Ovarian pregnancy: in the ovary. c) Abdominal pregnancy: in the abdominal cavity. ⎯ Ectopic pregnancy does not complete due to lack of adequate uterine nutrition. It invades the organ implanted at and causes its rupture or bleeding. INTRODUCTION TO BODY STRUCTURE 102 Chorionic vesicle Chorionic villi Parts of chorion and decidua Quizlet / Gateway to medical INTRODUCTION TO BODY STRUCTURE 103 CHORIONIC VESICLE Formation: Cytotrophoblast cells form a new layer inner to it called extraembryonic mesoderm (EEM). spaces appear inside EEM forming extraembryonic coelom (EEC) which divides EEM into outer layer lining the cytotrophoblast & inner layer surrounding the amniotic cavity and yolk sac. Part of the EEM will still connect the outer and inner layers (connecting stalk or future umbilical cord) dorsal to amniotic cavity. Now the blastocyst is called chorionic vesicle. The outer wall of the chorionic vesicle is called chorion, and is formed of (syncytiotrophoblast, cytotrophoblast and EEM lining the trophoblast). The chorion gives finger like processes called chorionic villi which invades the uterine wall. The spaces between the chorionic villi (intervillous spaces) are filled with maternal blood. The chorionic villi surround the whole chorionic vesicle, then the villi will enlarge deep in uterine wall (chorion frondosum), and atrophies towards the uterine cavity (chorion leave). Stages of chorionic villi: 1) Primary villi: formed of syncytiotrophoblast + cytotrophoblast. 2) Secondary villi: formed of syncytiotrophoblast + cytotrophoblast and extraembryonic mesoderm. 3) Tertiary villi: formed of syncytiotrophoblast + cytotrophoblast and extraembryonic mesoderm invaded by fetal blood vessels. It is further divided into: a) Stem (anchoring) villi: each extends from the base of the chorion (chorionic plate) towards the uterine wall. b) Branching (free absorbing) villi: small villi branching from the stem villi and surrounded by maternal blood. They are the sites of exchange of nutrients and gases between the maternal and fetal blood. Parts of chorion 1) Chorion frondosum: the part of the chorion invading the uterine wall. It shows numerous branching tertiary villi. 2) Chorion leave: The part of the chorion facing the uterine cavity. It is less developed. DECIDUA Definition: it is the endometrium after implantation. It maintains its secretory phase under the effect of progesterone (of corpus luteum and placenta). The stromal endometrial cells are called decidua cells. The decidua falls as a single mass during labor (decidua = fall). Features: 1) The endometrial stromal cells (decidua cells): enlarge and filled with nutrients. 2) The endometrial glands: becomes larger, tortuous and filled with glycogen and mucin. 3) The blood vessels: engorged, spiral. They are invaded by chorionic villi pouring maternal blood between them. Parts of decidua: 1) Decidua basalis: deep to the blastocyst. 2) Decidua capsularis: superficial to blastocyst (separating it from uterine cavity). 3) Decidua parietalis: the rest of the endometrium. INTRODUCTION TO BODY STRUCTURE 104 Primitive streak & primitive node Notochord Neural tube Pinterest / Quizlet INTRODUCTION TO BODY STRUCTURE 105 CHANGES IN THE EMBRYOBLASTS ❖ At the stage of blastocyst, the embryoblasts are the inner cell mass. During implantation the embryoblasts arrange into a bilaminar disc (ectoderm and endoderm). ❖ The disc is at first rounded, then oval, then pear shaped with a wide cranial end and narrow caudal end. PRIMITIVE STREAK & PRIMITIVE NODE Formation: Some midline caudal ectodermal cells proliferate to form a thick rod like structure called primitive streak. The cells at the cranial end of the primitive streak proliferate to form the primitive node. A depression occurs in the primitive node called primitive pit. NOTOCHORD Formation: Some cells of the primitive node proliferate and form a median solid cord called notochordal process, between the ectoderm and endoderm. It extends cranially to prechordal plate (prechordal = before the cord). The primitive pit deepens inside the notochordal process transforming it into notochordal canal. The floor of the notochordal canal degenerates with the underlying endoderm. So, a connection is formed between the amniotic cavity and the yolk sac (neuroenteric canal). The remaining roof and sides of the notochordal process form the notochordal plate, then enfolds to form solid definitive notochord (notochord = cord of the back) which is the primitive axis of the embryo. NEURAL TUBE Formation: The central part of the ectoderm between primitive node and prechordal plate thickens to form neural plate. The neural plate invaginates to form neural groove, which has two neural folds on its sides. The junction between the ectoderm and the neural groove on each side shows a longitudinal strip called neural crest. The neural folds fuse with each other to form the neural tube. The neural tube separates from the surface ectoderm and sinks down below it but above the notochord. The neural crest becomes dorsolateral to the neural tube. The neural tube has two openings at its ends called the cranial and caudal neuropores, which close by the end of the 4th week. The neural tube differentiates forming the central nervous system. While the neural crest differentiates forming dorsal root ganglion and other structures. INTRODUCTION TO BODY STRUCTURE 106 Intraembryonic mesoderm Somites Research gate / Slide share / Quizlet INTRODUCTION TO BODY STRUCTURE 107 INTRAEMBRYONIC MESODERM (IEM) Formation: Cells from primitive streak and primitive node migrate between ectoderm and endoderm and form intraembryonic mesoderm (IEM). IEM separates ectoderm from endoderm except at: Prechordal plate (Buccopharyngeal membrane): it is the future mouth, lies cranial to notochord, here, the endodermal cells become columnar and fuse tightly with the overlying ectoderm. Cloacal membrane: it is the future anal canal and urethra, it lies caudal to primitive streak. The IEM of both sides meat ant to prechordal plate (the most cranial part is called septum transversum (future diaphragm) followed caudally by cardiogenic area (future heart). It also crosses midline post to cloacal membrane (here it is connected to connecting stalk (future umbilical cord). Derivatives of IEM: on each side of the notochord, the IEM is divided into three parallel craniocaudal masses (from medial to lateral): 1) Paraxial mesoderm: It is divided into cubical masses called somites, which appear in a craniocaudal sequence. The first pair of somites appear on the 20th day of development and the last one in the 5th week. There are 42–44 pairs of somites; 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 8–10 coccygeal. Each somite is divided into three parts: a) Sclerotome: ventromedial, it surrounds the notochord and differentiates into vertebral column and ribs. b) Myotome: intermediate, it differentiates into the muscles of the back. c) Dermatome: dorsolateral, it differentiates into the dermis of the skin, which cover the muscles of the back. (both the skin and the muscles of the back are supplied by dorsal rami of the spinal nerves). 2) Intermediate cell mass: it is the nephrogenic area, it differentiates into most of the urogenital system. 3) Lateral plate mesoderm: Many small cavities appear in that plate. These cavities fuse together to form a u shaped cavity called intraembryonic coelom (IEC) whose base is cranial to the oropharyngeal membrane. The IEC divides the lateral plate mesoderm into: a) Somatic (parietal) layer: dorsal layer in contact with ectoderm, differentiates into the dermis, bones, joints, muscles and vessels of the limbs and ventral part of the trunk. b) Splanchnic (visceral) layer: ventral layer in contact with endoderm, differentiates into the connective tissue, smooth muscles and vessels of the viscera (gastrointestinal, respiratory and urogenital systems). INTRODUCTION TO BODY STRUCTURE 108 Folding Quizlet INTRODUCTION TO BODY STRUCTURE 109 FOLDING Timing: 4th week of pregnancy. Causes: Rapid growth of the cranial part of the neural tube. Expansion of amniotic cavity. Results of folding: Formation of folds: 1) Head fold (cranial). 2) Tail fold (caudal). 3) Two lateral folds. Fusion of similar germ layers: the three layers of the embryonic disc fold together. After complete folding each layer unites with the similar one of the opposite side, all around the connecting stalk (future umbilical cord). The ectoderm becomes the outer layer surrounding the embryo (future epidermis). Formation of the gut and definitive yolk sac: The endoderm and most of the yolk sac invaginates within the folds. The endoderm will form a tube inside the folds (gut) which is further divided into: a) Foregut: in the head fold. b) Midgut: between the lateral folds. c) Hindgut: in the tail fold. A small part of the yolk sac becomes outside the fold but still connected to the midgut by vitelline (vitello intestinal) duct (vitellus = Yolk sac). This part is called definitive Yolk sac. Rearrangement of positions: The cranial part of neural tube (future brain) becomes the most cranial structure and forms a large forebrain bulge. The prechordal plate (future mouth) lies caudal to the developing brain. The cardiogenic area (future heart) becomes caudal to the prechordal plate and forms a large Pericardial bulge. Septum transversum (future diaphragm) becomes caudal to the cardiogenic area. The connecting stalk (future umbilical cord) becomes ventral near the definitive yolk sac. The cloacal membrane (future anal canal and urethra) becomes caudal to connecting stalk. The amniotic cavity surrounds the whole embryo. INTRODUCTION TO BODY STRUCTURE 110 Placenta Research gate INTRODUCTION TO BODY STRUCTURE 111 CHANGES IN THE TROPHOBLASTS PLACENTA Formation: formed of chorion frondosum and decidua basalis. Microscopic features: The chorion projects chorionic villi invading decidua basalis. The part of the chorion forming the base of chorionic villi is called chorionic plate. The chorionic villi deepen inside the decidua but does not reach the muscular layer of the uterus, to prevent that, the cytotrophoblasts of the stem villi pierce their overlying syncytiotrophoblasts and join each other deep to the chorionic vesicle, forming cytotrophoblastic shell separating it from the deeper layers of decidua. Placental barrier: it is the layers separating the fetal blood (inside the fetal blood vessels of chorionic villi) from the maternal blood between the villi. At the first 20 weeks of pregnancy, the barrier is formed of syncytiotrophoblast, cytotrophoblast, extraembryonic mesoderm and endothelium of the fetal vessels. After 20 weeks of development, the barrier is formed of syncytiotrophoblast and endothelium of fetal vessels, to facilitate material exchange between maternal and fetal blood. Macroscopic features (at birth): Discoid in shape. 500 gm in weight. 15 – 20 cm in diameter. Occupying 20-30% of the endometrium. It is about 3 cm thick in the center, and gradually thins towards the periphery (1 cm). Surfaces: Maternal: irregular, formed of 15–20 irregular projections called cotyledons. The cotyledons are separated by placental septa, arising from the decidua basalis. Fetal: smooth, covered by amniotic membrane, the umbilical cord is attached near the center. Functions of the placenta: 1) Respiratory: O2 passes from maternal to fetal blood and CO2 passes from fetal to maternal blood (replacing lungs function). 2) Nutritive: nutrients pass from maternal to fetal blood (replacing GIT function). 3) Excretory: waste products pass from fetal to maternal blood (replacing kidneys function). 4) Endocrinal (secretory): the placenta secretes: a) Human chorionic gonadotrophins (HCG): which maintains the corpus luteum for the 1st half of pregnancy. b) Estrogen and progesterone. c) Lactogen, relaxin and other hormones. 5) Immunological: Some antibodies pass from maternal to fetal blood. 6) Protective: The placental barrier prevents passage of some infective agents and harmful drugs from maternal to fetal blood. However, some bacteria (e.g., syphilis), viruses (e.g., German measles), parasites (e.g., toxoplasma) and drugs (e.g., morphine) may pass the placental barrier causing harmful effects on fetus. INTRODUCTION TO BODY STRUCTURE 112 Placenta previa Placental anomalies Slide share / Pinterest INTRODUCTION TO BODY STRUCTURE 113 Anomalies of the placenta: 1) Position anomalies (placenta previa): the placenta is lower than the fetus due to implantation in the lower part of the uterus. Further divided into: a) lateral placenta previa: the placenta is attached to the lower uterine segment but above the cervix. b) Marginal placenta previa: the placenta is attached to the margin of the uterine cervix. c) Central placenta previa: the placenta is covering the internal os of cervix. N.B.: the lower uterine segment is the most growing and stretched part of the uterus. This may cause repeated tears in the placenta and repeated hemorrhages during pregnancy. It is completely forbidden to perform a PV (per vaginal) examination in that case as it may cause placental injury and fatal bleeding. 2) Shape anomalies: a) Bilobed (bipartite - bidiscoid) placenta: the placenta is formed of two discoid equal parts. b) Trilobed (tripartite - tridiscoid) placenta: the placenta is formed of three discoid equal parts. c) Accessory (succenturiate) placenta: small part of the placenta is separated from the main part. N.B.: in these cases, a part of the placenta may remain in the uterus after delivery, leading to postpartum hemorrhage. d) Diffuse (membranous) placenta: placenta occupying a wide area of endometrium, due to spreaded chorion frondosum. 3) Umbilical cord attachment anomalies a) Battledore placenta: the umbilical cord is attached to the periphery of the placenta. b) Velamentous attachment of umbilical cord: the umbilical cord is attached to fetal membranes away from placenta. However, the umbilical vessels are still connected to the placenta. 4) Placenta accreta (= adherent): abnormally fixed placenta to the uterus, due to deep invasion of chorionic villi reaching the muscle layer of the uterus. INTRODUCTION TO BODY STRUCTURE 114 Umbilical cord Quora INTRODUCTION TO BODY STRUCTURE 115 UMBILICAL CORD Formation and progress: At the stage of chorionic vesicle, the extraembryonic mesoderm lines the trophoblasts and covers the amniotic cavity and yolk sac. The two layers are separated by extraembryonic coelom except a part dorsocaudal to amniotic cavity called the connecting stalk. After folding, the connecting stalk moves to the ventral aspect of the embryo. It elongates gradually with the growth of the fetus allowing free movements. Contents: 1) Warton’s jelly: the EEM of the connecting stalk. 2) Umbilical vessels: a) Two (Rt & Lt) umbilical arteries: carries non oxygenated blood from the fetus to the placenta. b) Two (Rt & Lt) umbilical veins: the Rt rapidly disappears. The Lt vein carries oxygenated blood from the placenta to the fetus. 3) Vitelline duct: which connects the midgut to the definitive yolk sac. Later this duct disappears. 4) Urachus (distal part of allantois). Macroscopic features (at birth): 50–60 cm in length. 1–2 cm in diameter. Covered by amniotic membrane. Anomalies of umbilical cord: 1) Very long cord: may wind around the neck of the fetus causing hypoxia and may cause true knots. 2) Very short cord: may cause early separation of the placenta. 3) Knots of the cord: a) True knots: usually accompanied by long cord and excessive movement of the fetus. It is a fatal condition. b) False knots: localized collections of Wharton’s jelly. 4) Abnormal attachment of the cord to the placenta (battledore placenta and velamentous attachment of umbilical cord). INTRODUCTION TO BODY STRUCTURE 116 Amniotic cavity Science direct INTRODUCTION TO BODY STRUCTURE 117 AMNIOTIC CAVITY, AMNION AND AMNIOTIC FLUID Formation and progress: At the stage of blastocyst, the amniotic cavity is dorsal to embryonic disc. Its roof is formed of amnioblasts (cells derived from cytotrophoblasts) and the floor is formed by ectoderm. At the stage of chorionic vesicle, the EEM separates the amniotic cavity from the cytotrophoblasts. After folding, the amniotic cavity surrounds the whole embryo. And reflects at the umbilical ring to cover the umbilical cord. The amnion is the membrane enclosing the amniotic cavity. Sources of amniotic fluid: Maternal: diffusion through the chorion. Fetal: amniotic cells and fetal urine. Circulation of the amniotic fluid: the fetus swallow the amniotic fluid → the fluid is absorbed through fetal GIT → circulating in fetal circulation → secreted through fetal kidney as urine to the amniotic fluid → swallowed again. Functions of the amniotic fluid : 1) Protection against external trauma (water cushion). 2) Symmetrically distributes the pressure on the embryo preventing asymmetrical growth. 3) Prevents adherence between embryoblasts and its derivatives and trophoblasts and its derivatives. 4) Keeps aseptic environment around the embryo. 5) Keeps constant temperature of the embryo. 6) Allows free movement of the fetus encouraging muscular development. 7) Development of vital mechanisms before birth: a) Swallowing. b) GIT absorption. c) Circulation. d) Kidney functions and urine formation. e) Urination. 8) During labor, the amniotic sac enters the cervix of the uterus helping its dilatation. 9) Before birth, the amniotic cavity ruptures and its sterile fluid cleans the birth canal. Anomalies of amniotic fluid: The normal amount of amniotic fluid is 750-1500 ml (at full term). Oligohydramnios: it is less than 400 ml amniotic fluid. Causes: ⎯ Renal agenesis. ⎯ Urinary tract obstruction. Risk: adhesions Polyhydramnios: it is more than 2000 ml amniotic fluid. Causes: ⎯ Idiopathic. ⎯ Maternal diabetes. ⎯ Neural tube defect (e.g., anencephaly): defective development of the brain with defective swallowing. ⎯ GIT obstruction. Risk: premature rupture of amniotic membrane and premature labor. INTRODUCTION TO BODY STRUCTURE 118 Yolk sac Anomalies of vitelline canal Anomalies of urachus Quizlet / Stuartsumida / Research gate / Slide share INTRODUCTION TO BODY STRUCTURE 119 Yolk sac Formation and progress: Primary Yolk sac: at the stage of blastocyst, the yolk sac is ventral to embryonic disc. Its roof is formed by endoderm and the rest of the wall is formed of Heuser's membrane (cells derived from endoderm). Secondary yolk sac: at the stage of chorionic vesicle, the EEM surrounds the yolk sac. In this stage, a small pouch (called the allantois) projects from the yolk sac inside the connecting stalk. Tertiary (definitive) yolk sac: after folding, the yolk sac divides into The gut (sucked inside the embryo). Definitive yolk sac: pushed to the connecting stalk. Vitelline duct: connecting both, it disappears separating the gut from the yolk sac. Functions of the yolk sac: 1) Nutritive function: of minimal importance in humans. 2) The endoderm of its roof forms the epithelial lining of the gut and respiratory system. 3) The allantois forms part of the urinary bladder. 4) Primary germ cells (spermatogonia and oogonia) develop from yolk sac and migrate to gonads. Anomalies of vitelline duct: 1) Vitelline ligament: the duct is fibrosed but does not disappear. 2) Vitelline cyst: the duct is fibrosed but a middle part of it is patent. 3) Meckel’s diverticulum: the part of the duct attached to the gut is patent, the rest disappears. 4) Vitelline sinus: the part of the duct attached to the umbilicus is patent, the rest disappears. 5) Vitelline fistula: the whole duct is patent. ALLANTOIS Formation and progress: It is a diverticulum from the secondary yolk sac inside the connecting stalk. It is lined with endoderm. After folding it connects the hind gut (derived from endoderm) to the umbilical cord (derived from connecting stalk). The part connected to the hind gut will form part of the urinary bladder. The part extending to the umbilical cord is called the urachus which will obliterate forming median umbilical ligament connecting the urinary bladder to the umbilicus. It is surrounded by allantoic vessels. Anomalies of allantois: 1) Urachal cyst: the urachus is fibrosed but a middle part of it is patent. 2) Urachal diverticulum: the part of the urachus attached to the urinary bladder is patent, the rest is fibrosed. 3) Urachal sinus: the part of the urachus attached to the umbilicus is patent, the rest is fibrosed. 4) Urachal fistula: the whole urachus is patent.