Lecture Notes On Genetics/Embryology - ANA 212 - February 2024 - PDF

DEPARTMENT OF ANATOMY FACULTY OF BASIC MEDICAL SCIENCES AMBROSE ALLI UNIVERSITY, EKPOMA LECTURE NOTE COURSE TITLE: GENETICS/EMBRYOLOGY COURSE CODE: ANA 212 CLASS: YEAR TWO TOPIC; ORGANOGENESIS, FETAL MEMBRANE, PLACENTA FORMATION AND FUNCTION...

DEPARTMENT OF ANATOMY FACULTY OF BASIC MEDICAL SCIENCES AMBROSE ALLI UNIVERSITY, EKPOMA LECTURE NOTE COURSE TITLE: GENETICS/EMBRYOLOGY COURSE CODE: ANA 212 CLASS: YEAR TWO TOPIC; ORGANOGENESIS, FETAL MEMBRANE, PLACENTA FORMATION AND FUNCTION LECTURER: DR. (MRS) OVIOSUN, E. C. FEBRUARY, 2024 1  OUTLINE ORGANOGENESIS, FETAL MEMBRANE, PLACENTA FORMATION AND FUNCTION 2 Organogenesis 3  Organogenesis the process by which the ectoderm, endoderm, and mesoderm develop into the internal organs of the organism. Cells of each germ layer proliferate, migrate, reaggregate and differentiate into various tissues that form the organs (organogenesis) 4 How does it occur? The germ layers in organogenesis differ by three processes: folds, splits, and Condensation Cells of each germ layer proliferate, migrate, reaggregate and differentiate into various tissues that form the organs (organogenesis) 5 When does it occur? Internal organs initiate development in humans within the 3rd to 8th weeks in utero. 6 7 8 9  Embryonic stage Week 3: Beginning development of the brain, heart, blood cells, circulatory system, spinal cord, and digestive system. Week 4: Beginning development of bones, facial structures, and limbs (presence of arm and leg buds); continuing development of the heart (which begins to beat), brain, and nervous tissue. Week 5: Beginning development of eyes, nose, kidneys, lungs; continuing development of the heart (formation of valves), brain, nervous tissue, and digestive tract. 10  Embryonic stage Week 6: Beginning development of hands, feet, and digits; continuing development of brain, heart, and circulation system. Week 7: Beginning development of hair follicles, nipples, eyelids, and sex organs (testes or ovaries); first formation of urine in the kidneys and first evidence of brain waves. Week 8: Facial features more distinct, internal organs well developed, the brain can signal for muscles to move, heart development ends, external sex organs begin to form. By the end of the embryonic stage, all essential external and internal structures have been formed. The embryo is now referred to as a fetus. 11 12 CRITICAL PERIODS OF DEVELOPMENT It is a specific time during which the environment has its greatest impact on an individual's development. 13 if you drink, I will also drink 14 Why is it critical period? Because it will disturb the processes of developments: Control of cell division, Apoptosis, Gene expression, and Cellular metabolism 15 During pre-embronic stage The effects of maternal drug use on the development of the fetus are dependent upon the stage of embryonic and fetal development, as well as the dosage of the substance. After fertilization occurs, the embryo is highly resistant to birth defects, but the results of a high dose teratogen could result in death. Intake of teratogens during this preembryonic stage could also result in normal development if the teratogenic substance is stopped before the embryonic stage of development. 16  During embryonic stage During the embryonic stage of development (weeks 3-8), the effects of teratogens are increasingly detrimental. Organs begin forming during the embryonic stage, and harmful teratogens can result in miscarriage, and structural abnormalities. Once the embryonic stage is complete, the fetal period begins at week 9 and continues until full term at week 38. Teratogens taken during this period can result in improper organ functioning, delayed growth, but seldom result in birth defects. 17 18 19 20  By the end of the lecture the student should be able to: List the components of the fetal membranes. Describe the stages of development of the components. Describe the structure and function of the components. Describe their fate and the possible congenital anomalies. 21  Umbilical cord (Connecting Stalk) Amnion Amniotic Fluid Yolk Sac Allantois Functions 1. Protection 2. Nutrition 3. Respiration 4. Excretion 5. Synthesis of Hormones 22 It is a pathway which connects the ventral aspect of the embryo with the placenta (chorion) It is a soft tortuous cord measuring (30- 90) cm in length (average 55) ,(1-2) cm in diameter. It has a smooth surface because it is covered by the amnion 23  1-Connecting stalk: Alantois + two Umbilical arteries + two Umbilical veins The extra embryonic mesoderm forms Wharton’s jelly  2-Yolk stalk (Vitello-intestinal duct): A narrow, elongated duct which connects gut to yolk sac It contains Vitelline Vessels Later on , it is obliterated and the vitelline vessels disappear. 24  Normally, it is attached to a point near the centre of the fetal surface of the placenta 25  (1) Abnormal Attachment: a-Battledore placenta : The UC is attached to the margin of the placenta (it is not dangerous). b-Velamentous insertion of the cord : UC is attached to the amnion away from placenta, (It is dangerous to the fetus due to rupture of blood vessels during labor) 26  (2) Abnormalities in Length: a-Very Long Cord: It is dangerous , it may surround the neck of the fetus and causes its death. b-Very Short Cord: It is dangerous because it may cause premature separation of placenta, or the cord itself may rupture 27  (3) False and True knots of umbilical cord: a-False knots: UC looks tortuous due to twisting of umbilical vessels (umbilical vessels are longer than the cord), these knots are normal and do not cause any harm to the fetus b-True knots: Are rare (1%) of pregnancy, but very dangerous because they may cause obstruction to blood flow in umbilical True Knots in 20-weeks fetus vessels, leading to fetal anoxia & fetal death 28 3rd week: Appears as a diverticulum from caudal wall of Y.S. that extends into connecting stalk. 2nd month: Its extra- embryonic part degenerates. 3rd month: Its intra-embryonic part extends from UB to UC as thick tube , ‘(urachus) ’ After birth: the urachus is obliterated and fibrosed to form median umbilical ligament, that extends from apex of UB to umbilicus. 29 Blood formation in its wall during 3rd to 5th week. Its blood vessels persist as the umbilical vein & arteries. 30  It is essential in the transfer of nutrients to the embryo during 2nd & 3rd weeks, when the uteroplacental circulation is not established. It does not contain any yolk. Its development passes through three stages: Primary yolk sac. Secondary yolk sac. Definitive yolk sac. 31 Appears in the Blastocyst stage at 10-days, it lies ventral to the embryonic plate. Its roof is formed by hypoblast (primary endoderm), Its wall is formed by exocoelomic membrane, it lines the inner surface of the cytotrophoblast, and separated from it by the extraembryonic mesoderm 32  Appears in the chorionic vesicle stage Its roof is formed by hypoblast (embryonic endoderm), its wall is formed by exocoelomic membrane + inner layer (splanchnic layer) of the extraembryonic mesoderm. At day 16: a diverticulum appears from its dorsocaudal end (Allantois) into the substance of the connecting stalk 33  After folding, part of Yolk Sac is enclosed within the embryo to form the Gut (Foregut, Midgut & Hindgut). The remainder of Yolk Sac that remains outside the embryo becomes the Definitive Yolk Sac  The midgut is temporarily connected to Definitive Yolk Sac by a narrow duct Vitello-intestinal duct (Yolk stalk), which is incorporated inside the umbilical cord.  This is fibrosed and degenerated by the end of (6th week) 34  3rd week:  (a) Blood formationt First formed in the extra-embryonic mesoderm covering the wall of the yolk sac, until hemopoietic activity begins in the liver during 6th week 4th week: endoderm of yolk sac is incorporated into the embryo to form primordial gut Epithelium of Respiratory system &G.I.T. (b)Primordial germ cells in the endodermal lining of the wall of caudal end of the yolk sac migrate into the developing sex glands to differentiate into germ cells (spermatogonia or oogonia) 35 Yolk stalk detached from midgut by the end of 6th week. In (2%) of adults, its proximal intra- abdominal part persists as ileal diverticulum (Meckel diverticulum). At 10 week, small definitive yolk sac lies in the chorionic cavity between amniotic & chorionic sacs At 20 weeks, as pregnancy advances, definitive yolk sac atrophies and becomes a very small cyst. In unusual cases, it persists under the amnion near the attachment of Umbilical cord, on the fetal surface of the placenta. Its persistence is of no significance 36  It is a thin, transparent & tough fluid- filled, membranous sac surrounding the embryo.  At First : It is seen as a small cavity lying dorsal to the embryonic plate.  At Stage of Chorionic Vesicle: The amnion becomes separated from the chorion by chorionic cavity or extra embryonic coelom.  After Folding: the amnion expands greatly and is becomes on the ventral surface of the embryo.  As a result of expansion of the amnion, the extra embryonic coelom is gradually obliterated and amnion forms the epithelial covering of umbilical cord. 37  It is a watery fluid inside the amniotic cavity (sac). It has a major role in fetal growth & development It increases slowly, to become (700- 1000) ml by full term (37) weeks. Composition: 99% of amniotic fluid is water It contains un-dissolved material of desquamated fetal epithelial cells + organic + inorganic salts As pregnancy advances, composition of amniotic fluid changes as fetal excreta (meconium = fetal feces & urine) are added 38  Fetal & Maternal Sources: Initially, some amniotic fluid is secreted by amniotic cells. Most of fluid is derived from Maternal tissue by: 1-Diffusion across amnio- chorionic membrane from placenta. 2-Diffusion across chorionic plate (chorionic wall related to placenta) from the maternal blood in the intervillous spaces. Later, it is derived from Fetus through: Skin, Fetal Respiratory Tract & mostly by Excreting Urine (at beginning of 11th week) 39  Provides symmetrical external growth of the embryo Acts as a barrier to infection (it is an aseptic medium) Permits normal fetal lung development Prevents adherence of embryo to amnion It protects embryo against external injuries Keeps the fetal body temperature constant Allows the embryo to move freely, aiding muscular development in the limbs It is involved in maintaining homeostasis of fluids & electrolytes It permits studies on fetal enzymes, hormones and diagnosis of fetal sex and chromosomal abnormalities 40  Amniotic fluid remains constant & in balance --Most of fluid is swallowed and few passes into lungs by fetus, and absorbed into fetal blood, where it is metabolised -- Part of fluid passes through placental membrane into maternal blood in intervillus space, Other part of fluid is excreted by fetal kidneys into amniotic sac 41  (1) Oligohydramnios: The volume is less than ½ liters Causes : Placental insufficiency with low placental blood flow Preterm rupture of amnio-chorionic membrane occurs in 10% of pregnancies Renal Agenesis (failure of kidney development) Obstructive Uropathy (urinary tract obstruction) lead to absence of fetal urine (the main source) Complications : Fetal abnormalities (pulmonary, facial & limb defects) 42  (2) Polyhydramnios (Hydramnios): The volume is more than 2 liters, it is diagnosed by Ultrasonography. Causes Fetal ( 1-20% ) : Esophageal atresia. Maternal (2-20%) : defects in maternal circulation. Idiopathic (3-60%) 43  PLACENTA This is a fetomaternal organ. It has two components: Fetal part – develops from the chorionic sac ( chorion frondosum ) Maternal part – derived from the endometrium ( functional layer – decidua basalis ) The placenta and the umbilical cord are a transport system for substances between the mother and the fetus.( vessels in umbilical cord ) Function Of The Placenta: 1. Protection 2. Nutrition 3. Respiration 4. Excretion 5. Hormone production 44  DECIDUA is the DECIDUA endometrium of the gravid (pregnant) uterus. It has four parts: Decidua basalis: it forms the maternal part of the placenta Decidua capsularis: it covers the conceptus Decidua parietalis: the rest of the endometrium Decidua reflexa: Junction between capsularis & parietalis. 45 DEVELOPMENT OF PLACENTA Until the beginning of the 8th week, the entire chorionic sac is covered with villi. After that, as the sac grows, only the part that is associated with Decidua basalis retain its villi. Villi of Decidua capsularis compressed by the developing sac. Thus, two types of chorion are formed: Chorion frondosum (villous chorion) Chorion laeve – bare (smooth) chorion About 18 weeks old, it covers 15-30% of the decidua and weights about 1\ 6 of fetus 46 DEVELOPMENT OF PLACENTA The villous chorion ( increase in number, enlarge and branch ) will form the fetal part of the placenta. The decidua basalis will form the maternal part of the placenta. The placenta will grow rapidly. By the end of the 4th month, the decidua basalis is almost entirely replaced by the fetal part of the placenta. 47 FULL-TERM PLACENTA Cotyledons –about 15 to 20 slightly bulging villous areas. Their surface is covered by shreds of decidua basalis from the uterine wall. After birth, the placenta is always inspected for missing cotyledons. Cotyledons remaining attached to the uterine wall after birth may cause severe bleeding. Grooves – formerly occupied by placental septa Maternal side 48 FULL-TERM PLACENTA Fetal surface: ( Discoid shape -500- 600 gm- Diameter 15-20 cm – Thickness of 2-3 cm) This side is smooth and shiny. It is covered by amnion. The umbilical cord is attached close to the center of the placenta. The umbilical vessels radiate from the umbilical cord. They branch on the fetal surface to form chorionic vessels. Fetal side They enter the chorionic villi to form arteriocapillary-venous system. 49  This is a composite structure that PLACENTAL MEMBRANE consists of the extra-fetal tissues separating the fetal blood from the maternal blood. It has four layers: Syncytiotrophoblast Cytotrophoblast Connective tissue of villus Endothelium of fetal capillaries After the 20th week, the cytotrophoblastic cells disappear and the placental membrane consists only of three layers. 50 TRANSFER ACROSS THE PLACENTAL MEMBRANE Viruses: measles;poliomyelitis Microorganism: treponema pallidum of syphilis ; T.g which produce destructive change in the eye; brain. IgG( gamma globulin) , IgS;IgM ( immunoglobulin S;M ) 51 Placental endocrine synthesis The syncytiotrophoblast synthesizes protein & steroid hormones The protein hormones 1- human chorionic gonadotropin 2- human chorionic somatomammotropin 3- human chorionic thyrotropin 4- human chorionic corticotropin The steroid hormones Progesterone & Estrogens 52  Placental anomalies Third trimester bleeding is the common sign of these anomalies 53 54 55 56