Gross Anatomy (3rd Month to Birth) PDF

GROSS ANATOMY Topic: Third Month to Birth Clinical Instructor/s: Doctor Barza Created by: Castro, Wilson Brent E., Shinji, Naomi., Agag, Alexandre, Abiera, Lenz, Isha, Esquinas (M1A ‘27)...

GROSS ANATOMY Topic: Third Month to Birth Clinical Instructor/s: Doctor Barza Created by: Castro, Wilson Brent E., Shinji, Naomi., Agag, Alexandre, Abiera, Lenz, Isha, Esquinas (M1A ‘27) LEGENDS 6th week Book Knowledge From Lecturer From OT - Intestinal loops cause a large swelling (herniation) in the umbilical cord. OUTLINE 12th week - The loops have with drawn into the abdominal cavity. I. DEVELOPMENT OF THE FETUS  Third month  Fourth and fifth month THIRD MONTH  Sixth month  Face becomes more human looking.  Last two months  Eyes, initially directed laterally, move to the ventral aspect of the II. TIME OF BIRTH face. III. FETAL MEMBRANE AND PLACENTA  Ears come to lie close to their definitive position at the side of the  Chorion frondosum and Decidua basalis head. IV. STRUCTURE OF THE PLACENTA  Limbs reach their reactive length in comparison with the rest of  Beginning of fourth month the body.  Fourth month  Primary ossification centers are present in the long bones and  Between fourth month and fifth month skull.  Full term placenta  External genitalia develop to such a degree that the sex of the  Circulation of placenta fetus can be determined by external examination (ultrasound).  Functions of placenta At the end of the third month, reflex activity can be evoked in aborted  Amnion and umbilical cord fetuses, indicating muscular activity. V. AT THE END OF 3RD MONTH VI. PLACENTAL CHANGES AT THE END OF PREGNANCY  Amniotic fluid  Fetal membrane in twins FOURTH AND FIFTH MONTH  Parturition  Fetus lengthens rapidly Crown-Rump Length (CRL) is now approximately 15 cm. The weight increases to more than 500g at  Three stages of labor the end of the fifth month. VII. CLINICAL CORRELATION  Fetus is covered with fine hair called lanugo hair, eyebrows, and head hair are also visible.  Movement of fetus can be felt by the mother. DEVELOPMENT OF FETUS *During the second half of intrauterine life, weight increases considerably, particularly during the last 2.5 months, when 50% of the Fetal period full-term weight (approximately 3,200 g) is added. - Period from the beginning of the ninth week to birth. - characterized by maturation of tissues and organs and rapid SIXTH MONTH growth of the body  Skin of the fetus is reddish and has a wrinkled appearance because of the lack of underlying connective tissue.  A fetus born early in the sixth month has great difficulty surviving. Length of fetus  Several organ systems are able to function, but the respiratory system and the central nervous system have not differentiated - Crown-rump length (CRL) - Sitting Height sufficiently, and coordination between the two systems is not yet - Crown heel length (CHL) - the measurement from the vertex of well established. the skull to the heel (standing height) 6 TO 7 MONTHS Length of pregnancy is considered to be 280 days, or 40 weeks - CRL is about 25 cm, and weight is approximately 1,100g after the onset of the last normal menstrual period (LNMP) - Infant has a 90% chance of surviving. More accurately, 266 days or 38 weeks after fertilization. 1 of 9  GROSS ANATOMY COLLEGE OF MEDICINE (BATCH 2027) LAST TWO MONTHS Changes in Trophoblast The fetus obtains well-rounded contours as the result of deposition of subcutaneous fat. By the beginning of the second month:  Trophoblast characterized by great number of secondary and - Vernix caseosa is a whitish fatty substance composed of tertiary villi, which give it a radial appearance. secretory products from sebaceous glands that covers the skin  Stem (anchoring) villi extend from mesoderm of the chorionic by the end of intrauterine life. plate to the cytotrophoblast shell.  Surface of the villi is formed by the syncytium, resting on a layer of cytotrophoblastic cells that in turn cover a core of vascular END OF NINTH MONTH mesoderm. - Skull has the largest circumference of all parts of the body, an  The capillary system developing in the core of the villous stems important fact with regard to its passage through the birth canal. soon comes in contact with capillaries of the chorionic plate and connecting stalk, thus giving rise to the extraembryonic system. TIME OF BIRTH o Maternal blood is delivered to the placenta by the spinal  At the time of birth, the weight of a normal fetus is 3,000 to 3,400 arteries in the uterus. g, its CRL is about 36 cm, and its CHL is about 50 cm. o Erosion of these maternal vessels to release blood into  Sexual characteristics are pronounced, and the testes should be the intervillous spaces is accomplished by in the scrotum. endovascular invasion by the cytotrophoblastic cells.  Most accurately indicated as 266 days, or 38 weeks, after o Cytotrophoblastic cells invade the terminal ends of fertilization. The oocyte is usually fertilized within 12 hours of spiral arteries, where they replace maternal ovulation; however, sperm deposited in the reproductive tract up endothelial cells in the vessel’s walls, creating hybrid to 6 days prior to ovulation can survive to fertilize oocytes. vessels containing both fetal and maternal cells.  The obstetrician calculates the date of birth as 280 days or 40 (cytotrophoblast cells undergo an epithelial- to- weeks from the first day of the LNMP. endothelial transition)  Most fetuses are born within 10 to 14 days of the calculated delivery date. If they are born much earlier, they are categorized During the following months: as premature; if born later, they are considered postmature.  Numerous cell extensions grow out from existing steam villi and extend as free villi into the surrounding lacunar or intervillous Ultrasound spaces. - Can provide an accurate (1 to 2 days) measurement of CRL during the 7th to 14th weeks. Beginning of the fourth month: Biparietal Diameter  Cytotrophoblastic cells and some connective tissue cells - Measurements commonly used in the 16th to 30th weeks. disappear.  Syncytium and endothelial wall of the blood vessels are then the - Measures head and abdominal circumference, and femur only layers separating maternal and fetal circulations. length.  Syncytium becomes very thin, and large pieces containing several nuclei may break off and drop into the intervillous blood *An accurate determination of fetal size and age is important for lakes, (pieces known as syncytial knots) managing pregnancy, especially if the mother has a small pelvis or if the baby has a birth defect.  These pieces enter the maternal circulation and degenerates.  Disappearance of cytotrophoblastic cells progresses from the smaller villi to larger villi. FETAL MEMBRANES AND PLACENTA Placenta - Organ that facilitates nutrient and gas exchange between the maternal and fetal compartments. - As the fetus begins the 9th week of development, it demands for nutritional, and other factors increase causing major changes in placenta. - Increase in surface area between maternal and fetal components to facilitate exchange. - Disposition of fetal membranes is also altered as production of amniotic fluid increases. - Fetal components: derived from trophoblast and extra embryonic mesoderm (chorionic plate) - Maternal components: derived from the uterine endometrium 2 of 9  GROSS ANATOMY COLLEGE OF MEDICINE (BATCH 2027) Decidua - Functional layer of the endometrium which is shed during parturition. - Decidua basalis: the decidua over the chorion frondosum which consists of compact layer of large cells (decidual cells) with abundant amount of lipids and glycogen. - Decidual plate: layer tightly connected to the to the chorion. - Decidual capsularis: decidual layer over the abembryonic pole. With the growth of the chorionic vessels, this layer becomes stretched and degenerates. - Decidua parietalis: Uterine wall that comes in contact with chorion leave, on the opposite side of the uterus, and the two fuses, obliterating the uterine lumen CHORION FRONDOSUM AND DECIDUA BASALIS Chorion Frondosum Early week of development:  Villi cover the entire surface of the chorion.  As pregnancy advances, villi on the embryonic pole continue to grow and rise giving rise to the chorion frondosum. By the third month:  Villi on the abembryonic pole degenerate the side of the chorion, Formation of placenta now known as the chorion leave o The difference between embryonic and abembryonic - Only a portion of the chorion participating in the exchange of ples of the chorion is also reflected in the structure of process is the chorion frondosum, which together with the the decidua. decidua basalis, makes up the placenta. - Placenta commonly known as “Inunan” in Filipino Formation of amniochorionic membrane - Fusion of amnion and chorion; obliterates the chorionic cavity - Pertains to the membrane that ruptures during labor (breaking of water) Placenta Previa - Normal previa - Low implantation: placenta lies in the lower uterine segment, but its lower edge does not abut the internal cervical os (os pertains to the opening between the cervix and the corpus) 3 of 9  GROSS ANATOMY COLLEGE OF MEDICINE (BATCH 2027) - Partial placenta previa: placenta partially covers the internal - As a result of this septum formation, the placenta is divided into cervical os a number of compartments, or cotyledons - Total/Complete placenta previa: placenta completely covers the internal cervical os FULL- TERM PLACENTA - Discoid with a diameter of 15 to 25 cm, is approximately 3 cm thick, and weighs about 500 to 600 g. Maternal Side - 15 to 20 slightly bulging areas, the cotyledons, covered by a thin layer of decidua basalis, are clearly recognizable. - Grooves between the cotyledons are formed by decidual septa. Fetal Surface - covered entirely by the chorionic plate. - A number of large arteries and veins, the chorionic vessels, converge toward the umbilical cord - Chorion is covered by the amnion CIRCULATION OF THE PLACENTA STRUCTURE OF THE PLACENTA  Cotyledons receive their blood through 80 to 100 spiral arteries Beginning of Fourth Month: that pierce the decidual plate and enter the intervillous spaces at Two components of the Placenta: more or less regular intervals.  Fetal Portion  Pressure in the arteries forces the blood deep into the intervillous - formed by the chorion frondosum bordered by the spaces and bathes the numerous small villi of the villous tree in chorionic plate oxygenated blood.  Maternal Portion  As the pressure decreases, blood flows back from the chorionic - formed by the decidua basalis bordered by the decidua plate toward the decidua, where it enters the endometrial veins basalis  Blood from the intervillous lakes drains back into the maternal circulation through the endometrial veins. Trophoblast and Decidual cells intermingle in the junctional zone.  Placental exchange takes place only in those that have fetal vessels in intimate contact with the covering syncytial membrane. Junctional zone is characterized by decidual and syncytial giant cells,  In these villi, the syncytium often has a brush border consisting of is rich in amorphous extracellular material. numerous microvilli that greatly increases the surface area; consequently, the exchange rate between maternal and fetal Most cytotrophoblast cells have degenerated. circulations. Intervillous spaces between the chorionic and decidual plates are filled with maternal blood. THE PLACENTAL MEMBRANE FOURTH AND FIFTH MONTH - separates maternal and fetal blood - Decidua forms a number of decidual septa, which project into - also called the placental barrier, despite that, it is not an actual intervillous spaces but do not reach the chorionic plate barrier as many substances can freely pass through it - These septa have a core of maternal tissue, but their surface is - Ideally, maternal and fetal blood should not mix, but small covered by a layer of syncytial cells, so that at all times, a numbers of fetal blood cells occasionally escape across syncytial layer separates maternal blood in intervillous lakes microscopic defects in the placental membrane from fetal tissue of the villi. 4 of 9  GROSS ANATOMY COLLEGE OF MEDICINE (BATCH 2027) Initially composed of four layers: - the fetus gains passive immunity against various  the endothelial lining of fetal vessels infectious diseases  the connective tissue in the villus core - newborns begin to produce their own IgG which  the cytotrophoblastic layer reaches adult levels at the age of three years  the syncytium 4. Hormone Production - all hormones are produced in the syncytial trophoblast Progesterone - produced in sufficient amounts before the end of the fourth month - maintains pregnancy if the corpus luteum is removed/fails to - During the 4th month onwards, it thins because the function properly endothelial lining of the vessels comes in close contact with the syncytial membrane, which increases the rate of Estrogenic hormones, mainly Estriol: exchange greatly - produced in increasing amounts until just before the end of - The human placenta is considered to be hemochorial pregnancy when it reaches the maximum level because the maternal blood in the intervillous spaces is - high levels stimulate uterine growth and development of separated from the fetal blood by a chorionic derivative the mammary glands Hemochorial - having the maternal blood in direct Human Chorionic Gonadotropin (hCG): contact with the fetal epithelium/chorion - produced during the first 2 months of pregnancy - excreted by the mother in the urine - its presence is used as an indicator of pregnancy FUNCTIONS OF PLACENTA Somatomammotropin: - formerly placental lactogen - exchange of metabolic and gaseous products between - growth hormone-like substance that gives the fetus priority maternal and fetal bloodstreams on maternal blood glucose making the mother somewhat - hormone production diabetogenic - also promotes breast development of milk production MAIN FUNCTIONS 1. Exchange of Gases: - intake of oxygen and output of carbon dioxide and Amnion and Umbilical Cord carbon monoxide, through simple diffusion across the fetal membrane. Primitive umbilical ring - at term, the fetus extracts 20 to 30 ml of oxygen - an oval line of reflection between the amnion and the per minute from the maternal circulation embryonic ectoderm (amnio-ectodermal junction) - short term interruption of the oxygen supply is fatal to the fetus Structures that pass through the ring at the 5th week of development: - placental blood flow is critical to oxygen supply - the amount of oxygen reaching the fetus depends 1. The connecting stalk on the rate of blood flow, not diffusion - contains the allantois and the umbilical vessels, consisting of 2 arteries and 1 vein 2. Exchange of Nutrients and Electrolytes - it is rapid and increases as pregnancy advances 2. The yolk stalk (vitelline duct) - includes amino acids, free fatty acids, carbohydrates, - accompanied by the vitelline vessels and vitamins 3. The canal connecting the intraembryonic and 3. Transmission of Maternal Antibodies extraembryonic cavities - immunological competence develops late into the - The yolk sac proper occupies a space in the chorionic first trimester cavity, that is, the space between the amnion and the - during this time the fetus makes all its components of chorionic plate complement - During further development, the amniotic cavity enlarges rapidly at the expense of the chorionic cavity, and the - maternal Immunoglobulin G (IgG) begins to be amnion begins to envelop the connecting and yolk sac transported from mother to fetus at approximately 14 stalks, crowding them together and giving rise to the weeks primitive umbilical cord 5 of 9  GROSS ANATOMY COLLEGE OF MEDICINE (BATCH 2027) - The abdominal cavity is temporarily too small for the rapidly  the walls of umbilical arteries are muscular and contain many developing intestinal loops, and some of them are pushed elastic fibers, which contribute to the rapid constriction and into the extraembryonic space in the umbilical cord forming contraction of the umbilical vessels after the cord is tied off the physiological umbilical hernia. PLACENTAL CHANGES AT THE END OF PREGNANCY  increase in fibrous tissue in the core of the villus  thickening of basement membranes in fetal capillaries  obliterative changes in small capillaries of the villi  deposition of fibrinoid on the surface of the villi in the junctional zone and the chorionic plate  excessive fibrinoid formation frequently causes infarction of an intervillous lake or sometimes of an entire cotyledon causing the cotyledon to assume a whitish appearance Amniotic Fluid - clear, watery fluid produced in part by amniotic cells but is derived primarily from maternal blood - increases from approximately 30 mL at 10 weeks of gestation to 450 mL at 20 weeks to 800 - 1,000 mL at 37 weeks. Primitive umbilical cord - serves as a protective cushion by absorbing jolts, - Its distal portion contains the yolk sac stalk and the umbilical prevents adherence of the embryo to the amnion, and vessels allows for fetal movements - Its proximal portion contains some intestinal loops and the - the volume of amniotic fluid is replaced every 3 hours remnants of the allantois - From the beginning of the fifth month, the fetus swallows about 400 mL of amniotic fluid a day Amnion - Fetal urine is added daily to the amniotic fluid which is - a large sac containing amniotic fluid in which the fetus is mostly water because the placenta functions as an suspended by its umbilical cord exchange for metabolic wastes - During childbirth, the amnion chorionic membrane forms a Amniotic fluid hydrostatic wedge that helps to dilate the cervical canal. - prevents adherence of the embryo to the surrounding tissue, absorbs jolts, and allows fetal movements FETAL MEMBRANES IN TWINS Dizygotic Twins AT THE END OF THIRD MONTH  Approximately 90% of twins  Also called FRATERNAL TWINS  the amnion expanded until it comes in contact with the chorion,  Incidence increases with maternal age obliterating the chorionic cavity  Two oocytes were fertilized by two spermatozoa  the yolk sac shrinks until it is gradually obliterated  Develops its own placenta, amnion, and chorionic sac.  the intestinal loops are withdrawn into the body of the embryo, and the cavity in the cord is obliterated  the allantois and the vitelline duct and its vessels are also obliterated leaving just the umbilical vessels surrounded by Wharton jelly which is a connective tissue rich in proteoglycans that functions as a protective layer for the umbilical vessels. 6 of 9  GROSS ANATOMY COLLEGE OF MEDICINE (BATCH 2027) Monozygotic Twins  Intrauterine Growth Restriction (IUGR) and Small for  Also called IDENTICAL TWINS Gestational Age (SGA) consider gestational age  Develops from a single fertilized ovum  Possible question: What are the risk of IUGR? Intrauterine  Strong resemblance in blood groups, fingerprints, sex, and Growth Restriction (IUGR) - Have an increased risk for external appearance neurological problems, congenital malformations, meconium  Has common placenta and a common chorionic cavity but aspiration, hypoglycemia, hypocalcemia, and respiratory distress separate amniotic cavity syndrome (RDS)  Possible question: What is Barker’s Hypothesis? Barker’s Hypothesis - Development of metabolic disorders in later life (obesity, hypertension, hypercholesterolemia, cardiovascular PARTURITION - Commonly known as childbirth - Tissue undergoes a transitional phase during the last 2-4 weeks disease, and type 2 diabetes) of gestation - Ends with thickening of the myometrium in the upper region of the uterus and a softening and thinning of the lower region and cervix Small for Gestational Age (SGA)  Below 10th percentile for their gestational age  May be pathologically small (they may have IUGR) or they may be constitutionally small (healthy, but smaller size).  Pathologically (Sick). vs Constitutionally (Healthy, It’s just a small baby)/ SGA not always unhealthy  Symmetric growth restriction vs Asymmetric growth restriction  SGA infants are at risk of perinatal asphyxia, meconium aspiration, hypoglycemia, polycythemia, and hypothermia 3 STAGES OF LABOR 1. Effacement and Dilation of the Cervix Insulin-like growth factor-1 (IGF-1) 2. Delivery of the Fetus 3. Delivery of the Placenta and Fetal Membranes  Major growth-promoting factor during development before and after birth due to mitogenic and anabolic effects Contractions usually begin about 10 minutes apart (