Embryology L5 Past Paper PDF

Embryology L5 - P1  PART 01 Third Month to Birth: The Fetus and Placenta  DEVELOPMENT OF THE FETUS The period from the beginning of the ninth week to birth is known as the fetal period. It is characterized by maturation of tissues and organs and rapid growth of the body. The length...

Embryology L5 - P1  PART 01 Third Month to Birth: The Fetus and Placenta  DEVELOPMENT OF THE FETUS The period from the beginning of the ninth week to birth is known as the fetal period. It is characterized by maturation of tissues and organs and rapid growth of the body. The length of the fetus is usually indicated as the crown-rump length (CRL) (sitting height) or as the crown-heel length (CHL), the measurement from the vertex of the skull to the heel (standing height). These measurements, expressed in centimeters, are correlated with the age of the fetus in weeks or months. Growth in length is particularly striking during the third, fourth, and fifth months, whereas an increase in weight is most striking during the last 2 months of gestation. In general, the length of pregnancy is considered to be 280 days, or 40 weeks after the onset of the last normal menstrual period (LNMP) or, more accurately, 266 days or 38 weeks after fertilization. For the purposes of the following discussion, age is calculated from the time of fertilization and is expressed in weeks or calendar months.  Monthly Changes One of the most striking changes taking place during fetal life is the relative slowdown in growth of the head compared with the rest of the body. At the beginning of the third month, the head constitutes approximately half of the CRL. By the beginning of the fifth month, the size of the head is about one-third of the CHL, and at birth, it is approximately one-quarter of the CHL. Hence, over time, growth of the body accelerates but that of the head slows down. During the third month, the face becomes more human looking. The eyes, initially directed laterally, move to the ventral aspect of the face, and the ears come to lie close to their definitive position at the side of the head. The limbs reach their relative length in comparison with the rest of the body, although the lower limbs are still a little shorter and less well developed than the upper extremities. Primary ossification centers are present in the long bones and skull by the 12th week.  Monthly Changes Also, by the 12th week, external genitalia develop to such a degree that the sex of the fetus can be determined by external examination (ultrasound). During the 6th week, intestinal loops cause a large swelling (herniation) in the umbilical cord, but by the 12th week, the loops have withdrawn into the abdominal cavity. During the fourth and fifth months, the fetus lengthens rapidly, and at the end of the first half of intrauterine life, its CRL is approximately 15 cm, about half the total length of the newborn. The weight of the fetus increases little during this period and by the end of the fifth month is still < 500 g. The fetus is covered with fine hair, called lanugo hair; eyebrows and head hair are also visible. During the fifth month, movements of the fetus can be felt by the mother. During the second half of intrauterine life, weight increases considerably, particularly during the last 2.5 months, when 50% of the full-term weight (approximately 3,200 g) is added.  Monthly Changes During the sixth month, the 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. Although several organ systems are able to function, the respiratory system and the central nervous system have not differentiated sufficiently, and coordination between the two systems is not yet well established. By 6.5 to 7 months, the fetus has a CRL of about 25 cm and weighs approximately 1,100 g. If born at thist ime, the infant has a 90% chance of surviving. Some developmental events occurring during the first 7 months are indicated in Table 8.2. During the last 2 months, the fetus obtains well-rounded contours as the result of deposition of subcutaneous fat. By the end of intrauterine life, the skin is covered by a whitish, fatty substance (vernix caseosa) composed of secretory products from sebaceous glands.  Monthly Changes At the end of the ninth month, the skull has the largest circumference of all parts of the body, an important fact with regard to its passage through the birth canal. At the time of birth, the weight of a normal fetus is 3,000 to 3,400 g, its CRL is about 36 cm, and its CHL is about 50 cm. Sexual characteristics are pronounced, and the testes should be in the scrotum.  Time of Birth The date of birth is most accurately indicated as 266 days, or 38 weeks, after fertilization. The oocyte is usually fertilized within 12 hours of ovulation; however, sperm deposited in the reproductive tract up to 6 days prior to ovulation can survive to fertilize oocytes. Thus, most pregnancies occur when sexual intercourse occurs within a 6-day period that ends on the day of ovulation. A pregnant woman usually will see her obstetrician when she has missed two successive menstrual bleeds. By that time, her recollection about coitus is usually vague, and it is readily understandable that the day of fertilization is difficult to determine. The obstetrician calculates the date of birth as 280 days or 40 weeks from the first day of the LNMP. In women with regular 28-day menstrual periods, the method is fairly accurate, but when cycles are irregular, substantial miscalculations may be made. An additional complication occurs when the woman has some bleeding about 14 days after fertilization as a result of erosive activity by the implanting blastocyst. Hence, the day of delivery is not always easy to determine. Most fetuses are born within 10 to 14 days of the calculated delivery date. If they are born much earlier, they are categorized as premature; if born later, they are considered postmature.  Time of Birth Occasionally, the age of an embryo or small fetus must be determined. By combining data on the onset of the last menstrual period with fetal length, weight, and other morphological characteristics typical for a given month of development, a reasonable estimate of the age of the fetus can be formulated. A valuable tool for assisting in this determination is ultrasound, which can provide an accurate (1 to 2 days) measurement of CRL during the 7th to 14th weeks. Measurements commonly used in the 16th to 30th weeks are biparietal diameter (BPD), head and abdominal circumference, and femur length. An accurate determination of fetal size and age is important for managing pregnancy, especially if the mother has a small pelvis or if the baby has a birth defect.  FETAL MEMBRANES AND PLACENTA The placenta is the organ that facilitates nutrient and gas exchange between the maternal and fetal compartments. As the fetus begins the ninth week of development, its demands for nutritional and other factors increase, causing major changes in the placenta. Foremost among these is an increase in surface area between maternal and fetal components to facilitate exchange.  Changes in the Trophoblas the fetal component of the placenta is derived from the trophoblast and extraembryonic mesoderm (the chorionic plate); the maternal component is derived from the uterine endometrium. By the beginning of the second month, the trophoblast is characterized by a great number of secondary and tertiary villi, which give it a radial appearance. Stem (anchoring) vill extend from the mesoderm of the chorionic plate to the cytotrophoblast shell. The surface of the villi is formed by the syncytium, resting on alayer of cytotrophoblastic cells that in turn cover a core of vascular mesoderm. The capillary system developing in the core of the villous stems soon comes in contact with capillaries of the chorionic plate and connecting stalk, thus giving rise to the extraembryonic vascular system.  Changes in the Trophoblas Maternal blood is delivered to the placenta by spiral arteries in the uterus. Erosion of these maternal vessels to release blood into intervillous spaces, is accomplished by endovascular invasion by cytotrophoblast cells. These cells, released from the ends of anchoring villi, invade the terminal ends of spiral arteries, where they replace maternal endothelial cells in the vessels’ walls, creating hybrid vessels containing both fetal and maternal cells. To accomplish this process, cytotrophoblast cells undergo an epithelial-to- endothelial transition. Invasion of the spiral arteries by cytotrophoblast cells transforms these vessels from small-diameter, high-resistance vessels to larger diameter, low-resistance vessels that can provide increased quantities of maternal blood to inter- villous spaces.  Changes in the Trophoblas During the following months, numerous small extensions grow out from existing stem villiand extend as free villi into the surrounding lacunar or intervillous spaces. Initially, these newly formed free villi are primitive, but by the beginning of the fourth month, cytotrophoblastic cells and some connective tissue cells disappear. The syncytium and endothelial wall of the blood vessels are then the only layers that separate the maternal and fetal circulations. Frequently, the syncytium becomes very thin, and large pieces containing several nuclei may break off and drop into the intervillous blood lakes. These pieces, known as syncytial knots, enter the maternal circulation and usually degenerate without causing any symptoms. Disappearance of cytotrophoblastic cells progresses from the smaller to larger villi, and although some always persist in large villi, they do not participate in the exchange between the two circulations.  CHORION FRONDOSUM AND DECIDUA BASALIS In the early weeks of development, villi cover the entire surface of the chorion. As pregnancy advances, villi on the embryonic pole continue to grow and expand, giving rise to the chorion frondosum (bushy chorion). Villi on the abembryonic pole degenerate, and by the third month, this side of the chorion, now known as the chorion laeve, is smooth. the decidua: the functional layer of the endometrium, which is shed during parturition. The decidua over the chorion frondosum, the decidua basalis, consists of a compact layer of large cells, decidual cells, with abundant amounts of lipids and glycogen. This layer, the decidual plate, is tightly connected to the chorion. The decidual layer over the abembryonic pole is the decidua capsularis. With growth of the chorionic vesicle, this layer becomes stretched and degenerates.  CHORION FRONDOSUM AND DECIDUA BASALIS Subsequently, the chorion laeve comes into contact with the uterine wall (decidua parietalis) on the opposite side of the uterus, and the two fuse, obliterating the uterine lumen. Hence, the only portion of the chorion participating in the exchange process is the chorion frondosum, which, together with the decidua basalis, makes up the placenta. Similarly, fusion of the amnion and chorion to form the amniochorionic membrane obliterates the chorionic cavity. It is this membrane that ruptures during labor (breaking of the water). summary THANK YOU！ Embryology L5 - P2  PART 02 Third Month to Birth: The Fetus and Placenta  STRUCTURE OF THE PLACENTA By the beginning of the fourth month, the placenta has two components: (1) a fetal portion, formed by the chorion frondosum, and (2) a maternal portion, formed by the decidua basalis. On the fetal side, the placenta is bordered by the chorionic plate; on its maternal side, it is bordered by the decidua basalis, of which the decidual plate is most intimately incorporated into the placenta. In the junctional zone, trophoblast and decidual cells intermingle. This zone, characterized by decidual and syncytial giant cells, is rich in amorphous extracellular material. By this time, most cytotrophoblast cells have degenerated. Between the chorionic and decidual plates are the intervillous spaces, which are filled with maternal blood. They are derived from lacunae in the syncytiotrophoblast and are lined with syncytium of fetal origin. The villous trees grow into the intervillous blood lakes.  STRUCTURE OF THE PLACENTA During the fourth and fifth months, the decidua forms a number of decidual septa, which project into intervillous spaces but do not reach the chorionic plate. These septa have a core of maternal tissue, but their surface is covered by a layer of syncytial cells so that at all times, a syncytial layer separates maternal blood in intervillous lakes from fetal tissue of the villi. As a result of this septum formation, the placenta is divided into a number of compartments, or cotyledons. Because the decidual septa do not reach the chorionic plate, contact between intervillous spaces in the various cotyledons is maintained. As a result of the continuous growth of the fetus and expansion of the uterus, the placenta also enlarges. Its increase in surface area roughly parallels that of the expanding uterus, and throughout pregnancy, it covers approximately 15% to 30% of the internal surface of the uterus. The increase in thickness of the placenta results from arborization of existing villi and is not caused by further penetration into maternal tissues.  Full-Term Placenta At full term, the placenta is discoid with a diameter of 15 to 25 cm, is approximately 3 cm thick, and weighs about 500 to 600 g. At birth, it is torn from the uterine wall and, approximately 30 minutes after birth of the child, is expelled from the uterine cavity as the afterbirth. When the placenta is viewed from the 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. the fetal surface of the placenta is covered entirely by the chorionic plate. A number of large arteries and veins, the chorionic vessels, converge toward the umbilical cord. The chorion, in turn, is covered by the amnion. Attachment of the umbilical cord is usually eccentric and occasionally even marginal. Rarely, however, does it insert into the chorionic membranes outside the placenta (velamentous insertion).  Circulation of the Placenta Cotyledons receive their blood through 80 to 100 spiral arteries that pierce the decidual plate and enter the intervillous spaces at more or less regular intervals. Pressure in these arteries forces the blood deep into the intervillous spaces and bathes the numerous small villi of the villous tree in oxygenated blood. As the pressure decreases, blood flows back from the chorionic plate toward the decidua, where it enters the endometrial veins. Hence, blood from the intervillous lakes drains back into the maternal circulation through the endometrial veins. Collectively, the intervillous spaces of a mature placenta contain approximately 150 mL of blood, which is replenished about three or four times per minute. This blood moves along the chorionic villi, which have a surface area of 4 to 14 m2. Placental exchange does not take place in all villi, however, only in those that have fetal vessels in intimate contact with the covering syncytial membrane. In these villi, the syncytium often has a brush border consisting of numerous microvilli, which greatly increases the surface area and, consequently, the exchange rate between maternal and fetal circulations.  Circulation of the Placenta the placental membrane, which separates maternal and fetal blood, is initially composed of four layers: (1) the endothelial lining of fetal vessels, (2) the connective tissue in the villus core, (3) the cytotrophoblastic layer, and (4) the syncytium. From the fourth month on, the placental membrane thins because the endothelial lining of the vessels comes in intimate contact with the syncytial membrane, greatly increasing the rate of exchange. Sometimes called the placental barrier, the placental membrane is not a true barrier, as many substances pass through it freely. Because the maternal blood in the intervillous spaces is separated from the fetal blood by a chorionic derivative, the human placenta is considered to be of the hemochorial type. Normally, there is no mixing of maternal and fetal blood. However, small numbers of fetal blood cells occasionally escape across microscopic defects in the placental membrane.  Function of the Placenta Main functions of the placenta are: (1) exchange of metabolic and gaseous products between maternal and fetal bloodstreams and (2) production of hormones.  exchange of gases Exchange of gases—such as oxygen, carbon dioxide, and carbon monoxide—is accomplished by simple diffusion. At term, the fetus extracts 20 to 30 mL of oxygen per minute from the maternal circulation, and even a short-term interruption of the oxygen supply is fatal to the fetus. Placental blood flow is critical to oxygen supply, as the amount of oxygen reaching the fetus primarily depends on delivery, not diffusion.  Exchange of Nutrients and Electrolytes Exchange of nutrients and electrolytes, such as amino acids, free fatty acids, carbohydrates, and vitamins, is rapid and increases as pregnancy advances.  Transmission of Maternal Antibodies Immunological competence begins to develop late in the first trimester, by which time the fetus makes all of the components of complement. Immunoglobulins consist almost entirely of maternal immunoglobulin G (IgG), which begins to be transported from mother to fetus at approximately 14 weeks. In this manner, the fetus gains passive immunity against various infectious diseases. Newborns begin to produce their own IgG, but adult levels are not attained until the age of 3 years.  Hormone Production By the end of the fourth month, the placenta produces progesterone in sufficient amounts to maintain pregnancy if the corpus luteum is removed or fails to function properly. In all probability, all hormones are synthesized in the syncytial trophoblast. In addition to progesterone, the placenta produces increasing amounts of estrogenic hormones, predominantly estriol, until just before the end of pregnancy, when a maximum level is reached. These high levels of estrogens stimulate uterine growth and development of the mammary glands. During the first 2 months of pregnancy, the syncytiotrophoblast also produces human chorionic gonadotropin (hCG), which maintains the corpus luteum. This hormone is excreted by the mother in the urine, and in the early stages of gestation, its presence is used as an indicator of pregnancy. Another hormone produced by the placenta is somatomammotropin (formerly placental lactogen). It is a growth hormone— like substance that gives the fetus priority on maternal blood glucose and makes the mother somewhat diabetogenic. It also promotes breast development for milk production. summary THANK YOU！ Embryology L5 - P3 PART 03 Third month to birth: The fetus and placenta  AMNION AND UMBILICAL CORD The oval line of reflection between the amnion and embryonic ectoderm (amino-ectodermal junction ) is the primitive umbilical ring. At the fifth week of development, the following structures pass through the ring, (1) connecting stalk; (vessels ); (2) yolk stalk; (3) canal connecting the intraembryonic and extraembryonic cavities. The yolk sac occupies a space in the chorionic cavity, that is, the space between the amnion and chorionic plate. During the development, the amniotic cavity enlarges rapidly at the expense of the chorionic cavity, and the amnion begins to envelop the connecting and yolk sac stalks, crowding them together and giving rise to the primitive umbilical cord. The cord contains the yolk sac stalk and umbilical vessels. The yolk sac, found in the chorionic cavity, is connected to the umbilical cord by its stalk, at the end of the third month, the amnion has expanded so that it comes in contact with the chorion, obliterating the chorionic cavity. The yolk sac then usually shrinks and is gradually obliterating. All that remains in the cord are the umbilical vessels surrounded by Wharton , s jelly. This tissue, which is rich in proteoglycans, functions as a protective layer for the blood vessels.  PLACENTAL CHANGES AT THE END OF PREGNANCY At the end of pregnancy a numbers of changes that occur in the placenta. These changes include: - an increase in fibrous tissue in the core of the villus, - thickening of basement membranes in fetal capillaries, - obliterative capillaries of the villi, - deposition of fibrinoid on the surface of the villi in the junctional zone and in the chorionic plate.  AMNIOTIC FLUID The amniotic cavity is filled with a clear , watery fluid that is produced in part by amniotic cells but is derived primarily from maternal blood. The amount of fluid increases from approximately 30 mL at 10 weeks of gestation to 450 mL at 20 weeks to 800 to 1000mL at 37 weeks. During the early months of pregnancy, the embryo is suspended by its umbilical cord in this fluid, which serves as a protective cushion. The fluid (1) absorbs jolts, (2) prevents adherence of the embryo to the amnion, and (3) allows for fetal movements. The volume of amniotic fluid is replaced every 3 hours. From the beginning of the fifth month, the fetus swallows its own amniotic fluid, and it is estimated that it drinks about 400 mL a day, about half of the total amount. Fetal urine is added daily to the amniotic fluid in the fifth month, but this urine is mostly water, because the placenta is functioning as an exchange for metabolic wastes. During childbirth, the amniochorionic membrane forms a hydrostatic wedge that helps to dilate the cervical canal.  FETAL MEMBRANES IN TWINS Arrangement of fetal membranes in twins varies considerably, depending on the type of twins and on the time of separation of monozygotic twins.  DIZYGOTIC TWINS Approximately two thirds of twins are dizygotic, or fraternal. The rate for dizygotic 7 to 11 per 1000 births, increases with maternal age. They result from simultaneous shedding of two oocytes and fertilization by different spermatozoa. Because the two zygotes have totally different genetic constitution, the twins have no more resemblance than other brothers or sisters. They may or may not be of different sex. The zygotes implant individually in the uterus, and usually each develops its own placenta, amnion, and chorionic sac. Sometimes, the two placentas are so close together that they fuse. Similarly, the walls of the chorionic sacs may also come into close apposition and feus.  MONOZYGOTIC TWINS The second type of twins, which develops from a single fertilized ovum, is monozygotic, or identical, twins. The rate for monozygotic twins is three to four per 1000. They result from splitting of the zygote at various stages of development. The earliest separation is believed to occur at the two-cell stages, in which case two separate zygotes develop. The blastocysts implant separately, and each embryo has its own placenta and chorionic sac. Although the arrangement of the membranes of these twins resembles that of dizygotic twins, the two can be recognized as partners of a monozygotic pair by their strong resemblance in blood groups, fingerprints, sex, and external appearance, such as eye and hair color.  MONOZYGOTIC TWINS Splitting of the zygote usually occurs at the early blastocyst. The inner cell mass splits into two separate groups of cells within the same blastocyst cavity. the two embryos have a common placenta and a common chorionic cavity but separate amniotic cavities. In rare cases, the separation occurs at the bilaminar germ disc stage, just before the appearance of the primitive streak. This method of splitting results in formation of two partners with a single placenta and a common chorionic and amniotic sac.  PARTURITION (BIRTH) for the first 34 to 38 weeks of gestation, the uterine myometrium does not respond to signals for parturition (birth). During the last 2 to 4 weeks of pregnancy, however, this tissue undergoes a transitional phase in preparation for the onset of labor. This phase ends with a thickening of the myometrium in the upper region of the uterus and a softening and thinning of the lower region and cervix. Labor itself is divided into three stages: (1) effacement (thinning and shortening ) and dilatation of the cervix, (2) delivery of the fetus, and (3) delivery of the placenta and fetal membranes SUMMARY THANK YOU！