Exam 3 Terms menstrual cycle to parturition

fetus

An individual from the the beginning of the 9th week when all of the organ systems are present, through the time of birth.

chorion

Another role of the trophoblast is to secret human chorionic gonadotropin (HCG). HCG stimulates the corpus luteum to secrete estrogen and progesterone, and progesterone suppresses menstruation. The level of HCG in the mother's blood rises until the end of the second month. During this time, the trophoblast develops into a membrane called the chorion which takes over the role of the corpus luteum and makes HCG unnecessary. The ovaries become inactive for the rest of the pregnancy, but estrogen and progesterone levels rise dramatically as they are secreted by the ever-growing chorion.

cleavage

Cleavage refers to mitotic divisions that occur in the first 3 days, while the conceptus migrates down the uterine tube. The first cleavage occurs about 30 hours after fertilization and produces the first two daughter cells, or blastomeres.

estrogen

Estrogen secretion increases to about 30 times the usual amount by the end of gestation. The corpus luteum is an important source of estrogen for the first 12 weeks; after that it comes mainly from the placenta. The adrenal glands of the mother and fetus secrete androgens, which the placenta converts to estrogens. The most abundant estrogen is estriol, but its effect are relatively weak; estradiol is less abundant but 100 times as potent. Estrogen stimulates tissue growth in the fetus and mother. It causes the mother's uterus and external genitalia to enlarge, and the mammary ducts to grow and the breasts to increase to nearly twice their former size. It makes the pubic symphysis more elastic and the sacroiliac joints more limber, so the pelvis widens during pregnancy and the pelvic outlets expands during childbirth.

cortical granules

Fertilization by two or more sperm called polyspermy would produce a triploid and the egg would die. The egg has two ways to prevent this. In the fast block, binding of sperm to the egg opens Na+ channels in the egg membrane and depolarizes the membrane to inhibit more sperm attachment. The slow block involves secretory vesicles called cortical granules just beneath the membrane. Sperm penetration releases an inflow of Ca2+; this in turn, stimulates a cortical reaction in which the cortical granules release their secretion beneath the zona pellucida. The secretion swells with water, pushes any remaining sperm away from the egg, and creates an impenetrable fertilization membrane between the egg and the zona pellucida.

FSH/LH

Follicle stimulating hormone and luteinizing hormone are gonadotropins. During the follicular phase, FSH stimulates continued growth of all follicles in the cohort, but of the dominant follicle above all. FSH stimulates the granulosa cells of the antral follicles to secrete estradiol. In response to estradiol, the dominant follicle up-regulates its receptors for FSH, LH, and estradiol itself, thereby becoming increasingly sensitive to these hormones. The anterior pituitary secretes less and less FSH but an increasing amount of LH. Most antral follicles suffer from the reduced FSH level and degenerate (undergo atresia). The dominant follicle however, has the richest blood supply and the greatest density of FSH receptors, so it becomes the mature, preovulatory follicle. Ovulation is the rupture of the mature follicle and the release of its egg around day 14. Dramatic changes over the preceding day signify its imminence. Estradiol stimulates a surge of LH and a lesser spike in FSH secretion by the pituitary. LH induces several momentous events. The primary oocyte completes meiosis I, producing a haploid secondary oocyte and the first polar body. The transformation from ruptured follicle to corpus luteum is regulated by LH; hence, LH is also called the luteotropic hormone. LH stimulates the corpus luteum to continue growing and to secrete rising levels of estradiol and progesterone. There is a 10-fold increase in progesterone level, which has a crucial role in preparing the uterus for pregnancy. Despite its luteinizing role, LH secretion declines steadily over the rest of the cycle, as does FSH. This is because high levels of extradiol and progesterone , along with inhibin from the corpus luteum, have a negative feedback effect on the pituitary. If pregnancy does not occur, the corpus luteum shrinks down to an inactive bit of scar tissue called the corpus albicans. With the waning of ovarian steroid secretion, the pituitary is no longer inhibited and FSH levels begin to rise again, stimulating a new cohort of follicles.

trophoblast

Meanwhile, the zona pellucida disintegrates and releases the conceptus, which is now in a stage called the blastocyst- a hollow sphere with an outer layer of squamous cells called the trophoblast. The trophoblast is destined to form part of the placenta and play an important role in nourishment of the embryo.

extraembryonic membranes

Some mesoderm overflows the embryonic disc and becomes an extensive extraembryonic mesoderm, which contributes to formation of the placenta. Several accessory organs develop alongside the embryo: a placenta; umbilical cord; and four embryonic membranes called the amnion, yolk sac, allantois, and chorion.

acrosome

The acrosome is a lysosome in the form of a thin cap covering the apical half of the nucleus around the head of sperm. It contains enzymes that are later used to penetrate the egg if the sperm is successful.

allantois

The allantois begins as an outpocketing of the yolk sac, but eventually becomes an outgrowth of the caudal end of the gut. It forms the foundation for the umbilical cord and becomes part of the urinary bladder. It can be seen in cross sections cut near the fetal end of a mature umbilical cord.

amnion

The amnion is a transparent sac that develops from cells of the epiblast. It grows to completely enclose the embryo and is penetrated only by the umbilical cord. The amnion fills with amniotic fluid which protects the embryo from trauma, infection and temperature fluctuations; allows the freedom of movement needed for muscle development; enables the embryo to develop symmetrically; prevents body parts from adhering to each other, such as arm to trunk; and stimulates lung development as the fetus "breathes" the fluid. At first, the amniotic fluid forms by filtration of the mother's blood plasma, but beginning at 8 or 9 weeks, the fetus urinates into the amniotic cavity about once an hour and contributes substantially to the fluid volume. The volume grows slowly, however, because the fetus swallows amniotic fluid at a comparable rate.

amniotic cavity

The amniotic cavity forms during embryogenesis when the embryoblast separates slightly from the trophoblast and creates a narrow spaced between them. This space is the amniotic cavity.

chorionic villi

The chorion is the outermost membrane of the embryo and has shaggy outgrowths called chorionic villi around its entire surface, but as pregnancy advances, the villi of the placental region grow and branch while the rest of them degenerate. At the placental attachment, the chorion is then called the villous chorion, and the rest is called the smooth chorion. The villous chorion forms the fetal potion of the placenta.

blastomeres, morula

The first cleavage occurs about 30 hours after fertilization and produces the first two daughter cells, or blastomeres. These divide simultaneously at shorter and shorter time intervals, doubling the number of blastomeres each time. By the time the conceptus arrives in the uterus, about 72 hours after ovulation, it consists of 16 or more cells and somewhat resembles a mulberry- hence, it is called a morula. The morula is no larger than the zygote; cleavage merely produces smaller and smaller blastomeres and a large number of cells from which to form different embryonic tissues.

menstrual cycle

The menstrual cycle runs concurrently with the ovarian cycle. It consists of a buildup of the endometrium, followed by its breakdown and vaginal discharge. It is divided into: 1. proliferative phase 2. secretory phase 3. premenstrual phase (ischemic phase) 4. menstrual phase. 1. at the end of menstruation, the endometrium is about 0.5 mm thick and consists of only a basal layer. But as new follicles develop they secrete more and more estrogen. Estrogen stimulates mitosis in the basal layer and the prolific regrowth of blood vessels, thus generating the functional layer. By the end of day 14, the endometrium is 2 to 3 mm thick. Estrogen also stimulates endometrial cells to produce progesterone receptors, priming them for the next secretory phase. 2. The endometrium still thickens but as a result of fluid accumulation, not mitosis. This phase extends from day 15 to day 26 typically. After ovulation the corpus luteum secrete mainly progesterone which stimulates endometrial glands to secrete glycogen. The glands grow wider, longer, more coiled, and the lamina propia swells with tissue fluid. By the end of this phase, the endometrium is 5 to 6 mm thick- a soft wet nutritious bed available for embryonic development in the event of pregnancy. 3. The last 2 days of the cycle are the premenstrual phase, a period of endometrial degeneration. When there is no pregnancy, the corpus luteum atrophies and progesterone levels fall sharply. This drop triggers spasmodic contraction of the spiral arteries of the endometrium, causing endometrial ischemia. Ischemia brings on tissue necrosis and menstrual cramps. As the endometrium degenerates, pools of blood accumulate in the functional layer. Necrotic endometrium falls away from the uterine wall, mixes with flood and serous fluid in the lumen, and forms menstrual fluid. 4. When enough menstrual fluid accumulates in the uterus, it begins to be discharged from the vagina for a period called the menstrual phase (menses). The first day of discharge, marks day one of a new cycle. The average women expels about 40mL of blood and 35mL of serous fluid over a 5-day period. Menstrual fluid contains fibrinolysin, so it does not clot.

blastocyst

The morula lies free in the uterine cavity for 4 or 5 days and divides into 100 cells or so. Meanwhile, the zona pellucida disintegrates and releases the conceptus, which is now in a stage called the blastocyst- a hollow sphere with an outer layer of squamous cells called the trophoblast, an inner cell mass called the embryoblast, and an internal cavity called the blastocoel.

zona pellucida

The oocyte is surrounded by a layer of glycoprotein gel, the zona pellucida.

umbilical cord

The placenta is the fetus's life-support system- a disc-shaped organ attached to the uterine wall on one side, and on the other, attached to the fetus by way of the umbilical cord.

placenta

The placenta is the fetus's life-support system- a disc-shaped organ attached to the uterine wall on one side, and on the other, attached to the fetus by way of the umbilical cord. It is the fetus's source of oxygen and nutrients, and its means of waste disposal. The diffusion of nutrients from the mother's through the placenta into the fetal blood is called placental nutrition. The placenta begins to develop about 11 days after conception, becomes the dominant mode of nutrition around the beginning of week 9 and is the sole mode of nutrition from the end of week 12 until birth. The period from week 9 until birth is called the placental phase of the pregnancy.

progesterone

The placenta secrets a great deal of progesterone, and early in the pregnancy, so does the corpus luteum. Progesterone and estrogen suppress pituitary secretion of FSH and LH, thereby preventing more follicles from developing during pregnancy. Progesterone also suppresses uterine contractions so the conceptus is not prematurely expelled. It prevents menstruation and promotes the proliferation of decidual cells of the endometrium, on which the blastocyst feeds. Once estrogen has stimulated growth of the mammary ducts, progesterone stimulates development of the secretory acini-another step toward lactation.

afterbirth

The placenta, amnion and other fetal membranes are expelled by uterine contractions, which may be aided by a gentle pull on the umbilical cord. The membranes (afterbirth) must be carefully inspected to be sure everything has been expelled. If any of these structures remain in the uterus, they can cause postpartum hemorrhaging. The umbilical blood vessels are counted because an abnormal number in the cord may indicate cardiovascular abnormalities in the infant.

parturition

The process of giving birth. True labor contractions mark the onset of parturition.

yolk sac

The yolk sac arises from hypoblast cells oppostie the amnion. It is a small sac suspended from the ventral side of the embryo. It contributes to the formation of the digestive tract and produces the first blood cells and forerunners of the future egg or sperm cells.

embryo

a developing individual from the 16th day of gestation when the three primary germ layers have formed, through the end of the eighth week when all of the organ systems are present. Preembryo> embryo> fetus Preembryo: individual from time of fertilization to the 16th day when the three primary germ layers have formed. Embryo: individual from 16th day to end of 8th week when all organ systems are present. Fetus: individual from beginning of 9th week to birth.

inner cell mass

the inner cell mass of the blastocyst is called the embryoblast. The embryoblast is destined to become the embryo itself.

blastocoel

the internal cavity of the blastocyst.

implantation

Implantation is when bout 6 days after ovulation, the blastocyst attaches to the endometrium, usually on the fundus or posterior wall of the uterus. The trophoblast (outer layer of blastocyst) separate into two layers. The superficial layer, in contact with the endometrium, the trophoblast cells fuse into a multinucleate mass called the syncytiotrophoblast. The deep layer, is called the cytotrophoblast because it retains individual cells divided by the membranes. The syncytiotrophoblast grows into the uterus like little roots, digesting endometrial cells along the way. The endometrium reacts to this injury by growing over the blastocyst and eventually covering it so the conceptus becomes completely buried in endometrial tissue. Implantation takes about a week and is completed about the time the next menstrual cycle would have occurred if the woman had not become pregnant. Another role of the trophoblast is to secret human chorionic gonadotropin (HCG). HCG stimulates the corpus luteum to secrete estrogen and progesterone, and progesterone suppresses menstruation. The level of HCG in the mother's blood rises until the end of the second month. During this time, the trophoblast develops into a membrane called the chorion which takes over the role of the corpus luteum and makes HCG unnecessary. The ovaries become inactive for the rest of the pregnancy, but estrogen and progesterone levels rise dramatically as they are secreted by the ever-growing chorion.