Medical brain notes 44

  Maturation and Fertilization of the Ovum While still in the ovary, the ovum is in the  primary oocyte  stage. Shortly before it is released from the ovarian follicle, its nucleus divides by meiosis and a  firstpolar body  is expelled from the nucleus of the oocyte. The primary oocyte thenbecomes the  secondary oocyte.  In this process, each of the 23 pairs of chromo-somes loses one of its partners, which becomes incorporated in a  polar body  that is expelled. This leaves 23  unpaired  chromosomes in the secondary oocyte. It is at this time that the ovum, still in the secondary oocyte stage, is ovulated into the abdominal cavity. Then, almost immediately, it enters the fimbriated end of one of the fallopian tubes. Entry of the Ovum into the Fallopian Tube (Oviduct).  When ovulation occurs, the ovum,along with a hundred or more attached granulosa cells that constitute the  corona radiata,  is expelled directly into t...

  Early Nutrition of the Embryo We pointed out that the progesterone secreted by the ovarian corpus luteum during the latter half of each monthly sexual cycle has an effect on the uterine endometrium, converting the  endometrial stromal cells into large swollen cells con-taining extra quantities of glycogen, proteins, lipids, and even some minerals necessary for development of the conceptus.   Then, when the conceptus implants in the endometrium, the continued secretion of proges-terone causes the endometrial cells to swell further and to store even more nutrients. These cells are now called  decidual cells,  and the total mass of cells is called the  decidua. As the trophoblast cells invade the decidua, digest-ing and imbibing it, the stored nutrients in the decidua are used by the embryo for growth and development. During the first week after implantation, this is the only means by which the embryo can obtain nutrients; the embryo continues to obtain at l...

  Function of the Placenta Developmental and Physiologic Anatomy of the Placenta While the trophoblastic cords from the blastocyst are attaching to the uterus, blood capillaries grow into the cords from the vascular system of the newly forming embryo. By the 16th day after fertilization, blood also begins to be pumped by the heart of the embryo itself. Simultaneously,  blood sinuses  supplied with blood from the mother develop around the outsides of the tro-phoblastic cords. The trophoblast cells send out more and more projections, which become  placental villi  into  which fetal capillaries grow. Thus, the villi, carrying fetal blood, are surrounded by sinuses that contain maternal blood. The final structure of the placenta is shown in Figure 82–5. Note that the fetus’s blood flows through two  umbilical arteries,  then into the capillaries of the villi,and finally back through a single  umbilical vein  into the fetus. At the same time,...

  Hormonal Factors in Pregnancy In pregnancy, the placenta forms especially large quantities of  human chorionic gonadotropin, estrogens,progesterone,  and  human chorionic somatomam-motropin,  the first three of which, and probablythe fourth as well, are all essential to a normal pregnancy. Human Chorionic Gonadotropin and Its Effect to Cause Persistence of the Corpus Luteum and to Prevent Menstruation Menstruation normally occurs in a nonpregnant woman about 14 days after ovulation, at which time most of the endometrium of the uterus sloughs away from the uterine wall and is expelled to the exterior. If this should happen after an ovum has implanted, the pregnancy would terminate. However, this is prevented by the secretion of human chorionic gonadotropin by the newly developing embryonic tissues in the following manner. Coincidental with the development of the tro-phoblast cells from the early fertilized ovum, the hormone  human chorionic gonadotropin ...

  Response of the Mother’s Body to Pregnancy Most apparent among the many reactions of the mother to the fetus and to the excessive hormones of pregnancy is the increased size of the various sexual organs. For instance, the uterus increases from about 50 grams to 1100 grams, and the breasts approximately double in size. At the same time, the vagina enlarges and the introitus opens more widely. Also, the various hormones can cause marked changes in a pregnant woman’s appearance, sometimes resulting in the development of edema, acne, and masculine or acromegalic features. Weight Gain in the Pregnant Woman The average weight gain during pregnancy is about 24 pounds, with most of this gain occurring during the last two trimesters. Of this, about 7 pounds is fetus and 4 pounds is amniotic fluid, placenta, and fetal membranes. The uterus increases about 2 pounds and the breasts another 2 pounds, still leaving an average weight increase of 9 pounds. About 6 pounds of this is extra fluid i...