Medical brain notes 27

  HUMAN DEVELOPMENT   During the 40 weeks of  gestation , the embryo-fetus is protected and nourished in the uterus of the mother. A human being begins life as one cell, a fertilized egg called a zygote, which develops into an individual human being consisting of billions of cells organized into the body systems with whose functions you are now quite familiar.   FERTILIZATION   Although millions of sperm are deposited in the vagina during sexual intercourse, only one will fertil-ize an ovum. As the sperm swim through the fluid of the uterus and fallopian tube, they undergo a final metabolic change, called  capacitation . This change involves the  acrosome , which becomes more fragile. When sperm and egg make contact, the acrosomal enzymes will digest the layers of cells and membrane around an ovum.   Once a sperm nucleus enters the ovum, changes in the egg cell membrane block the entry of other sperm. The nucleus of the ovum completes the second m...

  EMBRYO AND EMBRYONIC MEMBRANES   An  embryo  is the developing human individual from the time of implantation until the eighth week of gestation. Several stages of early embryonic development are shown in Fig. 21–3. At approximately 12 days, the  embryonic disc  (the potential person) is simply a   plate of cells within the blastocyst. As cells keep divid-ing, the plate of cells begins to fold. At 14 days twin-ning is believed to be no longer possible.     Figure 21–3.  Embryonic development at 12 days (after fertilization), 14 days, 20 days,   and 4 to 5 weeks. By 5 weeks, the embryo has distinct parts but does not yet look definitely human. See text for description of embryonic membranes. QUESTION:  At 14 days, what structures shown will become the fetal portion of the   placenta? Very soon thereafter, three primary layers, or germ layers, begin to develop: the  ectoderm ,  mesoderm , and  endoderm . Thi...

  PLACENTA AND UMBILICAL CORD   The  placenta  is made of both fetal and maternal tissue. The chorion of the embryo and the endometrium of the uterus contribute, and the placenta is formed by the third month of gestation (12 weeks). The mature placenta is a flat disc about 7 inches (17 cm) in diameter. The structure of a small portion of the placenta is shown in Fig. 21–5. Notice that the fetal blood vessels are within maternal blood sinuses, but there is no direct connection between fetal and maternal vessels. Normally, the blood of the fetus does not mix with that of the mother. The placenta has two functions: to be the site of exchanges between maternal and fetal blood and to produce hormones to maintain preg-nancy. We will consider the exchanges first.   The fetus is dependent upon the mother for oxygen and nutrients and for the removal of waste products. The  umbilical cord  connects the fetus to the pla-centa. Within the cord are two umbilical ar...

  PARTURITION AND LABOR   Parturition  is the rather formal term for birth, and  labor  is the sequence of events that occur during   birth. The average gestation period is 40 weeks (280  days), with a range of 37 to 42 weeks.   Toward the end of gestation, the placental secretion of progesterone decreases while the estrogen level remains high, and the myometrium begins to contract weakly at irregular intervals. At this time the fetus is often oriented head down within the uterus (Fig. 21–6). Labor itself may be divided into three stages:   Figure 21–6.  Full-term fetus positioned head down within the uterus.   QUESTION:  Why is it beneficial that the fetus is positioned head down? First stage —dilation of the cervix. As the uterus con-tracts, the amniotic sac is forced into the cervix, which dilates (widens) the cervical opening. At the end of this stage, the amniotic sac breaks (rupture of the “bag of waters”) and the fluid l...