Prolactin preferentially enters amnionic luid, and little enters maternal blood. Consequently, prolactin levels in amnionic luid are extraordinarily high and may reach 10,000 ng/mL at 20 to 24 weeks' gestation (Tyson, 1972). This compares with fetal serum levels of 350 ng/mL and maternal serum levels of 150 to 200 ngl mL. As a result, decidual prolactin is a classic example of paracrine function between maternal and fetal tissues.
The ferus is dependent on the placenta for pulmonary, hepatic, and renal functions. These are accomplished through the anatomical relationship of the placenta and its uterine interface. In overview, maternal blood spurts from uteroplacental vessels into the placental intervillous space and bathes the outer syncytiotrophoblast. his allows exchange of gases, nutrients, and other substances with fetal capillary blood within the core of each villus. hus, fetal and maternal blood does not normally mix in this hemochorial placenta. A paracrine system also links mother and ferus through the anatomical and biochemical juxtaposition of the maternal decidua parietalis and the extraembryonic chorion laeve, which is fetal. This is an extraordinarily important arrangement for communication between fetus and mother and for maternal immunological acceptance of the conceptus (Cuzeloglu-Kayisli, 2009).
With ovulation, the secondary oocyte and adhered cells of the cumulus-oocyte complex are freed from the ovary. Although technically this mass of cells is released into the peritoneal cavity, the oocyte is quickly engulfed by the fallopian tube infundibulum. Further transport through the tube is accomplished by directional movement of cilia and tubal peristalsis. Fertilization, which normally occurs in the oviduct, must take place within a few hours, and no more than a day after ovulation. Because of this narrow window, spermatozoa must be present in the fallopian tube at the time of oocyte arrival. Almost all pregnancies result when intercourse occurs during the 2 days preceding or on the day of ovulation.
Fertilization is highly complex. Molecular mechanisms allow spermatozoa to pass between follicular cells; through the zona pellucida, which is a thick glycoprotein layer surrounding the oocyte cell membrane; and into the oocyte cytoplasm. Fusion of the two nuclei and intermingling of maternal and paternal chromosomes creates the zygote.
Early human development is described by days or weeks postfertilization, that is, postconceptional. By contrast, in most chapters of this book, clinical pregnancy dating is calculated from the irst day of the last menstrual period (LMP). hus, 1 week postfertilization corresponds to approximately 3 weeks from the LMP in women with regular 28-day cycles. As an example, 8 weeks' gestation refers to 8 completed weeks following the LMP.
After fertilization, the zygote-a diploid cell with 46 chromosomes-undergoes cleavage, and zygote cells produced by this division are called blastomeres (Fig. 5-7). In the two-cell
FIGURE 5-7 Zygote cleavage and blastocyst formation. The morula period begins at the 12-to 16-cell stage and ends when the blastocyst forms, which occurs when there are 50 to 60 blastomeres present. The polar bodies, shown in the 2-cell stage, are small nonfunctional cells that soon degenerate.