Estrogen secretion during the follicular phase prepares the female reproductive tract for efficient sperm transport to the site of fertilization in the ampulla of the oviducts. Ciliary action and muscular contractions of the oviduct propel the ovum and later the blas-tocyst toward the uterus. Implantation occurs about 5 days after ovulation and involves invasion of the endometrium and differentiation of the outer portion of the blastocyst into the primitive placenta.
The placenta is a highly vascular organ specialized for bidirectional transport of metabolites and gases and for hormone synthesis. Secretion of human chorionic gonadotropin (hCG) by the early trophoblast prolongs the life of the corpus luteum and maintains its capacity for progesterone production for most of the first trimester.
Human chorionic somatomammotropin (hCS) is secreted by the placenta in large quantities, particularly in the latter half of pregnancy, and
Essential Medical Physiology, Third Edition
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