Fertilization, Implantation, and Endocrinology of Pregnancy

Abstract

The complex and coordinated set of events leading to sperm and egg maturation and transport in the female genital tract that culminates in fertilization is one of the most remarkable phenomena in nature. This set of events is followed by the equally important unique processes of implantation, fetal maturation, and parturition. The hormonal changes that regulate these events are dependent on the close interaction of the fetal-placental-maternal unit. Just before ovulation, the egg, which has been arrested in the diplotene stage, completes the first meiotic division and forms the first polar body. The second meiotic division starts at the time of ovulation but ends only after fertilization by a sperm. The process of egg maturation is regulated through a closely interrelated set of hormonal events, most notably involving follicle-stimulating hormone (FSH), luteinizing hormone (LH), and estrogen. At the time of ovulation the fimbria of the oviduct are closely applied to the surface of the ovary. The extruded oocyte and adherent granulosa cells, known as the cumulus oophorus, is collected by the ciliated fimbrial end of the fallopian tube. The transport of the egg into the end of the fallopian tube occurs within minutes and is regulated primarily by ciliary action. The cumulus cells are able to communicate with one another via a network of intercellular bridges through the zona pellucida to the perivitelline space. The cumulus cells have also been reported to play a role in nutrition and maintenance of the ovum. There are three different stages of passage of the ovum through the fallopian tube. The first stage includes the transfer of the ovum from the fimbriated end of the fallopian tube until the egg reaches and is retained at the ampullary-isthmic junction. The ampullary-isthmic junction is a functional block but is not a clearly defined anatomical structure. The ovum remains at this junction for 1–2 days, during which time fertilization occurs.