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Reproductive System

The Processes of Fertilization, Preembryonic Development and Implantation

Each primary oocyte undergoes the first meiotic division while still in the ovarian follicle. This division forms a secondary oocyte and the first polar body, each containing 23 chromosomes. At ovulation, the secondary oocyte and first polar body, still enclosed within a sphere of granulosa cells, are

Embryonic Development – Germ Layers, Extraembryonic Membranes and Placenta

The embryonic stage of development begins at the start of the third week of development and is completed at the end of the eighth week. During this time, the embryo undergoes rapid development, forming the rudiments of all body organs, extraembryonic membranes, and the placenta. By the

Fetal Development Stages

At the beginning of the ninth week of development, the developing offspring has a distinctively human appearance and is now referred to as a fetus. Weeks of Development Changes 5-8 Weeks Recognizable human shape; head as large as the body; eyes far apart; upper and lower limbs

Hormonal Control of Pregnancy

Without the formation of a preembryo, the corpus luteum degenerates about two weeks after ovulation as a result of the decline in luteinizing hormone from the anterior lobe of the pituitary gland. The resulting decline in blood levels of estrogens and progesterone causes the endometrium to break

Lactation – The Control of Lactation and Milk Ejection

High blood levels of estrogens and progesterone during pregnancy stimulate the development of the mammary glands and enlargement of the breasts in preparation for milk secretion, or lactation (lak-ta’-shun). Although the mammary glands are capable of secreting milk, the high levels of estrogens and progesterone inhibit the

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