B

Figure 2.5 A. Scanning electron micrograph of sperm binding to the zona pellucida. B. The three phases of oocyte penetration. In phase 1, spermatozoa pass through the corona radiata barrier; in phase 2, one or more spermatozoa penetrate the zona pellucida; in phase 3, one spermatozoon penetrates the oocyte membrane while losing its own plasma membrane. Inset. Normal spermatocyte with acrosomal head cap.

Corona Radiata Oocyte

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Figure 2.6 A. Oocyte immediately after ovulation, showing the spindle of the second meiotic division. B. A spermatozoon has penetrated the oocyte, which has finished its second meiotic division. Chromosomes of the oocyte are arranged in a vesicular nucleus, the female pronucleus. Heads of several sperm are stuck in the zona pellucida. C. Male and female pronuclei. D and E. Chromosomes become arranged on the spindle, split longitudinally, and move to opposite poles. F. Two-cell stage.

Figure 2.6 A. Oocyte immediately after ovulation, showing the spindle of the second meiotic division. B. A spermatozoon has penetrated the oocyte, which has finished its second meiotic division. Chromosomes of the oocyte are arranged in a vesicular nucleus, the female pronucleus. Heads of several sperm are stuck in the zona pellucida. C. Male and female pronuclei. D and E. Chromosomes become arranged on the spindle, split longitudinally, and move to opposite poles. F. Two-cell stage.

scar tissue, the corpus albicans. Simultaneously, progesterone production decreases, precipitating menstrual bleeding. If the oocyte is fertilized, degeneration of the corpus luteum is prevented by human chorionic gonadotropin (hCG), a hormone secreted by the syncytiotrophoblast of the developing embryo. The corpus luteum continues to grow and forms the corpus luteum of pregnancy (corpus luteum graviditatis). By the end of the third month, this structure may be one-third to one-half of the total size of the ovary. Yellowish luteal cells continue to secrete progesterone until the end of the fourth month; thereafter, they regress slowly as secretion of progesterone by the trophoblastic component of the placenta becomes adequate for maintenance of pregnancy. Removal of the corpus luteum of pregnancy before the fourth month usually leads to abortion.

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