Maturation and Fertilization of the Ovum

While still in the ovary, the ovum is in the primary oocyte stage. Shortly before it is released from the ovarian follicle, its nucleus divides by meiosis and a first polar body is expelled from the nucleus of the oocyte. The primary oocyte then becomes the secondary oocyte. In this process, each of the 23 pairs of chromosomes loses one of its partners, which becomes incorporated in a polar body that is expelled. This leaves 23 unpaired chromosomes in the secondary oocyte. It is at this time that the ovum, still in the secondary oocyte stage, is ovulated into the abdominal cavity. Then, almost immediately, it enters the fimbriated end of one of the fallopian tubes.

Entry of the Ovum into the Fallopian Tube (Oviduct). When ovulation occurs, the ovum, along with a hundred or more attached granulosa cells that constitute the corona radiata, is expelled directly into the peritoneal cavity and must then enter one of the fallopian tubes to reach the cavity of the uterus. The fimbri-ated ends of each fallopian tube fall naturally around the ovaries. The inner surfaces of the fimbriated tentacles are lined with ciliated epithelium, and the cilia are activated by estrogen from the ovaries, which causes the cilia to beat toward the opening, or ostium, of the involved fallopian tube. One can actually see a slow fluid current flowing toward the ostium. By this means, the ovum enters one of the fallopian tubes.

It seems likely that many ova might fail to enter the fallopian tubes. However, on the basis of conception studies, it is probable that as many as 98 per cent succeed in this task. Indeed, in some recorded cases, women with one ovary removed and the opposite fallopian tube removed have had several children with relative ease of conception, thus demonstrating that ova can even enter the opposite fallopian tube.

Fertilization of the Ovum. After the male ejaculates semen into the vagina during intercourse, a few sperm are transported within 5 to 10 minutes upward from the vagina and through the uterus and fallopian tubes to the ampullae of the fallopian tubes near the ovarian ends of the tubes. This transport of the sperm is aided by contractions of the uterus and fallopian tubes stimulated by prostaglandins in the male seminal fluid and also by oxytocin released from the posterior pituitary gland of the female during her orgasm. Of the almost half a billion sperm deposited in the vagina, a few thousand succeed in reaching each ampulla.

Fertilization of the ovum normally takes place in the ampulla of one of the fallopian tubes soon after both the sperm and the ovum enter the ampulla. But

Corona radiata

Dispersed corona radiata

Corona radiata

Dispersed corona radiata

Female pronucleus

Centrosome

Figure 82-1

Fertilization (day 1)

Cell division

Blastocyst

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Essentials of Human Physiology

Essentials of Human Physiology

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