Formation Of Eggs

A female is born with more than 400,000 eggs in her ovaries. These eggs are immature and cannot be fertilized. Typically, a female will release 300 to 400 mature eggs during her lifetime, averaging one egg about every 28 days from puberty to about age 50. Thus, less than 1 percent of her eggs will mature.

Like sperm formation, egg formation occurs through meiosis. So, each mature egg cell has 23 chromosomes (the haploid number). Unlike sperm formation—in which four functional sperm result from each cell that begins meiosis—egg formation results in one functional egg from each cell that begins meiosis. All immature eggs begin meiosis but stall in prophase I until the female reaches puberty, when the sex hormones stimulate egg maturation. Then, every 28 days these hormones signal 10 to 20 immature eggs to resume meiosis. Generally, only one of these eggs completes meiosis I and is released from an ovary. Meiosis I produces two haploid cells. One cell receives most of the cytoplasm and can go on to become a mature egg. The second haploid cell, or first polar body, contains a very small amount of cytoplasm. In humans, the first polar body usually dies without dividing again. Meiosis II is not completed unless a sperm fertilizes the egg. If fertilized, the egg completes the final meiotic division by dividing into a mature egg and a second polar body. The mature egg, or ovum (OH vuhm), retains most of the cytoplasm, which provides nutrients for the egg through the early stages of development. The second polar body dies. An ovum, shown in Figure 51-7, is about 75,000 times larger than a sperm and is visible to the unaided eye.

The female reproductive system consists of several internal and external structures that enable fertilization and development.

figure 51-7

This ovum is being approached by a single sperm. Notice the tremendous size difference between the egg and the sperm.

Word Roots and Origins menstrual from the Latin mensis, meaning "month"

figure 51-8

During the 28-day ovarian and menstrual cycles, an egg matures and is released by an ovary, and the uterus prepares for a possible pregnancy. The events of the menstrual cycle are regulated by hormones that are produced by the anterior pituitary and the ovaries.

Luteinizing

Follicle-stimulating

hormone (LH)

hormone (FSH)

Estrogen

Progesterone

Follicular phase

Follicular phase

Luteal phase

Menstruation

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