Human Biology

1.70. Human Egg Cells http://www.ck12.org

1. Before birth, special cells in the ovaries go throughmitosis(cell division).


2. The daughter cells then start to divide bymeiosis. But they only go through the first of the two cell divisions of
   meiosis at that time. They go through the second stage of cell division after the female goes through puberty.


3. In a mature female, an egg develops in an ovary about once a month. The drawing below shows how this
   happens (Figure1.132).

As you can see from the figure, the egg rests in a nest of cells called afollicle. The follicle and egg grow larger and
go through other changes. After a couple of weeks, the egg bursts out of the follicle and through the wall of the
ovary. This is calledovulation. The moving fingers of the nearbyfallopian tubesweep the egg into the tube.

FIGURE 1.132

This diagram shows how an egg and its

follicle develop in an ovary. After it devel-

ops, the egg leaves the ovary and enters

the fallopian tube. (1) Undeveloped eggs,

(2) Egg and follicle developing, (3) Egg

and follicle developing, (4) Ovulation.

Fertilization occurs if a sperm enters the egg while it is passing through the fallopian tube. When this happens, the
egg finally completes meiosis. This results in two daughter cells that are different in size. The smaller cell is called
apolar body. It contains very little cytoplasm. It soon breaks down and disappears. The larger cell is the egg. It
contains most of the cytoplasm. This will develop into a child.

Vocabulary

• cytoplasm: Entire contents of the cell inside the plasma membrane, excluding the nucleus.


• fallopian tube: Female reproductive organs that carry eggs from the ovary to the uterus.


• follicle: Structure in the ovary where eggs mature.


• meiosis: Process in cell division during which chromosome number is halved in order to produce gametes.


• mitosis: Division of the nucleus.


• ovulation: Release of the egg out of the follicle and through the wall of the ovary.


• polar body: Small cells formed by the unequal meiotic divisions of cytoplasm as an egg develops.

Summary

• Eggs are female gametes that form in the ovaries and are released into the fallopian tubes.


• The eggs are formed before a baby girl is born, but these eggs are not fully developed.