Module 5 Lesson 4 - 2

Spermatogenesis

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Read page 569

To consider common chromosome disorders in people, you will consider first the formation of human gametes. Recall from your studies in Unit B that the male gamete is produced in the testes. The process of generating sperm is spermatogenesis, and it occurs in males at the rate of millions of sperm per day after puberty is reached.

In the seminiferous tubules, diploid germ cells known as spermatogonia can divide by mitosis for growth, repair, and replacement, or they divide by meiosis to produce four haploid sperm cells (each with only 23 chromosomes).

The process of spermatogenesis begins with the mitosis of a diploid germ cell called a spermatogonium. The spermatogonium contains 46 chromosomes and a complete copy of the genetic material of the male .

During mitosis, two identical diploid spermatogonia are produced. One remains a spermatogonium and the other goes through the process of meiosis to produce gametes.

After meiosis, a single diploid spermatogonium produces four haploid sperm cells.

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Consider the following video illustrating the meiotic process of spermatogenesis. While watching, be sure you are able to recognize differences among the spermatogonium, the spermatocyte, the spermatids, and the sperm.

Oogenesis

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Read pages 569 - 570

Formation of gametes in females is called oogenesis. Although the process is similar to spermatogenesis, several key differences occur in the two gametogenesis. The process of oogenesis occurs in the female ovaries.   

Consider the following video illustrating the meiotic process of oogenesis.  While watching, note when and where the two meiotic divisions occur.

In females, oogenesis begins before birth. During fetal development, the diploid germ cells called oogonia divides through mitosis and form primary oocytes in the fetal ovaries. In other words, all the primary oocytes that eventually will mature into eggs during a woman's lifetime are formed before that female is born. At birth, approximately 2 million primary oocytes are in the ovaries and about 400 000 remain at puberty.

These primary oocytes stop at prophase I of meiosis, and oogenesis remains unchanging at this stage until puberty. At the onset of puberty, one primary oocyte continues through meiosis I approximately every 28 days.

The cell division at the end of meiosis I results in an unequal cell division. One daughter cell called a secondary oocyte is much larger and leaves the ovary during ovulation. The smaller cell called a polar body may divide to produce a second polar body. These polar bodies eventually disintegrate. Ovulation marks the end of meiosis I.

The secondary oocyte is released into a Fallopian tube for fertilization. This is the egg cell or ovum that is matured each month as part of the menstrual cycle studied in Unit B. Although the secondary oocyte is considered haploid, the sister chromatids have not separated because meiosis II has not yet occurred.

When the outer membrane of the egg is penetrated by a sperm, the secondary oocyte completes meiosis II. At the end of meiosis II, the large haploid egg is fertilized by a haploid sperm to form a diploid zygote. The smaller polar body produced from meiosis II deteriorates and is absorbed.

Unlike spermatogenesis, one primary oocyte produces up to four daughter cells, but only one gamete survives. The remaining three daughter cells become polar bodies.

Comparison of Spermatogenesis and Oogenesis

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Similarities

• Location of the gametogenesis: Both processes occur in the gonads.
• Both reduce the chromosomal number from 2n to n.
  

Differences

• Spermatogenesis is an equal cytokinesis, but oogenesis is an unequal cytokinesis.
• Timing: Spermatogenesis begins at puberty and is a rapid process. Oogenesis begins before birth and the meiosis continues after the onset of puberty. After the egg is fertilized, it proceeds through meiosis II.

Watch and Listen

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Consider this video of both spermatogenesis and oogenesis. Notice how both result in a reduction division that produces gametes needed for fertilization. However, the timing and process of oogenesis is more complex and is characterized by long periods of rest, or waiting.