Gametogenesis, the production of sperm and eggs, takes place through the process of meiosis.
During meiosis, two cell divisions separate the paired chromosomes in the nucleus. They then separate the chromatids that were made during an earlier stage of the cell’s life cycle, resulting in gametes that each contain half the number of chromosomes as the parent. The production of eggs and sperm are called oogenesis and spermatogenesis, respectively.
Oogenesis occurs in the outermost layers of the ovaries. As with sperm production, oogenesis starts with a germ cell, called an oogonium (plural: oogonia). Still, this cell undergoes mitosis to increase in number, eventually resulting in up to one to two million cells in the embryo.
The cell starting meiosis is called a primary oocyte. This cell will begin the first meiotic division, but be arrested in its progress in the prophase I stage. At the time of birth, all future eggs are in the prophase stage. At adolescence, anterior pituitary hormones cause the development of several follicles in an ovary. This results in the primary oocyte finishing the first meiotic division. The cell divides unequally, with most of the cellular material and organelles going to one cell, called a secondary oocyte, and only one set of chromosomes and a small amount of cytoplasm going to the other cell. This second cell is called a polar body and usually dies. A secondary meiotic arrest occurs, this time at the metaphase II stage. At ovulation, this secondary oocyte will be released and travel toward the uterus through the oviduct. If the secondary oocyte is fertilized, the cell continues through the meiosis II, completing meiosis, producing a second polar body and a fertilized egg containing all 46 chromosomes of a human being, half of them coming from the sperm.
Oogenesis. A primary oocyte begins the first meiotic division but then arrests until later in life when it will finish this division in a developing follicle. This results in a secondary oocyte, which will complete meiosis if it is fertilized.
Spermatogenesis occurs in the wall of the seminiferous tubules. Immediately under the capsule of the tubule are diploid, undifferentiated cells called spermatogonia (singular: spermatagonium), go through mitosis with one offspring growing into a sperm cell and the other progressing to the next generation of sperm.
Meiosis begins with a cell called a primary spermatocyte. At the end of the first meiotic division, a haploid cell called a secondary spermatocyte is produced. This haploid cell must go through another meiotic cell division. When the spermatid (cell produced at the end of meiosis) reaches the lumen of the tubule and grows a flagellum (or “tail”), it is called a sperm cell. Four sperms result from each primary spermatocyte that goes through meiosis.
Spermatogenesis. During spermatogenesis, four sperms result from each primary spermatocyte, which divides into two haploid secondary spermatocytes; these cells will go through a second meiotic division to produce four spermatids.
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Key Points
• Gametogenesis, the production of sperm (spermatogenesis) and eggs (oogenesis), takes place through the process of meiosis.
• In oogenesis, diploid oogonium goes through mitosis until one develops into a primary oocyte, which will begin the first meiotic division, but then arrest; it will finish this division as it grows in the follicle, giving rise to a haploid secondary oocyte and a smaller polar body.
• The secondary oocyte begins the second meiotic division and then arrests again; it will not finish this division unless a sperm fertilizes it, and then produces a mature ovum and another polar body.
• In spermatogenesis, diploid spermatogonia go through mitosis until they begin to develop into gametes; eventually, one develops into a primary spermatocyte that will go through the first meiotic division to form two haploid secondary spermatocytes.
• The secondary spermatocytes will go through a second meiotic division to each produce two spermatids; these cells will eventually develop flagella and become mature sperm.
Key Terms
meiosis: cell division of a diploid cell into four haploid cells, which develop to produce gametes
mitosis: the division of a cell nucleus in which the genome is copied and separated into two identical halves. It is normally followed by cell division
polar body: one of the small cells that are by-products of the meiosis that forms an egg
oocyte: a cell that develops into an egg or ovum; a female gametocyte
spermatocyte: a male gametocyte, from which a spermatozoon develops
oogenesis: growth process in which the primary egg cell (or ovum) becomes a mature ovum
oogonium: an immature female reproductive cell that gives rise to primary oocytes by mitosis
prophase: the first phase of mitosis, the process that separates the duplicated genetic material carried in the nucleus of a parent cell into two identical daughter cells
metaphase: chromosomes line up at the metaphase plate
ovulation: the release of an egg during menstruation in females
diploid: a full set of chromosomes, 46 in humans
spermatogenesis: the process by which haploid spermatozoa develop from germ cells
haploid: half the number of chromosomes