Anatomy & Physiology

Anatomy & Physiology Study Guide

25.2 Spermatogenesis The process of spermatogenesis produces spermatozoa. Spermatogenesis begins at the outermost layer of cells in the seminiferous tubules and proceeds toward the lumen. First, stem cells called spermatogonia, divide by mitosis to produce two daughter cells, one of which remains at that location as a spermatogonium , while the other differentiates into a primary spermatocyte . Primary spermatocytes are the cells that begin meiosis, a specialized form of cell division involved only in the production of gametes (spermatozoa in males, ova in females). Primary spermatocytes give rise to secondary spermatocytes that divide and differentiate into spermatids –immature gametes that subsequently differentiate into spermatozoa. Each seminiferous tubule contains spermatogonia , which are spermatocytes at various stages of meiosis. Spermatogenesis involves three integrated processes: • Step 1 Mitosis: Throughout adult male life, spermatogonia undergo cell divisions. One daughter cell from each division is pushed toward the lumen of the seminiferous tubule and the other remains in place. The displaced cells differentiate into primary spermatocytes, which prepare to begin meiosis. • Step 2 Meiosis: Meiosis is a special form of cell division involved in gamete production. In humans, gametes contain half the normal set, 23 chromosomes. As a result, the fusion of the nuclei of a female gamete and a male gamete produces a cell that has the normal number of chromosomes (46), rather than twice that number. In the seminiferous tubules, meiotic divisions that begin with primary spermatocytes produce the undifferentiated gametes, spermatids. • Step 3 Spermiogenesis: Spermatids are small, relatively unspecialized cells. In spermiogenesis, physically mature spermatozoa develop from spermatids. The mature spermatozoa are the most highly specialized cells in the body. Spermiogenesis involves major changes in a spermatid's internal and external structures. Mitosis and Meiosis Somatic cells contain 23 pairs of chromosomes. Each pair consists of one chromosome provided by the father and another provided by the mother, at the time of fertilization. Mitosis is part of the process of somatic cell division, producing two daughter cells, each containing identical pairs of chromosomes. Since daughter cells contain bothmembers of each chromosome pair (for a total of 46 chromosomes), they are called diploid cells . Meiosis involves two cycles of cell division (meiosis I and meiosis II) and produces four cells, each of which contains 23 individual chromosomes. These cells are called haploid cells because they contain only one member of each pair of chromosomes. As a cell prepares to begin meiosis, DNA replication occurs within the nucleus just as it does in a cell preparing to undergo mitosis. This similarity continues as prophase I arrives; the chromosomes condense and become visible with a light microscope. As in mitosis, each chromosome consists of two duplicate chromatids. Four spermatids are produced from each primary spermatocyte that enters meiosis.

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