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Spermatogenesis

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This process occurs in the seminiferous tubules of the testis. It begins at puberty and continues throughout life.

Type A spermatogonia, which are derived from the primordial germ cells that migrated from the wall of the yolk sac, undergo mitosis. Some of the daughter cells remain as type A spermatogonia to serve as a reservoir of stem cells while others differentiate into type B spermatogonia, which contain the diploid number of chromosomes (46), but which will undergo meiosis to produce sperm cells which contain the haploid number of chromosomes (23).

One
Type B spermatogonium will produce 2 primary spermatocytes. Note that the primary spermatocytes are the result of a mitotic division and are therefore diploid.

At the end of the first meiotic division, each
primary spermatocyte will produce 2 secondary spermatocytes. During meiosis I, the chromosomal DNA is replicated and each chromosome pair consists of four strands of DNA called chromatids. At the end of the first meiotic division, each secondary spermatocyte contains 23 chromosomes. Each of these chromosomes consists of paired chromatids.

Each secondary spermatocyte completes the second meiotic division without the replication of DNA and produces 2 spermatids each containing 23 chromosomes.
Spermatids undergo morphologic alteration (spermiogenesis) to become mature spermatozoa.

These changes include:
  • formation of the acrosome
  • condensation of the nucleus
  • formation of a neck, middle piece and tail
  • shedding of most of the cytoplasm.
Read about the relationship between the developing spermatocytes and Sertoli cell in the textbook

Stages in Spermatogenesis

  • Spermatogonia

    Type A spermatogonia serve as a stem cell population in the testis. They replenish the cell line (Type B spermatogonia) which undergo meiosis to form the spermatozoa. Note that the spermatogonium pictured is diploid, containing 46 total chromosomes, or 23 pairs. Of these, 22 pairs are autosomes and one pair is a pair of sex chromosomes. Since the individual is male, the pair of sex chromosomes consists of one X chromosme derived from the individual's mother and one Y chromosome derived from the individual's father.

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  • Primary Spermatocytes

    Note that the primary spermatocyte, which is a daughter of the spermatogonium contains the diploid number of chromosomes (46). Of these 46 chromosomes, two are sex chromosomes (XY), which determine the gender of the individual. During prophase of the first meiotic division the pairs of chromosmes align with each other. Each chromosome consists of two strands of DNA. These strands are called chromatids. At the end of the first meiotic division, half of the chromosomes, each consisting of two chromatids, are distributed to each daughter cell.

  • Secondary Spermatocytes

    At the end of the second meiotic division, each secondary spermatocyte produces two spermatids. Each spermatid receives 23 chromatids from the parent cell. In the spermatids, the chromatids represent single strands of DNA and become the chromosomes of the gametes. Subsequently, each spermatid undergoes a radical morphological alteration to become a spermatozoon. This process of morphological change is called spermiogenesis. Note that half of the spermatids will development into spermatozoa carrying an X chromosome and half will develop into spermatozoa carrying a Y chromosome

  • Spermatids & Spermiogenesis

    During spermiogenesis, the morphology of the spermatid radically changes. The cell develops the acrosome, which covers the sperm head and contains enzymes that assist in penetrating the zona pellucida during fertilization. The cell becomes elongated forming a neck, connecting the head to the middle piece and tail. The middle piece contains mitochondria for generating energy to propel the cell through the female reproductive tract by action of the flagellum in the tail. The cell loses most of its cytoplasm and must depend on secretions in the semen for sustenance on its journey through the female reproductive tract.

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