Spermatogenesis

Spermatogenesis occurs in the seminiferous tubules of the testis; beginning at puberty and continuing throughout life.

Type A spermatogonia undergo mitosis. While some of the daughter cells remain as type A spermatogonia, serving as a reservoir of stem cells, others differentiate into type B spermatogonia. Mitotic division of type B spermatogonia produces primary spermatocytes.

In meiosis I, primary spermatocytes, that are diploid, replicate the entire compliment of 46 chromosomes to make 46 double stranded chromosomes. Upon completion of meiosis I, they produce 2 secondary spermatocytes, that are haploid, each possessing 23 double stranded chromosomes.

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Fig 1. Summary of the stages in spermatogenesis.

Secondary spermatocytes rapidly undergo meiosis II without further replication of DNA to produce 2 spermatids, each of which possesses a single set of 23 chromosomes, and so are haploid.

In genetic males, the sex chromosomes consists of one X chromosome derived from the individual's female parent and one Y chromosome derived from the individual's male parent.
At the end of meiosis, half of the spermatids carry an X chromosome and half carry a Y chromosome.

Spermatids undergo a radical morphogenesis to prepare them for the task of finding and fertilizing an ovum. This transformation is called spermiogenesis.

Stages in Spermatogenesis

The four panels making up the slide show below review the process of spermatogenesis using iron-hematoxylin stained images of the seminiferous tubules to illustrate the stages. They are value added for those who are interested in matching the histologic appearance of each stage with the chromosomal profile but not crucial to understanding of the overall process of spermatogenesis at this time.

You can advance the slide by a mouse click or using the arrows at the right or left side of the panel.

  • Start Slide Show

    Spermatogonia

    Spermatogonia are diploid, containing 46 total chromosomes, or 23 pairs. Of these, 22 pairs are autosomes and one pair of sex chromosomes.

    Type A spermatogonia     are stem cells that replenish the cell line by mitosis, producing new Type A spermatogonia as well as Type B spermatogonia.

    It is Type B spermatogonia that undergo mitosis to form primary spermatocytes.


    Sertoli cells, provide protection and sustenance to the developing spermatocytes.

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    Fig 2a. Section of seminiferous tubule showing type A and type B spermatogonia in the base of the tubule.

  • Primary Spermatocytes

    Primary spermatocytes begin meiosis I by replication of the cell's DNA. Each of the 46 chromosomes then consists of two strands of DNA called chromatids.

    Completion of the first meiotic division results in two     secondary spermatocytes, each containing 23 double stranded chromosomes.

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    Fig 2b. Section of seminiferous tubule showing primary spermatocytes in progression of spermatogenesis.

  • Secondary Spermatocytes

    Secondary spermatocytes are haploid because they possess only one set of chromosomes, even though each chromosome is double stranded.

    Each     secondary spermatocyte rapidly undergoes meiosis II to produce two spermatids and so are not readily detected in histologic sections of the seminiferous tubules.

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    Fig 2c. Diagram showing spermatogenesis and the chromosomal composition of secondary spermatocytes as they enter meiosis II.

  • Spermatids

    Completion of meiosis II by division of the secondary spermatocytes forms spermatids, each with a single set of 23 chromosomes (haploid) and 1N DNA.

    Spermatids must undergo transformation to a form more suitable for navigation of the female reproductive tract in search of an ovum in order for fertilization to take place.

    This     process of morphological change is called spermiogenesis.

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    Fig 2d. Section of seminiferous tubule showing spermatids in various stages of spermiogenesis.

Spermiogenesis

Spermatids undergo radical 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.

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Fig 3. The diagram shows several stages in the process of spermatogenesis and the morphologic changes that transform the round spermatid to a spermatozoan.

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Fig 4. Animated gif showing the transformation of a spermatid to a spermatozoan. This morphologic transformation is necessary for sperm to navigate the female reproductive tact and effect fertilization.

During spermiogenesis, the spermatid morphs into a shape that is suitable for navigation through the uterus and uterine tube.

The cell nucleus moves into one side of the spermatid to form the head of the spermatozoan. The acrosome, which covers the sperm head, 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 power to whip the flagellum in the tail and propelling the spermatozoan through the female reproductive tract.

The cell loses most of its cytoplasm and must depend on secretions in the semen and mucosa of the female reproductive tract for sustenance on its journey.

Clinical Correlates

Abnormal Gametes
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Fig 5. Several morphologic alterations and malformations of sperm cells that negatively affect their viability.

Up to 10% of spermatozoa have notable defects, such as more than one flagellum, multiple heads, oversized heads or microscopic heads. Deformed spermatozoa generally have poor motility and are considered not to be viable contenders for fertilization.