Spermatogenesis

Unlike oogenesis, spermatogenesis occurs at puberty. Spermatogenesis occurs in the seminiferous tubules of the testes, and takes between 65 and 75 days to occur. However about 300 million sperm complete spermatogenesis each day.

Spermatogenesis begins with spermatogonia (spermatogonium s.)  found beneath the basement membrane of the seminiferous tubules. The spermatogonia are sperm cells which are undifferentiated and undergo mitosis. The cells which remain near the basement membrane remain as a reservoir of undifferentiated cells, whereas those which loose contact squeeze through a blood-testis barrier and differentiate into primary spermatocytes.

Primary spermatocytes undergo meiosis I to become secondary spermatocytes with the haploid number of chromasomes. Secondary spermatocytes then undergo meiosis II forming spermatids. These have the right amount of genetic material but have not developed fully into sperm cells yet.

During the process of spermatogenesis, the daughter cells from meiosis do not undergo full cytokenesis such that they are joined by cytoplasmic bridges through out their development.

After meiosis, the spermatids undergo a stage called spermiogenesis where each spermatid becomes a single sperm cell. Aided by sertoli cells, the spermatids develop an acrosome, the nucleus condenses and elongates, the flagellum develops, mitochondria multiply, and sertoli cells dispose excess cytoplasm. Finally, the sperm cells are released from the sertoli cells through a process known as spermiation. The sperm are released into the lumen of the seminiferous tubule where fluids push them to the ducts of the testes. At this point sperm are unable to swim.

Spermatogonium (undifferentiated stem cells)

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mitosis – some are pushed away from basement membrane and begin spermatogenesis.

Spermatogonium

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DNA replication and Diffrentiation occurs

Primary spermatocyte

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Meiosis I- 2 chromosomes split + crossing over.

Secondary spermatocyte

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Meosis II – sister chromatids split

Spermatid

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Spermiogenesis

Sperm cells

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Spermiation

Structure of Sperm

Sperm is about 60 mcm long, and is made up of the head and the tail. The head is about 4-5 mcm long with a highly condensed nucleus and an acrosome covering its anterior two thirds. The tail is subdivided 4 parts, the neck, the middle piece, the principle piece and the end piece. The neck refers to region behind the head which contains the centrioles, where the microtubules of the tail are formed. The middle piece contains mitochondria arranged in a spiral and the principle piece is the main portion of the tail. The end piece a thinning portion of the tail.

Hormonal control of spermatogenesis

1. at puberty the hypothalamic neurosecretory cells increase their secretion of gonadotrophin-releasing hormone (GnRH).
2. This stimulates the gonadotrophs (specialised cells in the anterior pituitary gland) to release more Luteinising hormone (LH) and follicle stimulating hormone (FSH)
3. LH causes Leydig cells located between the seminiferous tubules to release testosterone.
4. FSH and testosterone synergistically causes the secretion of androgen binding protein (ABP) from sertoli cells into the interstitial fluid and lumen of the seminiferous tubules
5. ABP binds to testosterone imaintaining its concentration
6. testosterone stimulates the final stages of spermatogenesis.
7. When the level of spermatogenesis is high enough, sertoli cells release inhibin which inhibits FSH secretion.
8. High levels of testosterone inhibits the release of LH by the anterior pituary gland and GnRH by the hypothalamus.

Some target cells convert testosterone to dihydro-testosterone via the enzyme 5 alpha reductase. Both of these androgens attach to the same receptors affecting gene expression. This causes several changes. This includes the development of the male reproductive system before birth, the development of male sexual characteristics at puberty, including secondary characteristics, development of sexual features and stimulates anabolism.