Spermatogenesis

Spermatogenesis is the process by which spermatozoa are produced from spermatogonial stem cells by way of mitosis and meiosis. The initial cells in this pathway are called spermatogonia, which yield primary spermatocytes by mitosis. The primary spermatocyte divides meiotically (Meiosis I) into two secondary spermatocytes; each secondary spermatocyte divides into two spermatids by Meiosis II. These develop into mature spermatozoa, also known as sperm cells. Thus, the primary spermatocyte gives rise to two cells, the secondary spermatocytes, and the two secondary spermatocytes by their subdivision produce four spermatozoa.

Spermatogenesis is highly dependent upon optimal conditions for the process to occur correctly, and is essential for sexual reproduction. DNA methylation and histone modification have been implicated in the regulation of this process. It occurs in several stages that take place in the seminiferous tubules of the testes, and takes approximately 64 days.

Phases of spermatogenesis

Spermatogenesis is broken up into several stages, each corresponding to a specific series of cellular events. The entire process can be broken up into several distinct stages, each corresponding to a specific phase of development.

Spermatocytogenesis

During spermatocytogenesis, a diploid spermatogonium, which resides in the basal compartment of seminiferous tubules, divides mitotically to produce two diploid intermediate cells called primary spermatocytes. Each primary spermatocyte then moves into the adluminal compartment of the seminiferous tubules and duplicates its DNA and subsequently undergoes meiosis I to produce two haploid secondary spermatocytes.

Spermatidogenesis

Spermatidogenesis is the creation of spermatids from secondary spermatocytes during meiosis II. Secondary spermatocytes produced earlier rapidly enter meiosis II and divide to produce haploid spermatids. The brevity of this stage means that secondary spermatocytes are rarely seen in histological preparations.

Spermiogenesis

During spermiogenesis, the spermatids begin to form a tail, or flagellum, and develop a thickened midpiece, where the mitochondria gather and form an axoneme. Spermatid DNA also undergoes packaging, becoming highly condensed. The DNA is packaged with specific nuclear basic proteins, which are subsequently replaced with protamines during spermatid elongation. The resultant spermatozoon is now mature but lacks the ability to move, which it will gain during the process of spermiation.

Regulation

Spermatogenesis is regulated by the endocrine system. In particular, luteinizing hormone (LH) and follicle stimulating hormone (FSH) play key roles in this process.

See also

• Oogenesis
• Gametogenesis
• Seminiferous tubule
• Sertoli cell
• Leydig cell

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