Spermatozoa

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Histology of the testis.

The term spermatozoa (singular: spermatozoon), also known as sperm, refers to the male sex cells or gametes. They are formed through the process of spermatogenesis, which begins at puberty with the proliferation progenitor (stem) cells called spermatogonia. This process takes place within the seminiferous tubules of the testes, supported by sustentocytes or nurse cells, previously referred to as Sertoli cells, a popular eponym.

Diploid spermatogonia, each containing 46 chromosomes (23 pairs), undergo meiosis to form male gametes called spermatids, each of which have 23 chromosomes. The spermatids then undergo a maturation phase known as spermiogenesis, resulting in the production of haploid spermatozoa. The spermatozoa undergo final maturation in the epididymis, where they acquire motility. On average, the entire process of spermatogenesis takes approximately 70 days.

The mature spermatozoon measures approximately 65 μm in length and is structurally designed for rapid and efficient motility. It is composed of three main parts: head, neck and tail.

The head has a distinctive flattened pear shape and houses a large elongated nucleus with scanty cytoplasm. This nucleus contains condensed, dark-staining chromatin a varying number of regions of dispersed chromatin known as nuclear vacuoles. Covering the anterior two-thirds of the nucleus is the acrosomal cap, a specialized membrane-bound vesicle. This vesicle contains several hydrolytic enzymes and is crucial in oocyte fertilization, as it helps in dispersing the corona radiata cells and breaking down the zona pellucida.

The neck of the spermatozoon is a short segment measuring about 0.3 μm in length. It connects the head with the tail and usually retains a significant amount of cytoplasm. It contains the remnants of centrioles, one of which forms the axoneme of the tail that is essential for the propulsion of the sperm.

The tail of the spermatozoon (flagellum) is subdivided into three regions: the middle (mid) piece, principal piece and end piece.

• The middle piece is a long cylinder roughly the same length as the head (4-7 μm). It consists of the axoneme, an axial bundle of microtubules, encased by a cylinder of nine longitudinally arranged dense outer microtubules. Surrounding this core structure is a tightly packed helical sheath of elongated mitochondria which supplies the energy required for tail movement. Thus, the middle piece is sometimes referred to as the 'powerhouse' of the spermatozoon. Beneath the plasma membrane, a dense fibrous layer known as the annulus serves as a barrier, preventing the mitochondria from sliding into the principal piece.
• The principal piece, as its name suggests, makes up most of the length of the tail. It measures about 40 μm and is primarily responsible for motility. It is composed of a central axoneme core, surrounded by and the nine
  coarse fibres that extend from the middle piece to form the axonemal complex. 
• The end piece forms a short narrowing segment of the tail, approxmately 5–7 μm and contains only the axoneme.

Spermatozoa along with the secretions of the accessory reproductive glands constitute the semen.