Spermatogenesis (Spermatocytogenesis and Spermiogenesis) in Male Animals

Spermatogenesis (spermatocytogenesis and spermiogenesis) is a complex process of cell division and differentiation resulting in the formation of spermatozoa.

Spermatozoa are formed in the seminiferous tubules of the testes by a series of cell divisions followed by a metamorphosis produces a highly differentiated and potentially motile cell, the spermatozoa.

Spermatogenesis can be divided into two phases:

1. Spermatocytogenesis
2. Spermiogenesis

Spermatocytogenesis is the proliferative stage in which primitive germ cells are multiplied by a series of miotic divisions followed by the meiotic divisions which produce the haploid stage.

Spermiogenesis is the differentiative phase in which the nucleus and cytoplasm undergo morphologic changes to form the germ cells. Meatmorphosis of spermatid into spermatozoa is called as spermiogenesis.

1. Spermatocytogenesis

• Spermatocytogenesis begins with the mitotic division of spermatogonia on the basement membrane of seminiferous tubules and proceed towards the lumen.
• Spermatogonia are activated to form active type A spermatogonia. There may be several generations of type A spermatogonia.
• Most of type A spermatogonia divide to form intermediate spermatogonia.
• Some of the type A are retained as resting type A spermatogonia. In this way, the type A cells provide daughter cells for the formation of spermatozoa but are not depleted in the process.
• Intermediate spermatogonia divide to form type B spermatogonia, which undergo the last of the miotic divisions to form primary spermatocytes. Spermatocytogenesis is concluded by the miotic divisions, which produce secondary spermatocytes, then spermatids.
• The formation of spermatids marks the end of spermatocytogenesis and the beginning of spermiogenesis.
• The entire divisional process of spermatocytogenesis, from spermatogonia to spermatid takes approximately 45 days.

2. Spermiogenesis

Spermiogenesis consists of 3 major changes:

1. Condensation of nuclear chromatin
2. Formation of tail / flagellum
3. Formation of acrosomal cap / membrane

Spermiogenesis consists of four phases namely:

1. Golgi phase
2. Cap phase
3. Acrosomal phase
4. Maturation phase

Golgi Phase

• The newly formed spermatid (A) has a well-developed golgi apparatus.
• Small vesicles of the golgi fuse, giving rise to larger secretory granules called proacrosomic granules (B).
• Vesicle fusion continues until a large acrosomic vesicle is formed that contains a dense acrosomic granule (C).
• During the last half of the Golgi phase the centrioles migrate to a position opposite to the acrosomic vesicle.
• Proximal centriole (PC) gives rise to the attachment point for the tail.
• Dostal centriole (DC) will give rise to the developing axoneme inside the cytoplasm of the spermatid.
• The axoneme is the central portion of a flagellum.

Cap Phase

• The acrosomic vesicle flattens and begins to form a distinct cap (B) consisting of an outer acrosomal membrane (OAM) and inner acrosomal membrane (IAM).
• The golgi (A) migrates towards the caudal part of the cell (B).
• Dostal centriole (DC) form the axoneme (AX) or flagellum which projects away from the nucleus toward the lumen of the seminiferous tubule.

Acrosomal Phase

• Spermatid nucleus begins to elongate (A).
• Acrosome covers the majority of the anterior nucleus (B).
• A unique system of micotubules known as manchette extends from posterior nucleus poetions of manchette attach to the region of the nucleus just posterior to the acrosome.
• Some of the microtubules of the manchette will become the post nuclear cap.
• During the acrosome phase, spermatid become deeply embedded in sertoli cells with their tails protruding towards the lumen of the seminiferous tubule.

Maturation Phase

• Manchtte migrate towards the tail and begin to disappear.
• Mitochondria are assembled around flagellum to form the middle piece.
• Dense outer fibres form around the flagellum.
• Postnuclear cap is formed from manchette microtubules.
• Annulus forms the juncture between the middle piece and principal piece.
• During later stages of spemiogenesis, sertoli cells forms the cytoplasm remaining after elongation of spermatid into a spheroidal lobule called residual body.
• Lobule of cytoplasm is connected to the elongated spermatid by a slender thread cytoplasm and is inter connected with other residual bodies by intercellular bridges.
• Formation of residual bodies completes final maturation and spermatids are ready for release.
• Sertoli cells also remove degenerating germ cells.

Spermiation

• Release of formed germ cells into lumen of the seminiferous tubules is known as spermiation.
• Elongated spermatids are gradually extruded into the lumen of the tubule.
• Extrusion continues until only a slender stalk of cytoplasm connectes the neck of the spermaid to the residual body.
• Breakage of the stalk results in formation of cytoplasmic droplet in the neck region of the released spermatozoa (proximal droplet) and retention of interconnected residual bodies Residual body are disposed by sertoli cells.

Duration of Spermatogenesis

Entire spermatogenesis takes about 50-60 days in boars, 60-70 days in bull and ram.

Spermatogenesis comprised of four to five cycles of seminiferous epithelium. Cycle of seminiferous epithelium is defined as series of stages / changes between two similar cellular associations in the given area of seminiferous tubule.

The time necessary to complete a cycle of the seminiferous epithelium is called duration of spermatogenesis and varies among domestic animals.

Duration of the cycle in different species:

• 9 days: boar
• 10 days: Ram
• 12 days: horse
• 14 days: bull

Spermatogenic Wave

The distance between two similar cellular associations in the seminiferous epithelium is called spermatogenic wave.