Spermatid

The spermatid is represented by a haploid male gamete that is produced by secondary spermatocyte division. Each spermatid possesses just half of the genetic material contained in the original primary spermatocyte as a result of meiosis. Cytoplasmic material connects spermatids, and there is excess cytoplasmic material around their nuclei. Early round spermatids must go through further maturational events in order to mature into spermatozoa, a process known as spermiogenesis. The spermatids begin to build a live thread, form an acrosome, and create a thicker mid-piece where the mitochondria become localised. Spermatid DNA is also packaged, resulting in a highly compressed form. During spermatid elongation, the DNA is first bundled with certain basic nuclear proteins, which are then replaced by protamines. The tightly packed chromatin that results is transcriptionally inactive.

Sperm

In anisogamous forms of sexual reproduction, sperm represents the male reproductive cell, or gamete (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce spermatozoa, these are motile sperm having a flagellum tail, whereas red algae and fungi result in the formation of spermatia, that are non-motile sperm cells. Pollen from flowering plants contains non-motile sperm, but pollen from more basal plants such as ferns and gymnosperms contains motile sperm. 

Sperm cells are formed during spermatogenesis, which occurs in the seminiferous tubules of the testes in amniotes (mammals and reptiles). Starting with spermatogonia, this process produces a series of sperm cell precursors that eventually differentiate into spermatocytes. The spermatocytes next undergoes meiosis, this reduces the number of chromosomes in half and leads to the formation of spermatids. After this the spermatids mature and form a tail, or flagellum, in animals, which gives rise to the motile and mature sperm cell. This entire process occurs on a continuous basis and takes about three months from start to end.

Structure of Sperm

The sperm cell of a mammalian is basically divided into two regions:

• Head: includes the nucleus, which has densely coiled chromatin fibres, and is bordered anteriorly by the acrosome, a narrow, flattened sac that contains enzymes for entering the female egg. It also has vacuoles in it.

• Tail: The longest component, also known as the flagellum, is capable of a wave-like action that drives sperm for swimming and aids in egg entry. Previously, it was believed that the tail usually moved in a helical pattern.

One conventional centriole and one atypical centriole, like those of the proximal centriole, are generally found in the neck or connecting component. The midpiece possesses a central filamentous core that has many mitochondria spiralled around it, which is used to produce ATP along the passage via the female cervix, uterus, and uterine tubes.

The sperm supplies three critical components to the oocyte during fertilisation: (1) a signalling or activation agent that induces the metabolically inactive oocyte to activate; (2) the haploid paternal genome; and (3) the centriole, which forms the centrosome and microtubule system.

Spermatogenesis

The biological process of formation of sperm cells is known as spermatogenesis. It happens in a sexually reproducing organism’s male gonad. Through a series of processes, undifferentiated male germ cells evolve into spermatozoa these processes are as follows (1) spermatocytogenesis, (2) spermatidogenesis, and (3) spermiogenesis and spermiation are the stages. These stages take place in the testis’ seminiferous tubules. Spermiation, on the other hand, will continue in the epididymis, where mature spermatozoa released into the lumen are driven to the epididymis and stored. 

When spermatozoa enter the female reproductive canal in animals, they acquire full motility (hence, maturity). Spermatogenesis in humans usually begins at puberty and lasts for the rest of one’s life. The complete procedure might take up to 64 days.

Steps of Spermatogenesis

Spermatocytogenesis

The initial stage of spermatogenesis is spermatocytogenesis. The spermatogonia in the convoluted seminiferous tubule’s basal lamina divide repeatedly through mitosis, creating identical spermatogonia. Others enter the first meiotic division as primary spermatocytes in the adluminal compartment of the convoluted seminiferous tubule. Diploid spermatogonia and main spermatocytes are both present.

Spermatidogenesis

The next stage after spermatocytogenesis is spermatidogenesis. Thus, it represents the intermediate stage of spermatogenesis. The importance of this stage is meiosis, a type of cell division consisting mainly of two succeeding divisions: first meiotic division (meiosis I) and second meiotic division (meiosis II). In humans and other mammals, the seminiferous tubule is the only site in a male body where meiosis takes place. At this stage, the primary spermatocyte along with its duplicated DNA enters meiosis I in order to give rise to two haploid secondary spermatocytes. Each of the secondary spermatocytes will immediately enter meiosis II in order to form four genetically non-identical, haploid spermatids.

Spermiogenesis and Spermiation

Spermatidogenesis is followed by spermiogenesis. The spermatids mature into mature spermatozoa during this stage of spermatogenesis. Spermiogenesis is divided into four stages:

• Phase of the Golgi

• Phase of the cap

• Phase of the tail

• Phase of maturation

Nuclear condensation, acrosome formation, mid-piece formation, and tail creation will all occur in the spermatids. The spermatozoa, on the other hand, are not fully functioning at this stage. They are not yet motile, despite having acquired traits necessary for fertilising an ovum (e.g., becoming compressed and streamlined rather than the initial spherical shape with leftover cytoplasm). 

Non-motile spermatozoa leave the seminiferous tubules after spermiogenesis and proceed to the rete testis in the mediastinum testis, the efferent ducts, and eventually the epididymis. Spermiation is the process of spermatozoa migrating to the epididymis. Because the spermatozoa aren’t yet motile, the Sertoli cells produce testicular fluid to help them along their journey. In the epididymis, the spermatozoa will become more motile. Full motility is obtained in animals by capacitation, which occurs when spermatozoa enter the female reproductive canal.

Conclusion

This mechanism appears to be quite sensitive, and even minor changes in hormone levels can have a significant impact. The Leydig Cells, the hypothalamus and pituitary gland, for example, are all involved in testosterone production. Changes in temperature, a lack of food, drunkenness, drug exposure, and the presence of disease can all have a negative impact on the pace of sperm generation. After hypophysectomy, spermatogonia continues to divide, although in less quantities. In hypophysectomized animals, spermatocytogenesis is entirely halted at the primary spermatocyte stage; testosterone restores this process. Within the seminiferous tubule, androgen-binding protein (the testicular equivalent of SHBG) sequesters testosterone (and caput epididymis). Meiosis II is not affected by hormones. Spermiogenesis is aided by follicle-stimulating hormones. The spermatogenic process is additionally aided by estradiol and DHT.

Hormonal impacts on sperm cells are mediated through Sertoli cells rather than being direct. In spermatogenesis, the biochemical and biophysical aspects of sperm-Sertoli interactions remain mainly unexplored. Endocrine medication has no effect on the rate of spermatozoa production.