testicles, testicular cancer, penis

Where Do Sperm Cells Mature?

Your body produces sperm in a system of tubes called seminiferous tubules located in your testicles. These cells start out as simple round cells and eventually transform into sperm with tails and other specializations that help them effectively fertilize an egg cell.

This remarkable development is triggered by hormones such as testosterone and luteinising hormone. This process is known as spermatogenesis and takes around 74 days to reach maturation.

Seminiferous tubules

The seminiferous tubules are coiled tubes found inside the testes. They provide support and nourishment to the sperm cells (male gametes). A transverse section of the seminiferous tubule reveals that it is packed with a mix of specialized epithelial cells, including sustentacular columnar Sertoli cells and proliferating and differentiating germ cells that give rise to spermatozoa. The progression from an undifferentiated spermatogonium to a spermatid takes place within the adluminal compartment of the tubule. It is during this process, called spermatogenesis, that the 23 chromosomes of the sperm cell are duplicated in two cellular divisions through meiosis.

The adluminal compartment of the seminiferous tubules is lined by Sertoli cells. These tall simple columnar cells act as a supporting and nourishing structure for the proliferating and differentiating germ cell, helping them to adhere to the basement membrane, and secreting the male sex hormone testosterone. During the spermiogenesis process, these cells associate with the proliferating and differentiating spermatocytes and spermatids through cadherin-like molecules. They form pockets around these cells and provide nutrients and phagocytose excess spermatid cytoplasm not needed in the formation of sperm.

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Interstitial or Leydig cells are also located in the connective tissue surrounding the seminiferous tubules. These cells produce testosterone, which is essential for the proliferation of the germinal epithelium. The rete testis connects the seminiferous tubules to the ductus deferens, which leads to the rest of the male genital tract. The ductus deferens is another muscular tubule that stores the sperm until they reach the epididymis, where they acquire motility through a pseudostratified epithelium containing stereocilia.

Epididymis

Sperm cells move from the testes to the epididymis, which is a long, coiled tube that stores immature sperm until they are ready to leave the body. When the penis is stimulated with sexual arousal, muscle contractions propel sperm into the lower portion of the epididymis, known as the tail. Once the sperm reach the tail, they are released from the epididymis into the vas deferens and then the ejaculatory duct. Hundreds of millions of sperm are released during each ejaculation. The sperm enter the prostate gland, where they are mixed with milky fluid to form semen. The sperm then travels to the urethra.

In a process called spermatogenesis, the Sertoli cells in the seminiferous tubules produce sperm. Diploid primary germ cells undergo meiosis, a cell division that halves the number of chromosomes, to become haploid secondary spermatocytes. The haploid secondary spermatocytes then develop tails and other specializations to become mature sperm cells, or spermatozoa, which can fertilize an egg cell.

In addition to its tail, a mature sperm cell has several other structures that help it reach and penetrate an egg. These include mitochondria, which provide energy for sperm to swim, and the head of the sperm cell, which is covered partially by a structure called the acrosome. A mature sperm cell also releases enzymes that help it dissolve the egg’s outer layer and penetrate its nucleus.

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Egg

The final destination for sperm cells is the egg. The egg is a sessile organic vessel that contains the fertilized gamete cell that gives rise to a new organism. All sexually reproducing life forms, including both plants and animals, produce two types of gametes: the male gamete cells (sperm) and the female gamete cells (ovum). Both male and female gametes combine to form a single fertilized cell known as an embryo.

Immature sperm cells, called spermatogonia, are found in the outer wall of the seminiferous tubules. These immature cells undergo a series of cell divisions and other changes, called spermatogenesis, which result in the formation of primary sperm cells. Each primary sperm cell has 46 chromosomes. Half of these chromosomes are used to fertilize the egg, while the other half remain in the primary sperm cell as a source of future sperm cells.

In most animals, the head of a sperm cell has a specialized secretory vesicle that encapsulates hydrolytic enzymes. When the sperm cell approaches an egg, these enzymes are released by a process of exocytosis. This action, called the acrosome reaction, changes the surface of the egg. It also exposes proteins that help bind the sperm to the egg.

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In humans, the acrosome reaction takes about 20 hours to complete. Once the acrosome reaction is complete, the sperm cell becomes motile and is ready for fertilization. The sperm cell is propelled by flagella, which are tiny microtubule-like structures located in the sperm’s tail.

Spermiation

During this final stage of the spermatogenesis cycle, known as spermiation, mature spermatids are released into the tubule lumen. This happens after a series of steps that include remodeling of the head and tail, removing specialized adhesion structures, spermatid disengagement from Sertoli cells, and finally sperm transportation into the epididymis. These changes are mediated by hormones (especially androgens).

Histologically, this process is known as spermatidogenesis or spermiation. It starts when a somatic cell called a 1o spermatocyte divides to give rise to two short-lived secondary spermatocytes, each of which produces a single spermatid. The spermatids have a haploid number of chromosomes.

At this point, the spermatids begin to develop polarity. At one end of the spermatid, the Golgi apparatus develops enzymes that will gather inside the future acrosome (cap phase). Meanwhile, at the other end of the spermatid, the mid-piece develops and thickens as mitochondria fill this area. Spermatid DNA also undergoes packaging, becoming highly condensed and transcriptionally inactive.

In addition, the distal centriole develops a thickening at its anterior end. This becomes the axoneme, a structure that forms part of the sperm’s tail. At the same time, the spermatid’s tail grows longer and develops a temporary structure called a manchette. Excess cytoplasm is shed as a residual body and is phagocytized by the Sertoli cells that surround the sperm.