Spermatogenesis

Spermatogenesis is the process by which stem cells develop into mature spermatozoon (sperm) cells. It is perhaps one of the most important and delicate processes that occur in the body, and is essential for sexual reproduction. This process is occurs in four stages:

Multiplication
Growth
Maturation or Meiosis
Differentiation

What's the point?

Since sexual reproduction involves the union of two sex cells, or gametes, then this will also involve a union of chromosomal material. To preserve the number of chromosomes of the offspring, which is characteristic to each species (e.g., normal humans have 46 chromosomes), each gamete must have half the usual number of chromosomes present in other cells. Otherwise, the offspring will have twice the normal number of chromosomes.

In males, spermatogenesis solves this problem by allowing the production of sperm cells with n chromosomes from stem cells with 2n chromosomes. Oogenesis, on the other hand, does the same to create an ovum, the female sex cell.

Where and When?

Spermatogenesis takes place inside a male’s testes, specifically in the walls of the seminiferous tubules. It starts at puberty and usually continues uninterrupted till death, although a slight decrease can be discerned in the quantity of produced sperm with increase in age.

Stages: from Stem Cell to Sperm Cell

Multiplication

Each stem cell divides by mitosis to produce two daughter cells with 2n chromosomes each. One daughter cell, known as a Type A spermatogonium, does not participate in spermatogenesis and is needed to ensure that stem cells never run out of supply, which are needed in large quantities since the average male produces trillions of sperm cells throughout his lifetime. The other daughter cell, known as Type B spermatogonium, is the one which actually enters into spermatogenesis.

The Type B spermatogonium initiates another mitotic division that results in two daughter cells, each equipped with 2n chromosomes. The daughter cells of spermatogonium move away from the basal lamina.

Growth

The two produced daughter cells initially have chromosomes with just one chromatid. These cells enter into a period of growth, known as interphase, which allows the cells to replenish their chromosomal material. The end result is two cells with double-chromatid chromosomes. These cells are known as primary spermatocytes.

Maturation or Meiosis

The primary spermatocytes enter into meiosis, which is a double division. The first division, meiosis I, is called the reductional division. It produces two daughter cells each with n chromosomes (half the starting number, 2n). These daughter cells are dubbed secondary spermatocytes. It is worth noting that the chromosomes of these cells have two chromatids.

The second division, meiosis II, is called the equational division, and is nearly identical to mitosis. However, it is very important to note that there is no period of "intermission," or interphase, between the meiosis I and meiosis II. The previously obtained daughter cells each produce two new daughter cells, which brings the total of daughter cells (or granddaughter cells, to be exact) to four. The newly formed daughter cells have n chromosomes but with one chromatid. They are called spermatids.

In short, what was accomplished during meiosis was first, dividing the chromosomes in half, then second, breaking up the double-chromatid chromosomes into single-chromatid chromosomes. All of these progeny cells remain attached to each other by cytoplasmic bridges. The bridges remain until sperm are fully differentiated.

Differentiation or Spermiogenesis

Once all the previous divisions are done, and spermatids are formed, differentiation starts. Differentiation is essentially the process of transforming spermatids into bona fide, mature sperm cells, with all the features necessary for the task they’re designed to do—most important of all, motility.

Differentiation is also known as spermiogenesis, not to be confused with spermatogenesis. During differentiation, the spermatid is slowly molded into an elongated shape, and large portions of the cytoplasm (the residual cytoplasm) are shed off. Most notably, the spermatid develops an acrosome, produced by the Golgi apparatus, and a flagellum that looks roughly like a long tail and is responsible for motility. The flagellum is produced by the centrioles present in the spermatid. The nucleus is squeezed into an elongated shape and forms with the acrosome the head of the sperm, the acrosome occupying the topmost part of the head. The mitochondria which act as power plants for cells are arranged in the middle piece of the sperm along with the centriole.

Once differentiation is complete, the sperm cell separates from the Sertoli cell to which it was bound during spermatogenesis and migrates into the lumen of the seminiferous tubules.

Duration

The overall process takes 74 days; the maturation phase, two weeks; and the differentiation phases, about three weeks.

Sertoli Cell & Developing Sperm Cells

At all stages of differentiation, the spermatogenic cells are in close contact with Sertoli cells which are thought to provide structural and metabolic support to the developing sperm cells. A single Sertoli cell extends from the basement membrane to the lumen of the seminiferous tubule although its cytoplasm is difficult to distinguish at the light microscopic level. They are characterized by the presence of a vesicular, oval, basally positioned nucleus which contains a prominent nucleolus. The nuclear envelope often contains a definite fold. The significance of the very close association of the two types of cells is unknown. Sertoli cells are endocrine cells - they secrete the polypeptide hormone, inhibin. Inhibin acts at the level of the pituitary to reduce the secretion of follicle stimulating hormone.

Conditions and Regulators

Testosterone

Temperature

Developmental biology | Animal physiology | Reproductive system

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