Spermatogenesis

Now we will relate meiosis to sperm production (fig. 27.15). The first cells destined to become sperm are primordial germ cells. Like the first blood cells, these form in the yolk sac, a membrane associated with the developing embryo. In the fifth to sixth week of development, they migrate into the embryo itself and colonize the gonadal ridges. Here they differentiate into spermatogonia, which lie along the periphery of the seminiferous tubule, outside the blood-testis barrier (BTB).

Spermatogonia multiply by mitosis, producing two types of daughter cells called type A and type B sper-matogonia. Type A cells remain outside the BTB and continue to multiply from puberty until death. Thus men never exhaust their supply of gametes and normally remain fertile throughout old age.

Type B spermatogonia migrate closer to the tubule lumen and differentiate into slightly larger cells called primary spermatocytes. These cells must pass through the BTB and move toward the lumen of the tubule. Ahead of the primary spermatocyte, the tight junction between two sus-tentacular cells is dismantled, while a new tight junction forms on the other side, like closing the door behind the spermatocyte (fig. 27.16). The spermatocyte moves forward toward the lumen and is now separated from blood-borne agents, such as antibodies, held back by the tight junction.

\ Mitosis

spermatogonium Ljg

^Hip^asJh

Type B

spermatogonium

Spermatogonium

Spermiogenesis

Primary spermatocyte

Secondary spermatocytes

Spermatids

Spermiogenesis

Figure 27.15 Spermatogenesis. 2n indicates diploid cells and n indicates haploid cells. Note that the daughter cells from secondary spermatocytes through spermatids remain connected by slender cytoplasmic processes until spermiogenesis is complete and individual spermatozoa are released.

Now safely isolated from the blood, the primary sper-matocyte undergoes meiosis I, which gives rise to two equal-sized, haploid secondary spermatocytes. Each of these undergoes meiosis II, dividing into two spermatids— or a total of four for each spermatogonium. Each stage is a little closer to the lumen of the tubule than the earlier stages. All stages on the lumenal side of the BTB are bound to the sustentacular cells by tight junctions and gap junctions and are closely enveloped in tendrils of the sustentac-ular cells. Throughout these meiotic divisions, the daughter cells do not completely separate, but remain connected to each other by narrow cytoplasmic bridges.

Saladin: Anatomy & I 27. The Male Reproductive I Text I © The McGraw-Hill

Physiology: The Unity of System Companies, 2003 Form and Function, Third Edition

Chapter 27 The Male Reproductive System 1035

Chapter 27 The Male Reproductive System 1035

Cytoplasmic Bridges Spermatogenesis

Figure 27.16 Spermatogenesis in Relation to the Sustentacular Cells.

Why must the primary spermatocyte move through the blood-testis barrier before undergoing meiosis?

Figure 27.16 Spermatogenesis in Relation to the Sustentacular Cells.

Why must the primary spermatocyte move through the blood-testis barrier before undergoing meiosis?

The rest of spermatogenesis is called spermiogenesis (fig. 27.17). It involves no further cell division, but a gradual transformation of each spermatid into a spermatozoon. A spermatid sprouts a flagellum (tail) and discards most of its cytoplasm, becoming as small and lightweight as possible. Eventually, the sperm cell is released and is washed down the tubule by fluid from the sustentacular cells. It takes about 74 days for a spermatogonium to become a mature spermatozoon. A young man produces about 300,000 sperm per minute, or 400 million per day.

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Responses

  • Blaine
    Why spermatogonia and primary spermatocyte are on edge of tubule outside blood testis barrier?
    8 years ago

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