Wk4 Ch17.6: male reproductive- spermatogenesis

• (singular, spermatogonium)
• The undifferentiated germ cells, called spermatogonia (singular, spermatogonium), begin to divide mitotically at puberty.

• 
• Figure 17.10
• Summary of spermatogenesis, which begins at puberty.
• Each spermatogonium yields, by mitosis, a clone of spermatogonia; for simplicity, the figure shows only two such cycles, with a third mitotic cycle generating two primary spermatocytes.
• The arrow from one of the spermatogonia back to a stem cell spermatogonium denotes the fact that one cell of the clone does not go on to generate primary spermatocytes but reverts to an undifferentiated spermatogonium that gives rise to a new clone.
• Each primary spermatocyte produces four spermatozoa.
• The entire process of spermatogenesis, from primary spermatocyte to sperm, takes approximately 64 days.
• The typical human male manufactures approximately 30 million sperm per day.

• The undifferentiated germ cells, called spermatogonia (singular, spermatogonium), begin to divide mitotically at puberty.
• The daughter cells of this first division then divide again and again for a specified number of division cycles so that a clone of spermatogonia is produced from each stem cell spermatogonium.
• Some differentiation occurs in addition to cell division.
• The cells that result from the final mitotic division and differentiation in the series are called primary spermatocytes, and these are the cells that will undergo the first meiotic division of spermatogenesis.

• It should be emphasized that if all the cells in the clone produced by each stem cell spermatogonium followed this pathway, the spermatogonia would disappear—that is, they would all be converted to primary spermatocytes.
• This does not occur because, at an early point, one of the cells of each clone “drops out” of the mitosis– differentiation cycle to remain a stem cell spermatogonium that will later enter into its own full sequence of divisions.
• One cell of the clone it produces will do likewise, and so on. Therefore, the supply of undifferentiated spermatogonia is maintained.

• The cells that result from the final mitotic division and differentiation in the series are called primary spermatocytes, and these are the cells that will undergo the first meiotic division of spermatogenesis.
• Each primary spermatocyte increases markedly in size and undergoes the first meiotic division (see Figure 17.10) to form two secondary spermatocytes, each of which contains 23 two-chromatid chromosomes.

• Each primary spermatocyte ungerdoes the first meiotic division to form two secondary spermatocytes, each of which contains 23 two-chromatid chromosomes.
• Each secondary spermatocyte undergoes the second meiotic division to form
• spermatids.

• Each secondary spermatocyte undergoes the second meiotic division to form spermatids.
• In this way, each primary spermatocyte, containing 46 two-chromatid chromosomes, produces four spermatids, each containing 23 one-chromatid chromosomes.

• The final phase of spermatogenesis is the differentiation of the spermatids into spermatozoa (sperm).
• This process involves extensive cell remodeling, including elongation, but no further cell divisions.
• The head of a sperm cell (Figure 17.11) consists almost entirely of the nucleus, which contains the genetic information (DNA).
• The tip of the nucleus is covered by the acrosome, a proteinfilled vesicle containing several enzymes that are important in fertilization.
• Most of the tail is a flagellum—a group of contractile filaments that produce whiplike movements capable of propelling the sperm at a velocity of 1 to 4 mm per min.
• Mitochondria form the midpiece of the sperm and provide the energy for movement.
• 

The tip of the nucleus (of a sperm) is covered by the acrosome, a protein-filled vesicle containing several enzymes that are important in fertilization.

Most of the tail is a flagellum—a group of contractile filaments that produce whiplike movements capable of propelling the sperm at a velocity of 1 to 4 mm per min

• Each seminiferous tubule is bounded by a basement membrane.
• In the center of each tubule is a fluid-filled lumen containing the mature sperm cells, called spermatozoa.
• The tubular wall is composed of developing germ cells and their supporting cells, called Sertoli cells (also known as sustentacular cells).
• Each Sertoli cell extends from the basement membrane all the way to the lumen in the center of the tubule and is joined to adjacent Sertoli cells by means of tight junctions (Figure 17.12).
• Thus, the Sertoli cells form an unbroken ring around the outer circumference of the seminiferous tubule.
• The tight junctions divide the tubule into two compartments—a basal compartment, between the basement membrane and the tight junctions, and a central compartment (or adluminal compartment), beginning at the tight junctions and including the lumen.
• 

• The ring of interconnected Sertoli cells forms the Sertoli cell barrier (blood–testes barrier), which prevents the movement of many chemicals from the blood into the lumen of the seminiferous tubule and helps retain luminal fluid.
• This ensures proper conditions for germ cell development and differentiation
• in the tubules.
• The arrangement of Sertoli cells also permits different stages of spermatogenesis to take place in different compartments and, therefore, in different environments.

• The Leydig cells, or interstitial cells, which lie in small, connective-tissue spaces between the tubules, synthesize and release testosterone.
• Therefore, the sperm-producing and testosterone-producing functions of the testes are carried out by different structures—the seminiferous tubules and Leydig cells, respectively.

• As shown in Figure 17.12, spermatogenesis is ultimately controlled by the gonadotropins that stimulate local testosterone secretion from Leydig cells and increase the activity of Sertoli cells.
• Mitotic cell divisions and differentiation of spermatogonia to yield primary spermatocytes take place entirely in the basal compartment.
• The primary spermatocytes then move through the tight junctions of the Sertoli cells (which open in front of them while at the same time forming new tight junctions behind them) to gain entry into the central compartment.
• In this central compartment, the meiotic divisions of spermatogenesis occur, and the spermatids differentiate into sperm while contained in recesses formed by invaginations of the Sertoli cell plasma membranes.
• When sperm formation is complete, the cytoplasm of the Sertoli cell around the sperm retracts and the sperm are released into the lumen to be bathed by the luminal fluid.

• Sertoli cells serve as the route by which nutrients reach developing germ cells, and they also secrete most of the fluid found in the tubule lumen.
• This fluid contains androgen-binding protein (ABP), which binds the testosterone secreted by the Leydig cells and crosses the Sertoli cell barrier to enter the tubule.
• This protein maintains a high concentration of total testosterone in the lumen of the tubule.
• The dissociation of free testosterone from ABP continuously exposes the developing spermatocytes and Sertoli cells to testosterone.

• In response to FSH from the anterior pituitary gland and to local testosterone produced in the Leydig cell, Sertoli cells secrete a variety of chemical messengers.
• These function as paracrine agents to stimulate proliferation and differentiation of the germ cells.
• In addition, the Sertoli cells secrete the protein hormone inhibin, which acts as a negative feedback controller of FSH, and paracrine agents that affect Leydig cell function.
• The many functions of Sertoli cells, several of which remain to be described later in this chapter, are summarized in Table 17.2.