Animal Science 434 - 3/12/98
I. The Site of Spermatogenesis and the Functional Cell Types Participating (Fig. 1).
Spermatogenesis is the process of producing sperm with half the number of chromosomes (hapliod) as somatic cells. The process of spermatogenesis then allows the recombination of male and female haploid gametes at fertilization. This provides genetic contributions from both parents without increasing the number of chromosomes each generation. Spermatogenesis occurs in medullary sex cords which are called seminiferous tubules (totaling about 3 miles in length in the bull).
1) The germ cells progress first from the diploid to haploid state and then change shape to become spermatozoa.
2) The Sertoli cells which may also be called "sustentacular cells or nurse cells" provide:
II. Spermatogenesis in the Sexually Mature Male
A. The gametogenic function of the testes is to produce the male gametes or spermatozoa. This process is termed, spermatogenesis. The sites of spermatozoa production are the seminiferous tubules. The spermatozoa originate from precursor cells that are called spermatogonia, and these cells line the basement membrane of the seminiferous tubule. Spermatogenesis can be divided into three portions:
Discussion of spermatogenesis will be based on the adult male mammal that is a continuous breeder. The stages are outlined in Figure 2.
B. Spermatocytogenesis and Meiosis
This phase begins with the division of the spermatogonia, that line the seminiferous tubule, near the basement membrane. Spermatogonia originate at puberty by the proliferation of the gonocytes and are the descendants of the primordial germ cells. One or two divisions of spermatogonia occur to maintain their population in a stem cell pool. Of the cells resulting from these mitotic divisions, some spermatogonia stay in the "resting" pool, while the remaining type A spermatogonia proliferate several times and undergo 1 to 5 stages of division and differentiation. After the last division, the resulting cells are termed primary spermatocytes and this ends spermatocytogenesis. The primary spermatocytes then undergo the first of the two division that constitute meiosis. The first meiotic division produces two secondary spermatocytes. Division of the secondary spermatocytes completes meiosis and produces the spermatids (Figure 2).
2. The "resting" or stem cell spermatogonia remain dormant for a time and then join a new proliferation of spermatogonia. Since this new wave of spermatogonial divisions does not wait for the previous generation of cells to complete spermatogenesis, the result are an overlapping of generations in any one area of the seminiferous tubules. The purpose of this phenomena is to ensure a residual population of spermatogonia, without which the testis would exhaust its ability to produce sperm. The time required for one spermatogonium to divide and form spermatozoa requires about 4.5 to 5 times that time span between divisions of the stem cell spermatogonial.
This part of spermatogenesis is defined as the nuclear and cytoplasmic changes in the spermatid that results in the spermatozoa. Some aspects of the restructuring of the cell are:
The process of spermiogenesis ends in the testis with release of the spermatozoa from the Sertoli cell. The spermatozoa has been embedded up until now in the Sertoli cell. The process by which spermatozoa are shed into the lumen of the seminiferous tubule for transport out of the testis is spermiation. The overall results of spermatogenesis is:
D. Cycles of the Seminiferous Epithelium (Figure 3)
1. In the simplest sense, a spermatogenic cycle represents the time required for one cell to go through all the stages shown in Figure 2. However, you have already seen that an overlapping of generations occurs in the germinal epithelium of the testes. Since the spermatogenic cycle takes a set length of time (duration) and each step occupies a certain length of that time, a finite number of cellular associations results. In other words, specific groupings of cells in different steps of spermatogenesis will occur together.
2. The series of changes between two successive appearances of the same cellular associations constitutes one cycle of the seminiferous epithelium. One cellular association is a stage of the seminiferous cycle. Stages of the cycle may be recognized by:
The cycle is useful in quantitating the effects of drugs, hormones, nutritional factors, etc., on the different steps of spermatogenesis. Keep in mind that the number of cellular associations in a cycle of the epithelium will differ depending on the species, the number of spermatogonial divisions in that species and the number of distinguishable stages of spermatid development in that species.
3. The Wave of the Seminiferous Epithelium (Figure 4): The seminiferous tubules are a series of loops in the testis with each end of a loop entering the rete testis. There is an orderly progression along the tubule of stages. This progression of stages is called a wave and ensures that a portion of the seminiferous tubule is releasing sperm at any specific time.
III. Sertoli Cells (Figure 5)
Sertoli cells are large and because of their shape and tight junctions functionally divide the seminiferous tubules into two compartments or environments for the development of spermatozoa. The basal compartment below the tight junctions has contact with the circulatory system and is the space in which spermatogonia develop to primary spermatocytes. The tight junctions open at specific times and allow progression of spermatocytes to the adluminal compartment. It is in the adluminal compartment where meiosis is completed and spermatid development is sustained by the Sertoli cells. At the appropriate time, the cytoplasm of the spermatid is actively pinched off by the Sertoli cells and spermatozoa are shed into the lumen of the tubules. The Sertoli cell serves several functions among which are:
IV. Hormonal Control of Spermatogenesis
There are three hormones produced by the testis that directly or indirectly effect spermatogenesis. These hormones are testosterone, estradiol and inhibin. The Leydig cells produce testosterone and are located adjacent to seminiferous tubules. The Sertoli cells are located within the seminiferous tubule and produce both estradiol and inhibin.
The hormonal mechanisms controlling spermatogenesis are not completely understood, but the following facts are known: The development of spermatogenesis at puberty depends on the hypothalamus, pituitary gland and functional Leydig cells (to produce testosterone) in the testes. In the absence of the pituitary gland, spermatogenesis can be initiated by FSH and testosterone. FSH is necessary to develop the androgen binding protein production by Sertoli cells and to develop the blood-testis barrier and other functions of these cells. Once the Sertoli function is developed, testosterone alone will maintain spermatogenesis. The yield of spermatozoa, however, is increased if FSH is present. FSH is known to increase the yield of spermatogonia by preventing the atresia of differentiating A type spermatogonia. Normally 50% of A spermatogonia degenerate. The loss of A type seprmatogonia can also be reduced by increased sexual activity. FSH levels in males are environmentally influenced, are increased by sexual activity and decreased by inhibin.
Androgens are transported from the site of production (Leydig cells) to influence the developing germ cells. Androgen binding protein produced by the Sertoli cell and shed into the adluminal compartment assists in this role as well as transporting large amounts of androgen to the caput epididymis. Synthesis of androgen binding protein by the Sertoli cell is dependent on FSH stimulation but only after the Sertoli cell has been under androgen influence.
V. Spermatogenic Rate and Prevention of Spermatogenesis
Spermatogenic cycles can be stopped and the yield of spermatozoa can be influenced but the time required for a spermatogenic cycle cannot be altered and is characteristic for genetic groups of species or subspecies. This time constant for each species, agents blocking spermatogenesis and factors influencing ejaculatory yield will be considered in a later lecture.
VI. Development of Seminiferous Tubules
The functional units of the testes in spermatogenesis are the seminiferous tubules. In the fetus, seminiferous tubules develop from the medullary sex cords of the differentiating gonad (about 6 weeks in humans and by the 4th to 5th week in farm animals). They contain multiplying embryonic germinal cells called gonocytes; descendants of the primordial germ cells which migrated to the undifferentiated gonads. These gonocytes are embedded in the supporting cells of the seminiferous tubules. The supporting cells are developed from the mesonephric mesenchyme. They multiply in number during fetal and adolescent life and at sexual maturity are directly transformed under pituitary hormone (FSH) influence to functional Sertoli cells.
In most species, the gonocytes are transformed to Type A spermatogonia
during the first month after birth. In the early postnatal period (first
2 weeks), Type A spermatogonia proliferate rapidly by mitotic division but
do not differentiate to more advanced spermatogonial stages until the second
month in farm animals. It is after this time that all stages of spermatogonial
division are evident. Primary spermatocytes develop between the second
and third month and mature sperm are formed by 4-6 months in the boar and
ram, and 6 months in the bull. The development of spermatogenesis is hormonally
dependent on LH and FSH from the pituitary gland. LH functions to initiate
androgen production while FSH develops the Sertoli cells initially and then
controls the production of spermatogonia.