GONADS AND THE FORMATION OF GAMETES

Now that you have examined the gross anatomy of the fetal pig reproductive organs, we will focus on the formation of gametes, or sex cells. Most people think of development as a continuous process which begins with fertilization of the mature female gamete or egg and ends with death. The formation of mature male and female gametes from their respective stem cells is called gametogenesis. Spermatogenesis refers to the production of spermatozoa and oogenesis refers to the production of ova.

Histology of the Testes and spermatogenesis

The image to the right is a cross section of an entire testis and contains transverse sections of many seminiferous tubules. Examine the slide first on low power. Though not labeled, you should be able to distinguish the round or slightly oval tubules. Each tubule is bounded by a basement membrane. The tubules are surrounded by connective tissue. In this matrix also are the Leydig cells. They are usually found along with blood vessels due to their endocrine function.

Spermatogenesis occurs in the seminiferous tubules of the testes. You should be able to see all stages of sperm development. The high magnification image was captured using a 40x lens (higher magnification than any other images in this lab). In the high magnification image, you will find the most immature cells, the spermatogonia, forming the cell layers closest to the basement membrane of the tubule (top left in this image). These are the cells that divide mitotically to produce more spermatogonia. Successive steps in the formation of spermatozoa are seen progressing from the periphery or basal end of the tubule toward its lumen. A number of spermatogonia will differentiate (does not involve cell divisions) into cells termed primary spermatocytes, which are double their original size. Each primary spermatocyte now goes through two meiotic divisions to form two secondary spermatocytes and then four spermatids. Each spermatid is haploid. Since the secondary spermatocytes divide rapidly, it is difficult to locate them in sections.

Following the formation of the spermatids, these cells will differentiate into mature spermatozoon or -zoa, the motile cells specialized to carry and introduce into the egg the genetic complement of the father. The process of spermatid differentiation to form spermatozoa is termed spermiogenesis. Mature spermatozoa can be seen in clusters with their tails in the tubule lumen and their heads directed toward an elongate Sertoli cell. Sertoli cells are sessile, individual columnar cells that function in the regulation of the spermatogenic cycle.