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1. Spermatocytogenesis (also called Mitosis): Stem cells (Type A spermatogonia; singular = spermatogonium) divide mitotically to replace themselves and to produce cells that begin differentiation (Type B spermatogonia).  Spermatogonia have spherical or oval nuclei, and rest on the basement membrane. (You are not responsible for distinguishing between Type A and Type B spermatogonia in lab.) 

Click on any brown labels to view the histological image.

2. Meiosis: Cells in prophase of the first meiotic division are primary spermatocytes.  They are characterized by highly condensed chromosomes giving the nucleus a coarse chromatin pattern and an intermediate position in the seminiferous epithelium. This is a long stage, so many primary spermatocytes can be seen.  Primary spermatocytes go through the first meiotic division and become secondary spermatocytes.  The cells quickly proceed through this stage and complete the second meiotic division. Because this stage is short there are few secondary spermatocytes to be seen in sections. You are not responsible for identifying secondary spermatocytes in lab.  Meiosis is the process by which the diploid number of chromosomes present in spermatogonia (the stem cells) is reduced to the haploid number present in mature spermatozoa.

The products of the second meiotic division are called spermatids. They are spherical cells with interphase nuclei, positioned high in the epithelium.  Since spermatids go through a metamorphosis into spermatozoa, they occur in early through late stages.  You are not responsible for distinguishing the different stages of spermatids, but you are required to identify a spermatid.

All of these progeny cells remain attached to each other by cytoplasmic bridges. The bridges remain until sperm are fully differentiated.

Click on any brown labels to view the histological image.

3. Spermiogenesis: 
This is the metamorphosis of spherical spermatids into elongated spermatozoa. No further mitosis or meiosis occurs.  During spermiogenesis,  the acrosome forms, the flagellar apparatus forms, and most excess cytoplasm (the residual body) is separated and left in the Sertoli cell. Spermatozoa are released into the lumen of the seminiferous tubule. A small amount of excess cytoplasm (the cytoplasmic droplet) is shed later in the epididymis.

Click on any brown labels above to view the histological image(s).

Spermiogenesis:  a process of metamorphosis from a round cell with typical organelles to a highly specialized, elongated cell well adapted for traversing the male and female reproductive tracts and achieving fertilization of an egg.

Transition: Spermatid to spermatozoan
(Click anywhere on the image to see the process animated.)

Sertoli Cell & Developing Sperm Cells: an interaction

The Interaction  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.

Blood-testis barrier. Large molecules cannot pass from the blood into the lumen of a seminiferous tubule due to the presence of tight junctions between adjacent Sertoli cells. The spermatogonia are in the basal compartment (deep to the level of the tight junctions) and the more mature forms such as primary and secondary spermatocytes and spermatids are in the adluminal compartment. The function of the blood-testis barrier (red highlight in diagram above) may be to prevent an auto-immune reaction. Mature sperm (and their antigens) arise long after immune tolerance is established; therefore, a male animal is capable of making antibodies against his own sperm. Injection of sperm antigens causes inflammation of the testis (autoimmune orchitis) and reduced fertility. Thus, the blood-testis barrier may reduce the likelihood that sperm proteins will induce an immune response.