Page 792 - Atlas of Histology with Functional Correlations

FORMATION OF SPERM:

SPERMATOGENESIS

The process of sperm formation is called spermatogenesis. This process
includes mitotic divisions of spermatogenic cells located at the base of the
seminiferous tubules. Spermatogenic cells are subdivided into type A

spermatogonia and type B spermatogonia. Dark type A spermatogonia are stem
cells that continue to divide and give rise to other dark and pale type A
spermatogonia. Pale type A spermatogonia replicate themselves and give rise to
type B cells. Type B cells proliferate by mitosis and give rise to primary
spermatocytes, which undergo the first meiotic division to produce secondary

spermatocytes. The secondary spermatocytes complete the second meiotic
division right away and produce round spermatids. During these meiotic
divisions, there is a reduction in the number of chromosomes and the amount of

DNA in each cell. After the completion of the second meiotic division, the
spermatids now contain 23 single chromosomes (22 + X or 22 + Y). Spermatids
do not continue to divide but instead undergo spermiogenesis, an extensive and
complex morphological transformation of a round cell into an elongated sperm
with a nucleus and a motile tail (flagellum). Upon fertilization of the egg by the

sperm, the total normal number of chromosomes is restored to 46.

Once the spermatogenic cells in the germinal epithelium start to differentiate
and begin to mature, they are held together by thin intercellular or cytoplasmic
bridges during further development and differentiation. These intercellular

bridges are broken when the developed spermatids are released (spermiation)
into the fluid-filled seminiferous tubules as fully formed sperm from the luminal
regions of the supportive Sertoli cells.

TRANSFORMATION OF SPERMATIDS:

SPERMIOGENESIS

During spermiogenesis, the size and shape of the spherical spermatids are
altered, the nuclear chromatin condenses, and the heads become elongated (Fig.

20.2). In the initial Golgi phase, small granules accumulate in the Golgi
apparatus of the spermatids and form an acrosomal granule within a membrane-

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