The series of changes resulting in the transformation of spermatids into spermatozoa is spermiogenesis. These changes include (a) formation of the acrosome, which covers half of the nuclear surface and contains enzymes to assist in penetration of the egg and its surrounding layers during fertilization (Fig. 1.25); (b) condensation of the nucleus; (c) formation of neck, middle piece, and tail; and (d) shedding of most of the cytoplasm. In humans, the time required for a spermatogonium to develop into a mature spermatozoon is approximately 64 days.
When fully formed, spermatozoa enter the lumen of seminiferous tubules. From there, they are pushed toward the epididymis by contractile elements in the wall of the seminiferous tubules. Although initially only slightly motile, spermatozoa obtain full motility in the epididymis.
Figure 1.24 Sertoli cells and maturing spermatocytes. Spermatogonia, spermatocytes, and early spermatids occupy depressions in basal aspects of the cell; late spermatids are in deep recesses near the apex.
In humans and in most mammals, one ovarian follicle occasionally contains two or three clearly distinguishable primary oocytes (Fig. 1.26A). Although these oocytes may give rise to twins or triplets, they usually degenerate before reaching maturity. In rare cases, one primary oocyte contains two or even three nuclei (Fig. 1.26B). Such binucleated or trinucleated oocytes die before reaching maturity.
In contrast to atypical oocytes, abnormal spermatozoa are seen frequently, and up to 10% of all spermatozoa have observable defects. The head or the tail may be abnormal; spermatozoa may be giants or dwarfs; and sometimes they are joined (Fig. 1.26C). Sperm with morphologic abnormalities lack normal motility and probably do not fertilize oocytes.
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