Spermatids and Spermiogenesis

Spermiogenesis is the long phase of morphogenesis during which the haploid sper-matids become highly motile cells ultimately capable of fertilizing ova. The major morphological changes occurring during this phase of spermatogenesis are the formation of the head of the spermatozoon, formation of the tail, and loss of cytoplasm (13). The head of the mature spermatozoon in higher primates consists of the acrosome, a specialized lysosome containing enzymes needed at the time of fertilization, and the nucleus, which contains highly condensed chromatin. Several distinct steps in the formation of the acro-some during spermiogenesis, which are revealed by periodic acid-Schiff's staining of the putative organelle, are used to identify the temporal sequence of spermiogenesis.

The newly formed spermatid is distinguished from other germ cells by the presence of a small spherical nucleus in the cell cytoplasm. The Golgi region of the newly formed spermatid is pink under PAS-Schiff's staining and lies near the nuclear surface, producing a polarity in the spermatid. Spermatids with this morphology are identified as Step 1. Step 2 spermatids have two or more small red granules, usually between the Golgi region and the nucleus. Coalescence of the granules into a single large acrosomic granule indicates Step 3. Step 4 spermatids have a single red granule in a vesicle that contacts the nucleus of the cell. The formation of Step 4 spermatids marks the end of the first, or Golgi, phase of spermiogenesis. The vesicle and granule of the putative acrosome begin to spread along the nuclear surface. The next four steps of spermiogenesis, Steps 5, 6, 7, and 8, are distinguished by the extent of the cap-like formation, and these steps constitute the cap phase of acrosome formation. The next four steps of spermiogenesis, Steps 9-12, are characterized by formation of the final shape of the acrosome, condensation of the nuclear chromatin, and further growth of the spermatid tail. These steps form the acrosomic phase of spermiogenesis. The final phase of spermiogenesis, maturation phase, comprises Steps 13 and 14 in nonhuman primates. By Step 13, the spermatids have achieved the morphology of the spermatozoa, and little morphological change occurs during these steps, except for a further reduction of cytoplasm. Step 14 sper-matids are often called testicular spermatozoa, and, with completion of this last step of spermiogenesis, the spermatozoa are released from the seminiferous epithelium.

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