In the spermatid phase, spermatids undergo extensive cell remodeling as they differentiate into mature sperm
Each spermatid that results from the second meiotic division is haploid in DNA content and chromosome number (22 autosomes and an X or Y chromosome). No further division occurs. The haploid spermatids undergo a differentiation process that produces mature sperm, which are also haploid. The normal diploid condition is restored when a sperm fertilizes an oocyte.
The extensive cell remodeling that occurs during differentiation of the spermatid population into mature sperm (spermiogenesis) consists of four phases. These phases occur while the spermatids are physically attached to the Sertoli cell plasma membrane by specialized junctions. The morphologic changes in all four phases that occur during spermiogenesis are described below and summarized in Figure 21.11.
• Golgi phase. This phase is characterized by the presence of periodic acid—Schiff (PAS)-positive granules that accumulate in the multiple Golgi complexes of the spermatid. These proacrosomal granules, rich in glycoproteins, coalesce into a membrane-bounded vesicle, the acrosomal vesicle, adjacent to the nuclear envelope. The vesicle enlarges and its contents increase during this phase. The position of the acrosomal vesicle determines the anterior pole of the developing sperm. Also during this phase, the centrioles migrate from the juxtanuclear region to the posterior pole of the spermatid, where the mature centriole aligns at right angles to the plasma membrane. The centriole initiates the assembly of the
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