The cells derived from the first meiotic division are called secondary spermatocytes. These cells immediately enter the prophase of the second meiotic division without synthesizing new DNA (i.e., without passing through an S phase; see pages 89–93). Each secondary spermatocyte has a reduced number of chromosomes to (1n), which is represented by 22 autosomes and an X or a Y chromosome. Each of these chromosomes consists of two sister chromatids. The secondary spermatocyte has the (2d), diploid amount of DNA. During metaphase of the second meiotic division, the chromosomes line up at the metaphase plate, and the sister chromatids separate and move to opposite poles of the spindle. As the second meiotic division is completed and the nuclear membranes re-form, two haploid spermatids, each containing 23 single-stranded chromosomes (1n) and the (1d) amount of DNA, are formed from each secondary spermatocyte (Fig. 22.10).
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 (1d) and chromosome number (1n) represented by 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 here and summarized in Figure 22.11.
 Golgi phase. This phase is characterized by the presence of in the multiple Golgi complexes of the spermatid. These two pairs of homologous chromosomes G1 phase G2 phase *note: prophase II, anaphase II, and telophase II are not shown ovum
FIGURE 22.10 • Comparison of mitosis and meiosis in a spermatogonial cell. The two pairs of chromosomes (2n) of maternal and paternal origin are depicted in red and blue, respectively. The mitotic division produces daughter cells that are genetically identical to the parental (2n) cell. The meiotic division, which has two components, a reductional division and an equatorial division, produces a cell that has only half the number of chromosomes (n). In addition, during the chromosome pairing in prophase I of meiosis, chromosome segments are exchanged, crossing-over, creating genetic diversity. In humans, the first polar body does not divide, but it does so in other species. *Note that prophase II, anaphase II, and telophase II are not shown.