spermatid

Meaning: In spermatogenesis, a diploid spermatogonium divides by mitosis to give a primary spermatocyte (2n). Meiosis I produces two haploid secondary spermatocytes (n, each chromosome still made of two sister chromatids). Meiosis II separates the sister chromatids and produces four haploid spermatids (n, each chromosome a single chromatid). So one primary spermatocyte gives four spermatids, and all four become functional sperm.

Spermiogenesis is the maturation of a spermatid into a sperm cell. It involves no division. The cell forms a flagellum (tail) with a mitochondria-rich midpiece for ATP, and it condenses its nucleus. The Golgi forms the acrosome, a cap of digestive enzymes used to penetrate the egg. Most cytoplasm is shed. Sertoli cells in the seminiferous tubules support and nourish these developing cells. The sperm then matures and is stored in the epididymis.

On the MCAT: Questions ask you to order the stages (spermatogonium → primary spermatocyte → secondary spermatocyte → spermatid → sperm), give the ploidy or chromosome number at each stage, or compare with oogenesis. Expect a figure of a seminiferous tubule or a passage about a defect, such as missing mitochondria or acrosome, and ask you to predict the effect on motility or fertilization.

Common confusion: Students call the spermatid a mature sperm cell. It is not. It is haploid but still immature, and it needs spermiogenesis. Also, meiosis is finished at the spermatid stage. Unlike oogenesis, which gives one egg and polar bodies, spermatogenesis gives four equal-fate gametes.