sperm

Meaning: A sperm is the mature male gamete, a haploid (n) cell that delivers the paternal genome to the egg. It forms by spermatogenesis in the seminiferous tubules: a primary spermatocyte (2n) completes meiosis I to give two secondary spermatocytes (n), and meiosis II gives four spermatids that differentiate into sperm. Sertoli cells nourish the developing cells. The head holds the nucleus and an acrosome of enzymes that help penetrate the egg's outer layers; the midpiece packs mitochondria for ATP; the flagellum provides motility. Sperm mature and are stored in the epididymis.

On the MCAT: Expect questions on where each stage occurs, the chromosome number at each step, or which structure explains a defect (few mitochondria or a faulty flagellum lowers motility). Know the path: seminiferous tubules, epididymis, vas deferens, ejaculatory duct, urethra. Fusion with the egg triggers changes that help block polyspermy.

Common confusion: Sperm are not semen. Semen is the fluid that carries sperm plus secretions of the seminal vesicles, prostate and bulbourethral glands. Also, a secondary spermatocyte is haploid in chromosome number, though each chromosome still has two chromatids. One primary spermatocyte yields four sperm, whereas oogenesis yields one functional ovum.