spermatozoon

A spermatozoon, commonly called a sperm cell, is a male gamete in animals. It is a haploid cell specialized for fertilization, carrying paternal genetic material and contributing to the zygote after fusion with an oocyte. The term is singular; the plural is spermatozoa.

In most species, spermatozoa are produced by spermatogenesis in the testes. Germ cells undergo mitotic divisions and two successive meiotic divisions to form haploid spermatids, which differentiate into mature spermatozoa. Spermiation releases them into the seminiferous tubule lumen; they gain motility and structural maturation as they transit through the epididymis.

Anatomy: A typical spermatozoon has a compact nucleus capped by an acrosome, a slender cytoplasm-reducing head, a midpiece densely packed with mitochondria, and a flagellum (tail) that powers movement. The nucleus carries paternal DNA, and the acrosome contains enzymes that help penetrate the oocyte’s surrounding layers during fertilization.

Function and fertilization: After ejaculation, spermatozoa are deposited in the female reproductive tract and must become capable of fertilization through a process called capacitation. The acrosome reaction enables enzymatic penetration of the zona pellucida, followed by fusion of the sperm and oocyte membranes to deliver the paternal genome. In humans, paternal mitochondria are typically not inherited.

Size and numbers (humans): Mature human sperm are about 50–60 micrometers long, with heads ~5 micrometers long. An ejaculation contains millions of sperm, but only a small fraction reach the oocyte, and fertilization success depends on multiple factors, including motility and acrosomal integrity. Lifespan in the female tract is limited to days at most.