A sperm cell, or spermatozoon, is the male reproductive cell and the counterpart of the egg cell, or ovum, in females. Produced in the testicles, it is distinct from the body's other cells in that it is self-propelling and appears only in sexually mature animals. As is usually the case in nature, its function is to fertilize an egg cell to create what ultimately becomes a living being.
A sperm cell resembles a tiny tadpole and has three parts -- the head, neck and tail; the last is often described as divided into a middle piece and an end piece. A mature human sperm cell is about 60 micrometers long. In humans, these cells are produced at a fantastic rate -- 2,000 new sperm per second between the two testes. This results in a density of 20 million or more sperm cells per milliliter of semen in normal adult males. The head stores the sperm cell's important information in a nucleus, while the tail provide propulsion.
The head of a spermatozoon, which is flattened and measures about 5 micrometers long by 3 micrometers at its widest point where it meets the neck, has two main features -- the acrosome and the nucleus. The acrosome, at the apical or forward half of the head, is a de facto chemical battering ram. It is a membrane-bound sac containing enzymes that dissolve the exterior membrane of the egg cell, thereby aiding the sperm in penetrating the "shell" of the egg so that the genetic material in the sperm's nucleus reaches the interior of the ovum. The nucleus takes up most of the volume of the sperm cell's head, and, unlike the nuclei of other cells in the male body, is haploid, meaning that in humans it contains 23 single chromosomes instead of 23 paired chromosomes.
The neck, or middle portion, of a sperm cell is very short -- about 1 micrometer -- but essential to what happens both in front of and behind it. The neck is connected to the head via a membranous structure called the basal plate. It contains the spermatozoon's two centrioles, which are necessary for cell division. In "successful" sperm, the proximal (anterior) centriole participates in the division of the newly fertilized zygote; the distal (posterior) centriole serves as an anchor point for the flagellum of the tail section of the sperm cell.
The tail of a spermatozoon, which accounts for about 90 percent of the cell's length, consists of two parts -- a middle piece and an end piece. The middle piece, which is rich in energy-producing mitochondria, is the biological engine that drives the propulsive flagellum. An axial filament extends from the neck and throughout the tail. This axial filament is composed of nine filamentous microtubules surrounding a core of two central microtubules; this "9 + 2" structure is typical of flagella. This bundle of filamentous microtubules is also called the axoneme. The end piece is the longer portion of the tail. The entire tail moves the sperm through liquid media using lashing, or whip-like, movements.
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