The spermatozoon has two parts: a pear-shaped head and a long tail (fig. 27.18). The head, about 4 to 5 ^m long and 3 ^m wide at its broadest part, contains three structures: a nucleus, acrosome, and flagellar basal body. The most important of these is the nucleus, which fills most of the head and contains a haploid set of condensed, genetically inactive chromosomes. The acrosome31 is a lysosome in the form of a thin cap covering the apical half of the nucleus. It contains enzymes that are later used to penetrate the egg if the sperm is successful. The basal body of the tail flagellum is nestled in an indentation at the basal end of the nucleus.
The tail is divided into three regions called the mid-piece, principal piece, and endpiece. The midpiece, a cylinder about 5 to 9 ^m long and half as wide as the head, is the thickest part. It contains numerous large mitochondria that spiral tightly around the axoneme of the flagellum. They produce the ATP needed for the beating of the tail when the sperm migrates up the female reproductive tract. The principal piece, 40 to 45 ^m long, constitutes most of the tail and consists of the axoneme surrounded by a sheath of fibers. The endpiece, 4 to 5 ^m long, consists of the axoneme only and is the narrowest part of the sperm.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.