ANATOMY OF A SPERM
The mature spermatozoon is an elaborate, specialized cell produced in massive quantity, up to 1200 per second. Spermatogenesis begins when Type B spermatogonia divide mitotically to produce diploid primary spermatocytes (2n), which then duplicate their DNA during interphase. After a meiotic division, each daughter cell contains one partner of the homologous chromosome pair, and they are called secondary spermatocytes (2n). These cells rapidly enter a second meiotic division in which the chromatids then separate at the centromere to yield haploid early-round spermatids (n). Thus, theoretically, each primary spermatocyte yields four spermatids, although fewer actually result, as the complexity of meiosis is associated with germ cell loss.
The process by which spermatids become mature spermatozoa within the
Sertoli cell takes several weeks and consists of several events: the acrosome
is formed from the Golgi apparatus; a flagellum is constructed from the
centriole; mitochondria reorganize around the midpiece; the nucleus is
compacted to about 10% of its former size; and residual cell cytoplasm is
eliminated. With completion of spermatid elongation, the Sertoli cell cytoplasm
retracts around the developing sperm, stripping it of unnecessary cytoplasm and
extruding the sperm into the tubule lumen. The mature sperm has remarkably
little cytoplasm.
The human spermatozoon is approximately 60 µm in length and is divided
into three anatomic sections: head, neck, and tail. The oval sperm head, about
4.5 µm long and 3 m wide, consists of a nucleus containing highly compacted
chromatin, and an acrosome, a membrane bound organelle
harboring the enzymes required for penetration of the outer vestments of the
egg before fertilization. The sperm neck maintains the connection between the
sperm head and tail. It consists of the connecting piece and proximal
centriole. The axonemal complex extends from the proximal centriole through the
sperm tail. The tail harbors the midpiece, principal piece, and endpiece. The
midpiece is 7 to 8 µm long and is the most proximal segment of the tail, terminating
in the annulus. It contains the axoneme, which is the 9 +2 microtubule arrangement, and surrounding outer dense fibers. It also
contains the mitochondrial sheath helically arranged around the outer dense
fibers. The outer dense fibers, rich in disulfide bonds, are not contractile
proteins but are thought to provide the sperm tail with the elastic rigidity
necessary for progressive motility. Similar in structure to the midpiece, the
principal piece has several columns of outer dense fibers that are replaced by
the fibrous sheath. The fibrous sheath consists of longitudinal columns
and transverse ribs. The sperm terminates in the endpiece, the most distal
segment of sperm tail, which contains axonemal structures and the fibrous
sheath. Except for the end-piece region, the spermatozoon is enveloped by a
highly specialized plasma membrane that regulates the trans-membrane movement of ions and other molecules.
The spermatozoon is a remarkably complex metabolic and genetic machine.
The 75 mitochondria that surround the axoneme contain enzymes required for
oxidative metabolism and produce adenosine triphosphate (ATP), the primary
energy molecule for the cell. Mitochondria are semiautonomous organelles that
produce cellular energy and can also cause apoptotic cell death through the
release of cytochrome c. Mitochondria are composed of double (outer and inner)
membranes. Five distinct respiratory chain complexes span the width of the
inner membrane and are necessary for oxidative
phosphorylation: the NADPH dehydrogenase, succinate
dehydrogenase, cytochrome bc1, cytochrome c oxidase, and ATP synthase complexes. The sperm axoneme contains enzymes and structural proteins necessary for the chemical transduction of ATP into mechanical movement. The plasma membrane covering the sperm-head region harbors specialized proteins that participate in sperm–egg interaction.
The axoneme is the true motor assembly and requires 200 to 300 proteins to function. Among these, the microtubules are the best-understood components. Sperm microtubules are arranged in the classic “9+2” pattern of 9 outer doublets encircling an inner central doublet. The protein dynein extends from one microtubule doublet to the adjacent doublet and forms both the inner and outer “arms” of the axoneme. Sperm with outer arm mutants have reduced motility and those with inner arm mutants have no motility. Radial links or spokes connect a microtubule of each doublet to the central inner doublet and consist of a complex of proteins. Tektins are proteins associated with the outer microtubular doublets, and nexin links are proteins that connect the outer doublets to each other and maintain the cylindrical axonemal shape.
