Axoneme of the axial filament stretches across the full length of the flagellum and constitutes the motor apparatus of the sperm tail. The organization of the axoneme is similar to that of cilia and flagella of all eukaryotic cells. It has two central microtubules surrounded by nine evenly spaced microtubular doublets – the classic 9 + 2 pattern. The nine peripheral doublets are numbered in a clockwise direction from 1 to 9. The first doublet is situated on a place perpendicular to that of the two central microtubules. Each doublet consists of an A subunit with a complete microtubule of 26 nm in diameter and a B subunit which is an incomplete microtubule (C shaped in cross section) that is attached to the A subunit. Tubulin is the structural component of the microtubules (Farrell 1982; Curry and Watson 1995). The A tubule is made of 13 protofilaments that are aligned side by side. The B tubule is made from 10 protofilaments. Extending from A microfilament to B microfilament are ‘arms’ which play a crucial role in flagellar movement. The principal component of the arms is dynein (Porter and Johnson 1989; Holzbaur and Vallee 1994; Milisav 1998). Activation of the axonemal dynein ATPase results in sliding of adjacent outer doublet microtubules and it has been proposed that these sliding results in flagellar bending (Tash and Means 1982). The doublets are connected by the protein nexin (Clermont et al. 1990). The central microtubules are interconnected by linkages and are surrounded by a pair of spiral fibres that are attached to the microtubules. The spiral fibres form the central sheath from which radial spokes go out to the A subunit of the doublet (Pederson 1970).

The connecting piece connects the flagellum and the sperm head. It is about 0.5 μ in length and consists of:

The capitulum is a dome-shaped fibrous structure. Proteinaceous filaments run between the capitulum and the caudal surface of the nucleus. The capitulum is the site of the centrioles. It is positioned at right angles to the axis of the flagellum. Only the proximal centriole is seen in a mature spermatozoon as the distal centriole disintegrates during spermiogenesis. The centriole plays a major role in the formation of the axoneme during spermiogenesis and also a major part in fertilization and the events following fertilization. The structure of the centriole is the same as that of an axoneme but without the central pair of microtubules and has peripheral triplet in place of doublets.

The segmented columns fuse proximally into major and minor columns and attach to the capitulum. The columns attach to the outer dense fibre in the midpiece distally.

As early as 1887, it was postulated by Theodor Boveri that the oocyte has all elements required for embryonic development except the active division centre (Baltzer 1967). In a somatic diploid cell, the mitotic spindle is the key to the distribution of the genomic material. The spindle is derived from the centrosome. The centrosome and centriole are a part of the MTOC (microtubule-organizing centre). The centrosome consists of two centrioles and the pericentriolar material (PCM). The centriole is a pair of cylinders arranged perpendicularly, whereas the aster and the spindle fibres are derived from PCM (Palermo et al. 1994). The centriole displays the distinct 9 + 0 pattern of nine triplet microtubules. This differs from the axoneme by the absence of the central pair of microtubule. Earlier reports suggested that mammalian gametes lack centrioles. It was proven beyond doubt that centrioles are indeed part of the mitotic division in humans (Sathananthan et al. 1991) and in other species too (Guen and Crozet 1989). Sathananthan et al. described in detail the anatomy of centrioles in human reproduction. They showed that the human oocyte does not possess any centriole and the sperm has two centrioles – proximal and distal. The proximal centriole is located in the connecting piece, next to the basal plate of the sperm head. It has a pinwheel structure of nine microtubules surrounded by electron dense material referred to as the ‘black box’. The distal centriole is located perpendicular to the proximal and is aligned with the flagellum that forms the axoneme during spermiogenesis (Sathananthan et al. 1991; 1996).

The two major functions of the centrosome are (1) nucleation of microtubule and (2) mitotic spindle formation (Schatten 1994; Bornens et al. 1990). In all mammalian species, except the mice, the sperm centrosome nucleates the aster. This brings about the apposition of the female and male pronuclei. It has been demonstrated that injection of the tail alone can induce aster formation (Van Blerkom and Davis 1995). After the fusion of the gametes, the tail of the sperm is incorporated into the ooplasm. The centriole duplicates during the pronuclear stage. Centrioles have been detected up to the stage of blastocyst (Sathananthan et al. 1996). In humans, only the male gamete has an active centrosome and is the structure responsible for the first mitotic division (Palermo et al. 1994).

The axoneme of the sperm is surrounded by the outer dense fibre. Each of the peripheral microtubule doublets has an outer dense fibre. They are individual fibres that are teardrop shaped, with an outer rounded edge that tapers towards the axoneme. Cranially, these fibres fuse with the connecting piece. These are suggested to facilitate sperm movement, mediated by protein phosphorylation, and serve as protector of the sperm during its passage in the male and female tracts (Tash and Means 1983). It is also suggested to act as the stiffening rods within the sperm tail.

The midpiece of the flagellum is about 3.5 μm in length and runs from the distal end of the connecting piece to the annulus. It is an electron dense circumferential band marking the junction between the midpiece and principal piece. A recent report by Guan and colleagues in BMC Developmental Biology 2009 described the development of the annulus, in the formation of the mature spermatozoon. Its function has not clearly been established, but it may constitute a diffusion barrier between the two compartments and/or facilitate mitochondria migration and alignment along the axoneme.

The midpiece has a mitochondrial sheath with a species-specific number of mitochondria. The mitochondria are arranged in a helical pattern around the axoneme. The human spermatozoon has a helix composed of 11–15 gyres. The mitochondrial structure in the sperm is the same as that in other cell types but has greater stability. It is resistant to osmotic changes which might help resist stretching and compression of the mitochondria during flagellar beat. It is the site of energy production of spermatozoa and its position allows ready supply of ATP to the axoneme. The flagella activity requires energy which is obtained in the form of ATP. The ATP is supplied by the mitochondria and is hydrolyzed by ATPase in the dynein arms in the presence of magnesium.

This is the longest flagellum extending from the annulus to the proximal end of the terminal piece. It is approximately 55 μm in length. It has a fibrous sheath which is the cytoskeletal structure surrounding the axoneme and the outer dense fibres. The sheath consists of two columns that is circumferentially connected by a series of closely packed filaments called ribs. In the human sperm, the ribs are 10–20 nm apart and 50 nm thick. The function of the fibrous sheath appears to be similar to that of outer dense fibres – to act as stiffening rods and provide rigid support to the flagellum and determining its planar beat (Lindemann et al. 1992). It has been seen that sperms with disorganized fibrous sheath have disrupted motility indicating its importance in motility of the sperm.

Beyond the fibrous sheath is the terminal piece of the flagellum, which is about 3 μm in length. The microtubules of the axoneme terminate in this region. The dynein arms disappear first and the A subunit takes a hollow appearance. The central pair of the microtubules terminates, after which two of the peripheral outer doublets move to the centre. The doublets separate and the open B tubule disappear. The pattern that remains is a single central tubule surrounded by a circle of single microtubule covered by plasma membrane.