Anatomy

A spermatozoon may be divided into five main sections: the head, the neck, the mid-piece, the principal piece and the end-piece (Fig. 4.2) (Dott, 1975; Johnson et al., 1980; Johnson, 1991a; Amman and Graham, 1993; Christensen et al, 1995).

Head

The head of the stallion spermatozoon is illustrated in Figs 4.3-4.6. Several authors have reported various dimensions for the head. Its length has been reported to vary from 4.9 to 10.6 pm, with a maximum width of 2.5-3.91 pm.

Post Acrosomal Lamina
Fig. 4.2. A diagrammatic representation of a stallion spermatozoon.

The length of the post-acrosomal region (post-acrosinal lamina or cap) has been reported to be 1.65 pm, and the width of the base of the head to be 1.45-1.8 pm, with a total surface area for the spermatozoon head of 10.3-16.5 pm2 (Nishikawa et al., 1951; Dott, 1975; Johnson et al., 1978b, 1980; Bielanski and Kaczmarski, 1979; Massanyi, 1988; Gravance et al., 1996, 1997; Casey et al., 1997). The work by Gravance et al. (1996) and Casey et al. (1997) reported an association between large heads and subfertility.

The head of the spermatozoon may be divided into three sections: the nucleus, the acrosome and the post-acrosomal region.

The nucleus, the contents of which are electron dense with a few nuclear vacuoles, is contained within a double-layered nuclear membrane, an envelope

Nuclear Membrane
Fig. 4.3. Scanning electron micrograph of a fresh stallion spermatozoon. (Bar = 10 m.) (Photograph by Jennifer Jones.)

with a few pores (Bielanski and Kaczmarski, 1979). The nucleus contains highly condensed chromatin, which is DNA complexed with protamine, the genetic material for fusion with the corresponding oocyte nuclear material. The shape of the head is determined by the shape of the nucleus and tends to be broad, flat and asymmetrical (Figs 4.3, 4.4 and 4.5). When viewed from the side (narrowest aspect) it has a total length of up to 10.6 pm and is thicker at the caudal end, tapering to 0.6 pm at the cranial end (Johnson et al., 1978b, 1980; Massanyi, 1988). When viewed from the front (broadest aspect) the head is similarly tapered at the cranial end, with the widest (2.36 pm) section being in the centre aspect (Johnson et al., 1978b, 1980). This shape is at variance with other mammalian spermatozoa, which tend to be widest at the base (Fawcett, 1970, 1975).

The acrosome region of the spermatozoon head (Fig. 4.6) lies over the caudal two-thirds of the nucleus and can be subdivided into the apical ridge, the principal segment and the equatorial segment. The apical ridge tends to be thicker (that is, a greater distance separating the inner and outer acrosomal membranes) than the principal segment, which in turn is thicker than the equatorial segment (Bielanski and Kaczmarski, 1979). The acrosome also appears to be smaller and more uniform in shape in the stallion than thickened to one side, a characteristic of bull, boar and ram spermatozoa (Massanyi, 1988; Amann and Graham, 1993). Occasionally a perforatum is seen

Ram Head Anatomy
Fig. 4.4. Scanning electron micrograph of fresh stallion spermatozoa illustrating the broad flat shape of the head with both perpendicular (central) and abaxial (non-central) attachment of the head to the neck region. (Bar = 2 m.) (Photograph by Jennifer Jones.)

at the cranial end of the acrosome, lying in the region of the apical ridge. This is postulated to be a stabilizing structure (Amann and Graham, 1993). The membrane of the acrosome comprises an inner and an outer membrane. The inner membrane is in juxtaposition to the nuclear membrane. The outer membrane lies under the plasma membrane, with which it fuses during the acrosome reaction to release the acrosomal contents. The acrosome region contains glycolipids and hydrolytic enzymes, including hyaluronidase, proacrosin/acrosin and lipases, all of which are released during the acrosome reaction and are required for subsequent passage of the spermatozoon through the surrounding investments of the oocyte (Goodpasture et al., 1981; Eddy, 1988; Bedford and Hoskins, 1990; Johnson, 1991a). The membrane of the acrosome region is particularly high in cholesterol, when compared with other parts of the spermatozoon plasma membrane (Lopez and Souza, 1991). The equatorial segment of the acrosome region does not contain enzymes and so is not involved in the acrosome reaction, but its membranes and the associated plasma membrane do fuse with the oocyte membrane at fertilization.

The post-acrosomal region is the region of the head not covered by the acrosome; it lies caudally to the acrosome region and runs down to the

Head Neck Sarcoma Electron Microscopy

Fig. 4.5. Transmission electron micrograph of a stallion spermatozoon showing the narrowest aspect of the head, neck and beginning of the mid-piece. The tear apparent in the plasma membrane on the right-hand side of the spermatozoon is a common artefact of transmission electron microscopy of stallion spermatozoa. (Bar = 0.36 m.)

Fig. 4.5. Transmission electron micrograph of a stallion spermatozoon showing the narrowest aspect of the head, neck and beginning of the mid-piece. The tear apparent in the plasma membrane on the right-hand side of the spermatozoon is a common artefact of transmission electron microscopy of stallion spermatozoa. (Bar = 0.36 m.)

connection with the neck. It is composed of characteristically tight lamellae of high electron density (Bielanski and Kaczmarski, 1979) but its function is unclear (Fig. 4.5). The plasma membrane in the post-acrosomal region may also be involved in fusion with the oocyte plasma membrane. Small protrusions from the base of the nuclear membrane may be evident at the junction between the post-acrosomal region and the neck. These are remainders of nuclear membrane left after the contraction and condensation of nuclear material during spermatogenesis (Bielanski and Kaczanaski, 1979).

Neck

The spermatozoon neck is the connection between the mid-piece and the head. Its connection is at the implantation (or articular) fossa, at which there is a thickened area of the double nuclear membrane, forming a basal plate for articulated attachment (Bielanski and Kaczmarski, 1979; Katz, 1991) (Figs 4.5, 4.7 and 4.8).

Attachment in the stallion spermatozoon differs from other spermatozoa in two ways. Firstly, the attachment seems to be particularly fragile, accounting for the high numbers of detached heads and tails, or inappropriate attachments of the two, seen in stallion spermatozoa. Secondly, in about 50% of

Capitulum Spermatozoa

capitulum caudal ring segmented column proximal centriole mitochondria dense fibre doublet central pair

Fig. 4.6. Longitudinal cross-section through neck and head region of a stallion spermatozoon, taken through the narrowest aspect, as seen in Fig. 4.5.

stallions, the attachment of the head and the mid-piece in the neck region is not perpendicular but abaxial; that is, the neck is attached off-centre (Fig. 4.9) (Bielanski, 1951; Dott, 1975; Pickett et al., 1987).

The neck itself is only a short area, 0.8 pm in diameter, containing the connecting piece, the proximal centriole and several small mitochondria. The connecting piece is the area where the connection between the head and the neck is seen. The attachment area lying on the neck side of the basal plate of the head is called the capitulum (Figs 4.7 and 4.8). The capitulum is formed primarily from the two major, segmented columns within the neck. In total, the neck region contains nine segmented columns formed from fibrous protein and arranged like a pile of 15 plates. Four of these segmented columns fuse into two pairs, forming an area termed the two major segmented columns. These two segmented columns fuse and largely form the capitulum. There is a difference between these two major segmented columns. The primary one gives rise to the major portion of the capitulum, its two segmented columns being fused along the entire length. The secondary major segmented column

Capitulum Spermatozoa

Fig. 4.6. Longitudinal cross-section through neck and head region of a stallion spermatozoon, taken through the narrowest aspect, as seen in Fig. 4.5.

Spermatozoon

Fig. 4.7. Transmission electron micrograph of a stallion spermatozoon showing a longitudinal cross-section through the broadest aspect of the neck region, to illustrate the attachment of the head and mid-piece. (Bar = 0.25 pm.) A diagrammatic representation of this area is given in Fig. 4.8. (Photograph by Jennifer Jones.)

Fig. 4.7. Transmission electron micrograph of a stallion spermatozoon showing a longitudinal cross-section through the broadest aspect of the neck region, to illustrate the attachment of the head and mid-piece. (Bar = 0.25 pm.) A diagrammatic representation of this area is given in Fig. 4.8. (Photograph by Jennifer Jones.)

also gives rise to, and attaches to, the capitulum, but its two component segmented columns do not fuse until they are cranial to the proximal centriole. The remaining four unpaired segmented columns also merge with the capitulum at right angles to the attachment of the two major segmented columns (Fig. 4.8) (Amann and Graham, 1993; James, 1998).

The proximal centriole is seen between the two major segmented columns, towards the cranial region of the connecting piece. It is situated oblique to the long axis of the spermatozoon at an angle of 45-60° to the mid-piece axis, with one end near the plasma membrane. It is typically larger in stallion spermatozoa than observed in other mammals. The distal centriole is not evident, having been lost during the development of the connecting piece.

Small mitochondria along with numerous microtubules may also be seen in the area of the connecting piece and proximal centriole (Bielanski and Kaczmarski, 1979). The presence of numerous microtubules is of interest, as their presence has not been reported in other mammals and their function is

Small Section The Plasma Membrane

Fig. 4.8. Longitudinal section of neck and initial part of the mid-piece region of a stallion spermatozoon, taken through the broadest aspect, as seen in Fig. 4.7. A surface view (lower portion) is given to illustrate the helical arrangement of mitochondria around the central inner dense fibres and axenome of the mid-piece of the spermatozoa.

Fig. 4.8. Longitudinal section of neck and initial part of the mid-piece region of a stallion spermatozoon, taken through the broadest aspect, as seen in Fig. 4.7. A surface view (lower portion) is given to illustrate the helical arrangement of mitochondria around the central inner dense fibres and axenome of the mid-piece of the spermatozoa.

Electron Micrographs Testes
Fig. 4.9. Scanning electron micrograph of stallion spermatozoa, illustrating the abaxial attachment of the mid-piece to the head of the spermatozoon in the neck region. (Bar = 1 m.) (Photograph by Jennifer Jones.)

unclear. On leaving the testis the spermatozoon neck is surrounded by cytoplasmic droplets, remnants of old spermatozoon cytoplasm that remains from the original gamete cell prior to concentration. This cytoplasmic droplet, which migrates down the mid-piece and is normally lost by the time the spermatozoa are ejaculated, is rich in hydrolytic and glycolytic enzymes (Harrison and White, 1972; Dott, 1975).

The attachment of the neck to the mid-piece is via dense fibres. The caudal end of each segmented column within the neck is fused, but not continuous, with one of nine dense fibres originating in the mid-piece.

Mid-piece

The mid-piece of the spermatozoon connects the neck and the principal piece and is characterized by a high concentration of mitochondria. Its length, including the neck region, is reported to be 10.5 ± 1.27 pm (excluding the neck, 9.83 ± 3.3 pm), with a diameter of 0.6 ± 0.11 pm (Nishikawa et al., 1951; Bielanski and Kaczmarski, 1979). The mitochondria are arranged circumventrally around the outer limits of the mid-piece (Figs 4.8, 4.10, 4.11 and 4.12), in a double layered spiral, each mitochondrion being half the

Peripheral Doublets Composed

Fig. 4.10. Transverse cross-section through the mid-piece of the stallion spermatozoon. Both the doublets and dense fibres within the mid-piece are numbered 1-9. Number 1 dense fibre and doublet is the one bisected by a line running at right angles to the cross bridge connecting the two central tubules. The remaining dense fibres and doublets are numbered in order in the direction in which the arms of the doublet point. In this diagram, dense fibre and doublet 1 are at 12 o'clock; the remainder are numbered in a clockwise fashion.

Fig. 4.10. Transverse cross-section through the mid-piece of the stallion spermatozoon. Both the doublets and dense fibres within the mid-piece are numbered 1-9. Number 1 dense fibre and doublet is the one bisected by a line running at right angles to the cross bridge connecting the two central tubules. The remaining dense fibres and doublets are numbered in order in the direction in which the arms of the doublet point. In this diagram, dense fibre and doublet 1 are at 12 o'clock; the remainder are numbered in a clockwise fashion.

Transverse Direction Membrane

Fig. 4.11. Tranmission electron micrograph of a transverse cross-section through the mid-piece of the spermatozoon, illustrating the outer helical arrangement of the mitochondria and the inner dense fibres and axenome. Using the convention given in Figure 4.10, doublet and dense fibre 1 are situated at 10 o'clock; the remaining doublets and dense fibres are evident in an anticlockwise fashion. The large dense fibres 5 and 6 are evident at 6 and 4 o'clock, respectively, the intermediate-size dense fibre number 9 is evident at 12 o'clock. (Bar = 0.09 m.) (Photograph by Jennifer Jones.)

Fig. 4.11. Tranmission electron micrograph of a transverse cross-section through the mid-piece of the spermatozoon, illustrating the outer helical arrangement of the mitochondria and the inner dense fibres and axenome. Using the convention given in Figure 4.10, doublet and dense fibre 1 are situated at 10 o'clock; the remaining doublets and dense fibres are evident in an anticlockwise fashion. The large dense fibres 5 and 6 are evident at 6 and 4 o'clock, respectively, the intermediate-size dense fibre number 9 is evident at 12 o'clock. (Bar = 0.09 m.) (Photograph by Jennifer Jones.)

circumference (that is, half a turn) in length. The junction between two mitochondria in one layer is positioned directly above and below the centres of the adjacent mitochondria (Fig. 4.8).

In the stallion's spermatozoon the mitochondria are typically arranged in 50-60 helical turns (Bielanski and Kaczmarski, 1979; Amann and Graham, 1993). These mitochondria are responsible for the production of ATP and as such contain the enzymes and cofactors necessary for this process (Gibbons and Gibbons, 1972).

In the centre of the tube of mitochondria are nine dense fibres involved in fusion with the segmented columns of the neck (Figs 4.8, 4.10 and 4.11). These provide support, by way of their tough keratin-like fibrous structure, and probably provide stability and so reduce the wobble factor that the whipping of the spermatozoon tail would normally generate, but still allowing some flexibility to be maintained. These dense fibres are not equal in size, numbers one, five and six being the largest, and number nine intermediate in size, compared with the others.

Fig. 4.12. Transmission electron micrograph showing a longitudinal cross-section through the mid (upper) and principal (lower) piece of the stallion spermatozoon, illustrating the helical arrangement of the mitochondria around the central core of the mid-piece. No such arrangement of mitochondria is observed in the principal piece region shown in the lower part of the photograph. Also shown is the annulus region at thejunction of the mid-piece and the principal piece. (Bar = 0.25 m.) (Photograph by Jennifer Jones.)

Fig. 4.12. Transmission electron micrograph showing a longitudinal cross-section through the mid (upper) and principal (lower) piece of the stallion spermatozoon, illustrating the helical arrangement of the mitochondria around the central core of the mid-piece. No such arrangement of mitochondria is observed in the principal piece region shown in the lower part of the photograph. Also shown is the annulus region at thejunction of the mid-piece and the principal piece. (Bar = 0.25 m.) (Photograph by Jennifer Jones.)

Running down the centre of these dense fibres is the axoneme (Figs 4.8, 4.10 and 4.11). The axenome consists of a central pair of microtubules surrounded by a ring of nine doublets. Each of the doublets is made up of two microtubules, A and B. The A tubule is the major one, with the 'C'-shaped B microtubule attached to it. Each doublet has four arms or attachment points from the A microtubule. Two of these are longitudinal arms pointing towards the next doublet; a third arm, called the radial spoke, attaches each doublet to the central pair (nine radial arms in total); and finally a nexin link attaches each doublet to its neighbour around the circumference (Fig. 4.13).

The longitudinal arms contain dynein, rich in ATPase, which transduces chemical energy into mechanical action. The nexin links are likely to regulate the relative displacements of the doublets during their sliding and contraction and so maintain symmetry. The radial spokes also contain dynein, and are also likely to provide structural support and stability. The microtubules themselves are made up largely of tubulin molecules arranged in 13 and nine or ten protofilaments (Gibbons and Gibbons, 1972; Kimball, 1983; Beford and Hoskins, 1990; Amann and Graham, 1993).

The end of the mid-piece is marked by the annulus or Jensen's ring, an electron-dense area marking the junction between the mitochondria of the mid-piece and the fibrous sheath of the principal piece. At this point the spermatozoon plasma membrane is firmly attached to the underlying annulus (Fig. 4.12) (Bielanski and Kaczmarski, 1979; Amann and Graham, 1993).

Principal piece

The principal piece is in essence a continuation of the mid-piece, resembling it internally with the continuation of the dense fibres and the axenome, but having a smaller diameter (0.45 ^m) (Johnson et al, 1978b, 1980; Amann and

^^-microtubule B

microtubule A .—nexin link

- radial spoke

■outer longitudinal dynein arm inner longitudinal dynein arm central sheath central microtubule

Fig. 4.13. Doublets of the middle and principal piece of the stallion spermatozoon numbered as shown. The relationship between the A and B microtubules is illustrated, along with the positioning of the two longitudinal arms, inner and outer, (pointing towards the next doublet), the radial spoke (attaching each doublet to the central pair) and a nexin link attaching each doublet to its neighbour around the circumference. (Satir, 1974.)

Fig. 4.13. Doublets of the middle and principal piece of the stallion spermatozoon numbered as shown. The relationship between the A and B microtubules is illustrated, along with the positioning of the two longitudinal arms, inner and outer, (pointing towards the next doublet), the radial spoke (attaching each doublet to the central pair) and a nexin link attaching each doublet to its neighbour around the circumference. (Satir, 1974.)

Graham, 1993) (Figs 4.12, 4.14 and 4.15). However, towards the caudal end of the principal piece the dense fibres taper away.

The one major difference between the mid-piece and the principal piece is the loss of the surrounding mitochondria and the gain of the fibrous outer sheath in their place. The fibrous sheath is arranged in two fibrous ribs connecting two longitudinal columns, which run along the ventral and dorsal aspects of the principal piece, overlying the dense fibres three, four and eight. Towards the end of the principal piece the dense fibres taper away. The first fibres to terminate are numbers three and eight, followed by number nine and then numbers one, five and six. All fibres terminate prior to the end piece. As the dense fibres taper away towards the caudal end of the principal piece, the fibrous sheath lies increasingly closer to the axenome. It is likely that the function of the fibrous sheath is to provide support, and yet allow flexibility for the translation of the contraction and sliding of the doublets within the axenome into controlled tail movement (Amman and Graham, 1993).

End-piece

The end-piece is the caudal end of the spermatozoon and is reported to measure 2.79 ± 0.9 pm in length (Dott, 1975; Bielanski and Kaczmarski, 1979)

Cross Section Axenome

Fig. 4.14. Cross-section through the mid-piece of one spermatozoon (on the right) and the cross-section through the principal piece of another (to the left). Despite the poor focus, the replacement of the outer mitochondria with a fibrous sheath is clearly evident in the principal-piece cross-section compared with that of the mid-piece. (Bar = 0.17 m.) (Photograph by Jennifer Jones.)

Fig. 4.14. Cross-section through the mid-piece of one spermatozoon (on the right) and the cross-section through the principal piece of another (to the left). Despite the poor focus, the replacement of the outer mitochondria with a fibrous sheath is clearly evident in the principal-piece cross-section compared with that of the mid-piece. (Bar = 0.17 m.) (Photograph by Jennifer Jones.)

doublet 1

dense fibre 1

dense fibre 8

Plasma Membrane Cross Section

dense fibre 3

dense fibre 4

fibrous rib

Fig. 4.15. Transverse cross-section through the principal piece of the stallion spermatozoon. Dense fibre and doublet 1 are positioned at 12 o'clock and the remainder are numbered in a clockwise direction. Note the loss of the outer mitochondria helix.

doublet 1

dense fibre 1

plasma membrane dense fibre 8

dense fibre 3

longitudinal column dense fibre 4

fibrous rib

Fig. 4.15. Transverse cross-section through the principal piece of the stallion spermatozoon. Dense fibre and doublet 1 are positioned at 12 o'clock and the remainder are numbered in a clockwise direction. Note the loss of the outer mitochondria helix.

(Fig. 4.16). The nine doublets plus the two central microtubules continue through the first half of the end-piece. They then taper out over a short distance, leaving just the fibrous sheath, which then tapers away, but not necessarily evenly, disappearing on one side before the other. The microtubules of the doublet do not disappear evenly, as the B microtubule terminates prior to the A microtubule (Bielanski and Kaczmarski, 1979; Kimball, 1983).

Plasma membrane

The whole of the spermatozoon is contained within its plasma membrane, which is anchored in specialized areas and forms the outermost component of the spermatozoon. The plasma membrane remains intact, except in the region of the acrosome as a preliminary to fertilization or as a result of senescence or death of the spermatozoon.

The structure of the membrane is consistent throughout, in that it is composed of three layers or zones: lipid bilayer, phospholipid-water interface and glycocalyx (Fig. 4.17).

The lipid bilayer (5 nm in thickness) is subdivided into polar phospholipids, which orientate themselves so that the hydrophilic polar head groups are situated externally, and the hydrophobic fatty acid chains orientated internally towards each other (Amann and Pickett, 1987; Hammerstedt et al., 1990). The major lipids present are phospholipids and cholesterol in a ratio of 0.64:0.36 (Chow et al., 1986; Parks and Lynch, 1992). It is suggested, by some workers, that although this bilayer configuration is the one most commonly observed and an efficient one for maintaining a permeability

Structure Plasma Membrane
Fig. 4.16. Transverse cross-section through the first half of the end-piece of the stallion spermatozoon. As in Fig. 4.15, doublet 1 is at 12 o'clock and the remainder are numbered in a clockwise fashion. Note the loss of the dense fibres.

barrier, an alternative hexagonal phase II configuration may be present. In this configuration the lipids are arranged in a cylindrical form with their hydrophobic phospholipid heads orientated towards the centre of the

Plasma Membrane Structure
Fig. 4.17. Simplified structure of the plasma membrane of the equine spermatozoon. (Adapted from Amann and Graham, 1993.)

cylinder. It is postulated that this configuration reduces the efficiency of the membrane to act as a barrier but allows membrane fusion (Hammerstedt et al., 1990). The amounts of cholesterol relative to phospholipids determine the fluidity of the membrane. In general, the higher the relative concentration of phospholipids, the more fluid is the membrane. Cholesterol therefore acts, along with integral proteins, as a stabilizer ensuring a normal lamella configuration of the phospholipids and the bilayer. The concentration of cholesterol is known to vary with location within the plasma membrane, being highest at the acrosome region (Lopez and Souza, 1991). It has been shown that in other domestic species the class of phospholipids and the nature of their side chains also vary with their position. The major classes include choline, ethanolamine and sphingomyelin. It is likely that a similar pattern is to be found in equine spermatozoa (Amann and Picket, 1987; Hammerstedt et al., 1990). Indeed, work by Parks and Lynch (1992) demonstrated that, as with boars, bulls and roosters, these were the major phos-pholipids in the horse, with the addition of phosphoglycerides. Glycolipids in all four species were minor, making up less than 10% of the total polar lipids. The peak phase transition temperature for phospholipids in horses is 20.7°C, compared with 24.0°C, 25.4°C and 24.5°C for boars, bulls and roosters, respectively. Similarly the horse demonstrated the lowest peak phase transition temperature for glycolipids at 33.4°C, compared with 36.2°C and 42.8°C for boars and bulls (no peak phase transition temperature was recorded for roosters). It is possible that these differences reflect the differing tolerance of spermatozoa to rapid drops in temperature.

Proteins are also found amongst the lipids and make up 50% of the weight of the plasma membrane. These proteins act as either structural (integral) proteins or as attachment points for other peripheral proteins. The structural proteins may also act as channels or pores through which small molecules may pass to the cytoplasm of the spermatozoon; the remaining structural proteins are found between the two bilayers of the membrane. Attachment proteins act as surface receptors for other peripheral proteins from the surrounding medium by means of their negatively charged carbohydrate side chains. Proteins which attach to, or are part of, the membrane are known to participate in spermatozoon-ovum interactions. These proteins differ between species, though their function in binding is similar (Calvete et al., 1994; Dobrinski et al., 1997; Thomas et al., 1997).

In addition, a-1,4-galactosyltransferase has been located on the equine spermatozoon membrane where it mediates the binding of the spermatozoon with the glycoconjugate residues in the zona pellucida of the ovum (Fayrer-Hosken et al., 1991).

The next area of the membrane is the phospholipid-water interface, which is the junction between the hydrophilic polar head groups of the lipid layer and the surrounding medium (largely water) and in which the glyco-calyx is found. The glycocalyx is a polysaccharide outer coat of the equine spermatozoon, 15 nm in depth. Its exact function is unclear but it is likely to be involved in antigenicity, cell adhesiveness, specific permeability and ATP

activity (Winzler, 1970). It is known that within the glycocalyx there are attachments for peripheral proteins (Hammerstedt et al., 1990). These proteins are likely to be provided by seminal plasma and act as a stabilizing influence on the spermatozoon during its passage through the male, and subsequently the female, tract. They may also be involved in capacitation (Hernandez-Jauregui et al., 1975).

As stated previously, the plasma membrane of the spermatozoon conforms to the above structure regardless of its location. However, as seen, minor differences do occur in the class of phospholipids and their associated fatty acid chains. The function and significance of these differences is unclear. Some variations may be associated with areas of specialization or attachment to the underlying spermatozoon structure. There are three such areas: over the caudal ring of the head (the junction of the head and the neck), the annu-lus (the junction of the mid-piece and the principal piece) and the principal piece (Eddy, 1988; Bedford and Hoskins, 1990). These areas are characterized by densely packaged particles, presumably involved in the attachment. In the caudal end of the head region and the annulus these densely packaged particles are arranged in a band around the circumference of the spermatozoon. In the principal piece, however, they are arranged in a zipper fashion along the length of the principal piece, again presumably providing the attachment. It has also been suggested that a further point of attachment may be found overlying the post-acrosomal region, as this is a relatively stable area (Hancock, 1957). Proteins within the bilayer also apparently change: for example, Calvete et al. (1994) isolated a zona pellucida protein similar to AWN-1 previously found in boars, termed stallion AWN protein, which was restricted to just the equatorial segment.

The structure of the plasma membrane may, under certain circumstances, be altered or changed - for example, during cooling or freezing. Under such circumstances the stability of the membrane may be affected due to induced changes to the lipids of the bilayer. Such disturbances are potentially irreversible and so lead to spermatozoon metabolic malfunction and death. The effect of cooling on spermatozoon membranes is discussed in more detail in Chapter 7.

0 0

Post a comment