The Morphology of Boar Spermatozoa

SYNOPSIS The morphological features of boar spermatozoa in freshly ejaculated semen and in semen exposed to various experimental treatments have been studied in stained smears and with the phase‐contrast microscope and the electron‐microscope. The spermatozoon of the boar closely resembles that of other domestic ungulates. The head is approximately 8.5μ long; it is twice as long as it is wide. The mid‐piece is approximately 10μ long and the main tail‐piece is 30μ long. The head is covered anteriorly by a cap‐like structure—the acrosome. Evidence is presented to show that the acrosome has two components: the outer acrosome is the larger, the smaller inner acrosome is a crescentic structure. The segment‐shaped area of overlap between the two parts of the acrosome forms the equatorial segment which is a feature of living boar spermatozoa. The surface of the head behind the acrosome impregnates with silver; this argentophil area is the post‐nuclear cap. Three neck granules can be distinguished at the posterior border of the head and these are apparently connected to three bundles of neck fibres which the head receives from the mid‐piece. The structure of the spermatozoon is altered by post‐mortem changes; the most prominent changes occur in the acrosome. Evidence is presented that the post‐nuclear cap is absent from dead spermatozoa. The development of the boar acrosome has been studied in sections stained by the PAS method and in unfixed tissues with the phase‐contrast microscope. The development of the post‐nuclear cap has been studied in sections impregnated with silver. The morphological features of epididymal spermatozoa have been studied in spermatozoa from different levels of the epididymis. The development of the acrosome is shown to resemble its development in the bull. Two separate components of the acrosome of the spermatid are distinguished. It is suggested that these develop into the two components of the acrosome of the mature spermatozoon. Migration of the cytoplasmic droplet from the neck to the mid‐piece occurs at varying levels in the epididymis, but it is shown that it may occur in the head of the epididymis. The findings are discussed in relation to current ideas about the structure of mammalian spermatozoa.