Comparative ultrastructural studies of spermiogenesis and spermatozoa in some species of Polyplacophora (Mollusca)

Ultrastructural studies of spermatogenesis in Antalis entalis (Scaphopoda, Mollusca)

Mature spermatozoa and spermatogenesis of Antalis entalis were studied by light, scanning and transmission electron microscopy. During spermatogenesis the spherical proacrosome arises from a single Golgi complex, migrates toward the anterior pole of the spermatid, and there differentiates to form an electron-dense collar. This is unique among examples of the simple acrosome of primitive spermatozoa. Nuclear condensation does not occur until the proacrosome and mitochondria migrate towards the future anterior and posterior poles of the spermatid. Chromatin condensation occurs by the formation of heterochromatin granules. No manchette of microtubules is observed in the course of nuclear condensation and elongation. Mitochondria achieve their mature spherical shape by decreasing in number and increasing in diameter at an early stage of spermiogenesis. No inter-mitochondrial or mitochondrianuclear (or centriolar) junctions occur. The centriolar satellite is formed in association with the distal centriole late in spermiogenesis. Earlier, the typical ‘9 + 2 ’ microtubular axoneme is coiled within the spermatid cytoplasm and the flagellum emerges only late in spermiogenesis. Beta glycogen granules appear late in the development of the spermatid, and are distributed in the centriolar and periacrosomal regions.

Comparative spermatology of 11 species of Polyplacophora (Mollusca) from the suborders Lepidopleurina, Chitonina and Acanthochitonina

Transmission electron microscopy of the spermatozoa and spermatogenesis of 11 species (in three suborders Chitonina, Acanthochitonina, Lepidopleurina) of chiton has shown that each species has a sperm with a unique morphology indicating that spermatozoa can be used as a taxonomic character. Although structure is species-specific, similarities between species within suborders and subfamilies can be recognized. The spermatozoa of species from the suborders Chitonina and Acanthochitonina have a head comprising nuclear material only, the anterior portion of which is in the form of a long thin (approximately 80 nm diameter) filament. In many species the centrioles and mitochondria of the mid-piece are lateral in position, the mitochondria often being sited anteriorly alongside the nucleus. By contrast,Leptochiton asellus, a member of the more ancient suborder Lepidopleurina, has a sperm with a head comprising a nucleus and an acrosome. The mid-piece is also more conven­tional in structure with a ring of five or six spherical mitochondria (sited behind the nucleus) that surround the centrioles. The presence of the acrosome inL. asellussuggests that in the more recent chitons the acro­some has been secondarily lost. It is proposed that loss of the acrosome is correlated to a modification in egg-coat thickness. A preliminary examination of the structure of the eggs of three species has shown that those ofL. asellusare surrounded by a very thick chorion (14-30 μm) whereas inAcanthochitona crinitusandDinoplax gigasthere are regions of the chorion that are less than 2 μm thick. The morphological changes that occur during spermatogenesis are very similar in the Chitonina and Acanthochitonina. During spermiogenesis the nucleus elongates to develop a long anterior filament. Chro­matin condensation within the nucleus involves the formation of fibrils that become orientated along its long axis. Closely associated with the elongating nucleus is a manchette. InL. asellus a spherical proacrosomal vesicle appears in the spermatocytes. This vesicle becomes compressed as it matures and simultaneously it migrates to the presumptive anterior end of the spermatid where it invaginates and elongates. Although the pattern of chromatin condensation in the nucleus is similar to that described above, a manchette has not been observed.

Fertilization in a chiton: acrosome-mediated sperm-egg fusion

Contrary to the widely accepted view that chiton sperm lack acrosomes and that fertilization in this group occurs via a micropyle, we demonstrate here that fertilization in Tonicella lineata occurs by acrosome-mediated sperm-egg fusion. The acrosome is a small vesicle containing two granules located at the tip of the sperm. The eggs have an elaborate hull (= chorion), which is formed into cupules that remain covered by follicle cells until maturity. When dissected ripe eggs were exposed to sperm in vitro, the sperm were attracted only to open cupules, inside which they swam through one of seven channels to the base where they penetrated the hull. The acrosome fired on contact with, or in, the hull, and during passage through it the apical granule was exhausted while the basal granule was exposed. If sperm contacted follicle cells between the cupules the acrosome did not react. The vitelline layer beneath the hull contains pores arranged in a regular pattern. Embedded in the base of each pore is an egg microvillus. Having penetrated the hull the sperm anterior filament located a pore and fused with the tip of the egg microvillus projecting into it. This created a membranous tube, through which the sperm nucleus was injected into the egg. The egg membrane appeared to be raised up into a small fertilization cone around the penetrating sperm, the vitelline layer became slightly elevated, and some cortical granules were released by exocytosis.