Summary
Sperm and spermiogenesis of L. perforatum and D. listerianum have been investigated by light- and electron microscopy. The study revealed basic ascidian organization of sperm cells with the nucleus flanked by a unique mitochondrion, but with specializations not previously reported: a dense groove on the head surface, extended nucleus-mitochondrion association, and well developed endoplasmic derivatives.
The groove is a long furrow of the plasmalemma stretching the nucleus towards the cell periphery. At least two parallel, 20 nm high, ridges emerge from the bottom of the groove. At the opposite side of the groove the long mitochondrion embraces the nucleus by extending its outer membrane over the nuclear envelope; a restricted cytoplasmic space is always found between the two organelles. Filamentous inclusions organized in two strands are present in the mitochondrial matrix. Endoplasmic derivatives appear as a single, extended cistern in L. perforatum, and series of long tubules in D. listerianum. They appear to originate in the spermatid from the Golgi complex and endoplasmic reticulum. The spermatozoon lacks an evident acrosomal complex, but the possible homology of small apical densities with an acrosomal complex is considered. It is suggested that the functional role of these specialized structures is related to the special mode of internal fertilization of these species.
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Burighel, P., Martinucci, G.B. & Magri, F. Unusual structures in the spermatozoa of the ascidians Lissoclinum perforatum and Diplosoma listerianum (Didemnidae). Cell Tissue Res. 241, 513–521 (1985). https://doi.org/10.1007/BF00214570
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DOI: https://doi.org/10.1007/BF00214570