Sperm morphology,spermatogenesis and spermiogenesis of three species of chitons (Mollusca,Polyplacophora) |
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Authors: | E R Sakker |
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Institution: | (1) School of Biological Sciences, FO7, The University of Sydney, 2006 Sydney, NSW, Australia |
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Abstract: | Summary Mature sperm of the three species, Onithochiton quercinus, Chiton pelliserpentis and Plaxiphora paeteliana are eupyrene and basically of the primitive type. The sperm are small, with a distinct head, midpiece with a few spherical to oval mitochondria and a long tail with a (2×9)+2 axoneme. They are unusual among primitive sperm in being bilaterally symmetrical, with a long anterior filament containing an extension of the nucleus and lacking an acrosome. Spermatogenesis occurs synchronously throughout the testis in inwardly folded tissue plates. Spermatogonia arise adjacent to the central blood sinus in each tissue plate. Cells in successive stages of spermatogenesis are displaced towards the luminal surface. The cytoplasm of all stages contains ribosomes, rough endoplasmic reticulum, lysosomes and mitochondria. A Golgi complex is present in secondary spermatocytes and spermatids but does not form an acrosome. During spermiogenesis Golgi complexes are confined to the posterior region of developing sperm and are eventually shed in the residual cytoplasm behind the midpiece. Preacrosomal vesicles are not formed. The long anterior filament of the sperm and lack of an acrosome are features associated with the fertilization of eggs surrounded by a chorion which may have pores or a micropyle. The exact method of fertilization in chitons remains to be elucidated.Abbreviations
af
anterior filament
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bh
body of the head
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bn
body of the nucleus
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bs
blood sinus
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c
collar
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dc
distal centriole
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esg
early spermatogonium
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fc
fibrous chromatin
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gc
granular chromatin
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if
implantation fossa
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lsg
later spermatogonium
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m
mitochondrion
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mc
muscle cell in blood sinus
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mm
midpiece mitochondrion
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mt
microtubule
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mI
primary spermatocyte undergoing first meiotic division
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mII
secondary spermatocyte undergoing second meiotic division
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n
nucleus
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ncc
nuclear condensing chromatin
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ne
nuclear envelope
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pc
proximal centriole
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rc
resorbing cell
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s
spermatozoon
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1°sc
primary spermatocyte
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2°sc
secondary spermatocyte
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st
spermatid
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t
tail
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tc
thinning cytoplasm
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tf
tail flagellum
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tpec
tissue plate epithelial cell |
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