Dual origin of yolk nuclei in the lesser spotted dogfish,Scyliorhinus canicula (Chondrichthyes)
✍ Scribed by Lechenault, Henri ;Mellinger, Jean
- Publisher
- John Wiley and Sons
- Year
- 1993
- Tongue
- English
- Weight
- 962 KB
- Volume
- 265
- Category
- Article
- ISSN
- 0022-104X
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✦ Synopsis
Abstract
Using plastic embedding techniques and semithin sections in order to overcome the difficulties encountered in the sectioning of yolky eggs, we made a histological study of the developing blastodisc and early embryo of the oviparous dogfish Scyliorhinus canicula (L.) (Pisces: Chondrichthyes) from Roscoff (Brittany, France). According to our observations, several conclusions based on classical studies on elasmobranch development have to be revised as follows: (i) Cleavage of the germinative disc into blastomeres lags considerably behind the mitotic activity of diploid nuclei derived from amphimixy. As a result, the number of surface blastomeres does not correspond in any way to the actual number of nuclei, even at the start of segmentation. Thus, segmentation is primarily syncytial. (ii) The first set of yolk nuclei corresponded to the deepest nuclei that remained beneath the segmentation cavity after complete cellularization of the blastodisc. They became polyploid, some of them degenerated, and the others multiplied while remaining close to the extraembryonic endoderm during epiboly. (iii) A second set of yolk nuclei was derived from the embryonic endoderm along its boundary with the extraembryonic endoderm. Endoderm cells escaped into the yolk layer nearby. They then give off free nuclei, which also became very large and finally were indistinguishable from the first set. (iv) No yolk nuclei were derived from supernumerary sperm nuclei, even when the latter were able to divide several times in synchrony. Nuclei from this haploid lineage were always smaller than those in the first set, and they were finally located in the deepest layer of the germinative disc, where they degenerated. It is concluded that this chondrichthyan fish generates its yolk syncytium in a very different way from teleosts, in which all yolk nuclei are derived from the blastodisc. © 1993 Wiley‐Liss, Inc.