Posthatching development of the gut and lateral muscle in the Sole, Solea solea (L).

At hatching the gut of sole Solea solea was a primitive epithelial tube which showed vigorous cellular proliferation but only initial regionalization, and no secretory cells. By contrast, at this stage lateral muscle already consisted of a clearly distinct superficial monolayer of parvalbumin-negative fibres, and a deeper layer of parvalbumin-positive (presumptive fastwhite) fibres. There were no very small-diameter fibres present and very few proliferating cell nuclear antigen (PCNA)-positive nuclei, indicating that new fibres were not being produced at this time. The gut consisted of a distinct pharynx, oesophagus, stomach and intestine by 12 days, further differentiation of the anterior and posterior intestine was seen at 21 days, and morphological differentiation was complete at 2.5 months. Goblet cells containing acidic glycoconjugates were present at 12 days in the pharynx, oesophagus and anterior intestine, and increased in number at later ages. Neutral glycoconjugates first appeared in absorptive (not goblet) cells at 21 days. From 2.5 months neutral glycoconjugates were found in secretory cells throughout all parts of the gut except the stomach and the oesophagus, and by the adult stage the neutral glycoconjugates were the predominant type in the intestinal goblet cells. The difference in parvalbumin expression between the superficial (negative) and deep (positive) muscle layers seen at hatching persisted at all ages, and from 21 days immunostaining for slow isoforms of myosin gave a positive reaction in the superficial monolayer and subsequent slow muscle layer. Hypertrophic growth continued up to the largest subject examined, by posthatching myogenesis occurred in two distinct and relatively brief phases. The distribution of very small diameter muscle fibres and PCNA-positive nuclei indicated that muscle hyperplasia occurred initially in apical areas of the myotomes (at 12 and 21 days), but this was followed (at 2.5 months) by a second ‘mosaic’ phase of myogenesis as new fibres appeared scattered throughout the deep muscle layer. Hyperplastic growth had ceased by 1 year.