Development of head and trunk mesoderm in the dogfish, Scyliorhinus torazame: I. Embryology and morphology of the head cavities and related structures.

Vertebrate head segmentation has attracted the attention of comparative and evolutionary morphologists for centuries, given its importance for understanding the developmental body plan of vertebrates and its evolutionary origin. In particular, the segmentation of the mesoderm is central to the problem. The shark embryo has provided a canonical morphological scheme of the head, with its epithelialized coelomic cavities (head cavities), which have often been regarded as head somites. To understand the evolutionary significance of the head cavities, the embryonic development of the mesoderm was investigated at the morphological and histological levels in the shark, Scyliorhinus torazame. Unlike somites and some enterocoelic mesodermal components in other vertebrates, the head cavities in S. torazame appeared as irregular cyst(s) in the originally unsegmented mesenchymal head mesoderm, and not via segmentation of an undivided coelom. The mandibular cavity appeared first in the paraxial part of the mandibular mesoderm, followed by the hyoid cavity, and the premandibular cavity was the last to form. The prechordal plate was recognized as a rhomboid roof of the preoral gut, continuous with the rostral notochord, and was divided anteroposteriorly into two parts by the growth of the hypothalamic primordium. Of those, the posterior part was likely to differentiate into the premandibular cavity, and the anterior part disappeared later. The head cavities and somites in the trunk exhibited significant differences, in terms of histological appearance and timing of differentiation. The mandibular cavity developed a rostral process secondarily; its homology to the anterior cavity reported in some elasmobranch embryos is discussed.

Calretinin immunoreactivity in the developing retina of sharks: comparison with cell proliferation and GABAergic system markers.

The calcium-binding protein calretinin (CR) has been widely used as a marker of neuronal differentiation. In the present study we analyzed the distribution of CR-immunoreactive (CR-ir) elements in the embryonic and postembryonic retina of two elasmobranchs, the lesser spotted dogfish (Scyliorhinus canicula) and the brown shyshark (Haploblepharus fuscus). We compared the distribution of CR with that of a proliferation marker (the proliferating cell nuclear antigen, PCNA) in order to investigate the time course of CR expression during retinogenesis and explored the relationship between CR and glutamic acid decarboxylase (GAD), the synthesizing enzyme of the gamma-aminobutyric acid (GABA), which has been reported to play a role in shark retinogenesis. The earliest CR immunoreactivity was concurrently observed in subsets of: a) ganglion cells in the ganglion cell layer; b) displaced ganglion cells in the inner plexiform layer and inner part of the inner nuclear layer (INLi); c) amacrine cells in the INLi, and d) horizontal cells. This pattern of CR distribution is established in the developing retina from early stage 32, long after the appearance of a layered retinal organization in the inner retina, and coinciding with photoreceptor maturation in the outer retina. We also demonstrated that CR is expressed in postmitotic cells long after they have exited the cell cycle and in a subset of GABAergic horizontal cells. Overall our results provide insights into the differentiation patterns in the elasmobranch retina and supply further comparative data on the development of CR distribution in the retina of vertebrates. This study may help in understanding the possible involvement of CR in aspects of retinal morphogenesis.

Spatially distinct domains of cell behavior in the zebrafish organizer region.

To determine the sequence of cell behaviors that is involved in the morphogenesis of the zebrafish organizer region, we have examined the dorsal marginal zone of vitally stained zebrafish embryos using time-lapse confocal microscopy. During the late-blastula stage, the zebrafish dorsal marginal zone segregates into several cellular domains, including a group of noninvoluting, highly endocytic marginal (NEM) cells. The NEM cell cluster, which lies in a superficial location of the dorsal marginal zone, is composed of both enveloping layer cells and one or two layers of underlying deep cells. The longitudinal position of this cellular domain accurately predicts the site of embryonic shield formation and occupies a homologous location to the organizer epithelium in Xenopus laevis. At the onset of gastrulation, deep cells underneath the superficial NEM cell domain undergo involution to form the nascent hypoblast of the embryonic shield. Deep cells within the NEM cell cluster, however, do not involute during early shield formation, but instead move in front of the blastoderm margin to form a loose mass of cells called forerunner cells. Forerunner cells coalesce into a wedge-shaped mass during late gastrulation and eventually become overlapped by the converging lateral lips of the germ ring. During early zebrafish tail elongation, most forerunner cells are incorporated into the epithelial lining of Kupffer's vesicle, a transient teleostean organ rudiment long thought to be an evolutionary vestige of the neurenteric canal. Owing to the location of NEM cells at the dorsal margin of blastula-stage embryos, as well as their early segregation from other deep cells, we hypothesized that NEM cells are specified by an early-acting dorsalizing signal. To test this possibility, we briefly treated early-blastula stage embryos with LiCl, an agent known to produce hyperdorsalized zebrafish embryos with varying degrees of expanded organizer tissue. In Li(+)-treated embryos, NEM cells appear either within expanded spatial domains or in ectopic locations, primarily within the marginal zone of the blastoderm. These results suggest that NEM cells represent a specific cell type that is specified by an early dorsal patterning pathway.

Distribution and ontogeny of glucagon-like cells in the gastrointestinal tract of the cartilaginous fish Scyliorhinus stellaris (L.).

The ontogeny and distribution of glucagon-like cells were studied in the gastrointestinal tract of embryos, neonates, and adults of the cartilaginous fish Scyliorhinus stellaris (L.) by immunocytochemistry. The results indicate that they appear early during embryonic development, and, in some portion of the gastrointestinal tract, even before the mucosa morphological differentiation. Immunoreactive glucagon-like cells were observed both in gastric and intestinal epithelium, being present in the pyloric portion only at a particular period of its differentiation. Some differences were observed between the embryonic and adult distributive pattern. They were more numerous in proliferative zone and sometimes were situated near other endocrine epithelial cells. These findings together with available information on trophic effects of some gastrointestinal hormonal peptides suggest a possible regulatory role of this peptide on the growth and differentiation of the gastrointestinal tract.

[Stages of development and growth of the thyroid gland and adenohypophysis of the dogfish embryo (Scillium canicula Cuv.), a chondrichthyan fish]. / Phases du développement et de la croissance de la t-yroïde et de l'adénohypophyse de l'embryon de roussette (Scyllium canicula Cuv.), chondrichthyen

The thyroid morphology, histology, biometry and its anatomical connections are studied during the whole embryonic life in the small spotted dogfish (Scyllium canicula Cuv.). The first differentiation of the gland appears as a thickening of the pharyngeal floor, containing PAS inclusions, at stage 10 mm. The first follicles are apparent at stage 30 mm. The thyroid volumetric relative growth is studied during the whole development and is compared with the absolute embryonic length growth. Volumetric variations of the hypophysis upper parts and ventral lobe are also studied during the whole embryonic life.