The development of adrenal homolog of rainbow trout Oncorhynchus mykiss: an immunohistochemical and ultrastructural study

Differentiation of steroidogenic cells in the developing adrenal gland of Testudo hermanni Gmelin, 1789 (chelonian reptiles)

The aim of this study was to investigate the development and differentiation of steroidogenic cells in the embryonic adrenal gland of Testudo hermanni using histological, histochemical, immunohistochemical and ultrastructural methods. The 26 developmental stages were divided into three periods: early (stages 1-18, up to 20 days of incubation), intermediate (stages 19-22, incubation days 21-35) and advanced (stages 23-26, from incubation day 36 to hatching). A small presumptive bud of steroidogenic cells was visible at the end of the early period, protruding into the coelom from the lateral wall of intermediate mesoderm. Ultrastructural characteristics suggested that young and scarcely differentiated cells could already be able to perform steroidogenic activity: lipid droplets, large amount of SER and RER, small rounded mitochondria with variously shaped cristae and dense matrix. The cell membrane showed microvilli and coated pits. During the intermediate period, the interrenal bud deepened into the haemopoietic tissue, close to the mesonephros and the newly formed metanephros. The ultrastructural, immunohistochemical and immunocytochemical characteristics pointed to enhanced steroidogenic activity. The contact with both kidney types (mesonephros and metanephros) continued in the advanced period, and chromaffin cells were also extensively mixed with steroidogenic cells. This is a peculiar feature of chelonian adrenal gland, in comparison with that of other reptiles. The variable cytological characteristics of embryonic steroidogenic cells in the advanced period suggest a four-phase cycle of steroidogenic activity.

Morphological and histological study of larval development of the Senegal sole Solea senegalensis: an integrative study

This study provides a comprehensive description of the main morphological and histological events that take place during larval and post-larval development of Senegal sole Solea senegalensis in order to establish a reference for its normal developmental organogenesis. Five stages have been described. Before gill development at the onset of metamorphosis (eye migration process, stage 4c), the skin was the main site of gas and ion exchange, whereas during stages 3 and 4, the skin begins differentiating into the definitive juvenile structure. The timing of development of the endocrine system depends on each organ, the endocrine pancreas and thyroid gland being the first to differentiate (stages 2 and 3, respectively), followed by the interrenal tissue and stannius corpuscles that develop at metamorphosis (stages 4 and 4c, respectively). The differentiation and maturation of the lymphohaematopoietic organs was coupled with the increase in complexity of the cardiovascular system and the presence of mature erythrocytes (stage 4b), which might be attributed to the change in respiration and the development of fully functional gills. In the differentiation of sensory structures, the development of eyes, inner ear, neuromasts and olfactory organs was rapid, with most of these organs becoming fully developed soon after hatching (stage 1). Vision, chemo- and mechano-reception developed very early in ontogeny, in parallel with the development of the central nervous system and changes in feeding habits. Although the general pattern of development in S. senegalensis appeared similar to most marine fish larvae already described, there were species-specific ontogenetic characteristics probably derived from the species' particular environment (subtropical waters) and behaviour (nocturnal, benthic, omnivorous feeding habits). These results on the organogenesis of larvae are a useful tool for establishing the functional systemic capabilities and physiological requirements of larvae to ensure optimal welfare and growth under aquaculture conditions.

The ontogeny of regulatory control of the rainbow trout (Oncorhynchus mykiss) heart and how this is influenced by chronic hypoxia exposure

Salmonid embryos develop in cool waters over relatively long periods of time. Interestingly, hypoxic conditions have been found to be relatively common in some nesting sites (redds). The goals of this study were to determine the ontogeny of cardiac regulation in rainbow trout early life stages and how this is influenced by chronic hypoxia. The heart rate response to cholinergic and adrenergic receptor stimulation or inhibition was measured in individuals reared in normoxic (100% O2 saturation) or hypoxic (30% O2 saturation) conditions from fertilization to embryonic stages 22, 26 and 29, and larval stages 30 and 32. In normoxia, heart rate increased in response to β-adrenergic receptor stimulation (isoproterenol) as early as embryonic stage 22, and decreased with the antagonist propranolol after this stage. Cholinergic stimulation (acetylcholine) was ineffective at all stages, but atropine (acetylcholine antagonist) increased heart rate at larval stage 32. This demonstrates that cardiac β-adrenergic receptors are functional at early life stages, while cholinergic receptors are not responsive until after hatching. Collectively, embryos had cardio-acceleration control mechanisms in place just after the heartbeat stage, while cardio-inhibitory control was not functional until after hatching. Chronic hypoxia exposure triggered bradycardia, increased the response to adrenergic stimulation in embryos and larvae, and delayed the onset of cholinergic control in larvae. In non-motile stages, therefore, survival in chronic low oxygen may depend on the ability to alter the cardiac ontogenic program to meet the physiological requirements of the developing fish.

Degeneration and possible renewal processes related to the interrenal cells in the head kidney of the stickleback Gasterosteus aculeatus

The ultrastructural aspect of degeneration and recovery processes involving the steroidogenic interrenal cells of the stickleback was studied. Together with the adrenergic cells, the interrenals constitute the adrenal homolog in teleosts. From our study it appears that a process of massive cell death may lead to temporary disappearance of the gland. Moreover, our E.M. observations suggest two main ways, each leading to morphological dedifferentiation of the cells, no longer recognizable as interrenals: the first way involves elimination of organelles and recovery of the nucleus surrounded by a thin rim of cytoplasm; the second involves fragmentation of the cytoplasm by other pyknotic star-shaped interrenals, together with autophagocytosis processes. Our E.M. observations also suggest that the subsequent reconstitution of the tissue can occur in two ways. In the first, the interrenals appear mainly to differentiate from mesenchymatic-like electron-light cells, while in the second, the new interrenals appear mainly raising from some macrophagic electron-dense cells. Some data obtained with Mallory's trichrome staining of histological sections, and localization of the enzyme 3beta hydroxysteroid dehydrogenase in thin sections, support the above-mentioned results. A hypothesis is advanced on the origin of the electron-dense differentiating interrenals, and a possible role of dedifferentiated cells in restoration of the interrenal gland is also discussed.

Chromaffin cells in the adrenal homolog ofAphanius fasciatus (teleost fish) express piecemeal degranulation in response to osmotic stress: A hint for a conservative evolutionary process

The effect of severe osmotic stress on the ultrastructural morphology of chromaffin cells in the adrenal homolog of Aphanius fasciatus, a small eurhyaline teleost living in saltpans, was evaluated by electron microscopy quantitative analysis. Fishes were transferred from salt water, whose salinity was 3.7%, to dechlorinated tap water and chromaffin cells were studied at resting condition and after 2 and 48 hr from the beginning of the experiment. Ultrastructural examination revealed a series of granule and cytoplasmic changes highly specific for piecemeal degranulation (PMD), a secretory process based on vesicular transport of cargoes from within granules for extracellular release, which was previously described in chromaffin cells of the mouse, rat, and human adrenal medulla. There was indeed a significant trend toward loss of content material from chromaffin granules accompanied by enlargement of granule size. Remarkably, chromaffin granules maintained their individual close structure during the whole releasing process and eventually transformed into large empty containers. A dramatic increase in the density of small, membrane-bound, variably electron-dense vesicles free in the cytoplasm or attached to granules was recognized during the first 2 hr of stress response. These features fell to control levels after 48 hr. A similar time-course pattern was observed concerning the formation of budding projections from the surface of chromaffin granules. This study provides new insight into PMD physiology and suggests that PMD is part of an adaptive secretory response to severe osmotic stress in fishes. From an evolutionary point of view, this study lends support to the concept that PMD is a secretory mechanism highly conserved throughout vertebrate classes.

Differentiation of chromaffin cells in the developing adrenal gland of Testudo hermanni

The aim of this study was to investigate the development and differentiation of chromaffin cells in the adrenal gland of the turtle Testudo hermanni during ontogenesis using histological, immunocytochemical and ultrastructural methods. The 26 developmental stages were divided into three periods: in the early period (stages 1-18, up to 20 days of incubation at 37 degrees Celsius and 85% humidity), the chromaffin cells were observed from stage 12. They followed a ventro-lateral migration pathway with respect to the notochord and dorsal aorta, forming groups embedded in undifferentiated mesenchymal tissue. They reached the kidney surface only at the end of this period. Under the EM the chromaffin cells showed typical embryonic characters, such as rounded shape, high nucleus/plasmatic ratio, cell membrane with elongated processes; the cytoplasm contained a large number of free ribosomes, Golgi complexes, RER and a few chromaffin granules distributed in small sets. The granules were small and displayed a high electrondensity. Numerous unmyelinated fibres ran close to the chromaffin cells. At the end of this period both nervous elements and chromaffin cells were positive to the antigen for DbetaH. The intermediate period (stages 19-22, incubation days 21-35) was characterized by the first occurrence of steroidogenic cells on the ventro-medial kidney surface. Some chromaffin cells were still found in the same position, whereas other cells were still migrating, maintaining their embryonic character. It was possible to divide the secretory granules into two types according to their shape and electrondensity: the more numerous N-type granules had a dark content, whereas the small number of A-type granules (consistent with the scarce PNMT reaction) displayed a light content. They occurred for the first time in this period. In the advanced period (stages 23-26, from incubation day 36 to hatching) the adrenal gland reached its definitive shape, although remaining immature; groups of variously sized chromaffin cells intermingled with steroidogenic cells, both lying on the kidney surface. Chromaffin granules were more numerous and larger than in the previous stages, frequently mingling in the same cell. A migration pathway of the chromaffin cells along the nerve fibres can be hypothesized on the basis of their common origin and closeness. The polymorphic shape of chromaffin cells with long cytoplasmic processes also accounts for their migrating fitness. We can assume that steroidogenic differentiation from the mesodermic blastema begins after the first chromaffin cells have completed their migration.