The steroidogenic interrenal cells in the adrenal homologue of the male stickleback (Gasterosteus aculeatus) were studied in relation to the reproductive cycle by means of histological and ultrastructural observations, and using histochemical methods for the localisation of 3β-hydroxysteroid dehydrogenase (3βHSD) and 17β-hydroxysteroid dehydrogenase (17βHSD). To determine the various stages of the reproductive cycle, the testes were also examined by histological and histochemical methods (3βHSD). The results indicate that in this teleost the interrenal cells can undergo a cycle in which phases characterised by different cytological aspects are observed. During this cycle there is a renewal of organelles, in particular mitochondria and SER. Periodic degenerative processes are also found. Organelle cytology showed that the cell cycle has at least 3 different aspects during the year. An analogy with some cytological aspects of the adrenal zonation in mammals is possible. It is postulated that the interrenal gland activity could substitute or supplement androgen production by the testes.

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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 timecourse 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.

In this work we describe the adrenal homolog of the rainbow trout Oncorhynchus mykiss during development. At the histological level, the interrenal primordium is clearly evident in larvae 25 days after fertilization (dpf), and the immunohistochemical reactions for tyrosine hydroxylase (TH) and phenylethanolamine-N-methyltransferase (PNMT), which mark the chromaffin cells, appear as early as 27 dpf. Both reactions are evident in cells localized in the head kidney and in some, probably migrating, cells close to the notochord. In 27-dpf larvae, the ultrastructural analysis shows the presence of the interrenal cells with mitochondria with tubulovesicular cristae, typical of steroidogenic cells, sometimes surrounded by smooth endoplasmic reticulum (SER) cisternae, indicating that in this stage the cells have the capacity for steroid synthesis and secretion. In the same stage the chromaffin cells are characterized by few and small membrane-bound granules containing cores of heterogeneous electron density. Both types of cells show large nuclei, numerous free or clumped ribosomes, developed rough endoplasmic reticulum (RER), and scarce SER. Rare nerve endings contacting chromaffin cells are present. In the subsequent developmental stages, a further differentiation of both types of cells is evidenced by modifications of cell organelles as mitochondria, chromaffin granules, RER, SER, and so on. A clear discrimination of the two types of catecholamine-containing cells, adrenaline and noradrenaline cells, is evident only 5 days after hatching. The presence of different interrenal cell types in larvae at 5 and 10 days after hatching probably indicates the activation of a physiological cellular cycle. The immunohistochemical and ultrastructural results are compared with those obtained by other authors in the same and other vertebrate species.