Immunocytochemical and ultrastructural characterization of endocrine cellsin the larval stomach of the frog Rana temporaria tadpoles: a comparison with adult specimens

According to immunostaining and ultrastructural patterns, Rana temporaria t adpole stomach displays a well-differentiated endocrine population comprisi ng, at least, six cellular types: ECL, EC [serotonin], D [somatostatin] - a ll three of them abundant -, P [bombesin] - less numerous -, CCK-8 [cholecy stokinin/gastrin] and A [glucagon/glicentin] - both very scarce. Larval end ocrine cells are mainly located in the surface epithelium and show open or closed morphologies. Cellular diversity is similar in tadpoles and frogs, w ith the exception of immunoreactivity for gastrin-17, found in adults in nu merous cells. Larval cells display mature ultrastructural traits, although with smaller secretory granules. The different distribution of endocrine ce lls, which in adults are preferentially located in the glands, probably ref ers to different functional requirements. However, the rich vascular plexus present in larval mucosa may be an efficient transport medium of surface h ormones to gastric targets. The enhancement in adults of endocrine populati on and correlative increase in hormonal secretion indicates a more active f unctional role, probably related to the shift from herbivorous to carnivoro us habits. In summary, the tadpole gastric endocrine population, although n ot as numerous as that of adult frogs, displays histological traits that in dicate a relevant (immunoreactive and ultrastructural properties, cellular diversity) and specific (surface location, relative abundance of open-type cells) role of local regulatory factors in amphibian larval gastric functio n. (C) 2001 Harcourt Publishers Ltd.