[Ultrastructure of the secretory parenchyma of the postanal gland of the coelacanth, Latimeria chalumnae Smith].

Latimeria chalumnae Smith, the only surviving crossopterygian, possesses a well-developed, unpaired, postanal gland whose function is as yet undocumented. The availability of tissues suitable for ultrastructural and histochemical studies from the recent International Comores Expedition has permitted a reappraisal of the comparative morphology and functional hypotheses relating to the postanal gland. The richly vascularized tubuloacinar postanal gland is composed of cells which show the characteristic specializations of active ion transport. These include complex and extensive development of basal and lateral cell membrane infolding (cisternae) which are closely associated with numerous large, dense mitochondria. Histochemical studies have shown a high level of ATPase activity within the gland cells. These features of the postanal gland correlate well with those of the rectal glands of elasmobranchs and suggest a similar role in ion excretion and osmoregulation.