The Harderian gland of anuran amphibians: Morphological and functional aspects

The Harderian gland (HG) is an orbital gland present in all vertebrates possessing nictitating membrane. In this chapter we discuss current knowledge on HG of anuran amphibians. The primordium of the HG appears during metamorphosis at the time of development of the nictitating membrane. In adult frog it is an oval acinous gland that lies at the medial corner of the orbit and, in absence of lacrimal, it represents the only orbital gland. The single excretory duct begins at the hilus and opens ventrally in the conjunctival fornix. In some anuran species an anatomical- functional inter-relation between the HG, the nasolacrimal duct, the external naris, the inward leading vestibular channel and the vomero nasal organ has been recently described. In the HG of all examined anuran species the mucopolysaccharides are always present in the acini. This secretory product, similar to that of the lacrimal gland, is responsible for lubrication of the eyeball. In some anurans the HG shows seasonal changes as well as sexual dimorphism. Lipid secretions occurring only in females of Bufo viridis suggest that they could act as a pheromonal stimulus. In P. esculentus the columnar cells show considerable variation in height during the annual cycle, depending upon its functional state. The annual variations in secretory activity are consistent with changes in environmental temperature. In the hottest months the glandular cells appear elongated and filled with secretory granules, indicating a high secretory activity; the secretory activity drastically declines in September, during which time the cells become shorter and almost devoid of secretory granules and then recrudesce slowly from October onward. The annual morphological changes in the secretory activity of frog HG are strongly correlated to that the annual variations in the kinase activities. A new gene, named harderin, isolated from a cDNA library of P. esculentus, shows differential expression between the sexes during the annual cycle. To date its function remains to clarify. Pituitary, thyroid and sex steroid hormones are the major endogenous factors regulating the secretory activity of the frog HG. Estrogens appear to be responsible of the sexual dimorphism in B. viridis and P. esculentus HG. Preliminary molecular approaches to the study of HG secretion appear to be promising, notably the recent demonstration in some species of anurans of Prot-a transcripts and of antimicrobial peptides (preprotemporin-CBa and preprochensirin-2CBa) strongly suggests a role of the gland in the host's system of innate immunity defending against microbial invasion.