The orbital glands of the chelonians Pseudemys scripta and Testudo graeca: comparative histological, histochemical and ultrastructural investigations.

The orbital glands of the chelonians Pseudemys scripta and Testudo graeca were investigated at the histological, histochemical and ultrastructural levels. Four acinar cell types were seen in the harderian gland of P. scripta on the basis of histochemical reactions and ultrastructure. Secretory granules were of 2 types, one showing moderate electron density with an electronlucent core, the other being smaller and more osmiophilic with an electron-dense core. In the harderian gland of T. graeca only 2 glandular cell types were found; one type contained secretory granules with a dense core surrounded by a wide zone of lower density. Acinar cells of the anterior lacrimal gland in both species were of 2 types, one being of mucous type. In the harderian gland and in the lacrimal gland of both species, one cell type appeared not to be involved in the secretion of organic material. These cells contained numerous tightly packed mitochondria among which were abundant clumps of glycogen; the cell membrane was specialised at both edges. This cell type was similar ultrastructurally to the 'salt cells' described in the salt-secreting glands of various marine vertebrates, i.e. of the cells involved in transport processes. These combined histological, histochemical and ultrastructural studies have allowed us to distinguish orbital glands. In the past, the harderian and lacrimal glands in chelonians have often been mistaken for one another.