The orbital glands of the terrapin Pseudemys scripta in response to osmotic stress: a light and electron microscope study.

The histochemical and ultrastructural features of both orbital glands--the anterior lacrimal and harderian glands--were compared in the terrapin Pseudemys scripta following adaptation either to distilled water or to seawater. Seawater adaptation activated the harderian gland and caused a major structural reorganisation. Type I cells increased in number, whereas type II cells became smaller and less numerous. The most striking change was shown by the type IV cells which display the features of a salt-secreting cell. This cell type increased in number and changed from a unicellular form to multicellular complexes. Transfer from fresh water to distilled water caused a decrease in alcianophilia in type I and type III glandular cells and degenerative phenomena were often seen in type I and type IV cells. The anterior lacrimal gland showed only minor changes either in distilled or seawater adapted terrapins. Transfer to distilled water also caused degenerative phenomena in the lacrimal gland. These results establish for the first time the involvement of the harderian gland of a terrapin in osmoregulation.