Ultrastructural characteristics of canine transmissible venereal tumor at various stages of growth and regression.

Ultrastructural study of canine transmissible tumors in developing, mature, and regressing stages from 6 dogs revealed the presence of healthy and degenerating tumor cells in all neoplasms. The total number of neoplastic cells seemed to decrease, and the number of degenerating neoplastic cells seemed to increase in mature tumors. Macrophages, lymphocytes, and plasma cells infiltrated mature and regressing tumors. Alteratons in degenerating tumor cells consisted mainly of cytoplasmic changes in early stages and of both nuclear and cytoplasmic changes in cells in which degeneration was more advanced. Amounts of endoplasmic reticulum and ribosomes were decreased. There were swelling and vacuolation of mitochondria. Nuclear chromatin was clumped along the nuclear envelope, and the perinuclear space was widened. Degenerating cells often contained membrane-bound granules and clusters. Lamellar complexes were observed in tumor cells from 2 dogs. Virus particles were not seen.