[The ultrastructure of mixed mammary gland tumours in bitches. III. The early stages of the myoepithelial proliferation (author's transl)].

The early stages of myoepithelial proliferation in 14 mixed canine mammary tumours were studied by light- and electron microscopy. Two different early stages can be distinguished: 1. Proliferations inside the basal lamina with partial maintenance of the organoid pattern. 2. Proliferations with a break through the basal lamina and transposition of tumour cells into the surrounding connective tissue. The intracytoplasmic fibrillae of myoepithelial tumour cells are striped periodically and thicker than those of normal myothelia. It is suggested that the amorphous as well as the fibrous intercellular substance in the early stages is derived from the described myoepithelial tumour cells.