Mucin-producing carcinomas of the breast: ultrastructural observations.

The ultrastructural characteristics of mucin production in mammary mucoid carcinoma, signet ring cell carcinoma (Harris, Wells & Vasudev 1978), papillary carcinoma and lobular carcinoma are compared. The mucin in lobular carcinoma is confined to intracytoplasmic lumina whereas it is present as membrane bound granules in the other three types, although intracytoplasmic lumina also occur in the latter. A possible origin of intracytoplasmic lumina from distended Golgi cisternae is proposed. Possibly, extracellular mucin acts as a mechanical barrier between tumour cells and lymphatics, thus explaining the good prognosis of typical mucoid carcinomas.