ULTRASTRUCTURE OF EPITHELIAL CELLS IN THE MEDULLA OF THE GUINEA‐PIG THYMUS

Summary An ultrastrutural study of the thymic medulla of near‐term foetal and young post‐natal guinea‐pigs revealed the presence of a variety of epithelial cells. Three groups of structurally specialized medullary epithelial cells were distinguished and were referred to as the ‘ squamous ',‘ villous ’ and ‘ cystic ’ cells. Structurally undifferentiated epithelial cells were also seen and cells intermediate in appearance between these and each of the other epithelial cell types were also noted. The “squamous” cells formed Hassall's corpuscles and closely resembled keratinizing stratified squamous epithelium.“Villous” cells were seen forming an epithelial lining to cleft‐like spaces around the periphery of some Hassall's corpuscles. These cells resembled intestinal lining cells. The “cystic” cells were characterized by the presence of intracytoplasmic cystic spaces, frequent mitochondria and numerous profiles of smooth‐surfaced endoplasmic reticulum. In “thick” sections stained with the periodic acid‐Schiff (P.A.S.) technique and examined with the light microscope these cells were seen to contain periodic acid‐ Sehiff positive material within the cysts. The morphology of the cystic cells is consistent with a secretory function and they may be the source of a thymic humoral Factor. No evidence was seen of any cells intermediate in structure between epithelial cells and lymphocytes.