Ultrastructural study of synovial membrane from the antebrachiocarpal joint of calves.

The synovial intima from the antebrachiocarpal joint of 4-month-old calves was between 1 and 3 cells in thickness and did not have a basal lamina. Numerous areas of the intimal matrix were in direct contact with the joint lumen. The synovial membrane was comprised mainly of A-type synoviocytes usually located adjacent to the joint lumen. These cells were characterized by numerous filopodia (or lamellipodia), large, empty-appearing vacuoles, numerous lysosomes, large vacuoles containing granular material separated from the vacuolar membrane by a radiolucent band, and coated micropinocytotic vesicles. Smooth micropinocytotic vesicles were seen only rarely in these cells. In contrast, B-type cells had few filopodia, numerous smooth micropinocytotic vesicles, few coated micropinocytotic vesicles, a well-developed Golgi apparatus and rough endoplasmic reticulum, mitochondria that were longer and had a denser matrix than that of A cell mitochondria, and surprisingly, only few maturing or fully formed secretory granules. A distinct intermediate (C or AB) type synoviocyte could not be unequivocally identified. Desmosome-like structures were present between synoviocytes, although it was considered questionable if these were true intercellular junctions. No other junctions were present.