Primary culture of duck salt gland. I. Morphology of confluent cell layers.

Dissociated avian salt gland secretory cells were maintained in primary culture after plating on hydrated collagen gels. When seeded at 3 X 10(6) cells/cm2, confluent cell sheets formed within 2-3 days, whereas cultures seeded at lower densities formed a complex reticulum of cell aggregates, which remained nonconfluent even after 7 days. Scanning electron microscopy showed that the free surface of 3-day confluent cultures consisted of intermixed convex and flattened cell membranes with prominent junctional boundaries and abundant microvilli. Transmission electron microscopy indicated that these cultures were multilayers of 1-4 cells in thickness. The plasma membranes of the superficial cells were polarized into apical and basolateral regions displaying, respectively, microvilli and interdigitating lateral membrane folds. These membrane domains were separated by shallow occluding junctions, which consisted of both single strands and simple net-like arrays in freeze-fracture images. Underlying epithelial cells retained lateral membrane folds and formed desmosomal contacts with superficial and neighboring cells. These cultures, unlike the intact tissue, allow direct access to the apical and basolateral cell surfaces for electrophysiological analysis of transmural active ion transport.