Freeze-fracture studies of the developing cell surface. I. The plasmalemma of the corneal fibroblast.

Abstract

The freeze-fracture technique was used to study changes in the corneal fibroblast cell membrane during morphogenesis in chick embryos. Fibroblasts migrated into the acellular primary corneal stroma on day 6 of embryogenesis, moving between the orthogonal layers of collagen fibrils which serve as their substratum. Intramembrane particles (IMP) were morphometrically analyzed. Their concentration on the P [protoplasmic] face decreased from 756 to 534/.mu.m2 from days 6-14. After day 14, fibroblast migration and cell division ceased and the stroma condensed due to dehydration, so that by day 18 all of the layers of fibroblasts were extremely flattened and the cornea had taken on its mature, transparent form. The cell membranes of the terminally differentiated, highly compacted fibroblasts were rich in IMP (1300/.mu.m2, P face). The synthetic events occurring at this time were analyzed, but no correlation with 25SO4, or 3H-proline incorporation was found. The event which best correlated with the doubling of P face particles between days 15-18 was the dehydration and condensation of the stroma, an event which was associated with cessation of both cell division and migration. Thyroxine stimulated premature condensation of the stroma, whereas thiouracil delayed condensation, but neither of these treatments affected IMP concentration, IMP concentration on the filopodia of migrating fibroblasts was similar to that on the cell bodies, suggesting that the new membrane had the same composition as the pre-existing membrane. Tight and gap junctions between fibroblasts and the relationship of extracellular matrix to the outer etched surface of the fibroblast plasmalemma were also studied.