Wound healing in the imaginal discs of Drosophila. II. Transmission electron microscopy of normal and healing wing discs.

Abstract

AbstractThe normal elements of the cell surface and the cytoskeleton of imaginal wing disc cells in third‐instar larvae of Drosophila melanogaster are described. This serves as a background for understanding the process of wound healing after experimentally cutting the two disc epithelia (columnar epithelium and peripodial membrane).The cells of both epithelial layers display microvilli at the apical surface, and there is a microvillar ridge at the periphery of each cell. The lateral cell surfaces display zonulae adhaerens, gap junctions, and septate junctions in characteristic patterns. A terminal web of 60–70‐Å microfilaments is located in the apical cell periphery, extends into the microvillar ridge, and is connected with the zonula adhaerens and probably also with the underlying belt of microtubules.Immediately after an incision is made in a normal disc, the two wound edges move apart to form a gape of about 45°. This fast wounding reaction is reversibly inhibited by treatment with cytochalasin B (0.5 μg/ml), suggesting that the wounding reaction is achieved by actinlike microfilaments. Initial wound closure results in an apposition of the columnar epithelium and the peripodial membrane (heterotypic contacts). Close (10–20 nm) heterotypic membrane contacts are found beginning at 12 hours after wounding, but cell junctions are not observed at such contact sites.Within 24 hours after wounding, the first focal cell membrane contacts are made between cells of corresponding epithelial types across the wound (homotypic contacts), at which time the heterotypic contacts are broken. At the time of epithelial healing, the wound surfaces shorten (“wound contraction”), and this is associated with cell‐shape changes within the epithelial cells along the wound. Within 48 hours after wounding, homotypic cell junctions and nonjunctional membrane contacts are fully reestablished, and wound healing is completed.We propose that the homotypic contacts between cells of the columnar epithelium are involved in the intercellular interactions controlling pattern regulation in the imaginal wing disc.