Replacement of the larval basement lamella by adult-type basement membrane in anuran skin during metamorphosis

Abstract

The basement lamella in the head of a 75-mm tadpole of Rana pipiens contained 46 orthogonally aligned layers, while the basement lamella over the muscular part of the proximal end of the tail in the same animal had about 32 layers. Invasion of the basement lamella by subdermal mesenchyme cells was first detected in the head skin of a tadpole at Taylor-Kollros stage XI. Invading cells, which appear to be fibroblasts, move upward toward the epidermis, then may spread horizontally between layers of the basement lamella. Mesenchyme cells are abundant within the membrane by stage XIV. Invasion of the basement membrane does not occur in the tail. At about stage XIV or XV, the basement lamella of head skin becomes detached from the epidermis. Connective tissue cells, principally fibroblasts and melanophores, then move into the subepidermal space and epidermal glands grow into it. The layer thus created is the stratum spongiosum. The basement lamella, which may now be called the dermal lamella, becomes the inner layer of the dermis, the stratum compactum. A new basement membrane develops under the epidermis after the larval lamella becomes detached. At stages during resorption of the tail, the basement membrane in the head consists only of granules or short filaments embedded in ground substance. Filaments are again abundant in the membrane by stage XXV when metamorphosis is complete. The adult frog retains the type of basement membrane developing at stage XXV. Filaments are grouped into bundles that appear to run in several directions beneath the highly folded border of basal epidermal cells. The regular layered pattern of the larva is not re-established.