Transformations of epithelial monolayers during wing development of Manduca sexta

Abstract

The two epithelial monolayers of the insect wing undergo striking morphogenetic changes during the course of adult development, but the exact interactions between these monolayers were not evident until the ultrastructure of the cells was carefully examined. The interaction of the dorsal monolayer with the ventral monolayer continually changes as the two initially separate monolayers first lose their pupal basal laminae and then come together along a sharp interface to form microtubule-associated junctions. As blood space between the two monolayers expands 2 days later, new adult basal laminae and cuticle form.

Concomitantly the epithelial cells stretch along their apicobasal axes to create a thin cellular M layer halfway between the dorsal and ventral surfaces of the wing that represents the site where connections between the monolayers are maintained at specialized basal junctions. The elongated processes of each monolayer that make up this M layer first fasciculate and then span the space separating the two monolayers, but only at relatively widely-spaced intervals. During later stages of adult development, dense aggregates of microtubules appear in these epithelial processes and presumably contract as cells dramatically shorten along their apicobasal axes during expansion of the wing. Examination of the ultrastructure of the developing adult wing has revealed how certain cellular events can account for the mechanics of cuticle and wing expansion after adult emergence.

Introduction

Most tissues and organs of insects are shaped and molded by transformations of epithelial monolayers. The cells of epithelial monolayers are polarized along their apicobasal axes and undergo a variety of morphogenetic movements that retain the polarity of the cells but that alter the arrangement of these cells. Sheets of epithelial cells can thicken, thin, or bend. Epithelia can bend with their apical surfaces outermost (evagination) or with their basal surfaces outermost (invagination). During these transformations, connections are maintained among the cells of the epithelium, but during other transformations the coherence of the epithelium is disrupted. Sometimes a contiguous population of cells can bud off the sheet as a hollow vesicle; these cells retain their apicobasal polarity even though they switch from their initially planar configuration to a spherical configuration. At other times not only is the defined apicobasal polarity of the epithelial monolayer lost but also the coherence of the epithelial monolayer. Whenever epithelial monolayers transform into loose aggregates of mesenchyme cells, the original connectivity of the cells is drastically altered. Mittenthal (1989) classified these various epithelial transformations as either: (1) connectivity-preserving or (2) connectivity-altering.

During adult development of the Manduca wing, the initially noncontiguous dorsal and ventral epithelial monolayers become juxtaposed after their pupal basal laminae break down. The basal processes from each monolayer shorten after they meet and become united by specialized junctions to form an epithelial bilayer. This cellular arrangement is an ephemeral structure, and the space separating the two monolayers soon widens as new basal laminae form on each monolayer. The transformation in epithelial morphology clearly changes the spatial relationship of the two monolayers. These novel epithelial transformations are probably unique to invertebrates and may also represent transformations that are unique to epithelia of insect wings. Cellular bilayers are the exception rather than the rule among the insects.

Finding out what cellular events accompany the morphogenetic transformations of epithelial monolayers and bilayers necessitated obtaining high resolution images of wing cells as they rearranged and changed shape. This claim is certainly evident from the few existing descriptions of epithelial transformations during lepidopteran wing development that are solely based on examination of tissue sections with the light microscope (Mayer, 1896, Mercer, 1900, Köhler, 1932). The corresponding transformations of wing epithelia during Drosophila pupal and adult development were first described by Waddington (1941). Many of the subcellular events observed with the electron microscope were not predicted from earlier examinations of epithelial cells at the resolution of the light microscope and were not noted in another ultrastructural study of wing development (Greenstein, 1972).