The structure of an epidermal cell during the development of the protein epicuticle and the uptake of molting fluid in an insect

Abstract

AbstractA structure for a generalized insect epidermal cell during the formation of the epicuticle is proposed, based on studies of several different epidermal cell types. The protein epicuticle is defined as the dense homogeneous layer below the cuticulin. The formation of the protein epicuticle involves secretory vesicles arising in Golgi complexes, and marks an interlude in the involvement in cuticle formation of plasma membrane plaques. The plaques are concerned in cuticulin formation before and in fibrous cuticle formation after the deposition of the protein epicuticle.The epidermis is characterized by the possession of a cytoskeleton of microtubules and a matrix of microfibers. In the elongated cells forming bristles and spines, the microfibers are often oriented in bundles with an axial banding which repeats every 120 Å. The microtubules are also arranged in columns with a trigonal packing and center to center spacing of about 800 Å. These cytoskeletal structures separate the other organelles into channels which may restrict the pathways open for the movement of secretory and pinocytotic vesicles. The protein epicuticle arises from the secretory vesicles which discharge at the apical surface. The contents disperse and reaggregate below the cuticulin. The Golgi complexes in the basal and central regions have many secretory vesicles and a small saccular component, differing from those nearer the apex which are smaller and have fenestrated saccules. The small coated vesicles (800 Å in diameter) associated with both sorts of complex, probably move to the apical and basal faces of the cell where they may give rise to the large coated vesicles (2000 Å in diameter) inserted in the plasma membrane. Pinocytosis occurs from both apical and basal faces but most lytic activity is in the apical region. Plant peroxidase injected into the haemocoel is taken up basally and transported to the apical MVBs. The large coated vesicles on the apical face may be concerned in the control of the extracellular subcuticular environment. They appear to fill up and detach, fusing to become the apical MVBs.