Formation of the inner branchiostegal cuticle of the blue crab, Callinectes sapidus

The noncalcified inner branchiostegal cuticle, which lines the branchial chamber, was examined histologically and ultrastructurally over the molt cycle in the blue crab, Callinectes sapidus. In intermolt crabs (stage C₄ ) the epithelium underlying the inner cuticle is cuboidal and has abundant intercellular spaces and a prominent basement membrane. Apolysis occurs at stage D₀ and dissolution of the cuticle is accompanied by the formation of numerous lysosomes in the epithelium. During stage D₁ , cells increase in height, apical mitochondria become more abundant, and the cuticle continues to be resorbed. An epicuticle is formed in early D₂ , arising from a fusion of small subunits apparently attached to short apical microvilli. Cuticle deposition continues through D₂ and is complete by stage D₃ . By the time cuticle deposition is complete, the epithelium has become extremely columnar and cells are filled with bundles of microtubules. In stage D₄ , an amorphous electron-dense core appears in the microtubule-filled cells, which are attached to the cuticle at their apical end and anchored to their basement membrane at the basal surface. These microtubule-filled cells persist through ecdysis, stage E, but during stage A₁ the cores disappear and some organelles begin to reappear in the cytoplasm. By stage A₂ , the cells return to the cuboidal morphology seen in intermolt and remain so throughout the remainder of the molt cycle. This new pattern of cuticle deposition resembles that observed in the gills of crustaceans in that the cuticle is uncalcified and there is no postecdysial cuticle formation. However, instead of apolysis being delayed until just before ecdysis, the inner cuticle is formed during the first half of premolt, allowing the epithelial cells time to differentiate into a morphology that provides tensile strength for the stress of ecdysis. These new observations demonstrate that cuticle formation can follow very diverse structural and temporal patterns. In order to integrate and coordinate these diverse patterns, it is suggested that a suite of feedback mechanisms must be present. J. Morphol. 240:267-281, 1999. © 1999 Wiley-Liss, Inc.