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The midgut of most insects is lined with a peritrophic matrix, which is thought to facilitate digestion and protect the midgut digestive epithelial cells from abrasive damage and invasion by ingested micro-organisms. The type 2 peritrophic matrix is synthesised by a complex and highly specialised organ called the cardia typically located at the junction of the cuticle-lined foregut and midgut. Although the complex anatomy of this small organ has been described, virtually nothing is known of the molecular processes that lead to the assembly of the type 2 peritrophic matrix in the cardia. As a step towards understanding the synthesis of the peritrophic matrix, the synthesis and secretion of the intrinsic peritrophic matrix protein, peritrophin-15 has been followed in the cardia of Lucilia cuprina larvae using immuno-gold localisations. The protein is synthesised by cardia epithelial cells, which have abundant rough endoplasmic reticulum, Golgi, and vesicles indicative of a general secretory function. Peritrophin-15 is packaged into secretory vesicles probably produced from Golgi and transported to the cytoplasmic face of the apical plasma membrane. The vesicles fuse with the plasma membrane at the base of the microvilli and release peritrophin-15 into the inter-microvilli spaces. The protein then becomes associated with the nascent peritrophic matrix, which lies along the tips of the epithelial cell microvilli. It is proposed that peritrophin-15 binds to the ends of chitin fibrils present in the nascent peritrophic matrix, thereby protecting the fibril from the action of exochitinases., (Copyright 2001 Wiley-Liss, Inc.)

Previous studies have shown that certain sulfated polysaccharides, such as dextran sulfate (DxS), can induce a rapid and sustained increase in the free intracellular calcium ion concentration ([Ca2+]i) of thymocytes, whereas they cannot induce a change in the [Ca2+]i of peripheral lymphocytes. This study examined this phenomenon in more detail and clearly demonstrated that only immature thymocytes can respond to DxS. Cortisone-resistant thymocytes failed to exhibit an increase in [Ca2+]i when exposed to DxS. Furthermore, analysis of [Ca2+]i changes at the single cell level using a flow cytometer demonstrated that only a subpopulation of immature thymocytes responded to DxS. Additional flow cytometry studies revealed that DxS and Con-A induce a [Ca2+]i response in different subpopulations of thymocytes, approximately 30% of thymocytes responding to Con-A and 15-20% to DxS. Thus, responsiveness to Con-A and DxS, in terms of increases in [Ca2+]i, may represent a novel marker for two different thymocyte subpopulations. Such metabolic markers could be used to complement the conventional serological methods currently employed to define stages in thymocyte differentiation.