Ultrastructural localization of the site of hemoglobin synthesis in Chironomus thummi (Diptera)

Abstract

Light and electron microscopy illustrate that the fat body of 4th instar larvae of Chironomus thummi is composed of a loose association of homogenous cells. The fat body is surrounded by a capsule-like basal sheath. Cells immediately adjacent to the basal sheath sometimes display digitiform surface amplifications. Fat body cells contain numerous lipid droplets and glycogen deposits, evidence of storage function. The presence of glycogen was verified by the periodic acid-thiocarbohydrazide-silver proteinate cytochemical technique. Additionally, the synthetic capacity of fat body cells is indicated by the presence of abundant rough endoplasmic reticulum (RER) and free polysomes. The localization of hemoglobin pseudoperoxidase activity was shown by means of a 3-3′-diaminobenzidine cytochemical technique. Reaction product was found in cisternae of RER, perinuclear cisternae. Golgi complexes, and Golgi-derived vesicles of all fat body cells examined. Autoradiography demonstrates that the heme precursor, δ-aminolevulinic acid, is incorporated into a fixable tissue product. This study confirms and expands upon previous biochemical studies showing that the fat body is the major site of hemoglobin synthesis in C. thummi .