A study of the metabolism of 20-hydroxyecdysone in Homarus americanus

Abstract

De-eyestalking female intermolt lobsters brings them to the premolt (D_0) stage in an average of 66 days at 18°C. The use of microcrystalline collagen hemostat, Avitene, greatly reduces mortality during the de-eyestalking procedure. ln vivo incubation of labeled cholesterol for 5 days permits its ramification into at least 7 labeled metabolites. Evidence is suggestive that one of these is 7-dehydrocholesterol. ln vivo label and in vitro chase experiments with specific tissues suggest that intermolt antennae, the yellow body, the heart, and the ovary have a cholesterol turnover that exceeds their premolt analogs suggesting that conversion of cholesterol to metabolites is accelerated in these organs and tissues in the intermolt state. The same experiments indicate that cholesterol metabolism is more rapid in premolt Y-organ, hepatopancreas, mandibular gland, green gland, gills, and muscle. The neuronal metabolism of cholesterol was little influenced by molt cycle. The concept that the conversion of cholesterol to ecdysones might be accelerated in some. tissues in the intermolt period is strengthened by the results of hemolymph titer determinations for the two ecdysones, alpha-ecdysone and 20-hydroxyecdysone. Both hormone titers peak in the intermolt period in the de-eyestalked lobster. The hemolymph titer of a-ecdysone peaks before the hemolymph titer of 20-hydroxyecdysone, supporting the proposition that a-ecdysone is the precursor of 20-hydroxyecdysone. Both hormones return to low levels by the time the animal enters the premolt stage. The gross morphology of the mandibular gland has many characteristics of an endocrine gland. Ultrastructural changes in the gland suggest changes with the position in the molt cycle. Ramification of smooth endoplasmic reticulum and the appearance of lipid droplets suggest a steroidogenic or lipid synthetic ability. Two different cell types are present and each may have a unique function. Dark-staining, compact cells possess more steroidogenic characteristics while larger, light-staining cells demonstrate lipid droplets. The nature and mechanism of secretion is not known. The morphology of the Y-organ is seen to vary with the molt cycle. The nuclei of the cells become light-staining due to appearance of euchromatin in the premolt gland. The cells Y-organ cells apparently serve a holocrine secretory function. The yellow body is a novel tissue described here for the first time. It also appears to serve an endocrine function. Cells located at the free border of the tissue contribute apocrine secretion to the hemocoel. The activity of the yellow body may fluctuate with the molt cycle as indicated by development of nuclei with diffuse chromatin during the premolt stage. Tegumental glands are noted in both Y-organ and the yellow body. These glands are thought to contribute to the maintenance of the exoskeleton. Their morphology is altered during the molt cycle with the glands becoming atretic as the animal approaches molt. New glands support the new shell.