MIDGUT-GLAND DEVELOPMENT DURING EARLY LIFE-HISTORY STAGES OF THE AMERICAN LOBSTER HOMARUS-AMERICANUS
Currently there are at least 2 hypotheses regarding function of the different cell types in the midgut gland of decapod crustaceans. Both agree that E-cells are undifferentiated cells which mature into the other cell types. One hypothesis holds that F-cells synthesize digestive enzymes and subsequently differentiate into B-cells, which, in turn, secrete the enzymes into the midgut-gland lumen. R-cells are believed to function in the absorption of digested nutrients, intracellular digestion, and storage of lipid and glycogen, among other roles. The alternate hypothesis states that F-cells synthesize and secrete digestive enzymes and then take up partially digested material to yield B-cells, which are involved in intracellular digestion. B-cells are later discharged into the lumen; R-cells subsequently absorb nutrients from the gut lumen and store lipid and glycogen. In the present study, the midgut glands of embryos, prelarvae, and stage I larvae of the American lobster Homarus americanus were examined histologically. R-cells of all the early developmental stages sampled had only 1 or 2 lipid vacuoles, presumably representing resorbed yolk lipids, in contrast to the numerous lipid vacuoles in R-cells of adult lobsters. E-, R-, and F-cells, but not B-cells, were present in embryos sampled approximately 3 days before their siblings hatched as stage I larvae. B-cells had developed in prelarvae sampled approximately 12 h before their siblings hatched and also occurred in prelarvae sampled approximately 3 h before hatching and in newly hatched stage I larvae. B-cell numbers increased by the time stage I larvae reached intermolt, regardless of whether they were fed or starved. Since embryos, prelarvae, and newly hatched larvae have not ingested exogenous nutrients, the presence of B-cells in these early developmental stages was triggered by something other than uptake of partially digested food and is probably genetically programmed. These results provide indirect evidence supporting 1 hypothesis regarding midgut-gland cell function: that F-cells mature into B-cells before, rather than after, digestion has begun.