A DESCRIPTION OF THE MUSCLE NET SURROUNDING THE DIGESTIVE EPITHELIUM IN THE MIDGUT GLAND OF THE LOBSTER HOMARUS AMERICANUS

Abstract

The organization of the muscle cell layer surrounding the tubules of the midgut gland in Homarus americanus was examined with transmission and scanning electron microscopy. This layer contains a uniform network of broad circular myofilaments interconnected by smaller longitudinal connectives. The primary means of constriction of the lumen of the tubule appears to be by contraction of the circular muscle fibers. The longitudinal myofilaments may provide both structural support and a means for conducting the contractile impulse along the tubule. The decapod hepatopancreas, or midgut gland, is a multilobed organ comprised of numerous blindly ending tubules, connected by a graduated series of collecting ducts. The collecting ducts coalesce into two main ducts which articulate at the pyloric-intestinal junction. As the primary organ for digestion and absorption of nutrients, both digestive enzymes and the products of digestion are moved through the tubules; the food brei moves from the stomach to the tubules and the enzymes and digestive waste products return from the tubules to the stomach. In a general sense, the midgut gland may be equated to the vertebrate small intestine with associated digestive organs, while the true decapod intestine may be equated with the large bowel. Although Gibson and Barker (1979) have published a thorough review on the hepatopancreas, including the basic cell types, sequence of digestive epithelial cell development, and enzymatic activity of the cells involved in both extracellular and intracellular digestion, the mechanism for moving the enzymatic secretions out and the food solution into the tubules of the hepatopancreas is poorly known and often casually dismissed. For example, Stanier et al. (1968) described only a discontinuous stellate network of contractile fibrils around each tubule of Carcinus maenas, and Young (1959) did not mention muscular activity associated with the hepatopancreas of Penaeus setiferus. The musculature associated with the tubules of the midgut gland in Astacus fluviatilis were reported as a series of parallel circular fibers of striated muscle with smaller longitudinal connectives (Pump, 1914). The striated muscle found in the hepatopancreas of crayfish was described as tonic muscle (Loizzi, 1971). The contractile function of tonic invertebrate striated muscle can be equated to that observed in vertebrate intestinal smooth muscle (Prosser, 1973). Van Weel (1974) speculated that the action of the muscle fibers surrounding each tubule of the midgut gland was similar to the squeezing and releasing of a rubber syringe during filling and emptying. This study examines the musculature around the tubule of the midgut gland in the American lobster Homarus americanus, confirming Pump's (1914) work at the level of scanning electron microscopy and provides indirect evidence for a peristaltic transport mechanism to move materials within the tubules of the hepatopancreas.