Cellular Development of the American Lobster Heart

The heart of the American lobster, Homarus americanus, representsa relatively simple nerve-muscle system. A nine-celled cardiacganglion functions to initiate the contractions of the transverselystriated myocardium. To describe the development of this nerve-musclesystem, hearts of lobster embryos in different stages of developmenthave been examined with an electron microscope. The heart wallconsists of an outer adventitial layer of fibroblast cells andan inner layer of transversely striated myocardial cells. Adjacentfibroblast cells form fasciae adherent and gap junctions. Intercalateddiscs and membrane regions of close apposition (4 nm) occurbetween adjacent myocardial cells. A cardiac ganglion that containsnine neurons and synaptic neuropil is present. Neuromuscularsynapses occur in the myocardium. Cells that we have termedstorage cells contain dense cytoplasmic inclusions that resembleyolk material and form gap junctions with myocardial cells.The heart at six weeks is about 200 µm long and 160 µmwide; at six months it is 300 and 250 /m respectively. The myocardiumgrows by an increase in the size and in the number of myocardialcells. The myocardium contains pre-myoblast, myoblast and well-differentiatedmyocardial cells. Sarcomere formation occurs first adjacentto the inner surface of the sarcolemma where the myofilamentsalign and anchor. Distinct A and I bands appear first, followedby the Z lines. The myocardium is sparsely innervated when theinitial heartbeats occur; a preliminary quantitative analysishas shown that three days later, the density of innervationhas increased six-fold.