© 1979 by The Society for Integrative and Comparative Biology
Cellular Development of the American Lobster Heart
Department of Biology, Clark University Worcester, Massachusetts 01610
The heart of the American lobster, Homarus americanus, represents a relatively simple nerve-muscle system. A nine-celled cardiac ganglion functions to initiate the contractions of the transversely striated myocardium. To describe the development of this nerve-muscle system, hearts of lobster embryos in different stages of development have been examined with an electron microscope. The heart wall consists of an outer adventitial layer of fibroblast cells and an inner layer of transversely striated myocardial cells. Adjacent fibroblast cells form fasciae adherent and gap junctions. Intercalated discs and membrane regions of close apposition (4 nm) occur between adjacent myocardial cells. A cardiac ganglion that contains nine neurons and synaptic neuropil is present. Neuromuscular synapses occur in the myocardium. Cells that we have termed storage cells contain dense cytoplasmic inclusions that resemble yolk material and form gap junctions with myocardial cells. The heart at six weeks is about 200 µm long and 160 µm wide; at six months it is 300 and 250 /m respectively. The myocardium grows by an increase in the size and in the number of myocardial cells. The myocardium contains pre-myoblast, myoblast and well-differentiated myocardial cells. Sarcomere formation occurs first adjacent to the inner surface of the sarcolemma where the myofilaments align and anchor. Distinct A and I bands appear first, followed by the Z lines. The myocardium is sparsely innervated when the initial heartbeats occur; a preliminary quantitative analysis has shown that three days later, the density of innervation has increased six-fold.