Electron Microscope Study of Mammalian Cardiac Muscle Cells

Abstract

The cellular theory of heart muscle is supported by a detailed description of the intercalary discs. The discs are adjacent plasma membranes separated by an interspace while the sarcolemma appears as plasma membrane, interspace plus basement membrane of the interstitium. The nucleus of the cell is closely associated with the entire cell by way of the endoplasmic reticulum. Transversely it connects the outer nuclear membrane at the level of the Z and M bands with the contractile material and the sarcolemma. Longitudinally it connects the outer nuclear membrane with the plasmalemma at the intercalated discs. The description of the spiral attachment of the endoplasmic reticulum on the outer nuclear membrane supplements earlier observations on the helicoidal structure of the heart muscle cell. Plasma membranes and endoplasmic reticulum are considered to be carriers of membrane potentials and to conduct excitation.

ER-plasma membrane junctions: Why and how do we study them?

“…To transport lipids and Ca 2+ for cell homeostasis and signaling, the ER forms minute membrane junctions with other organelles, where the ER membrane closely apposes other membrane compartments within a 30 nm gap distance [5, 6]. Evidence of ER-organelle junctions was first demonstrated in classical electron microscopy (EM) studies sixty years ago [7, 8]. Recently, these ER-organelle junctions have been shown to support inter-organelle signaling between the ER and the trans-Golgi apparatus, mitochondria, endosomes, and the plasma membrane (PM) in yeast and in mammalian cells [9–24].…”

The Sarcoplasmic Reticulum

“…That the rest of the sarcoplasmic reticulum, apart from the T system, represents the ER of the muscle cell has been generally assumed and is sup-ported by evidence of connections between it and the nuclear envelope (30), by the morphology and distribution of the component parts, and by the character of the limiting membrane. It, unlike the plasma membrane, is only about 50 A thick and does not appear to show the three-layered structure of Robertson's unit membrane.…”

“…These early observations were soon supple-merited by increasingly convincing and detailed images--products of greatly improved preparation techniques, and it was in the following period of 2 or 3 years that most of the major structural features of the reticulum were described (1,16,27,30,42). Thus there emerged a picture of the component as a lacework of tubules and vesicles within the sarcoplasrn between the myofibrils.…”

A Brief History of Tissue Models For Cardiac Electrophysiology

“…One of the early cardiac histologists, H. Jordan noted in 1911 that "the designation of cardiac muscle as a tissue of discrete cells or as a syncytium depends upon the interpretation given to the so-called intercalated disks or junctional lines (also, bands of Eberth), i.e., whether or not they mark cell boundaries" [5]. With the improvement and increased use of electron microscopy in the 40s and 50s, there was growing evidence that cardiac tissue was not a syncytium but rather was composed of discrete, rod like cells, bounded by a membrane, which in places was continuous with the intercalated disk [6]- [8]. Analyses of the images revealed that the cell membrane was greatly folded in the region of the intercalated disk and spacing between cells between opposing faces of the disk was tighter than for nondisk membrane [6].…”