Telocytes in endocardium: electron microscope evidence

Abstract

The term TELOCYTES was very recently introduced, for replacing the name Interstitial Cajal-Like Cells (ICLC). In fact, telocytes are not really Cajal-like cells, they being different from all other interstitial cells by the presence of telopodes, which are cell-body prolongations, very thin (under the resolving power of light microscopy), extremely long (tens up to hundreds of micrometers), with a moniliform aspect (many dilations along), and having caveolae. The presence of telocytes in epicardium and myocardium was previously documented. We present here electron microscope images showing the existence of telocytes, with telopodes, at the level of mouse endocardium. Telocytes are located in the subendothelial layer of endocardium, and their telopodes are interposed in between the endocardial endothelium and the cardiomyocytes bundles. Some telopodes penetrate from the endocardium among the cardiomyocytes and surround them, eventually. Telopodes frequently establish close spatial relationships with myocardial blood capillaries and nerve endings. Because we may consider endocardium as a 'blood-heart barrier', or more exactly as a 'blood-myocardium barrier', telocytes might have an important role in such a barrier being the dominant cell population in subendothelial layer of endocardium.

Fig 1

Digitally coloured electron micrograph of mouse atrial endocardium shows a telocyte (TC, blue) beneath endothelial cells (End, burgundy). CM, cardiomyocytes; rbc, red blood cells; el, elastic fibres; coll, collagen fibres.

Fig 2

Digitally coloured electron micrograph of mouse ventricular endocardium (burgundy). Telocytes (coloured in blue) make an interstitial network in the heart. Subendocardial telocytes (TC₁) send telopodes between cardiomyocytes (CM) and communicate with myocardial telocytes (TC₂). Cap, capillary.

Fig 3

Digitally coloured electron micrographs of mouse atrial endocardium (End, burgundy). Small amount of elastin (el) is visible beneath the endothelial cells (burgundy). (A) Electron micrograph of mouse atrial endocardium shows a 30-μm-long telocyte (TC, blue) with two thin (50–500 nm) telopodes (Tp) beneath endothelial cells of endocardium. (B) Telopode with dichotomous pattern of branching (arrow). (C) Telopode with a sinuous segment having less than 50 nm thickness. (D) Dilatation of telopode hosting mitochondria (m), endoplasmic reticulum (er) and caveolae (arrowhead). (E) Telopode in close apposition with a nerve ending (n); CM, cardiomyocytes.

Fig 4

(A,B). Digitally coloured electron micrographs of mouse atrial endocardium (End, burgundy). Square marked area in A is enlarged in B. A gap junction (arrows) connects a subendocardial fibroblast (Fb) with a telopode (Tp) of a myocardial telocyte. The fibroblast has large rough endoplasmic reticulum (rer). The nerve endings (n) could be observed in between telopode and fibroblast; CM, cardiomyocytes.