Endocardium

The endocardium is a thin inner lining overlying the myocardium. The simple squamous endotheliumThe innermost layer of blood vessels, composed of simple squamous epithelial cells, which reduces f appears as a delicate sheet of flattened cellsThe basic structural and functional units of life. with dark, elongated nucleiClusters of neurons in the CNS responsible for processing information. that rest directly on the connective tissue beneath. This layer provides a smooth surface in direct contact with the blood and reduces friction of the flowing erythrocytes(RBCs): Red blood cells responsible for oxygen transport..

Beneath the endothelium lies the subendothelial connective tissue, which stains blue/green due to its collagenA structural protein in the dermis that provides strength and elasticity. content. This region varies in thickness depending on location and may contain small elastic fibers and capillariesThe smallest blood vessels where gas, nutrient, and waste exchange occurs between blood and tissues.. At 100x, it appears looser and paler than the dense myocardium it overlies. In some sections, you can also see small bundles of Purkinje fibers embedded near the junction with the myocardium—these appear larger, paler-staining cells compared to the surrounding muscle fibers.

Overall, the endocardium contrasts with the red-stained myocardium beneath it: it is thinner, more delicate, and dominated by collagenous connective tissue with its endothelial covering. Masson’s trichrome makes this distinction especially clear, highlighting the structural layering that ensures both a smooth blood-contacting surface and a supportive framework for conductionThe transmission of nerve impulses along neurons. fibers.

Heart, ventricle, H&E, 40X  Slide 098-1 Right heart wall, H&E, 40X
All by University of Michigan Histology, licensed under CC BY-NC-SA 3.0.