Defining the myofibroblast: normal tissues, with special reference to the stromal cells of Wharton's jelly in human umbilical cord.

Cells differing widely in tissue distribution, immunophenotype and ultrastructure have been described as myofibroblasts. The definition of the myofibroblast was analysed as applied to normal tissues, with original observations on Wharton's jelly stromal cells as an example. Stromal cells in Wharton's jelly were studied by conventional histology, immunohistochemistry, and electron microscopy. The normal architecture of the cord was confirmed by light microscopy. Stromal cells and the smooth-muscle cells of the umbilical vessels were positive for vimentin, desmin and alpha-smooth muscle actin, while only the stromal cells were positive for prolyl 4-hydroxylase. Electron microscopy revealed variable but sometimes only moderate amounts of rough endoplasmic reticulum, bundles of smooth-muscle type filaments with focal densities, a large Golgi apparatus with collagen secretion granules, lipid and glycogen. There was no convincing evidence for either lamina or fibronexus junctions. The nature of the stromal cell was discussed in the light of these findings. It was concluded that a myofibroblastic designation was inappropriate and that these cells had phenotypic similarities to vascular smooth muscle cells. The possibility is proposed that most examples of spindle cells cited in the literature as being myofibroblasts and seen in normal tissues not subjected to trauma or showing pathology may be pericytic or smooth-muscle in nature.