Fibroblast EM

This electron micrograph shows several fibroblasts, which constitutively secrete collagen. Note the large nuclei, with heterochromatin and euchromatin, as well as the abundant rough endoplasmic reticulum. One of these cells is actually in the process of releasing tropocollagen into the extracellular matrix. This will aggregate to form parallel collagen fibers. Fibroblasts also secrete proteoglycans and precursors of elastic fibers.