Fibroblast cells. Coloured transmission electron micrograph (TEM) of a section through mammalian fibroblast cells. These cells synthesise tropocollagen. This sub-unit of the structural protein collagen is formed into collagen fibres outside the cell. Collagen provides high tensile strength in tissues like tendons, and forms up to one third of the total body protein of humans. Each cell contains a nucleus (blue circle) and a cytoskeleton (green). Fibroblasts are found in the connective tissue of vertebrate animals. Magnification unknown.