Fibroblasts are the most common cells in connective tissue proper. They are elongated or stellate shaped and produce both fibers and ground substance. These images compare connective tissues in which the majority of the fibroblasts are active (upper panel) with one in which the fibroblasts are inactive. The cytoplasm of fibroblasts is not always visible by light microscopy, and thus, cells must be identified primarily by their nuclei. 1000x, 1000x
Active fibroblast nuclei >
The nucleus of an active fibroblast is oval-shaped and mostly euchromatic. The cytoplasm is difficult to distinguish as it blends with the surrounding collagen fibers. These cells are actively producing the extracellular matrix around them.
Inactive fibroblast nuclei >
The nucleus of an inactive fibroblast is elongated, highly heterochromatic and flattened between collagen bundles. The sparse cytoplasm can be seen extending beyond the ends of the nucleus. These cells are mostly quiescent with only minimal extracellular matrix production.
Collagen bundles >
The extracellular matrix surrounding active fibroblasts consists of ground substance containing highly interlaced collagen bundles of various sizes. The matrix surrounding inactive fibroblasts contains large, parallel bundles of collagen with very little ground substance.