Lifetime ultrastructural characteristics of fibroblasts and collagen. The dermis in the skin of 15-day-old rats is occupied with numerous elongated fibroblasts (a). The extracellular matrix is poorly organized, and sparse thin collagen bundles are visualized between fine fibroblast extensions (arrows). The mature skin dermis in adult 3-month-old rat (b) consisted of large fibroblasts with well-developed rough endoplasmic reticulum (ER). Dilated ER cisterns are filled with proteinaceous material; the extracellular matrix is well organized, showing compact and regularly oriented collagen bundles (b). In the dermis of 21-month-old rat, the fibroblasts reduced in number and their long cytoplasmic extensions wrap large collagen bundles with thicker and tightly packed, but poorly oriented and electron-lucent, fibers (c). Magnification: semifine section, toluidine blue staining: ×100, orig.; framed area analyzed on transmission electron microscopy, magnification: ×3000, orig.; far right: ×8800, orig.