Development of dermal connective tissue in human embryonic and fetal skin

The primordial dermis begins as a cellular mesenchyme and transforms into a fibrous, sparsely cellular network. At six weeks gestation a fine meshwork of collagen fibrils underlies the dermal-epidermal junction and adheres to the dermal mesenchymal cell surfaces. With increasing gestational age, the amount of extracellular collagen matrix increases, fibrils associate into collagen fiber bundles, cells are spaced further apart and their elongated axes tend to orient parallel to the plane of the skin. The fine collagen meshwork persists throughout development at the dermal-epidermal junctions and ensheathes epidermal appendages as they extend into the dermis. A similar network encases developing vessels. The overall thickness of the dermis increases from 0.1 to 0.15 mm at 7 weeks to greater that 0.7 mm at 20 weeks. By 16 weeks gestation the epidermal appendages extend deeply into the dermis and the dermis is organized into two regions: an upper, finely fibrillar papillary region and a deep reticular region that contains larger collagen fiber bundles. A vascular plexus separates the dermis from the hypodermis. The dermis of the 20-week fetus is adult-like in overall appearance, although it is still smaller in total thickness and size of collagen fibrils and fiber bundles than in the newborn skin.