The Dermis (Corium)
The dermis (derm = skin/Gk also corium) forms a well-defined border with the epidermis and a less well-defined border with the subcutis (subcutaneous fatty tissue (i)).
The stratum papillare forms a well-defined, wave-shaped border with the epidermis.
1 Stratum papillare
2 Basal membrane
3 Basal cells
Formation and function of the dermis
Stratum papillare and stratum reticulare
The stratum reticulare (stratum = covering, layer; reticular = net-like/Lat.) makes up the lower part of the dermis and shows a continuous transition to the subcutis below. The stratum papillare (papillae = protuberance/Lat.) is the upper layer. It forms the sharp, wave-shaped border with the epidermis. The wavy structure increases the contact area with the epidermis, thus ensuring optimal nourishment of the deepest epithelial layer of the epidermis - the basal cells - by way of the blood vessels running through the papillae.
The connective tissue of the dermis
The main constituent of the dermis is the proteinous connective tissue made up of arc-shaped, elastic fibres and undulated, nearly inelastic collagen fibres. These are responsible for the high elasticity and tensile strength of the dermis.
Young collagen fibre-glycosaminoglycan (i) complexes can bind large amounts of water and so determine the high intrinsic tension of young skin. As the skin ages, the interweaving of the collagen fibres increases and the water-binding capacity decreases. The skin tends to wrinkle.
Connective tissue, glycosaminoglycans and water-binding capacity
The interfibre space of the dermal fibre network contains a sort of "filling" made of long chains of sugar molecules (polysaccharides; poly = many, sacchar = sugar/Gk.). These are known as glycosaminoglycans (also mucopolysaccharides). With the help of fibronectins, a type of "glue", they bind to the proteinous connective tissue matrix to form proteoglycans, which can bind water molecules. This gel-like mass functions like a sponge. Under pressure it can expel the bound water and in a reverse process take it up again. This process probably helps supply the dermis with nutrients.
Hyaluronic acid (hyalo = glass/Gk.) belongs to the group of glycosaminoglycans and so is part of the water-binding unit of the connective tissue. Glycosaminoglycans are constantly being produced and degraded. In contrast, new collagen fibres are produced only when needed, such as when the skin is injured.
Other constituents of the dermis are various types of cells such as the fibroblasts, mast cells and other tissue cells, as well as numerous blood and lymph vessels, nerve endings, hot and cold receptors as well as tactile sensory organs.