The skin is the largest organ of the human body. It protects us from the invasion of microorganisms and harmful radiation from the sun. It is also essential in preventing dehydration and regulation of body temperature. There are two types of skin in humans: most frequently thin skin, but also thick skin covering hands palms, ventral surface of fingers and feet soles.
On a microscopic level, the skin is divided into three main structures:
The epidermis, forming the outermost layer, is in humans on average 50 μm thick. It is highly cellular (50,000 cells per mm2), which results in a darker staining than the underlying dermis. Thin skin epidermis is further stratified into 4 layers, listed from the deepest to the most superficial:
- The stratum basale or stratum germinativum
- The stratum spinosum
- The stratum granulosum
- The stratum corneum
The dermis, forming the tissue beneath the epidermis, consists primarily of dense connective tissue, which results in a lighter staining than the overlying epidermis. Separated from the overlying epidermis by the basement membrane, the dermis is the vascularized connective tissue that provides nutritional and structural support. It is composed of a mucopolysaccharide gel fortified by collagen and elastin fibers. It contains different cells (fibroblasts, dermal dendritic cells, macrophages and mast cells), as well as different structures (vessels, nerves, and skin adnexa). The dermis can be roughly divided into:
- Papillary dermis, an upper located layer consisting of finer fibrillar collagen
- Reticular dermis, a deeper located layer with dense collagen fibers.
The hypodermis or subcutis, forming the deepest layer and separating the skin from underlying organs. The hypodermis consists of mature adipose tissue arranged in lobules separated by thin fibrous septa.
Skin also contains adnexal structures: hair follicles, nails, eccrine and apocrine sweat glands, sweat ducts, and sebaceous glands. Hair follicles include sebaceous glands and are connected to arrector pili muscle fibers. The development of skin appendages is based on epithelial-mesenchymal interactions to produce miniature organs, containing cells that follow specialized routes of differentiation, to add important functions to the skin.