Skin - Anatomy & Physiology
Development
Origin of the epidermis
Following neurulation, the precursor of the epidermis is formed from the cells covering the embryo. This then becomes 2 layers, the outer forming the periderm and the inner forming the basal layer or stratum basale. A further layer is formed from the basal layer called the spinous layer or stratum spinosum. The basal and spinous layers are termed the Malphigian layer, the cells of which then divide to produce the granular layer or stratum granulosum. As the cells of the granular layer differentiate and keratinise, they form the outer cornified layer or stratum corneum.
Structure and Function
Epidermis
The epidermis is a stratified squamous epithelium and is composed of 4 cell layers anchored to a basal lamina of connective tissue. Keratinocytes migrate through the epidermis from the basal layer. This migration begins in the stratum basale, then moves up through the stratum spinosum, stratum granulosum and the stratum corneum.
- Stratum basale
The deepest layer of the epidermis is the stratum basale. It consists of a single layer of columnar epithelium which is adhered to the basal lamina. Nutrition is supplied from the underlying capillaries of the dermis. Migration of cells towards the surface begins here.
- Stratum spinosum
Above the stratum basale is the stratum spinosum. Spinous cells are large polygonal cells with prominent desmosomal intercellular filaments. When cells move into this layer, they shrink and move apart. Keratinisation begins in the stratum spinosum and continues in the stratum granulosum.
- Stratum Granulosum
External to the stratum spinosum, lies the stratum granulosum. Granular cells are thin, flattened keratinocytes with variably prominent keratohyaline granules. At its outer surface, the stratum granulosum secretes oragnelles called lamellar bodies, which are composed of lipid and enzymes which fuse with the plasma membrane and help to form the water concentration gradient that leads to transepidermal water loss.
- Stratum Corneum
The outer layer, stratum corneum, consists of anucleate, thin cells filled with keratin filaments and proteins that form a cell envelope resistant to solvents and enzymes; there is also a permeability barrier to water and ions. The cells of the stratum corneum are embedded in a lipid-rich matrix which helps to hold the cells together, while the desmosomes that were present in lower layers have disappeared. For the stratum corneum to maintain a constant thickness, there is a constant turnover of exfoliated corneocytes, being replaced by new corneocytes. Enzymes in the lamellar bodies from the stratum granulosum help to break down the intercellular lipid 'glue' that holds the cells in place and there is also some degradation of desmosomes.
Cells of the Epidermis
- Langerhans' cells
These are dentritic cells derived from bone marrow. They act as antigen presenting cells in the epidermis as part of the mononuclear phagocyte immune system. Langerhans' cells express MHC I and II molecules and immunoglubulin Fc receptors and are a key feature of delayed hypersensitivity responses.
Dermis
Glands
- Sweat Glands
- Mammary Glands
- Sebaceous Glands