The integument is often said to be the largest organ in the body, comprising 16% of total body weight.
The skin consists of two layers, the epidermis, and the dermis.
The epidermis is a terminally differentiated stratified squamous epithelium, the major cell type of which is the keratinocyte.
Keratinocytes synthesize keratin, a protein-containing coiled polypeptides chains which combine to form supercoils of several polypeptides linked bydisulphide bonds between adjacent cysteine amino acids.
Stratum basale (basal cell layer)
This layer is generally only one cell thick.
The main cell type is the keratinocyte that may be dividing or non-dividing.
Melanocytes are present in the basal layer and make up 5-10%of the cell population.
Stratum spinosum (spinous or prickle cell layer)
Basal cells move towards the surface and form a layer of polyhedral cells which are connected by desmosomes.
Within this layer, Langerhans cells can be identified.
Stratum granulosum (Granular cell layer)
Keratinocytes in the granular layer contain intracellular granules of keratohyalin.
The cells discharge their lipid components into the intercellular space which plays an important role in barrier function and intercellular cohesion within the stratum corneum.
Stratum corneum (horny layer)
The cells (now called corneocytes) have lost their nuclei and cytoplasmic organelles, and appear flattened and the keratin filaments align into disulphide cross-linked macrofibres.
In palmoplantar skin, there is an additional zone, the stratum lucidum.
The cells found in this layer are still nucleated and are termed transitional cells.
The time from cell division to shedding from the horny layer is approximately 28 days, but this can be altered in various disease processes.
The dermis varies widely in its thickness being less than 1 mm thick on the eyelids but over 5 mm on the back, it's a tough, resilient layer that protects the body against mechanical injury and contains specialized structure.
The papillary dermis is the thin upper layer and the reticular dermis is the deeper of the dermis.
Being connective tissue, the dermis contains cells, ground substance and fibres.
The cells are fibroblasts that synthesize collagen and elastin fibres.
Collagen represents 75% of the dry weight and up to 30% of the volume of the dermis.
75% is type I collagen and 15% type III collagen.
The properties of collagen change both qualitatively and quantitatively with ageing.
Elastin Fibres are also present within the dermis and these provide a degree of elasticity to the skin.
Hair is also derived from keratin.
Anagen lasts approximately 1000 days in men and 2-5 years longer in women.
In catagen, the hair follicle degenerates, a process taking 2-3 weeks.
In telogen, the hair is shed and the hair follicle enters a resting phase that lasts 3-4 months.
Ten percent of hairs are usually in this phase leading to 50-100 hairs falling out on a daily basis.