A) The Skin


The skin covers the external surface of the body. It is continuous with the mucosa of the nasal cavity, oral vestibule, conjunctiva, and the skin of the genital regions. The genital skin is typically considered as belonging to those organs (penis, vulva) than the rest of the skin.

Microscopic Appearance


The epidermal layer is made up of keratinising stratified squamous epithelium, and usually stains darker than the dermis. The stratified epithelium can be divided into layers:

  • The stratum basale is the deepest layer and lies against the basement membrane. Cuboidal or columnar basal cells are found in this region.
  • The stratum spinosum is characterised by polygonal cells with intervening clefts. The cells digitate in these clefts through cytoplasmic extensions, creating the ‘spine-like’ appearance that gives the layer its name.

Important: This appearance, known as intercellular bridging, is an important feature of squamous cell carcinoma.

  • The stratum granulosum varies in size between thick and thin skin. The cytoplasm of cells in this layer contains fine grains on light microscopy.
  • The stratum lucidum is a thin layer formed by thin, dead cells with no nuclei. This layer is usually not visible in thin skin.
  • The stratum corneum is the most superficial of the layers, containing thin dead cells containing keratin. The cells are linked together with a lipid material making identification of individual cells difficult. The most superficial layers are more loosely connected.

Melanocytes, T-lymphocytes and Langerhan’s cells are also found within the epidermis, usually in the basal layer. The melanocytes and Langerhan’s cells have small cytoplasmic processes which travel some distance from the cell body.



The dermis is a thick connective tissue layer beneath the epidermis. There are two distinct layers:

  • The papillary layer (papillary dermis) is more superficial and forms papillae which protrude into the epidermal layer. It contains numerous capillaries which supply the avascular epidermal layer.
  • The reticular layer (rete dermis) is denser and less cellular than the papillary layer, and contains thick collagen fibres. These fibres usually run parallel to the surface of the skin and are arranged along lines of greatest tension. The reticular layer contains larger blood vessels which feed the capillary network of the papillary layer.

The dermis contains blood vessels, lymphatics, nerves and the skin appendages.

Variations in the Skin


The epidermis serves as an impervious barrier to fluid and infective agents.
The dermis supports the epidermis with nutrient vessels. It is also involved in thermoregulation.
Sweat glands have several functions.

  • Apocrine glands secrete oily residue and are found in the axilla and groin
  • Merocrine glands secrete normal sweat which is used for thermoregulation

Generation and Differentiation of Keratinocytes

Skin keratinocytes, the cells which make up the epidermal layer, are all generated within the stratum basale by division of the basal cells. One daughter cell remains along the basement membrane, whereas the other enters the stratum spinosum.
Within this layer, the keratinocytes form the cytoplasmic digitations with their neighbours as they move superficially. Within these digitations, the cells form lamellar granules containing a lipid material.
Also within this layer and the stratum basale, melanocyte dendrites distribute melanin granules to the maturing keratinocytes.
Within the stratum granulosum, the cells begin to form keratin, which appears as fine grains. The lamellar granules are released into the clefts, sealing the gaps between the cells and providing an impervious barrier.
The nucleus is fully degenerate within the stratum lucidum and the cells are ‘dead’.
The cells gradually progress upwards through the stratum corneum, and eventually the lipid membrane between them breaks down. The cells are then shed, to be replaced by other cells progressing through the maturation cycle.
The process takes approximately 3 – 4 weeks to complete.