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The Epidermis

Epidermis

The epidermis is made up of stratified squamous epithelium tissue, there are no blood vessels there and nourishment comes from interstitial fluid. The epidermis represents every transition from basal cells with well defined nuclei (the living part of the cell), to superficial flaky debris in which the nuclei and all evidence of cell structures have disappeared (the cell is dead). The epidermis lies on top of the dermis, a dense fibrous layer, into which it interlocks by a series of finger-like projections called papillae.

The epidermis is the outer covering of the skin and consists of the following 5 layers, each layer can either be called by its Latin name (starting with stratum or its more common name:

  1. The stratum corneum/horny layer
  2. The stratum lucidum/clear layer
  3. The stratum granulosum/granular layer
  4. The stratum spinosum/prickle layer
  5. The stratum basal/germinative layer

The stratum corneum or horny layer

This layer is the outer layer of the skin and forms the thickest part of the epidermis in many parts of the body. This surface layer of skin, consisting of 25 to 30 cells deep of flattened irregular dead keratinised cells. This layer of skin is a waterproof protective covering, which prevents excessive dehydration of the tissues, hence the term ‘horny’ layer. The cells contain an epidermal fatty material which keeps them waterproof and helps the skin from cracking and becoming open to infection. These cells are continuously shedding, this is called desquamating

The stratum lucidum or clear layer

This layer derives its name from its clear/translucent, almost transparent appearance. Only a few cells deep in the stratum lucidum lie between the outer horny layer and the inner granular layer. This layer lies below the stratum corneum and is 4 cells deep with a narrow transparent layer consisting of flattened and closely packed cells, with an indistinct outline and no nuclei hence the term clear layer. This layer is more pronounced in thick hairless skin eg on the palms of the hands and soles of the feet. It is thought that this layer is the site of the barrier zone controlling the transmission of water through the skin.

 

The stratum granulosum or granular layer

This is the third layer of the epidermis and is between one and three cells deep, consisting of flattened spindle shaped cells. This layer forms the main part of all horny tissue. The nucleus begins to break down here (basically the cell dies here) and granules of keratin (protein found in the skin) appear, hence the term granular layer. Keratinisation is the change of living cells into dead horny flat cells with no nucleus. Loss of fluid is an essential process in the stages of keratinisation and the stratum granulosum cells are believed to represent the first stage in the transformation of the epidermal cells into the horny keratin material. The thickness of this layer may vary from one to several cells deep and is thickest on the palms of the hands and the soles of the feet.

The stratum spinosum or prickle cell layer

This layer is 8 cells deep with round nucleated cells, which vary in size and shape and produce fibrils which interconnect them with neighbouring cells, hence this is often referred to as the prickle cell layer. The prickle cell layer is often classed with the stratum germinativum to form the malpighian layer, the living layers of the skin

The stratum basal or germinative layer

This is the deepest section of the epidermis and is in contact with the dermis from which it derives its nutrient fluids via the capillary blood vessels. Within this layer cells are one cell deep, rounded and clearly defined with a nucleus and rest on the papillae of the dermis. These epithelial cells are able to divide and reproduce themselves by a process called mitosis hence the term ‘germinative’. As these cells form they push themselves up towards the skin surface . As the development of new cells it leads to a gradual displacement of the older cells towards the surface. The epidermis is a reproductively self-sufficient system. For example, it generates entirely from the cells resident within it. The basal layer is at its most productive between midnight and four am. This could explain the term ‘beauty sleep’. Within this layer there are also melanocyte cells, which produce the pigment melanin, which gives both the hair and skin its colour density. Melanocytes, melanin forming cells, are found in abundance in the stratum germinativum. One in every ten cells is a melanocyte. Langerhan cells are also found within this layer and absorb and remove foreign bodies that enter the skin.