The Skin

Weighs approximately one ninth of the individual’s total body weight. It is a multi-layered membrane which covers, contains and protects the delicate tissue of the body. Its composition is complex, its functions – numerous. It is the sensory organ of touch and an organ of absorption, excretion and respiration.

It is composed of two basic layers: Epidermis and Dermis.

The degree of its attachment to the underlying tissue varies according to the location. At the palms and soles it is tight. At the trunk and joint flexure aspects it remains relatively loose. Its degree of thickness varies.

THE EPIDERMIS

The skin’s outer protective covering is cellular and avascular.

  • It consists of multi-layers of stratified epithelium with varying degrees of keratinisation. The surface cells are continually being worn away.
  • It has a thickness of 0.05 – 0.1 mm in most regions but somewhat thicker on the back and significantly more so on callus regions.
  • It arises from embryonic ectoderm (undeveloped outermost germ layer of the embryo) and forms: the pilo sebaceous follicle (the hair and the sebaceous glands), and the nails which are however nourished by the Dermis into which they distend.
  • It perpetuates itself by the proliferation of cells which modify via degrees of keratinisation. This is illustrated by the following table of the five layers (strata) through which cells pass commencing from the innermost nucleated basal layer and finally reaching the uppermost squamous keratinised layer which is subsequently shed. A process taking 14-21 days. The existence of the Stratum Lucidum is contested by some authorities.
  1. Stratum Basale (the germinating layer) the lowest layer which consists of a single layer of roughly cylindrical cells with basophilic cytoplasm (cellular substance surrounding the nucleus containing large granules that stain with basic dyes) and elongated nuclei. These cells are joined to the base membrane which forms the junction between the dermis and epidermis. These Cells divide at their upper extremes by mitosis forming the second layer Stratum Spinosum (Prickle Cell layer).
  2. Stratum Spinosum (Prickle Cell layer) consists of 2-5 rows of polyhedral cells which become more flattened as they pass through each of the following strata during their journey towards the skins surface. St. Spinosum takes its name from the spiny intercellular bridges (dentrites) which are visible during electron microscopic examination. Above this is the:
  3. Stratum Granulosum (granular layer) consists of several layers of diamond -shaped cells. These have nuclei which appear to be packed with microscopic granules of keratohyalin – the precursor of the protein filaggrin (filament-aggregating protein), which is believed to be associated with keratin distribution. This layer is most highly developed at the palms and soles.
  4. Stratum Lucidum (transparent layer) a thin layer of a-nucleated cells due to the presence of hydrolysing enzymes (lysomal enzymes) capable of breaking down most of the constituents of living matter. Lysomes are present in all animal cells, and are particularly abundant in white blood cells.
    The existence of the St Lucidum as a separate layer is debatable.
  5. Stratum Corneum (layer of dead cells) consists of numerous layers of flat, a-nucleated, dead, keratin-rich cells, varying in thickness according to the region of the body. Thickest at the palms and soles at which sites the cells seem denser and more compact to withstand pressure. In other regions the cells are looser and flexible. Keratin renders the skin tough, pliable and relatively impermeable. 

THE DERMIS

The Dermis (syn’s. Corium, Cutis Vera) provides/includes:

  • Protection  A structure of tough resilient tissue made up of strong fibres viz: collagen and elastin (proteins) which cushions/protects underlying tissue/organs plus   adipose tissue, arteries and veins, fibrous tissue, hair follicles, lymphatics and nerves, sebaceous glands, sweat glands and ducts which protect against mechanical injury. 
  • Nutriment for the epidermis and its cutaneous appendages viz Nails and Hair follicles.
  • Respiration : via Sudiferous (sweat) glands and ducts.
  • Lubrication : via the production of oily sebum from Sebaceous glands.
  • Lymphatics : lymph nodes (a filtering system) and lymph vessels utilised in the conveyance of Lymph a transparent or slightly       opalescent fluid also containing white   blood cells (chiefly lymphocytes) and a few red blood cells.
  • Lymphocytes (syn. lymph cells): white blood cells formed in lymphoid tissue constituting 25-33% of all white blood cells in    adult peripheral blood.
  • Macrophages (syn. Neutrophils) – large white cells which ingest bacteria, foreign particles, cellular debris, degenerated cells and blood tissue.
  • Melanocytes : pigment cells within the skin which produce melanin.
  • Melanophages : phagocytic cells which engulf particles of melanin.
  • Nerves (somatic afferent nerves – sensory nerves) cord like structures carrying impulses from the periphery, muscles and joints to the brain and spinal chord. The supply is via a network of interlocking fibres surrounding the upper part of the follicle forming a collar. Fibres extend to the sebaceous glands, epidermis, arrector pili and sweat glands. These nerves (both motor and sensory) contain cholinesterase an enzyme found throughout body tissues which acts to remove acetylcholine at neuro-muscular junctions.
  • The Blood Supply is via arteries paired with veins, these enter the lower regions of the dermis and rise to supply the pilo-sebaceous follicles and the sub-epidermal network which also supplies the epidermis.
    Blood flow within the skin operates as an aid to vital respiration. The smallest arteries have powerful muscular walls enabling the short-circuiting of capillary circulation to prevent body heat losses.
  • The Skin Colour is associated with melanin production, carotene, capillary dilation or constriction, hormonal activity.

    Flora and Fauna. viz bacteria, fungi, lice, scabies, demodex follicularum hominis. 

© 2003 – B Stevens    Contact the author

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