2002-03-13T17:47:00.000-08:00

Skin

The skin consists of four layers:

1. Epidermis – Major protective layer.
2. Dermis – Major support layer.
3. Skin Appendages – Nails, Follicles, Subaceous glands, and Sweat glands.
4. Subcutaneous fat.

1. The Epidermis

Consists of four layers:

1. Cornified (or Horny) layer – Outer non-nucleated barrier.
2. Granular layer – The layer where epidermal nuclei disintegrate.
3. Germinitive (or Prickle Cell) layer) – Contains the bulk of living epidermal keratinocytes.
4. Basal Layer – The only layer of keratinocytes in normal skin
which undergo cell-division.

There is also sometimes a possible fifth layer called the Stratum Lucidum in thick skin.

Cell Types in the Epidermis:

A. Keratinocyte – Main cell.
B. Melanocyte – Pigment-producing cell in the BASAL layer.
C. Langerhans Cell – Important immunological cell found in the middle layers.
D. Merkel Cell – Member of the APUD system found in and around the BASAL layer.

A. Keratinocytes

- Main type of cell found in the epidermis.
- Make their way from the BASAL layer to the CORNIFIED layer through the process of ORTHOKERATOSIS. To get an idea of this
process, follow the process below from the bottom layer upward:

^ CORNIFIED layer -Non-nucleated stratum corneum – Dead and flat. Cells get squashed together.
^ GRANULAR layer -Cell nuclei disintegrate.
^ PRICKLE-CELL layer – Cells connect to each-other with DESMOSOMES.
^ BASAL layer -- Cell division. One daughter cell heads up to the next layer, while the
other stays to repeat the process of cell division.

- HEMIDESMOSOMES connect the left-over keratinocytes to the underlying basement membrane.

- In some mucousal sites, PHYSIOLOGICAL PARAKERATOSIS avoids the granular-layer disintegration of nulcei.

- PATHOLOGICAL PARAKERATOSIS – This is the accelerated and imperfect development of the Stratum Corneum:

- In normal skin, the whole process shown above takes 50-75 days.
- For people suffering from psoriasis – This process takes 8 – 10 days.

- The strength of the epidermis relies on a structural protein called Alpha-Keratin, which forms TONOFILAMENTS that are continuous
with desmosomes.

- GAP JUNCTIONS are communication channels connecting the cytoplasm of neighbouring cells.

B. MELANOCYTES

- Responsible for pigmentation in the skin.
- Each melanocyte has multiple dendrites connecting to adjacent keratinocytes.

Ratio of melanocytes to keratinocytes:
- In the face – 1:5
- In the back – 1:20

- Chronic light exposure increases the prevalence of melanocytes.
- These cells synthesise the pigment Melanin from Dopa, on PRE-MELANOSOMES, using the enzymes DOPA-OXIDASE and
TYRONISINASE which are only found in active melanocyte cells.
- The presence of TYRONISINASE is required for a cell to be classed as a melanocyte.
- Melanin granules are distributed to keratinocytes along the dendrites.
- Melanocytes are found in the epidermis, hair bulb, eye and brain.
- The function of melanocytes in normal skin is THOUGHT to involve protection from ultra-violet radiation. The basis for this is the
much lower prevalence of skin cancer among darker-skinned peoples.

C. LANGERHANS CELL

- Dendritic cells derived from the bone marrow, usually in the middle area of the epidermis in large numbers. The dendrites link these
cells to each-other.
- Also linked by highly-specific intracytoplasmic structures called BIRBECK GRANULES which look tennis-racket shapish.
FUNCTION UNKOWN.
- In normal skin, langerhans cells are the only ones to express MHC Class 2 antigens, and carry receptors for complement.
- Seem to have the capacity to act as antigen-presenting cells, but don’t phagocytose very efficiently.
- COULD be important in allergic contact dermatitis.

D. MERCKEL CELL

- This cell’s cytoplasm is packed with electron dense granules.
- Found in large numbers in touch-sensitive sites such as the finger-tips and around the lips.
- Seem to be concentrated around the basement membrane area, and in association with nerve endings.
- THOUGHT to be part of the APUD (Amine Precursor Uptake and Decarboxylation) system, as granules contain large amounts of
catecholamines.(Note: Find out what the hell APUD is about)
- Exact function is NOT KNOWN, but thought to be related to cutaneous sensation.

CELL ADHESION MOLECULES

- Haven’t got time to go into details at the moment, but this group includes ICAM, Integrins, Cadherins and cells expressing MHC
Class 2 antigens.