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Skin Function and Structure of Skin

  1. Skin Function
    Skin is actually the largest organ in the body. Its wet weight can be as heavy as 4 kg and covers an area of 1.4 to 2 sq metres. It also plays many important functions in the body as summarized in Table I.

Table I: Function of the Skin




Protection from harmful agents of external environment: biological germs, ultraviolet light & chemicals


Preservation of a balanced internal environment


Shock absorber

Subcutaneous fat

Temperature regulation

Blood vessels & eccrine sweat glands


Subcutaneous fat


Nerve endings


Sebaceous glands

Protection & grip


Calorie reserve

Subcutaneous fat

Vitamin D synthesis

Epidermis (Keratinocytes)

Body odour

Apocrine sweat glands


Hair & Nails

  1. Structure of Skin
    1. Epidermis:
      1. Thickness varies from 0.1 mm at the eyelids to nearly 1 mm on the palms and soles.
      2. The outermost is the horny layer (stratum corneum) which is made up of flattened dead keratinocytes. It acts as the major physical barrier in the epidermis.
      3. Stratum lucidum – Present on palms and soles.
      4. The granular layer ( stratum granulosum ) is 1-2 layers thick. Cells contain granules known as keratohyline granules.
      5. Prickle cell layer (stratum spinosum ) is the thickest layer of the epidermis. It is 5-7 layers thick.
      6. The basal layer (stratum basale ) is the deepest, most active and single layer of the epidermis. It is the germinative layer for the epidermis. There is a gradual differentiation from basal layer to the horny layer.
Layers of epidermis (From deep to superficial)
In palm & sole (5 layers)
i) Stratum basale
ii) Stratum spinosum
iii) Stratum granulosum
iv) Stratum lucidum
v) Stratum corneum
Elsewhere (4 layers)
i) Stratum basale
ii) Stratum spinosum
iii) Stratum granulosum
iv) Stratum corneum
  • Dyskeratosis is abnormal keratinization occurring prematurely within the cells below the stratum granulosum (normally intracellular keratin is accumulated in keratinocytes in stratum granulosum layer). 
  • Diseases causing dyskeratosis :-
    1. Benign          → Darrier's disease, Hailey - Hailey disease (chronic benign pemphigus).
    2. Malignant      → Bowen's disease, Squamous cell carcinoma, Paget's disease.
  • Keratinocytes are connected to each other by intracellular desmosomal bridges.  Separation of keratinocytes due to loss of intracellular desmosomal bridges → Acantholysis.
  • Acantholytic cells (separated keratinocytes) of pemphigus vulgaris are derived from Stratum basale.
  • Intracellular edema of keratinocytes → Ballooning
  • Basal cell degeneration is seen in lichen planus.
Intercellular edema in stratum spinosum → Spongisis (characteristic of eczemA.
Thickening of stratum spinosum → Acanthosis
  • Normally nucleus disappears in stratum corneum and cells are anucleated.
    Retention of nuclei by cells of stratum corneum → Parakeratosis.
  • Parakeratosis is seen in - 
  1. Normally → Mucous membrane of mouth and vagina.
  2. Disorders → Actinic keratosis, Psoriasis, Squamous cell carcinoma, Seborrheic dermatitis, Eczema.

    o Thickening of stratum corneum → Hyperkeratosis. 
  • Separation of keratinocytes due to loss of intracellular bridges → Acantholysis
  • Intracellular edema of keratinocytes → Ballooning

    Stratum basale
  • Acantholytic cells of pemphigus vulgaris are derived from stratum basal.
  • Basal cell degeneration occurs in Lichen planus.

    Stratum spinosum
  • Intercellular (in between the cells) edema       → Spongiosis
  • Thickening        → Acanthosis

    Stratum granulosum
  • Thickening → Hypergranulosis

    Stratum corneum
  • Retention of nuclei within cells   → Parakeratosis
  • Thickening    → Hyperkeratosis
  • Stratum corneum is involved in  → Micromunro abscess, Dermatophytes
  1. Cells of Importance
    1. Melanocyte
    2. Langerhans Cells
    3. Merkel Cell
      Development of cells of epidermis 
    • Cells in epidermis are derived from two sources: -
    1. Local cells → Develop from ectoderm along with epidermis, e.g., keratinocytes.
    2. Immigrant cells → Develop elsewhere and migrate to epidermis e.g., : -
      1. Melanocytes and merkel cells develop from neural crest and migrate to epidermis.
      2. Langerhans cells are derived from mesenchyme and migrate into epidermis.

Description: skin%20layers

Skin appendages: Hair, nail, eccrine sweat glands, apocrine sweat glands, sebaceous glands.

  1. Hair: Types and Growth Cycle
    The first hair to be produced by the fetal follicles, so called lanugo, is fine, soft, unmedullated, and usually unpigmented. Lanugo is normally shed in utero in the seventh or eighth month of gestation.
    Postnatal hair can be divided into
    1. Vellus, which is soft unmedullated, usually unpigmented, and seldom more than 2cm long, and
    2. Terminal hair, which is longer, coarser, and often medullated and pigmented. There is, however, a range of intermediate types.
  1. The type of hair produced by any particular follicle can change.

The most striking example is the replacement of vellus by terminal hair at puberty which starts in the pubic regions. This leads us to the definition of androgen-dependent hair. It is obvious from the events of puberty that pubic, axillary, facial, and body hair are hormone-dependent. So, paradoxically, is pattern baldness (male), in which terminal hair is replaced by fine, short hair resembling vellus. The growth of male beard depends on testicular hormones. The action of testosterone involves its reduction to 5 alphadihydro-testosterone and binding to an intracellular receptor.

The most important feature of hair follicles is that their activity is intermittent (cyclical). As the hair reaches a definitive length, it is shed to be replaced by a new hair.


Thus a hair follicle will pass into three stages: an

  1. Active (anagen) stage,
  2. A transitional (catagen) stage, and
  3. A resting (telogen) stage where the hair stops growing to be finally shed.

In human scalp hair, the anagen stage takes about three years, the catagen stage takes three weeks, and the telogen phase takes three months. Anagen to telogen ratio = 12:1


The hair cycle occurs in different hair follicles asynchronously, i.e., at a given time, each individual hair follicle is at a different stage of the hair cycle.

Dermo-epidermal junction: At the interface between the epidermis and dermis lies the basement membrane zone. Electron microscopy shows that it can further be divided to.

  1. Hemidesmosomes,
  2. Lamina lucida (20-40 nm),
  3. Lamina densa (30-60 nm, Type IV collagen).
  4. Sublamina densa with anchoring fibrils (Type VII collagen),
  5. Dermal microfibril bundles,
  6. Types I & III collagen fibres.

The structures of the dermo-epidermal junction provide good mechanical support, adhesion and growth of the basal layer unless it is diseased.

Dermis: It is thickest at the palms, soles and back (3 mm), thinnest at the eyelids (0.3 mm) and penis. There are papillary dermis and deeper reticular dermis. It contains many cells, fibres and amorphous ground substance.

Fibroblast for synthesis of collagen, reticulin, elastin, glycosaminoglycans is the major cell in the dermis. The ground substance consists of two glycosaminoglycans:

  1. Hyaluronic acid
  2. Dermatan sulphate.
  3. Other structures found in the dermis include: blood vessels, lymphatics, nerves, nerve endings and receptors, dartos muscles in scrotum, appendageal glands and their accessories e.g. arrector pili muscles.

Subcutaneous fat: It is absent from the eyelids and the male genitalia.


It has abundant blood and lymphatic supplies.

  1. Cell Types
    1. Epidermis:
      1. Keratinocytes (85% of cells in epidermis),
      2. Langerhans cells (800 per sq mm), (From bone marrow)
      3. Melanocytes (from neural crest, wedged between basal keratinocyte in 1 to 10 ratio; 1 melanocyte supplies pigments to 36 keratinocyte),
      4. Merkel Cells.
    2. Dermis:
      1. Fibroblast
      2. Mononuclear phagocytes
      3. Lymphocyte
      4. Langerhans cell
      5. Mast cell
  2. Kinetics of Skin
    Epidermis: Cell cycles of keratinocytes: 300 hours (from G1, S, G2 to M phases); keratinocytes need 28 days to move from basal layer to horny layer and another 28 days to slough off from horny layer (i.e. from the basal layer to the environment requires 52-75 days).

In psoriasis the cell cycle time is 36 hrs and the cell transition time is 4-7 days.


Skin appendages: Appendages of Skin include glands (sweat and sebaceous), hair and nails. Appendages of skin are usually  located in the dermis.

Hair (scalp): growth rate: 0.4 mm/day, 80% in anagen at any one time; anagen phase: 3 years, catagen phase: 3 weeks, telogen phase: 3 months.

Nail: fingernail growth rate: 3 mm/month, toenail growth rate : 1 mm/month


Sebaceous glands are holocrine glands, i.e., their secretion (consisting of lipids) is formed by complete destruction of cells. Sebaceous glands harbour following organisms: - 
  1. Pityrosporum ovale (fungus)  → Causes dandruff (Seborrheic dermatitis) 
  2. Propionibacterium acnes (bacterium)  → Cause Acne vulgaris
  3. Demodex folliculorum (mitE.  
o Sebaceous glands are small in childhood, and at puberity become prominent under the influence of androgens.
Some sebaceous glands are not associated with hair follicles: - 
  1. Meibomian glands → Eyelids
  2. Tyson's glands  → Prepuce
  3. Montgomery's glands  → Nipple
  4. Fordyce's spots   → Oral epithelium
Terminology Used in Dermatology
  1. Macule: flat area of altered colour or texture (less than 0.5 cm)
  2. Patch: large macule (more than 2 cm)
  3. Papule: elevated solid lesion (less than 0.5 cm)
  4. Nodule: elevated solid lesion (more than 0.5 cm)
  5. Plaque: elevated area of skin of more than 2 cm in diameter, a disc shaped lesion, formed by extension or coalescence of papules or nodules.
  6. Vesicle: fluid filled blister (less than 0.5 cm)
  7. Bulla: larger blister (more than 0.5 cm)
  8. Pustule: collection of free pus
  9. Wheal or weal: accumulation of dermal oedema
  10. Angioedema: diffuse oedema of deep dermis extending to subcutaneous tissue
  11. Comedo (comedones): a plug of keratin and sebum wedged in a dilated pilosebaceous orifice
  12. Burrow: a small tunnel in the skin that houses scabies acarus
  13. Poikiloderma: combination of atrophy, reticulate hyperpigmentation and telangiectasia
  14. Sclerosis: induration of the subcutaneous tissues
  15. Scale: flake from the horny layer
  16. Crust: dried serum, exudate or tissue fluid
  17. Ulcer: loss of the epidermis and dermis
  18. Excoriation: linear erosion or ulcer produced by scratching
  19. Erosion: loss of the epidermis
  20. Fissure: slit in the skin
  21. Scar: result of healing, normal structure replaced by fibrous tissue
  22. Atrophy: thinning of skin due to diminution of the epidermis, dermis or subcutaneous fat
  23. Lichenification: comprises of thickening, hyperpigmentation, accentuated skin-fold markings
  24. Crust: Dried exudate of body fluids that may be either yellow (serous exudate) or red (hemorrhagic exudate).
  25. Milia: Small, firm, white papules that are filled with keratin (and may in part resemble pustules).
  26. Scar: normal tissue is replaced by fibrous tissue. A change in the skin secondary to trauma or inflammation. Sites may be erythematous, hypopigmented, or hypertrophic depending on their age or character. Sites on hair-bearing areas may be characterized by destruction of hair follicles.
  27. Pruritus: A sensation that elicits the desire to scratch. Pruritus is often the predominant symptom of inflammatory skin diseases (e.g., atopic dermatitis, allergic contact dermatitis); it is also commonly associated with xerosis and aged skin. Systemic conditions that can be associated with pruritus include chronic renal disease, cholestasis, pregnancy, malignancy, polycythemia vera, and delusions of parasitosis.
  1. Special Techniques Used in Clinical Examination
    1. Magnification: Magnification is essential to examine follicular plugging in lupus erythematosus, Wickham's striae in lichen planus, telangiectasia and translucence in basal cell carcinoma, changes in colour in malignant melanoma.
    2. Diascopy consists of pressing a transparent slide or plastic spatula over a skin lesion. Examiner will find this of special value to distinguish erythema or purpura. It is useful to detect the glassy yellow-brown appearance of papules in sarcoidosis, tuberculosis and other granuloma.
      Diascopy is designed to assess whether a skin lesion will blanch with pressure or not. For instance, erythema will blanch with pressure, whereas a purpuric lesion will not. Lupus vulgaris will have an “apple jelly” appearance on diascopy.
    3. Darier's sign is positive when a brown macular or papular lesion of urticaria pigmentosa becomes palpable wheal after being rubbed with the blunt end of an instrument. It is present in urticaria pigmentosa.
    4. Auspitz's sign is positive when slight scratching or curetting of a scaly lesion reveals punctate bleeding points within the lesion which suggests of psoriasis.
    5. Nikolsky's sign is positive when a new blister is generated with ease by applying shearing force to skin. Causes of Nikolsky sign: e.g pemphigus vulgaris, TEN, steuen johnson’s synd., ssss

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