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Skin/integument covers the body and serves many functions. It consists of a thick, protective, cornified, stratified squamous epithelium (epidermis), on a firm, dense CT lamina propria (dermis), and has special appendages, hair and nails, and accessory glands, sweat, sebaceous, and mammary glands.

The Epidermis
The epidermis is composed of stratified squamous epithelium and is composed of 4 layers in thin skin but 5 in thick skin.

Cells of the Epidermis
The Keratinocyte
This is the predominant cell of the epidermis.
On leaving the basal layer has two functions:
  1. The production of keratin;
  2. And creation of an extracellular water barrier.
    The keratinocytes are engaged in intermediate filament production (tonofilaments).
    This is the protein of keratin production, which is bundled into tonofibrils.
    In the upper part of the stratum spinosum they start to make keratohyalin granules and lamellar bodies.
    Keratinisation is the conversion of granular cells into cornified cells.
    It involves the breakdown of the nucleus and the thickening of the plasma membrane.
    The lamellar bodies are secreted and coat the cell with a glycolipid forming a water barrier.
    Here it is soft keratin compared with the hard keratin of the nails and hair.
The Melanocyte
This is a dendritic cell among the basal cells.
Its long processes extend into the stratum spinosum between the keratinocytes.
  • The melanocyte represents a small proportion of the total epidermal cells.
  • They are also present in dermis where they are stellate cells with long processes and an elongated nucleus.
  • They are derived from the neural crest and with the keratinocytes form an epidermal-melanin unit.
  • The melanin granule is called a melanosome.
  • These melanosomes are concentrated near the bases of the cell processes when they are nearly mature and in the processes or at the ends if they are mature.
  • Melanosomes are transferred to the keratinocyte through phagocytosis of the ends of the processes. The Langerhans’ Cell
  • This cell doesn’t form desmosomes with the neighbouring keratinocytes.
  • Its nucleus is characteristically indented in many places.
  • It has granules that appear as rods with a striated band.
  • It is involved in the initiation of cutaneous contact hypersensitivity reactions.
The Merkel Cell
  • This is a modified epidermal cell located in the stratum basale.
  • It is most abundant in skin where sensory perception is acute, e.g. the fingertips.
  • The Merkel cell is bound to nearby keratinocytes by desmosomes.
  • They have keratin in the cytoplasm and their nucleus is lobed.
  • These cells are characterised by the presence of dense-cored neurosecretory granules.
  • The combination of the neuron and epidermal cell is called a Merkel’s corpuscle, a very sensitive mechanoreceptor.

Epidermis (epithelium)

  1. Layers
    1. Stratum comeum of keratinized cells (outermost).
    2. Stratum lucidum, a thin pale layer of keratin seen when the stratum comeum is very thick.
    3. Stratum granulosum of cells with basophilic granules.
    4. Stratum spinosum of keratinocytes/prickle epithelial cells.
    5. Stratum germinativum, bordering on the BL.
  2. Cytological details of the layers
    1. Stratum germinativum/basale
      1. Keratinocyte precursor cells, cuboidal or columnar in form, lie on a BL.
      2. Cells project down many small basal processes.
      3. The whole underside of the epithelium is indented by CT dermal papillae for effective attachment, nutrition, and sensation.
      4. Cells proliferate to replace lost surface cells.
    2. Stratum spinosum
      1. Keratinocytes/prickle cells
        • Principal cell kind; ectodermal in origin; move upwards in the layer and continue to proliferate, despite the many desmosomes holding them together (which, with processing shrinkage, lead to the cells’ spiny, prickly appearance).
        • Cytoplasm is rich in keratin filaments, bundled into tonofilaments and increasing in number towards the keratin layer, and formed from prekeratin monomers.
      2. Melanocytes
        • Ectodermal; but migrated neural crest cells.
        • Constitute 1 in 4 to 1 in 10 of basal epithelial cells.
        • Deficient in tonofilaments and desmosomes.
        • Synthesize melanin and transfer it via their long dendritic processes to neighbouring keratinocytes.
        • EM shows that the Golgi apparatus participates in forming the melanosome granules.
        • Melanin is formed from tyrosine by the enzyme tyrosinase. Cells with melanin-forming ability can be revealed in a section by treating it with dihydroxyphenylalanine (DOPA), which is oxidized to melanin.
        • Albinos have an inborn error of metabolism, making them unable to synthesize melanin in the skin and eye.
        • UV light causes greater melanin formation and a thickening of the keratin layer. Pituitary and adrenal hormones also increase pigmentation, which is a useful sign for diagnosis.
      3. Langerhans cells are poorly phagocytic, marrow-derived, specialized macrophages, with long dendrites. They are antigen-presenting cells, accessory to T-cell immunity.
      4. Merkel cells are sensory cells with vesicles and a poly lobulated nucleus. They attach to disc-shaped endings of some of the axons that penetrate the epithelium.
    3. Strata granulosum and comeum.
      1. Stratum granulosum cells form a kerato-hyaline matrix from their basophil granules, binding together packed tonofilaments within the cells to convert the cells to soft keratin. Other organelles and the nucleus vanish, while the plasmalemma thickens and toughens, to build a cornified envelope.
      2. Flattened, dead, keratinized, surface cells desquamate.
      3. Only with EM is keratin seen to be cellular. In the usual HE preparation it is eosinophilic, and often splits and breaks.
      4. Epidermis is thrown up into ridges - cristae cutis - on the palmar and plantar surfaces of the hands and feet : the basis of finger and palm prints.
      5. At the top of the ridges, spiralling holes open through the keratin to let out the sweat.
      6. Keratin layer may be very thick, for instance on the soles and palms. Such thick skin is hairless, and lacks sebaceous glands.
      7. The molecular epidermis: - Filaggrin is the protein of keratohyaline granules, and aggregates ‘keratinoeyte’ keratins Nos. 5/14. These acidic-basic combinations of keratins are characteristic for particular classes of epithelia, e.g., simple versus squamous, and help in interpreting pathological changes. Directly under the plasmalemma is a complex of proteins that are made dense and insoluble - to constitute the “envelope” - by transglutaminase-mediated cross-linking. One protein of the cornified cell envelope is involucrin. Ceramide and other extracellular lipids surround the envelope to boost the barrier function.

Dermis (Corium)

  1. Divided into layers: papillary, fine-textured CT adjacent to the epidermis, and a deeper reticular layer.
  2. Reticular layer is thick collagenous CT of a variable thickness, not always related to that of the overlying epidermis.
  3. Elastic fibres of the dermis give skin its elasticity, but cause wounds to gape. Ruptured dermis often heals as a white line visible through the epidermis, e.g., a mother’s stretch marks.
  4. Has the usual cells of CT - fibroblasts, macrophages and other defensive cells, and sometimes pigment-bearing chromatophores/dermal melanocytes.
  5. Smooth muscle of arrectores pilorum, nipples and scrotal dartos, and skeletal muscle in the scalp and face, are attached in the dermis.
  6. Blood vessels are derived from arterial plexuses: a deep cutaneous plexus/rete, and a subpapillary plexus sending capillary loops up into dermal papillae. Lymphatics accompany blood vessels. Blood flow is varied greatly by shunts through glomi (coiled arteriovenous anastomoses), and by the constriction or relaxation of arterioles.
  7. Nervous receptors, with sensory nerve fibres are present; and autonomic (sympathetic) nerve fibres:
    vasomotor to vascular smooth muscle,
    pilomotor to hair arrector muscles,
    sudomotor to sweat glands.
  8. Hair follicles and glands lie mostly in the dermis.

Sweat Glands (Glandulae sudoriparae)

  1. Single coiled tubules, lined by simple cuboidal light and dark cells; distributed over the body except for the lips, glans penis and inner prepuce.
  2. Secretory part lies in the lower dermis, or subcutaneously in the hypodermis/superficial fascia. One tubule is cut through many times in one section.
  3. The secretion, mainly water and electrolytes plus some lipids, is led to the epidermis through a duct, lined by stratified cuboidal epithelium, then through the living/Malpighian layer and a spiralling hole in the keratin. The gland’s chloride channel is one that is impaired in cystic fibrosis.
  4. Myoepithelial cells are seen within the basal lamina of the secretory tubule. Their contraction is under autonomic control.
  5. The larger variety of gland seen in the axillary, perianal and perigenital regions is termed apocrine, in contrast to the eccrine glands in the majority. Apocrine glands become active with pubertal development of the ambosexual hair, and may be related to animals’ scent glands.
  6. The ceruminous glands of the external auditory meatus seem to be enlarged sweat glands, producing a secretion of pigmented lipids.
Some additional points:
There are 2 types of sweat glands:
  1. Eccrine sweat glands, all over body except lips and part of external genitalia ;
  2. Apocrine sweat glands, only in axilla, areola, nipple of mammary gland, and circumanal region and the external genitalia. The ceruminous glands of ear and glands of Moll of eyelid are also apocrine.
Both the eccrine and the apocrine sweat glands are innervated by the sympathetic* nervous system.
Eccrine glands respond differently to heat and nervous state.
The apocrine glands respond to emotional and sensory stimuli but not heat.

Eccrine Sweat Glands
  • These are simple coiled glands that regulate body temperature.
  • The secretory segment is deep in the dermis or upper hypodermis.
  • Its duct leads to surface.
  • In the secretory region there are clear cells that produce the watery component of sweat and dark cells that produce a proteinaceous secretion.
  • There are also myoepithelial cells that are responsible for the expression of sweat from the gland.
  • Duct cells form the walls from the secretory portion to the area near the surface where the epidermal cells form the wall.
  • The duct is stratified cuboidal.
  • There is both thermoregulatory sweating and emotional sweating.
  • Resorption of some minerals takes place in the duct.
  • Myoepithelial cells are present in the duct.
Apocrine Sweat Glands
  • These are large lumen glands associated with hair follicles.
  • They develop from the same down growths that give rise to hair follicles.
  • The connection is retained and they are coiled tubular glands, sometimes branched.
  • The secretory portion is in the dermis or upper hypodermis.
  • The secretory product is stored in the lumen.
  • Myoepithelial cells facilitate the expulsion of the secretory product from the gland.
  • The duct has a narrow lumen.
  • This duct has a stratified cuboidal epithelium.
  • Resorption does not take place in the duct.
  • Myoepithelial cells are also not present in the duct.
  • Apocrine secretions contain protein, carbohydrate, ammonia and lipid.

Sebaceous Glands

  1. Pear-shaped, simple, branched alveolar, with large cells, usually looking vacuolated because their fatty content is dissolved out.
  2. Several glands are clustered by the side of a hair follicle, into which they discharge the secretion - sebum. Their short duct is lined by stratified squamous epithelium.
  3. Sebum, formed in a holocrine manner by the total breakdown of the cells, may lubricate the hair shaft, protect the skin from drying and moisture, and be bacteriostatic.
  4. Lie independently of hairs on the labia minora, glans penis, in the oral mucosa by the red margin of the lips, and as the Meibomian glands of the eyelid. They are absent from the palms and soles.

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