The epidermis and adnexae originate from the ectoderm



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Maria Lobo

The Skin
1. Explain the concept of the skin as a single organ of the body with its complex intra-relationships with other organ systems of the body.

The skin is the largest organ of the body and essential for life. Diseases of other organs eg jaundice can affect the skin and skin can show the first signs of systemic disease. Functions of the skin include barrier function, peripheral outposts to the immune system, sexual attraction, temperature control.


2. Describe the basic anatomical structure of the skin and the intra-relationships between the epidermis, dermis and subcutis.

There are three layers to the skin- epidermis (stratified squamous epithelium), dermis (structural and support), deep subcutaneous adipose tissue. Some cell types migrate through the different layers of the skin.


3. Describe the structure of the epidermis and associated adnexal structures, its foetal development and regulation of growth.

The epidermis and adnexae originate from the ectoderm.

Epidermis:

3rd week- single layer of glycogen-filled cells making up the epidermis

6th week- periderm (outer layer) and basal layer (inner layer) making up epidermis

21st week- periderm disappears, basal layer forms epidermal cells for rest of life.

Hair follicles:



9th week- hair follicles appear in eyebrows, upper lip, chin. Appear as a cluster of cells in the basal layer of the epidermis, and then grow downwards into the dermis (hair pegs). Associated with fibroblasts and mesenchymal cells to form hair papilla. The tip of the hair peg becomes progressively bulbous and surrounds the dermal papilla. Lower bulge (arrector pili), middle bulge (sebaceous glands 16th week), upper bulge (apocrine glands and ducts). Glands numerous in whole body surface of foetus, most disappear in 3rd trimester.



Stratum corneum- 15-20 layers keratinised cells. Anucleated and cannot divide. Change in type of keratin.
Stratum granulosum- flat cells with dense keratohyalin granules containing fillagrin (promotes aggregation of keratin gilaments).
Stratum spinosum- prickle cells with many tonofilaments (keratin intermediate filaments that attach to desmosomes).
Stratum basale- 1-3cells thick. Small cells with large nuclei. Numerous desmosomes give strength. Function is to divide.
Keratinocytes- labile cells that are produced in the stratum basale and move upwards.

Cell division in the basal layer is controlled by stimulatory signals (EGF epidermal growth factors, TGFα transforming growth factor alpha, IL-1 interleukin 1, IL-6 interleukin 6, GM-CSF granulocyte macrophage colony stimulating factor) and inhibitory signals (TFG β transforming growth factor beta, IFN α and γ interferons alpha and gamma, TNF α tumour necrosis factor alpha). Cytokines and growth factors produced by keratinocytes, langerhans cells and lymphocytes.


Adnexal structures include hair follicles, sebaceous glands and sweat glands and are in continuity with the epidermis.

Sebaceous glands are lobulated. Small germinative basophilic cells at the periphery. Daughter cells migrate to centre, accumulate lipid droplets in cytoplasm, disintegrate and discharge cellular debris (sebum) into sebaceous duct. pH protects against bacterial infection.
Sweat glands

  • Eccrine- whole skin, thermoregulation. Convoluted tube between dermis and subcutaneous fat secretes sweat, excreted by a straight duct crossing the dermis, acrosyringium in epidermis.

  • Apocrine- scent glands, axillae, anogenital skin, around umbilicus. Secretory coil 10x bigger. Surrounded by myoepithelial cells. Apoecrine duct ends in the hair follicle above the sebaceous duct. Enlarge under hormonal influences.

  • Apoeccrine glands- axillae, duct opens onto skin surface.


4. Outline the mechanisms by which the integrity of the dermo-epidermal junction is maintained and the results of failures of these mechanisms.

Hemidesmosomes are the adhesion units between the basal layer of the epidermis and the dermis. Their main function is attachment but also control permeability across the dermis. Keratins 5 and 14 attach to the hemidesmosome. 6 key proteins span the dermo-epidermal junction (plectin, 230 kDa BP antigen, α6β4integrin, type 17 collagen, laminin 5, type 7 collagen) and bind to the lamina densa and anchoring fibrils in the dermis.


5. Describe the migrant cell populations within the epidermis and detail their origins.





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