Dermatology - Background knowledge Flashcards
Outline the structure of the skin. What are skin appendages?
The skin is the largest organ in the body. It is composed of the epidermis and dermis overlying subcutaneous tissue.
The skin appendages (structures formed by skin derived cells) are hair, nails, sebaceous glands and sweat glands.
What are the 4 cell types found in the epidermis?
Each of the 4 cell types have specific functions:
1) Keratinocytes - produce keratin as a protective barrier
2) Langerhan’s cells - present antigens and activate T lymphocytes
3) Melanocytes - produce melanin, which gives pigment to the skin and protects the cell nuclei from ultraviolet radiation induced damage
4) Merkel cells - contain specialised nerve endings for sensation
What are the 4 layers of the epidermis?
When thinking about the epidermis, think of 4s - 4 cell types, and 4 layers. Each layer of the epidermis represents a different stage of keratinocyte maturation. The average epidermal turnover time (migration of cells from the basal cell layer to the horny layer) is 30 days.
Stratum basale - actively dividing cells, deepest layer
Stratum spinosum - differentiating cells
Stratum granulosum - so called because cells lose their nuclei and contain granules of keratohyaline. They secrete lipid into the intercellular spaces
Stratum corneum - layer of keratin, most superficial layer
In what areas of the body is the stratum lucidum present?
In areas of thick skin, such as the soles of the feet, there is a fifth layer to the epidermis called the stratum lucidum, beneath the stratum corneum. This consists of paler, compact keratin.
What pathological process can affect the epidermis?
1) Changes in epithelial turnover time - e.g. psoriasis (reduced epidermal turnover time)
2) Changes in the surface of the skin or loss of epidermis - e.g. scales, crusting, exudate, ulcer
3) Changes in pigmentation - e.g. hypo- or hyperpigmented skin
What is the dermis composed of?
The dermis is made up of collagen (mainly), elastin, and glycosaminoglycans, which are synthesised from fibroblasts. Collectively they provide strength and elasticity to the dermis.
The dermis also contains immune cells, nerves, skin appendages as well as lymphatic and blood vessels.
What pathological processes can affect the dermis?
1) Changes in the contour of the skin or loss of the dermis - e.g. formation of papules, nodules, skin atrophy and ulcers
2) Disorders of skin appendages e.g. disorder of hair, acne (disorder of sebaceous glands)
3) Changes related to lymphatic and blood vessels - e.g. erythema (vasodilation), urticaria (increased permeability of capillaries and small venules), purpura (capillary leakage)
What are the three main types of hair?
i) Lanugo hair (fine long hair in fetus)
ii) Vellus hair (fine short hair on all body surfaces)
iii) Terminal hair (coarse long hair on the scalp, eyebrows, eyelashes, and pubic areas)
What is the structure of hair?
Each hair consists of modified keratin and is divided into the hair shaft (a keratinised tube) and hair bulb (actively dividing cells, and melanocytes which give pigment to the hair).
What is the hair growth cycle?
Each hair follicle has its own growth cycle. This occurs in 3 main phases:
1) Anagen (long growing phase)
2) Catagen (short regressing phase)
3) Telogen (resting/ shedding phase)
What pathological processes can affect the hair?
1) Reduced or absent melanin pigment production - e.g. white or grey hair
2) Changes in duration of the growth cycle - e.g. hair loss (premature entry of hair follicles into telogen phase)
3) Shaft abnormalities
What is the nail composed of?
The nail is made up of a nail plate (hard keratin) which arises from the nail matrix at the posterior nail fold, and rests on the nail bed.
The nail bed contains blood capillaries which gives the pink colour of the nails.
Pathology can involve:
a) abnormalities of the nail matrix - e.g. pits and ridges
b) abnormalities of the nail bed - e.g. splinter haemorrhage
c) abnormalities of the nail plate - e.g. discoloured nails , thickening of nails
What is a sebaceous gland?
Sebaceous glands produce sebum via hair follicles (collectively called a pilosebaceous unit). They secrete sebum onto the skin surface which lubricates and waterproofs the skin.
Sebaceous glands are stimulated by the conversion of androgens to dihydrotestosterone and therefore become active at puberty.
Pathology of sebaceous glands may involve:
a) increased sebum production and bacterial colonisation - e.g. acne
b) sebaceous gland hyperplasia
What are sweat glands? How many types are there?
Sweat glands regulate body temperature and are innervated by the sympathetic nervous system.
They are divided into eccrine and aprocrine sweat glands:
- eccrine sweat glands are universally distributed in the skin
- apocrine sweat glands are found in the axillae, areolae, genitalia and anus and modified are found in the external auditory canal. They only function from puberty onwards and action of bacteria on sweat produces odour
Pathology of sweat glands may include:
- inflammation/ infection of apocrine glands - e.g. hidradenitis suppurativa
- overeactivity of sweat glands - e.g. hyperhidrosis
What are the 4 phases of wound healing?
1) Haemostasis - vasoconstriction and platelet aggregation; clot formation
2) Inflammation - vasodilation; migration of neutrophils and macrophages; phagocytosis of cellular debris invading bacteria
3) Proliferation - granulation tissue formation (synthesised by fibroblasts) and angiogenesis; re-epithelialisation (epidermal cell proliferation and migration)
4) Remodelling - collagen fibre re-organisation; scar maturation
