Lecture 28

Portals of entry in the skin:

• Epidermal barrier

  • During breakage of the barrier such as animal bites, injuries

• Adnexal barrier

  • Hair follicles, sebaceous glands and apocrine glands

• Through vessels

• Through support structures

Epidermis Response to Injury

Changes in Growth, Differentiation or Development

These changes are non-inflammatory

Abnormal Cornification

Hyperkeratosis is the result of excess keratin formation leading to a thickened stratum corneum

Clinical signs: Scaling, crusting or greasiness of the skin

Can result from either:

• Primary lesions

• Secondary lesions

  • Disease progression, manipulation or treatment

Histologically there are two types:

Orthokeratotic = keratinocytes undergo complete cornification and the cells are anuclear

Parakeratotic = keratinocytes undergo partial/incomplete cornification and the cells are nuclear

Epidermal Hyperplasia

Also known as acanthosis

This results as an increase in the number of cells in the epidermis and occurs most often in the stratum spinosum

The stratum spinosum can invaginate into the dermis layer as well.

This is a non-specific lesion seen in chronic diseases of all causes

Epidermal Dysplasia

This is the abnormal development of tissues or organs

This is a preneoplastic stage which can progress to neoplasia

Dysplasia is characterised by the loss of stratification and organisation of the kertatinocytes

• They lose both their shape, size and normal organisation

Apoptosis

A non-inflammatory response

Side note idk what the arrows are pointing at

Atrophy

This is a decrease in the number and size of the cells as a result of sublethal injury

• May be due to starvation, immune mediated diseases, ageing, endocrine diseases etc.

Can be seen clinically as thinned skin

Changes in the Fluid Barrier and Cellular Adhesion

These changes are non-inflammatory

Oedema

There are two types of oedema:

• Ballooning degeneration which is intracellular

  • The fluid accumulates inside the cells

• Spongiosis which is intercellular

  • The fluid accumulates between the keratinocyte cells which creates a sponge-like appearance to the epidermis

Acantholysis

This is the disruption of intercellular junctions between keratinocytes of the epidermis

Common in immune-mediated disorders and bacterial infections resulting in pustules and vesicles

Changes in Pigmentation

These changes are non-inflammatory

Hyperpigmentation

This can be caused by:

• Increased number of melanocytes

• Increased melanin production despite the number of melanocytes being the same

• Clusters of pigment cells

• Endocrine dermatoses such as hypothyroidism

Hypopigmentation

This can be caused by:

• Decreased number of melanocytes

• Reduced melanin production despite the number of melanocytes being the same

• Loss of exisiting melanocytes

• Copper deficiency → reduced functioning of the tyrosinase → poor melanin production

Inflammatory Responses of the Epidermis

Acute inflammation can occur from:

• Burns

• Ectoparaistes

• Trauma

• Vasculitis

• Immune-mediated disease

Dermis Response to Injury

Changes in Growth, Differentiation or Development

Dermal Atrophy

This is a decreased quantity of collagen fibres and fibroblasts in the dermis

Clinical signs: thin, translucent skin which can easily visualise the vasculature

Causes include endocrine disorders and starvation

Fibrosis

This is formation of granulation tissues in response to injuries such as ulceration of the epidermis leading to scar formation

Proud flesh, on the right, is when the proliferation of granulation tissue doesn’t stay within the boundaries

Collagen Dysplasia

A rare condition which is the inherited abnormality of collagen that results in decreased tensile strength and increased ability of the skin to stretch

Minor trauma can lead to tears

Abnormal Deposits in the Dermis

These deposits include mucin, calcium (leading to calcinosis cutis) and amyloid (rare but seen in horses)

Mucin deposits results in thick puffy skin

Inflammatory Lesions

Dermatitis = inflammation of the dermis

Acute is characterised by erythema, swelling and migration of leukocytes

Outcomes of acute:

• Complete resolution

• Abscess

• Chronic dermatitis

• Scarring

Chronic is caused by foreign bodies, granulomas or autoimmune reactions

Chronic Dermatitis

Neutrophils are seen in bacterial infections leading to suppurative inflammation

Macrophages are seen in infections such as with mycobacterium spp leading to granulomatous inflammation

Allergic and parasitic infections will lead to eosinophil accumulation

Inflammatory Patterns

A = inflammatory cells are around the blood vessels

B = inflammatory cells are present in the endothelial wall

C = inflammatory cells present at the interface of dermis and epidermis

D = inflammatory cells present at the interface of dermis and epidermis

E = inflammatory cells are present in nodular aggregations

Adnexa Response to Injury

Changes in Growth, Differentiation or Development

Atrophy

This is a gradual reduction in size of the hair follicle

This can be part of the normal hair cycle or can be pathological due to:

• hormonal imbalance

• nutritional abnormalities

• inadequacy of blood supply

• inflammation

• general state of health, including stressful events or systemic illness

Inflammatory Responses

Folliculitis = Inflammation of the hair follicle, affects most domestic animals.

Sebaceous adenitis = Inflammation of sebaceous glands

Hidradenitis = Inflammation of the apocrine glands

Panniculus Response to Injury

Panniculitis is the common response which can be caused by:

• Physical injury (trauma, injection of irritant material, foreign bodies)

• Infectious agents (bacteria, fungi)

• Immune-mediated disorders

• Nutritional disorders (vitamin E deficiency)

• Pancreatic disease (pancreatitis, pancreatic carcinoma)

• Idiopathic

Skin Lesion Morphology

Primary Lesions

These are direct results of underlying diseases processes originating from the normal skin

Macule – A circumscribed flat area of discoloration of the skin, up to 1 cm in diameter

• Can be red or black colour or other colours

Patch – A macule over 1 cm in diameter

Papule – A solid elevation of the skin, up to 1 cm in diameter

Plaque – A large, flat-topped elevation formed by extension or coalescing of papules

Nodule – A circumscribed solid elevation extending to the deep layers, >1 cm diameter

Tumour (neoplasm)

Vesicle – A circumscribed, elevated, fluid-filled lesion, < 1cm in diameter

Bulla – A vesicle over 1 cm in diameter

Pustule – A circumscribed, small, pus filled elevation of the epidermis

Abscess – A well demarcated cavity resulting from pus accumulation in dermis and subcutis

Wheal – A firm, circumscribed raised elevation of the skin due to dermal oedema

Primary OR Secondary lesions

Scale – an accumulation of loose fragments of cornified skin

• Primary occurs in icthyosis

• Secondary can be seen in condiions such as fungal dermatitis

Crust – dried exudate composed of cell debris, scale, fluid blood on the skin surface

• Can occur in photosensitisation or staphylococcal infection etc

Comedo – distended hair follicle with plug of stratum corneum and sebum

• Clinically these look like pimples

Alopecia

Hypertrichosis - more hair than normal

Hypotrichosis - less hair than normal

Secondary Lesions

These evolve from primary lesions and have less diagnostic importance compared to the primary lesions

Epidermal collarette – a circular rim of keratin scale after loss of the roof of vesicle or pustule

Erosion - Epidermal defect that spare the basement membrane

Ulcer - Loss of epidermis, basement membrane, and at least the superficial portion of dermis

Lichenification – thickening and hardening of the skin

Callus – a thickened, rough, alopecic, often lichenified plaque that develops on the skin → usually develops at pressure points