Endocrine effects on the skin - Pathology

The endocrine system typically has effects on the hair growth cycle so alopecia is a common finding. Many types of endocrine skin disease cause similar pathology. Affects dogs more than any other species.

The hair growth cycle

Hair follicles grow in repeated cycles in a mosiac pattern so that the whole hair coat isn't lost at one time.

• Anagen: Growth phase.
  The majority of hair follicles will be in this phase. The hair grows in length.
• Catagen: Transition phase.
  The dermal papilla is broken away and the follicle shrinks.
• Telogen: Resting phase.
  The hair doesn't grow but stays attached while the dermal papilla is resting.

After telogen the follicle re-enters anagen and the dermal papilla reattaches to the base of the follicle. If the old hair has not already epilated it will be pushed out by the new growing hair.

Pathogenesis

General skin changes (endocrine alopecia) seen with endocrine disease include:

• Hypotrichosis.
• Alopecia; usually bilaterally symmetric and non-pruritic.
• Remaining coat is dry and dull
• Pigment disturbance.
• Seborrhoea.

Histologically many of the follicles will be in telogen where the haircoat is easily epilated and then fails to regrow. This is why alopecia occurs over areas of friction (trunk face etc) while the legs and tail are spared. Follicular and epidermal atrophy may be evident along with atrophy of the adnexa E.g sebaceous atrophy.

• Often complicated by pyoderma or secondary seborrhea

Specific changes associated with disease

Hypothyroidism

• Grossly: endocrine alopecia mainly over trunk and neck
• Microscopically: endocrine alopecia plus mucin within the dermis -> myxoedema (dermal thickening), acanthotic epidermal and follicular infundibular epithelium
• Secondary pyoderma is common

Hyperadrenocorticism

• Grossly: endocrine alopecia except on head and extremities, thin skin, comedones, easy bruising and poor wound healing, sometimes involves calcinosis cutis
  • In cats, skin is very easily torn
• Microscopically: epidermal, dermal and follicular atrophy, comedones, calcinosis cutis often with granulomatous inflammation and epidermal hyperplasia

GH deficiency (hyposomatotropism)

Flame Follicles. Courtesy of A. Jefferies

• Retained puppy hair coat, endocrine alopecia, decreased dermal elastin and catagen follicles (Flame follicles).

Excess GH (hypersomatotropism)

• Grossly: thick, folded skin on head, neck and extremities, possibly with hypertrichosis
• Microscopically: epidermal hyperplasia, dermal fibrosis, mucinous degeneration of dermis

Hyperoestrogenism

• Grossly: endocrine alopecia, enlarged vulva and abnormal oestrus cycle in females, gynecomastia, pendulous prepuce or enlarged prostate in males
• Microscopically: telogen follicles, hyperkeratosis and acanthosis or epidermis and follicular infundibulum

Castration responsive dermatosis

• Mainly involves intact male dogs
• Grossly: fluffy and wool like hair coat, symmetrical alopecia over perineum, thighs, ventral abdomen and thorax, neck; possible hyperpigmentation
• Microscopically: increased nmbe of catagen and telogen hair follicles and flame follicles
• Castration may temporarily remove symptoms

Superficial Necrolytic Dermatopathy

• Also known as diabetic dermatopathy, hepatocutaneous syndrome
• In older dogs with diabetes mellitus, hepatic dysfunction or pancreatic lesions
• Grossly: scaling and crusting, erythema, facial, genital and distal extremities alopecia, crusting, fissures and ulceration of foot pads
• Microscopically: trilaminar thickening of epidermis (outermost layer is keratotic, intermediate oedematous and vacuolated, inner is hyperplastic)