Endocrine effects on the skin - Pathology

()Map ENDOCRINE SYSTEM (Map) INTEGUMENTARY SYSTEM

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:
  

  Flame Follicles. Courtesy of A. Jefferies
  • Retained puppy hair coat, endocrine alopecia, decreased dermal elastin and catagen follicles (Flame follicles).