Pilomatricoma

16F. Scalp mass

'Ghost' keratinocytes

Psoriasiform reaction pattern

Psoriasiform reaction pattern

Regular elongation of rete pegs ('elephant feet')

Parakeratosis, thinning of suprapapillary epidermis, dilated vessels in papillary dermis

Dilated vessels in papillary dermis

Keratoacanthoma

A symmetric crater in the skin filled with keratin (really low power, slide held over condenser)

With peripheral 'lipping' of keratinocytes

Usually in elderly, face-head-neck region, the crater is visible on gross. It is the smooth roundish outline of the lesion that distinguishes it from a squamous carcinoma (plus that there's no dysplasia in spite of cell enlargement towards the center)

Epidermal nevus

'Nevus' is a hamartoma of any element of skin.  So why not the epidermis itself?

Note the prominent papillomatosis, acanthosis and lamellated keratin