Psoriasis. Light micrograph of a section through a psoriasis pustule. Psoriasis is a chronic skin disease causing inflammation and scaly lesions. In this section the surface of the skin (stratum corneum) is at top. There is a parakeratotic layer with abnormal keratinisation and microabcesses (upper centre) immediately below the cornified layer. Beneath this, the stratum spinosum layer of the epidermis is greatly thickened (acanthosis). This is a major feature of psoriasis. The rete pegs (sections of epidermis that protrude into the dermis) and dermal papillae (sections of dermis that protrude into the epidermis) are elongated and pronounced.