Parakeratosis
Parakeratosis refers to retained keratinocytic nuclei in the stratum corneum. It is seen in proliferating keratinocytic disorders such as psoriasis, and in keratinocytic malignancies and premalignant conditions such as Bowen's disease and Solar keratosis. The nuclei are flattened and run parallel to the epidermal surface.
When you see it look carefully at the underlying epidermis. You may see hyperkeratosis, hypogranulosis and acanthosis. Histological parakeratosis usually equates to surface scale so think of the mnemonic for the red scaly diseases PMs PETAL for likely diseases.
Patterns of parakeratosis can be helpful.
Confluent in Bowen's disease and Psoriasis
Alternating in Solar keratosis
Parakeratosis in a heaped up column in the cornoid lamellae of DSAP
Checkerboard parakeratosis in PRP
Sandwich parakeratosis over orthokeratosis in Tinea
There is also the unusual axillary granular parakeratosis with granules in the stratum corneum.
