Understanding various dermatopathology terms is mandatory for proper interpretation of findings and hence reaching a proper diagnosis. Sometimes residents and trainers find it difficult to discriminate between various terms. A common area of confusion is the concept of horn cysts, horn pearls, squamous eddies and squamous morules.

Horn cysts represent foci of abrupt complete keratinization (with only a very thin surrounding granular cell layer and without retained nuclei). It may be pseudo or true. Pseudo horn cysts results from sectioning of a markedly papillomatous skin. True horn cysts represent foci of orthokeratosis within the substance of the lesion, by time they enlarge, move upwards and merge with the invagination of the surface keratin. Both types are characteristic of seborrheic keratosis. Small keratinous cysts described also as horn cysts are characteristic of trichoepitheliomas. They are made of keratinous centres (abrupt and complete) surrounded with basophilic cells. Sometimes we might find one or few layers of esinophilic cells with large oval vesicular nuclei situated between the basophilic cells and the keratinous centres. In all these types, the keratinization is surface in type (lamellated keratin).

On the other hand, horn pearls represent concentric layers of keratinocytes showing gradually increasing keratinization towards the centre. The centre shows incomplete keratinization (retained nuclei). Complete keratinization is rare. Keratohyaline granules within a pearl are sparse or absent. Horn pearls are seen in well differentiated and moderately differentiated SCC.

Squamous eddies represent circumscribed esinophilic flattened angulated squamous cells with prominent desmosomal attachments, arranged in an onion peel fashion with no keratinization in the centre. They have more uniform more basaloid cells at the periphery and they lack atypia. Squamous eddies are seen characteristically in irritated seborrheic keratosis (inverted follicular keratosis) and can be seen also in trichilemmoma (especially the desmoplastic variant) and in squamoid eccrine ductal carcinoma.

I haven’t come across the term squamous morules except 2 years ago during studying for my Diploma Exam and from the Spot Diagnosis section of Dr Philip McKee’s first website. It was Dr Richard Carr’s case of follicular SCC that brought up this concept. From what I have learned then, squamous morules refers to characteristic rounded balls of squamous cells with very abundant eosinophilic cytoplasm. They often show central abrupt homogenous complete bright orange keratin (trichilemmal, pilar, or follicular keratinization). In contrast, the above described horn pearls of ordinary SCC are more oval and not pilar. According to Dr Carr, these structures are seen in follicular/infundibular SCC, some variants of BCC and some hair follicle related tumors such as trichilemmoma, pilar sheath acanthoma and inverted follicular keratosis, reflecting a unique pattern of keratinization in follicle derived tumors. He has also seen them in some hidradenomas and porocarcinomas. According to Weedon’s skin pathology, this unique pattern of keratinization characterizes also keratoacanthomas. Dr Carr has proposed this term because these morules closely resembled those commonly seen in endometrial adenocarcinomas.

Understanding the term is the first step to proper identification and correct diagnosis!!