CD10 is a characteristic marker of tumours forming morules with biotin-rich, optically clear nuclei that occur in different organs.

Recently, Chiarelli et al.1 have demonstrated that CD10 immunoreactivity represents a useful marker of morules in endometrioid lesions of the female genital tract, allowing identification of various low-grade lesions which are associated with these nodular structures. Cytologically, morules exhibit bland cells lacking intercellular bridges and keratinization and express neither high-molecular-weight keratins2–5 nor involucrin.6 They are not related to human papillomavirus infection6 and are thus different from both squamous metaplasia and carcinoma.1 Morules with a characteristic nuclear clearing have been reported in endometria,ovarian endometrioid tumours,colorectal adenomas and carcinomas,13–15 gastric polyps,16 gallbladder adenomas,17 pulmonary blastomas, low-grade adenocarcinomas of fetal lung type, some papillary lung carcinomas,18–24 pancreatoblastomas25,26 and, finally, in the cribriform-morular variant (C-MV) of papillary thyroid carcinomas.2–5,27–29 The so-called ‘peculiar nuclear clearing’ so frequently found in these morules is due to biotin-rich intranuclear inclusions which ultrastructurally have the appearance of thread-like fibrils and consequently should not be misinterpreted as intranuclear cytoplasmic pseudoinclusions or as viral inclusion bodies.2,3,15,19,30,31 In order to demonstrate the usefulness of CD10 in the diagnosis of morular metaplasia in neoplasms in extragenital locations, the immunoexpression of CD10 and β-catenin was studied in seven cases of tumours containing morules with cells displaying biotin-rich optically clear nuclei (BROCN; BROCN-family tumours), corresponding to four instances of CM-V of papillary thyroid carcinoma (one of them in the setting of familial adenomatous polyposis), one case of low-grade adenocarcinoma of fetal lung type, one case of pulmonary blastoma and one pancreatoblastoma in a child with Beckwith–Wiedemann syndrome (Figure 1). Immunohistochemistry was performed on paraffin sections using a universal secondary antibody kit that used a peroxidase-conjugated labelled-dextran polymer (Dako EnVision Peroxidase/diaminobenxidene; Dako, Glostrup, Denmark), in order to avoid misinterpreting endogenous biotin or biotin-like activity in the cell cytoplasm or nuclei as immunopositivity.32 All cases revealed strong CD10 membranous positivity (clone 56C6, dilution 1:10, microwave oven, Tris—ethylenediamine tetraaceticacid; NovoCastra, Newcastle-upon-Tyne, UK) in morules (Figure 1), with some cytoplasmic staining, probably due to diffusion. As a result, CD10 positivity allowed easy identification of morules at low power in the different samples. CD10 immunopositivity also outlined the luminal borders of the neoplastic cells in the glandular formations of all tumours. Aberrant nuclear expression of β-catenin (β-catenin-1, 1:300, microwave oven; Dako) was detected only in neoplastic cells of all samples examined (Figure 1). Squamous metaplasia was negative for CD10, and a membranous but no nuclear pattern of reactivity for β-catenin was seen in a case of the diffuse sclerosing variant (DSV) of papillary thyroid carcinoma concurrently studied as a control.