Morules in endometrial carcinoma and benign endometrial lesions differ from squamous differentiation tissue and are not infected with human papillomavirus

Abstract

Background: Squamous differentiation/squamous metaplasia is often associated with endometrial adenocarcinoma and benign lesions, such as endometrial hyperplasia and chronic endometritis. Morules have distinct histological characteristics, and are referred to as squamous metaplasia or squamoid metaplasia.

Aim: To focus on the histological characteristics of morules and clarify the difference between morules and squamous differentiation.

Materials/methods: Twenty endometrioid carcinomas with morules or squamous differentiation, five adenosquamous carcinomas, and eight non-carcinomatous endometrial lesions with morules were investigated. Numerous antibodies for epithelial membrane antigen (EMA), involucrin, cytokeratins, neuropeptides, and oncofetal antigens were used for immunohistochemistry. In situ hybridisation and polymerase chain reaction were used to detect human papillomavirus (HPV).

Results: The morules observed were uniform cell clusters, with no squamous differentiation. They were immunonegative for epithelial antigens including involucrin, EMA, and cytokeratins, but were positive for neurone specific enolase. A few morules were immunopositive for acetylcholine esterase, and one case was positive for somatostatin; neither oncofetal nor proliferative cell markers, including blood group A, B, and AB, or other neuropeptides were demonstrated in the morules. HPV DNA was not found in either the morules in the carcinomas or in the benign lesions. However, true squamous differentiation tissue in four endometrioid carcinomas and two adenosquamous carcinomas was HPV positive using in situ hybridisation.

Conclusion: Morules are histologically distinct from squamous metaplasia/squamous differentiation tissue. Morules are thought to be neuroectodermal-like cell clusters, and are not infected with HPV. In contrast, some of the true squamous differentiation tissue was associated with HPV infection.

Figure 1

(A) Morules in endometrioid carcinoma. The cells of the morules have a large amount of cytoplasm, which is faintly eosinophilic. Arrows indicate the morules that seem to be linked to glandular epithelial cells. Arrowheads indicate morules outside the glandular structures. The nuclei are round or oval with the chromatin forming a thin peripheral rim. Haematoxylin and eosin (H&E) staining; original magnification, ×100. (B) Morules in a benign endometrial lesion (atypical endometrial hyperplasia). Arrows indicate the morules, one of which is found outside the glands. The morphology of the morules is the same as that seen in the carcinoma cases. H&E staining; original magnification, ×150. (C) Squamous differentiation in an endometrioid carcinoma. The asterisks indicate squamous differentiation. Note the prickle cell differentiation and keratinisation. H&E staining; original magnification, ×100. (D) Squamous differentiation and morules simultaneously found in the endometrioid carcinoma. Arrows indicate squamous differentiation. Arrowheads indicate morules. H&E staining; original magnification, ×100.

Figure 2

(A) Immunohistochemistry for cytokeratins (AE1/AE3 antibodies) in morules. The asterisk indicates the morules. No immunoreactivity was seen in the morules, although the glandular epithelial cells (atypical endometrial hyperplasia) were positive. Original magnification, ×100. (B) Immunohistochemistry for cytokeratin (antibody to cytokeratin 10) in morules. The morules and the adenocarcinoma components (atypical endometrial hyperplasia) were immunonegative. The arrows indicate the morules. Original magnification, ×150. (C) Immunohistochemistry for epithelial membrane antigen (EMA) in morules. The morules were negative for EMA (asterisk), but the glandular epithelial cells (endometrioid carcinoma) were positive. Original magnification, ×150. (D) Immunohistochemical demonstration of neurone specific enolase (NSE) in morules. Staining for NSE was strong in the morules (arrowheads) (endometrioid carcinoma). Original magnification, ×150. (E) Immunohistochemical demonstration of cytokeratin in morules. Cytokeratin (AE1/AE3 antibodies) was found in a few cells (asterisk) in morules, but serial sections revealed that they were glandular epithelial cells. Glandular epithelial cells were positively stained (atypical endometrial hyperplasia case). Original magnification, ×150.

Figure 3

(A) Immunohistochemical demonstration of cytokeratin (AE1/AE3 antibodies) in cells with squamous differentiation. The asterisk indicates the keratin positive cells with squamous differentiation. All cells with squamous differentiation stained positively. The adenocarcinoma cells neighbouring the squamous differentiation tissue (arrowheads) were also immunopositive (endometrioid carcinoma). Original magnification, ×150. (B) Immunohistochemical demonstration of involucrin in tissue with squamous differentiation. The asterisk indicates involucrin positive cells (squamous differentiation tissue). Original magnification, ×150. Arrowheads indicate morules that are negative for involucrin (same case as that shown in fig 1D). (C) Immunohistochemical demonstration of proliferating cell nuclear antigen (PCNA) in morules. The arrow indicates morules. PCNA was detected in the nuclei of the cancer cell, but not in the morules (endometrioid carcinoma). Original magnification, ×100.

Figure 4

(A) Demonstration of human papillomavirus (HPV) by the use of in situ hybridisation. HPV DNA (arrows and arrowheads) is demonstrated in the nuclei of cells with squamous differentiation in an endometrioid carcinoma. According to Cooper’s criteria, HPV was present in the integrated (arrowheads) and episomal (arrows) forms (type 3, Cooper’s criteria). Original magnification, ×150. (B) Demonstration of HPV DNA by use of in situ hybridisation. Neither the neoplastic glandular cells nor the morules harbour HPV DNA (endometrioid carcinoma case). The arrows indicate morules. Original magnification, ×150.