3‑D 'morula' clusters flagged

Cytology is the practice of reading single cells and small cell groups on a slide, and one red flag is a tight three-dimensional cluster that looks like a compact ball. A cytology educator highlighted that pattern this week in a post about “morula”-like groups seen in malignant smears. (x.com) On fluid and smear specimens, pathologists often start with shape before they move to stains: malignant epithelial cells can exfoliate as smooth-edged, crowded clusters, while benign mesothelial cells more often appear dispersed or in smaller groups. In pleural cytology, Pathology Outlines notes that clusters of more than 12 mesothelial cells are unusual in simple hyperplasia. (pathologyoutlines.com) The everyday comparison is spacing. Reactive mesothelial cells often keep tiny gaps, called “windows,” between neighboring cells, and their cluster borders can look scalloped or knobby rather than packed solid. (acsjournals.onlinelibrary.wiley.com) That is why a dense ball of cells gets attention on a smear. Reviews of effusion cytology say metastatic adenocarcinoma usually has smooth cluster contours and often stands out as a second, distinct cell population against the background cells in the specimen. (acsjournals.onlinelibrary.wiley.com) The distinction matters because serous fluid from the pleura, peritoneum, or pericardium is often the first — and sometimes the only — sample available for diagnosis. A 2023 study from MD Anderson said ruling in or out an epithelial component, meaning metastatic carcinoma, is the first step in working up these effusions. (acsjournals.onlinelibrary.wiley.com) Morphology alone is not always enough. Reactive mesothelial cells can become enlarged, multinucleated, and form three-dimensional groups, which is why cytology texts describe malignant-versus-reactive interpretation as a routine diagnostic problem. (pmc.ncbi.nlm.nih.gov, pmc.ncbi.nlm.nih.gov) When the picture is equivocal, labs usually add immunostains on a cell block, which is a paraffin-embedded pellet made from the fluid so it can be tested more like a biopsy. In the MD Anderson series of 229 cytology specimens, claudin-4 was positive in 134 of 143 carcinoma samples and in none of 39 mesotheliomas or 47 benign effusions. (acsjournals.onlinelibrary.wiley.com) Preparing that extra material can improve the read. In a 60-sample pleural fluid study, the cell-block method produced 15% more malignant yield than conventional smear review alone and gave better architectural patterns for interpretation. (pmc.ncbi.nlm.nih.gov) So the “morula” cue is not a diagnosis by itself; it is an architectural warning sign that tells a cytologist to look harder for metastatic carcinoma and, if needed, confirm it with ancillary testing. That is why a compact three-dimensional cluster on a slide still carries outsized weight in daily cytology practice. (acsjournals.onlinelibrary.wiley.com, acsjournals.onlinelibrary.wiley.com)