Which histologic feature distinguishes pancreatic acinar cells from pancreatic islets?

• A. Acinar cells have basophilic cytoplasm with conspicuous rough endoplasmic reticulum and zymogen granules.
• B. Islets have basophilic cytoplasm with conspicuous rough endoplasmic reticulum and zymogen granules.
• C. Islets appear highly vascularized and densely eosinophilic.
• D. Acinar cells lack zymogen granules.

Answer: (A)

Pancreatic acinar cells are enzyme-secreting units of the exocrine pancreas, and their histology reflects that role. Their cytoplasm is rich in rough endoplasmic reticulum, which makes it basophilic, because ribosomes on the RER stain darkly. They also contain zymogen granules—membrane-bound secretory granules packed with digestive enzymes—at the apical region of the cell. This combination of basophilic cytoplasm with conspicuous zymogen granules is the hallmark that sets acinar cells apart from the islets of Langerhans, which are endocrine and have little to no zymogen granules and a less basophilic cytoplasm, reflecting their different secretory needs and vascularization. Thus, the feature described in the correct option—basophilic cytoplasm due to abundant rough endoplasmic reticulum and presence of zymogen granules—best distinguishes acinar cells from islets.