Exocrine secretions

Pulmonary surfactant
Pulmonary surfactant secretion is located in type II pneumocytes in cytoplasmic bodies recognized by a more or less rounded profile and multiple concentric laminae. Often the laminae are closely apposed giving an overall dense appearance to the secretion.

Mucigen granules
Mucigen granules are rounded, single-membraned granules containing abundantly glycosylated or sulfated glycoproteins. They are typically pale-staining, often having a flocculent, finely textured interior; there may also be a reticulate sub-structure and a small dense granule which may be centrally or eccentrically placed. Mucigen granules may fuse together to produce intracellular pools of mucin.

Serous or zymogen granules
The serous granules of salivary gland and the zymogen granules of exocrine pancreas are ultrastructurally similar. They are often large (as big as 1 µm), and tend to aggregate near lumina. They are single membrane-bound, but this is sometimes not obvious because of the homogeneously and highly electron-dense content which abuts the membrane, typically without a halo.


Figure 1. A. Surfactant inclusions in a normal type II pneumocyte: lung biopsy for metastatic adenocarcinoma. X12,600. B. Individually recognizable but closely juxtaposed mucigen granules with a lightly textured and in some instances reticulate interior, along with a small central density, in a secretory cell from nasal epithelium. X13,300.

Figure 2. A. Zymogen granules, in acinar cells with extensive rER, from normal exocrine pancreas. X5,400. B. Higher magnification of zymogen granules. Note large size (many about 1 µm across), high density and uniform internal substructure, clustering in apical pole of cell, and presence around a narrow lumen with microvilli (arrow). X38,900.
  
See also Acinic cell carcinoma, Bronchioloalveolar carcinoma and Whatizzit #19