Serous demilune

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A serous demilune is a cap in the shape of a half-moon (hence the name, "demilune") on some salivary glands.

The serous demilune is an artifact from traditional methods of preparing samples. Samples are traditionally preserved and fixed in formaldehyde. When samples were preserved by quick-freezing in liquid nitrogen and then fixed with osmium tetraoxide in acetone, no demilunes are found. Examination showed that the serous cells and mucosal cells were aligned in the acinus. The traditional preparation caused serous cells to swell during fixation, and they popped out of their alignment. After sectioning the cells resembled the common demilune shape, and were so named.

When the gland has this demilunar structure, it produces both serous and mucous secretions, and is thus classified as "mixed".