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Epithelia invaginate during fetal development to form exocrine glands that most often have two types of secretory units called adenomeres or acini. Regardless of whether they are serous or mucous, the epithelial secretory cells in the acini are roughly pyramidal in shape because they are arranged around a small, central lumen, which is often difficult to locate. The serous cell’s cytoplasm is stained deeply pink, and its nucleus is round. The mucous cell’s cytoplasm often stains very lightly and its nucleus is dark and flattened against the base of the cell. Both types of exocrine glands are found in the respiratory and digestive systems.