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and cell debris are discharged into the lumen of the gland. This mechanism is found in sebaceous glands of skin and the tarsal (Meibomian) glands of the eyelid.

Exocrine glands are classified as either unicellular or multicellular

Unicellular glands are the simplest in structure. In unicellular exocrine glands, the secretory component consists of single cells distributed among other nonsecretory cells. A typical example is the goblet cell, a mucus-secreting cell positioned among other columnar cells (Fig. 4.26). Goblet cells are located in the surface lining and glands of the intestines and in certain passages of the respiratory tract.

Multicellular glands are composed of more than one cell. They exhibit varying degrees of complexity. Their structural organization allows subclassification according to the arrangement of the secretory cells (parenchyma) and the presence or absence of branching of the duct elements.

The simplest arrangement of a multicellular gland is a cellular sheet in which each surface cell is a secretory cell. For example, the lining of the stomach and its gastric pits is a sheet of mucus-secreting cells (Fig. 4.27).

Other multicellular glands typically form tubular invaginations from the surface. The end pieces of the gland contain the secretory cells; the portion of the gland connecting the secretory cells to the surface serves as a duct. If the duct is unbranched, the gland is called simple; if the duct is branched, it is called compound. If the secretory portion is shaped like a tube, the gland is tubular; if it is shaped like a flask, the gland is alveolar or acinar; if the tube ends in a sac-like dilation, the gland is tubuloalveo-lar. Tubular secretory portions may be straight, branched, or coiled; alveolar portions may be single or branched. Various combinations of duct and secretory portion shapes are found in the body. Classification and description of exocrine glands may be found in Table 4.3.

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