In some epithelia, individual cells secrete a substance that does not reach the bloodstream but rather affects other cells within the same epithelium. Such secretory activity is referred to as paracrine. The secretory material reaches the target cells by diffusion through the extracellular space or immediately subjacent connective tissue.
TABLE Types of Glands 5.5 Exocrine Glands Endocrine Paracrine Merocine Apocrine Holocrine Glands Glands
Cells of exocrine glands exhibit different mechanisms of secretion.
The cells of exocrine glands have three basic release mechanisms for secretory products (see Table 5.5):  Merocrine secretion. This secretory product is delivered in membrane-bounded vesicles to the apical surface of the cell. Here vesicles fuse with the plasma membrane and extrude their contents by exocytosis. This is the most common mechanism of secretion and is found, for example, in pancreatic acinar cells.
 Apocrine secretion. The secretory product is released in the apical portion of the cell, surrounded by a thin layer of cytoplasm within an envelope of plasma membrane. This mechanism of secretion is found in the lactating mammary gland, where it is responsible for releasing large lipid droplets into the milk. It also occurs in the apocrine glands of skin, ciliary (Moll’s) glands of the eyelid, and the ceruminous glands of the external auditory meatus.
 Holocrine secretion. The secretory product accumulates within the maturing cell, which simultaneously undergoes programmed cell death. Both secretory products 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. 5.38). 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. 5.39).
FIGURE 5.38 • Unicellular glands. Photomicrograph of intestinal epithelium showing single goblet cells (arrows) dispersed among absorptive cells. Each goblet cell may be regarded as a unicellular gland—the simplest exocrine type gland. 350.
FIGURE 5.39 • Mucus-secreting surface cells of stomach.
Photomicrograph of stomach surface. The epithelial cells lining the surface are all mucus-secreting cells, as are the cells lining the gastric pits (P). The cells of the gastric pit form simple tubular glands. 260.