Structural Types and Secretion Modes of Glandular Epithelium

Unicellular Glands

The simplest form of a gland is the unicellular gland. This consists of a single, specialized secretory cell scattered among other non-secretory cells. A classic example is the mucous-secreting cell (often called a goblet cell), which produces mucus to lubricate surfaces.

Multicellular Glands

Most glands in the human body are multicellular, meaning they are composed of many cells working together. These are further categorized by the structure of their ducts.

1. Simple Glands

In a simple gland, the duct does not branch before it reaches the secretory portion of the gland. Depending on the shape of the secretory part, they are subdivided into:

• Simple tubular: A straight, tube-like structure that opens onto a surface, such as the intestinal glands of the small intestine.
• Simple branched tubular: A tube-like gland where the secretory portion branches, such as the gastric glands in the stomach.
• Simple coiled tubular: A long, coiled tube with a long duct, exemplified by merocrine sweat glands in the skin.
• Simple branched alveolar: The secretory portions expand into saclike compartments along the duct, such as the sebaceous (oil) glands of the skin.

2. Compound Glands

Compound glands are more complex; their ducts branch repeatedly before finally reaching the secretory portions.

• Compound tubular: The secretory portions are tubules that extend from the various branches of the duct system, such as the bulbourethral glands in males.
• Compound alveolar: These consist of irregularly branched tubules that end in numerous saclike outgrowths, such as the mammary glands.

Modes of Secretion

Beyond their physical shape, exocrine glands are also classified by their mode of secretion. This describes the biological mechanism the cell uses to release its product.

Merocrine Secretion

In merocrine glands, the secretory product is released via exocytosis. The cell remains completely intact throughout the process, simply releasing small vesicles containing the product to the cell surface. This is a highly efficient method used by sweat glands.

Apocrine Secretion

Apocrine secretion is more "destructive" to the cell's structure. In this mode, the secretory product accumulates near the top of the cell, and the apex (the top portion) of the cell is lost or pinched off along with the secretion.

Holocrine Secretion

Holocrine secretion is the most extreme mode. To release the product, the entire cell ruptures and dies. The contents of the cell become the secretion itself. A primary example is the sebaceous gland, where cells fill with lipids and then burst to release oil onto the skin.

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