MULTICELLULAR EXOCRINE GLANDS:

Multicellular exocrine glands are multicelled glands and they occur in several forms. They are formed by invagination from an epithelial sheet (in blue) (like the endocrine glands) but will never lose the duct connecting them to the free surface of the epithelial sheet (surface of the skin or lumen of the open body cavities). The epithelium dips or grows down from the surface into the underlying tissues to form a simple or ramified tube (in green). The blind ends of these tubes constitute the secretory parts of the gland (in red)) and may stay tubular or expand to form round bags called acini or alveoli.

To classify multicellular exocrine glands look at

1. the structure of their duct systems :
• if the gland has a single non ramified duct connecting its secretory part to the surface, it will be called simple
• if the gland has a ramified duct system, it will be called compound.

2. the structure of their secretory parts:
• if the secretory parts form little tubes, the gland will be called tubular. Tubes can be coiled and in this case glands will be called coiled tubular.
• if the secretory parts form little bags, it will be called alveolar or ascinar
• Both shapes (tubes and bags) can be present in a compound exocrine glands. In such a case the gland will be called tubulo-alveolar.
• Most of the time each bag (or tube) is connected to its own private duct. However, some simple exocrine glands have several bags (or tubes) sharing the same duct: they are called branched.

3. their secretory behavior:
• in merocrine (or eccrine) glands, the secretory product is expelled out of the cells into the lumen through exocytosis;
• in holocrine glands, the secretory product is accumulated in the cytoplasm of the secretory cells. Then, the cells die and are released with their content in the lumen of the glands. The dead cells will rupture and the secretory product will be released. The dead cells will be replaced by division of the underlying cells.
• in apocrine glands, the product of secretion accumulates in the apex of the secretory cells (the cell part adjacent to the lumen). The cells will then then pinch and cut off their top part. The top parts full of secretory product will be discharged into the lumen of the glands. The secretory cells repair their damage and repeat the process again and again.

"Compound - acinar - holocrine - branched - merocrine - simple - tubular - coiled tubular - tubulo-alveolar - apocrine"
Are you getting all mixed up?

The following exercise will squelch any confusion lingering in your mind.