The intestinal mucosa contains numerous small tubular exocrine glands. In these glands, the same tubular structure performs both secretory and conducting functions. In most exocrine glands, however, the secretory and conducting portions are distinct structures.
Secretory Portion
A gland may contain one or more types of secretory cells. The characteristics and arrangement of these cells vary among different types of glands.
In most exocrine glands, the secretory cells are arranged around a central space called the lumen. Each secretory cell releases its product into this lumen. The lumen of the secretory portion is continuous with the lumen of the excretory duct, allowing the secretion to pass from the secretory portion into the duct.
The intestinal mucosa has a large amount of small tubular exocrine glands that execute both the secretory and excretory functions. In most exocrine glands, however, these portions are distinct structures.
Secretory portion
The gland may be formed by one or more cell types whose characteristics and arrangements are specific to the various gland types.
The cells of the secretory portions of most exocrine glands gather around central space: its lumen into which the secretory product of each cell is released. This lumen is continuous with the lumen of the excretory ducts.
Excretory portion
The wall of an excretory duct is formed by a lining epithelium arranged as a hollow tube around a central lumen.
The epithelium lining the initial portion of the duct is often simple cuboidal epithelium. In small glands with short ducts, this type of epithelium may be maintained throughout the entire length of the duct.
In larger and more complex glands, the ducts are longer and usually join with other ducts, forming progressively larger and wider ducts. The epithelium lining these larger ducts may be simple columnar epithelium, whereas the largest ducts may be lined by stratified epithelium
(click to view an excretory duct lined by stratified epithelium)
Longer and bigger ducts of more complex glands enlarge, blend with other ducts of the gland, and form even larger and wider ducts. The epithelial wall of these larger ducts may be a simple columnar epithelium and in even larger ducts a stratified epithelium (click to view an excretory duct whose wall is a stratified epithelium).
Observe the Gland and Its Duct
The image on this page shows a section containing an entire small gland. Its secretory portion, formed by several groups of secretory cells, is highlighted in light blue when you place the mouse cursor over the image or click on it.
The excretory duct, highlighted in dark blue, is lined by simple cuboidal epithelium. Because the image was obtained at very low magnification, however, this epithelial layer cannot be easily distinguished.
The gland and its duct are embedded in connective tissue that forms part of the wall of the esophagus. Small areas of smooth muscle can also be seen within this connective tissue.
The lumen of the esophagus is lined by stratified squamous epithelium, highlighted in red.
Small gland in the wall of the esophagus. Staining: hematoxylin and eosin. Very small magnification.
