Unicellular and Multicellular Glands
Glandular cells can function individually or assemble into complex structures.
- Unicellular glands are scattered independently directly within the covering epithelial sheet (endoepithelial). A classic example is the goblet cell (exocrinocytus caliciformis), found in the intestinal mucosa and respiratory tract. They produce polysaccharide-rich mucus and are identified on histological preparations using the PAS (Periodic Acid–Schiff) reaction, which stains their cytoplasm magenta.
- Multicellular glands form independent anatomical structures or complex organs.
Exocrine and Endocrine Glands
Based on the direction of secretion release, all multicellular glands are divided into two fundamentally different groups:
- Endocrine glands (ductless glands). They synthesize hormones and release them directly into the blood. They lack excretory ducts, and their cells (endocrinocytes) are organized into clusters or cords closely entwined with blood capillaries.
- Exocrine glands. They release secretions into the external environment (onto the skin or into organ cavities). They consist of two structural components:
- Secretory end pieces (acinus/tubule): hollow pouches or tubes resting on a basement membrane where exocrinocytes produce the secretion.
- Excretory ducts: lined by covering epithelium, serving to transport the product out of the gland.
Types of Secretion
Depending on what happens to the cell during the release of secretion, three mechanisms are distinguished:
- Merocrine (eccrine): the cell completely preserves its structural integrity (e.g., salivary or uterine glands).
- Apocrine: partial destruction occurs — the apical (top) part of the cell detaches and is released along with the secretion (e.g., mammary glands).
- Holocrine: the cell completely disintegrates and becomes the secretion itself (e.g., sebaceous glands of the skin).
Morphological Classification of Exocrine Glands
A gland's name is constructed based on the structure of its two components — the excretory duct and the secretory end piece:
- By duct branching:
- Simple: the duct does not branch.
- Compound: the duct branches.
- By end piece arrangement:
- Unbranched: a single end piece drains into a single duct.
- Branched: multiple end pieces drain into a single excretory duct.
- By the geometric shape of end pieces:
- Tubular (cylindrical).
- Alveolar (sac-like/rounded).
- Tubuloalveolar (combined).
For example, a sebaceous gland is a simple branched alveolar gland. This means it has a short, unbranched duct into which a cluster of several secretory acini opens.
End Pieces and Auxiliary Apparatus
Using a compound branched tubuloalveolar gland (such as the submandibular salivary gland) as an example, we can examine different types of secretory end pieces:
- Serous (protein) end pieces: alveolar in shape. They consist of dark cells (serocytes) with prominent cytoplasmic basophilia and round nuclei.
- Mixed (protein-mucous) end pieces: tubular in shape. Most of the end piece is composed of light mucous cells (mucocytes) with flattened basal nuclei, while serocytes cap the end of the tubule to form characteristic "demilunes" (serous demilunes).
Glands use a specialized apparatus to expel their secretions. Stellate myoepithelial cells lie between the basement membrane and the secretory cells. With their branching processes, they embrace the secretory end piece and, upon contraction, literally squeeze the secretion into the ducts.