Unicellular and multicellular glands
Glandular cells can function individually or assemble into complex structures.
- Unicellular glands are located independently of each other directly within the covering epithelium layer (endoepithelially). A classic example is goblet cells (exocrinocytus caliciformis), which are found in the intestinal mucosa and respiratory tract. They produce mucus rich in polysaccharides. On histological preparations, they are detected using the PAS reaction (cytoplasm stains magenta).
- Multicellular glands form independent anatomical structures or complex organs.
Exocrine and endocrine glands
Depending on the direction of secretion release, all multicellular glands are divided into two fundamentally different groups:
- Endocrine (ductless) glands. They produce hormones and release them directly into the blood. They lack excretory ducts, and their cells (endocrinocytes) are organized into groups or cords closely intertwined 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: hollow pouches or tubules on the basement membrane where exocrinocytes produce the secretion.
- Excretory ducts: lined with covering epithelium and serving to transport the product out of the gland.
Types of secretion
Depending on what happens to the cell during the secretion process, three mechanisms are distinguished:
- Merocrine (eccrine): the cell completely preserves its integrity (e.g., salivary or uterine glands).
- Apocrine: partial destruction occurs — the apical (upper) part of the cell detaches and is released along with the secretion (e.g., mammary glands).
- Holocrine: the cell is completely destroyed and itself becomes the secretion (e.g., sebaceous glands of the skin).
Morphological classification of exocrine glands
The name of a gland is based on the structure of its two components — the excretory duct and the end piece:
- By excretory duct:
- Simple: the duct does not branch.
- Compound: the duct branches.
- By end pieces:
- Unbranched: there is one end piece per duct.
- Branched: several end pieces open into a single excretory duct.
- By geometric shape of the end pieces:
- Tubular (cylinder).
- Alveolar (pouch/acinus).
- Alveolar-tubular (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 pouches opens.
Secretory end pieces and auxiliary apparatus
Using the example of a compound branched alveolar-tubular gland (submandibular salivary gland), we can examine different types of end pieces:
- Serous (protein) end pieces: have an alveolar shape. They consist of dark cells (serocytes) with prominent cytoplasmic basophilia and rounded nuclei.
- Mixed (protein-mucous) end pieces: have a tubular shape. The majority consist of light mucocytes with flattened basal nuclei, while serocytes are located at the bottom of the end piece, forming characteristic "demilunes".
To expel the secretion, glands use a special apparatus. Star-shaped myoepithelial cells lie between the basement membrane and the secretory cells. With their processes, they wrap around the end piece and, upon contraction, literally squeeze the secretion into the ducts.
Briefly: answers to topical questions
What phases does the secretory cycle of a glandular cell include?
The secretory cycle of a glandular cell includes five sequential phases.
- Uptake of precursors — necessary substances enter the secretory cell from the bloodstream.
- Synthesis — formation of the secretion inside the cell.
- Transport and maturation — movement of the product through the cytoplasm and its final formation.
- Accumulation — storage of the formed secretion.
- Release — elimination of the product from the cell into the external or internal environment (often via exocytosis).
From which embryonic germ layers does the glandular epithelium develop?
Glandular epithelium develops from various embryonic germ layers depending on its localization.
- Surface ectoderm — gives rise to merocrine and apocrine sweat glands, as well as sebaceous glands.
- Endoderm — the epithelium of the pancreas (exocrine acini and endocrine islets) develops from the endoderm of the middle section of the intestinal tube.
- Specific embryonic primordia — serve as the source of development for thyroid follicles.
What are the chemical composition features of the secretion of serous and mucous glands?
The chemical composition of the secretion depends on the type of secretory cells (end pieces).
| Type of end piece | Secretion characteristics |
|---|---|
| Serous (protein) | Protein secretion; for serous end pieces of salivary glands, watery saliva with a high content of amylase, peroxidase, glycosaminoglycans, and salts is indicated. Serocytes also synthesize the secretory component — a glycoprotein. |
| Mucous | Mucous, viscous secretion; mucocytes produce mucous saliva containing glycoproteins and mucins. Mucin imparts viscosity to the saliva and forms the mucous component of saliva. |
To easily remember the mechanisms of secretion based on cell fate, use this association: Merocrine — Mild (the cell remains intact), Apocrine — Amputation (the apex detaches), Holocrine — Death (the cell is completely destroyed).
Class and exam questions
What is the main difference between endocrine and exocrine glands?
Endocrine glands lack excretory ducts and release their products (hormones) directly into the surrounding blood capillaries. Exocrine glands have ducts and release secretions outward or into organ cavities.
What does the term "simple" mean in the name of a gland?
The word "simple" describes exclusively the excretory duct, meaning it has no branches. If the duct branches in a tree-like manner, the gland is called compound.
Which gland serves as an example of a simple unbranched tubular gland?
A typical example is the endometrial uterine glands. They appear as straight tubules (which become coiled in the secretory phase) opening into a short, unbranched excretory duct.
What else to study on this topic
- Lobular structure of compound glands and the system of interlobular and intralobular ducts.
- Chemical composition of the secretion: protein, mucous, sebaceous, and mixed glands.
- Principles of the PAS reaction for histochemical detection of mucus (mucins) in cells.
- Histological picture of uterine glands during different phases of functional activity.
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