General characteristics and structure of microvilli
Derivatives of the cell membrane (plasmalemma) represent specific cytoplasmic projections. They are always surrounded externally by the cell membrane. The most significant and widespread types of such structures are microvilli and cilia.
Microvilli are cylindrical, finger-like cytoplasmic evaginations. Electron micrographs clearly show that they are covered by the plasmalemma. Their shape and strictly vertical position are maintained by a reliable internal framework. This supporting apparatus consists of a dense bundle of actin microfilaments extending along the entire longitudinal axis of the projection.
The main task of microvilli is to dramatically and repeatedly increase the surface area of the apical pole of the epitheliocyte. This is of critical importance for ensuring the efficiency of the absorption process of various substances. A typical example of the localization of such structures is the epithelial cells of the small intestine, whose surface faces directly into the lumen of the digestive tract.
Visualization and striated border
Due to their microscopic dimensions (the diameter is only about 100 nm), it is impossible to resolve an individual microvillus under a standard light microscope. A vast multitude of such closely arranged projections visually merge into a single optical structure—a continuous narrow strip at the apical pole of the cell.
This structure is called the brush (or absorptive) border. With standard hematoxylin and eosin staining, it demonstrates pronounced tinctorial properties, namely, oxyphilia (staining pink with acidic dyes). Epitheliocytes possessing such a surface apparatus are classified as bordered or absorptive cells.
When studying histology, it is extremely important to perform differential diagnosis between the concepts of cellular and tissue levels of organization:
- Microvilli (microvilli) are the tiniest cytoplasmic evaginations of a single specific epithelial cell.
- Villi (villi) are large projections of the entire mucous membrane that are externally covered only by an epithelial layer.
Ultrastructure and functions of cilia
Cilia (cilia) are larger specialized cytoplasmic projections that are also covered by the plasmalemma. According to electron microscopy data, they consist of three main components:
- Axoneme — the internal axial shaft (rigid skeleton).
- Plasmalemma — the membrane covering the structure externally.
- Basal body — a formation at the base of the cilium serving for its firm attachment to the cell cytoplasm.
Unlike microvilli, the basis of the axoneme is formed not by microfilaments, but by a complex system of microtubules. It is this organization that provides the cilium with the ability for active wave-like movement. The main function of such an apparatus is the transport of surrounding fluid or mucus in a strictly defined direction (so-called mucociliary clearance).
A striking example of localization is the ciliated cells of the respiratory tract epithelium. If a hematoxylin and eosin-stained section of the trachea is examined under a light microscope at high magnification, the cilia are clearly distinguishable. They are directed straight into the organ lumen.
Morphological organization of the tracheal epithelium
The epithelial layer lining the tracheal lumen is characterized by pronounced heterogeneity. The cells here differ in both shape and size. Due to these morphological differences, cell nuclei are located at different levels, forming so-called nuclear stratification. In total, three levels of nuclear arrangement are distinguished.
Interestingly, the presence of organelles is strictly related to the position of the nucleus in the layer. Far from all epithelial cells possess cilia. The ciliary apparatus is characteristic exclusively of those epitheliocytes whose nuclei form the most superficial (upper) row of cells.
Key differences between microvilli and cilia
To differentiate the two main types of specialized cell surface structures, three main criteria are used, which are conveniently presented in a comparative table.
| Criterion | Microvilli | Cilia |
|---|---|---|
| Dimensions (diameter) | About 100 nm. Individual projections are indistinguishable under a light microscope. | About 200 nm (twice as thick). Clearly visible under light microscopy. |
| Internal cytoskeleton | The supporting framework consists of bundles of actin microfilaments. | The framework (axoneme) is formed by a complex system of microtubules. |
| Functional activity | Increase surface area to facilitate absorption. | Perform active movements to transport fluid and mucus. |
Briefly: answers to topical questions
What is the microtubule arrangement formula in the ciliary axoneme?
The microtubule arrangement formula in the ciliary axoneme is $(9 \times 2) + 2$.
The axoneme represents the internal axial shaft of the cilium, a microtubule framework. Its ultrastructure includes:
- Peripheral microtubules — 9 microtubule doublets.
- Central microtubules — 1 pair of microtubules enclosed in a central sheath.
Radial spokes extend from the peripheral doublets to the central sheath. The space between microtubules is filled with hyaloplasm.
Which proteins are involved in ciliary movement (motor proteins)?
The protein dynein is involved in ciliary movement.
- Dynein forms dynein arms/bridges extending from the peripheral microtubule doublets to adjacent ones.
- The movement mechanism is based on the attachment and detachment of dynein bridges.
- This causes the displacement, or sliding, of adjacent microtubule doublets relative to each other, leading to the bending of the entire structure.
- Impaired formation of dynein arms is associated with primary ciliary dyskinesia, also called immotile cilia syndrome.
To avoid confusing the cytoskeletal basis of these structures, remember: Microvilli are supported by Microfilaments (similar words), while cilia rely on microTubules (rigid "tubes" for powerful wave-like motion).
Class and exam questions
What is the difference between villi and microvilli?
Villi are projections of the entire mucosa (tissue level), whereas microvilli are tiny cytoplasmic protrusions of a single individual epithelial cell (cellular level).
Why are individual microvilli not visible under a light microscope?
Their diameter is only about 100 nm. Due to such microscopic dimensions, numerous microvilli optically merge into a single strip called the brush border.
Do all tracheal epithelial cells possess cilia?
No, only those epitheliocytes whose nuclei form the most superficial (upper) row of the epithelial layer possess a ciliary apparatus.
What color is the brush border stained with standard staining?
When stained with hematoxylin and eosin, the brush border exhibits oxyphilic properties, meaning it stains pink with acidic dyes.
What else to study on this topic
- Detailed structure of the axoneme and spatial arrangement of microtubules.
- Organization of the basal body and its connection with the centrosome.
- The molecular mechanism providing active ciliary movement.
- Structural and localization features of stereocilia (modified microvilli).
- Cell types in the pseudostratified ciliated respiratory epithelium.
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