Specialized Cell Surface Structures

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 outgrowths. Electron micrographs clearly show that they are covered by the plasmalemma. Their shape and strictly vertical position are maintained by a robust internal skeleton. 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 sharply and multiply increase the area of the apical (top) surface of the epithelial cell. This is critical for ensuring the efficiency of absorption 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 the Brush Border

Due to their microscopic dimensions (diameter is only about 100 nm), visualizing a single microvillus under a standard light microscope is impossible. A vast multitude of such closely packed projections optically merge into a single structural unit — 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 (H&E) staining, it demonstrates pronounced tinctorial properties, specifically oxyphilia (it stains pink with acidic dyes). Epithelial cells possessing such a surface apparatus are classified as brush-border or absorptive cells.

When studying histology, it is crucial to differentiate between concepts at the cellular and tissue levels of organization:

• Microvilli (microvilli) are the tiny cytoplasmic projections of one specific epithelial cell.
• Villi (villi) are large folds of the entire mucous membrane that are merely covered externally by an epithelial sheet.

Ultrastructure and Functions of Cilia

Cilia (cilia) are larger specialized cytoplasmic projections that are also covered by the plasmalemma. According to electron microscopy, they consist of three main components:

1. Axoneme — the internal axial shaft (rigid skeleton).
2. Plasmalemma — the membrane covering the structure externally.
3. Basal body — a structure 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. This exact organization provides the cilium with the capacity for active wave-like movement. The main function of this apparatus is the movement 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 point directly into the lumen of the organ.

Morphological Organization of the Tracheal Epithelium

The epithelial sheet lining the lumen of the trachea is characterized by marked 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. There are three distinct levels of nuclear positioning.

Interestingly, the presence of organelles is strictly related to the position of the nucleus within the sheet. Not all epithelial cells possess cilia. The ciliary apparatus is characteristic exclusively of those epithelial cells whose nuclei form the most superficial (upper) layer of cells.

Key Differences Between Microvilli and Cilia

To differentiate the two main types of specialized cell surface structures, three main criteria are used, conveniently summarized in the comparative table below.