Classification and Structure
Histologically, venules are divided into three sequential types. As the vessel increases in caliber, its wall progressively thickens and acquires new cellular elements.
- Postcapillary venules
- Diameter: up to 30 µm.
- Structure: direct continuation of capillaries. The wall has a similar structure to a capillary but contains significantly more pericytes (by contrast, sinusoids lack pericytes entirely).
- Function: the primary site of leukocyte diapedesis (migration) from the bloodstream. In lymphoid organs (thymus, lymph nodes), this role is performed by specialized high endothelial venules (HEVs). In hemopoietic organs, migration occurs through venous sinuses.
- Collecting venules
- Diameter: up to 50 µm.
- Structure: pericytes become numerous and form a continuous layer over the endothelium. Single smooth muscle cells (myocytes) begin to appear in the wall, and a loose adventitial layer forms externally.
- Muscular venules
- Diameter: up to 100 µm.
- Structure: pericytes disappear completely. The three classic layers (tunicae) appear. The tunica media consists of 1–2 continuous layers of smooth myocytes. An important distinction from arterioles is the absence of an internal elastic lamina and the often longitudinal (rather than circular) orientation of the muscle cells.
Features of Pial Vessels
Histology has exceptions to general rules. One such exception is the pia mater.
In this tissue, venules (and even veins) contain no smooth myocytes, regardless of their caliber. Therefore, the term "muscular venules" does not apply here.
The microvasculature here is studied using whole-mount (flat) preparations: the membrane is not sectioned into thin slices, but carefully spread out on a glass slide. This allows simultaneous visualization of arterioles, capillaries, and venules within a single interconnected network.
How to Differentiate a Venule from an Arteriolar on a Slide
When examining a whole-mount preparation of the pia mater, it is important to visually differentiate incoming and outgoing vessels.
Arterioles:
- Main feature — transverse striation. This results from the circularly arranged smooth myocytes in the media.
- Cell nuclei have different orientations: endothelial and adventitial cells are oriented along the vessel axis, while myocytes are transverse.
- Red blood cells in the lumen are often invisible due to the denser, less transparent wall.
Venules:
- Main feature — the vessel appears as a tube densely packed with red blood cells, which easily shine through the thin wall.
- Transverse striation is completely absent (due to the lack of myocytes).
- The wall is exceptionally simple, consisting only of endothelium and outer loose connective tissue (adventitia).