Classification and structure
Histologically, venules are divided into three consecutive types. As the vessel increases in size, its wall gradually thickens and acquires new cellular elements.
- Postcapillary venules
- Diameter: up to 30 µm.
- Structure: they are a direct continuation of capillaries. The wall has a similar structure to that of a capillary but contains significantly more pericytes (by comparison, sinusoids completely lack pericytes).
- Function: this is the primary site of leukocyte diapedesis (emigration) from the bloodstream. In lymphoid organs (thymus, lymph nodes), this task is performed by specialized high endothelial venules. In hematopoietic organs, migration occurs via 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 begin to appear in the wall, and a loose adventitial layer forms on the outside.
- Muscular venules
- Diameter: up to 100 µm.
- Structure: pericytes completely disappear. The three classical tunics appear. The middle tunic (tunica media) is represented by 1–2 continuous layers of smooth muscle cells. An important difference from arterioles is the absence of the internal elastic lamina and often the longitudinal (rather than circular) orientation of muscle cells.
Features of leptomeningeal vessels
In histology, there are exceptions to the general rules. One such exception is the pia mater (leptomeninges).
In this organ, venules (and even veins) do not contain smooth muscle cells regardless of their caliber. Therefore, the term "muscular venules" is not applicable here.
The study of the microvasculature here is carried out on a whole-mount (flat) preparation: the membrane is not cut into thin sections, but carefully spread out on a glass slide. This allows simultaneous visualization of arterioles, capillaries, and venules within a single network.
How to distinguish a venule from an arteriole on a slide
When studying a whole-mount preparation of the pia mater, it is important to be able to visually differentiate between incoming and outgoing vessels.
Arterioles:
- The main feature is transverse striation. It arises due to circularly arranged smooth muscle cells in the media.
- Cell nuclei are oriented differently: in endothelial cells and adventitial cells — along the vessel axis, and in smooth muscle cells — across it.
- Erythrocytes in the lumen are often not visible due to the denser and less transparent wall.
Venules:
- The main feature is that the vessel looks like a tube densely packed with erythrocytes, which are clearly visible through the thin wall.
- Transverse striation is completely absent (due to the lack of smooth muscle cells).
- The wall is extremely simple: it consists only of endothelium and loose connective tissue on the outside (adventitia).
Briefly: answers to topical questions
What specific tunics make up the wall of a muscular venule?
The wall of a muscular venule has three classical tunics:
- Tunica interna.
- Tunica media (tunica media) — fully developed, consisting of 1–2 continuous layers of smooth muscle cells.
- Tunica externa.
Additional features of a muscular venule by origin: pericytes disappear; internal elastic lamina is absent; orientation of smooth muscle cells is often longitudinal rather than circular.
Which adhesion molecules are involved in leukocyte diapedesis in postcapillary venules?
Leukocyte emigration through postcapillary venules involves the following adhesion molecules.
The following are expressed on activated vascular endothelium:
- P-selectins and E-selectins.
- L-selectin receptors — addressins.
- ICAM-1 and VCAM-1.
The following are expressed on blood leukocytes:
- L-selectins.
- P- and E-selectin receptors — PSGL-1 and other addressins.
- Integrins — LFA-1, Mac-1, p150,95, and αDβ2; they are in an inactive form.
During the rolling phase, leukocyte L-selectins interact with endothelial addressins, while endothelial E- and P-selectins interact with leukocyte addressins.
To distinguish a muscular venule from an arteriole on a section, look for the "accordion" pattern. An arteriole has an internal elastic lamina, which often appears as a wavy line on a slide. A venule has a thin, collapsed wall and lacks an elastic lamina.
Class and exam questions
Through which vessels do leukocytes enter tissues?
The primary site of leukocyte diapedesis is postcapillary venules. In lymph nodes and the thymus, this occurs through specialized high endothelial venules.
What is the main difference between the wall of a muscular venule and an arteriole?
The internal elastic lamina is absent in a muscular venule. In addition, smooth muscle cells in it are often oriented longitudinally, whereas in an arteriole they lie circularly.
Why do venules lack transverse striations in a whole-mount preparation of the pia mater?
Because the vessels of the pia mater completely lack smooth muscle cells, which are what create the visual striation in arterioles.
What else to study on this topic
- Structure and functions of high endothelial venules
- Features of sinusoids and venous sinuses in hematopoietic organs
- Pericytes: functions and their distribution in the microvasculature
- Method for preparing whole-mount flat specimens
Master the entire topic in the "Histology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Venules" and 3,485 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 137 video lessons on the subject
Other topics in the section "Cardiovascular system: arteries, microvasculature"
Free video lessons: histologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.