Macroscopic anatomy and topography
The organs are located along the course of lymphatic vessels, most often grouped into regional clusters. Their sites of localization are physiologically justified:
- In mobile areas of the body (popliteal, inguinal, axillary, cervical) — muscle contractions during movement help pump lymph through the nodes.
- At the hilum of internal organs (lung roots, mesentery, liver and spleen hila) and along large main vessels of the abdominal cavity and pelvis.
The lymph node has a characteristic polarity. On its convex side, afferent lymphatic vessels enter the organ. On the opposite, concave side lies the hilum of the node: efferent lymphatic vessels and veins exit here, while arteries and nerve trunks enter.
General Plan of Histological Structure
The organ's architecture includes a stromal framework and parenchyma. The stroma of local origin (developing from embryonic mesenchyme) is represented by dense fibrous connective tissue. It forms the outer capsule and septa extending inward from it — trabeculae.
The intertrabecular space is filled with parenchyma, which comprises three anatomical zones (from the periphery to the center):
- Cortex (dark on a histological section).
- Paracortical zone (intermediate).
- Medulla (lighter central part).
Lymphoid tissue (clusters of immune cells) and a system of lymphatic sinuses are present in all zones of the parenchyma. The microenvironment (stroma) for the cells is provided by reticular tissue, consisting of specialized cells and fibers.
Immunological specialization (T- and B-zones)
Lymphoid tissue is organized differently depending on the zone and is populated by derivatives of various central organs of the immune system.
- B-dependent zones: Located in the cortex as rounded lymphoid nodules (follicles) and in the medulla as medullary cords. These zones are populated by B-lymphocytes coming from the red bone marrow.
- T-dependent zone: Occupies the paracortical region, where lymphoid tissue is distributed diffusely. T-lymphocytes migrate here from the thymus (among them, about 40% are T-helper cells and 20% are T-killer cells).
It is important to note that zonation is not absolute. Cells constantly migrate, which ensures the necessary contact interaction between T- and B-lymphocytes to trigger antigen-dependent differentiation.
System of lymphatic sinuses
Sinuses are intraorgan channels for lymph flow that operate as a highly efficient biological filter. Lymph sequentially follows this pathway: afferent vessels → marginal (subcapsular) sinus → perifollicular (trabecular) sinuses (between trabeculae and follicles) → medullary sinuses (between trabeculae and medullary cords) → efferent vessels.
The walls of the sinuses are lined with reticuloendothelial ("littoral") cells with slit-like areas for the free migration of lymphocytes. Within the lumen of the sinuses, stellate reticular cells form a three-dimensional network ("sieve") that mechanically slows down the lymph flow, while resident and free macrophages present here perform phagocytosis of antigens.
Briefly: answers to topical questions
What specific vessels ensure the migration of lymphocytes from the blood into the paracortical zone?
The migration of lymphocytes from the bloodstream into the paracortical zone is provided by specialized postcapillary venules.
They are classified as high endothelial venules. The morphological feature of these vessels is that the endothelial cells in their walls have a cuboidal or prismatic shape rather than the typical flat one. It is precisely through the walls of these venules that diapedesis occurs—the main exit of cells from the bloodstream into the tissue of the lymph node, after which T-lymphocytes settle in the deep layers of the cortex.
What types of antigen-presenting cells are present in lymphoid nodules (follicles)?
In lymphoid nodules (follicles), sources explicitly indicate the following cells:
- Macrophages — are part of the germinal center; also indicated in the mantle zone (corona) of the follicle.
- Dendritic cells — are part of the cell population of the germinal center.
- Follicular dendritic cells — sources indicate that lymphotoxins are necessary for the formation of follicular dendritic cells and lymphoid follicles.
B-lymphocytes "prefer" extremes: they settle at the very periphery (cortical follicles) and in the very center of the organ (medullary cords). T-lymphocytes occupy the intermediate territory — the paracortical zone.
Class and exam questions
Why are many lymph nodes located in mobile parts of the body?
Localization in such areas (e.g., popliteal or axillary regions) has a physiological purpose: muscle movement in these areas facilitates the mechanical pumping of lymph through the system of intraorgan sinuses.
Are the T- and B-zones of the lymph node isolated from each other?
No, zonation is relative. T-cells can be found in B-zones and vice versa. This migration is critical for intercellular cooperation during the formation of an immune response.
How does lymph enter and leave the node?
Lymph enters via afferent vessels entering from the convex side of the node into the marginal sinus, and leaves the organ via efferent vessels exiting from the concave part — the hilum of the node.
What else to study on this topic
- Features of response and localization of B1-lymphocytes (a population with a simplified response type).
- Statistics on the distribution of lymphocytes across all central and peripheral organs.
- The role of the embryonic liver as a source of hematopoietic stem cells for the development of the node.
- Mechanisms of antigen-dependent differentiation and cell cooperation in the parenchyma.
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 "Structure of the lymph node" and another 3,485 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 "Hematopoietic and lymphoid organs"
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.