Histological methods for studying lymph nodes
Various approaches are used to visualize different structures of the organ:
- Standard hematoxylin and eosin staining. Allows visualization of the general architecture: capsule, marginal and medullary sinuses, follicles (including the mantle zone and germinal center), paracortical zone, and medullary cords.
- Silver nitrate impregnation. Used to demonstrate the stroma. Reticular fibers are argyrophilic (affinity for silver) and stain black-brown, forming a dense framework.
- Functional macrophage staining. Based on vital (in vivo) administration of a dye to the animal. The dye accumulates within typical macrophages located in the lumen of medullary sinuses and along their walls (so-called "littoral cells"). Meanwhile, lymphoid cells of the medullary cords remain free of dye particles.
Paracortical zone and migration pathways
The paracortical zone is examined first when studying the immune response because it lacks ordered structures like follicles (cells are arranged diffusely), yet it is precisely here that key initial reactions unfold.
The population is primarily composed of T lymphocytes. Macrophages and interdigitating cells (IDCs)—elements with long finger-like processes derived from lymph-borne dendritic cells—are also present here.
A crucial structure of this zone is the high endothelial venule (HEV). Their endothelial cells are so tall that they narrow the lumen. Their surface features receptors that capture lymphocytes. This ensures massive cell migration from the blood into the node, allowing the entire cellular composition of the lymph node to be completely renewed within a few days.
When an antigen enters (via lymph, blood, or within activated macrophages releasing interleukins IL-1 and IL-6), local inflammation develops in the paracortical zone, accompanied by increased blood flow and lymphocyte influx, clinically manifesting as node enlargement and tenderness.
Lymphatic follicles and B-cell response
Follicles provide the development of humoral immunity. They are divided into two types:
- Primary follicles. Found in fetuses and gnotobiotes (germ-free animals). They are small, homogeneous, lacking a germinal center, and filled with resting B cells. In the postnatal period, they rapidly disappear, transforming into secondary ones.
- Secondary follicles. Have a complex structure. The periphery features a dark corona (mantle) composed of small lymphocytes, while the center contains the germinal (reactive) center, where active proliferation takes place.
The germinal center is divided into three zones: the dark zone (at the base, facing the paracortical zone), the basal light zone, and the apical light zone.
Antigen-dependent differentiation begins in the dark zone. Dendritic cells retain "IgM + native antigen" complexes on their processes. ICosomes—corpuscles bearing these complexes—shed from the processes. B lymphocytes recognize the antigen on icosomes, engulf it, and present peptides to $T_{h2}$ cells. This contact leads to B-cell activation: they begin to actively proliferate, and somatic mutagenesis of immunoglobulin genes is triggered in these cells (at this stage, the cells are called centroblasts). Subsequently, the cells migrate to the basal light zone, where they are designated as centrocytes.
Briefly: answers to topical questions
What types of sinuses are distinguished in a lymph node, and what is their sequence?
The lymph node features a system of sinuses through which lymph flows sequentially.
Types of sinuses along the path of lymph flow:
- Marginal (subcapsular) sinus — the primary entry point for lymph, located between the node capsule and the lymphatic follicles.
- Perifollicular (trabecular) sinuses — located between the trabeculae (septa) and the cortical follicles.
- Medullary sinuses — localized in the medulla between trabeculae and medullary cords.
Lymph enters from afferent vessels, passes sequentially through the sinus system, and collects into efferent vessels.
What cell types form the medullary cords of the lymph node?
The medullary cords (B zone) serve as the site for the final maturation of antibody producers. They comprise the following cell populations:
- B-lineage cells: proplasmacytes and mature plasma cells (plasmacytes), which migrate ("drain") here from the follicular mantle.
- Macrophage lineage cells: typical macrophages.
Zones of the germinal center from bottom to top: Dark — Basal light — Apical light (the immune reaction rises from the paracortical zone like a candle flame from a dark wick).
Class and exam questions
What is the role of high endothelial venules?
They serve as the main "gates" for lymphocyte migration from the blood into the lymph node due to specific receptors on the surface of the high endothelium.
What are icosomes and where do they form?
These are microscopic corpuscles covered with immune complexes (IgM + native antigen). They detach from the processes of dendritic cells in the dark zone of the follicular germinal center and are necessary for antigen presentation to B cells.
What is the difference between centroblasts and centrocytes?
Centroblasts are located in the dark zone of the germinal center, where they actively divide and undergo somatic mutagenesis. Upon moving to the basal light zone following selection, they are renamed centrocytes.
What else to study on this topic
- Mechanisms of somatic mutagenesis of immunoglobulin genes
- Specifics of receptor interaction between T and B lymphocytes
- Cytokine profile of activated macrophages (IL-1, IL-6)
- Functional significance of the apical light zone of the follicle
- Architectonics of the marginal and medullary sinus system
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