Lymph nodes

The lymph node, abbreviated LN, is very important in pathology. It is the site of mets and may be the site of a primary lesion, i.e. lymphoma.

Haematopathology and lymphoma is dealt with in the haematopathology article. This article covers the basics of lymph nodes. Lymph node pathology is dealt with in the lymph node pathology article.

LN architecture

Lymph node architecture (superficial to deep):

• Capsule.
• Subcapsular sinus.
  • Place where metastatic cells often are found.
  • In a normal LN the vessels should be "open", i.e. have a discernible lumen.
• Cortex:
  • Follicles (superficial).
  • Paracortex (deep).
  • Cortical sinuses.
• Medulla:
  • Medullary cords.
  • Medullary sinuses.

Notes:

• Trabeculae - segment the LN radially, i.e. are spoke-like structures.

LN follicles

Types of follicles:

1. Primary follicle - no germinal center (pale area).
2. Secondary follicle - have a germinal center (pale area) and mantle zone (rim of darker staining lymphocytes).

Germinal centre

Architecture:

• Paler at medullary aspect. (???)
• Darker at capsular aspect. (???)

Location and cell types

Germinal centers:

• Site of B lymphocyte proliferation; follicle center cells.
  • Centroblasts - usually superficial aspect.
    • Darker, larger, nucleoli.
  • Centrocytes (mid germinal center).
    • Cells with cleavage.
  • Centrocytes (transforming to immunoblasts) - usually deeper aspect.
    • Cells with cleavage.
• Tingible body macrophages - mid germinal center.
• Follicular dendritic cells.

Paracortex:

• Site of T lymphocytes.
• Interdigitating dendritic cell.

Medullary cords (site of immunoglobulin synthesis):

• B lymphocytes.
• Plasma cells.

Mantle zone:

• Memory B cells.

B cell cycle

B cell cycle/maturation:

1. Centroblast (CB):
   • Appearance:
     • Large cells.
     • Round nucleus, dark.
     • Nucleolus (nucleoli).
     • Mitotically active.
   • Location:
     • Medullary aspect of germinal center.
2. Centrocytes (CC), AKA cleaved cells.
   • Appearance:
     • Cleaved nucleus
     • NOT mitotically active.
   • Location:
     • Mid germinal center.
3. Immunoblasts or memory B cells.
   • Appearance:
     • Small lymphocytes.
   • Location:
     • Peripheral blood, mantle zone.

Notes:

• Immunoblasts may be T cells or B cells; it is not possible to tell on morphologic grounds, i.e. without IHC.

Image:

• Lymphoid organs (healthsystem.virginia.edu).

Cell types

Follicular dendritic cells:

• Classically found in pairs.
• Large nucleus with:
  • Basketweave chromatin pattern.
  • Small central nucleolus.

Lymphocytes

• See B cell cycle section.

See also

• Haematopathology.
• Lymph node pathology.
• Basics.

External links

• Lymph node architecture (umdnj.edu).