DEFINITION AND DISTRIBUTION

RETICULOENDOTHELIAL SYSTEM

STRUCTURES HAVING RETICULOENDOTHELIAL CELLS

1. Endothelial lining of:
   • Vascular channels
   • Lymph channels
2. Connective tissue and some organs such as:
   • Spleen
   • Liver
   • Lungs
   • Lymph nodes
   • Bone marrow
   • Etc.
3. Reticular cells in the above tissues form the tissue macrophage system.

MACROPHAGE

CLASSIFICATION OF RETICULOENDOTHELIAL CELLS

Reticuloendothelial cells are classified into 2 types:

1. Fixed reticuloendothelial cells
2. Wandering reticuloendothelial cells

1. FIXED RETICULOENDOTHELIAL CELLS

• Fixed reticuloendothelial cells are also called:
• Tissue macrophages
• Fixed histiocytes
• These cells are usually located in the tissues.

DISTRIBUTION OF TISSUE MACROPHAGES

1. Reticuloendothelial cells in connective tissues and serous membranes like:
   • Pleura
   • Omentum
   • Mesentery
2. Endothelial cells of blood sinusoids in:
   • Bone marrow
   • Liver
   • Spleen
   • Lymph nodes
   • Adrenal glands
   • Pituitary glands

   Kupffer cells in liver belong to this category.

3. Cells in the reticulum of:
   • Spleen
   • Lymph node
   • Bone marrow
4. Meningocytes of meninges and microglia in brain.
5. Alveolar cells in lungs.
6. Subcutaneous tissue cells.

2. WANDERING RETICULOENDOTHELIAL CELLS

• Wandering reticuloendothelial cells are also called free histiocytes.
• They are of 2 types.

1. Free Histiocytes of Blood

• Neutrophils
• Monocytes
• Become macrophages.
• Migrate to the site of injury or infection.

2. Free Histiocytes of Solid Tissue

• During emergency:
• Fixed histiocytes from connective tissue and other organs become wandering cells.
• They enter the circulation.

FUNCTIONS OF RETICULOENDOTHELIAL SYSTEM

1. PHAGOCYTIC FUNCTION

• Macrophages are large phagocytic cells.
• They play an important role in defense mechanism by phagocytosis.
• When any foreign body invades:
• Macrophages ingest it by phagocytosis.
• Antigenic products (antigens) of the organism are liberated.
• Antigens activate:
• Helper T lymphocytes
• B lymphocytes
• Lysosomes of macrophages contain:
• Proteolytic enzymes
• Lipases
• These digest:
• Bacteria
• Other foreign bodies

2. ANTIGEN PRESENTATION

Macrophages present the antigenic substances to helper T cells during the development of immunity.

3. SECRETION OF BACTERICIDAL AGENTS

• In addition to proteolytic enzymes, tissue macrophages secrete many bactericidal agents which kill bacteria.
• Important bactericidal agents secreted by macrophages are free radicals or reactive oxygen species (ROS).
• Free radicals are potent bactericidal agents.
• Even bacteria which cannot be digested by lysosomal enzymes are destroyed by them.

Free Radicals Secreted by Macrophages

1. Superoxide (O₂)⁻
2. Hydrogen peroxide (H₂O₂)
3. Hydroxide ion (OH)⁻

4. SECRETION OF INTERLEUKINS

Tissue macrophages secrete interleukins which help in immunity.

i. Interleukin-1 (IL-1)

Accelerates maturation and proliferation of specific:

• B lymphocytes
• T lymphocytes

ii. Interleukin-6 (IL-6)

Induces:

• Growth of B lymphocytes
• Production of antibodies

iii. Interleukin-12 (IL-12)

Influences T helper cells.

5. SECRETION OF TUMOR NECROSIS FACTOR

Tissue macrophages secrete 2 types of tumor necrosis factors (TNFs):

i. TNF-α

• Causes necrosis of tumor.
• Activates immune responses in the body.

ii. TNF-β

• Stimulates immune system.
• Stimulates vascular response.
• Also causes necrosis of tumor.

6. SECRETION OF TRANSFORMING GROWTH FACTOR

• Tissue macrophages secrete transforming growth factor.
• It prevents rejection of transplanted tissues or organs by immunosuppression.

7. SECRETION OF COLONY-STIMULATING FACTOR

• Colony-stimulating factor (CSF) secreted by macrophages is MCSF.
• It accelerates the growth of:
• Granulocytes
• Monocytes
• Macrophages

8. SECRETION OF PLATELET-DERIVED GROWTH FACTOR

• Tissue macrophages secrete platelet-derived growth factor (PDGF).
• PDGF accelerates:
• Repair of damaged blood vessels
• Wound healing

9. REMOVAL OF CARBON PARTICLES AND SILICON

• Macrophages ingest substances such as:
• Carbon particles
• Silicon
• These substances enter the body.

10. DESTRUCTION OF SENILE RBC

• Reticuloendothelial cells, particularly those in the spleen, destroy senile RBCs.
• Hemoglobin is released from the destroyed RBCs.

11. DESTRUCTION OF HEMOGLOBIN

Hemoglobin released from broken senile RBCs is degraded by reticuloendothelial cells.

SPLEEN

FUNCTIONAL ANATOMY OF SPLEEN

COVERINGS AND SUPPORTING STRUCTURES

• Spleen is covered by:
• An outer serous coat
• An inner fibromuscular capsule
• From the capsule, trabeculae and trabecular network arise.
• Capsule, trabeculae and trabecular network contain:
• Collagen fibers
• Elastic fibers
• Smooth muscle fibers
• Reticular cells

PARENCHYMA OF SPLEEN

Parenchyma of spleen is divided into:

1. Red pulp
2. White pulp

RED PULP

Red pulp consists of:

• Venous sinuses
• Cords containing:
• Blood cells
• Macrophages
• Mesenchymal cells

WHITE PULP

• Structure of white pulp is similar to that of lymphoid tissue.
• It has a central artery surrounded by:
• Splenic corpuscles
• Malpighian corpuscles
• These corpuscles contain:
• Lymphocytes
• Macrophages

FUNCTIONS OF SPLEEN

1. FORMATION OF BLOOD CELLS

• Spleen has a hematopoietic function in embryo.
• During hepatic stage:
• Spleen produces blood cells along with liver.
• In myeloid stage:
• Spleen produces blood cells along with:
• Liver
• Bone marrow

2. DESTRUCTION OF BLOOD CELLS

Older:

• RBCs
• Lymphocytes
• Thrombocytes

are destroyed in spleen.

DESTRUCTION OF OLD RBCs

• When RBCs become old (120 days):
• Cell membrane becomes more fragile.
• Diameter of most capillaries is less than or equal to that of RBC.
• Fragile old cells are destroyed while trying to squeeze through capillaries because they cannot withstand the stress of squeezing.
• Destruction occurs mainly in capillaries of spleen because splenic capillaries have a thin lumen.
• Therefore, spleen is called the graveyard of RBCs.

3. BLOOD RESERVOIR FUNCTION

• In animals, spleen stores a large amount of blood.
• However, this function is not significant in humans.
• A large number of RBCs are stored in spleen.
• RBCs are released from spleen into circulation during emergency conditions like:
• Hypoxia
• Hemorrhage

4. ROLE IN DEFENSE OF BODY

• Spleen filters the blood by removing microorganisms.
• Macrophages in splenic pulp destroy:
• Microorganisms
• Other foreign bodies
• This occurs by phagocytosis.
• Spleen contains about:
• 25% of T lymphocytes
• 15% of B lymphocytes
• Spleen forms the site of antibody production.

APPLIED PHYSIOLOGY

SPLENOMEGALY AND HYPERSPLENISM

Splenomegaly

Splenomegaly refers to enlargement of spleen.

Hypersplenism

Hypersplenism means increase in the activities of spleen.

• Some diseases cause splenomegaly resulting in hypersplenism.

CAUSES OF SPLENOMEGALY

1. Infectious diseases such as:
   • Malaria
   • Typhoid
   • Tuberculosis
2. Inflammatory diseases like rheumatoid arthritis
3. Pernicious anemia
4. Liver diseases
5. Hematological disorders like spherocytosis
6. Cysts in spleen
7. Hodgkin’s disease
8. Glandular fever

EFFECTS OF SPLENOMEGALY

1. Hemolysis resulting in anemia
2. Leukopenia
3. Thrombocytopenia
4. Increase in plasma volume

HYPOSPLENISM AND ASPLENIA

HYPOSPLENISM / HYPOSPLENIA

ASPLENIA

• Asplenia means absence of spleen.
• Functional asplenia means absence of splenic functions.

CAUSES OF ASPLENIA

1. Congenital absence of spleen function
   • Congenital asplenia
2. Acquired through surgical removal of spleen
   • Splenectomy
3. Acquired through some diseases
   • Diseases destroy the spleen to such an extent that it becomes non-functional.
   • This process is called autosplenectomy.
   • Autosplenectomy occurs in:
   • Sickle cell anemia
   • Spherocytosis