In this article we will discuss about:- 1. Introduction to Reticulo-Endothelial 2. Classification, Varieties and Distribution of Reticulo-Endothelial Cells 3. Functions.
Introduction to Reticulo-Endothelial:
It has been known for many years that if a vital dye (viz., Trypan blue, Lithium carmine, etc.) be injected into a living animal, certain cells engulf the dye particles, whereas other do not.
It is further seen that these phagocytic cells generally occupy the endothelial lining and the reticular spaces of the connective tissues. From this distribution, Aschoff and Landou (1913) have introduced the name of reticulo-endothelial (R.E.) system to these cells.
These cells develop from the mesenchyme. It is to be noted that while most of the cell in our body have specialized for special functions, the cells of this system have resisted further differentiation and have retained to a very large extent, their primitive embryonic characteristics.
For this reason, it will be seen that these cells are endowed with remarkable powers of altering their shape, size, mode of living and thereby undergo differentiation into other types of tissue cells (mutation).
Classification, Varieties and Distribution of Reticulo-Endothelial Cells (Fig. 5.10):
The cells of the R.E. system may be divided into two large groups:
(a) Fixed Reticulo-Endothelial Cells:
They include the following varieties:
i. The Tissue Histiocytes:
They are found in the connective tissues and loose areolar tissue of the serous membranes, viz., omentum, pleura, etc. Although they are generally fixed, yet, under suitable stimulus (such as inflammation), these cells may become actively motile.
ii. The Reticulum Cells of the Spleen, Lymphatic Node and Bone Marrow:
They remain in the reticular spaces of these organs. They are large cells and remain joined to one another by long branching processes. Under suitable stimulus they may be actively motile.
iii. The Endothelial Cells:
They are present in the lining membrane of the blood sinuses of spleen, bone marrow, adrenal cortex, pituitary, liver, etc. In the liver they appear as large, flat, stellate cells lining the blood capillaries and are known as the Kupffer cells.
iv. Microglia:
These are small cells found in the central nervous system.
(b) Wandering Reticulo-Endothelial Cells:
They comprise the following varieties:
i. The Wandering Histiocytes:
These are large wandering cells found in the omentum, splenic pulp, lymph nodes, bone marrow, etc. Due to their large size they are also called macrophages. Sometimes they may contain many nuclei, for instance, osteoclasts and megakaryocytes of the bone marrow. In addition to these the fixed tissue histiocytes, described above, may become wandering.
ii. The Wandering R.E. Cells of the Blood Stream:
They are of two varieties:
1. The normal monocytes of blood.
2. Abnormal foreign cells which generally remain in the tissues but enter the blood stream under the influence of some pathological stimulus. Such cells are found in Leukaemia, bacterial endocarditis, etc.
Functions of the Reticulo-Endothelial System:
i. Phagocytosis:
This is the main characteristic property of this system of cells. They engulf foreign particles, bacteria and parasites and in this way, act as a great defensive mechanism.
ii. Formation of Antibodies:
The antitoxic and antibacterial bodies are manufactured by this system. This is also a great protective mechanism.
iii. Formation of R.B.C:
Red blood cells develop from the R.E. cells. Turnbull holds that they develop from the extravascular R.E. cells (haemocytoblasts).
iv. Formation of Leucocytes:
The granulocytes, lymphocytes and monocytes—are all derived from the R.E. system.
v. Destruction of Red Cells and White Cells:
The senile red cells and white cells are engulfed and destroyed by the R.E. cells. In adult life, it takes place chiefly in the spleen and liver.
vi. Scavengers of Tissue Debris and Bacteria in Infections Processes:
The R.E. cells help in the scavenging process.
vii. Formation of Bile Pigments:
The R.E. cells manufacture bilirubin from haemoglobin.
viii. Storage Function:
A large amount of lipids, cholesterol and iron are stored in the R.E. cells.
ix. Manufacture of Plasma Proteins:
Serum globulin and certain other plasma proteins are manufactured, to some extent, by the R.E. cells.
x. Formation of Tissue Cells:
Since the cells of this system are undifferentiated, they can be converted into ordinary connective tissue cells, such as fibroblasts, under suitable stimulus. This is seen during repair stage of inflammatory process.