Cells of LICT.

Chapter 7: The connective tissues.

The connective tissues are the second group of tissues derived from mesenchyme. They have two kinds of tissue elements – cells and intercellular ground substance. The last one presented by fibers and amorphous substance.

Classification of connective tissues.

1. Fibrous connective tissues.

A) Loose irregular connective tissue

B) Dense connective tissue:

è Regular: tendons, ligaments.

è Irregular: reticular layer of dermis.

2. Connective tissues with special properties.

A) Adipose tissue

B) Reticular tissue.

C) Mucous tissue.

D) Pigment tissue.

The fibrous connective tissues have a typical feature – they have a lot of fibers in ground substance. It is clear from the table above that fibrous tissues are of two types: loose and dense. In the loose connective tissue the amorphous substance dominates over fibers. In the dense connective tissues, the concentration of fibers is much higher than amorphous substance. In the loose connective tissue we can find a wide range of cell types. However, in the dense connective tissue the cellular composition is homogenous. The tissues can be regular and irregular. If the fibers are positioned in a special order (parallel to each other), it is regular tissues (like in ligament). If the fibers are disordered in a tissue, it is irregular connective tissue. The irregular connective tissue is subjected to mechanical load in different directions. That is why fibers are located to resist the load in any direction. In the regular connective tissues the mechanical load has one direction. So, fibers chose the best position to cope it.

Connective tissues with special properties (adipose, reticular, pigment, mucous) perform special functions and in majority of cases they have a limited distribution in the organism.

Loose irregular connective tissue (LICT).

LICT has the largest distribution in an organism among connective tissues. It accompanies the smallest vessels making a stroma of parenchymal organs. It is included to the coats of layered organs. The functions of the LICT are the following: defense, support, feeding, regulating, homeostasis maintaining, shape formation of organs, participation in regeneration and inflammation.

Structure. As all tissues of internal environment the LICT has cells and intercellular ground substance.

Cells of LICT.

LICT contains several cellular differons.

1. Differon of fibroblasts (stem cells → semistem cells → undifferentiated fibroblasts → differentiated fibroblasts → fibrocytes, fibroclasts, myofibroblasts).

2. Differon of macrohages (blood stem cell → …→ blood monocytes → macrophages of LICT).

3. Differon of plasma cells (blood stem cell → …→ B-lymphocytes of blood → B-lymphocytes of LICT → plasmablasts → proplasmacyte → plasma cell).

4. Differon of mast cells (Blood stem cell → …→ basophilic leucocyte of blood → mast cell of LICT).

5. Differon of lipocytes (undifferentiated fibroblasts → lipocytes).

6. Differon of pigmentocytes (precursor cells → pigmentocytes).

7. Adventitial cells.

8. Perycytes.

9. Leucocytes.

All cells of LICT can be classified into two groups.

1. Local cells. This group includes all differons exept leucocytes.

2. Foreign cells. They are the leucocytes.

All cells of LICT have three sources of development.

1. Cells from mechanocytes line. There are adventitial cells, perycytes, cells of fibroblast differon and lipocytes. They develop from special mechanocytes stem cell of red bone marrow origin. It is different from blood stem cell

2. Cells derived from blood stem cell. There are macrophages, mast cells, plasma cells and leukocytes. Both stem cells have mesenchymal origin. They undergo divergent differentiation.

3. Cells of neuroectodernal origin. Such cells are developed from neural crest. They are pigmentocytes.

FIBROBLASTS (FB). During embryogenesis, the fibroblasts are differentiated directly from mesenchymal cells. In postnatal ontogenesis, they are developed from the mechanocytes stem cell. It is located in red bone marrow. Arriving to connective tissue it is transformed to adventitial cell. The fibroblast population is the most numerous population in connective tissue. Their function is to produce intercellular ground substance containing fibers and amorphous substance.

The differon of fibroblasts contains the following cells as:

1. Immature FBs – they are cells with high NCR, moderate organelles number, large oval basophilic nucleus containing 1-2 nucleoli. They have ability to produce ground substance, but they still able to division. Consequently, their functions are synthesis of ground substance and enlargement of population size.

2. Mature FBs – they have low NCR, light euchromatin nucleus with 1-2 nucleoli. In light microscope, the cytoplasm of the cells is lightly basophilic. There are two regions: the first is internal, around the nucleus with more basophilic cytoplasm (endoplasm), the second is external with less basophilic cytoplasm (ectoplasm). The endoplasm contains well developed organelles of protein synthesis: routh EPR, Golgi complex, mitochonria, lysosomes. In the ectoplasm there are well developed components of cytoskeleton providing formation of proceses and active movement of FB. Ectoplasm may be not distinguishable from ground substance while staining. It looks like it gradually transforms into ground substance.

Functions of specialized FB are:

è Active synthesis of intercellular ground substance components.

è Regular influence on functioning of other cells of the connective tissue by constant releasing of wide range of mediators.

è Degradation of collagen, which was produced by fibroblasts themselves. It is performed both inside and outside of the cell. The collagenase enzyme is needed for this. The degradation of produced collagen is important process for controlling ground substance state, for tissue homeostasis maintaining. By destroying and modifying collagen the fibroblasts can modify scar connective tissue.

3. Fibrocytes. Aging FBs transform to fibrocytes. They are inactive cells of spindle shape with dense hyperbasophilic nucleus and with narrow band of cytoplasm. It is poor in organelles.

Functions. Fibrocytes are terminal point of fibroblasts development. And finally they die by apoptosis. But contemporary researches shows that fibrocytes can participate in some degree in renewing of ground substance components, and so in tissue homeostasis maintaining. During reparation after injury, the cells can transform to the cells with active protein synthesis similar to fibroblasts.

4. Myofibroblasts. They are the fibroblasts which have well developed contractive elements. They resemble smooth myocytes. But they are not surrounded by basement membrane and they have more developed EPR and Golgi complex. In light microscope myofibroblasts are same as smooth myocytes.

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