Connective tissue l General Histology & Biology revision for dental students

 

Connective tissue l General Histology & Biology revision for dental students

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Introduction in connective tissue

• Connective tissue is a mesodermal structure concerned with connection and support of other types of tissues.
• Connective tissue is formed of: CT cells and extracellular matrix formed of CT
• fibers and ground substance.
• NB: mesoderm ® mesenchymal tissues (CT, muscular tissue and vascular tissue).

Connective tissue fibers:

• There are 3 types of CT fibers: all are formed by fibroblasts.

1. White collagenous fibers (the flexible type of CT fibers):

Light microscope :

• wavy non-branching colorless soft fibers ® flexible branching white strong bundles.

Electron microscope :

• bundles Formed of ® fibers Formed of ® fibrils Formed of ® tropo-collagen Formed of ® 3 α peptides (2α1 & 1α2)

Staining:

• blue by Mallory, pink by eosin and red by van Gieson.

Types of collagen:

1. Type I collagen (for tension): Present in bones and tendons,  Formed by osteo-blasts.
2. Type II collagen (for pressure): Present in cartilage, Formed by chondroblasts.
3. Type III collagen (for expansion): Present in skin, muscles and blood vessels , Function: form reticular fibers.
4. Type IV collagen (for support): Present in basement membranes, Function: support and filtration.
5. Type V collagen as type I.

2. Reticular fibers = type III collagen (the delicate type of CT fibers):

Light microscope :

• single delicate fibers that branch to form the network (reticulum) of stroma present in glands.

Electron microscope :

• as collagenous fibers (3 α peptides) but does not form bundles.

Staining:

• dark brown by silver and purple by PAS.

3. Yellow elastic fibers (the stretchable type of CT fibers):

Light microscope :

• single branched yellow stretchable fibers (formed by fibroblasts and smooth muscle cells).

Electron microscope :

• each fiber is formed of elastin (protein) surrounded by fibrils (glyco-protein).

Staining:

• brown by Orcein, blue black by Verhoeff and yellow by van Gieson.

Connective tissue cells:

• There are 12 types of cells in CT proper which are classified by two ways:

Fixed or free Connective tissue cells:

Fixed (resident) CT cells (7 types):

• they are stable (produced in CT) and long-lived.
• Undifferentiated mesenchymal  cells - fibroblasts - pericytes - endothelial cells - reticular cells - fixed macrophages - fat cells.

Free (wandering) CT cells (5 types):

• they are motile (coming to CT from other areas) and short-lived.
• Leucocytes - plasma cells - mast cells - free macrophages - melanophores.

Branched or rounded Connective tissue cells:

Branched CT cells (8 types): they are mainly fixed cells.

• Undifferentiated mesenchymal  cells - fibroblasts - pericytes - endothelial cells - reticular cells - fixed macrophages - free macrophages - melanophores.

Rounded CT cells (4 types): they are mainly free cells.

• Leucocytes - plasma cells - mast cells - fat cells.

1. Un-differentiated mesenchymal cells = UDMCs (fixed branched CT cells):

• They are present in umbilical cord (Warton’s jelly) , vitreous humor of eyes ,  pulp of growing teeth.

Light microscope :

• pale basophilic cells with central oval nuclei , cells are surrounded by jelly-like matrix.

Electron microscope :

• many free ribosomes.

Function:

• embryonic stem (mother) cells that can differentiate to other CT cells.

NB: Un-differentiated mesenchymal cells (UDM cells) :

1. Fibroblasts ® reticular cells.
2. Pericytes.
3. Endothelial cells.
4. Lipoblasts ® lipocytes (fat cells).
5. Osteoblasts ® osteocytes.
6. Chondro-blasts ® chondrocytes.
7. Mesothelial cells

2. Fibroblasts (fixed branched mesenchymal cells arising from  Un-differentiated mesenchymal cells):

• The most active cells in the CT proper. They are protein-forming cells.

Light microscope :

• deep basophilic cells with large oval nuclei , nuclei are pale with prominent nucleolus.

Electron microscope :

• many mitochondria, many rough endoplasmic reticulum , well-developed Golgi apparatus and many secretory granules.

Function:

1. Synthesis of CT fibers and CT matrix.
2. Healing of CT after injury.

3. Pericytes (fixed branched Connective tissue cells arising from UDM cells):

• Adult mesenchymal cells present in blood capillaries (between BM and endothelium).

Light microscope :

• pale basophilic cells with central oval nuclei.

Electron microscope :

• many free ribosomes , many actin and myosin microfilaments.

Function:

• can change to endothelial cells or smooth muscle cells (that cause vasoconstriction).

4. Endothelial cells (fixed branched Connective tissue cells arising from UDM cells):

• They line vascular system by simple squamous endothelium.

Light microscope :

• flat cells with flat nuclei.

5. Reticular cells (fixed branched Connective tissue cells arising from fibroblasts):

• Modified fibroblasts that synthetize reticular CT fibers.

Light microscope :

• pale basophilic cells with central rounded nuclei and many long thin processes.

Function:

1. Formation of stroma that support various organs as glands, lymph nodes, spleen .
2. Change by antigens to phagocytic cells to remove of cellular debris of lymphatic tissue.

6. Fixed macrophages = Histeocytes

• fixed branched CT cells arising from monocytes

7. Free macrophages :

• migrated monocytes (free branched CT cells arising from monocytes): Phagocytic cells (of mono-nuclear phagocytic system) , Can engulf vital stains (as trypan blue).

Light microscope :

• pale granular amoeboid cells with dark kidney-shaped eccentric nuclei.

Electron microscope :

• many lysosomes (interferon, collagenase and elastase) and many phagosomes.

Function:

1. Phagocytosis of micro-organisms and small foreign body.
2. Can fuse together to form giant phagocytic cells for large foreign body).
3. Destruction of old RBCs in liver and spleen.

8. Melanophores = pigment cells (free branched mesenchymal cells arising from monocytes):

• Free CT macrophages present in the dermis of skin.

Electron microscope :

• their cytoplasm is rich in melanosomes.

Function:

• phagocytosis and storage of melanin pigment (produced by melanocytes present in the epidermis of skin).

9. Leucocytes (free rounded Connective tissue cells arising from PHS cells):

• Leucocytes include basophils, eosinophils, neutrophils, monocytes and lymphocytes.
• They migrate from blood to CT in presence of infection.

NB: pluri-potential hemato-poetic stem cells (PHS cells) :

1. Erythrocytes
2. Thrombocytes
3. Granulocytes (basophils, eosinophils and neutrophils)
4. Monocytes ® fixed macrophages, free macrophages and melanophores…
5. Lymphocytes ® plasma cells
6. Mast cells
7. Osteoclasts, microglia and Langerhan’s cells

10. Plasma cells = activated B-lymphocytes (free rounded Connective tissue cells arising from B-lymphocytes):

• They are protein-forming cells present mainly in lymphoid tissues.

Light microscope :

• large oval basophilic cells with eccentric cartwheel (clock-face) nuclei.

Electron microscope :

• many mitochondria, many rER and well-developed Golgi apparatus (with Hx & E stain Golgi apparatus unstained ® negative Golgi image).

Function:

• secretion of antibodies (IgG- IgM- IgA- IgE- IgD = Russell bodes) ® humoral immunity.

11. Mast cells (free rounded CT cells arising from PHS cells):

• Paracrine cells present in group around blood vessels in the CT of skin, GIT.

Light microscope :

• oval cells with eccentric rounded nuclei and basophilic granules (stained purple with toluidine blue ® meta-chromasia). These granules mask other contents of the cell.

Electron microscope :

• cytoplasmic processes (with receptors for IgE) and many electrondense granules.

Types:

• Heparin secreting cells (in CT of skin) that secrete heparin (anticoagulant).
• Histamine secreting cells (in CT of RT, GIT…) that secrete histamine (­ capillary permeability).

12. Fat cells = Adipocytes = Lipocytes 

• fixed rounded mesenchymal  cells arising from Undifferentiated mesenchymal  cells cells

Types of CT proper:

1. Loose (areolar) mesenchymal tissue :

• It binds tissues together and supports various organs (all over the body except brain).

Structure:

• it is formed of CT cells (fibroblasts, macrophages, mast cells, fat cells …) and CT fibers in a loose matrix containing areolae (fluid spaces).

Sites:

• subcutaneous tissues - submucosa - serous membranes – around organs and blood vessels …

2. Mucous (mucoid) mesenchymal tissue:

• Present mainly in embryonic tissues.

Structure:

• it is formed of UDM cells and fine collagenous fibers in a soft jelly-like matrix (rich in mucin and hyaluronic acid).

Sites:

• umbilical cord (Warton’s jelly) , vitreous humor of eyes , pulp of growing teeth.

3. Adipose (fatty) Connective tissue:

Structure: 

• it is similar to loose CT but very rich in adipocytes (that present in groups separated by fibrous septa).

Types:

1. White adipose CT that appears yellow (due to presence of few blood vessels and few mitochondrial pigments).
2. Brown adipose CT that appears brown (due to presence of many blood vessels and many mitochondrial pigments).

4. Dense (collagenous) Connective tissue :

Structure:

• it is formed of collagenous fibers and fibroblasts in a soft matrix (characterized by few amounts and poor blood supply).

Types: it has two types:

Regular dense Connective tissue :

• Collagenous bundles arranged regularly
• Colorless CT
• Function: stretch in one direction
• Sites:  Cornea of eye , Tendon of muscle

Irregular dense Connective tissue :

• Collagenous bundles arranged irregularly
• White CT
• Function: stretch indifferent directions
• Sites:

1. Sclera of eye, dura mater and dermis of skin
2. Capsule and septa of gland stroma
3. Perichondrium and Periosteum

5. Reticular Connective tissue:

• The fine type of CT that forms the network of stroma in bone marrow, various glands and various organs.

Structure:

• it is formed of reticular fibers and reticular cells in a soft matrix (reticular fibers are enveloped by the cytoplasm of reticular cells).
• Reticular CT contains cells of parenchyma.

Sites:

1. Bone marrow, lymph nodes, spleen, lungs .
2. Salivary glands, liver, testis, ovary .

6. Elastic Connective tissue:

• A yellow stretchable CT that form elastic membranes (as in aorta) or elastic ligaments (as in vocal cords).

Structure:

• it is formed of condensed regular elastic fibers and fibroblasts in a soft matrix.

Sites:

1. aorta and arteries
2. bronchi, bronchioles and alveoli
3. ligaments and skin.

Function of mesenchymal tissue Proper:

• Tissue connection and organ support.
• CT proper carries blood vessels, lymphatics and nerves to all parts of various organs.
• Tissue growth and wound healing.
• Specific functions for CT cells as:

1. Formation of ground substance and CT fibers (by fibroblasts).
2. defense through allergic reactions (by mast cells, basophils and eosinophils)
3. Phagocytosis (by neutrophils and macrophages).
4. Humoral and cell-mediated immunity (by plasma cells and lymphocytes).
5. Energy reservoir and heat production (by fat cells).