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CONNECTIVE TISSUE (GENERAL ASPECT)

Histology 13,369 Views

Introduction

‘Connective tissue’ is the term traditionally applied to a basic type of tissue of mesodermal origin which provides structural and metabolic support for other tissues and organs throughout the body, also known as  “Supporting tissue”.

Functions

•         Establishing a structural framework

•         Supporting, surrounding and interconnecting tissues

•         Transporting fluids and dissolved materials between cells & their blood supply

•         Storing energy reserves

•         Defending the body from microorganisms

•         Protecting delicate organs

Connecting tissue is broadly classified in to

1)      Connective tissue proper

2)      Embryonic connective tissue

Composition

  • Cells
  • Extracellular Matrix

n      Fibers

n      Ground substance

Cells

v     Residents

  • Fibroblasts
  • Macrophages
  • Reticular cells
  • Mesenchymal cells

v     Visitants

  • Mast cells
  • Plasma cells
  • Leukocytes
  • Fat cells
  • Melanocytes

Extra-cellular Matrix

•         Fibers

-Collagen fibers

-Reticular fibers

-Elastic fibers

Ground Substance

-Proteoglycans

-Glycosaminoglycans

-Glycoproteins

Resident Cells

1. FIBROBLASTS

One of the two most numerous cells of areolar C.T.

Large, flat cells with  branching processes  and appear  fusiform in profile.

Active fibroblasts

Mature/inactive fibroblasts

Most common cells

Synthesize proteins, such as collagen and elastin that forms collagen, reticular, and elastic fibers

Secrete “Ground Substance”

Involved in production of “Growth Factors”

Wound healing

Functions Of Fibroblasts

Fibers producing cells.

Recent research suggests that a single fibroblast is capable of producing all of the extra cellular matrix components

They  can  change  in  to  myofibroblasts which are  rich  in actin  filaments  and help in approximation of wound margins during healing. Myofibroblasts display properties of both fibroblasts & smooth muscle cells

2. Macrophages (Histiocytes)

Almost as numerous as fibroblasts

Most abundant in richly vascularized areas

Fixed/resting macrophages

Free/wandering macrophages

In resting phase

Appear as irregular cells with  short  and  blunt  processes but sometimes the processes may be long, slender and branching.

Nucleus is rounder, smaller and more heterochromatic than that of fibroblast.

Cytoplasm stains darkly & may contain few small vacuoles, stain supravitally with neutral red

In active state

•         Cell becomes  larger with bigger nucleus and prominent nucleolus

•         Cytoplasm becomes filled with granules  and vacuoles, containing ingested material

•         Size 10-30 µm

•         Oval or kidney shaped nucleus located eccentrically

•         Monocytes-Marophages

•         Kupffer cells-liver, microglial cells-CNS, osteoclasts- bone tissue

•         Phagocytosis

•         Have well-developed Golgi complex, many lysosomes, and a prominent rough endoplasmic reticulum

•         ‘Foreignbody giant cell’

Functions Of Macrophages:

Important agent of defense and part of Mononuclear phagocyte system.

These are secretory cells that produces several important substances including enzymes, two proteins of complement system, an important antiviral agent, interferon

They play very important role in immune system and act as antigen presenting cells.

They are capable of motility and when suitably stimulated grouped around a large foreign body to form multinucleated foreign body giant cells

3. ADIPOSE CELLS

The large mesenchymal cells after accumulation of fat droplets in their cytoplasm are called adipocytes

Occur singly or in clumps along small blood vessels

Also called “Fat Cells”

Size 50-150 µm

Cytoplasm is dispalced to periphery  by a single large fat droplet

Nucleus is pressed against the cell membrane “Signet ring”

Functions in energy reserves, insulation, protection, and support

Under L/M:

Prior to the storage of fat they are stellate shape cells and difficult to distinguish from fibroblasts but after storage of fat they become rounded.

Consists of peripheral rim of cytoplasm in which oval nucleus compressed against the cell wall is present.

4. Mast cells

Mast means well fed. Their cytoplasm is full of coarse granules so this name is given.

Tend to occur in small groups around blood vessels.

Irregularly oval in outline, have short pseudopodia

Nucleus is small being crowded by large number of prominent granules

Oval/round, Size 10–13 µm

•         Secretory granules (0.3-2 µm)

•         Histamine & Heparin

•         Metachromatic granules

•         Two populations of mast cells in CT

•         ‘Connective tissue mast cells’ (skin, peritoneal cavities)-heparin

•         ‘Mucosal mast cells’ (intestinal mucosa, lungs)-chondroitin sulfate

Functions Of Mast Cells

As these granules contain heparin (a powerful anticoagulant) and histamine (a potent vasodilator) so they play very vital role in homeostasis.

•         Mast cells are especially numerous in C.T. of skin & mucous membranes, but are not present in brain & spinal cord. These are numerous in thymus, & to a lesser degree, in other lymphatic organs, but they are not present in spleen.

The release of chemical mediators (such as ECF-A), Leukotriene C, PAF by the mast cells  against antigens promotes the allergic reaction known as immediate hypersensitivity reaction.

5. PERICYTES

•         These are cells which are present external to endothelium of blood capillaries and small venule.

•         Cytoplasm is pale staining and possesses long processes.

•         They can differentiate into fibroblasts, smooth muscle cells and myofibroblasts during wound healing.

Wandering Cells

1. LYMPHOCYTES

Smallest of the free cells of C.T.

Majority being 7-8µm in diameter

Spherical darkly staining nucleus

Thin rim of homogenous basophilic cytoplasm

Numerous in C.T. that supports epith lining of respiratory and elementary tracts

2. PLASMA CELLS

Resemble to lymphocytes

More cytoplasm which is basophilic

Eccentric nucleus

Cartwheel appearance

Cytoplasm contains a clear, rounded area (site of centrosphere and golgi apparatus

Extensive RER

Frequently found in serous membranes and lymphoid tissue and plentiful in sites of chronic inflammation

Represent a special differentiation of lymphocyte

Principle function is the production of antibodies which are synthesized in RER

Acidophilic inclusions called Russel bodies are present in cytoplasm

Large, ovoid cells

Nucleus is spherical and eccentrically placed

‘cart-wheel appearance’

Basophilic cytoplasm due to abundant RER

Rare in most connective tissues

Produce antibodies

Average life is short, 10–20 days

3. EOSINOPHILS

Not numerous in human C.T.

Plentiful in C.T. of lactating breast and of respiratory and alimentary tracts

Marked feature of loose C.T. of rat, mouse and guinae pig

Nucleus is usually reniform or bilobed

Cytoplasm contains spherical granules that are highly refractile and stain with acid dyes

Accumulate in blood and tissues in certain allergic and subacute inflammatory conditions

Wandering cells

leukocytes that migrate from the blood vessels by diapedesis

This process increases greatly during inflammation

Fibers

•         Formed by proteins which polymerize into elongated structures

•         Three types

•         Collagen Fibers

•         Reticular Fibers

•         Elastic Fibers

•         Collagen and reticular fibers are formed by the protein collagen, and elastic fibers are composed mainly of the protein elastin

Collagen

  • Family of closely related proteins
  • Composed of long molecules (tropocollagen)
  • Each molecule is 280 nm in length & 1.5 nm in width and consists of three polypeptide chains in the form of right handed triple helix
  • 14 varieties of collagen, type I – type XIV collagen

Collagen Fibers

•         Most abundant protein in human body

•         30% of body weight

•         Made up of collagen type-1

•         Diameter is variable(2-10 µm) and have great indefinite lengths

•         Tough and flexible but in-elastic (non-extensible)

•         EM shows that each collagen fiber is composed of parallel fibrils (50-90 nm in diameter) which appear to be cross striated

Reticular Fibers

•         Mainly consist of type III collagen

•         Extremely thin fibers (diameter 0.5-2 µm)

•         Form extensive network

•         Not visible  by H&E stain

•         stained by impregnation with silver salts (argyrophilic)

•         PAS positive

•         Abundant in smooth muscle, endoneurium, hemopoetic organs (spleen, lymph nodes, bone marrow)

Elastic Fiber System

•         Three types of fibers—oxytalan, elaunin, and elastic

•         Oxytalanic fibers (zonule fibers of eye) contain

•            ‘fibromodulin I and II’ and ‘fibrillin’

•         Elaunin  fibers (sweat glands, dermis)

•         Elastic fibers (lungs, large arteries, ligamentum flavum)

Ground Substance

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