View source for 8. connective tissue general structure

'''Connective Tissue: General Structure and Classification'''

Connective tissue plays a crucial role in supporting and connecting various tissues and cells within our body, ultimately forming organs. Here are some key points about connective tissue:

# '''Functions     of Connective Tissue:'''
#* '''Support:''' Connective      tissue provides structural support to the body. Examples include bone and      cartilage.
#* '''Pathways:''' It      furnishes pathways for nerves and blood vessels.
#* '''Exchange:''' Connective      tissue acts as a compartment for exchange between capillaries and active      cells.
#* '''Immune      Activities:''' Connective tissue is involved in immune responses.
# '''Properties     of Connective Tissue:'''
#* Constituents      are mainly extracellular.
#* Collagen      fibers are present.
#* Tissues      and cells are not polarized.
#* Cells      generally act independently.
# '''Components     of Connective Tissue:'''
#* '''Cells:''' These      include fibroblasts, adipocytes, osteoblasts, and macrophages.
#* '''Extracellular      Matrix (ECM):'''
#** ECM       consists of protein fibers (collagen, reticular, and elastic) and       amorphous ground substances.
#** Ground       substances include anionic hydrophilic proteoglycans, glycosaminoglycans       (GAGs), and multiadhesive glycoproteins.
#** Water       in the ECM allows for nutrient exchange between cells and the blood       supply.

# '''Mesenchyme:'''
#* Mesenchyme      is a type of connective tissue found mainly during embryonic development.
#* It      is composed mainly of ground substance with few cells or fibers.
#* Mesenchymal      cells are undifferentiated and spindle-shaped.

# '''Cell     Types in Connective Tissue:'''
#* '''Fibroblasts:'''
#** Key       cells originating from mesenchymal cells.
#** Synthesize       and maintain most of the ECM components (collagen, elastin, GAGs, etc.).
#** Fibrocytes       are dormant fibroblasts.
#** Myofibroblasts       play a role in wound healing.

#* '''Adipocytes:'''
#** Specialized       for lipid storage (white adipocytes) or heat production (brown       adipocytes).

#* '''Osteoblasts:'''
#** Bone       cells that produce collagen type I and calcium for bone matrix.
#* '''Macrophages:'''
#** Phagocytic       cells involved in turnover of fibers and removal of debris. '''Macrophages:'''
#** Macrophages       are highly specialized cells with a remarkable phagocytic ability. They       play a crucial role in the turnover of protein fibers, removal of dead       cells, and other debris.
#** These       cells are present in connective tissue throughout most organs and are       often referred to as histiocytes.
#** Macrophages       have an irregular surface with protrusions, indentations, and folds.       Their well-developed Golgi complexes and numerous lysosomes enable       efficient cellular processes.
#** Derived       from monocytes (bone marrow precursor cells), macrophages circulate in       the blood. They cross the epithelial wall of small venules to enter       connective tissue, where they differentiate, mature, and acquire       features necessary for phagocytic functions.
#** Macrophages       are involved in debris removal, antigen processing, and presentation for       lymphocyte activation.

#** '''Mast       Cells (Heparinocytes):'''

§  Mast cells are oval or irregularly shaped cells, ranging from 7 to 20 micrometers in size.

§  Their cytoplasm is filled with basophilic secretory granules that obscure the central nucleus.

§  Due to the high content of acidic radicals in their sulfated glycosaminoglycans (GAGs), mast cells exhibit metachromasia (a color change with basic dyes).

§  The granules are not well-preserved by fixatives, making their identification challenging at times.

§  Mast cells release substances important for inflammatory responses, immunity, and tissue repair:

§  '''Heparin:''' Acts locally as an anticoagulant.

§  '''Histamine:''' Promotes increased vascular permeability and smooth muscle contraction.

§  '''Serine proteases:''' Activate various mediators of inflammation.

§  '''Eosinophil and neutrophil chemotactic factors:''' Attract eosinophils and neutrophils to specific areas.

§  '''Cytokines:''' Polypeptides that direct activities of leukocytes and other immune system cells.

§  '''Phospholipid precursors:''' Converted to prostaglandins, leukotrienes, and other lipid mediators of the inflammatory response.

§  Immediate hypersensitivity reactions (allergic reactions) occur due to the release of certain chemical mediators stored in mast cells.

#** '''Plasma       Cells:'''

§  Plasma cells are derived from lymphocytes and are responsible for producing antibodies.

§  These large, egg-shaped cells have basophilic cytoplasm rich in rough endoplasmic reticulum (RER) and a large Golgi apparatus near the nucleus.

§  The nucleus contains heterochromatin alternating with regions of euchromatin (lightly packed).

§  Plasma cells play a vital role in the adaptive immune response by producing antibodies specific to antigens encountered by the immune system

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#** '''Fibers       of Connective Tissue:'''

§  Connective tissue contains elongated structures formed from proteins secreted by fibroblasts. The three main types of fibers are:

§  '''Collagen (20-300 nm):'''

#** Collagen       is a family of proteins that form fibers, sheets, and networks.
#** It       is resistant to normal shearing and tearing forces.
#** Collagen       is the most abundant protein in the human body and a major product of       fibroblasts.
#** Staining       characteristics:
#** Eosin:       Pink
#** Alcian       blue: Red
#** Mallory       trichrome: Blue
#** Mason’s       trichrome: Green
#** Sirius       red: Red
#** Types       of fibrillar collagens:
#** Type       I: Most abundant in skin, tendons, bone, ligaments, and cornea (forms       collagen fibers).
#** Type       II: Found in vertebral disks (forms fibrils).
#** Type       III: Forms reticular fibers.

§  '''Network or sheet-forming collagens (e.g., Type IV):'''

#** Subunits       produced by epithelial cells; major structural proteins of external       laminae and all epithelial basal laminae.
#** Type       IV collagen composes the basement membrane.


# '''Linking/Anchoring     Collagens:'''
#* Linking/anchoring      collagens are short collagen molecules that connect fibrillar collagens      together.
#* These      specialized collagens play a crucial role in maintaining the integrity      and stability of the extracellular matrix (ECM).
# '''Collagen     Synthesis:'''
#* Collagen      synthesis primarily occurs in fibroblasts, although various cell types      contribute.
#* The      process begins with procollagen alpha chains, which are polypeptides      synthesized in the rough endoplasmic reticulum (RER).
#* In      the RER, three alpha chains align and stabilize through disulfide bonds,      forming a triple helix.
#* This      procollagen helix is then exocytosed and cleaved into procollagen      molecules, which serve as the building blocks for collagen fibers or      sheets.
#* Key      steps in collagen synthesis:
#*# Procollagen       chains are produced on polyribosomes in the RER.
#*# Hydroxylase       enzymes add hydroxyl groups to prolines and lysines.
#*# Glycosylation       of hydroxylysine residues can occur.
#*# The       amino and carboxyl terminal sequences of alpha chains lack glycine-X-Y       repeats.
#*# In       the RER, the C-terminus regions of the three selected chains stabilize       through disulfide bonds.
#*# Procollagen       peptidases remove globular peptides, converting procollagen to collagen.
#*# Certain       proteoglycans and other collagens associate with new collagen fibrils,       promoting larger fibril formation.
#*# Cross-links,       catalyzed by lysyl oxidase, reinforce fibril structure and prevent       disassembly.

# '''Collagen     Characteristics:'''
#* Collagen      appears white due to its regular orientation, making it birefringent      under polarizing microscopy.
#* Staining      characteristics:

#** Eosin:       Pink
#** Mallory       trichrome: Blue
#** Sirius       red: Red

#* To      study collagen length and diameter, spread preparations (e.g., mesentery)      are used to allow light to pass through.
#* Collagenase      enzymes degrade collagen, contributing to proteolytic degradation outside      cells.
# '''Fibrilogenesis:'''
#* Collagen      molecules align head-to-tail, forming covalent bonds between adjacent      rows.
#* Fibrils      consist of more than one type of collagen (e.g., Types I, II, III, VIII).
#* Type      V and XI collagen form a fibrillar core, with Type I collagen deposited      on the surface.
#* Type      IX collagen links to glycosaminoglycan (GAG) molecules, stabilizing the      network of fibrils.
#* Fibrilogenesis      occurs in tissues like hyaline and elastic cartilage.

# '''Reticular     Fibers:'''
#* Mainly      composed of collagen Type III, reticular fibers form a network of thin      fibers (diameter between 0.5 and 2 micrometers).
#* They      support various cells and are visible in H&E preparations.
#* Staining:      Black with silver salt impregnation (argyrophilic) and PAS positive due      to sugar chains.
#* Found      in reticular lamina of basement membranes, surrounding adipocytes, smooth      muscle, nerve fibers, and small blood vessels.
#* Characterize      the stroma of hemopoietic tissue, spleen, and lymph nodes.

# '''Elastic     Fibers:'''

#* Elastic      fibers, which are thinner than Type I collagen fibers, form sparse      networks in organs that experience stretching or bending.
#* Within      the walls of large blood vessels, elastic lamellae contain elastin in a      fenestrated (containing many cavities) sheet.
#* Elastic      fibers are not strongly acidophilic and stain poorly with H&E      (hematoxylin and eosin). However, they stain darker with other stains      such as orcein (red-brown) and aldehyde fuchsin (violet).
#* These      fibers are a composite of fibrillin, forming microfibrils that are      embedded in a larger mass of elastin.

#* Microfibrils,      with a diameter of approximately 10 nm, scaffold upon one another,      allowing elastin to be deposited and accumulate, ultimately making up the      elastic fibers.

7.      '''Ground Substance:'''

#* Ground      substance is a highly hydrated, transparent complex mixture found in      connective tissue.
#* It      consists of:
#*# '''Glycosaminoglycans       (GAGs):''' Long polymers of repeating disaccharide units (usually       hexosamine and uronic acid). The most abundant GAG is hyaluronan       (hyaluronic acid), synthesized directly by hyaluronan synthase.

#*# '''Proteoglycans:''' GAGs       linked to protein cores. They allow for hydration, swelling pressure,       and compression handling.

#*# '''Multiadhesive       Glycoproteins:''' These stabilize and link the ECM to cell       surfaces, regulate cell movement, and play a role in cell proliferation       and differentiation.
# '''Major     Proteoglycans:'''
#* '''Aggrecan:''' Found      in cartilage, responsible for ECM hydration.
#* '''Decorin:''' Present      in connective tissue proper, cartilage, and bone. It binds to neighboring      collagen molecules and plays a role in fibrillogenesis.
#* '''Versican:''' Found      in fibroblasts, skin, smooth muscle, brain, and kidney mesangial cells.
#* '''Syndecan:''' Links      cells to the ECM.
# '''Multiadhesive     Glycoproteins:'''
#* These      glycoproteins stabilize the ECM and link it to cell surfaces. Some      examples include:
#*# '''Fibronectin:''' Abundant       and has binding sites for integrin and collagen Type IV.
#*# '''Laminin:''' Present       in basal lamina, anchoring cell surfaces.
#*# '''Tenascin:''' Modulates       cell attachment to the ECM.
#*# '''Osteopontin:''' Binds       to osteoclasts.
# '''Types     of Connective Tissue:'''
#* '''Connective      Tissue Proper:'''
#*# Classified       as loose or dense based on collagen content.
#*# '''Loose       Connective Tissue (Areolar Tissue):''' Contains cells, fibers, and       ground substance in equal proportions. Fibroblasts are the most numerous       cells. It is flexible and not resistant to stress.

#*# '''Dense       Connective Tissue:'''
#*## Fewer        cells and a predominance of bundled Type I collagen fibers.
#*## Subtypes:
#*### '''Dense         Irregular Tissue:''' Collagen fibers appear randomly without         orientation (e.g., deep dermis and organ capsules).

#*## '''Dense         Regular Connective Tissue:''' Type I collagen bundles and         fibroblasts align parallel to withstand repeated and prolonged         stresses (e.g., tendons, aponeuroses, and ligaments).

# '''Special     Connective Tissue:'''
#* Includes      mucous, reticular, elastic, adipose, and hematopoietic tissues.
#* '''Supporting      Connective Tissue:'''
#*# Examples       include bone, dentin (calcified tissue of the body), and cementum       (surface layer of tooth roots).
#* '''Embryonic      Connective Tissue:'''
#*# Mesoderm       gives rise to most connective tissues, except in the head where special       ectoderm-derived cells contribute.

#*# '''Mesenchyme:''' Primitive       connective tissue composed of small spindle-shaped cells with processes       forming a 3D network. Gap junctions facilitate communication between       processes.       '''Mesenchyme:'''
#*# Mesenchyme       is a primitive connective tissue established during early embryonic       development.
#*# It       consists of small, spindle-shaped cells with uniform appearances.
#*# These       cells extend processes that form a 3D network.
#*# Gap       junctions facilitate communication between these processes.
#*# '''Mucous       Connective Tissue:'''

#*# Mucous       connective tissue is found in the umbilical cord and is also known as       Wharton’s jelly.
#*# The       ground substance in this tissue is called Wharton’s jelly.
#*# The       cells within mucous connective tissue resemble fibroblasts and are       separated.
#*# Overall,       mucous connective tissue provides support and flexibility in the       developing embryo.