Connective tissue

Connective Tissue

• One of the basic tissues of the body.

• Connects various tissues of the body and gives them support.

• Develops from mesoderm or mesenchyme.

• Contains:

  • Cells

  • Extracellular matrix (fibers and ground substance)

Cells of Connective Tissue

Fibroblasts

• Most common cells of connective tissue in animals.

• Responsible for production and maintenance of fibers and ground substance.

• Spindle-shaped cells with basophilic cytoplasm, large round nucleus, many processes.

• Known as fibrocytes in resting stage.

Adipose Cells (Adipocytes)

• Specialized for synthesis and storage of fat.

• Large lipid droplet; nucleus is flat, pushed to the periphery; cytoplasm is very thin.

• Cells appear singly or more often in groups.

Macrophages

• Also called histiocytes.

• May be fusiform, stellate, or spheroidal in shape.

• Cytoplasm contains variety of granules and vacuoles; phagocytic in nature.

Plasma Cells

• Provide resistance to the body against diseases.

• Specialized for the manufacture of antibodies against antigens.

• Abundant in lamina propria of digestive and respiratory tracts; found in lymphoid tissue.

• Characterized by a cartwheel-shaped nucleus.

Mast Cells

• Widely distributed in connective tissue; seen along the course of blood vessels.

• Contain large granules in cytoplasm; secrete histamine and heparin.

Undifferentiated Mesenchymal Cells

• Multipotent stem cells that can differentiate into any cell type under proper stimulus.

• Usually found along blood vessels; also located in bone marrow.

• Precursors of all types of cells.

Blood Cells

• Lymphocytes and neutrophils that migrated from the bloodstream.

Fibers in Connective Tissue

Types of Fibers

1. Collagen Fibers

   • Inelastic and have great tensile strength.

   • Contain protein collagen; seen in all types of connective tissue.

   • Fibers run in bundles that split into branches; individual fibers are unbranched.

   • Synthesized by fibroblasts.

   • Fresh fibers are colorless; appear pink when stained with hematoxylin and eosin.

2. Elastic Fibers

   • Highly elastic; can be stretched to 150% of their resting length without breaking.

   • Found in the walls of large arteries; synthesized by fibroblasts.

   • Yellowish when grouped; contain protein elastin; usually run individually.

   • Appear pink when stained with hematoxylin and eosin.

3. Reticular Fibers

   • Type III collagen fibers; seen in lymph nodes, spleen, thymus, liver, bone marrow, endocrine glands.

   • Fine branching fibers form a network (reticulum).

   • Cannot be stained with hematoxylin and eosin; stained with silver staining methods.

Ground Substance

• Semisolid gel also known as intercellular substance.

• Cells and fibers are embedded in it.

• Consists of water, carbohydrates, and proteins.

• Carbohydrates are in the form of mucopolysaccharides (glycosaminoglycans).

• Proteins are in the form of glycoproteins.

Functions of Connective Tissue

• Providing structural support.

• Compartmentalization.

• Aiding in defense and protection of the body.

• Forming a site for storage of fat.

Classification of Connective Tissue

Connective Tissue Proper

1. Loose Connective Tissue

   • Areolar Tissue: Protects and binds deeper tissue.

   • Adipose Tissue: Fat storage.

   • Myxomatous Tissue: Fetal type, contains hydrated mucosubstances.

   • Reticular Tissue: Found in lymph organs.

2. Dense Connective Tissue

   • Regular Dense: Ligaments and tendons.

   • Irregular Dense: Binds epidermis to skin.

   • Elastic: Present in aorta.

Supporting Connective Tissue

• Provides protection and structure.

  • Cartilage:

    • Elastic (ears)

    • Hyaline (ribs)

    • Fibrocartilage (vertebral discs)

  • Bone

Fluid Connective Tissue

• Blood

• Lymph

Supporting Connective Tissue: Cartilage

• Specialized, supportive type of connective tissue resistant to compressive forces.

• Forms skeletal basis of body parts like the auricle of the ear and outer nose.

Composition of Cartilage

• Contains cells (chondrocytes and chondroblasts) and extracellular matrix (ground substance and fibers).

• Matrix covered by perichondrium:

  • Outer fibrous layer with collagen fibers.

  • Inner cellular (chondrogenic) layer with chondroblast cells.

Types of Cartilage

1. Hyaline Cartilage

   • E.g., costal cartilage, thyroid cartilage, trachea.

   • Has perichondrium; contains chondrocytes in lacunae.

2. Elastic Cartilage

   • E.g., auricle of ear, epiglottis.

   • Contains branching elastic fibers and is elastic in nature.

3. Fibrocartilage

   • Known as white fibrocartilage; e.g., intervertebral discs, pubic symphysis.

   • No perichondrium present; matrix has bundles of collagen fibers.

Bone Tissue

• Hardest form of connective tissue; provides protection and support.

• Highly vascular and living tissue making up the skeleton.

• Matrix is calcified by calcium phosphate deposits.

• Contains 206 bones in the human skeleton.

Functions of Bone

• Provides support and shape to the body.

• Protects vital organs.

• Helps in weight transfer and muscle attachment.

• Stores calcium salts.

• Involved in erythropoiesis (red blood cell production).

Classification of Bones

According to Position

1. Axial Bones: Forming the axis of the body (e.g., skull, vertebrae).

2. Appendicular Bones: Bones of the limbs (e.g., scapula, humerus).

According to Size and Shape

1. Long Bones: E.g., humerus, femur.

2. Short Bones: Small cuboid shapes (e.g., carpal bones).

3. Flat Bones: Expanded and flat (e.g., sternum).

4. Irregular Bones: E.g., vertebrae.

5. Pneumatic Bones: Contain hollow spaces with air (e.g., maxilla).

6. Sesamoid Bone: Develop within tendons (e.g., patella).

According to Development

1. Membranous Bones: Develop from mesenchymal tissue (e.g., clavicle).

2. Cartilaginous Bones: Start as cartilage then ossify (e.g., femur).

According to Structure

1. Compact Bones: Contains bony plates tightly arranged (e.g., shaft of long bones).

2. Spongy Bones: Made of irregularly arranged bony plates with spaces (e.g., ends of long bones).

Structure of Long Bone

• Diaphysis: Develops into the shaft; primary ossification center.

• Epiphysis: Forms ends of the bone; secondary ossification center.

• Epiphyseal Plate: Cartilage separating epiphysis and diaphysis.

• Metaphysis: Part of diaphysis adjacent to epiphyseal plate; region of active growth.

Ossification Process

• The process of bone formation from mesenchymal tissue.

Types of Ossification

1. Membranous Ossification: Directly transforms mesenchyme into bone (e.g., mandible).

2. Cartilaginous Ossification: Mesenchyme first becomes cartilage, then ossifies (e.g., femur).

Bone Structure

• Bone covered by Periosteum: Double-layered membrane rich in blood vessels, lymphatics, and nerves.

• Endosteum: Membrane lining internal surfaces of bone.

• Bone matrix (osteoid) composed of organic (collagen fibers) and inorganic matter (hydroxyapatite crystals).

Types of Bone Cells

1. Osteoblasts: Bone-forming cells.

2. Osteocytes: Inactive osteoblasts in lacunae, involved in nutrition.

3. Osteoclasts: Large cells that remove bone tissue.

Structure of Compact Bone

• Contains plates called lamellae with lacunae for osteocytes.

• Canaliculi connect lacunae and lamellae; contain osteocyte processes.

• Different types of lamellae: concentric, interstitial, and circumferential lamellae.

Osteon (Haversian System)

• Comprised of Haversian canal and surrounding concentric lamellae.

• Cylindrical structures running parallel to long axis of bone.

• Haversian canals contain blood vessels and nerves.

Structure of Spongy Bone

• Composed of trabeculae with interconnecting plates of bone.

• Spaces between trabeculae filled with bone marrow.

• Osteocytes sit in lacunae within the lamellae.

Bone Marrow

• Vascular connective tissue found in medullary cavities and spaces of spongy bone.

• Two types:

  • Red Bone Marrow: Active in blood cell production.

  • Yellow Bone Marrow: Contains fat cells; replaces red marrow with age.