Chapter 4B: Tissue - The Living Fabric

Connective Tissue

Connective tissue is the most abundant and widely distributed of primary tissues. Its major functions include binding and support, protection, insulation, storing reserve fuel, and transporting substances like blood. The four main classes are connective tissue proper, cartilage, bone, and blood.

Comparison of Classes of Connective Tissues

Connective Tissue Proper

  • Subclasses: Loose connective tissue (areolar, adipose, reticular) and dense connective tissue (regular, irregular, elastic).

  • Cells: Fibroblasts, fibrocytes, defense cells, and adipocytes.

  • Matrix: Gel-like ground substance with collagen, reticular, and elastic fibers.

  • General Features: Varies in density and fiber types; functions as binding tissue, resists mechanical stress, provides water and salt reservoir, and stores energy (fat).

Cartilage

  • Subclasses: Hyaline cartilage, elastic cartilage, and fibrocartilage.

  • Cells: Chondroblasts (in growing cartilage) and chondrocytes.

  • Matrix: Gel-like ground substance with collagen and elastic fibers in some.

  • General Features: Resists compression due to water content; cushions and supports.

Bone Tissue

  • Subclasses: Compact bone and spongy bone.

  • Cells: Osteoblasts and osteocytes.

  • Matrix: Gel-like ground substance calcified with inorganic salts and collagen fibers.

  • General Features: Hard tissue resisting compression and tension; functions in support.

Blood

  • Cells: Red blood cells (erythrocytes), white blood cells (leukocytes), and platelets.

  • Matrix: Plasma (fluid).

  • General Features: Fluid tissue that carries oxygen, carbon dioxide, nutrients, wastes, and hormones.

Common Characteristics of Connective Tissue

Connective tissues differ from other primary tissues in three main characteristics:

  1. Embryonic Origin: All arise from mesenchyme tissue.

  2. Vascularity: Varying degrees; cartilage is avascular, while bone is highly vascularized.

  3. Extracellular Matrix (ECM): Cells are suspended/embedded in a non-living protein-sugar mesh. Matrix supports cells, enabling them to bear weight, withstand tension, and endure abuse.

Structural Elements of Connective Tissue

All connective tissues have three main elements: ground substance, fibers, and cells. The ground substance and fibers together make up the extracellular matrix. The composition and arrangement of these elements vary.

Ground Substance

Unstructured gel-like material filling the space between cells; it serves as a medium for solute diffusion between blood capillaries and cells. Components include:

  • Interstitial fluid: acts as a molecular sieve for nutrient diffusion.

  • Cell adhesion proteins: act as "glue" for attachment.

  • Proteoglycans: sugar proteins forming large aggregates that trap water (e.g., chondroitin sulfate and hyaluronic acid).

Water content affects the viscosity of the ground substance.

Connective Tissue Fibers

Three types of fibers provide support:

  • Collagen: The strongest and most abundant type; provides high tensile strength, resisting being pulled apart.

  • Elastic Fibers: Networks of long, thin, elastin fibers allowing for stretch and recoil.

  • Reticular Fibers: Short, fine, highly branched collagenous fibers forming networks that offer more "give."

Cells

  • "Blast" Cells: Immature cells actively secreting ground substance and ECM fibers. Examples include: Fibroblasts (connective tissue proper), chondroblasts (cartilage), osteoblasts (bone), Hematopoietic stem cells (bone marrow)

  • "Cyte" Cells: Mature, less active forms of “blast” cells maintaining matrix health.

  • Fat Cells: Store nutrients.

  • White Blood Cells: Neutrophils, eosinophils, lymphocytes involved in tissue response to injury.

  • Mast Cells: Initiate local inflammatory response against foreign microorganisms.

  • Macrophages: Phagocytic cells that “eat” dead cells and microorganisms; function in immune system.

Types of Connective Tissues

Connective Tissue Proper

Consists of all connective tissues except bone, cartilage, and blood. Two subclasses:

  • Loose Connective Tissues:

    • Areolar: Widely distributed; supports and binds other tissues; universal packing material; contains fibroblasts secreting loose collagen fibers allowing increased ground substance which acts as water reservoir; contains macrophages and fat cells. Functions to wrap and cushion organs, phagocytize bacteria, plays role in inflammation and holds/conveys tissue fluid. Location: under epithelia of body, packages organs, surrounds capillaries.

    • Adipose: Similar to areolar but with greater nutrient storage; cells are adipocytes; scanty matrix, richly vascularized. Functions in shock absorption, insulation, and energy storage.

      • White Fat: Stores nutrients.

      • Brown Fat: Uses lipid fuels to heat bloodstream instead of producing ATP. Norepinephrine releases and triggers UCP1 which short circuits the batery and is activated in the mitochondria, brown fat cells soak up fat and sugars and burn them for heat.

    • Reticular: Resembles areolar but with thinner reticular fibers; fibroblast cells are reticular cells secreting reticular fibers forming a mesh-like stroma supporting blood cells in lymph nodes, spleen, and bone marrow.

  • Dense Connective Tissues:

    • Dense Regular: High tensile strength; withstands high tension and stretching; closely packed bundles of thick collagen fibers run parallel to pull direction; appears as white structures offering great pulling resistance; fibers are slightly wavy allowing to stretch a little; fibroblasts manufacture collagen fibers and ground substance; very few cells & ground substance, poorly vascularized. Example: tendons and ligaments

    • Dense Irregular: Same elements as dense regular but collagen bundles are thicker and irregularly arranged forming sheets resisting tension from many directions. Found in: dermis, fibrous joint capsules, coverings of some organs.

    • Elastic: Some ligaments are very elastic (e.g., ligaments connecting adjacent vertebrae); also found in walls of large arteries needing to stretch when blood enters and recoil to push blood out.

Cartilage

Matrix secreted from chondroblasts (during growth) and chondrocytes (adults); chondrocytes are found in cavities called lacunae. Cartilage is 80% water with packed collagen fibers and sugar proteins (chondroitin and hyaluronic acid), it is a tough yet flexible material lacking nerve fibers. Avascular, receiving nutrients from surrounding membrane (perichondrium) which gives rise to chondroblasts & chondrocytes.

Three types:

  • Hyaline: Most abundant; “gristle”; appears as shiny bluish glass; found at tips of long bones, nose, trachea, larynx, and cartilage of ribs.

  • Elastic: Similar to hyaline but with more elastic fibers; found in ears and epiglottis.

  • Fibrocartilage: Properties between hyaline and dense regular tissue; strong, found in intervertebral discs and knee.

Clinical – Homeostatic Imbalance 4.2

Avascular cartilage loses ability to divide as we age, so injuries heal slowly which is common in people with sports injuries; later in life cartilage can calcify or ossify (become bony), causing chondrocytes to die.

Bone

Also called osseous tissue. Supports and protects body structures, stores fat and synthesizes blood cells in cavities, has more collagen compared to cartilage, has inorganic calcium salts. Osteoblasts produce matrix, osteocytes maintain matrix and reside in lacunae (cavities in matrix). Osteons are individual structural units; richly vascularized.

Blood

Most atypical connective tissue because it is fluid consisting of cells surrounded by matrix (plasma). Red blood cells are the most common cell type; also contains white blood cells and platelets; fibers are soluble proteins that precipitate during blood clotting. Functions in transport and carrying nutrients, wastes, gases, and other substances.

Muscle Tissue

Highly vascularized; responsible for most types of movement. Muscle cells possess myofilaments (actin and myosin proteins) that bring about contraction. Three types of muscle tissues: skeletal muscle, cardiac muscle, and smooth muscle.

Skeletal Muscle

Attached to and causes movement of bones; also called voluntary muscle because skeletal muscles can be consciously controlled. Cells are called muscle fibers containing multiple nuclei and appear striated or banded.

Cardiac Muscle

Found only in walls of heart. An involuntary muscle. Like skeletal muscle, it contains striations but cells have only one nucleus. Cells can have many branches that join branches of other cardiac cells and intercalated discs are special joints where cardiac cells are joined.

Smooth Muscle

Found mainly in walls of hollow organs (other than heart). An involuntary muscle. Has no visible striations. Spindle-shaped cells with one nucleus.

Nervous Tissue

Main component of nervous system (brain, spinal cord, nerves). Regulates and controls body functions. Made up of two specialized cells: neurons and supporting cells. Neurons are specialized nerve cells that generate and conduct nerve impulses. Supporting cells support, insulate, and protect neurons.

Covering and Lining Membranes

Composed of at least two primary tissue types: an epithelium bound to underlying connective tissue proper layer. Three types: cutaneous membranes, mucous membranes, and serous membranes.

Cutaneous Membranes

Another name for skin that is keratinized stratified squamous epithelium (epidermis) attached to a thick layer of connective tissue (dermis). Unlike other membranes, skin is a dry membrane.

Mucous Membranes

Also called mucosae. Line body cavities that are open to the exterior (e.g., digestive, respiratory, urogenital tracts). Moist membranes bathed by secretions (or urine). Epithelial sheet lies over layer of loose connective tissue called lamina propria, may secrete mucus.

Serous Membranes

Also called serosae are found in closed ventral body cavities. Constructed from simple squamous epithelium (mesothelium) resting on thin areolar connective tissue. Parietal serosae line internal body cavity walls; visceral serosae cover internal organs. Cavity between layers is filled with slippery serous fluid, so these are moist membranes. Special names are given to show location: pleurae (lungs), pericardium (heart), peritoneum (abdomen).

Tissue Repair

When the body’s barriers are compromised, the inflammatory and immune responses are activated and then repair starts very quickly. Repair is the function of the inflammatory process occuring in two major ways: regeneration and fibrosis.

  • Regeneration: Same kind of tissue replaces destroyed tissue, so original function is restored.

  • Fibrosis: Connective tissue replaces destroyed tissue, and original function lost.

Steps in Tissue Repair

  1. Inflammation: Sets stage with release of inflammatory chemicals causing dilation of blood vessels and increase in blood vessel permeability and clotting of blood occurs.

  2. Organization: Restores blood supply as blood clot is replaced with granulation tissue (new capillary-enriched tissue), epithelium begins to regenerate and fibroblasts produce collagen fibers to bridge the gap until regeneration is complete and debris in area is phagocytized.

  3. Regeneration and Fibrosis: Affect permanent repair as the scab detaches, fibrous tissue matures and epithelium thickens and begins to resemble adjacent tissue resulting in a fully regenerated epithelium with underlying scar tissue (visible or not).

Regenerative Capacity of Different Tissues

  • Tissues that regenerate extremely well: epithelial tissues, bone, areolar connective tissue, dense irregular connective tissue, blood-forming tissue

  • Tissue with moderate regenerating capacity: smooth muscle and dense regular connective tissue

  • Tissues with virtually no functional regenerative capacity: cardiac muscle and nervous tissue of brain and spinal cord

Clinical – Homeostatic Imbalance 4.3

Scar tissue that forms in organs (particularly heart) can severely impair organ function which may cause the organ to lose volume capacity, may block substances from moving through organ and may interfere with ability of muscles to contract or may impair nerve transmissions. Scar adhesions may cause organs to adhere to neighboring structures preventing normal functions causing progressive failure of an organ (particularly heart).

Developmental Aspects of Tissues

Primary germ layers include ectoderm, mesoderm, and endoderm which formed early in embryonic development and specialize to form four primary tissues. Nerve tissue arises from ectoderm, muscle and connective tissues arise from mesoderm and epithelial tissues arise from all three germ layers.

Tissues function well through youth and middle age if given adequate diet & circulation and if wounds/infections are minimal. As the body ages, epithelia thin, so they are more easily breached; tissue repair is less efficient; bone, muscle, and nervous tissues begin to atrophy; DNA mutations increase cancer risk.