Tissue: The Living Fabric – Connective, Muscle, and Nervous Tissues

Connective Tissue

Overview of Connective Tissue

Connective tissue is the most abundant and widely distributed of the primary tissue types in the human body. Its major functions include binding and supporting other tissues, protecting organs, insulating the body, storing reserve fuel, and transporting substances such as blood.

• Four main classes: Connective tissue proper, cartilage, bone, and blood.

• Key functions: Support, protection, insulation, energy storage, and transport.

Common Characteristics of Connective Tissue

• Embryonic origin: All connective tissues arise from mesenchyme (an embryonic tissue).

• Vascularity: Varies from avascular (cartilage) to highly vascularized (bone).

• Extracellular matrix (ECM): Cells are suspended in a protein-sugar mesh that supports and protects them.

Structural Elements of Connective Tissue

All connective tissues share three main structural elements:

• Ground substance: Unstructured gel-like material that fills the space between cells and allows for diffusion of nutrients and waste.

• Fibers: Provide support and include collagen (strength), elastic (stretch and recoil), and reticular (supportive networks).

• Cells: "Blast" cells (immature, matrix-secreting) and "cyte" cells (mature, maintenance).

Types of Connective Tissue Proper

• Loose connective tissues: Areolar, adipose, and reticular.

• Dense connective tissues: Dense regular, dense irregular, and elastic.

Loose Connective Tissues

• Areolar: Most widely distributed; supports and binds other tissues, holds body fluids, defends against infection, and stores nutrients. Contains fibroblasts, macrophages, and fat cells.

• Adipose: Stores energy, insulates, and cushions organs. Adipocytes are the main cell type.

• Reticular: Forms a soft internal skeleton (stroma) that supports other cell types in lymphoid organs.

Dense Connective Tissues

• Dense regular: Parallel collagen fibers; found in tendons and ligaments; resists tension in one direction.

• Dense irregular: Irregularly arranged collagen fibers; found in dermis and joint capsules; resists tension from many directions.

• Elastic: High proportion of elastic fibers; allows tissues to recoil after stretching; found in large arteries.

Cartilage

Cartilage is a tough yet flexible tissue that lacks nerve fibers and is avascular. It is composed of chondroblasts (during growth) and chondrocytes (in lacunae), with a matrix that is 80% water.

• Hyaline cartilage: Most abundant; supports and reinforces; found at ends of long bones, nose, trachea, and ribs.

• Elastic cartilage: More elastic fibers; maintains shape of structures like the ear and epiglottis.

• Fibrocartilage: Thick collagen fibers; strong and resists compression; found in intervertebral discs and knee.

Bone (Osseous Tissue)

Bone tissue supports and protects body structures, stores fat, and synthesizes blood cells. It is richly vascularized and contains more collagen than cartilage, as well as inorganic calcium salts.

• Osteoblasts: Produce bone matrix.

• Osteocytes: Maintain bone matrix and reside in lacunae.

• Osteons: Structural units of compact bone.

Blood

Blood is a fluid connective tissue consisting of cells (red and white blood cells, platelets) suspended in plasma. It functions in the transport of respiratory gases, nutrients, wastes, and other substances.

Muscle Tissue

Overview of Muscle Tissue

Muscle tissue is highly vascularized and responsible for most types of body movement. Muscle cells contain myofilaments made of actin and myosin, which enable contraction.

• Three types: Skeletal, cardiac, and smooth muscle.

Skeletal Muscle

Skeletal muscle tissue is attached to bones and causes voluntary movement. The cells are long, cylindrical, multinucleated, and striated.

Cardiac Muscle

Cardiac muscle tissue is found only in the heart. It is involuntary, striated, and usually has one nucleus per cell. Cells are branched and connected by intercalated discs.

Smooth Muscle

Smooth muscle tissue is found mainly in the walls of hollow organs (except the heart). It is involuntary, non-striated, and composed of spindle-shaped cells with a single nucleus.

Nervous Tissue

Overview of Nervous Tissue

Nervous tissue is the main component of the nervous system (brain, spinal cord, nerves). It regulates and controls body functions and is composed of neurons and supporting cells.

• Neurons: Specialized cells that generate and conduct nerve impulses.

• Supporting cells: Support, insulate, and protect neurons.

Covering and Lining Membranes

Types of Membranes

Membranes are composed of at least two primary tissue types: an epithelium bound to underlying connective tissue. The three main types are cutaneous, mucous, and serous membranes.

• Cutaneous membrane: The skin; a dry membrane composed of keratinized stratified squamous epithelium and connective tissue.

• Mucous membranes (mucosae): Line body cavities open to the exterior; moist membranes bathed by secretions; found in digestive, respiratory, and urogenital tracts.

• Serous membranes (serosae): Line closed ventral body cavities; composed of mesothelium and areolar connective tissue; secrete serous fluid to reduce friction.

Tissue Repair

Overview of Tissue Repair

Tissue repair occurs when the body's barriers are compromised. The process involves inflammation, organization, and regeneration or fibrosis.

Steps in Tissue Repair

1. Inflammation: Inflammatory chemicals are released, blood vessels dilate, and clotting occurs to prevent infection and further injury.

2. Organization: The blood clot is replaced by granulation tissue, restoring the blood supply. Fibroblasts produce collagen fibers to bridge the wound.

3. Regeneration and fibrosis: The epithelium regenerates, the scab detaches, and scar tissue may form.

Regenerative Capacity of Different Tissues

• High regenerative capacity: Epithelial tissues, bone, areolar connective tissue, dense irregular connective tissue, blood-forming tissue.

• Moderate capacity: Smooth muscle, dense regular connective tissue.

• Low to none: Cardiac muscle, nervous tissue in brain and spinal cord.

Developmental Aspects of Tissues

Primary Germ Layers

During embryonic development, three primary germ layers form: ectoderm, mesoderm, and endoderm. These layers give rise to all tissues in the body.

• Ectoderm: Forms nerve tissue.

• Mesoderm: Forms muscle and connective tissues.

• Endoderm: Forms the inner lining of the digestive system.

• Epithelial tissues: Arise from all three germ layers.

Aging and Tissue Function

• Tissues function well through youth and middle age if diet and circulation are adequate and wounds/infections are minimal.

• With aging, epithelia thin, tissue repair is less efficient, and bone, muscle, and nervous tissues atrophy.

• DNA mutations increase cancer risk.