Histology: Connective, Muscle, and Nervous Tissues

Objectives

• Describe the general functions and properties of connective tissues.

• Compare and contrast the roles of individual cells and fiber types within connective tissues.

• Classify connective tissues based on distinguishing characteristics, describe their locations, and correlate structure with function.

• Summarize muscle and nervous tissue.

• Define organs and membranes.

• Briefly describe tissue repair.

Connective Tissue

General Functions and Properties

Connective tissues are a diverse group of tissues that provide structural and metabolic support for other tissues and organs throughout the body. They are characterized by abundant extracellular matrix (ECM) and a variety of cell types.

• Binding and Support:

  • Tendons bind muscle to bone.

  • Ligaments bind bones to bone.

  • Adipose tissue holds eyes and kidneys in place.

  • Bones support the body; cartilage supports the nose, ears, and larynx.

  • Fibrous tissues form the cardiac skeleton and framework for organs such as the spleen.

• Physical Protection:

  • Structures such as the cranium, rib cage, and adipose tissue protect vital organs.

• Immune Protection:

  • Leukocytes and other immune cells in connective tissues defend against pathogens.

  • Skin and mucous membranes act as barriers.

• Movement:

  • Bones act as levers for muscle action.

  • Cartilage moves vocal cords.

• Storage:

  • Adipose tissue stores energy.

  • Bones store calcium and phosphorus.

• Heat Production:

  • Brown fat in infants and hibernating animals generates heat.

• Transport:

  • Blood transports gases, nutrients, hormones, and wastes.

Key Properties:

• Cells usually occupy less space than the ECM.

• Cells are often not in direct contact with each other.

• ECM plays a major role in tissue function.

• Vascularity varies: loose connective tissue is highly vascularized, while tendons and ligaments are less so.

Cells and Fiber Types in Connective Tissues

Connective tissues contain a variety of cells and fibers, each contributing to tissue function.

• Fibroblasts: Produce fibers and ground substance.

• Adipocytes: Store triglycerides (fat).

• Immune Cells:

  • Leukocytes (white blood cells) defend against pathogens.

  • Macrophages and neutrophils phagocytose debris and microbes.

  • Mast cells and eosinophils participate in inflammation and allergic responses.

• Chondroblasts/Chondrocytes:

  • Chondroblasts secrete cartilage matrix; become chondrocytes when trapped in lacunae.

  • Cartilage is avascular and relies on the perichondrium for nourishment.

• Osteoblasts/Osteocytes/Osteoclasts:

  • Osteoblasts secrete bone matrix; become osteocytes when trapped in lacunae.

  • Osteoclasts dissolve bone tissue during remodeling.

• Blood Cells:

  • Red blood cells (erythrocytes) transport oxygen.

  • White blood cells (leukocytes) provide immune defense.

  • Platelets are involved in clotting; fibers (fibrin) form only during clotting.

Fiber Types:

• Collagen fibers: Strong, resist tension (found in tendons, ligaments, bone).

• Elastic fibers: Stretch and recoil (found in skin, lungs, blood vessels).

• Reticular fibers: Form supportive networks (found in lymphoid organs).

Classification of Connective Tissues

Connective tissues are classified based on their cellular composition and ECM characteristics. All are derived from a common embryonic origin (mesenchyme).

Additional info: Cartilage is 80% water, allowing resilience and quick recovery after compression. Bone ECM is 1/3 collagen, 2/3 mineral (mainly calcium phosphate).

Muscle and Nervous Tissue

Muscle Tissue

Muscle tissue is specialized for contraction and movement. Its cells are called myocytes or muscle fibers, and are excitable (respond to stimuli).

• Skeletal Muscle: Long, cylindrical, multinucleated cells with obvious striations; voluntary movement.

• Cardiac Muscle: Branching, striated cells with one or two nuclei; found in the heart; involuntary.

• Smooth Muscle: Fusiform (spindle-shaped), non-striated, single nucleus; found in walls of hollow organs; involuntary.

Nervous Tissue

Nervous tissue is specialized for communication via electrical impulses. It consists of two main cell types:

• Neurons: Transmit electrical signals; have a cell body (soma), dendrites (receive signals), and a single axon (transmits signals).

• Neuroglia: Support, protect, and nourish neurons.

Organs and Membranes

Organs

An organ is a structure composed of two or more tissue types working together for a common function. Example: skeletal muscle (contains muscle tissue, connective tissue, blood vessels, nerves).

Membranes

Membranes are thin sheets of tissue that line body surfaces or cavities. They may be true membranes or membrane-like structures.

• True Membranes:

  • Serous Membranes: Line pleural, pericardial, and peritoneal cavities; composed of simple squamous epithelium (mesothelium), basement membrane, and loose connective tissue. Have parietal and visceral layers with serous fluid between.

  • Synovial Membranes: Line cavities of freely movable joints (e.g., hip, knee, elbow, shoulder); composed of two layers of connective tissue; secrete synovial fluid.

• Membrane-like Structures:

  • Mucous Membranes (Mucosae): Line body surfaces that open to the outside (respiratory, digestive, reproductive tracts); consist of epithelium, basement membrane, and loose connective tissue (lamina propria).

  • Cutaneous Membrane: The skin; composed of keratinized stratified squamous epithelium (epidermis) and underlying connective tissue (dermis).

Tissue Repair

Tissue repair restores structure and function after injury. It occurs by two main processes:

• Regeneration: Damaged cells are replaced by the same type, restoring full function. Common in epithelial and most connective tissues.

• Fibrosis: Damaged cells are replaced by collagen (scar tissue) produced by fibroblasts. Function may be partially lost. Occurs in cardiac muscle, skeletal muscle, nervous tissue, and some cartilage.

Examples: Skin and liver can regenerate well; cardiac muscle and central nervous system tissue typically heal by fibrosis.