Chapter 4: Tissue—The Living Fabric

4.1 Tissue Preparation for Microscopy

To study tissues under a microscope, samples must be carefully prepared to preserve their structure and reveal cellular details.

• Fixation: Preserves tissue structure and prevents decay.

• Sectioning: Tissues are sliced thinly to allow light to pass through for microscopy.

• Staining: Dyes are applied to highlight different cellular components, making anatomical structures distinguishable.

Example: Hematoxylin and eosin (H&E) staining is commonly used to differentiate nuclei (blue) from cytoplasm (pink).

4.2 Epithelial Tissue

Epithelial tissue covers body surfaces, lines cavities, and forms glands. It is specialized for protection, absorption, filtration, excretion, secretion, and sensory reception.

Structural and Functional Characteristics

• Polarity: Has an apical (free) surface and a basal (attached) surface.

• Specialized Contacts: Cells are tightly joined by junctions (e.g., desmosomes, tight junctions).

• Supported by Connective Tissue: The basement membrane anchors epithelium to underlying tissues.

• Avascular but Innervated: Lacks blood vessels; nutrients diffuse from underlying tissues; contains nerve fibers.

• High Regeneration Capacity: Rapidly replaces lost cells by cell division.

Classification of Epithelia

• By Layers:

  • Simple: One cell layer (functions in absorption, secretion, filtration).

  • Stratified: Two or more layers (mainly for protection).

• By Cell Shape:

  • Squamous: Flat and scale-like.

  • Cuboidal: Box-like, as tall as they are wide.

  • Columnar: Tall and column-shaped.

Types of Epithelia and Locations

Glandular Epithelia

• Endocrine Glands: Ductless; secrete hormones into blood or lymph.

• Exocrine Glands: Secrete products onto body surfaces or into cavities via ducts.

  • Unicellular: E.g., goblet cells (secrete mucus).

  • Multicellular: Composed of a duct and secretory unit; classified by duct structure (simple/compound) and secretion method (merocrine/holocrine).

Note: Endocrine glands secrete hormones into the bloodstream; exocrine glands secrete onto surfaces or into ducts (e.g., digestive enzymes).

4.3 Connective Tissue

Connective tissue is the most abundant and widely distributed tissue type, providing support, binding, protection, insulation, and transport.

Common Characteristics

• Originates from mesenchyme (embryonic tissue).

• Varies in vascularity (from avascular to highly vascularized).

• Composed mainly of nonliving extracellular matrix (ground substance + fibers).

Structural Components

• Ground Substance: Fills space between cells; consists of interstitial fluid, cell adhesion proteins, and proteoglycans.

• Fibers:

  • Collagen: Strong, high tensile strength.

  • Elastic: Stretch and recoil (contain elastin).

  • Reticular: Fine, branching, support networks.

• Cells: Each connective tissue type has a fundamental cell (immature: -blast; mature: -cyte), plus adipocytes, leukocytes, macrophages, and mast cells.

Types of Connective Tissue

Example: Tendons (dense regular connective tissue) connect muscle to bone; adipose tissue stores energy and insulates the body.

4.4 Muscle Tissue

Muscle tissue is specialized for contraction and movement. It is highly cellular and well vascularized.

• Skeletal Muscle: Voluntary, long, cylindrical, multinucleate, striated; attaches to bones.

• Cardiac Muscle: Involuntary, striated, branched, uninucleate; found only in the heart.

• Smooth Muscle: Involuntary, non-striated, spindle-shaped; found in walls of hollow organs (e.g., intestines, blood vessels).

Example: Skeletal muscle enables limb movement; cardiac muscle pumps blood; smooth muscle moves food through the digestive tract.

4.5 Nervous Tissue

Nervous tissue forms the main component of the nervous system, responsible for regulating and controlling body functions.

• Neurons: Specialized cells that generate and conduct electrical impulses; have dendrites (receive signals) and axons (transmit signals).

• Supporting Cells (Neuroglia): Nonconducting cells that support, insulate, and protect neurons.

Example: Neurons in the brain process information; Schwann cells insulate peripheral nerves.

4.6 Body Membranes

Body membranes are simple organs composed of epithelial and connective tissues, serving as coverings or linings.

• Cutaneous Membrane (Skin): Dry; keratinized stratified squamous epithelium over dense irregular connective tissue.

• Mucous Membranes: Wet; line body cavities open to the exterior (e.g., digestive, respiratory tracts); consist of epithelium over lamina propria (areolar connective tissue).

• Serous Membranes: Moist; line closed body cavities (e.g., peritoneum, pleura, pericardium); simple squamous epithelium over areolar connective tissue.

4.7 Tissue Repair

Tissue repair restores tissue integrity after injury through two main processes: regeneration and fibrosis.

• Regeneration: Replacement of destroyed tissue with the same kind of cells.

• Fibrosis: Replacement with fibrous connective (scar) tissue.

Steps in Tissue Repair

1. Inflammation: Damaged cells release chemicals, increasing blood flow and permeability; prepares area for repair.

2. Organization: Blood clot replaced by granulation tissue, restoring blood supply.

3. Regeneration/Fibrosis: Surface epithelium regenerates; underlying area may form scar tissue if regeneration is incomplete.

Note: Tissues vary in regenerative capacity; epithelial and blood-forming tissues regenerate well, while cartilage and cardiac muscle regenerate poorly.

Developmental Aspects of Tissues

During embryonic development, three primary germ layers form all tissues:

• Ectoderm: Forms epithelium and nervous tissue.

• Mesoderm: Forms connective, muscle, and some epithelium.

• Endoderm: Forms epithelium (lining of digestive and respiratory tracts).

In adults, only epithelia and blood-forming tissues remain highly mitotic. Some tissues regenerate via mature cell division; others rely on stem cells.

Recent Discoveries and Research Topics

• Tissue Engineering: Creation of synthetic tissues and organs using bioprinting and stem cells.

• Stem Cell Therapy: Reverting fat cells to pluripotent stem cells for tissue repair.

• Research Topics: Cloning, bioprinting, advances in stem cell therapy, tissue harvesting for reconstructive surgery.

Key Terms and Definitions

• Epithelium: Sheet of cells covering surfaces or lining cavities.

• Gland: One or more cells that secrete a product.

• Extracellular Matrix: Nonliving material between cells in connective tissue.

• Osteon: Structural unit of compact bone.

• Chondrocyte: Mature cartilage cell.

• Neuron: Nerve cell specialized for impulse conduction.

Sample Comparisons

• Adipose vs. Bone Tissue: Both are connective tissues with rich blood supply and nutrient storage, but bone stores nutrients in the matrix, while adipose stores nutrients in cells.

• White vs. Brown Fat: White fat stores energy; brown fat generates heat (mainly in infants).

Sample Equations and Concepts

• Osmosis (relevant to epithelial absorption): Where: = flux, = hydraulic conductivity, = hydrostatic pressure difference, = reflection coefficient, = osmotic pressure difference.

Additional info: Tissue engineering and regenerative medicine are rapidly advancing fields, with research focusing on bioprinting organs and using stem cells for tissue repair. Understanding tissue structure and function is foundational for these innovations.