Tissues & Histology

Overview of Tissues

Tissues are groups of closely associated cells that are similar in structure and function. The study of tissues is called histology. Tissues are organized into organs, each with specialized roles in the body.

• Epithelial Tissue: Covers and lines body surfaces; forms glands.

• Connective Tissue: Provides support and structure.

• Muscle Tissue: Responsible for movement.

• Nervous Tissue: Controls and coordinates body activities.

Epithelial Tissue

Characteristics and Functions

• Polarity: Has an exposed (apical) surface, often with cilia or microvilli; basal surface attached to underlying tissues.

• Specialized Contacts: Cells joined by tight junctions and desmosomes, making the layer impermeable.

• Basement Membrane: Thin layer anchoring epithelium to connective tissue, resisting stretching and tearing.

• Avascular but Innervated: No blood vessels, but contains nerve fibers.

• Regeneration: High mitotic rate; can rapidly replace lost cells.

Functions: Protection, absorption, filtration, excretion, secretion.

Classification of Epithelial Tissue

• By Layers:

  • Simple: Single cell layer.

  • Stratified: Multiple cell layers.

• By Cell Shape:

  • Squamous: Flat, scale-like.

  • Cuboidal: Cube-shaped.

  • Columnar: Tall, column-like.

Types of Covering and Lining Epithelia

Glandular Epithelium

Glands are structures that produce and secrete substances, usually proteins.

• Endocrine Glands: Ductless; secrete hormones directly into blood or lymph (e.g., pituitary, thyroid, adrenal glands).

• Exocrine Glands: Secrete products into ducts (e.g., sweat, oil, salivary glands).

  • Unicellular: Single cells (e.g., goblet cells) secrete mucin.

  • Multicellular: Have ducts and secretory units surrounded by connective tissue.

Modes of Secretion

Connective Tissue

Characteristics and Functions

• Most abundant and widely distributed tissue type.

• Functions: Binding, support, protection, insulation, transport.

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

• All arise from embryonic mesenchyme.

• Vascularity varies: avascular (cartilage) to highly vascular (bone, others).

Structural Elements

• Ground Substance: Proteoglycans and interstitial fluid; allows diffusion of substances.

• Fibers:

  • Collagen: Strong, high tensile strength.

  • Elastic: Stretch and recoil.

  • Reticular: Fine, supportive networks.

• Cells:

  • Immature: -blasts (e.g., fibroblasts, chondroblasts, osteoblasts, hemocytoblasts).

  • Mature: -cytes (e.g., fibrocytes, chondrocytes, osteocytes).

Types of Connective Tissue

Muscle Tissue

Types and Functions

• Skeletal Muscle: Striated, voluntary, multinucleated; moves bones.

• Cardiac Muscle: Striated, branched, uninucleate; found in heart, involuntary.

• Smooth Muscle: Non-striated, spindle-shaped, uninucleate; in walls of hollow organs, involuntary.

Nervous Tissue

Structure and Function

• Composed of neurons (transmit electrical impulses) and supporting cells (glia).

• Forms brain, spinal cord, and nerves.

• Functions to receive, process, and transmit information; controls muscle and gland activity.

Covering and Lining Membranes

Types of Membranes

• Cutaneous Membrane: Skin; keratinized stratified squamous epithelium + dense irregular CT; dry membrane.

• Mucous Membranes (Mucosae): Line body cavities open to exterior (respiratory, digestive, urogenital tracts); wet; epithelium + loose CT (lamina propria); absorb/secrete substances.

• Serous Membranes: Line closed ventral body cavities; produce serous fluid for lubrication.

Tissue Repair

Steps of Tissue Repair

1. Inflammation: Injury triggers inflammation; histamine release increases capillary permeability; white blood cells and plasma enter area; clot forms and dries to form a scab.

2. Organization: Blood clot replaced by granulation tissue (capillaries, macrophages, fibroblasts); macrophages remove debris; fibroblasts produce collagen; granulation tissue becomes scar (fibrosis).

3. Regeneration: Surrounding cells divide and migrate to replace lost tissue; most tissue is restored, but some is replaced by scar tissue.

Regenerative Capacity of Tissues

Example: After a skin injury, the area becomes inflamed, a clot forms, and a scab develops. Granulation tissue replaces the clot, and new epithelial cells regenerate the surface, while deeper tissue may form a scar.