Tissues & Histology

Introduction to Tissues

Tissues are groups of structurally similar cells that perform related functions. The study of tissues is called histology. There are four primary tissue types in the human body: epithelial, connective, muscle, and nervous tissues.

• Tissue Preparation: For microscopic examination, tissues are fixed (preserved), sliced into thin sections, and stained with dyes. This process can introduce minor distortions known as artifacts.

Epithelial Tissue

Characteristics and Functions

Epithelial tissue covers body surfaces, lines internal cavities, and forms glands. Its main functions include:

• Protection

• Absorption

• Excretion

• Filtration

• Secretion

• Sensory reception

Special characteristics of epithelial tissue:

• Polarity: Cells have an apical (top) and basal (bottom) surface.

• Specialized contacts: Cells are closely joined by tight junctions and desmosomes.

• Supported by connective tissue: The basal surface rests on a basement membrane.

• Avascular: Contains no blood vessels but is innervated.

• High regenerative capacity: Rapidly replaces lost cells by cell division.

Classification of Epithelial Tissue

Epithelia are classified by the number of cell layers and cell shape:

• Layers: Simple (one layer) or stratified (multiple layers)

• Shapes: Squamous (flat), cuboidal (cube-shaped), columnar (tall)

Common types and their locations/functions:

• Simple squamous: Single layer of flat cells; allows diffusion/filtration; found in air sacs of lungs, lining of blood vessels (endothelium), and serous membranes (mesothelium).

• Simple cuboidal: Single layer of cube-shaped cells; secretion and absorption; found in kidney tubules and glands.

• Simple columnar: Single layer of tall cells; absorption and secretion; lines most of the digestive tract; may have microvilli and mucus-secreting cells.

• Pseudostratified columnar: Appears stratified but is a single layer; ciliated variety lines upper respiratory tract; contains mucus-secreting cells.

• Stratified squamous: Multiple layers; protects against abrasion; nonkeratinized type lines esophagus and vagina; keratinized type forms skin epidermis.

• Stratified cuboidal: Rare; found in ducts of large glands.

• Stratified columnar: Rare; found in male urethra and at transition areas.

• Transitional epithelium: Modified stratified squamous; stretches; lines urinary organs.

Glandular Epithelia

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

• Classification by secretion site: Exocrine (secrete onto surfaces or into ducts) and endocrine (secrete into blood).

• Structural classification: Unicellular (e.g., goblet cells) or multicellular (e.g., salivary glands).

• Multicellular exocrine glands: Classified by duct structure (simple or compound) and secretory unit shape (tubular, alveolar, or tubuloalveolar).

• Modes of secretion: Merocrine (secrete by exocytosis) and holocrine (accumulate product until rupture).

Connective Tissue

Functions and Characteristics

Connective tissue is the most abundant and widely distributed tissue type. Its functions include binding/support, protection, insulation, energy storage (fat), and transportation (blood).

• Originates from embryonic mesenchyme.

• Has an extensive extracellular matrix (ground substance + fibers).

• Vascularity varies: most are well vascularized, but cartilage is avascular and dense connective tissue is poorly vascularized.

Structural Components

• Extracellular matrix: Consists of ground substance (fluid, gel, or firm) and fibers (collagen, elastic, reticular).

• Cells: Each type has a primary cell in two forms: -blast (immature, matrix-secreting) and -cyte (mature, maintains matrix).

• Examples: Fibroblast (connective tissue proper), chondroblast (cartilage), osteoblast (bone), hematopoietic stem cell (blood-forming tissue).

Types of Connective Tissue

• Embryonic connective tissue: Mesenchyme.

• Connective tissue proper:

  • Loose connective tissue:

    • Areolar: Gel-like, all fiber types, supports organs, forms lamina propria.

    • Adipose: Mostly adipocytes, stores energy, insulates, protects organs; brown fat generates heat.

    • Reticular: Reticular fibers, forms stroma of lymphoid organs.

  • Dense connective tissue:

    • Dense regular: Parallel collagen fibers, high tensile strength, forms tendons/ligaments.

    • Dense irregular: Irregularly arranged fibers, resists tension from many directions, forms dermis and organ capsules.

    • Elastic connective tissue: Dense regular with many elastic fibers (e.g., in some ligaments).

• Cartilage:

  • Hyaline: Firm, resists compression, found in embryonic skeleton, bone ends, trachea.

  • Elastic: Flexible, found in external ear and epiglottis.

  • Fibrocartilage: Strong, resists tension/compression, found in intervertebral discs and knee cartilage.

• Bone (osseous tissue): Hard matrix with collagen and calcium salts, forms skeleton.

• Blood: Blood cells in fluid matrix (plasma), transports substances.

Muscle Tissue

Structure and Function

Muscle tissue consists of elongated cells specialized for contraction and movement. There are three types:

• Skeletal muscle: Attached to bones, voluntary movement, cylindrical and striated cells.

• Cardiac muscle: Forms heart walls, pumps blood, branched and striated cells, involuntary.

• Smooth muscle: In walls of hollow organs, propels substances, spindle-shaped cells, non-striated, involuntary.

Nervous Tissue

Structure and Function

Nervous tissue forms the organs of the nervous system. It is composed of two main cell types:

• Neurons: Branching cells that receive and transmit electrical impulses, essential for regulation and communication.

• Supporting cells (neuroglia): Support, protect, and insulate neurons.

Membranes

Types of Body Membranes

Body membranes are simple organs composed of an epithelium bound to underlying connective tissue. Types include:

• Cutaneous membrane: The skin; dry membrane.

• Mucous membranes (mucosae): Line body cavities open to the exterior; moist.

• Serous membranes (serosae): Line closed body cavities; moist; secrete serous fluid.

Tissue Repair

Process of Tissue Repair

Tissue repair occurs in response to injury and involves several steps:

• Inflammation: The body's initial response; prepares the area for repair.

• Organization: Blood clot is replaced by granulation tissue.

• Regeneration: Replacement of destroyed tissue with the same kind of tissue (if possible).

• Fibrosis: Replacement with dense connective (scar) tissue if regeneration is not possible.

If the wound is small and the tissue is mitotic, regeneration predominates. Large wounds or amitotic tissues heal by fibrosis (scar formation).

Developmental Aspects of Tissues

• Origin: Epithelium arises from all three germ layers (ectoderm, mesoderm, endoderm); muscle and connective tissue from mesoderm; nervous tissue from ectoderm.

• Aging: Most tissues decrease in mass and function with age, often due to poor nutrition or circulatory deficits.