The Tissue Level of Organization

Overview of Body Tissues

  • Body tissues are organized into four major categories, each with distinct structural properties, functions, and specific cell types:

    • Epithelial Tissue: Consists of sheets of cells that cover exterior surfaces of the body, line internal cavities and passageways, and form certain glands.

    • Connective Tissue: The most diverse category of tissue, which binds the cells and organs of the body together. It functions in the protection, support, and integration of all parts of the body.

    • Muscle Tissue: Contractile tissue that provides movement, including the skeletal movement required for walking or the contraction of the heart muscle to move blood through the body.

    • Nervous Tissue: Capable of short- and long-distance communication throughout the body.

Intercellular Junctions and Communication

  • Cellular junctions allow cells within a tissue to communicate and remain structurally stable. There are three primary classes:

    • Tight Junctions: These junctions fuse the membranes of two neighboring cells together tightly. This eliminates extracellular space between them and blocks the movement of substances through the extracellular space.

    • Gap Junctions: These form an intercellular passageway between the membranes of adjacent cells, facilitating the movement of small molecules and ions.

    • Anchoring Junctions: These help stabilize cells structurally and include:

      • Desmosomes: Connect to the cytoskeleton to provide structural integrity.

      • Hemidesmosomes: Stabilize the cell by anchoring it to underlying structures.

Epithelial Tissue Characteristics and Classification

  • General Functions:

    • Provides the body’s first line of protection from physical, chemical, and biological wear and tear.

    • Acts as the gatekeepers of the body by controlling permeability.

  • Apical Surface Features:

    • Cilia: Common features on the apical surface used for movement. For example, ciliated epithelium in the Fallopian tubes sweeps eggs from the ovaries to the uterus.

    • Microvilli: Surface features used to increase surface area for absorption.

  • Classification by Layers:

    • Simple Epithelium: A single layer of cells.

    • Stratified Epithelium: More than one layer of cells. The basal layer often consists of dividing stem cells, while the apical layers contain older, fully differentiated cells that display the mature cell shape.

    • Pseudostratified Epithelium: Appears stratified but consists of a single layer of irregularly shaped and differently sized columnar cells. It often contains goblet cells (mucus-secreting) and is found in the respiratory tract.

  • Classification by Shape:

    • Squamous: Flat and horizontal cells.

    • Cuboidal: Boxlike cells that appear round.

    • Columnar: Tall, column-like cells with elongated nuclei located at the basal end.

  • Specific Epithelial Types:

    • Simple Squamous: Facilitates rapid passage of chemical compounds; found in lung alveoli (275×275 \times magnification), segments of kidney tubules, and capillary linings.

    • Simple Cuboidal: Found in areas of transport, such as the lining of gland ducts and kidney tubules (250×250 \times magnification).

    • Simple Columnar: Active in absorption and secretion; lines sections of the digestive system (small intestine at 575×575 \times magnification) and parts of the reproductive tract.

    • Pseudostratified Ciliated Columnar: Lines the human trachea (560×560 \times magnification).

    • Stratified Squamous: Provides protection; can be keratinized or non-keratinized. Located in the skin and the lining of the cheeks and esophagus (140×140 \times magnification).

    • Stratified Cuboidal and Columnar: Uncommon in humans; found only in the linings of some large glands and ducts.

    • Transitional: A specialized stratified epithelium where cells change shape to accommodate volume (distension). Found in the urinary bladder and associated tubes (270×270 \times magnification).

Glandular Epithelia

  • A gland is a structure that synthesizes and secretes chemicals.

  • Endocrine Glands: Secrete hormones directly into the bloodstream or internal body fluids.

  • Exocrine Glands: Secrete substances onto an epithelium leading to the outside of the body. Examples include mucus, sweat, saliva, and breast milk.

  • Structural Classification of Exocrine Glands:

    • Simple Glands: Have an unbranched duct. Subtypes include simple acinar, simple tubular, simple branched acinar, simple branched tubular, and simple coiled tubular.

    • Compound Glands: Have branched ducts. Subtypes include compound tubular, compound acinar, and compound tubuloacinar.

  • Mechanisms of Exocrine Secretion:

    • Merocrine Secretion: Cells release contents through exocytosis (e.g., salivary glands).

    • Apocrine Secretion: The apical portion of the cell fills with product and pinches off from the basal portion.

    • Holocrine Secretion: The entire secretory cell is destroyed and consumed to become the secretory product.

Connective Tissue Diversity and Structure

  • General Properties: Connective tissues are the most structurally diverse, rarely have cells touching each other, and are typically vascularized (contain blood vessels).

  • Components:

    • Fibroblasts: The most common cell type, present in almost all connective tissue.

    • Ground Substance: Fills the space between cells and protein fibers; its consistency is unique to each tissue type.

    • Fibers: Includes loosely woven thick collagen fibers (for strength), thin branched elastin fibers (for flexibility), and reticular fibers.

  • Connective Tissue Proper:

    • Loose Connective Tissue:

      • Areolar: Loosely arranged collagen and elastin fibers; provides strength and flexibility.

      • Adipose: Consists of adipocytes storing lipids in a large central vacuole. Brown adipose tissue (common in infants) has more extracellular matrix (ECM) and lipids stored in multiple smaller vesicles.

      • Reticular: Predominantly reticular fibers; found in lymphoid organs and contains leukocytes.

    • Dense Connective Tissue:

      • Dense Regular: Collagen fibers oriented in one direction to withstand force in a single plane (e.g., tendons).

      • Dense Irregular: Collagen fibers oriented in every direction to withstand force in multiple planes (e.g., dermis).

  • Supportive Connective Tissue:

    • Cartilage: Includes Hyaline and Fibrocartilage.

    • Bone: The most rigid type. Divided into Compact Bone (forms the periphery) and Spongy Bone (forms the deeper portion and epiphyses, composed of trabeculae and marrow spaces).

  • Fluid Connective Tissue:

    • Blood: Contains erythrocytes, five types of leukocytes, and platelets in a liquid ECM.

    • Lymph: Mostly acellular fluid within lymphatic vessels.

Muscle Tissue Varieties

  • Skeletal Muscle: Voluntary movement; attached to bones and body openings. Cells are multinucleated (nuclei at perimeters) and striated (striped) due to the precise arrangement of contractile proteins. It generates heat and protects organs.

  • Cardiac Muscle: Involuntary; located in the heart. Cells have a single nucleus, are branched to connect to neighbors, and are striated. Contractions squeeze heart chambers to pump blood.

  • Smooth Muscle: Involuntary; located in the walls of hollow organs (GI tract, uterus) and blood vessels. Cells have a single nucleus, tapered ends, and no visible striations. It moves organ contents and controls blood flow.

Nervous Tissue and Logic

  • Neurons: The conducting cells of nervous tissue.

    • Cell Body (Soma): Contains the nucleus and organelles.

    • Dendrites: Short branches that receive signals or stimuli.

    • Axon: A long extension that carries electrical signals (action potentials) to other cells.

    • Axon Terminal: Releases chemical signals to other cells.

    • Myelin Sheath: Formed by Schwann cells; provides insulation.

    • Nodes of Ranvier: Gaps in the myelin sheath.

  • Glial Cells: Supportive cells. For example, Oligodendrocytes insulate neuronal axons to ensure rapid signal propagation.

Body Membranes

  • Mucous Membranes: Line cavities that open to the outside of the body; produce mucus.

  • Serous Membranes: Double-layered membranes surrounding organs (e.g., visceral and parietal pericardium surrounding the heart).

  • Cutaneous Membrane: The skin (comprising epidermis and dermis).

  • Synovial Membranes: Line joints and secrete synovial fluid.

Tissue Injury, Healing, and Pathology

  • Inflammation: Initiated when damaged cells or resident leukocytes release signaling molecules like histamine. This leads to local vasodilation and increased blood vessel leakiness, allowing leukocytes to enter tissue to fight pathogens.

  • Tissue Healing Process:

    1. Clotting: Platelets and clotting factors form a clot to block blood loss.

    2. Infection Control: White blood cells migrate to the site.

    3. Granulation: Granulation tissue (connective tissue) forms a temporary surface.

    4. Epithelial Migration: Epithelial cells migrate over the granulation tissue to cover the wound.

    5. Restoration: The epithelium is restored; however, the underlying connective tissue may remain thicker due to randomly laid collagen fibers from fibroblasts.

  • Cancer: A generic term for diseases where cells escape regulatory signals, resulting in a mass called a tumor. Tumors can be benign (non-spreading) or malignant (cancerous/spreading), altering the architecture of surrounding tissue.