Cells and Tissues

Composition of the Body’s Extracellular Environment

The extracellular environment consists of all substances outside the cells, providing structural and biochemical support to surrounding cells.

• Extracellular Fluid (ECF): Includes interstitial fluid, plasma, and transcellular fluids. It serves as the medium for nutrient, gas, and waste exchange.

• Extracellular Matrix (ECM): A network of proteins (such as collagen, elastin, and fibronectin) and polysaccharides that provide structural support and regulate cellular functions.

• Functions: Supports cell adhesion, migration, proliferation, and differentiation.

• Example: The ECM in connective tissue provides tensile strength and elasticity to skin and tendons.

Cell Adhesion Mechanisms

Cells adhere to each other and to the ECM through specialized structures and molecules.

• Cell Junctions: Include tight junctions (prevent leakage), desmosomes (provide mechanical strength), and gap junctions (allow communication).

• Adhesion Molecules: Such as integrins, cadherins, selectins, and immunoglobulin superfamily proteins mediate cell-cell and cell-ECM interactions.

• Example: Desmosomes are abundant in tissues subject to mechanical stress, such as the skin.

Cell Communication: Signaling Mechanisms

Cells communicate using various signaling processes to coordinate activities.

• Contact Signaling: Direct physical contact between cells via membrane-bound molecules (e.g., immune cell recognition).

• Chemical Signaling: Release of chemical messengers (hormones, neurotransmitters) that bind to receptors on target cells.

• Electrical Signaling: Changes in membrane potential, especially in excitable cells like neurons and muscle cells.

• Example: Neurotransmitters released at synapses enable rapid communication between neurons.

Definition of Tissue and Histology

Tissue is a group of similar cells performing a common function. Histology is the study of tissues.

• Four Basic Tissue Types:

  • Epithelial tissue

  • Connective tissue

  • Muscle tissue

  • Nervous tissue

Epithelial Tissues: Location, Structure, Functions, and Properties

Epithelial tissues cover body surfaces, line cavities, and form glands.

• Structure: Closely packed cells with minimal ECM, arranged in continuous sheets.

• Functions: Protection, absorption, filtration, excretion, secretion, and sensory reception.

• Specializations: Microvilli (increase surface area), cilia (move substances), tight junctions (prevent leakage).

• Properties: Avascular but innervated; high regenerative capacity.

Types of Epithelial Tissues: Structure and Function

Epithelial tissues are classified by cell shape and number of layers.

• Simple Squamous Epithelium: Single layer of flat cells; allows diffusion and filtration (e.g., alveoli, capillaries).

• Simple Cuboidal Epithelium: Single layer of cube-shaped cells; secretion and absorption (e.g., kidney tubules).

• Simple Columnar Epithelium: Single layer of tall cells; absorption and secretion (e.g., digestive tract lining).

• Pseudostratified Columnar Epithelium: Appears layered but all cells touch the basement membrane; secretion and movement of mucus (e.g., trachea).

• Stratified Squamous Epithelium: Multiple layers; protects against abrasion (e.g., skin, mouth lining).

• Stratified Cuboidal Epithelium: Typically two layers of cube-shaped cells; protection (e.g., sweat gland ducts).

• Stratified Columnar Epithelium: Several layers; rare, found in male urethra and some glandular ducts.

• Transitional Epithelium: Multiple layers, cells change shape; allows stretching (e.g., urinary bladder).

Glands: Definition and Types

Glands are epithelial structures that produce and secrete substances.

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

• Endocrine Glands: Ductless; secrete hormones directly into the bloodstream (e.g., thyroid gland).

• Structural Classes of Multicellular Exocrine Glands: Simple (unbranched ducts) vs. compound (branched ducts); tubular, alveolar, or tubuloalveolar secretory units.

• Functional Classes: Merocrine (secrete by exocytosis), holocrine (cell ruptures), apocrine (apical portion pinches off).

Connective Tissues: Location, Structure, Functions, and Properties

Connective tissues support, bind, and protect other tissues and organs.

• Structure: Cells scattered within an abundant ECM composed of fibers (collagen, elastic, reticular) and ground substance.

• Functions: Support, protection, insulation, storage, and transport.

• Specializations: Varying degrees of vascularity; diverse cell types (fibroblasts, adipocytes, chondrocytes, etc.).

Types of Connective Tissues: Structure and Function

Connective tissues are classified based on their structure and function.

• Mesenchyme: Embryonic connective tissue; gives rise to all other connective tissues.

• Loose Connective Tissue Proper:

  • Areolar: Gel-like matrix; wraps and cushions organs.

  • Adipose: Fat storage; insulates and protects organs.

  • Reticular: Network of reticular fibers; supports lymphoid organs.

• Dense Connective Tissue Proper:

  • Dense Regular: Parallel collagen fibers; resists tension (e.g., tendons, ligaments).

  • Dense Irregular: Irregularly arranged fibers; withstands tension from multiple directions (e.g., dermis).

  • Elastic: High proportion of elastic fibers; allows recoil (e.g., walls of large arteries).

• Cartilage:

  • Hyaline: Most common; supports and reinforces (e.g., nose, trachea).

  • Elastic: More elastic fibers; maintains shape (e.g., ear).

  • Reticular: Additional info: Reticular cartilage is not a standard type; likely refers to reticular connective tissue.

Serous and Mucous Membranes: Structure, Location, and Function

Body membranes line cavities and cover surfaces.

• Serous Membranes: Line closed body cavities; consist of simple squamous epithelium (mesothelium) and areolar connective tissue; secrete serous fluid to reduce friction (e.g., pleura, pericardium, peritoneum).

• Mucous Membranes: Line body cavities open to the exterior; consist of epithelium and lamina propria; secrete mucus for protection and lubrication (e.g., digestive, respiratory tracts).

Tissue Response to Injury

Tissues repair themselves through a series of coordinated steps.

• Growth Factor: Proteins that stimulate cell division and tissue repair.

• Inflammation: Initial response; blood vessels dilate, immune cells migrate to the site.

• Organization: Granulation tissue forms, restoring blood supply.

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

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

• Variables Influencing Repair: Type of tissue, severity of injury, nutrition, blood supply, age.

Review of Cell Structure and Function

Structure of the Plasma Membrane

The plasma membrane is a selectively permeable barrier that surrounds the cell.

• Phospholipid Bilayer: Hydrophilic heads face outward; hydrophobic tails face inward.

• Proteins: Integral and peripheral proteins serve as channels, receptors, and enzymes.

• Carbohydrates: Attached to proteins and lipids; involved in cell recognition.

Membrane Transport Mechanisms

Substances move across the plasma membrane by passive or active processes.

• Passive Processes:

  • Simple Diffusion: Movement of molecules from high to low concentration.

  • Osmosis: Diffusion of water across a semipermeable membrane.

  • Facilitated Diffusion: Transport via carrier or channel proteins.

  • Filtration: Movement due to hydrostatic pressure (e.g., in kidneys).

• Active Processes:

  • Active Transport: Movement against concentration gradient using ATP.

  • Bulk Transport: Movement of large particles via vesicles.

    • Exocytosis: Vesicles fuse with membrane to release contents.

    • Endocytosis: Membrane engulfs material to form vesicle.

    • Receptor-Mediated Endocytosis: Specific molecules are taken in after binding to receptors.

Cellular Structures: Structure and Function

Cell Cycle Events

The cell cycle is the series of events that cells go through as they grow and divide.

• Interphase: Cell grows, DNA replicates (G1, S, G2 phases).

• Mitosis: Division of the nucleus; includes prophase, metaphase, anaphase, telophase.

• Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

Phases of Mitosis:

• Prophase: Chromosomes condense, spindle forms.

• Metaphase: Chromosomes align at the cell equator.

• Anaphase: Sister chromatids separate to opposite poles.

• Telophase: Nuclear envelopes reform, chromosomes decondense.