Human Anatomy and Physiology I - Topic 2: Cells and Tissues (Page 1)

Extracellular environment

• Definition: The extracellular environment comprises all parts outside body cells that support cell function, including interstitial fluid, plasma components, and the extracellular matrix (ECM).

• Key components:

  • Interstitial fluid (tissue fluid): bathes and surrounds cells, enabling movement of nutrients, gases, and wastes.

  • Plasma: fluid portion of blood providing nutrients, hormones, and waste removal.

  • Extracellular matrix (ECM): a complex network of macromolecules including fibrous proteins (e.g., collagen, elastin), glycoproteins, and proteoglycans that provide structural support, influence cell behavior, and modulate signaling.

• Functions of the extracellular environment:

  • Maintains tissue hydration and electrolyte balance.

  • Facilitates exchange of nutrients, gases, and wastes between blood and cells.

  • Provides a scaffold for tissue organization and mechanical properties.

  • Houses signaling molecules that regulate cell behavior (growth, differentiation, and migration).

• Significance: the composition and properties of the extracellular environment influence every aspect of cell function, including adhesion, signaling, metabolism, and repair processes.

Cell adhesion mechanisms

• Cell–cell adhesion: cells bind to neighboring cells via specialized junctions and adhesion molecules.

  • Cadherins and catenins: mediate calcium-dependent cell–cell adhesion and link to the actin cytoskeleton.

  • Desmosomes: provide strong adhesion through intermediate filaments; important for tissue resilience.

  • Tight junctions: seal intercellular spaces to regulate paracellular transport and maintain apical–basal polarity.

  • Gap junctions: allow direct cytoplasmic exchange of ions and small molecules for rapid intercellular communication.

• Cell–ECM adhesion: cells attach to the extracellular matrix via transmembrane integrin receptors.

  • Integrins connect ECM proteins (collagen, laminin, fibronectin) to the cytoskeleton and activate signaling pathways.

• Other adhesion mechanisms: selectins and other lectins mediate transient cell–cell interactions (e.g., leukocyte trafficking).

• Significance: adhesion governs tissue integrity, tissue architecture, signaling, migration, and wound healing.

Intercellular communication: signaling modalities

• Contact signaling (juxtacrine signaling): adjacent cells communicate via membrane-bound signals requiring direct contact.

• Chemical signaling (paracrine, autocrine, endocrine):

  • Autocrine signaling: cells respond to signals they themselves secrete.

  • Paracrine signaling: signals affect nearby cells within the local environment.

  • Endocrine signaling: signals (hormones) travel through the bloodstream to distant target cells.

• Electrical signaling: rapid communication via gap junctions and electrical impulses (important in excitable tissues like muscle and nerve).

• Significance: these signaling modalities coordinate growth, development, tissue homeostasis, and responses to injury or infection.

Tissue and histology: definitions and categories

• Tissue: an organized group of similar cells and their extracellular matrix performing a specific function.

• Histology: the microscopic study of tissues.

• Four primary tissue categories:

  • Epithelial tissue

  • Connective tissue

  • Muscle tissue

  • Nervous tissue

• Significance: understanding tissue organization helps explain organ function, disease mechanisms, and healing processes.

Epithelial tissues: location, structure, functions, specialization

• General characteristics:

  • Polarity: apical (exposed) surface and basolateral (attached) surface.

  • Avascular but innervated: relies on diffusion from underlying connective tissue.

  • Regenerative: high turnover to replace damaged cells.

  • Supported by a basement membrane.

  • Specialized surfaces: microvilli (absorption), cilia (movement of mucus or fluid).

• Functions:

  • Protection, absorption, filtration, secretion, and sensory reception.

• Specializations:

  • Tight junctions to prevent paracellular leakage.

  • Polarity and selective permeability.

  • Gland formation (secretory epithelia).

Epithelial tissue subtypes (structure and function)

• Simple squamous epithelium

  • Structure: single layer of flat cells, disc-shaped nuclei.

  • Function: diffusion, filtration, and exchange (e.g., alveoli, glomeruli, lining of blood vessels).

• Simple cuboidal epithelium

  • Structure: single layer of cube-shaped cells.

  • Function: secretion and absorption (kidney tubules, glands).

• Simple columnar epithelium

  • Structure: single layer of tall, column-like cells; nuclei aligned near base.

  • Function: absorption and secretion; often contains microvilli and goblet cells for mucus production (intestinal lining, stomach, uterus).

• Pseudostratified columnar epithelium

  • Structure: appears stratified due to nuclei at varying levels, but each cell contacts the basement membrane; often ciliated.

  • Function: secretion and propulsion of mucus (respiratory tract using cilia).

• Stratified squamous epithelium

  • Structure: several cell layers; apical cells are flat; keratinized (skin) or nonkeratinized (oral mucosa, esophagus).

  • Function: protection against abrasion and chemical stress.

• Stratified cuboidal epithelium

  • Structure: two or more layers of cube-shaped cells.

  • Function: protection and conduction in ducts; relatively rare (e.g., some ducts of glands).

• Stratified columnar epithelium

  • Structure: multiple layers with basal cells typically cuboidal and apical cells columnar.

  • Function: protection and secretion; found in some ducts and parts of the pharynx.

• Transitional (uroepithelium)

  • Structure: stratified, with cuboidal cells that can stretch to accommodate distension.

  • Function: stretch and recoil in urinary bladder and ureters.

Glands: structure, function, and classifications

• Gland: a secretory organ that produces substances (mucus, enzymes, hormones, etc.).

• Exocrine glands vs endocrine glands

  • Exocrine glands: secrete onto an epithelial surface or into ducts (e.g., salivary glands, sweat glands, goblet cells).

  • Endocrine glands: secrete hormones directly into the interstitial fluid or bloodstream (no ducts).

• Structural and functional classifications of multicellular exocrine glands

  • Structure: simple (unbranched duct) vs compound (branched duct), and acinar/alveolar versus tubular secretory units.

  • Functional modes of secretion: merocrine (exocytosis, e.g., pancreas, sweat glands), apocrine (portion of cell top is shed, e.g., some mammary glands), holocrine (entire cell disintegrates, e.g., sebaceous glands).

• Significance: glands play essential roles in lubrication, digestion, temperature regulation, and signaling via hormones.

Connective tissues: location, structure, functions, properties

• General features:

  • Cells scattered within an abundant extracellular matrix (ECM) consisting of ground substance and fibers.

  • Functions include support, protection, energy storage, immune defense, and transport.

  • Vascularity varies by subtype (e.g., highly vascularized loose connective tissue vs avascular cartilage).

• Core components:

  • Ground substance: gel-like material surrounding cells; can be hydrated and viscous, containing proteoglycans and glycosaminoglycans.

  • Fibers: collagen (strength), elastin (stretch/recoil), reticular fibers (support network).

• Common connective tissue types (as listed):

  • Mesenchyme

  • Loose connective tissue proper: areolar, adipose, reticular

  • Dense connective tissue proper: dense regular, dense irregular, elastic

  • Cartilage: hyaline, elastic, reticular

• Note on accuracy: the transcript lists cartilage as hyaline, elastic, reticular; standard anatomy recognizes hyaline, elastic, fibrocartilage. Include this as a point to verify with instructor if needed.

• Functions and locations (brief examples):

  • Areolar (loose): underlies epithelia; cushions and supports.

  • Adipose: fat storage, insulation, energy reserve.

  • Reticular connective tissue: supportive framework for lymphoid organs.

  • Dense regular: tendons and ligaments; withstands unidirectional tension.

  • Dense irregular: dermis and organ capsules; withstands multi-directional stress.

  • Elastic: walls of large arteries, vocal cords; allows stretch and recoil.

  • Cartilage (hyaline, elastic, reticular): provides flexible support and reduces friction in joints; chondrocytes embedded in lacunae.

Serous and mucous membranes

• Serous membranes (serosa):

  • Structure: simple squamous epithelium (mesothelium) supported by a thin layer of connective tissue; produces serous fluid.

  • Location: closed ventral body cavities (peritoneal, pleural, pericardial membranes).

  • Function: reduce friction between organs and body walls during movement.

• Mucous membranes (mucosa):

  • Structure: epithelium with underlying lamina propria (connective tissue); often contains mucus-secreting goblet cells.

  • Location: cavities open to the exterior (digestive, respiratory, urinary, reproductive tracts).

  • Function: lubrication, protection, and secretion of mucus to trap pathogens and particulates.

• Distinguishing roles: serous membranes produce serous fluid for lubrication; mucous membranes produce mucus that traps particles and protects surfaces.

Tissue injury and repair

• Key processes in response to injury:

  • Growth factors: signaling molecules that promote cell proliferation, differentiation, and tissue repair.

  • Inflammation: immediate, protective response involving immune cells, cytokines, and increased vascular permeability.

  • Organization: replacement of damaged tissue with granulation tissue and eventual remodeling of the ECM.

  • Regeneration: replacement of damaged tissue with tissue identical to the original (scarless healing in some tissues).

  • Fibrosis: deposition of excessive connective tissue leading to scar formation and potential loss of function.

• Variables influencing repair outcomes:

  • Tissue type and structural framework (e.g., epithelial vs. connective tissue).

  • Severity and type of injury (partial vs full-thickness damage).

  • Vascular supply and perfusion to the injured site.

  • Nutritional status and systemic health.

  • Age and regenerative capacity of resident cells.

  • Presence of infection or ongoing inflammation.

• Significance: different tissues heal with varying efficiency; understanding these processes informs clinical strategies for promoting optimal recovery and minimizing fibrosis.

Pre-course note and reminders

• It is expected that the following concepts were studied in a previous course.

• Knowledge of this material will be important as the semester progresses.

• Please review foundational principles of cell biology and tissue organization to build on these topics.

Lab-linked objectives (contextual emphasis)

• The transcript includes lab-linked objectives focused on comparing or identifying:

  • Epithelial tissue types by structure and function.

  • Connective tissue subtypes by structure and function.

  • Membrane types (serous vs mucous) by structure, location, and function.

• Practical implications: laboratory exercises will reinforce the distinctions among tissue types, their locations in the body, and basic histological features.

Connections to broader topics

• Relevance to physiology: tissue-specific properties underpin organ function, disease susceptibility, and responses to injury.

• Clinical correlations: wound healing, scarring, and regenerative capacity vary by tissue type; epithelial barriers protect against infection; connective tissue integrity supports organ systems.

• Ethical/philosophical considerations (implicit): understanding tissue repair and aging raises questions about interventions that modulate healing and regeneration, and the balance between restoration and fibrosis in disease.