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Tissues and Histology - Seeley BIOL 319 Lab/Lecture Notes (Chapter 04)

4.1 Tissues and Histology

  • Tissue definition: A group of cells and/or fluids designed to perform a specific function.

  • Histology: Microscopic study of tissues.

  • Biopsy: Removal of tissue for diagnostic purposes; pathologist determines benign vs malignant.

  • Autopsy: Examination of organs of a dead body to determine cause of death.

  • Resting membrane potentials (RMP): All four tissue types have an RMP when applicable:

    • Epithelial (RMP)

    • Connective (RMP)

    • Muscle (RMP

    • AP)

    • Nervous (RMP

    • AP)

  • Pattern: form follows function; many subtypes per tissue type.

4.2 Embryonic Tissue

  • Embryonic germ layers (triploblastic): form all tissues of the body:

    • Endoderm (inner layer): forms lining of the GI tract and derivatives.

    • Mesoderm (middle layer): forms tissues such as muscle, bone, blood vessels.

    • Ectoderm (outer layer): forms skin and neuroectoderm (neural tissue), neural crest derivatives (peripheral nerves, skin pigment cells, adrenal medulla, facial tissues).

4.3 Epithelial Tissue

  • Characteristics of epithelial tissue:

    • Mostly cells; minimal extracellular matrix.

    • Covers body surfaces and forms glands; lines cavities and hollow organs.

    • Free/apical surface, basal surface, and lateral surfaces; basement membrane.

    • Avascular: relies on diffusion from underlying tissue; nutrients/gases diffuse from nearby capillaries.

    • High regeneration capacity.

    • Important: goblet cells secrete mucus; mucous glands contribute to defense.

  • Basement membrane anatomy:

    • Basal lamina and reticular lamina (basal lamina subdivides into lamina lucida and lamina densa).

    • Made by epithelial cells: collagen (most abundant), laminin, fibronectin, proteoglycans.

    • Functions: supports and guides cell migration during repair; porous to materials moving to/from epithelia.

  • Endothelial vs epithelial:

    • Endothelial tissue is a type of epithelium lining interior surfaces of blood and lymphatic vessels.

    • Epithelial tissue lines surfaces and is avascular; gas/nutrient exchange occurs via diffusion from connective tissue capillaries.

  • Classification basics:

    • By number of cell layers:

    • Simple: one cell layer from basement membrane to free surface.

    • Stratified: more than one layer; name based on apical cell shape.

    • Pseudostratified columnar: appears stratified but all cells contact basement membrane; typically simple.

    • Transitional: stratified epithelium that changes shape with stretch (e.g., bladder).

    • By cell shape:

    • Squamous: flat

    • Cuboidal: cube-shaped

    • Columnar: tall, narrow

  • Apical vs basolateral surfaces:

    • Apical: surface facing lumen.

    • Basolateral: basal + lateral surfaces; connected to basement membrane.

  • Epithelial surfaces and features:

    • Avascular; diffusion from underlying CT supplies nutrients.

    • Any epithelial cell layer should be within
      -6
      -8 cell lengths of a capillary to receive oxygen.

    • Free surface modifications:

    • Microvilli: increase surface area for absorption/secretion (e.g., intestinal lining).

    • Cilia: move mucus over surface (mucociliary escalator) in respiratory tract.

    • Stereocilia: sensory receptors in some tissues (not true cilia).

    • Transitional epithelium: changes shape from cuboidal/columnar to squamous when stretched.

  • Epithelial classifications

    • tissues and functions:

    • Simple Squamous Epithelium: diffusion/filtration; lines vessels and serous membranes; alveoli; kidney tubules.

    • Simple Cuboidal Epithelium: secretion/absorption; kidney tubules; gland ducts; choroid plexuses; gland surfaces; bronchioles (cilia) may be present.

    • Simple Columnar Epithelium: secretion/absorption; may have microvilli (intestines) or cilia (uterine tubes); lines stomach/intestines.

    • Pseudostratified Columnar Epithelium: pseudo-layered appearance; often ciliated with goblet cells; lines respiratory tract and nasal cavity.

    • Stratified Squamous Epithelium: multiple layers; protective barrier; keratinized (skin) vs nonkeratinized (mouth, esophagus, vagina, etc.).

    • Stratified Cuboidal Epithelium: multiple layers; secretory/ protective; glands ducts (sweat ducts, ovarian follicular cells, salivary ducts).

    • Stratified Columnar Epithelium: multiple layers with tall cells on basal layers; protective and secretory; found in mammary ducts, larynx, male urethra.

    • Transitional Epithelium: specialized stratified for stretching in urinary tract.

  • Free surface modifications (summary):

    • Microvilli: absorption/secretion; augmenter.

    • Cilia: mucociliary transport; impairment can contribute to coughing when escalator is lost.

  • Glands

    • Endocrine vs Exocrine:

    • Endocrine glands: no ducts; extensive vascularization; secrete hormones into blood.

    • Exocrine glands: open contact with exterior via ducts; include GI tract glands.

    • Exocrine glands classified by:

    • Duct structure (unicellular vs multicellular; simple vs compound)

    • Secretory portion (tubular vs acinar; tubuloacinar)

    • Secretion mode (merocrine, apocrine, holocrine)

    • Unicellular example: goblet cells secrete mucus (defense via mucus).

  • Important notes from structure examples (selected):

    • Simple tubular, simple branched tubular, simple acinar, simple branched acinar glands exist in various organs (stomach, colon, sebaceous glands, etc.).

    • Compound glands have multiple ducts and branched secretory portions (e.g., pancreas, duodenum glands, mammary glands).

    • Secretions can be merocrine (exocytosis), apocrine (bud off part of cell), or holocrine (entire cells released).

4.4 Connective Tissue

  • General features:

    • Abundant; found in every organ.

    • Composed of cells separated by extracellular matrix (ECM).

    • Derived from embryonic mesoderm.

    • Main components: elastin, collagen, ECM (ground substance), and various cells; supports body water compartments (Total Body Water, TBW).

  • Functions:

    • Enclose and separate organs; connect tissues to one another (e.g., tendons/ligaments).

    • Support and movement; bone framework.

    • Storage (lipids in adipose tissue).

    • Cushion and insulation (fat).

    • Transport (blood).

    • Protection (immune cells).

  • Cells and matrix terminology:

    • Specialized matrix-forming cells: blasts build matrix; cytes maintain; clasts break down (remodel).

    • CT cells for structural framework: osteoblasts/osteocytes/osteoclasts (bone); chondroblasts/chondrocytes (cartilage); fibroblasts/fibrocytes (fibrous CT).

  • ECM components:

    • Protein fibers: collagen (strong, flexible), reticular fibers, elastic fibers.

    • Ground substance: hyaluronic acid, proteoglycans, glycosaminoglycans; link proteins; adhesive molecules (fibronectin, chondronectin, osteonectin).

    • Fluid: interstitial fluid in the matrix.

  • Embryonic and adult CT classes:

    • Embryonic CT:

    • Mesenchyme

    • Mucous CT (Wharton
      -s jelly)

    • Adult CT Proper:

    • Loose CT (areolar, adipose, reticular)

    • Dense CT (regular, irregular; collagenous and elastic variants).

    • Supporting CT:

    • Cartilage (hyaline, fibrocartilage, elastic)

    • Bone.

    • Fluid CT:

    • Blood

    • Hematopoietic tissue (red and yellow marrow).

  • Loose CT

    • Areolar and Adipose:

    • Areolar: loose network of fibers with fibroblasts, macrophages, lymphocytes; supports and nourishes surrounding structures.

    • Adipose (fat): yellow adipose tissue (white at birth; turns yellow with age); brown adipose tissue generates heat.

    • Reticular tissue: fine network of reticular fibers; supports lymphoid organs (lymph nodes, spleen, bone marrow).

  • Dense CT:

    • Dense Regular Collagenous CT: parallel collagen fibers; supports tendons and ligaments; also in dermis and organ capsules.

    • Dense Irregular Collagenous CT: fibers in multiple directions; resists forces from many directions; in dermis and organ capsules.

    • Dense Regular Elastic CT: dense collagen with abundant elastic fibers; allows stretch and recoil (e.g., ligaments of the spine, vocal cords, vessel walls).

    • Dense Irregular Elastic CT: elastic fibers in various directions; found in elastic arteries.

  • Supporting CT

    • Cartilage:

    • Avascular and aneural; heals slowly; perichondrium surrounds cartilage.

    • Cartilage matrix rich in proteoglycans and hyaluronic acid; ground substance traps water.

    • Types:

    • Hyaline (articular cartilage, respiratory tract)

    • Fibrocartilage (intervertebral discs, pubic symphysis, knee menisci)

    • Elastic (external ear, epiglottis).

    • Chondrocytes reside in lacunae within the matrix.

  • Bone:

    • Hard connective tissue with organic (collagen) and inorganic (hydroxyapatite) components: \text{Ca}{10}(\text{PO}4)6(\text{OH})2 is the hydroxyapatite formula.

    • Osteocytes live in lacunae; osteoblasts build bone; osteoclasts resorb bone.

    • Two structural types: spongy (trabeculae) and compact bone; osteons are basic functional units in compact bone.

  • Blood and Hematopoietic tissue:

    • Blood is a fluid CT with formed elements: red blood cells, white blood cells, platelets, in a plasma matrix.

    • Red cells lack mitochondria and rely on glycolysis.

    • Hematopoietic tissue forms blood cells; red marrow is hematopoietic; yellow marrow is mostly adipose tissue.

  • Summary notes for CT:

    • CT is diverse in form and function, providing support, interaction, storage, transport, and protection throughout the body.

4.5 Muscle Tissue

  • General properties:

    • Contractile tissue; responsible for movement, posture, and circulation.

    • All muscle types contain actin and myosin.

  • Types:

    • Skeletal: attached to skeleton; striated; voluntary control; long cylindrical fibers with many peripheral nuclei.

    • Cardiac: heart muscle; striated; involuntary; connected by intercalated disks with gap junctions.

    • Smooth: nonstriated; involuntary; wall of hollow organs and skin; tapered cells with a single nucleus.

4.6 Nervous Tissue

  • Neurons: one cell that generates and conducts electrical signals (action potentials).

    • Parts:

    • Cell body (nucleus)

    • Axon (conducts impulses away)

    • Dendrites (receive impulses).

    • Types: multipolar, bipolar, pseudo-unipolar.

  • Glia (neuroglia): support cells; nourish, protect, and insulate neurons.

4.7 Tissue Membranes

  • Membranes are thin sheets composed of superficial epithelial tissue and underlying connective tissue.

  • Types:

    • Cutaneous (skin): external protection.

    • Mucous membranes: line cavities open to the exterior; secrete mucus; include goblet cells and lamina propria; found in respiratory, digestive, urinary, and reproductive systems.

    • Serous membranes: reduce friction; simple squamous epithelium (mesothelium) with a thin layer of loose connective tissue; line closed ventral body cavities (e.g., pleura, peritoneum).

    • Synovial membranes: line freely movable joints; secrete synovial fluid rich in hyaluronic acid.

4.8 Tissue Damage and Inflammation

  • Inflammation overview:

    • Purpose: mobilize defenses, destroy pathogens, remove debris, initiate repair.

    • Classic five manifestations: redness, heat, swelling, pain, disturbed function.

  • Chemical mediators:

    • Histamine, kinins, prostaglandins, leukotrienes, and others.

    • They increase blood flow and vascular permeability, recruiting WBCs.

  • Edema: tissue swelling due to fluid leakage.

  • EGF (Epidermal Growth Factor):

    • Stimulates cell growth and differentiation by binding to EGFR.

    • Human EGF: approximately 6 \text{-} kDa; about 53 amino acids; has three intramolecular disulfide bonds.

    • Originally described as urogastrone; found in platelets, submandibular glands, and parotid glands.

    • Role in skin repair and tissue regeneration.

  • Cancer tissue profiles (tissue of origin):

    • Carcinomas: cancers of epithelial tissue (lung, breast, colon, prostate, skin, etc.).

    • Adenocarcinomas: glandular epithelial origin.

    • Sarcomas: mesodermal tissues (rare).

  • Aging effects on tissues (important):

    • Slower cell division; slower blood cell production in elderly.

    • Healing is slower; collagen becomes more irregular; tendons/ligaments become less flexible; elastic fibers lose elasticity.

    • Arterial walls and ligaments become less elastic; risks of atherosclerosis and reduced tissue perfusion.

    • Wrinkling of the skin; bones become more prone to fracture due to reduced remodeling; aging hormones contribute to these changes.

4.9 Tissue Repair

  • Regeneration vs Replacement:

    • Regeneration: new cells of the same type restore function.

    • Replacement: new tissue of a different type forms, often resulting in scar tissue and partial loss of function.

  • Cell regeneration categories:

    • Labile: mitosis throughout life (e.g., surface epithelia, mucous membranes, hematopoietic tissue, lymphatic tissue).

    • Stable: limited mitosis after growth; can divide after injury (e.g., liver, some endocrine glands).

    • Permanent: very limited regeneration; nervous tissue, skeletal muscle, cardiac muscle.

  • Wound repair processes: primary vs secondary union

    • Primary union (primary intention): wound edges closely approximated; rapid healing.

    • Secondary union: edges not closed; greater risk of infection; more inflammation and scar formation.

  • Skin repair sequence (summary of the four stages):

    1. Clot formation.

    2. Influx of epithelial cells to bridge the wound.

    3. Formation of granulation tissue (fibroblasts proliferate, collagen laid down).

    4. Replacement by connective tissue and scar formation.

  • Secondary union details:

    • Greater inflammatory response, more granulation tissue, wound contraction, and more disfiguring scars.

    • Suturing may promote primary union to improve outcome.

Additional notes and cross-topic connections

  • Extracellular matrix (ECM) overview:

    • Three major components: protein fibers (collagen, reticular, elastic), ground substance (e.g., hyaluronic acid), and interstitial fluid.

    • Hyaluronic acid is a key lubricant and water-retaining component; contributes to skin tissue turgor and resilience.

    • Proteoglycans and glycosaminoglycans trap water and form hydrated gels that resist compression.

  • Cellular dynamics in CT:

    • Blasts build matrix; cytes maintain it; clasts remodel by breaking down matrix.

  • Important conceptual connections:

    • The pattern form follows function across epithelia (surface protection, absorption, secretion) and CT (support, transport, protection).

    • In aging, ECM remodeling and cellular turnover drive changes in tissue mechanics, healing capacity, and risk of disease.

  • Quick reference formulas and identifiers:

    • Hydroxyapatite in bone: Ca10(PO4)6(OH)2Ca10(PO4)6(OH)2

    • EGF details: \text{MW} \approx 6\text{-}kDa ; n \approx 53 \text{amino acids} ; 3 disulfide bonds.

    • Distance rule for diffusion: within 6
      -8 cell lengths of capillaries for oxygen delivery.

Tissue organization involves four primary types: epithelial, connective, muscle, and nervous tissue, all originating from embryonic germ layers (endoderm, mesoderm, ectoderm). Histology is the microscopic study of these tissues, with RMPs present in all types.

  • Epithelial Tissue: Primarily composed of cells, it covers surfaces, lines cavities, and forms glands. Key features include a free/apical surface, a basement membrane, avascularity, and high regeneration capacity. It's classified by cell layers (simple, stratified, pseudostratified, transitional) and cell shape (squamous, cuboidal, columnar). Modifications like microvilli (absorption) and cilia (movement) enhance function. Glands are either endocrine (ductless, secrete hormones into blood) or exocrine (with ducts, secrete externally or into cavities), classified by duct structure, secretory portion, and secretion mode.

  • Connective Tissue: Abundant and diverse, it supports, connects, protects, stores, cushions, and transports. It consists of cells (blasts build, cytes maintain, clasts break down) separated by an extracellular matrix (ECM) with protein fibers (collagen, reticular, elastic), ground substance (hyaluronic acid, proteoglycans), and fluid. Categories include embryonic CT, adult CT proper (loose and dense), supporting CT (cartilage and bone), and fluid CT (blood, hematopoietic tissue).

  • Muscle Tissue: Contractile, responsible for movement, posture, and circulation, all containing actin and myosin. Types include skeletal (striated, voluntary), cardiac (striated, involuntary, with intercalated disks), and smooth (nonstriated, involuntary).

  • Nervous Tissue: Composed of neurons, which generate and conduct electrical signals (action potentials), and glia, which support and protect neurons.

  • Tissue Membranes: Thin sheets of epithelial and connective tissue, including cutaneous (skin), mucous (lining open cavities), serous (lining closed cavities, reducing friction), and synovial (lining joints, secreting synovial fluid).

  • Tissue Damage and Repair: When tissues are damaged, inflammation occurs (redness, heat, swelling, pain) through chemical mediators to mobilize defenses and initiate repair. Repair occurs via regeneration (same cell type, function restored) or replacement (different tissue, scar formation, partial function loss). Cell regeneration capacity varies: labile (mitosis throughout life), stable (limited mitosis, can divide after injury), and permanent (very limited regeneration). Wound healing involves clot formation, epithelial influx, granulation tissue formation, and scar formation, with primary union for closed wounds and secondary union for open wounds leading to more scarring.

  • Aging Effects: Tissues age, resulting in slower cell division, reduced healing, less flexible collagen and elastic fibers, decreased bone remodeling, and increased risk of arterial issues.

  • Key Connectors: Form follows function across all tissues. The ECM, with hyaluronic acid as a key component, is central to tissue structure, hydration, and resilience, and its remodeling plays a crucial role in aging and repair.