Comprehensive Study Notes: Tissues Overview (5.1–5.6)
5.1 Epithelial tissue: surfaces, linings, and secretory functions
Overview: Epithelia (epithelial tissue) are composed of one or more layers of tightly packed cells with little to no extracellular matrix between cells; they are avascular (no blood vessels) and richly innervated. They cover body surfaces, line body cavities and organ cavities, and form glands.
Terminology:
Epithelium = surface sheet that lines or covers; “epithelial tissue” = epithelia.
Epithelia exhibit polarity: apical (free/superficial surface), lateral (cell-to-cell contacts), basal (attached to basement membrane).
Basal surface attaches to a basement membrane and underlying connective tissue.
Key structural features:
Cellularity: tightly packed cells with minimal extracellular matrix.
Polarity: apical, lateral, basal surfaces.
Apical surface: may bear cilia or microvilli (see §4.6 c).
Lateral surfaces: contain membrane junctions (tight junctions, desmosomes, gap junctions).
Basal surface: attached to basement membrane via basement membrane components (collagen, glycoproteins, proteoglycans).
Basement membrane: a complex molecular structure formed by secretions of epithelium and underlying connective tissue; strengthens attachment and forms a selective barrier.
Avascularity: nutrients reach epithelial cells by diffusion across the basal surface from underlying connective tissue or across the apical surface.
Innervation: epithelia are richly innervated to detect environmental changes.
High regeneration capacity: rapid cell division to continually renew tissue exposed to environment.
Attachment to basement membrane
The epithelial layer is bound at its basal surface to a thin basement membrane.
Basement membrane composition includes collagen, glycoproteins, and proteoglycans; produced by both epithelium and underlying connective tissue.
Basement membrane acts as glue and a selective barrier between epithelium and connective tissue.
Avascularity
Epithelial tissues lack blood vessels; nutrients diffuse from blood vessels in underlying connective tissue or across the apical surface.
Innervation
Epithelia are richly innervated to detect changes in environment (sensory input).
Regeneration
High regenerative capacity supports replacement after abrasion or damage.
5.1 b Functions of epithelial tissue
Four (or more) primary functions (noting that no single epithelium performs all):
Physical protection: guards external and internal surfaces against dehydration, abrasion, chemical/biological threats; all substances entering or leaving the body pass through epithelia (gatekeepers).
Selective permeability: permeability varies; filtration basics; substances may be allowed or restricted; example: kidney filtration.
Absorption: substances cross epithelium to enter blood/lymph; nutrients absorbed in the small intestine.
Secretion: specialized epithelial cells form glands; goblet cells are unicellular glands; multicellular glands secrete various substances.
Sensory reception: epithelia contain sensory receptors and are innervated to relay sensory input to CNS (e.g., touch, temperature, pain).
5.1 c Classification of epithelial tissue
Based on number of cell layers:
Simple epithelium: one layer; all cells contact basement membrane; primary functions in filtration, absorption, secretion; examples include simple squamous, simple cuboidal, simple columnar, and pseudostratified columnar (which is actually a simple epithelium that appears layered).
Stratified epithelium: two or more layers; only deepest cells contact basement membrane; provide protection and/or secretion; examples include stratified squamous, stratified cuboidal, stratified columnar, transitional.
Pseudostratified epithelium: appears stratified but is actually simple; all cells contact basement membrane; nuclei at different levels; some cells do not reach apical surface; typically includes ciliated forms with goblet cells; two forms: pseudostratified ciliated columnar epithelium and pseudostratified non-ciliated columnar epithelium.
Based on cell shape at the apical surface:
Squamous: flat, wide cells; nucleus flattened; poor thickness barrier; good for diffusion/filtration.
Cuboidal: about as tall as wide; spherical, centrally located nucleus.
Columnar: taller than wide; oval nucleus oriented lengthwise in basal region; apical microvilli and goblet cells may be present.
Transitional: shape changes between distended and relaxed states (e.g., urinary bladder, ureters, part of urethra).
Classification by types (typical examples):
Simple squamous: single layer of flattened cells; functions in rapid diffusion/filtration; locations: air sacs (alveoli), endothelium (lining of heart chambers, blood/lymph vessels), mesothelium (serous membranes).
Simple cuboidal: single layer of cube-like cells; functions in absorption and secretion; locations: kidney tubules, thyroid follicles, ovarian surface.
Non-ciliated simple columnar: tall, single layer; nuclei basal; may contain microvilli and goblet cells; functions in absorption/secretion; locations: lining of most of GI tract (stomach to anal canal).
Ciliated simple columnar: tall cells with cilia; goblet cells common; functions in movement of mucus; locations: larger bronchioles, uterine tubes (oocyte movement).
Pseudostratified columnar epithelium: appears layered but is simple; two forms: with cilia and goblet cells (pseudostratified ciliated); without cilia (rare, e.g., parts of male urethra/epididymis); functions: protection, secretion, movement of mucus.
Stratified squamous: multiple layers; basal cells cuboidal/polyhedral; apical cells flattened; keratinized (dead cells filled with keratin) vs non-keratinized (alive cells, nuclei visible); locations: keratinized on skin epidermis; non-keratinized in oral cavity, pharynx, esophagus, vagina, anus.
Stratified cuboidal: two or more layers; apical cells cuboidal; locations: ducts of most exocrine glands, ovarian follicles.
Stratified columnar: two or more layers; apical cells columnar; relatively rare; locations: large ducts of some glands, conjunctiva, membranous part of male urethra.
Transitional epithelium: varying appearance with distension/relaxation; lines urinary bladder, ureters, part of urethra; can stretch; presence of binucleated cells in relaxed state; function: accommodates urine volume changes.
5.1 d Glands
Definition and types:
Glands are structures composed of epithelial tissue that secrete substances for use within the body or for elimination.
Endocrine glands: secrete hormones directly into the blood (no ducts); examples include thyroid, adrenal glands.
Exocrine glands: secrete onto epithelial surfaces via ducts; examples include sweat, salivary, mammary glands; may be unicellular or multicellular.
Unicellular exocrine glands
Goblet cells: produce mucin, a glycoprotein that forms mucus when hydrated with water; typically found in simple columnar and pseudostratified epithelia.
Multicellular exocrine glands
Consist of acinus (secretory units) and ducts; encased by a fibrous capsule, with septa dividing gland into lobes.
Classified by anatomic form: simple (single unbranched duct) vs compound (branched ducts).
Classified by secretory portion shape: tubular (secretory portion and duct same diameter) vs acinar/sinous (secretory portion forms a sac) vs tubuloacinar (both).
Classified by method of secretion:
Merocrine (eccrine): secretions packaged in vesicles and released by exocytosis; gland cells remain intact
Examples: lacrimal glands, salivary glands, pancreas, gastric glands.
Apocrine: apical portion pinches off to release secretory material; cell repairs and repeats.
Examples: mammary glands, ceruminous glands of the ear.
Holocrine: entire cell disintegrates to release secretory product; gland cells replaced by division of stem cells.
Example: sebaceous glands.
5.2 Connective tissue: cells, fibers, and ground substance
Overview: Connective tissue is diverse, but all share three basic components that form the extracellular matrix (ECM): cells, protein fibers, and ground substance. It binds, supports, and protects organs and structures; it is often vascularized to varying degrees.
Major groups: connective tissue proper (loose, dense), supporting connective tissue (cartilage, bone), and fluid connective tissue (blood).
5.2 a Characteristics of connective tissue
Three components: cells, protein fibers, ground substance (ECM).
ECM forms the extracellular matrix; fibers and ground substance provide structure and function.
All connective tissues derive from embryonic mesenchyme.
Vascularity varies among tissues (e.g., highly vascularized areolar; limited in dense regular; avascular in mature cartilage).
Regenerative capacity varies (bone and blood highly regenerative; cartilage limited).
Examples: tendons, ligaments, adipose tissue, cartilage, bone, blood.
5.2 b Functions of connective tissue (general)
Physical protection.
Support and structural framework for the body.
Binding of structures together.
Storage of energy (adipose) and minerals (bone:
Transport of fluids and solutes (blood).
Immune protection (immune cells throughout connective tissues).
5.2 c Embryonic connective tissue
Two primary embryonic tissues: mesenchyme and mucous connective tissue (Wharton’s jelly).
Mesenchyme: the tissue from which all connective tissues originate; cells are stellate or spindle-shaped; contains immature protein fibers in a gel-like ground substance.
Wharton's jelly: mucus connective tissue in the umbilical cord; more immature fibers than mesenchyme.
Clinical note: cord blood contains many stem cells with therapeutic potential.
5.2 d Connective tissue cells in a supportive matrix (resident vs wandering cells)
Resident cells (permanently housed):
Fibroblasts: produce fibers and ground substance; most abundant in connective tissue proper.
Adipocytes: fat cells; form adipose tissue when in large clusters.
Mesenchymal cells: embryonic stem-like cells that can differentiate into other connective tissue cells; important for repair.
Fixed macrophages: engulf damaged cells/pathogens and release inflammatory signals.
Mast cells: release pro-inflammatory molecules; part of immune defense.
Wandering cells (migrate through tissue):
Various leukocytes (e.g., dendritic cells in dermis) and other immune cells; participate in immune defense and tissue repair.
Protein fibers in connective tissue
Collagen fibers: unbranched, strong, flexible, resist stretching; major component; ~25% of body protein; appear white in fresh tissue and pink in H&E-stained sections; abundant in tendons/ligaments.
Reticular fibers: thin, form a branching framework; composed of same subunits as collagen but arranged differently; form stroma in lymphoid organs (lymph nodes, spleen, liver).
Elastic fibers: composed of elastin; appear black with special stains; allow stretching and recoil; abundant in skin, arteries, lungs.
Ground substance
Molecular, non-cellular material produced by connective tissue cells; occupies ECM; can be viscous, semi-solid, or solid.
Glycosaminoglycans (GAGs): large, negatively charged, hydrophilic molecules that attract water and ions; contribute to viscosity of ground substance.
Common GAGs include chondroitin sulfate, heparan sulfate, and hyaluronic acid.
When GAGs link to a protein, they form proteoglycans (core protein + GAG chains).
Ground substance also contains adherent glycoproteins that help bind cells and fibers to the ECM.
ext{GAGs are negatively charged and hydrophilic, attracting cations (e.g., Na^+) and water.}
Proteoglycan: ext{Proteoglycan} = ext{core protein} + ext{GAG chains}
Clinical view: Scurvy (vitamin C deficiency) affects collagen synthesis and connective tissue integrity; symptoms include gum disease, poor wound healing, hemorrhages, abnormal bone growth.
5.2 a–d integrated learning prompts: relationships among ECM components and tissue properties; significance of GAGs and proteoglycans for tissue hydration and resilience.
5.2 b Functions (revisited): connective tissues provide protection, support, binding, energy storage, transport, and immune defense; specific roles depend on tissue type.
5.3 Muscle tissue: movement
Overview: Muscle tissue is contractile, conductive, elastic, extensible, and excitable; highly vascularized. Three types of muscle tissue: skeletal, cardiac, smooth.
Skeletal muscle tissue
Structure: long, cylindrical fibers; multinucleated; nuclei at periphery; striated appearance due to alternating thick/thin filaments.
Function: move skeleton; also contributes to thermal regulation via heat generation during contraction.
Location: attached to bones; some facial muscles; external urethral and anal sphincters.
Regeneration: limited capacity for repair via division of satellite cells.
Control: voluntary (somatic nervous system).
Cardiac muscle tissue
Structure: short, branched cells; usually 1–2 central nuclei; striated; connected by intercalated discs (desmosomes and gap junctions).
Function: pumps blood through heart; contracts as a unit due to electrical coupling.
Location: myocardium (heart wall).
Regulation: involuntary; pacemaker cells initiate contraction.
Regeneration: limited regenerative capacity; damage not readily replaced.
Smooth muscle tissue
Structure: short, fusiform (spindle-shaped) cells; 1 central nucleus; no striations; cells often arranged in sheets or muscle layers.
Function: moves materials through hollow organs; controls lumen size (e.g., intestines, stomach, airways, bladder, uterus, blood vessels).
Location: walls of most viscera and some vascular structures; iris (pupil size).
Control: involuntary.
Integrating notes
Distinguishing features under light microscopy: shape, nucleus location, presence/absence of striations, presence of intercalated discs (cardiac).
5.4 Nervous tissue: information transfer and integration
Location: brain, spinal cord, and peripheral nerves.
Cellular components:
Neurons: primary signaling cells; have a cell body (soma), dendrites (receptive), and an axon (signal transmission).
Glial (supporting) cells: nourish, protect, and support neurons; do not conduct nerve impulses themselves.
Neuron structure and function
Neuron: receives, processes, and relays nerve impulses.
Glial cells: provide nourishment, protection, and support for neurons.
5.5 Integration of tissues in organs and body membranes
5.5 A Organs
An organ is a structure composed of two or more tissue types that work together to perform complex functions.
Example: the stomach consists of epithelium, areolar and dense connective tissue, smooth muscle layers, and nervous tissue; glands associated with epithelium secrete digestive substances; connective tissue provides blood vessels and nerves; smooth muscle contracts to mix contents; nervous tissue regulates contraction and secretion.
5.5 B Body membranes
Body membranes are epithelial sheets bound to connective tissue that line cavities or cover surfaces; four main types:
Mucous (mucosa): lines passageways open to the exterior (GI tract, respiratory tract, urinary tract, reproductive tract). Composed of an epithelium and lamina propria (areolar connective tissue) forming mucus.
Serous membranes: line closed body cavities and cover many organs; consist of a simple squamous epithelium (mesothelium) and a thin areolar connective tissue layer; secrete serous fluid to reduce friction (serous cavity with parietal and visceral layers).
Cutaneous membrane: the skin; composed of keratinized stratified squamous epithelium (epidermis) and an underlying connective tissue (dermis); protects and minimizes water loss.
Synovial membranes: line joint cavities; composed of connective tissue only (no epithelial layer); secrete synovial fluid to reduce friction and provide nutrients to cartilage.
Serous membranes and serous fluid reduce friction in organ movement (e.g., pericardium with the heart, pleura with lungs, peritoneum with abdominal organs).
5.6 Tissue development, modification, and aging
5.6 a Tissue development (embryology)
Three primary germ layers form during development: ectoderm, mesoderm, and endoderm.
By week 3, these layers give rise to all tissues of the body.
The embryo’s tissue derivations are summarized in concept figures (e.g., Fig. 5.13).
5.6 b Tissue modification (growth and transformation of tissues)
Hypertrophy: increase in cell size (e.g., skeletal muscle hypertrophy with long-term exercise).
Hyperplasia: increase in cell number (e.g., callus formation from repeated friction).
Atrophy: shrinkage in tissue size due to decreased cell size/number, aging, or disuse (e.g., cast immobilization, bed rest).
Metaplasia: mature tissue changes to a different mature tissue type in response to stress (e.g., smokers’ tracheal epithelium switching from pseudostratified ciliated columnar to non-keratinized stratified squamous; reversal possible with smoking cessation).
Dysplasia: abnormal tissue development; precancerous changes (e.g., cervical dysplasia due to HPV); may progress to cancer or revert.
Neoplasia: uncontrolled growth forming tumors (benign or malignant).
Necrosis: tissue death due to irreversible damage; often accompanied by inflammation; gangrene is a form of necrosis.
5.6 c Aging of tissues
Tissues change with age; repair and maintenance become less efficient; collagen content often declines; epithelia thin; bones become brittle; muscles and nervous tissue atrophy.
Contributing factors: poor diet, circulation problems, smoking, alcohol use; cumulative minor damage increases risk of health problems.
Stem cells and regenerative biology (Clinical perspectives throughout)
Stem cells are immature cells capable of self-renewal and differentiation.
Two main characteristics: self-renewal and potency (differentiation potential).
Potency levels: totipotent, pluripotent, multipotent, unipotent.
Embryonic stem cells (totipotent → pleuripotent) have broad differentiation potential but raise ethical concerns; induced pluripotent stem cells (iPSCs) reprogram adult cells to a pluripotent state.
Adult stem cells are typically multipotent or unipotent and can renew themselves.
Common adult stem cell sources include bone marrow and various tissues; embryonic stem cells originate from blastocysts.
Hi-profile clinical example: HeLa cells (Henrietta Lacks) have contributed to medical research; controversy about consent and use of patient-derived cells.
Tissue transplantation (Clinical view)
Grafting is used to replace diseased or damaged tissue; four major graft types:
Autograft: tissue transplanted within the same individual (e.g., skin grafts); no rejection.
Isograft (from identical twin): tissue transplanted between genetically identical individuals.
Allograft: tissue transplanted between genetically different individuals (humans); risk of rejection; requires immunosuppression.
Xenograft: tissue transplanted between species (e.g., pig tissue for valves); also prone to rejection; genetic modification aims to reduce rejection.
Clinical notes and examples mentioned in the transcript
Scurvy as a result of impaired collagen synthesis due to vitamin C deficiency; seafood sailors historically used citrus fruits to prevent it.
Marfan syndrome: connective tissue disorder linked to mutations in fibrillin-1 gene; features include long limbs, chest deformities, joint laxity, aortic aneurysm risk.
Gangrene: necrosis due to loss of blood supply; several forms (dry, wet, gas) with distinct etiologies and treatments.
Metaplasia and dysplasia reviewed as precursors to cancer or reversals with environmental changes.
Grafting and transplantation ethics and advances (autograft, allograft, xenograft) with immunosuppressive therapy implications.
Quick study tips (from end-of-chapter activities mentioned in transcript)
Distinguish epithelial types by: Is it one layer or many? What is the apical cell shape? (simple vs stratified, squamous vs cuboidal vs columnar).
Compare transitional vs keratinized stratified squamous epithelia in terms of structure, function, and location.
Use a flowchart to trace connective tissue origin from mesenchyme and classify tissues as proper, supporting, or fluid; identify features such as resident vs wandering cells, and types of fibers.
Recognize tissue types by function and structure to map to organ systems and clinical conditions.
Recap: The four major tissue families (epithelial, connective, muscle, nervous) combine in organs and membranes to support body function, with diverse structural and functional specializations. Developmental biology and aging influence how tissues change over time, with implications for health, disease, and therapeutics.
Appendix: Key terms and examples to memorize
Endothelium vs. Mesothelium as specialized simple squamous epithelia.
Basal lamina as part of basement membrane.
Ground substance components: GAGs (e.g., chondroitin sulfate, heparan sulfate, hyaluronic acid), proteoglycans, and glycoproteins.
Gland secretion types: merocrine, apocrine, holocrine.
Tissue layers and organ examples: stomach (epithelium, connective tissue, smooth muscle layers, nervous tissue) as an organ with multiple tissue types.
Grafting types and their clinical implications.