Histology Chapter 4: Tissues and Epithelia (Vocabulary Flashcards)

Tissues

• Histology is the study of tissues — collections of specialized cells and cell products that perform specific functions.

• Tissues combine to form organs (e.g., heart, liver).

• Four types of tissue:

  • Epithelial

  • Connective

  • Muscle

  • Nervous

Epithelial Tissue

• Epithelial tissue includes epithelia and glands.

  • Epithelia (singular: epithelium): layers of cells covering internal or external surfaces.

  • Glands: structures that produce fluid secretions.

• Functions of epithelial tissue:

  • Provide physical protection

  • Control permeability

  • Provide sensation

  • Produce specialized secretions

• Characteristics of epithelia:

  • Polarity (apical and basal surfaces)

  • Cellularilty (little ground substance)

  • Attachment (basement membrane)

  • Avascularity (avascular)

  • Regeneration

• Integrity of epithelia is maintained by:

  • Intercellular connections

  • Attachment to the basement membrane

  • Epithelial maintenance and repair

• Polarity of epithelial cells:

  • Apical surface:

  • Microvilli increase absorption or secretion

  • Cilia on a ciliated epithelium move fluids

  • Basolateral surface

• Intercellular connections of epithelia:

  • Cells are firmly attached to each other via large areas of opposing plasma membrane (cell junctions)

  • Cell adhesion molecules (CAMs): transmembrane proteins

  • Proteoglycans act as intercellular cement (contain glycosaminoglycans such as hyaluronan)

• Hyaluronic acid (hyaluronic acid) in moisturizers: used to hydrate and lubricate skin, potentially helping to reduce signs of aging by maintaining moisture and viscosity.

• Cell junctions (types):

  • Gap junctions

  • Tight junctions

  • Desmosomes

• Gap junctions:

  • Allow rapid communication; cells held together by interlocking transmembrane proteins (connexons)

  • Allow small molecules and ions to pass; coordinate activities (e.g., beating of cilia)

• Tight junctions:

  • Encircle the apical portion of cells; adhesion belt attaches to terminal web and microfilaments

  • Prevent passage of water and solutes; keep enzymes, acids, and wastes in the lumen (e.g., digestive tract)

• Desmosomes:

  • Densely packed CAMs and proteoglycans link opposing membranes

  • Spot desmosomes tie cells together to allow bending and twisting

  • Hemidesmosomes attach cells to the basement membrane

• Functional significance of gap junctions:

  • They share ions between adjacent cells and coordinate cellular activities; they contribute to tissue resilience and integrated function.

• Basement membrane anatomy:

  • Basal lamina: closest to epithelium; disorganized extracellular matrix secreted by epithelium

  • Reticular lamina: deeper portion; secreted by underlying connective tissue; provides strength

• Epithelial maintenance and repair:

  • Epithelial cells are replaced by continual division of stem cells located near the basement membrane

• Quiz note (characteristics of epithelial tissue): NOT a characteristic

  • Options include:

  • It stores energy reserves. (Not a characteristic of epithelia; relates to connective tissue)

• Classification of epithelia (by shape and layers):

  • Shapes: squamous (thin and flat), cuboidal (square), columnar (tall, slender rectangles)

  • Layers: simple (single layer), stratified (several layers)

• Squamous epithelia

  • Simple squamous epithelia: functions in absorption and diffusion

  • Mesothelium: lines body cavities

  • Endothelium: inner lining of heart and blood vessels

  • Stratified squamous epithelia: provide protection against mechanical stresses; Keratin adds strength and water resistance

• Simple squamous locations and functions (examples):

  • Mesothelium: lining pleural, pericardial, and peritoneal cavities

  • Endothelium: inner lining of heart and blood vessels

  • Kidney tubules (thin sections of nephron loops)

  • Inner lining of cornea

  • Alveoli of lungs

  • Functions: reduces friction; controls vessel permeability; absorption and secretion

• Stratified squamous epithelium locations/functions (examples):

  • Surface of skin; lining of mouth, throat, esophagus, rectum, anus, vagina

  • Functions: physical protection against abrasion, pathogens, and chemical attack

• Structure of the epidermis (layered view):

  • Strata (from basal to surface): basale, spinosum, granulosum, lucidum (only in thick skin), corneum

  • Key cell types: keratinocytes; melanocytes; Merkel cells; dendritic cells (Langerhans cells)

• Keratinized vs non-keratinized stratified squamous epithelia

  • Function and appearance differences discussed; keratinized provides more protection and water resistance; non-keratinized lacks the keratin layer

• Cuboidal and transitional epithelia

  • Simple cuboidal epithelia: secretion and absorption; glands and kidney tubules

  • Stratified cuboidal epithelia: relatively rare; ducts of sweat and mammary glands

  • Transitional epithelia: tolerate stretching; appear cuboidal when relaxed and squamous when stretched; found in urinary bladder

• Columnar epithelia

  • Simple columnar epithelia: absorption and secretion; found in stomach, small intestine, large intestine

  • Pseudostratified columnar epithelia: typically have cilia; nuclei at multiple levels but all cells touch basement membrane; found in nasal cavity, trachea, bronchi; functions include protection, secretion, mucus movement with cilia

  • Stratified columnar epithelia: relatively rare; superficial layers are columnar; provide protection (pharynx, anus, urethra)

• Body cavities and vascular linings

  • Epithelia lining body cavities and blood vessels include simple squamous epithelia (mesothelium and endothelium)

Glandular Epithelia

• Glands are collections of epithelial cells that produce secretions

  • Endocrine glands: release hormones into bloodstream; no ducts

  • Exocrine glands: discharge secretions through ducts onto epithelial surfaces

• Exocrine glands: classification considers:

  • Structure of the gland (not essential here)

  • Secretion method

  • Secretory products

• Exocrine gland secretion methods (examples):

  • Merocrine: released by secretory vesicles via exocytosis (e.g., merocrine sweat glands)

  • Apocrine: released by shedding apical portions of cytoplasm packed with secretory vesicles (e.g., mammary glands)

  • Holocrine: released when gland cells burst and are replaced by stem cells (e.g., sebaceous glands)

• Secretory products types:

  • Serous glands: watery secretions

  • Mucous glands: mucins

  • Mixed exocrine glands: both serous and mucous

• Examples of exocrine glands:

  • Salivary glands (serous and mucous components)

  • Mammary glands (apocrine secretion in milk production)

  • Sebaceous glands (holocrine secretion)

Connective Tissue

• Components of connective tissues:

  • Specialized cells

  • Extracellular protein fibers

  • Ground substance (fluid)

• Extracellular matrix (ECM):

  • ECM = extracellular protein fibers + ground substance

  • ECM is the majority of tissue volume and determines specialized function

• Functions of connective tissue:

  • Structural framework for the body

  • Transportation of fluids and dissolved materials

  • Protection of delicate organs

  • Surrounding, interconnecting, and supporting other tissues

  • Energy storage (triglycerides in adipocytes)

  • Defense against invading microorganisms

• Types of connective tissue:

  • Connective tissue proper

  • Fluid connective tissues

  • Supporting connective tissues

• Connective tissue proper cellular components:

  • Fibroblasts: most abundant; secrete proteins and hyaluronic acid (cellular cement)

  • Fibrocytes: maintain fibers

  • Adipocytes: fat cells; store lipids

  • Mesenchymal cells: stem cells that differentiate into fibroblasts, macrophages, etc.

  • Melanocytes: synthesize/store melanin; also found in epidermis

  • Macrophages: phagocytic immune cells; fixed or free

  • Mast cells: trigger inflammation; release histamine and heparin

  • Lymphocytes: migrate; may become plasma cells

  • Microphages: neutrophils, eosinophils

• Protein fibers in connective tissue:

  • Collagen fibers: most common; long, straight, unbranched; strong and flexible; resist force in one direction; abundant in tendons and ligaments

  • Reticular fibers: thinner, highly branched; form a network (stroma); stabilize functional cells and structures within organs

  • Elastic fibers: contain elastin; branched and wavy; return to original length after stretching

• Ground substance:

  • Clear, colorless, viscous; rich in proteoglycans and glycoproteins; fills spaces between cells; slows pathogen movement

• Adipose tissue (fat tissue):

  • Adipocytes store fat; adults’ adipocytes do not divide; fat expands by lipid storage and shrinks as lipids are released

  • White fat: stores fat, absorbs shocks, insulates

  • Brown fat: more vascularized; many mitochondria; burns lipids to generate heat (important in newborns)

• Loose connective tissue (areolar, adipose, reticular):

  • Areolar tissue: least specialized; open framework; viscous ground substance; elastic fibers; holds capillary beds

  • Adipose tissue: deep to skin; padding around organs; cushions; insulation; energy storage

  • Reticular tissue: provides supportive framework in liver, kidney, spleen, lymph nodes, bone marrow

• Dense connective tissue:

  • Dense regular connective tissue: tightly packed, parallel collagen fibers; resists tension in one plane; tendons, ligaments, aponeuroses

  • Dense irregular connective tissue: interwoven collagen network; resists forces from many directions; provides dermal strength; surrounds cartilages, bones; organ capsules

  • Elastic tissue: predominantly elastic fibers; e.g., elastic ligaments of the spine; stabilizes vertebrae and other organs; allows expansion/contraction

• Fasciae:

  • Fasciae are connective tissue layers that support and surround organs

  • Superficial fascia (subcutaneous): areolar and adipose tissue; separates skin from underlying tissues

  • Deep fascia: dense regular connective tissue; strong internal framework

  • Subserous fascia: loose connective tissue between deep fascia and serous membranes

• Fluid connective tissue:

  • Blood: watery matrix (plasma); formed elements (RBCs, WBCs, platelets)

  • Lymph: derived from interstitial fluid; monitored by immune system; returned to veins near the heart

• Blood components:

  • Red blood cells (erythrocytes): transport oxygen; lack nucleus; ~hemoglobin gives color

  • White blood cells (leukocytes): defend against infection; various types

  • Platelets: cell fragments; essential for clotting

  • Monocytes, lymphocytes, neutrophils, eosinophils, basophils (phagocytes and immune roles)

• Cartilage (supporting connective tissue):

  • Matrix is a firm gel; chondroitin sulfates; cells are chondrocytes in lacunae

  • Avascular; heals poorly; perichondrium surrounds cartilage and helps supply nutrients via diffusion

  • Perichondrium has outer fibrous layer and inner cellular layer

  • Hyaline cartilage: most common; tough yet flexible; reduces friction; found in joints, rib tips, sternum, trachea

  • Elastic cartilage: bends easily due to elastic fibers; found in external ear and epiglottis

  • Fibrocartilage: very durable; limits movement; prevents bone-to-bone contact; found in knee pads, pubic symphysis, intervertebral discs

• Bone (osseous tissue):

  • For weight support; calcified matrix with collagen fibers; osteocytes in lacunae; central canals; canaliculi allow exchange with blood; periosteum covers bone (fibrous outer and cellular inner layers)

  • Osteon structure: central canal, lamellae, canaliculi

• Cartilage vs bone repair notes:

  • Herniated intervertebral disc involves fibrocartilage damage (disc contains fibrocartilage)

  • Bone can remodel and repair throughout life; cartilage has limited regenerative capacity

• Quiz/recall prompts:

  • If a person has a herniated intervertebral disc, which type of cartilage has been damaged? → fibrocartilage

  • Which type of connective tissue contains primarily triglycerides? → adipose

Membranes

• Tissue membranes are physical barriers that line or cover body surfaces; composed of an epithelium supported by connective tissue.

• Four types of tissue membranes:

  • Mucous membranes (mucosae): line passageways that have external connections (digestive, respiratory, urinary, reproductive); epithelial surfaces must be moist; lamina propria is areolar tissue

  • Serous membranes: line cavities that do not open to the outside; thin but strong; parietal portion lines the inner surface of the cavity; visceral portion (serosa) covers organs; serous fluid reduces friction

  • Cutaneous membrane: the skin; thick, waterproof, and usually dry

  • Synovial membranes: line synovial joint cavities; movement stimulates production of synovial fluid; lack a true epithelium

• Key contrasts:

  • Serous membranes line sealed internal subdivisions; mucous membranes line passages that connect to the exterior

Muscle Tissue

• Muscle tissue is specialized for contraction.

• Three types:

  • Skeletal muscle: large muscles; voluntary, striated; muscle fibers are long, multinucleated

  • Cardiac muscle: only in the heart; striated involuntary; cells form branching networks with intercalated discs; pacemaker cells regulate rhythm

  • Smooth muscle: in walls of hollow, contracting organs; non-striated, involuntary; cells are short and spindle-shaped

• Locations and functions vary by type; skeletal enables body movement; cardiac pumps blood; smooth moves food, urine, secretions and controls passageways

Nervous Tissue

• Specialized for conducting electrical impulses; concentrated in brain and spinal cord

• Two principal cell types:

  • Neurons: transmit electrical signals; parts include cell body (nucleus and organelles), dendrites (receive signals), and axon (sends signals)

  • Neuroglia (glial cells): supporting cells; provide nutrients, insulate, guide development, and maintain environment; can divide (unlike neurons)

Tissue Repair and Regeneration

• Tissue repair is wound healing: dead/damaged cells removed and replaced to restore homeostasis

• Regeneration vs fibrosis:

  • Regeneration: dead/damaged cells replaced with the same type of cells; tissue returns to normal function

  • Fibrosis: if regeneration is not possible, fibroblasts fill gaps with dense irregular connective tissue, forming scar tissue (reduced function)

• Capacity for repair by tissue type:

  • Epithelial tissues: typically regenerate well due to stem cell activity near basement membrane

  • Connective tissues: often heal by regeneration; cartilage has limited regeneration and often heals by fibrosis

  • Smooth muscle: usually regenerates; cardiac and skeletal muscles heal by fibrosis; skeletal muscle has satellite cells that provide limited regeneration

  • Nervous tissue: neurons rarely regenerate; neuroglia can divide and replace some tissue; axons outside CNS may regenerate under right conditions

• Other factors affecting repair:

  • Nutrition (protein, especially collagen precursors) and blood supply are critical for repair

  • Vitamin C is required by fibroblasts to produce functional collagen

  • Adequate oxygen and nutrients delivery and immune cells are essential for repair

• Practical takeaway:

  • After injury, maintaining good nutrition (adequate protein and vitamin C), hydration, and blood flow supports tissue repair and healing processes

Quick Review / Key Takeaways

• Epithelial tissue is avascular and highly regenerative, with distinct apical/basal polarity and multiple junction types (gap, tight, desmosomes).

• Epithelia classification includes simple vs stratified and squamous, cuboidal, and columnar shapes; transitional epithelia tolerate stretching.

• Glands can be endocrine or exocrine; exocrine glands secrete via merocrine, apocrine, or holocrine mechanisms; secretions include serous, mucous, or mixed products.

• Connective tissue types vary from loose to dense, supporting, and fluid (blood/lymph); ECM composition (fibers and ground substance) determines function.

• Cartilage and bone are supporting connective tissues with unique properties: cartilage is avascular; bone has a vascularized, remodelable matrix.

• Membranes include mucous, serous, cutaneous, and synovial; serous membranes secrete a lubricating fluid to reduce friction.

• Muscle tissue types differ in control, structure, and function (skeletal, cardiac, smooth).

• Nervous tissue comprises neurons and neuroglia; neurons conduct impulses, neuroglia support them.

• Tissue repair depends on cell division capacity, stem cells, nutrition, and blood supply; regeneration is preferred, fibrosis occurs when regeneration is limited.

• Practical dietary guidance after injury includes adequate protein and vitamin C intake, plus overall energy and hydration to support repair processes.

Suggested Connections to Foundational Principles

• Structure equals function: epithelial polarity and junctions enable barrier and selective transport roles.

• ECM composition governs tissue mechanics and resilience; collagen provides tensile strength, elastin permits stretch, and ground substance controls diffusion.

• Regeneration capacity reflects cell turnover rates and stem cell availability across tissues.

• Homeostasis maintenance relies on coordinated activities of epithelia, connective tissues, and muscular/nervous systems during repair and response to injury.

Equations and Notation

• Extracellular matrix (ECM) composition can be summarized as:
  \text{ECM} = \text{Collagen fibers} + \text{Reticular fibers} + \text{Elastic fibers} + \text{Ground substance}

• General roles of connective tissue types can be framed around key functions (structural support, transport, protection, energy storage, immunity) rather than numeric formulas.

End-of-Notes

• Quick recall prompts:

  • Which cartilage type is damaged in a herniated disc? fibrocartilage

  • Which connective tissue contains primarily triglycerides? adipose

  • Name the four tissue types and one primary function for each.

  • Differentiate merocrine, apocrine, and holocrine secretions with an example for each.

  • What are the main components of the basement membrane? basal lamina and reticular lamina