Histology: Epithelial, Connective, Muscle, and Nervous Tissues (HLTH115)

Tissue Organization

  • Tissues are groups of similar cells and extracellular material with a common function (e.g., providing protection).

  • Study of tissues is histology.

  • The four basic tissue types are: epithelial, connective, muscle, and nervous.

Epithelial Tissue: Characteristics

  • Also referred to as epithelium.

  • Composed of one or more layers of closely packed cells.

  • Contains little to no extracellular matrix.

  • Covers body surfaces and lines body cavities.

  • Forms the majority of glands.

  • Cellularity: composed almost entirely of tightly packed cells.

  • Polarity: has apical surface (exposed to external environment or internal body space; may have microvilli or cilia), lateral surface (intercellular junctions), and basal surface (attached to connective tissue).

  • Attachment to basement membrane: three layers – lamina lucida, lamina densa, reticular lamina; composed of collagen, glycoproteins, and proteoglycans; acts as a barrier between epithelium and connective tissue.

  • Avascularity: nutrients obtained across the apical or basal surface.

  • Extensive innervation: detects changes in the environment in that region.

  • High regeneration capacity: frequent cell division; continual replacement of lost cells due to environmental exposure; stem cell division.

  • Apical (free) surface, lateral surface, basal surface, basement membrane interplay are depicted in standard histology figures.

Functions of Epithelial Tissue

  • Physical protection: protects external and internal surfaces from dehydration, abrasion, and destruction.

  • Selective permeability: relatively non-permeable to some substances; promotes passage of others.

  • Secretions: some cells secrete; may be scattered among other cells or arranged in clusters.

  • Sensations: contain nerve endings and provide information to the nervous system (touch, pressure, temperature, pain); specialized epithelium (neuroepithelium) houses cells responsible for sight, taste, smell, hearing, balance.

Classification of Epithelial Tissue (5.1c)

  • Two-part name reflects: (1) number of epithelial cell layers and (2) shape of cells at the apical surface.

Classification by number of cell layers

  • Simple epithelium: one cell layer thick; all cells contact the basement membrane; functions primarily in filtration, absorption, or secretion; examples include lining of air sacs of lungs, intestines, and blood vessels.

  • Stratified epithelium: two or more layers; only basal layer contacts basement membrane; better resists wear and tear; examples include skin, esophagus lining, urinary bladder; basal cells continuously regenerate.

  • Pseudostratified epithelium: appears layered due to nuclei at different levels but all cells contact basement membrane; not all reach the apical surface; typically a simple epithelium in disguise.

Classification by cell shape

  • Squamous: flat, wide, irregular; floor-tile-like; nucleus flattened.

  • Cuboidal: about as tall as wide; edges somewhat rounded; nucleus spherical and centered.

  • Columnar: slender and taller than wide; nucleus oval and lengthwise at the base.

  • Transitional: can change shape with stretch; located where epithelium stretches and relaxes (e.g., lining of the bladder); cells are polyhedral when relaxed and flatter when stretched.

Epithelium organization by layers and shape (summary)

  • Simple epithelium: simple squamous, simple cuboidal, simple columnar, pseudostratified columnar (often ciliated/nonciliated).

  • Stratified epithelium: stratified squamous (keratinized and nonkeratinized), stratified cuboidal, stratified columnar, transitional.

  • Keratinized vs nonkeratinized forms: keratinized superficial cells are dead; nonkeratinized retain living cells at surface.

  • Some epithelia are ciliated or nonciliated depending on function.

Relationship between type and function

  • Simple epithelia: better suited for diffusion, absorption, and secretion due to thinner barrier.

  • Stratified epithelia: better suited for protection against abrasion, friction, and environmental hazards.

Glands (5.1d)

  • Glands are made of epithelial tissue and secrete substances used elsewhere or for elimination.

  • Secretions can include mucin, electrolytes, hormones, enzymes, urea, etc.

  • Endocrine glands: lack ducts; secrete hormones directly into the bloodstream (chemical messengers that influence activity elsewhere).

  • Exocrine glands: invaginated epithelium with ducts connecting to an epithelial surface (e.g., sweat glands, mammary glands, salivary glands).

  • Unicellular exocrine glands: single-cell glands (most common is goblet cell).

  • Multicellular exocrine glands: many cells organized into acini (secretory clusters) and ducts; surrounded by a fibrous capsule; secretion transported to epithelial surface via ducts.

  • General structure: secretory portion (acinus) and conducting portion (duct).

  • Modes of secretion (how products exit the cell):

    • Merocrine: secretions packaged in vesicles and released by exocytosis (e.g., lacrimal glands, salivary glands, some sweat glands, pancreas secretions, gastric glands).

    • Apocrine: apical membrane pinches off to release secretions; glandular cells repair and continue to secrete; examples include mammary glands and ceruminous glands of the ear.

    • Holocrine: secretion formed when the secretory cell disintegrates; secretory material is the disintegrated cell; replacement occurs; examples include sebaceous glands of the skin.

Connective Tissue (5.2): Cells in a Supportive Matrix

  • Connective tissue (CT) is the most diverse, abundant, and widely distributed tissue type; it supports, protects, and binds organs.

  • Components of CT: cells, protein fibers, ground substance (extracellular matrix).

  • CT cell types are grouped as resident cells (in CT) and wandering cells (move through CT).

  • Resident cells

    • Fibroblasts: flat cells with tapered ends; most abundant CT proper resident cell; synthesize fibers and ground substance of ECM.

    • Adipocytes: fat cells; appear in small clusters in some CT; adipose tissue is a dominant CT proper type.

    • Mesenchymal cells: embryonic stem-cell-like; can divide to replace damaged cells; one daughter becomes a committed cell.

    • Fixed macrophages: large, irregular-shaped cells; derived from monocytes; phagocytize damaged cells/pathogens; release immune-stimulating chemicals.

  • Wandering cells

    • Continuously move through CT; components of the immune system; repair damaged ECM; various leukocytes (white blood cells).

    • Types include mast cells, plasma cells, free macrophages, neutrophils, T-lymphocytes, etc.

    • Mast cells: small, near blood vessels; secrete heparin (anticoagulant) and histamine (vasodilation).

    • Plasma cells: derived from B-lymphocytes; produce antibodies.

    • Free macrophages: mobile phagocytic cells.

    • Other leukocytes: neutrophils, T-lymphocytes, etc.

  • Protein fibers in the ECM

    • Collagen fibers: unbranched, cable-like; strong, flexible, resistant to stretching; appear white in fresh tissue; abundant in tendons and ligaments.

    • Reticular fibers: thinner, form branching networks; provide framework; abundant in lymphoid organs (lymph nodes, spleen, liver).

    • Elastic fibers: contain elastin; branching, wavy fibers; stretch and recoil easily; found in skin, arteries, lungs; give resilience.

  • Ground substance

    • Noncellular material produced by CT cells; fills space between cells and fibers; serves as a medium for exchange; can be viscous (blood), semisolid (cartilage), or solid (bone).

    • Together with protein fibers, ground substance constitutes the extracellular matrix (ECM).

  • Connective Tissue Components and Organization

    • Blood vessels, capillaries, and ground substance with protein fibers (collagen, elastin, reticular).

    • Resident cells and wandering cells distributed within the ECM.

  • 5.2b Functions of Connective Tissue

    • Physical protection: bones protect organs; adipose tissue protects kidneys and eyes.

    • Support and structural framework: bones; cartilage keeps airways open; protective capsules around organs.

    • Binding of structures: ligaments (bone to bone); tendons (muscle to bone); dense irregular tissue anchors skin to underlying tissues.

    • Storage: adipose tissue stores energy; bone stores calcium and phosphorus.

    • Transport: blood transports nutrients, wastes, hormones, gases.

    • Immune protection: leukocytes; ECM restricts movement of disease-causing organisms.

  • All CT is derived from mesenchyme; mesenchyme differentiates into adult CT types: CT proper, supporting CT, and fluid CT.

  • CT proper is divided into Loose CT and Dense CT.

CT Proper: Loose CT

  • Loose CT has fewer cells and fibers than dense CT but abundant ground substance; acts as packing material and supports structures.

  • Three types:

    • Areolar CT: loose organization of collagen and elastic fibers; highly vascularized; contains fixed and wandering cells; abundant, viscous ground substance; found in the papillary layer of the dermis; major component of the subcutaneous layer; surrounds organs, nerves, and blood vessels.

    • Adipose CT (fat): predominantly adipocytes; two types – white (energy storage, insulation, cushioning) and brown (heat generation in newborns; declines with age); adipocyte number remains stable; weight changes occur via adipocyte size changes.

    • Reticular CT: meshwork of reticular fibers that forms the structural framework of lymphatic organs (spleen, thymus, lymph nodes, bone marrow).

CT Proper: Dense CT

  • Dense CT is dominated by protein fibers with relatively little ground substance; collagen fibers predominate.

  • Three categories:

    • Dense Regular CT: tightly packed, parallel collagen fibers; fibroblasts align between fibers; little ground substance; poor blood supply; withstands stress in one direction; found in tendons and ligaments.

    • Dense Irregular CT: collagen fibers arranged in many directions; more ground substance; extensive blood supply; withstands stress in multiple directions; found in most of the dermis, periosteum of bone, perichondrium of cartilage, and organ capsules.

    • Elastic CT: abundant elastic fibers; allows stretching and recoil; found in walls of large arteries (e.g., aorta), trachea, and vocal cords.

Supporting CT: Cartilage

  • Cartilage provides firm, semisolid ECM; collagen and elastic fibers; chondrocytes reside in lacunae; surrounded by a dense irregular CT covering called perichondrium (except fibrocartilage).

  • Avascular in the mature state.

  • Three types:

    • Hyaline cartilage: most common; glassy matrix; chondrocytes scattered in lacunae; usually has perichondrium; locations include tip of nose, trachea, most of larynx, costal cartilage, articulating ends of long bones, and most of the fetal skeleton.

    • Fibrocartilage: weight-bearing; bundles of collagen fibers between chondrocytes; sparse ground substance; no perichondrium; found in intervertebral discs, pubic symphysis, and menisci of knee joints.

    • Elastic cartilage: numerous elastic fibers; highly resilient and flexible; perichondrium present; found in external ear and epiglottis.

Bone

  • Bone is more solid than cartilage; provides greater support but less flexibility; ECM includes organic components (collagen and glycoproteins) and inorganic components (calcium salts).

  • Bone cells are osteocytes housed in lacunae; two types: compact bone and spongy bone.

  • Compact bone: organized into osteons with lamellae surrounding a central canal; a perforated network provides nutrients via canaliculi.

  • Spongy bone: latticework inside bones; lighter but strong; contains hemopoietic tissue.

  • Functions: levers for movement, supports soft tissues, protects organs, stores calcium and phosphorus, houses hemopoietic cells.

Fluid Connective Tissue

  • Two types: blood and lymph.

  • Blood: formed elements (erythrocytes, leukocytes, platelets) suspended in plasma; plasma contains dissolved proteins and nutrients; functions include transport of gases, nutrients, wastes, and immune protection.

  • Lymph: derived from blood plasma; lacks cellular components or fragments; returns to bloodstream.

Muscle Tissue: Movement (5.3)

  • Muscle tissue cells contract when stimulated, enabling movement:

    • Skeletal muscle: voluntary, moves skeleton; long cylindrical multinucleated fibers with striations; arranged in parallel bundles; does not contract without somatic nervous system input.

    • Cardiac muscle: involuntary; located in the heart wall (myocardium); short, branching cells with one or two nuclei; striated; intercalated discs connect cells to promote rapid conduction of electrical activity.

    • Smooth muscle: involuntary; nonstritated; spindle-shaped cells with a single central nucleus; found in walls of hollow organs (intestines, stomach, airways, bladder, uterus, blood vessels); propels contents and controls lumen size.

  • Table snapshots (Table 5.10): skeletal, cardiac, and smooth muscle tissue—structure, function, and location.

Nervous Tissue: Information Transfer and Integration (5.4)

  • Located in brain, spinal cord, and nerves.

  • Two major cell types:

    • Neurons: receive, transmit, and process nerve impulses.

    • Glial cells: supportive, nourishing, and protective roles; do not transmit nerve impulses.

  • Parts of a neuron:

    • Cell body: houses nucleus and organelles.

    • Dendrites: short, numerous processes that receive signals.

    • Axon: single long process that transmits signals away from the cell body.

  • Neurons are the longest cells in the body.

  • Table 5.11 summarizes nervous tissue structure and function.

Review: Type and Function Relationships in Epithelia

  • Simple epithelia are thinner and better for diffusion, absorption, and secretion.

  • Stratified epithelia are thicker and better for protection against abrasion and friction.

Summary of Glandular and Connective Tissue Concepts (Key Takeaways)

  • Glands can be endocrine or exocrine; exocrine glands can be unicellular (goblet cells) or multicellular (acini and ducts) with merocrine, apocrine, or holocrine secretory mechanisms.

  • Connective tissue comprises cells, fibers, and ground substance forming the ECM; it supports and protects the body, stores energy, transports substances, and defends against disease.

  • Cartilage and bone provide supportive CT, with cartilage being avascular and highly specialized; bone contains a mineralized ECM and supports vital functions like calcium storage.

  • Muscle and nervous tissues enable movement and information processing, respectively, with distinct structural and functional properties.