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Tissues and Epithelial Tissue - Key Terms

Tissues overview

  • Four major tissue types in the body: Muscle, Nervous, Epithelial, Connective.

  • Tissues are grouped into organs; organs with related functions form systems.

  • Key concept: tissues form the structural and functional units of the body, enabling movement, signaling, protection, secretion, and support.

Muscle Tissue

  • Origin and classification

    • Derived from the mesodermal germ layer.

    • Classified as skeletal, cardiac, or smooth muscle.

  • General function

    • Produce force and cause motion (locomotion or movement within internal organs).

  • Skeletal muscle

    • Striated and usually attached to the skeleton.

    • Contraction moves bones/joints; voluntary control.

  • Smooth muscle

    • Found in walls of hollow organs (esophagus, stomach, intestines, bronchi, uterus, urethra, bladder, blood vessels).

    • Not under conscious control; spindle-shaped cells with a single nucleus.

  • Cardiac muscle

    • Involuntary muscle unique to the heart.

    • Branching, striated cells; fused at plasma membranes to form intercalated discs; pumps blood through the circulatory system.

Muscle tissue—microscopic descriptions (from the figure)

  • Skeletal muscle

    • Type: Skeletal muscle

    • Description: Long, striated cells with multiple nuclei

    • Location: In skeletal muscles

    • Function: Contraction for voluntary movements

  • Smooth muscle

    • Type: Smooth muscle

    • Description: Long, spindle-shaped cells, single nucleus

    • Location: In hollow organs (e.g., stomach)

    • Function: Propulsion of substances along internal passageways

  • Cardiac muscle

    • Type: Cardiac muscle

    • Description: Branching, striated cells fused at plasma membranes

    • Location: Wall of heart

    • Function: Pumping of blood in the circulatory system

Nervous Tissue

  • Primary roles

    • React to stimuli and conduct impulses to various organs to elicit responses.

  • Neurons

    • Specialized nerve cells that generate and transmit electrical impulses rapidly.

  • Nerve structure

    • A nerve is composed of many nerve fibers (neurons) bound by connective tissue.

    • Epineurium: dense connective tissue sheath surrounding the entire nerve.

    • Perineurium: surrounds bundles of nerve fibers.

    • Endoneurium: thin layer of loose connective tissue around individual nerve fibers.

    • Blood vessels traverse the epineurium to supply nerves.

Epithelial Tissue: Structure and Functions

  • General function of epithelia

    • Sheets of cells covering body surfaces and cavities.

    • Protect, sense, secrete, absorb, excrete, diffuse, cleanse, and reduce friction.

  • Epithelia and glands

    • Epithelial tissue forms protective coverings and glandular secretory surfaces.

    • Glands can be exocrine (ducts to surfaces) or endocrine (ductless, secretions into interstitial space and blood).

  • Glands overview

    • Exocrine glands secrete onto body surfaces or cavities (e.g., goblet cells secreting mucus in the gut).

    • Endocrine glands secrete hormones into the bloodstream.

    • Pancreas as an example: endocrine portion secretes insulin and glucagon; exocrine portion secretes digestive enzymes.

  • Epithelial cell classification by shape and layers

    • Shapes: Squamous (flat), Cuboidal (cube-like), Columnar (tall and rod-like).

    • Layers: Simple (one layer) or Stratified (multiple layers).

    • Stratified epithelia are named according to the shape of the apical cells.

  • Specific epithelia: shapes, layers, functions, and locations

    • Simple squamous

    • Shape: flat; all cells rest on basement membrane.

    • Function: diffusion of gases and nutrients; reduce friction.

    • Location: lining body cavities, capillaries, alveoli.

    • Simple cuboidal

    • Shape: cube-like; central nuclei.

    • Function: secretion and absorption.

    • Location: glands, kidney tubules, ducts; germinal epithelium in ovaries and testes.

    • Simple columnar

    • Shape: elongated; basally located nuclei.

    • Function: secretion and absorption; mucus production via goblet cells (e.g., duodenum).

    • Location: lining of stomach and intestines; some sensory roles (nose, ears, taste buds).

    • Pseudostratified columnar epithelium

    • Appears multilayered due to nucleus position, but is a single layer.

    • Function: secretion and mucus.

    • Location: ducts of large glands; ciliated variety lines trachea and most of the upper respiratory tract; vas deferens.

    • Stratified cuboidal epithelium

    • Multi-layered; protects ducts of sweat glands and the male urethra.

    • Stratified columnar epithelium

    • Several cell layers; protection and secretion.

    • Location: small amount in the male urethra and large ducts of some glands.

    • Stratified squamous epithelium

    • Location: vagina (and skin in other contexts).

    • Transitional epithelium

    • Resembles both stratified squamous and stratified cuboidal; basal cells are cuboidal/columnar; surface cells dome-shaped or squamous-like depending on stretch.

    • Function: stretches readily; permits distension of urinary organs by contained urine.

    • Location: lines ureters, bladder, and part of the urethra.

  • Basal surface and basement membrane

    • Basal lamina: basal surface of epithelial cells; acts as filter and base for regeneration.

    • Basal lamina plus underlying reticular fibers form the basement membrane.

Glands: Exocrine and Endocrine

  • Exocrine glands

    • Retain ducts to body surfaces; goblet cells are unicellular exocrine glands secreting mucus.

    • Multicellular exocrine glands are classified by duct structure (simple vs compound) and by secretory unit structure (tubular, alveolar/acinar, or tubuloalveolar).

  • Endocrine glands

    • Do not have ducts (ductless); secrete hormones into interstitial spaces and then into capillaries.

  • Pancreas example

    • Has both endocrine (insulin, glucagon) and exocrine (digestive enzymes) parts.

Modes of Secretion in Exocrine Glands

  • Merocrine secretion

    • Secretory products are released by membrane-bound secretory vesicles that fuse with the apical membrane.

    • Most glands use merocrine secretion.

  • Apocrine secretion

    • Apical portions of cells are pinched off with the secretory product.

    • Secretions may contain membrane components; example: mammary glands.

  • Holocrine secretion

    • Secretory cells are released and break apart; the entire cell contents become the secretory product.

    • Produces more complex secretions; examples include some sweat glands in axillae, pubic areas, and around areolae, and sebaceous glands.

Cell Junctions and Epithelial Surface Specializations

  • Lateral cell surface features

    • Adhesion proteins link plasma membranes of adjacent cells.

    • Contours of adjacent cell membranes contribute to cell–cell adhesion.

    • Special junctions: Tight junctions, Adhering junctions, Desmosomes, Gap junctions.

  • Tight junctions (zonula occludens)

    • Formed by claudin and occludin proteins; connect cytoskeletons of neighboring cells.

    • Functions:

    • Hold cells together and seal intercellular space.

    • Block movement of membrane proteins between apical and basolateral surfaces; preserve transcellular transport.

    • Prevent passage of molecules/ions through space between cells.

  • Desmosomes

    • Two disc-like plaques connected across intercellular space.

    • Plaques are linked by cadherin proteins; intermediate filaments insert into plaques from the cytoplasmic side.

  • Gap junctions

    • Direct passageways between adjacent cells.

    • Allow small molecules to move through cytoplasmic bridges.

    • Connected by protein channels forming hollow cylinders.

Basal Epithelial Surface and Basement Membrane

  • Basal lamina

    • A proteinaceous sheet on which epithelial cells lie.

    • Acts as a filter and a foundation for regenerating epithelial cells.

  • Basement membrane

    • Formed by basal lamina plus underlying reticular fibers.

    • Provides structural support and selective barrier.

Connective Tissue

  • Core function and origin

    • Connects, supports, and binds other tissues.

    • Cells are scattered within an extracellular matrix.

    • Derived from the mesoderm.

  • Loose connective tissue

    • Contains collagenous, elastic, and reticular fibers.

  • Fibrous connective tissue

    • Includes dense, fibrous tissues (e.g., tendons, ligaments).

  • Specialized connective tissues

    • Adipose, Cartilage, Bone.

Connective Tissue Diseases (examples mentioned)

  • Marfan syndrome

    • Inheritance: autosomal dominant.

    • Clinical features: disproportionately long limbs, long fingers, tall stature.

    • Cardiovascular risk: abnormalities affecting heart valves and the aorta.

    • Pathogenesis: defect in the gene on chromosome 15 that encodes fibrillin-1, a glycoprotein essential for elastic fibers in connective tissue.

  • Ehlers-Danlos syndrome

    • Group of genetic disorders affecting collagen synthesis (notably type I and III).

    • Implications: connective tissue fragility with hyperextensible skin and joints (varies by subtype).

Connections to Principles and Real-World Relevance

  • Structure–function relationships

    • Tissue type determines function (e.g., epithelial layers for protection/absorption, connective tissue ECM for strength and support, nervous tissue for rapid signaling).

  • Developmental origins

    • Muscle, nerve, epithelial, and connective tissues arise from distinct germ layers and lineages, influencing their organization and repair.

  • Clinical relevance

    • Defects in structural proteins (e.g., fibrillin-1, collagen I/III) lead to systemic disorders with cardiovascular, skeletal, and skin manifestations.

  • Practical implications

    • Understanding epithelial junctions helps explain barrier function, selective permeability, and tumor metastasis pathways.

  • Ethical/philosophical considerations (brief)

    • Genetic disorders like Marfan and Ehlers-Danlos raise considerations about screening, genetic counseling, and the social impact of inherited conditions.

Quick reference: key terms and concepts

  • Epithelial tissue: protective coverings and glandular linings with eight functional roles (protection, sensation, secretion, absorption, excretion, diffusion, cleaning, reducing friction).

  • Gland types: Exocrine (ducts) vs Endocrine (ductless).

  • Secretory modes: Merocrine, Apocrine, Holocrine.

  • Junctions: Tight, Desmosomes, Adhering junctions, Gap junctions.

  • Basal lamina and basement membrane: foundational structures for epithelia.

  • Connective tissue: ECM-centric tissue types with roles in support and movement; principal diseases highlighted involve collagen and elastic fibers.