Tissues and Epithelial Tissue

epithelial

• Made of tightly packed polyhedral cells

• Adheres to a thin layer of ECM

• Forms sheets covering surfaces and lining cavities

■ Protection — e.g., epidermis

■ Absorption — e.g., intestinal lining

■ Secretion — e.g., glandular cells

■ Myoepithelial cells (contractile)

■ Sensory cells (e.g., taste buds, olfactory epithelium)

simple squamous

• Lining of vessels (endothelium); Serous lining of cavities: pericardium, pleura, peritoneum (mesothelium)

• Facilitates the movement of the viscera (mesothelium), active transport by pinocytosis (mesothelium and endothelium), secretion of biologically active

• blood vessel, bowman’s capsule, alveoli

simple cuboidal

• Covering the ovary, thyroid

• Covering, secretion

• kidney tubules

• thyroid follicles

simple columnar

• Lining of intestine, gallbladder

• Protection, lubrication, absorption, secretion

• stomach

• gall bladder

stratified squamous keratinized (dry)

• Epidermis

• Protection; prevents water loss

stratified Squamous nonkeratinized (moist)

• Mouth, esophagus, larynx, vagina, anal canal

• Protection, secretion; prevents water loss

stratified Cuboidal

• Sweat glands, developing ovarian follicles

• Protection, secretion

Transitional

• Bladder, ureters, renal calyces

• Protection, distensibility

stratified Columnar

• Conjunctiva

• Protection

PSEUDOSTRATIFIED

• layers of cells with nuclei at different levels; not all cells reach surface but all adhere to basal lamina

• Lining of trachea, bronchi, nasal cavity

• Protection, secretion; cilia-mediated transport of particles trapped in mucus out of the air passages

CHRONIC VITAMIN A DEFICIENCY

• Epithelial tissues of the type found in the bronchi and urinary bladder may gradually be replaced by stratified squamous epithelium.

CHRONIC BRONCHITIS

• Common among habitual smokers

• Number of goblet cells in the lining of airways in the lungs often increases greatly

• Leads to excessive mucus production in areas where there are too few ciliated cells for its rapid removal and contributes to obstruction of the airways

• Ciliated pseudostratified epithelium lining the bronchi of smokers can also be transformed into stratified squamous epithelium by metaplasia

celiac disease

• Also known as gluten-sensitive enteropathy or sprue

• One of the first pathologic changes is loss of the microvilli brush border of the absorptive cells

• Caused by an immune reaction against the wheat protein gluten during digestion, which produces:

• Diffuse enteritis (intestinal inflammation)

• Changes to the epithelial cells leading to malabsorption

• Pathologic changes in the intestinal wall

• Malabsorption problems and structural changes are reversible when gluten is removed from the diet

connective

• Provides structural and metabolic support

• Connects tissues and organs

• Contains interstitial fluid for nutrient or waste exchange

• Main component ⟶ Extracellular Matrix

• Less cellular than other types of tissue

muscle

• CONTRACTILITY

• Movement of the body, organs, and blood.

• Most muscle cells originate from the mesoderm and differentiate by elongating and producing actin and myosin.

neurons

glial cells

anatomical divisions

Cell Types in Nervous Tissue:

Neurons

⚬ form extensive networks

⚬ information processing and response generation.

Glial cells

• support, protect, and nourish neurons, and play roles in neural activity and immune defense.

• Central Nervous System (CNS)

• Peripheral Nervous System (PNS)

Anatomical Divisions:

nervous

• Processes and integrates sensory information to maintain homeostasis

• Regulates behavioral patterns

Action potential

• Neurons are excitable cells, capable of quickly reversing their ionic gradient across the membrane when stimulated.

• spreads along the neuron's membrane and transmits signals over long distances.

epithelial tissue

• Shapes vary and are related to function.

• Nuclear shape corresponds to cell shape

• Nuclei visibility helps determine:

  • Cell shape, cell density, and number of layers

• Lack blood vessels and receive nutrients and oxygen from underlying connective tissue.

lamina propria

papillae

polarity

KEY FEATURES OF EPITHELIAL CELLS

LAMINA PROPRIA

connective tissue beneath epithelial linings of organs

PAPILLAE

small projections from connective tissue to increase contact area

POLARITY

• Basal

• Apical

• Lateral

BASEMENT MEMBRANE

• A thin extracellular layer

• Acts as a semipermeable filter

• Filter and structural support.

• Maintain cell polarity

• Scaffold for repair and regeneration

• Facilitates endocytosis, signal transduction, and cell migration.

Basal lamina

• Type IV collagen

• Laminin: glycoproteins that attach to integrins

• Nidogen and perlecan: cross-link laminin to collagen

Reticular lamina

• Type III collagen.

• Connected to the basal lamina by type VII collagen

TIGHT JUNCTIONS

• Most apical cell junctions

• “Zonula” refers to their band-like structure

• Create an intercellular seal, forcing substances to cross the cell

• Regulate permeability and act as membrane fences

• Seen as a membrane fusion under TEM

ADHERENS JUNCTIONS

• Below tight junctions

• Anchoring junctions

• Cadherins ⟶ cell adhesion

  • Require calcium (Ca2+) to bind to cadherins of adjacent cells.

  • Bind to catenins, which connect to actin filaments

DESMOSOMES

• Anchoring junctions

•“spot-welds”, providing STRONG cell-to-cell adhesion.

• Composed of large cadherin family proteins

• Attach to cytokeratin filaments (tonofilaments)

• Mechanical strength and maintain structural integrity

HEMIDESMOSOMES

• Anchoring junctions are located on the basal surface

• Use integrins + cytokeratin (not cadherins)

• Firm attachment of epithelial cells

GAP JUNCTION

• Intercellular communication

• Made up of connexins, which form connexons

  • When two cells join, connexons from each membrane align to form channels connecting the cells.

• Allow exchange of ions/small molecules (<1.5 nm)

• Rapid signal transmission, supporting coordinated cell activity

MICROVILLI

• Densely packed in absorptive cells, forming a visible brush border or striated border.

• Increase surface area

• Each microvillus contains a core of bundled actin filaments..

• Actin-myosin interactions help to maintain optimal absorption conditions.

• Host channels, receptors, and enzymes essential for nutrient uptake.

CELIAC DISEASE

microvilli

STEREOCILIA

• Primarily found on absorptive epithelial cells in the male reproductive tract.

• Increase surface area for absorption, similar to microvilli.

• Inner ear serve a sensory, motion-detecting function.

• Contain actin filaments and actin-binding proteins, like microvilli.

CILIA

• Long, motile apical structures found on many epithelial cells.

• Contain microtubules, not microfilaments.

• Primary cilia:.

⚬ Non-motile, enriched with receptors and signal transduction

⚬ complexes for sensing light, odor, motion, or fluid flow

• Core structure is the axoneme, with a 9 + 2 arrangement

clia Basal bodies:

• Located just beneath the cell membrane.

• Structurally similar to centrioles (contain microtubule triplets).

• Anchor cilia to the cytoskeleton.

cilia flagella

• Found in sperm cells; structurally similar to cilia.

• Move via the same axonemal mechanism.

SECRETORY EPITHELIAL CELLS AND GLANDS

SECRETORY EPITHELIAL CELLS AND GLANDS

GLANDS

mixed-function epithelia or form specialized organs

Unicellular glands

scattered secretory cells (i.e Goblet cells)

exocrine

endocrine

GLANDS

Exocrine glands

• retain a duct connecting to the surface.

• The connection forming the tubular ducts lined with epithelium that deliver the secreted material where it is used

Endocrine glands

• lose connection to the surface; secrete hormones into nearby capillaries.

• Thin-walled blood vessels (capillaries) adjacent to endocrine cells absorb their secreted hormone products for transport in blood to target cells throughout the body.

SEROUS

MUCOUS

SEROMUCINOUS

glands TYPES OF SECRETION

EXOCRINE GLANDS

MEROCRINE SECRETION

• Serous Cells

• Secrete mostly non-glycosylated proteins

• Have well-developed RER and Golgi.

• Filled with secretory granules

• Found in pancreas and parotid glands.

EXOCRINE GLANDS

MEROCRINE SECRETION

• mucous cells

• Secrete glycosylated proteins (mucins).

• Also have RER and Golgi.

• Mucins hydrate to form mucus upon release.

• Poorly stain with eosin due to loss of mucins in processing.

• PAS stain highlights mucinogen granules.

• Example: goblet cells.

EXOCRINE GLANDS

MEROCRINE SECRETION

• mixed (seromucous) glands

Contain both serous acini and mucous tubules

Secrete a mixture of digestive enzymes and mucus.

EXOCRINE GLANDS

MYOEPITHELIAL CELLS

• Sweat, lacrimal, salivary, and mammary glands.

• Located at the base of secretory cells.

• Resemble octopus arms around acini.

• Contain actin and myosin; connected by gap junctions and desmosomes.

• Contract to help expel secretions into ducts.

ENDOCRINE

• Lack myoepithelial cells.

• Specialized for:

  • Protein hormone synthesis → prominent RER → secreted via exocytosis.

  • Steroid hormone synthesis → abundant SER → secreted via diffusion.

• Hormones transported via bloodstream to distant targets.

ion transport

transcellular

transcytosis

TRANSPORT ACROSS EPITHELIA

RENEWAL OF EPITHELIAL CELLS

• labile

• their cells are continuously renewed via mitosis and stem cells

• Stem cells and mitosis occur in the basal layer

• Rapidly repair and replace damaged or apoptotic cells

merocrine gland

• Exocytosis of secretory vesicles (e.g., proteins, glycoproteins).

• Most common method of protein or glycoprotein secretion

• Involves typical exocytosis from membrane-bound vesicles or secretory granules.

holocrine gland

• Entire cell disintegrates to release product and debris

• Cells accumulate product continuously as they enlarge and undergo terminal differentiation, culminating in complete cell disruption that releases the product and cell debris into the gland’s lumen.

• Ex: sebaceous glands producing lipid-rich material in

  skin

apocrine gland

• Apical part of the cell pinches off with product

• Product accumulates at the cells’ apical ends, portions of which are then pinched off to release the product surrounded by a small amount of cytoplasm and cell membrane.

• Secretory products are generally in larger vesicles, are not formed within multivesicular bodies, and are released into ducts.

• Epithelial cells of mammary glands use merocrine secretion to add proteins to milk, but their release of lipid droplets represents the best-studied example of blank

ACNE

• The holocrine sebaceous glands are the primary structure involved in the common form.

• Excessive holocrine secretion of sebum and keratin triggered by the surge of the steroid hormone testosterone that occurs in both genders at puberty frequently leads to blocked ducts within the gland. Activity of the normal commensal skin bacterium Propionibacterium acnes within the blocked duct commonly produces localized inflammation.

ION TRANSPORT

• Actively transport certain ions against concentration and electrical potential gradients

TRANSCELLULAR TRANSPORT

• Some epithelial cells specialize in the transfer of ions (by ion pumps) and water (via the membrane channels called aquaporins) in either direction across the epithelium, the process is known as transcellular transport

• Apical tight junctions prevent paracellular diffusion or backflow between the cells.

TRANSCYTOSIS

• Occurs between the apical and basolateral membranes domains in cells of simple cuboidal and columnar epithelia and is important in many physiologic processes.

CARCINOMA

Malignant tumors of epithelial origin

ADENOCARCINOMA

• Malignant tumors derived from glandular epithelial tissue

• Most common tumors in adults after age 45.

NEOPLASIA

• Epithelial cells are prone to abnormal growth or dysplasia, which can progress to precancerous growth

• Early neoplastic growth is often reversible and does not always result in cancer

METAPLASIA

• Epithelial tissue may undergo transformation into another type in another reversible process

• In heavy cigarette smokers, the ciliated pseudostratified epithelium lining the bronchi can be transformed into stratified squamous epithelium.