2: Epithelial & Connective Tissue, Integumentary System

epithelium (epithelia)

covering + lining

• covers exposed body surfaces + protects from external environment

  • ex. epidermis

• lines internal body cavities, hollow organs, organ passageways for absorption/secretion

  • ex. GI + respiratory tract lining
    
    

glandular

• collections of ____ cells that secrete substances into ducts or bloodstream

  • ex. sweat glands, sebaceous glands, salivary glands, pancreatic glands

covering/lining epithelium

type of epithelium

• has adjacent cells attached via cell junctions

• has 3 cell surfaces

  • lateral - side surface

  • apical - free surface

  • basal - bottom surface

• has basement membrane, separating from connective tissue

  • basal lamina (facing upwards)

  • reticular lamina (facing downwards)

• avascular - relies on diffusion from underlying loose connective tissue

cell junctions

composed of specialized transmembrane proteins; 3 types:

• tight

• gap

• anchoring

epithelial membrane

epithelial layer + underlying connective tissue layer

• consists of:

  • epithelial layer

  • basement membrane

  • connective tissue layer

• types:

  • mucous

  • serous

  • cutaneous (skin)

basement membrane

barrier separating epithelial layer from underlying connective tissue layer

• top layer facing epithelium = basal lamina

• bottom layer facing connective tissue = reticular lamina

• layer to which epithelial cells can attach + anchor

tight junctions

bind cells together to form leak-proof sheets

• happens closer to apical surface

• found on lateral surface of cells

gap junctions

allow communication b/w adjacent cells via open pore

anchoring junctions

prevent cells from being pulled apart + anchors cells to underlying membrane

• functions via 3 different proteins

  • desmosome

  • adherens

  • hemidesmosome

desmosome

forms anchoring junctions

• transmembrane proteins attach like velcro to e/o, connecting cells

• pass through PM

• proteins further anchored to intermediate filaments inside cells

adherens

forms anchoring junctions

• come all around cell + interact w/ e/o to keep cells together

hemidesmosome

forms anchoring junctions

• looks like half of a desmosome

• found on basal surface of cells

• anchors cells to basement membrane

• Velcro-like pieces reach down into basement membrane

• protein itself is anchored to cell

simple epithelium

1 layer of epithelial cells

• designed for exchange of materials

• all cells anchored to basement membrane

• classified as:

  • squamous

  • cuboidal

  • columnar (ciliated or non-ciliated)

  • pseudo stratified columnar (usually ciliated)

pseudostratified epithelium

1 layer of epithelial cells but looks like multiple

• designed for exchange of materials + protective barrier

• nuclei are scattered + cells are bunched together

stratified epithelium

multiple layers of epithelial cells

• designed for protective barrier

• only deepest layer anchors to basement membrane

• classified as:

  • squamous

  • cuboidal (uncommon)

  • columnar (uncommon)

  • transitional

squamous epithelium

flat-shaped epithelial cells

• designed for rapid exchange of materials

cuboidal epithelium

cube-shaped epithelial cells

• designed for production, secretion, absorption of materials

columnar epithelium

column-shaped epithelial cells

• designed for production, secretion, absorption of materials

simple squamous

single layer of flat cells

• all cells in contact w/ basement membrane

• ex. in lung alveoli → permits diffusion of gases b/w alveoli + pulmonary capillaries

simple cuboidal

single layer of cube-shaped cells

• nuclei centrally located

• most of digestive system lined w/ this

• ex. pancreatic duct lining → secrete H2O + bicarbonate

simple columnar

single layer of rectangular-shaped cells

• can be non-ciliated (small intestine epithelium) or ciliated (oviduct epithelium)

• nuclei located near basal surface of cell

• often features goblet cells

goblet cells

modified columnar cells that produce mucus

• mucus forms layer on apical surface

• lubricates, moistens, protects epithelium

pseudostratified columnar

single layer of cells that appear stratified

• nuclei located at different levels

• all cells attached to basement membrane BUT not all reach apical surface

• largely found in respiratory tract

  • ex. lining trachea → prepares air for gas exchange in lungs

stratified squamous

• layers closes to apical surface = squamous

• deeper cell layers = cuboidal or columnar

• can be keratinized or non-keratinized

  • keratinized = outside of body = protection

  • non-keratinized = inside of body (still has keratin, just not as much)

transitional epithelium

type of stratified epithelium

• apical layer changes shape depending on organ state

  • stretched = squamous (flat)

  • relaxed = cuboidal (square)

• ex. epithelium lining urinary bladder, ureter

gland

single cell or group of epithelial cells that:

• secrete substances into ducts or onto surface = exocrine (outside/indirect)

• secrete substances directly into bloodstream = endocrine (inside/direct)

exocrine pancreas

aka pancreatic acini

• secrete digestive enzymes into ductal system → empties to 2nd part of duodenum

endocrine pancreas

aka islet of Langerhans

• collection of cells that secrete hormones that regulate blood glucose homeostasis

  • hormones deposited into interstitium + enter blood vessels directly

  • main cells:

    • alpha = secrete GLUCAGON → ⬆ blood glucose

    • beta = secrete INSULIN → ⬇ blood glucose

connective tissue

functions in structural support + packaging

• abundant + widely distributed throughout body

• greater space b/w cells compared to epithelial tissue

• components:

  • several cell types → subset responsible for maintaining ECM

  • varying amounts ECM → determines quality/consistency

    • protein fibers

    • ground substance

  • does not occur on free surface

  • vascular + innervated except for cartilage

connective tissue classification

1. connective tissues

   1. connective tissue proper → packaging, repair, protection

      1. loose

      2. dense

   2. fluid connective tissue → transportation

      1. blood

      2. lymph

   3. supporting connective tissue → structure, framework

      1. cartilage

      2. bone

connective tissue cells

type caries according to connective tissue type

• fibroblasts

• macrophages

• plasma cells

• mast cells

• adipocytes

fibroblasts

cell found in connective tissue

• numerous - in many connective tissue types

• synthesize + secrete:

  • proteins → assembled into long fibres (ex. collagen)

  • ECM ground substance components (ex. proteoglycans)

adipocytes

cell found in connective tissue

• triglycerides stored in large fat droplet

• nucleus + organelles pushed to side

extracellular matrix (ECM)

material b/w cells in connective tissues

• composition varies b/w different tissues

  • ex. bone tissue → calcium phosphate (hydroxyapatite)

  • ex. tendon/ligament → tightly packed collagen fibres

• composed of:

  • protein fibers

    • collagen

    • elastic

    • reticular

  • ground substance

    • H2O + large organic molecules

    • hydroxyapatite (mineral deposition) in bone

collagen fibers

found in ECM of connective tissue

• most abundant protein in body (~25%)

  • type I and II most common

• synthesized by fibroblasts

• form long fibres → arranged in bundles

• provide tensile strength + flexibility

scurvy

condition due to vitamin C deficiency

• vitamin C required for collagen synthesis

• deficiency affects areas w/ high collagen turnover

• causes teeth loss → periodontal ligament holds teeth in place

• causes poor wound healing → collagen needed to form scars

elastic fibers

found in ECM of connective tissue

• thin, wavy fibres consisting of protein elastin + glycoprotein fibrillar

• provides elasticity

  • can be stretched + returns to normal when relaxed

reticular fibers

found in ECM of connective tissue

• thin fibres (type III collagen) that form interwoven, branched network

• synthesized by reticular cells (fibroblast)

• forms framework (storma) → strengthens, supports soft organs, ex. liver, spleen

ground substance

non-fibrous material b/w cells + protein fibres in connective tissue

• contains

  • water

  • proteoglycans

    • complexes of polysaccharides (ex. glycosaminoglycans - GAG) + proteins

    • have (-) charge → attracts cations, ie. Na+

      • Na+ pulls H2O w/ it

      • proteoglycans trap + hold H2O in matrix

    • forms gel-like, viscous substance

• provides resistance to compressive forces

connective tissue proper

classified based on [protein fibres] relative to ground substance

1. loose

2. dense

loose connective tissue

type of connective tissue proper

• loosely packed = ⬇ protein fibers + ⬆ ground substance

• functions in protection, support, insulation, energy storage

• types:

  • areolar

  • adipose

  • reticular

dense connective tissue

type of connective tissue proper

• densely packed = ⬆ protein fibers + ⬇ ground substance

• functions in structure, support, withstanding tensile strength

• types:

  • regular

  • irregular

  • elastic

areolar connective tissue

type of loose connective tissue proper

• widely distributed throughout body

• provides strength + support

• has all 3 protein fibres

  • collagen

  • elastic

  • reticular

• viscous ground substance

• ex. connective tissue underlying epithelium

  • lamina propria of mucous membranes

  • superficial dermis of skin

adipose tissue

type of loose connective tissue proper

• found within areolar connective tissue

• comprised of adipocytes

• highly vascularized

  • needs vascular network to get energy molecules out from stored triglycerides

• ex. surrounding organs → protection, thermal insulation, energy storage

reticular connective tissue

type of loose connective tissue proper

• contains network of interwoven reticular fibres

• ex. in lymph nodes → filters lymph + provides supporting framework

dense regular connective tissue

type of dense connective tissue proper

• bundles of collagen fibres arranged in parallel

• flattened fibroblasts

• sparse ground substance

• able to withstand tensile/pulling stress in one direction

• ex. tendons, ligaments

dense irregular connective tissue

type of dense connective tissue proper

• sheets of closely packed, irregularly arranged collagen fiber bundles

• able to within stand pulling forces in many directions

• ex. fibrous sheets surrounding organs, fibrous joint capsule, deep dermis of skin

elastic connective tissue

type of dense connective tissue proper

• densely packed elastic fibres (appear wavy)

• allows stretching + recoil back to original shape

• ex. aortic walls → stretching when heart contracts + recoil when heart relaxes

membrane

flat sheets of tissue covering body surface/structure or lining body cavity/hollow organ

• 2 types:

  • epithelial

  • connective tissue

connective tissue membrane

consists of connective tissue layer

• no epithelial layer, ex. synovial membrane

mucous membrane (mucosa)

type of epithelial membrane

• lines hollow passageways in body that are directly open to external environment

  • ex. respiratory tract, GI tract

• epithelial layer:

  • type of epithelium varies w/ location

  • basement membrane

• connective tissue layer

  • lamina propria = areolar connective tissue layer

respiratory mucosa

type of epithelial mucous membrane

• epithelial layer

  • pseudostratified ciliated columnar

    • cells anchored to basement membrane

  • goblet cells secrete mucous

    • precent apical surface from drying

    • traps fine particulates in incoming air

• connective tissue layer

  • lamina propria = areolar connective tissue

  • has blood vessels, lymphatics, nerves

  • support/anchor for epithelial layer

  • protection for underlying structures

serous membranes (serosa)

type of epithelial membrane

• lines internal body cavities + covers visceral organs within body cavities

• epithelial layer

  • simple squamous epithelium = mesothelium

  • secretion of serous fluid into cavity → ⬇ friction between/w organs, prevents adhesion

  • basement membrane

• connective tissue layer

  • areolar connective tissue

• two layers w/ cavity in b/w:

  • parietal

  • visceral

parietal layer

type of serous membrane (epithelial membrane)

• outer layer

• adheres to body cavity wall

visceral layer

type of serous membrane (epithelial membrane)

• inner layer

• adheres to organ surface

layers of skin

• epidermis

• dermis

• hypodermis

epidermis

superficial epithelial layer + basement membrane

• avascular keratinized stratified squamous epithelium

  • multiple layers of flattened cells w/ lots of keratin = protective layer

• thick + thin skin

  • thick → has extra layer = stratum corner

• cell types:

  • keratinocytes

  • melanocytes

  • Langerhans cells

  • Merkel cells

keratinocytes

most abundant cells in epidermis (90%)

• produces:

  • keratin

    • tough, fibrous protein that fills cell

  • lipid-rich lamellar granules

    • contains water repellent/sealant released into extracellular environment

• protects → abrasion, heat, injury, microorganism entry

• prevent excessive water loss

fungal infections

• affects hair, nails + top layer of skin

• fungi survive by feeding on keratin in these areas

  • make enzymes that digest keratin

melanocytes

comprises ~8% of epidermal cells

• found in deepest layer of epidermis = stratum basal

• produces melanin

  • black-brown/red-yellow pigment

  • contributes to skin colour

  • absorbs UV light + prevents DNA damage in skin cells

• have cytoplasmic extensions → deliver melanin to keratinocytes

• melanosomes cluster above nucleus toward skin surface → protect DNA

  • UV light = ⬆ melanocyte enzymatic activity

melanocytes nevus (mole)

benign tumour of melanocytes

melanoma

• superficial spreading → treated by excision

• can progress to vertical growth phase into dermis → metastatic potential

  • grows past basement membrane into dermis = cancerous

• ABCDE mnemonic

  • asymmetry

  • border irregularities

  • colour variation

  • diameter

  • evolution

langerhans cells (dendritic cells)

type of macrophage

• produced in bone marrow → migrates to epidermis (stratum spinosum)

• engulfs antigens that invade epidermis

  • travel via lymphatic vessels to lymph nodes

  • present antigens to lymphocytes = immune response

merkel cells

• least numerous epidermal cell

• in stratum basale → deepest epidermis layer

• cells contact specialized flattened sensory nerve endings → detect light touch sensations

epidermis cell layers

4 (thin) or 5 (thick) layers w/ keratinocytes in various stages of development:

• corneum

• lucidum (only in thick)

• granulosum

• spinosum

• basale

stratum basale

deepest epidermal layer w/ only single row of cells

• in contact w/ basement membrane

• contains:

  • Merkel cells

  • melanocytes

  • basal (stem) cells

    • divide via mitosis

    • 1 daughter cell pushed up to replace cells shed at apical surface

    • other daughter cell remains as stem cell

    • dysplasia of basal cells → basal cell carcinoma

psoriasis

autoimmune condition that stimulates rapid basal cell division = thickening of skin (plaques)

• cells shed prematurely → flaky, silver scales at surface

• appears red due to underlying inflammation

• chronic + recurring

stratum spinosum

8-10 layers of keratinocytes produced by stem cell vision in basal layer

• dysplasia of keratinocytes → squamous cell carcinoma

• packed closely + anchored together by desmosomes

• keratinocytes producing keratin

• contains:

  • melanocyte processes

  • Langerhans cells

stratum granulosum

3-5 layers of flattened keratinocytes

• keratin synthesis complete → cells release lipid-rich granules

• cells start to die = apoptosis

  • organelles degenerate + flatten

stratum lucidum

3-5 layers of flattened, dead keratinocytes

• cells filled w/ large amounts of keratin

• only in thick skin, eg. fingertips, palms, soles

stratum corneum

15 (face) to 100 (soles) layers of dead keratinocytes

• exposed layer → facing external environment

• cells continuously shed + replaced by underlying layer

• takes 21-30 days for cell to move from stratum basal to top layer

callus

thickening of stratum corneum

• form in response to repeated friction (protective)

atopic dermatitis (eczema)

chronic inflammatory skin condition characterized by dry skin, redness, pruritis (itching)

• weakness in skin barrier → underlying tissues susceptible to external irritants/allergens

dermis

connective tissue layer in cutaneous membrane

• 2 layers:

  • papillary layer

  • reticular layer

• contains:

  • blood vessels

  • sensory receptors (nerve endings)

    • nociceptors = pain/itch

    • thermoreceptors = temperature

    • mechanoreceptors = pressure/touch, vibration, stretch

  • sweat (sudoriferous) + sebaceous (oil) glands

  • hair follicles

papillary layer

layer of dermis → superficial 1/5

• areolar connective tissue

• forms fingerlike structures = dermal papillae

  • extend upwards into epidermis → ⬆ SA for attachment to epidermis for diffusion from capillary loops

• contains:

  • capillary loops → nourish epidermis

  • Meissner corpuscles → light touch encapsulated receptors

reticular layer

layer of dermis → deep 4/5

• dense irregular connective tissue

  • ⬆ collagen + elastic fibers = support, strength, elasticity

• contains:

  • vascular network → supplies capillary loops in papillary region

  • sensory receptors

    • Ruffini corpuscles = stretch

    • Pacinian corpuscles = pressure/vibration

  • sweat (sudoriferous) + sebaceous (oil) glands

  • hair follicles

hypodermis

subcutaneous layer (below skin) - NOT part of integument

• areolar connective tissue w/ adipose tissue deposits

• anchors skin to deep tissues