#12: Cell-cell and cell-matrix interactions

tissue

a group of similar cells, along w/ their extracellular matrix, that are organized together to perform a specific, specialized function

• structural level of organization, operating between individual cells and complete organs

4 main types of tissues in animals

• epithelial: covering

• connective: supporting

• muscle: movement

• nervous: control

organ

a specialized structure composed of different types of tissues that work together to perform specific functions essential for survival and health of a multicellular organism

extracellular matrix

material that cells secrete around themselves

• large component of connective tissue

• gives supportive tissues (such as bones) their strength

cell junctions that link

the human body is composed of over ___ cells

• there are more than ____ types of cells w/ ____ morphologies and functions

30 trillion

• 200; various

______ cells are considered the MOST abundant cell type in the human body

epithelial → estimated to make up a significant portion of the body’s cells — ~37.1% of total cell mass in skin alone!

how many unique cell types are there in the body?

142

ALL substances that ____ or _____ an organ MUST cross the epithelial tissue first

enter, leave

ex of epithelial tissue in body

• epidermis

• lining of intestines

• lining of respiratory tract

• lining of abdominal cavity

• sweat glands

most of the cells in multicellular organisms are organized into cooperative assemblies called ?

tissues!

tissues are organized into ?

organs! (heart, lung, brain, kidney, etc)

what are tissues composed of?

• cells — w/ their internal framework of cytoskeletal filaments

• extracellular matrix (LARGE component of CT) = material that cells secrete around themselves → gives supportive tissues (such as bone) their strength

what gives supportive tissues (such as bone) their strength?

extracellular matrix!

can cells attach to one another directly?

YES — cell junctions link cells together in the flexible epithelial tissues of animals

cell junctions transmit forces either from the cytoskeleton of one cell to ____ or to _____

• to the cytoskeleton of the next cell

• to the extracellular (EC) matrix

how are cells in tissues held together?

• cell-cell adhesions

• EC matrix

• both

epithelial cells

among the most abundant cells

• cover skin

• line organs, body cavities, and blood vessels

• HIGHLY specialized histologic features are CRITICAL for their physiological functions in diff organs

fibroblasts

type of cell that contributes to the formation of CT (connective tissue)

• secretes collagen and other proteins into the EC matrix

are mechanical stresses on tissues handled uniformly?

NO — handled DIFFERENTLY depending on tissue type

epithelial tissue

• cells are ____ bound together into sheets

• the cytoskeletons of the cells are linked from cell to cell by ?

• EC matrix may be _____, and the cells are directly joined to one another and carry the mechanical load THEMSELVES

• tightly

• anchoring junctions

• LIMITED

what attachments bond epithelial tissue to the CT beneath it

cell-matrix

basal lamina

the thin mat of EC matrix proteins (primarily laminin) that separates epithelial sheets from CT

• provides area for anchorage for epithelial tissue

• aka basement membrane

CT (connective tissue) ex

• bone or tendon

• submucosa in gut

CT is formed from ______ produced by cells that are distributed SPARSELY in the matrix

an EC matrix (primarily collagen and proteoglycan)

what bears most of the mechanical stress to which the tissue is subjected in CT?

EC MATRIX!! (rather than the cells)

are direct attachments between one cell and another in CT common?

NO — relatively rare, but cells have important attachments to the matrix through integrin receptors

in CT, cell cells have important attachments to the matrix through ?

integrin receptors

which cytoskeleton?

• smallest, aka microfilaments

• 2 globular protein strands that are intertwined

• determine the shape of the cell’s surface and are necessary for locomotion

• abundant in muscle cells

• resists tension (pulling forces); can stretch w/out breaking (like a rubber band)

actin filaments

which cytoskeleton?

• determine the positions of membrane-enclosed organelles and direct intracellular vesicle transport

• hollow structure built from 13 parallel strands

• resist compression → maintains cell shape under pressure

• pull apart replicated chromosomes during cell division

• structural elements of flagella and cilia

microtubules → highways for vesicle transport

which cytoskeleton?

• bear tension and maintain cell shape — NO role in movement

• anchor nucleus and other organelles in place

• diverse class w/ several types of fibrous proteins

• NOT found in cytoplasm of cells w/ cell walls

intermediate filaments

• side to side and end on end interactions

• VERY flexible

• resist pulling

actin filaments (aka microfilaments) and microtubules have faster / more polymerization growth on the plus or minus end?

PLUS end

apical

top of the cell b

basolateral

at side, near the base

basal

bottom of the cell → connected to the basal lamina

are relative positions of junctions the same in ALL epithelial cells?

YES — from apical to basal:

1. anchoring junctions

2. occluding (tight) junctions)

3. channel forming (gap) junctions

are epithelial cells polar or nonpolar?

polar

anchoring junctions

• cell-cell adhesions at ?

• cell-matrix adhesions at ?

________ stresses and are tethered to _________ inside the cell

• cell-cell adhesions at adherens junctions or desmosomes (transmembrane cadherins)

• cell-matrix adhesions at hemidesmosomes (transmembrane integrins)

transmit stresses and are tethered to cytoskeletal filaments inside the cell

occluding (tight) junctions

seal the gaps between cells in epithelia to make a cell sheet into an IMPERMEABLE or SELECTIVELY PERMEABLE BARRIER

• involve claudin proteins

occluding (tight) junctions involve ____ proteins

transmembrane claudin and occludin — hold adjacent cells together

channel forming (gap) junctions create _________ linking the ______ of adjacent cells for passage of ________ and ______

create passageways linking the cytoplasms of adjacent cells for passage of small molecules and ions

• composed of connexin or innexin proteins

channel forming (gap) junctions are composed of ___ or _____ proteins

connexin or innexin

5 types of junctions of epithelial cells (in order from apical to basal)

1. tight junction

2. adherens junction

3. desmosome

4. gap junction

5. hemidesmosome

1. tight junction

seals neighboring cells together in an epithelial sheet to PREVENT LEAKAGE of EC molecules between them

• helps polarize cells

2. adherens junction

• thousands of ________ molecules, packed into dense, regular arrays that are linked to the _______ cytoskeleton by a complex of intracellular anchor proteins on the __________ side

joins an ACTIN bundle in one cell to a similar bundle in a neighboring cell (think adherens = actin)

• thousands of transmembrane cadherin molecules, packed into dense, regular arrays that are linked to the actin cytoskeleton by a complex of intracellular anchor proteins on the cytoplasmic side

3. desmosome

joins the INTERMEDIATE FILAMENTS in one cell to those in a neighbor

4. gap junction

forms channels that allow SMALL, INTRAcellular, WATER-SOLUBLE molecules (including inorganic ions and metabolites) to pass from cell to cell

• allow very rapid flow of ions

• enable cells to communicate electrically and chemically → esp important in cardiac muscle cells

5. hemidesmosome

anchors INTERMEDIATE FILAMENTS in a cell TO the basal lamina

junctional complex proteins — what 3 types of epithelial cell junctions?

act as a coordinated unit to maintain tissue polarity — depend on each other for positioning

1. tight junction

2. adherens junction

3. desmosome

*provides STRONGEST sites of ATTACHMENT

anchoring junctions — what 3 types of epithelial cell junctions?

transmit stress, provide structure and maintain tissue integrity

2. adherens junction

3. desmosome (transmembrane cadherins)

5. hemidesmosome (transmembrane integrins)

*tethered to cytoskeletal filaments INSIDE the cell

what are anchoring junctions tethered to?

cytoskeletal filaments INSIDE the cell

are gap junctions used for cell adhesion?

NO — strictly for COMMUNICATION!!

______ junctions seal gaps between cells and separate apical and basolateral surfaces

tight

sealing strands in tight junctions completely encircle the ______ end of each cell

apical

epithelia are selective permeable barriers due to _____ junctions

tight → seal gaps between cells

will a low-molecular-weight tracer added to one side of an epithelium generally pass beyond the tight junction?

NO — this is very important physiologically

• think about GI tract → you want epithelial cells to take up nutrients → a tightly bound epithelium is important so that nutrients don’t just float through the intracellular spaces

is there a linkage from tight junctions to cytoskeleton?

NO

consider the epithelial cells that line the lumen of the gut. why is it important for the epithelia to form a selectively permeable barrier?

allows the absorption of essential nutrients from food while simultaneously preventing harmful substances (like bacteria and large molecules) from entering the bloodstream → maintains a crucial balance between nutrient uptake and protection against pathogens and toxins w/in the digestion tract

adherens junctions link epithelial cells via _______ to form an adhesion belt

transmembrane cadherin molecules

cadherins

calcium dependent adherins

where do adherens junctions form an adhesion belt

close beneath the apical face of the epithelium → encircling each of the interacting cells in the sheet

contractile bundle of actin filaments, tethered to the PM by the cadherins, can _____ w/ the help of myosin motor proteins

CONTRACT → providing the motile force for folding of epithelial cell sheets

what provides the motile force for folding of epithelial cell sheets?

contractile bundle of actin filaments (tethered to the PM by cadherins and associated intracellular anchor proteins) can contract w/ help of myosin motor proteins

contraction of epithelial sheets is important for ?

folding into tubes, vesicles, and other related structures during development

desmosomes and hemidesmosomes are connected to ________ (a type of intermediate filament)

keratin → provides tensile strength

link cells via cadherins

3. desmosomes

link cell to protein laminin in basal lamina through integrin receptors

5. hemidesmosomes

desmosomes form spots of ____cellular adhesion through what type of cadherins?

spots of INTERcellular adhesion through non-classical cadherins (desmoglein and desmocollin

desmosomes provide mechanical strength through linkage to ? and to ?

intermediate filaments (keratin filaments in epithelial cells) and similar bundles in adjacent cells

• creates a framework that extends throughout the tissue for tensile strength → resists pulling / twisting forces; can withstand mechanical stress w/out breaking

desmosomes play an important role in preserving tissue ?

integrity

hemidesmosomes are epithelial cells that are anchored on their basal side to the _______, which lies at the interface w/ the underlying CT

basal lamina

________ receptors anchor the cell to the EC matrix protein laminin in the basal lamina

integrin

integrin receptors anchor the cell to the EC matrix protein ________ in the basal lamina

laminin

hemidesmosomes — intracellular attachment is to ______ → provides _____ to epithelial tissues

intermediate filaments (keratin); mechanical strength

gap junctions are built from 6 _______ transmembrane proteins that form a ______ on each cell

connexin; connexon

connexon pores align to form a ______ between 2 cells — gap junctions

channel

gap junctions consist of _______ connexon pairs in _______

many; parallel

gap junctions couple the _______ between animal cells electrically and metabolically

cytoplasm

• heart muscle contraction; spread of action potentials between neurons

only molecules SMALLER THAN ______ DALTONS can pass through gap junctions

~1000

in the heart, cardiac cells are connected by ______, which allow for the rapid transmission of electrical impulses between cells, which ensures coordinated muscle contractions throughout the heart by facilitating the flow of _______ between adjacent cells

gap junctions; ions

• effectively acting as a functional syncytium → CRUCIAL for proper heart rhythm and conduction of electrical signals

the cytoplasms of adjacent plant cells are connected via ?

plasmodesmata

plasmodesmata

numerous cytosolic channels that pass between adjacent plant cell walls to connect their cytoplasm, and enable transport of materials from cell to cell (and thus throughout the plant)

• very similar to gap junctions

movement of ions between adjacent cells is proportional to the # of __________ that connect the cells

plasmodesmata!! (think: more openings = faster transport)

does plasmodesmata allow passage of viruses and larger molecules?

YES — including signaling molecules, RNAs, and transcription factors

cell walls are composed of EC matrix rich in _______ and _________

cellulose, polysaccharides

is there a need for anchoring junctions in plant cells? explain.

NO — cell walls of adjacent cells are FIRMLY CEMENTED to those of their neighbors → eliminates need for anchoring junctions to hold the cells in place

what is the role of the EC matrix in tissues?

network of proteins and carbs

• cells interact w/ ECM through integrin receptors for migration

________ tissues consist largely of ECM (EC matrix)

connective

the ECM is made and oriented by the cells _______ it

• primarily secreted by _________

WITHIN

• fibroblasts — cartilage = chondroblasts; bone = osteoblasts

_______ in the ECM make, excrete, and organize components of the ECM

fibroblasts (cells)

epithelial cells are separated from the underlying tissues by the ?

basal lamina — prevents fibroblasts and CT from migrating UP

_____ is the primary organizer in the basal lamina

laminin = large, cross-shaped glycoproteins

• essential for providing structural support

• promoting cell adhesion

• guiding cell migration and differentiation in processes like tissue development, wound repair, and the immune response

______ provides tensile strength in the ECM

collagen

true or false: ECM is dynamic and continuously remodels

TRUE!!

ECM is constructed of

• proteoglycans

• fibrous proteins (collagens and elastic fibers)

• glycoproteins (laminin and fibronectin)

glycoprotein

a protein w/ one or more carbohydrate molecules (called oligosaccharides) covalently bonded to it

is CT uniform?

NO — NOT orderly and cells distinctly diff shapes & composition

collagen chains are arranged in _______ and organized by ________

fibers; fibroblasts

the MOST abundant protein in the mammalian ECM (and in the entire human body) is

collagen — VERY LARGE

collagen structure

3 intertwined polypeptide chains → makes collagen VERY STRONG