Life 205 Multicellular Organism

How did multicellularity evolve?

Cell Differentiation: Cells multiply and differentiate to form distinctive tissues - making organs + systems

How do cells differentiate?

Each cell encodes a lot of genes (20,000) - only a subset are expressed. Some will be silenced, others expressed.

How is gene expression controlled?

External factors, cell-cell interactions, cell-extracellular matrix interactions, growth factors, hormones

Animal cells require extracellular signals to function. What do these include?

Humoral (soluble) factors - hormones.

Autocrine/panacrine factors - growth factors

Solid State factors - cell adhesion molecules, extracellular matrix

What is the effect of extracellular factors?

Trigger intracellular signalling pathways to alter gene expression

Signalling cascades downstream of receptors changes activity of transcription factors in the nucelus

(Transcription factors decide which genes are expressed in the cell)

C. elegans as case study for multicellularity

Fate of all their cells is completely predictable

Programmed cell death is genetically programmed

Tissues definition

A collection of similar cells from the same origin that together carry out a specific function

What causes disease?

Changes to a cells environment / response to environment - changes cells fate

Functions of Epithelia:

- Physical barrier protection

- Controls permeability

- Produces secretions from secretory cells

- Provide sensations

Structure of Epithelia:

- Tightly packed cells arranged as sheets

- Attached to a basement membrane

- Polar

- Specialised membrane/structures/cell junctions

- Na/K pump at basolateral surface allows cavitation (water pumped in causing expansion)

- Microvilli dramatically increase surface area for absorption

- Basal folds increase surface area for transport

- Cillia for motility of mucus (trachea) and cells involved in fallopian tubes + development

Epithelial cell junctions: Tight Junctions

2/3 surfaces interacting to form a continuous intercellular barrier

Linked to intercellular signalling e.g. transcription factors

Example = Claudins

Epithelial cell junctions: Cell-cell adhesion molecules (CAMs)

Cell-surface proteins that mediate interaction between cells and/or extracellular matrix.

4 families: Ig, integrins, cadherins + claudins

Weaker binding (compared to hormones/neurotransmitters)- possible to separate

Immunoglobulin

Weaker adhesion than cadherins

Found in tight junctions + cell-cell

Calcium independent CAMS

Useful for interactions during development

NCAMs (neural cell adhesion molecule)

Important for migration of cells to the olfactory bulb

Signals through tyr kinases

Comes in many forms due to splicing

Affinity changes in development due to sialic acid:

- High sialic acid = low affinity, more rearrangement of cells

- Low siaclic acid = high affinity, stable tissues

Cadherins

MAINTAIN EPITHELIAL INTEGRITY!

Calcium dependent CAMs often found in cell junctions

Important for differentiation of cells into specialised tissues/organs (histogenesis)

Signal to the nucleus + alter gene transcription

Homophillic.

Only bind to the same cadherin

Developmental origin of the kidney

Occurs after gastrulation, within the intermediate mesoderm

Overview of kidney development

- Begins with development of nephric duct in the intermediate mesoderm

- 1st kidney formed = pronephros

- 2nd kidney formed = mesonephros (causing degeneration of pronephros)

- 3rd kidney formed = metanephros (permanent, causes degeneration of mesonephros)

Metanephros gives rise to nephrons & collecting tubules/ureters.

If a cell isn't epithelial, it's mesenchymal + will form nephrons.

Role of tubules in kidney

Keeps things contained + compartmentalised.

Made of epithelial cells.

Development of intermediate mesoderm

- Diffusion gradient of signals/growth factor (BMP)

- Concentration of BMP morphogen drops with distance from source

- At high concentrations of morphogen, both genes A + B turned on. At moderate, only B turned on. Below threshold, neither gene is active.

Intermediate mesoderm forms when medium levels of BMP

Inductive Interactions in the kidney

Induction = one group of cells emits a signal that causes an adjacent set of cells to change their fate during development.

Requires signal from inducer + responder cells to be competent

Nephrons, collecting ducts + 3D structure of kidneys are formed by reciprocal inductive interactions between UB + MM (via Wnt + GDNF signals)

What is the first component of kidney formed?

Nephric Duct (within the intermediate mesoderm.)

Gives rise to ureteric bud (forms collecting tubules & ureters.)

→ first step of development in adult kidney (metanephros).

→ Dependent on inductive signals from MM, which secretes growth factor GDNF.

Ureteric Bud + Metanephric Mesenchyme are the 2 primordial cell types.

MM = nephrons

UB = collecting ducts

Induction of Metanephric Mesenchyme

Induced by signals from Ureteric Bud, to become polarized epithelial vesicle.

Mesenchymal cells near the bud become induced and convert to an epithelium which goes on to generate the nephron.

Wnt signalling factors expressed by UB cause the MM to undergo mesenchymal-to-epithelial transition.

Role of kidneys in homeostasis

- Regulates blood pressure

- Osmoregulation (maintains fluid balance + electrolytes)

- Blood pH constant, acid/base balance

- Removes soluble nitrogenous waste (renal tubules)

- Uses loop of Henle: proximal tubules reabsorb electrolytes, glucose, water.

GFR determines how quickly blood is cleansed of waste.

How do the kidneys regulate blood pressure?

If too low: Juxtaglomerular cells release Renin, triggering formation of angiotensin II. (Renin-angiotensin pathway). Stimulates aldosterone release from adrenal glands, which causes kidney (distal tubules) to reabsorb more Sodium (causes more water absorption).

Vasopressin = anti-diuretic hormone that causes more water absorption to raise blood pressure. High levels cause vasoconstriction.

If too high: All of the above hormones stop being released.

The heart can release ANP which antagonizes aldosterone + cause the kidney to excrete more sodium and water (causes vasodilation)

Where does primary filtration occur?

Glomerular Basement Membrane.

Contains electrolytes, metabolic wastes, metabolites, small proteins.

Kidney disease / injury

Creatinine clearance = measure of kidney function. A waste product of muscle cells.

Elevated serum creatinine level linked with acute kidney injury. Also reduction in urine.

CKD has 5 stages - progressive loss of renal function over time. Decline in GFR + creatinine clearance.

Causes of CKD

- Diabetes (nearly 50%) causes glomeular hyperfiltration

- Hypertension (glomerular + tubular damage)

- Glomerulonephritis

- 60+

- Smoker

- Obesity

How can the kidney repair itself?

- Endothelial repair and regeneration

- Resolution of inflammatory infiltrate

- Tubular proliferation

- Macrophage M1 to M2 switching

- Immune system uses macrophages, neutrophils

What does the muscular system consist of?

Cardiac, Skeletal + Smooth muscle cells.

Cardiac (Heart) + Smooth (Intestine) = involuntary, Skeletal (Bicep) = voluntary.

Structure of skeletal muscle

Consist of muscle fibres which are composed of parallel bundles of 1000 myofibrils.

Smallest unit is the sarcomere.

Thin Actin (Light) forms ladder for myosin to climb.

Thick Myosin (Dark) composes of globular head with ATP + Actin binding sites.

Sarcoplasm / Sarcolemma / Sarcoplasmic reticulum

Sarcoplasm = cytoplasm of muscle fibre

Sarcolemma = muscle fibre plasma membrane

Sarcoplasmic reticulum = smooth endoplasmic reticulum of the muscle cell

Fast Twitch vs Slow Twitch

Fast twitch: Faster contractions, greater force, no oxygen. Dominate pale/white muscle.

Slow twitch: Slower contractions, slower force, oxygen. Dominate dark red muscle with myoglobin.

Type 1 fibres (slow oxidative)

- Larger numbers of mitochondria

- Higher concentrations of myoglobin + red muscles

- High capacity for aerobic metabolism

- Higher resistance to fatigue

E.g. marathon runners/mountaineers

Type 2A/B fibres

Type 2A (Fast-oxidative fibres) = between type 1 and type 2B. Relatively fatigue resistant with intermediate levels of glycolytic activity. Combo of aerobic + anaerobic.

Type 2B (Fast-glycolytic fibres) = Smaller numbers of mitochondria, limited capacity for aerobic metabolism, less resistance to fatigue. Large amounts of glycolytic enzymes provide high capacity for anaerobic metabolism. White muscles contain lots of 2B fibres.

E.g. sprinters/high jumpers

Sequence of events at neuromuscular junction

1. Action potential arrives at pre-synaptic terminal

2. Voltage-gated calcium channels open

3. Ca2+ uptake releases Ach from vesicles into synaptic cleft

4. Ach travels across to post-synaptic membrane. Contains nicotinic receptors for Ach

5. Ach causes channels to open, increases permeability to Na+ and K+

6. End plate depolarized as more NA+ in than K+ out

7. EPP reaches threshold value, action potential goes along sarcolemma

How does Ach activity end at the neuromuscular junction?

Acetyl cholinesterase turns off the muscle cell electrical response.

Degrades Ach to Choline + Acetate

Muscle fibre can now relax before next action potential/Ach

How is force generated in the muscle?

Increased calcium = contracts muscle

Decreased calcium = stops muscle

Action potentials release calcium, which binds to troponin + moves tropomyosin away from binding site, so cross-bridges can form.

Rapidly repeated stimulations cause individual responses to fuse into one continuous contraction (tetanus) which produces maximum force. Used for most normal body movements.

Sliding-filament hypothesis

1. ATP hydrolysis (ADP + P) energizes the myosin head

2. Formation of cross-bridges: myosin head attaches to myosin-binding site on actin

3. Power stroke: The cross-bridge rotates, sliding the filaments

4. Detachment of myosin from actin: As the next ATP binds to the myosin head, the myosin head detaches from actin

The cycle applies force that shortens the sarcomere.

The contraction cycle repeats as long as ATP and Ca++ available.

What are the 3 types of contraction?

Concentric (Muscle activated + shortened)

Isometric (Muscle activated + remains same length)

Eccentric (Muscle activated + lengthened)

Muscle contraction: Direct Phosphorylation

Uses Creatine Phosphate to regenerate ATP very quickly, no oxygen needed.

Phosphate transferred from creatine to ADP to make ATP.

Muscle contraction: Anaerobic Glycolysis

Lactic acid system breaks down glucose to pyruvate, no oxygen needed.

Inefficient, short-term, not much ATP.

Muscle contraction: Aerobic System

Uses oxygen from RBCs + myoglobin to make ATP.

Produces a lot of ATP, long-term, utilises lots of substrates.

What are the 2 things muscle contraction is dependent on?

Calcium + ATP

What is bone?

A specialised form of connective tissue - living, calcified tissue with a rich blood supply

Withstands high amounts of load due to minerals in fibrous matrix

What are the 4 main functions of bone?

- Mechanical support/scaffold

- Locomotion

- Protection

- Metabolic reservoir of minerals

Compact vs Trabecular bone

Compact = 80%, dense, outer shell, important for strength, decreases with age.

Trabecular = 20%, light porous, provides support, houses bone marrow. Inner part of bones with a lot of space + large SA for metabolic activity.

Osteons/haversian systems

Subunits of compact bone.

An osteon has a haversian canal surrounded by rings of bone + osteocytes. Contain blood vessels + nerves.

Periosteum

• Outer, fibrous covering of bones.

• Condensed collagen layer.

• Inner layer of osteogenic cells for bone growth, remodelling + fracture healing.

• Outer layer of fibrous cells.

• Supply of blood vessels + sensory nerves.

Endosteum

• Inner surface of bone covered by a cellular layer.

• Thinner + more cellular than periosteum.

• Includes trabecular, medullary cavity, inside of haversian canals.

• Large SA - calcium homeostasis.

What forms the composition of bone?

- Support cells (osteoblasts + osteocytes)

- Remodelling cells (osteoclasts)

- Inorganic minerals salts in matrix

- Osteoid: organic unmineralized extracellular matrix (90% collagen, proteoglycans, glycosaminoglycans)

Which mineral salt gives bone its strength & rigidity?

Hydroxyapatite

Demineralised bone

No calcium - bone retains shape, very flexible, still has tensile strength.

Anorganic bone

No collagen - very brittle, can't withstand compression.

2 main patterns of bone involved in the pattern of collagen forming the osteoid

1. Woven Bone: Weak. Irregular, loosely intertwined pattern of collagen. Prevalent in fetuses + rapid new bone formation.

2. Lamellar bone: Strong. Parallel sheets of collagen. Makes up virtually all bone in healthy adults.

3 primary types of bone cells

1. Osteoblasts: Bone forming by making organic matrix (osteoid) + mineralising it. Lines bone cells.

2. Osteocytes: Bone maintenance. Osteoblasts that become trapped in bone matrix.

3. Osteoclasts: remove old bone via resorption. Dissolve minerals by releasing protons, dissolve organic matrix by releasing enzymes.

Osteoprogenitor cells

Stem cells of bone.

Precursors of osteoblasts.

What is Ossification?

The process of bone formation.

Intramembranous Ossification

Flat bones of skull + facial skeleton, from fibrous membranes.

Endochondral Ossification

Rest of skeleton, from hyaline cartilage templates. Starts from mesenchymal tissue

Interstitial vs Appositional growth

Interstitial: Progressively increases length of bone. Rapidly growing cartilage, calcify extracellular matrix which replaced by trabecular bone. Growth plate fuses/hardens once reaching adult length.

Appositional: Widens bone without becoming thicker. Bone deposited at surface and resorbed inner surface.

What is Wolff's Law?

Bone adapts to stresses/demands placed upon them

What is used for metabolic regulation of bone?

Vitamin D & PTH (parathyroid hormone) - linked to calcium absorption.

More PTH = Bone loss.

Disorders of bone

- Osteoporosis: Disorder of bone quantity, less of it, "porous bones".

- Paget's: Increased bone resorption, increased but disorganised bone formation, abnormal remodelling.

- Osteopetrosis: Brittle bones, fracture easily, despite more being formed not remodelled. Not degraded in usual way - can't acidify resorption pit.

- Hyperparathyroidism: Excess production of PTH, increasing serum calcium. Can be primary or secondary. Either problem with glands, or linked to Vit D deficiency/CKD.

- Rickets + Osteomalacia (vit D disorders): childhood or adult form, impaired bone mineralisation, inadequate calcium + phosphate. Defect in bone quality.

Functions of the skin

- Barrier

- Immune (antigens)

- Homeostais (temp + water loss)

- Sensory (pressure/pain/touch)

- Excretion (sweat)

What is the epidermis mainly populated by?

Keratinocytes (90%) - microbial + UV protection, skin hydration.

Also melanocytes (pigment cells, melanin) + dendritic/ langerhans cells (immune, pathogen defence, process antigens)

Where does cell division occur in the skin?

Basal layer: cells differentiate, new cells form + forced upwards

prickle cells -> granular -> corneocytes

Where are nerve endings + blood vessels most commonly found in the skin?

The dermis.

Nerve endings involve merkel cells, pacinian corpsucles + more.

What happens when the skin is grazed?

- Skin releases discharged lamellar body contents

- Increase in expression of Cx26 + 30. Downregulation of 43.

(this also occurs in skin disorders)

What does wound repair depend on?

Calcium gradient: needs to be restored.

There is a calcium ion gradient - less towards the bottom. A wound breaks down the calcium gradient, triggering the granular cells to secrete more lipids to restore permeability barrier.

Basal layer: extracellular Ca2+ lower

Granular cell layer: extracellular & intracellular Ca2+ higher.

Low levels favour proliferation, high levels favours differentiation.

What is the role of connective tissue?

- Provides structural + metabolic support

- Loose ones = biological packaging

- Dense ones = supports physical stresses + provides support

What is connective tissue made up of?

Non-cellular: extracellular matrix of fibrous proteins + proteoglycans

-> Gel cushioning to resist compression

Cellular: Immune cells, mast cells, macrophages, neutrophils, lymphocytes, nerves, blood vessels, adipocytes, fibroblasts

What do integrins do?

Adhesion receptors for extracellular matrix/ligands. Activate signalling into cell. Function bidirectionally.

Elastin

- More rubbery than collagen

- Stretch + recoil

- Cross-linked

Gap Junctions

Allow contents of one cell to connect to contents of another.

Only small molecules (ions, small sugars) can pass through - proteins, mRNA etc can't.

Can be shown through fluorescent dye - will fuse into adjacent cells.

Important in heart + smooth muscles - passes on depolarisation etc.

What are gap junctions inhibited by?

Calcium - too much can overload the cell and kill it.

What is the cerebellum responsible for?

Movement: motor skills, posture, balance.

What is the cerebral cortex responsible for?

Consciousness, language, thought, memory.

Key features of a neuronal cell

Dendrites: receive signals from axons of other neurons

Cell body: main site of protein synthesis, contain nucleus + organelles.

Axon: conduct action potential

Growth cone: guide axon to target.

Synapse: specialised contact between neurons/effectors. Synaptic vesicles-neurotransmitters-fast communication.

Neuronal cytoskeleton

Microtubules (Largest, stiff)

Actin (Semi flexible)

Intermediate/neurofilaments (Smallest, flexible)

How do you polymerise / depolymerise Actin

Polymerise: Add ATP at + end

Depolymerise: ADP at - end

Same but GTP/GDP for microtubules

Microtubule motor proteins

· Dynein moves towards the minus (non growing end) of the MT

· Kinesin moves towards the plus (or growing end) of the MT

· Motor proteins have 2 heads as each head must let go and rebind (with each cycle of ATP hydrolysis)

· Having 2 heads ensures there is always one attached

Axonal Transport

Most proteins made in cell body.

New material added at growth cone during axon extension.

New proteins must be transported down the axon.

Fast (FAT) + Slow (SAT) transport via neuronal cytoskeleton.

FAT + SAT

FAT = First wave within hours, very quick. New membrane proteins, vesicles, membrane-associated components.

SAT = Second wave in weeks, cytoskeletal compounds e.g. neurofilaments, mictrotubules, actin.

Both use kinesin + dynein as motor proteins.

What uses Retrograde transport and why?

FAT only.

Growth cone -> cell body

Used to recycle vesicles or for communication e.g. nerve growth factor.

1. Resting Membrane Potential

- Membrane impermeable to charged ions

- Inside more negative, outside more positive

- Concentration gradient due to Na/K pump (3 out 2 in)

- Around -70mV

- Some K+ channels open

2. Depolarization

- Action potential generated

- Na+ channels open & enters

- Voltage slightly more positive: reaches threshold required for action potential

- Triggers vesicle exocytosis

3. Repolarization

- Na+ channels close

- K+ channels open

- K+ leaves

- More negative than before: refractory period before returning to -70 resting state.

What is exocytosis?

Material released from cells, and new proteins inserted into plasma membrane.

What additional machinery / specialised properties have neurons evolved for exocytosis?

- Rab proteins (vesicle targeting)

- SM proteins (docking/fusion)

- SNARE proteins (membrane fusion)

- NSF/SNAPs (SNARE recycling)

Mechanism of synaptic vesicle exocytosis

Rab3 binds to GTP - activates RIM, which binds to calcium channels, providing the trigger for vesicle fusion.

Excitatory (depolarising) neurotransmitters in CNS

Glutamate in brain, aspartate in spinal cord.

Inhibitory (hyperpolarising) neurotransmitters in CNS

GABA in brain, Glycine in spinal cord.

Ionotropic vs Metabotropic receptors

Ionotropic = Fast, act directly (GABA, glutamate)

Metabotropic = Slow, act indirectly

Synaptic plasticity

Efficacy of synaptic transmission can change over time.

Strengthening: Repeated firing + stimulation induces changes in type + number of ion channels. Increase of positive ions = stronger response to same stimulation.

Weakening: Low calcium concentration from infrequent presynaptic stimulation. If rarely used, will weaken over time.

Insertions / changes that increase synaptic plasticity

- Insertion of new AMPA receptors

- Increased sensitivity of existing receptors

- Increased neurotransmitter conc in synaptic cleft

- Increased probability of release

- Retrograde messengers

- Number of synapses

What do Glial cells do?

Hold neurons together.

Capable of cell division (unlike neurons)

Lots in CNS - most stem cells in adult nervous system will differentiate into glia.

Embryonic origin

What is the main component of white matter?

Myelin - around 60%.

Myelin is 70-85% lipid, so phospholipid heads + membrane proteins interact.

Which glial cells are used for myelination?

CNS = Oligodendrocytes (has processes that can myelinate axons of multiple different neurons)

PNS = Schwann Cells (wraps around one/several small axons)

-> proliferate & migrate along axons

What is myelin made of?

CNS = proteolipid protein + myelin basic protein

PNS = PO protein + MBP (less abundant)

Myelin is made of compacted membrane - as cytoplasm is squeezed out the way to form mostly membrane.

Demyelination disorders include MS + guillain barre syndrome.

Nodes of Ranvier

- Unmyelinated region

- Voltage Na/K channels concentrated here (site of depolarisation)

- Action potential hops from one NOR to next.