Part 3 Flashcards
Cardiovascular disease
Main cause of death in UK and world wide, over 1 in 4
Myocardial infarction
Heart attack
Leading cause of death in developing countries, one person has a heart attack every 3 min in uk
Congenital heart diease
800 deaths per year
1/3 babies
Health care cost
around £11 billion per year in UK
Why do we study Drosophilla (lower organisms)?
Help understand human organogenesis
Drosophilla genome sequenced- March 2000
929 genes in humans- 548 have homologues in drosophila
Discovered- Hh, Notch, Wg, Dpp
Timings of cardiac development in different organisms
Drosophila (24h), mouse (19.5 dpc) and human (40 hrs) all have same steps different timings
Straight after gastrulation:
- fusion and formation of heart tube/looping
- Chamber form
- Heart functions
Where do cardiac cells originate from?
Ventral latera mesoderm called splanchnopleura
Somatopleura= Splanchic mesoderm and ectoderm Splancnopleura= Splanchnic mesoderm and foregut endoderm (lateral)
Heart precursor cells have 2 origins
First heart field
second heart field
Fate cells- identify in early development and follow to late to see what they make
What does the first heart field consist of?
- left ventricle
- right ventricle
- atrio-ventricular canal
What does the second heart field consist of?
- right ventricle
- outflow tract
- inflow tract
Cardiac specification
Production of angiognetic cell clusters
Above neural plate
- Arise immediately after gastrulation
- Crescent mannor is the anterior part of the embryo
Steps in heart formation for vertebrate and drosophila
Step 1
Precursor to cardioblast
(specification)
Drosophila- Tinman
Vertebrate- Nkx2.5
Heart formation
Step 2
Cardioblast to cardiomyocyte
(Differentiation)
Drosophila- Dmef2
Vertebrate- Mef2
Heart formation
Step 3
Cardiomyocyte to linear heart tube
Drosophila- Pannier
Vertebrate- GATA4
Heart formation
Step 4
This stage is where there is a difference between drosophila and vertebrate
Drosophila is linear heart tube to dorsal vessel
Vertebrate is linear heart tube to looping heart tube (InV)
Heart formation
Step 5
Only in vertebrates
Looped heart tube to multichambered heat
dhand and ehand
What drives the specification of cells in heart formation?
Drosophila
- Uses expression of tinman using dpp
- Tinmnan= protein code for TF homeobox
- Expressed through mesoderm however it becomes restricted as the embryo develops only being expressed in the dorsal vessel
Trunk mesoderm - dorsal mesoderm - dorsal vessel
Vertebrate
- Homologues of tinman= Nkx-2
- Nkx2.5 is the earliest to be expressed and specifically in cardiac crescent- restricted to angiogenetic cells
What happens when you have a mutant mice of Nkx 2.5?
Do not form a heart and show cardiac defects at looping stage- compensatory mechanism
Mutant humans- congenital heart defects
What regulates the expression of a gene?
- Onset
2. spatial mannor
What induces Nkx2.5 expression?
BMP
Express ectopically in gain or loss of function approach
1- electrooporate the BMP genes in the chick timing either immediately prior or same time as you would expect the genes to be expressed.
2-grafting cells expressing Bmp
3- adding beads (BMP bead soaked and control bead), allow chick embryo to develop, carry out in-situ hybridisation,
-Bmp bead expresses Nkx2.5 but at the paraxial mesoderm
-control expresses Nkx2.5 where it would normally be expressed
What signal is produced in the anterior mesoderm and neural plate?
Wnt so block BMP -Create an evironment which is favourable for induction -Block inhibitory factors - array of genes +proteins required
Endocardium
Contains precursors of endothelial lining of the heart and cushions cells that form valves
Myocardium
Contains myocytes of atria and ventricles and purkinjie fibres
What is used for cardiac muscle differentiation on drosophila and vertebrate?
Drosophila- Dmef2
- Induced by Tinman
- Protein expressed in all muscles of developing embryo
- Mutant= lose differentiation of muscle function but still see tinman expression
Vertebrates- Mef2A, Mef2B, Mef2C
- Earliest gene expressed in heart mef2C
- Mutant- no heart looping and no right ventricle up-regulation of ef2B
Cardiomyocyte to linear heart tube in vertebrate and drosophila
GATA- Vertebrates
- GATA1-3=Hematopoiesis
- GATA4-6= cardiogenesis with gata4 the earliest gene expressed
Gene 4 KO- failure to form heart tube
+/1- form endocardial tube and migrated to centre and fused to single tube by this point
-/- Instead of seeing single tube fused in ventral part, see 2 tubes remaining in bilateral position
Cardiac bilatera
Failure to form and join single tube- form but dont migrate
Gata involved migration (directed- opposite foregut)
Heart looping
First break in embryos symmetry
Heart loops to the right
Assymetic- cell division, death and shape
How does establishment of left and right asymmetry in the chick embryo occur?
Lefty and nodal two TGFbeta molecules are specifically expressed on the left side, lefty activates Shh
Activin expressed on the right, inhibits Shh
Linear heart tube to looping heart
IV and INV
Mutations in InV or IV cause random or complete reversal of heart looping (meant to be to right)
Lefty and nodal expressed on right not left
What do IV and InV do?
IV encodes a dynein, protein involved in the movement of cilia
InV encodes for inversion, a protein containing ankyrin repeats found in cilia
Both required for cilia movement/ rotation, useful as cilia establishes a professional flow of nodal and lefty molecules to the left
(mutation= dont flow)
How are the chambers formed?
dHand and eHand- bHLH TF with specific expression in right and left venticle
Conditional -/- eHand= Left ventricle defect but mice survive til birth
- /- eHand= placental defects (mice die E8.5)
- /- dHand in mice = RV hypoplasia (mice die at E10.5)
What is an organ?
Single functional unit in your body made up from different functional tissue types
What happens when the optic vesicle infolds?
Forms a bilayered optic cup
Inner wall of the optic cup= neural retina
Outer wall of the optic cup= pigment epithelium- produce melanin
What is the optic cup derived from?
Neuroepithelium
Inner part contains SC like populationwhich can either self renew or differentiate into progenitors that diff into ganglion, interneurons, photoreceptors
Where does the lens come from?
Derived from epiderm also containing sc like population which can either self-renew or differentiate to crystallin- producing fibre like cells
What type of cells behaviours underlie eye development?
- Changes in fate- neural to optic vesicle, epiderm to lens placode
- Proliferation
- Involution- Optic vesicle to optic cup
- Pinching off- lens placode to lens vesicle
- Migration- differentiation of retinal neuronal types
How do different tissues that form the eye assemble?
Changes in fate - neural to optic vesicle induces epiderm to lens placode
Involution- signal from lens placode induces optic vesicle to optic cup
Pinching off- optic cup induces lens placode to lens vesicle
Parts of the kidney
- papilla- tip of each pyramidal where urine drains from
- major/minor calyces- empty into renal pelvis
- Renal pelvis- transmits urine to bladder via ureter
- urether- empties into bladder
- cortex- peripheral layer
- medulla- inner layer ( arranged in multiple pyramidal structures that together overlying cortex comprise the renal lobe)
- Renal capsule- fibrous outer layer
What is the ureter?
Runs form the kidney to urinary bladder
Ends in renal pelvis a region characterised by major calyces
What are major calyces?
Divided into minor calyces
receive fluid outflow from many microscopic collecting tubes
What is the function of calyces, ureter and collecting tubules?
To collect urine and void it into the bladder
colllecting urine from large surface area
Where are nephrons found and what are their structures?
Within the cortex and medulla
Structure includes glomerular blood filtrate containing podocytes and a tubular epithelium that loops down into the medulla- Divided into proximal, intermediate and distal segment
What events do we need to understand to appreciate the kidney development?
Induce different cell types
induce these in certain place of the body
Proliferation
Form branches- branching morphogenesis
Form many nephrons-2 ends need to be different
Attract blood capillaries to one end of the nephron
Form and connect tubes
How is the definitive kidney formed?
Arises as results of reciprocal interactions between 2 structures, a small out-pocketing of an intermediate mesodermal structure, called the ureteric duct and adjacent mesenchyme called metanephric mesenchyme
What does the metanephric mesenchyme form?
The definitve nephric tubules
Interaction between ureteric bud epithelium and nephrogenic mesenchyme is an inductive interaction
Ureteric bud induces nephrogenic mesenchyme to CONDENSE around the bud and undergo a mesenchymal to epithelial cell transition and form renal epithelium, then renal vesicles
What do renal vesicles proliferate and differentiate to form?
Nephron
Proximally they fuse with bud, distally they will attract endothelial capillaries
Facts about the nephron
You have 10^4 in each kidney
This is because the ureteric bud undergoes branching morphogenesis forming 10^6 renal vesicle inducing buds in each kidney
Branching morphogenesis of the ureteric bud is governed by signals from the metanephric mesenchyme- the key steps are:
- Proliferation and outgrowth of the bud ‘tip cells’
- Arrest in proliferation of leading- edge tip cells, resulting in flattening
- Continued proliferation of lateral tip cells,resulting in formation of cleft and 2 tips
What is clefting in the ureteric bud of the kidney?
GDNF (neurotrophic factor, acts to promote proliferation, migration and outgrowth) signal required by mesenchyme
Luminal mitosis and dispersion to sides causes flattening of basement membrane
Clefting occurs as basement membrane accumulates and basment membrane holes move to the sides
Repeat again and again to form branches
What is the receptor of the bud?
A receptor tyrosine kinase called Ret
What is the sequence of events?
Signal from mesenchyme and epithelium - signalling pathways - gene expression - cellular response - bifurcation (reshaping)
When the mesenchymal condensation around the bud causes a mesenchymal to epithelial cell transition
Ureteric tips - pretubular aggregate - renal vesicle - comma-shaped body - s shaped body - distal collecting tube/loop of henle
How do we study this process?
In real time, using transgenic animals, organ cultures and sophisticated imaging
Branching morphogenesis of the ureteric bud underlies future calyces
What happens in different weeks of embryogenesis in humans?
Week 6- 16 branches
Week 7- minor calyces formed
Week 32- 10^6 branches formed
What gene is expressed in nephron stem cells?
TF six2
Six2 cells form the nephron and all cells within
Loss of function of Six2 depletes SC pool
Overexpression of Six2 prevents differentiation
Suggests six2 is not JUST a marker but regulates the events that keep the nephron stem cells in a self renewing state
How do you get the nephric duct and the metanephric mesenchyme
Build structures called pronephrons, mesonephros and metanephros
Give clues anout evolutionary development of segmented vertebrates and evolution kidneys
Which mesodermal line of cells do the kidneys form from?
Intermediate mesoderm
- line extends along the whole posterior body
- undergoes a mesenchymal to epithelial cell transition to form two structures the nephric duct and the nephric cord
How does the nephric duct and cord develop?
It develops in the lumen
Duct will eventually give rise to the ureteric bud and cord will give rise to metanephric mesenchyme
Recognise them through expression of different genes
ND and NC communicate
Attempt to build new components
What happens as the pronephric tubules degenerate?
- The middle portion of the nephric duct induces a new set of kidney tubules in the mesenchyme constituting the mesenchyme or mesonephric kidney
- Each meseonephric tubule attracts a blood supply from a branch near the aorta, ending at the capillary tuft
- Mesonephric tubule forms a capsule around this tuft, allowing for blood filtration
- mesenphros then largely degenerate but by then the mesenephric (definitive) kidney has been induced
What happens in the end to the mesonephric duct?
Attaches to the cloaca
Helps trigger change in the local metanephric duct, causing the formation of the bud, grows into mass adjacent- intermediate mesoderm/ metanephric blastema
Why are we keen to look at details of molecules?
- Provides understanding of kidney diseases
- Wnt1 was identified as a gene involved in wilms tumour (pediatric cancer) where kidney elements are incompletely differentiated instead proliferates to form tumours
- Polycystic kidney diseases
Which 2 tissues is the lung derived from?
- Ectoderm- gives rise to epithelial lining of the trachea, larynx and bronchi, alveoli through branching morphogenesis
- Mesoderm- gives rise to cartilage, muscle and connective tissue
What do the trachea, bronchi, broncioles and alveoli all arise from?
Respiratory diverticulum= for this the foregut splits into the oesophagus and trachea and the tip then begins to undergo branching morphogenesis
Wen does the lung bud form in humans?
Week 4
The respiratory diverticulum is a ventral outgrowth of the foregut endoderm, what is it composed of and what is it dependent on?
It is composed of endothelial, epithelial cells
The outgrowth is dependent on signals from adjacent mesoderm and mesenchyme
What is the sac of mesoderm that surrounds each limb bud?
Medial edge- Viscera pleura
outer edge- parietal pleura
Development of the human lung- day 41-44
- Trachea
- Right and left main bronchus
- Right middle, superior and inferior primordium
- Left superior and inferior primordium
Development of the human lung week 9
Trachea
Lobes
Left main bronchioles
Development of the human lung week 10
Lobes
What drives branching? step 1
Endothelial, epithelial cells expressing FGF receptor respond to secreted FGF from mesenchyme nearby bud formation and extension
Exposure of tip cells to high concs of FGF induce expression of secondary genes in the tip (including genes that code for BMP, Shh and sprouty)
Turns tip of bronchial branches into signalling centres
What drives branching? Step 2
- BMP4 is expressed at highest levels in the ‘leading edge’ tip cells and autonomously inhibits epithelial cells proliferation limitiing branching extension
- Shh expressed by the tip cells diffuses to the mesenchyme and inhibits FGF10 expression in the mesenchyme nearest the tip. Splits FGF10 expression promoting the next round of branching
Sprouty limits the action of FGF10 so that branching is restricted to the tip of the branch
Sprouty as a negative feeback loop
- FGF10 induces expression of genes that direct growth and proliferation
- Over slightly linger time-frame, FGF induces expression of another gene, sprouty
- Sprouty inhibits FGF signalling
- Negative feedback loop as a signal induces its own inhibitor, to limit the time of its own action
How is branching morphogenesis an interactive process?
Elongation
Terminal bifurcation
Total number of branching events
17 by the end of 6 months (gestation)
6 more formed during postnatal life
Step that goes on throughout development and into postnatal life
- late disruption= minor effects
- early disruption= results in hypoplasia or agenesis
Development of tissue involved in air exchange
- canicular period- 16th-20th week
- terminal sac period- 24th weeks to birth
- avleolar periods-late fetal thru childhood, type 2 sufactant producing cells
Alveoli are generated they attract endothelial blood cells, which come into close contact and provide expensive capillary network required for gas exchange
How much of your alveoli is present at birth?
1/6
How do we study complex cell behaviours in 3-D?
- In vivo approaches- investigate using conditional KO eg. Rac1KO phenotype in mammary gland
- specific ablation of Rac1 in vivo results in baobob tree with massive enlarged ducts
- Investigate how Rac1 and integrins regulate SC fate into luminal or basal compartments, through tracing of genetically mark sc using GFP reporter genes - Investigate in a 3-D dissected embryonic mouse lung
- right side cultured unpertubed after dissection
- left bronchial tip- mesenchyme removed (no braching occurs at tip as lack of FGF10)
Apicobasal polarity
If a cell loses apico-basal polarity it no longer recognises the need to be an epithelium and can leave
Branching morphogenesis happens to many tissues:
Lung, ureteric bud, salivary gland, prostate, mammary gland and pancreas
What are gonadal germ cells?
Cells put aside in an undifferentiated state for next generation
Which organisms can readily form new organisms?
Chidarians, flatworm and tunicates
Insects and vertebrates there is an early division between somatic and germ cells
Germ cells and the next generation- 2 step process
- Primordal germ cells (PGCs) are determined in a specific location just ‘in the edge’ or ‘outside’ of the developing embryo
- PGCs migrate to the gonad and become the progenitor population for eggs and sperm
What do you need for germ cell detemination?
Plastic cell type (totipotent)
A cell capable of undergoing meiosis
How have C.elegans helped us with stem cells?
In early cleavage and gastrulation
asymmetical division produce a specialised cell- the p lineage cell
P cell is different to others
Depends on plane of division- determining factor
-2 daughters both identical to mother cells
- two different cells. One a progenitor and one has different determinants
What does the P cell act as?
Pre-germ cell
Asymmetric division p cells inherit specialised ‘p-granules’ ( mixture of proteins and RNAs ) that are in the cytoplasm but can get into the nucleus
What can the p- granuales do?
1, bind to DNA of P-cell and block almost all transcription, thus all differentiation
- And in cytoplasm block transcription
- Promote stem cell fate, and cause cells to undergo meiosis (rather than mitosis)
Where does the P cell lie in vertebrates?
It ALWAYS lies at the posterior part of the developing embryo germ cells(= - equivalent to p cells) in all species express the transcriptional blocker
In all vertebrates in the very earliest stages of development, a germ cell is established - this shows:
- no transcription or translation
-therefore no differentiation
These cells therefore have a ‘plastic/multipotent’ identity, also they undergo meiosis
How do you control widespread transcriptional decision?
PGCs are ‘shut’ down transcriptionally/translationally
Epigenetic silencing= mechanism that governs widespread shutdown
-DNA methylation- methyl marks added to certain DNA bases to repress gene activity
-Histone modification- a combination of a different molecules can attach to tails of proteins
Where do PGCs (progenitor germ cells) form?
At the posterior end in extra embryonic epiblast and initially stay there
What is a stem cell?
Divides into a stem cell and a differentiated cell that becomes specialised (eg.muscle or nerve)
What is a progenitor?
Like a stem cell but has a tendency to differentiate into a specific cell type
Already more specific then a stem cell
Why does a stem cell self renew and differentiate?
- self renew- maintains stem cell pool
- specialised cell- differentiation replaces dead or damaged cells throughout your life, adds new cells through life to increase organ size or generate specific cells required
Where is a stem cell found?
Embryonic SC- blastocyst at early stage
Adult SC- fetus, baby and throughout life
How do you make specific cells from ES cells?
ES cells taken from inner mass inside blastocyst
Culture is lead to grow more cells (fluid with nutrients)
ES cells differentiate into all possible types of specialized cells, directed to produce one type by growing under specific conditions
Most body (somatic) cells differentiated and not mitotic but there are stem cells or tissue specific cells
These are multipotent or pluripotent
Recognised first in- Bone marrow, liver, gut + skin
Now recognised in brain/muscle
What is an adult stem cell?
Undifferentiated cell found in tissue and organs
Can self renew and differentiate to become most of all specialised cells types within their specific tissue lineages
What do adult stem cells do?
Maintain cell population
Help you heal
play a role in ageing
What is cellular homeostasis?
The ability to regulate internal conditions usually by a system of feedback controls
Constant or periodic generation of new cells to replace old damaged and dying cells
Example- pregnant females produce more RBC, continuously made throughout life in bone marrow
What is innate regeneration capability?
Tissues that regenerate more readily= more prone to cancer (gut, skin)
What are the types of adult stem cells?
- Hematopoietic- Blood and immune
- Mesenchymal- bone, cartilage, fat, muscle, tendon/ligament
- Epithelial- skin, gut, other lining
- Muscle
- Neural- neurons, glial, retinal
What are specialised niches that stem cells reside in?
Complex micro-environments around the stem cells, made up of many cells, that interact with environment and SC to decide whether to activate
What do hematopoietic stem cells give rise to?
Blood cell types:
- Myeloid= Monocytes, macrophages, neutrophils, blastopils, eosinphils, erythrocytes, megakaryoytes/platlelets
- Lymphoid=T cells, B cells, Nk cells
Found in bone marrow from early as in development, as well as in umbilical cord and blood and placental tissue
HPCs in the complex niche- that stem cells cycle between a quiescent and an active form. How do they decide?
Many adjacent cells in niche provide factors to regulate stem cell activity then divide to HSC or differentiate
Sc respond to physiological signals like hormones
What do mesenchmyal stem cells differentiate into?
Cartilage Muscle Fat tendons, ligaments, connective tissues Located throughout body- bone marrow, fat and cord blood
What is in a muscle stem cell niche?
Includes- post-mitotic, multi-nucleated muscle fibres, ensheathed by basement membrane
Complexity of SC niche increase by presence of other non-muscle, cell types including endothielial and blood cell in vasculature– motor neurons, apidocytes, fibroblasts
What happens when ligand is presented on the muscle fibre?
interact with transmembrane receptors displayed by muscle stem cells to regulate stem fate
Epithelial stem cells
Give rise to 60% of differentiated cells in the body
responsible for covering internal (intestinal) and external (skin)surfaces of body (including lining of vessels and glands)
found in buldge region of hair follicle cells
What are neural stem cells?
Located in sub-ventricular, subgranular and hypothalamus
Neural sc give rise to neural progenitors and then neurons, oligodendrocytes and astrocytes
How does regeneration work?
Adult SC remain quiescent (non-dividing) for long periods
Then they are activated by signals to maintain tissue
When activated divide symmetrical and asymmetric
through this maintain sc pool and differentiate to desired tissue
