biol 205 exam 4 notes
drosophila fertilization
* flies mate · fertilization is Internal · Females store sperm internally in spermatheca · Eggs are regulated by sperm release · Sperm is released as egg passes down oviduct *sperm enters through pre-fixed opening in egg (micropyle) o FERTILIZATION > The sperm tail is much LONGER than the adult fly · So some produce just a few sperm bc the long tail blocks polyspermy by blocking other sperms from entering >sexual selection may promote evolution of sperm giants
A-P patterning in flies: Where would a gene that had an enhancer with high affinity binding sites for bicoid (anterior morphogen) AND caudal (posterior morphogen) be expressed if it needs both to bind for expression?
- Highly expressed in the middle and gradient that fades to both the left anterior side and the right posterior side but a lil more towards the anterior side (stop where protein X ends on posterior side)
Invagination
> infolding of a sheet (epithelium) of cells ~indentation of a soft rubber ball whens it poked ex: sea urchin endoderm
Hans Spemann- Novel Prize in 1935 for work in early developmental processes
- If you artificially alter it so that the grey crescent (nucleus) is on one side, you do not get normal development > . Eventually (as late as 16 cell stage, a nucleus would escape across to non-nucleated side (then he constricted it fully - separated the halves. o Contains the instructions to form the dorsal region o Without it you can only form a belly piece - Fate maps show that dorsal blastopore lip is derived from grey crescent Experiment: transplant tissue from early gastrula dorsal blastopore lip to ventral epidermis region of later gastrula
A-P patterning in flies: Where would a gene that has an enhancer with a low-affinity binding site for bicoid be expressed?
- More anterior and less far across the embryo since there is a low affinity and you need more bicoid for expression
A-P patterning in flies: Where would a gene that has an enhancer with a high-affinity binding site for bicoid be expressed?
- More anterior since bicoid is anterior but still a lot of expression far across the embryo to the posterior side since the affinity is high
A-P patterning in flies: Where would a gene that has an enhancer with a binding site for protein X and a silencer with a low-affinity binding site for bicoid be expressed?
- Since it is being silenced on the anterior side, it will only be expressed on the posterior side where bicoid expression is low enough (stop where protein X ends again)
Level of dorsal protein activates different genes !
- Subdivision of the drosophila dorsal-ventral axis by the gradient of dorsal protein in the nuclei o Cascade of transcriptional regulation occurs regionally *** know how to draw expression patterns based on cascade of TF patterning table as a result of the cascade
Animal pole vs. vegetal pole
- Yolk in the vegetal pole - Animal pole is more animated (more division is happening) - Cleavage divisions take longer at vegetal pole bc of the yolk
ventral follicle cells
- cells build a signal into the egg shell that determines the dorsal/ventral axis of the embryo Ventral signal --> transmembrane receptor --> signaling pathway --> transcription factor - A signal from the ventral follicle cells leads to activation of different genes on the ventral side of the Drosophila embryo 1. Signal built into eggshell by ventral follicle cells 2. Signal processed by snake and easter proteases 3. Signal received by Toll receptor 4. Signal leads to release of dorsal from cactus 5. Dorsal enters nucleus and turns on genes
signaling in ventral region
- triggers the destruction of a cytoplasmic anchor (cactus) releasing Dorsal protein to enter the nucleus and alter gene expression
Vegetal cells:
--> Become endoderm --> Induce cells above them to become mesoderm (signaling) - E. cadherins holds cells together in frogs
*****Early cleavage embryo proteins needed for formation of A-P axis in embryo
1. Anterior: high bicoid & low caudal (higher hunchback bc of gradient/ more hunchback the less nanos) 2. Posterior: high nanos & low hunchback (higher caudal bc of gradient/ more caudal the less bicoid) · Bicoid: anterior morphogen ( blocks caudal mRNA from tanslating to caudal protein) - ACTIVATES Hb · Nanos: posterior morphogen ( blocks hunchback mRNA from translating to nanos protein) - activated posterior genes (caudal) both morphogens inhibits translation of mRNA for opposing polarity by binding 3'UTR and therefore proteins
What are 2 ways that dorsal could act in the nucleus to alter transcription?
1. Can cause ACTIVATION of TFs of certain genes 2. Can cause REPRESSION of TFs of other genes How? - via Enhancers (enhance genes txn or silence gene txn) 1. Certain genes have enhancers that don't bind dorsal with a high affinity, so they are only transcribed at high levels with HIGH CONCENTRATIONS of dorsal (snail, twist, etc) 2. Certain genes are transcribed at INTERMEDIATE levels of dorsal (sog) 3. Certain genes are REPRESSED by dorsal (zen, dpp) by: o A few silencers have a binding site for dorsal and they form a complex that allow dorsal to bind to TF repressors
The anterior-posterior axis is specified during oogenesis STEPS
1. Gurken mRNA is synthesized in nurse cells, transported into an oocyte · Translation is REPRESSED during transport · Translated into protein NEAR NUCLEUS between nucleus and follicle cells at the posterior pole · Cannot diffuse far 2. Gurken protein in egg signals receptor (torpedo) on follicle cells to results in posterization of those follicle cells · Local, signal does not travel far 3. Posterior follicle cell signals egg · Protein kinase A orients the microtubules Outcome: recruits PAR-1 to posterior 4. PAR-1 activity stabilizes microtubules (MT) organization > MT + end (growing) - posterior > MT - end (cap) - anterior >Motor proteins move on microtubules · Motor proteins move on MT o Kinesins move to + end o Dyneins move to - end > energy comes from sperm 5. Motor proteins move mRNAs loaded by nurse cells · Bicoid to ANTERIOR · Oskar to POSTERIOR · Nanos to POSTERIOR End result: Creates the AP axis]
Dorsal blastopore lip: THE ORGANIZER
1. Induced host's ventral tissues to change and form a neural tube and dorsal mesoderm tissue 2. Organized host and donor tissues into a secondary embryo with clear A-P and D-V axes
after the A-P axis is created
1. Oocyte nucleus moved to future dorsal anterior along MT 2. Gurken mRNA was localized b/w nucleus and follicle cells BUT now is located in the follicle cells at the dorsal side *cells on the outside now are FOLLICLE cells - somatic cells surrounding the egg, later these are gone and cells around the edge are cells derived from the zygote
Involution
> inward movement of an expanding outer layer so that it spreads over the internal surface of the remaining external cells ex: amphibian mesoderm
Mesenchyme
> loose collection of individual cells > lack cell polarity > lack cell junctions >cells migrate individually >lots of ECM
Mesenchymal- epithelial transition (MET)
> mesenchymal cells transition to epithelial cells
Ingression
> migration of individual cells from the surface into the embryo's interior. > individual cells become mesenchymal (seperate from one another) and migrate independently. ex: sea urchin mesoderm & drosophila neuroblasts
So we have A-P polarity now. How do we generate D-V polarity in Drosophila?
1. Oocyte nucleus moves to anteriordorsal side 2. Gurken mRNA is localized so Gurken is made locally · Gurken signals torpedo on Follicle Cells (FC) 4. FC closest to nucleus get Gurken signal o Becoming dorsal FC (differentiating) o Activating a repressor inhibits PIPE transcription . Gurken does NOT diffuse far, cannot get to ventral FC 5. Ventral FC make Pipe · Pipe signals vitelline envelope proteins to become sulfated **Pipe synthesis on ventral side NOT on dorsal side Cellularization: 1. follicle cells and nurse cells apoptosis when oocyte matures. 2. Gastrulation defective (GD) binds sulfated vitelline envelope proteins. 3. GD cleaves snake and activated it. GD and snake bind and cleave easter 4. cleaved (active) easter signal activated/cleaves spatzle 5. activated/cleaved spatzle act as a ligand and signals Toll receptor on plasma membrane 6. Toll receptor in endocytosed and signals Pelle and Tube. 7. Pelle and tube phosphorylate cactus triggering degradation of cactus. 8. dorsal is freed to enter nucleus. Dorsal cannot travel far and reaches only ventral-most nuclei.
How do cells choose fates before gastrulation? How did the embryo know where to put the blastopore?
1. The dorsal-ventral axis in the frog
Level of dorsal protein activates different genes -
1. highest levels of Dorsal activate type 1 genes, such as snail, thereby restricting the expression of the associated target genes to the presumptive mesoderm. - type 1 enhancers contain a disordered series of low-affinity Dorsal and/or Twist binding sites. {high Snail} w/ low affinity enhancer for dorsal or twist= more ventralized 2. ) Expression of the type 1A gene, sim, in the mesectoderm depends on Notch signaling, which is produced by the Snail-dependent inhibition of Tom. (D) The sim enhancer contains a high-affinity Dorsal binding site and Suppressor of Hairless Su(H) sites, which mediate Notch signaling. {sim} high = inhibition of TOP= high affinity for dorsal= ventralized & middle 3. Type 2 genes: vnd are activated by intermediate levels of the Dorsal gradient, and low levels of Twist. >a fixed arrangement of Dorsal and Twist binding sites > more middle 4. ) Intermediate levels of the Dorsal gradient induce EGF signaling, which is important for the activation of the Type 2A gene, ind, in the neurogenic ectoderm. (H) The ind enhancer contains a high-affinity Dorsal binding site and ETS sites, which mediate EGF signaling. [Ind] w high affinity enhancer of Dorsal - middle less ventral and more dorsal. 5.zen is repressed by the same low levels of the Dorsal gradient that activate sog. low levels of Dorsal : activate SOG and repress Zen > almost dorsal
animal half and vegetal half
> Animal half has darker pigment > Vegetal half has lighter pigment > grey crescent in b/w - Sperm brings microtubules
The noble prize for TLR and Toll
> Bruce A beutler: TLR > Jules A hoffman: Toll
Primary mesenchyme ingression
> Driven: by changes in adhesion > cells are adhering to Interior > release basal lamina so hemidesmosomes and focal adhesions don't connect them down
frog gastrulation step 3: cover embryo in ectoderm
> Epiboly: movement of epithelial sheets (usually ectodermal cells spreading as a unit (rather than individually) to enclose deeper layers of the embryo. > Can occur by cells dividing by cells changing their shape or by several layers of cells intercalating into fewer layers, often, all three mechanisms are used. Ex: ectoderm formation in sea urchins, tunicated and amphibians >Epiboly is driven by 1) cell division and 2) convergent extension in three dimensions >Convergence and extension occurs in three dimensions (slide 15 lecture 22 for three dimensions image)
In situ hybridization vs qPCR
> ISH gives you a highly parallel look at the expression of a gene in a cell, but it's hard to put a number to those expression levels. >qPCR: lets you quantitatively describe expression levels of genes in a sample
frog gastrulation step 2: gastrulation is internalization of the mesoderm
> Internalization of the mesoderm is accomplished using involution and convergent extension > Convergent extension of a sheet of cells: converge and make cells longer > Involution: inward movement of an expanding outer layer so that it spreads over the internal surface of the remaining external cells **Example: amphibian mesoderm
How to quantiy mRNA
> Quantitative PCR (qPCR) * Quantification of PCR fragments - relative -absolute (standard curve)
Gastrulation
> Xenopus laevis > Blastomeres more relative to one another, resulting in the formation of the three germ layers: > endoderm, mesoderm, & ectoderm
experiment 2
> animal cap cultured alone = ectoderm > animal cap + ventral vegetal cells = ventral and intermediate mesoderm (blood, kidney) > animal cap+ dorsal vegetal cells = dorsal mesoderm (muscle and notochord)
Gastrulation movement
> assemble of several of these movement > mix NOT one or the other
What does basal lamina do for expression to be on
> basal lamina directs genes expression in mammary gland > interacting w ECM causes signal transduction thus gene expression
Vegetal cells have two jobs:
> become endoderm > induce cells above to become mesoderm
> Quantitative PCR (qPCR) : Can be performed on RNA or DNA ?
> can be performed on RNA or DNA - RNA requires conversion to cDNA - DNA qPCR is a bit of a nice use case, it almost always uses RNA/cDNA as a template
Epithelium
> cells connected tightly as a sheet or tube > have cell polarity : apical and basal sides differ cell junctions > migrate as a sheet > little ECM
How do cells move through the ECM
> cells use integrin molecules to adhere to/move through ECM
EMT steps
> changes in gene expression occur > cadherin adhesions are dismantled/broken >basal lamina is dissolved and new mesenchyme specified ECM is made > cytoskeleton is rearranged > mobility is stimulated (cell released from basement membrane)
Cells plated with no basal lamina
> cyclins are ON but expression of genes specific for mammary glands are OFF
qPCR
> expression for our control to be 1 and other values are expressed relative to that value > units do not matter > MT and WT equal in placebo > MT and WT have different values in drug
Epiboly
> movement of epithelial sheets (usually ectodermal cells) spreading as a unit (rather than individually) to enclose deeper layers of the embryo. > can occur by cells dividing, by cells changing their shape, or by several layers of cells intercalating into fewer layers > all 3 mechanisms used ex: ectoderm formation in sea urchins, tunicated, amphibians
Drosophila early embryogenesis
> occurs in syncytium: Karyokinesis w/o cytokines
Thomas Hunt morgan
> pHD in zoology > studied sea spider, sea actors frogs > debate over inheritance > worked to test mendel's theory of inheritance by making mutations in flies > For two years, MT flies using physical, chemical radiations 1st generation: males from Red eyed females x white eyed males cross = ALL red-eyed females Second generation: red eyed female x red eyed male = white-eyed males
frog gastrulation step 1: form a blastopore
> prospective mesodermal cells form in bottle cells - Apical constriction, invagination, bottle cells --> bottle cells at the site of the blastopore undergo apical constriction and elongate, causing the involution of surrounding cells and the formation of a groove that defines the dorsal lip of the blastopore
Anterior - posterior axis specified in C. elegans
> specified based on point of sperm entry - PAR proteins therefore, SPECIFIED AFTER sperm entry
Delamination
> splitting of one cellular sheet into two or more or less parallel sheets. > cellular basis resembles ingression > result: formation of a new additional epithelial sheet of cells ex: hypoblast formation in birds and mammals
How does dorsal protein specify different layers? How can there be more than one layer specified?
>Mesoderm is on ventral side then various layers of ectoderm before you get to the amnioserosa is on the dorsal end so different layers are specified ! via the gradient
Simple observation suggested the point of sperm entry is critical asymmetric cue in setting up the dorsal-ventral axis in Xenopus (frog)
>Sperm enters animal cap → cortical rotation (outer rotates but inside remains intact) → opposite side of sperm entry is where the blastopore forms at DORSAL side (where gastrulation initiates) Fertilization triggers a cytoplasmic rearrangement and fixes the site of initiation of gastrulation o Rotates the cortical cytoplasm with regards to the internal cytoplasm (outside moves, inside doesn't) this exposes the grey crescent > microtubules involved
Cells plated with basal lamina
>adhesions developed > cyclins are ON and expressions of genes are ON for mammary gland
Given what we just learned, which embryo is: A) ventralized, B) dorsalized, and C) wild-type?
A: Fate map of a lateral cross section through the Drosophila embryo at division cycle 14. : WT B: Dorsalized C: Ventralized
grey crescent cytoplasm
As cell divisions occur, only a small group of cells inherit > these cells become the "organizer"
After specification of the A-P axis, how could we tell that Gurken protein is now localized dorsally?
Co-localization · Fluorescence · Stained for antibody for gurken protein & actin too bc actin is in the cortex which shows you the outlines of the membranes > Gurken protein can travel only a short distance > nucleus at dorsal > overlap = yellow
How is A-P axis established in the embryo?
Cytoplasmic polarity (MATERNAL EFFECT) · Ligation of embryo (separating the anterior half from the posterior half and then observe development): > if done early in development the anterior and posterior structures developed but no middle o Suggests that intermediate tissue need BOTH signals
What happens? What does this tell us about the specification/determination of the early dorsal blastopore lip tissue and what would the resulting frog larvae look like?
Dorsal blastopore lip: THE ORGANIZER > early dorsal blastopore lip tissue can instruct the tissue around it to become all of these normal structures to form embryonic axes (definitely DETERMINED and SPECIFIED) e.g: Two-headed tadpoles ( 2 dorsals)
Subdivision of the Drosophila dorsal-ventral axis by the gradient of Dorsal protein in the nuclei
Dorsal: turns on twist, snail genes, turns on zEN GENES Dorsal turns off zerknullt, tolloid, decapentaplegic ( dorsal structures) Twist: turns on mesodermal genes and snail gene snail: turns off ectodermal genes (including FGF8, rhombioid)
Thomas Hunt Morgan
Drosophila conclusion: using drosophila, explored the theory of GENES being on chromosomes, "crossing over" basis of heredity, mapping genes on chromosomes. Genes. established drosophila as an important model system. Nobel prize in 1933 for physiology or medical chromosomes
Cells exist in one of the two states during development
Epithelial and mesenchymal cells > they express DISTINCT proteins, esp. cell surface proteins
Fibronectin in ECM can signal cell via integrins
Fibronectin - part of extracellular matrix Integrins in the membrane link the fibronectin outside the cell to actin filaments inside the cell. Integrins: cell adhesion molecules that INTEGRATE intracellular and extraceullar matrices
Extracellular Matrices in the developing Xenopus - embryo during gastrulation
Fibronectin: General adhesion molecule - links cells to one another and other substrates (collagen, proteoglycans (ECM)) Fibronectin creates "roads" over which certain migrating cells travel
Why do we study development in frogs?
Frogs as a model system: · Have LARGE cells o Can transplantation experiments easily · Develop rapidly · Use many of the same processes to develop body axes and organs as other vertebrates Bad: - Long growth period before fertility - Some species are ploidy (4n) which means we would need 4 copies to study whereas xenopus tropicalis is 2n - easier to study
Even though no cell membranes, interphase and anaphase still show
Interphase: bulbs coming out - poking out Anaphase: diving parallel to corte (allows nucleis to stay in periphery ) : cycle 10 - no poking out
Types of cell movements during gastrulation
Invagination Involution Ingression Delamination Epiboly
Frog Gastrulation: Step 1, form a blastopore
Labels - slide 5 lecture 22
Now we have different mRNA's on anterior and posterior side of oocyte. Are all maternal mrna translated immediately? how might they be kept inactive
Not all maternal mRNA are translated immediately Oogenesis:Oocyte Growth = Poly-A tail present, immediate translation & bicoid and caudal can prevent translation of specific mRNA Cleavage = Short Poly-A tail, Blocks translation
Step 1 of gastrulation in sea urchins?
Primary mesenchyme cells INGRESS > LOCATION : move up >SHAPE: cells become more circular and smaller
experiment conclusion
a signal from the vegetal pole is needed to induce mesoderm in animal caps
Hans Spemann- Novel Prize in 1935 for work in early developmental processes exp 2
a. constrict in the plane of the first cleavage result : Grey crescent BISECTED = normal development b. constrict perpendicular to the plane of first cleavage Grey crescent ALL on one side result: belly piece only suggest that Something in the grey crescent is essential for proper embryonic development!
drosophila segments
anterior and posterior/ dorsal and ventral - Head - Thorax - Abdominal
Ventralized embryo cells
blocks cactus from binding to dorsal, allowing dorsal to enter nucleus and producing Ventral side.
Epithelial Mesenchymal Transition (EMT)
breakdown of epithelium cells into loosely organized mesenchyme cells
E-Cadherin
control embryo: involution Embryo + antisense oligonucleotide to a specific RNA = NO involution
Frog gastrulation:
covers the embryo in ectoderm · Ectoderm gives rise to skin - Invagination - Involution - Ingression - Determination - Epiboly
The dorsal protein has been stained black. Is the dorsal protein in the dorsal region or ventral region? (pic A)
dorsal has entered the ventral most nuclei
when is the anterior-posterior axis is specified in fruit flies?
during OOGENESIS · Bicoid and Nanos are segregated o Nurse cells: Mitotic sister, germline cells o Follicle cells: somatic cells § Contain a receptor for Gurken to receive the signal
without a blastocoel there is no
ectoderm
Experiment 1: animal pole tissue cultured in isolation
ectoderm formed
Length of life cycle of a fruit fly
fly is TEMPERATURE DEPENDENT (to slow down just put in the frig) · The cool, the longer it takes to survive. 25 : 8.5 days 18: 19 days 12: 50 days (not exact)
Gastrulation:
goal is to separate the germ layers to a more specified locations (endoderm, mesoderm, ectoderm) > Ball of cells -> ball of cells w DENT move layers to where they should be
Reorganization of the cytoplasm and cortical rotation produce
gray crescent the eggs of certain species of frog
Snail vs Sog
high levels of Dorsal: snail specifies mesoderm (ventral) intermediate levels of dorsal : Txn & Sog specifies neural ectoderm (vs epithelium)
ECM
insoluble network of extracellular macromolecules secreted by cells Major components: 1) collagens 2) proteoglycans: heparin sulfate 3) glycoproteins: fibronectin , laminin
***karyokinesis and cytokinesis in drosophila
karyokinesis happens WITHOUT cytokinesis · DNA is replicated and there is division of the genome (nuclei), but division of the cytoplasm isn't happening o No membrane to separate the cells that are divided · Nuclei move to periphery --> karyokinesis occurs >cytokinesis occurs in 2 main waves: 1. First pole cells (prospective germline) at posterior (~ cycle 10) 2. rest at cycle 13 (prospective somatic cells) · Even though no cell membranes, interphase and anaphase still show o They divide long, parallel to cortex allow nuclei to stay in periphery > cell membrane does form until the 13th division
what forms from the dorsal lip blastopore
mesoderm and endoderm
experiment 1: animal pole tissue cultured in combination with vegetal pole tissue
mesoderm formed
How do we generate D-V polarity in Drosophilia?
protein called Dorsal signals on the VENTRAL side
qPCR
qPCR can be used to quantify mRNA expression in different populations of cells, and is often used to compare levels of mRNA expression for a particular gene in untreated samples to levels of mRNA expression for that same gene in treated samples.
Reverse genetics:
start with a gene and then see what it does to the phenotype (what does this particular group of DNA do?) · Example: embryo cannot live without muscles very long (sns mutant- muscles dont form)
Forward genetics:
start with a phenotype and search for the genetic basis of that phenotype
Bicoid:
when you have something deficient in bicoid (knockout), you lose the anterior portion of the head (ACRON) ! · If you ADD bicoid to anterior, it'll be normal (bc this is where bicoid already is) · If you ADD bicoid to middle in a bicoid deficient MT, you get a head in the middle · If you ADD bicoid to the posterior in bicoid deficient embryo, you get a head at the tail too *If you ADD bicoid to the posterior in a WT, you get a head at the tail on anterior and posterior side : 2 heads
Syntactical A-P axis formation Fly embryo
· > Gradients of RNA and proteins are created in syncytial fly embryo. >distinct sets of nuclear proteins are found in nuclei from different regions of the embryo > nuclei are SPECIFIED > b/c of their nuclear different, when cells form, the cells are DIFFERENTIALLY SPECIFIED. >Opposing gradients: 1. High levels of bicoid on left anterior 2. High levels of Caudal on right posterior - Gradients of RNAs and proteins are created which causes more effects
Blastocoel:
· Can change shape and aid w cell migration during gastrulation · Can prevent premature interaction of cells in animal cap with cells at the base of the blastocoel o Without a blastocoel, there is NO ectoderm
Specification of cell fate by the gradient of Dorsal protein
· Dorsal mRNA / protein is uniform · Zygotic translation starts 90 mins after fertilization (~cycle 11-12) · Once cactus is degraded on VENTRAL SIDE, dorsal is free to enter the VENTRAL nuclei · It cannot travel far, so it does most reach the dorsal nuclei
How can you start to determine which genes are involved in mutations? christiane nusslein volhard, Erin Weishaus and Ed lewis Noble prize 1995
· Genetic screens for mutants altering embryonic development o Used mutagens (Random Mutagenesis) o Looked for phenotypes: 1) § Take flies w mutagens and cross w wild type 2) § Take progeny and back cross to the mutated parent 3) § Cross parents that were both heterozygous to create some progeny with 2 copies of the mutation · Once they found the mutant by looking for particular phenotypes (what did they mess up) · Then they worked backwards to figure out where the mutation was
drosophila mid blastula transition
· Synchronous for a long time cycle 1-10 : 8 min synchronous cycle 13: 25 min cycle 14: 75-115 min : asynchronous · Then cellularize and become asynchronous Change in cell cycle regulation: · From cycles w only M and S cycles with G1 and G2 phases
dorsal protein
· isn't translated until 90 mins post-fertilization o Dorsal proteins is found throughout the embryo, not just in a certain region § How does it cause the dorsal-ventral patterning in the embryo? > Inactive when outside the nucleus and bound to Cactus > It enters the nucleus on the ventral side > A dorsal knockout would have NO VENTRAL SIDE and only be dorsal
In embryos mutant for Toll, dorsal, or any other positive component of the signal
· transduction pathway ALL cells take on dorsal fates o This led to the realization that conserved signal transduction pathways exist in us § Important roles in pathogen immunity § The receptors are different (Toll isn't same as IL-1) even though the pathway is the same · Toll still holds an important role - Mammals also have a Toll-like pathway and similar receptors !
