D104 Exam 2

Definitions

• Archenteron: The precursor to the gut that forms during gastrulation.

• Notochord: A rod-like structure in the embryo that gives rise to the spine.

• Somites: Segmented blocks of mesoderm that will develop into vertebrae and muscles.

• Epiblast: The layer of the blastoderm that contributes to the formation of the embryo.

• Hypoblast: The layer of the blastoderm that contributes to the extraembryonic structures or yolk.

• Henson’s Node: The structure at the anterior end of the primitive streak in avian embryos, analogous to the organizer in amphibians.

• Primitive Streak: A structure that forms during the early stages of embryonic development and establishes the A/P axis.

• Wnt Pathway: A signaling pathway involved in regulating cell-to-cell interactions during development.

• Organogenesis: The formation of organs from the germ layers during development.

• Spemann Organizer: A group of cells that directs the development of surrounding tissues in amphibian embryos.

Xenopus Development

Overview

Xenopus is an important model organism for developmental biology.

Cell Signaling Pathways

• Wnt/β-catenin pathway:

  • Wnt-11: Localized to the dorsal side, crucial for specifying the future dorsal axis.

  • Activates β-catenin which stabilizes and promotes expression of target genes involved in dorsal structures formation.

• Nodal:

  • Secreted from the endoderm; important for mesoderm induction and establishing the A/P and D/V axes.

  • Localization occurs predominantly in the vegetal hemisphere during early cleavage.

• Vg-1:

  • Located in the vegetal pole, a member of the TGF-β superfamily promoting mesoderm formation.

• VegT:

  • a T-box transcription factor, endoderm and mesoderm specification

Cleavage and Blastula Stage

1. Fertilization and Cleavage:

   • Sperm enters egg in the animal region.

   • The fertilized egg completes meiosis and cortical rotation begins after fertilization (60 minutes post-fertilization).

2. Cleavage Stages:

   • 1st Cleavage: Occurs along the animal-vegetal (A/V) axis

   • 2nd Cleavage: Also along A/V axis, perpendicular to the first

   • 3rd Cleavage: Is equatorial

   • The cleavage produces 4 small animal and 4 large vegetal blastomeres.

   • After 12 synchronous cleavages, a blastula with several thousand cells and a blastocoel forms.

Gastrulation

1. Initiation:

   • Begins opposite sperm entry; formation of the blastopore marks the future dorsal side.

2. Cell Movement:

   • Involution of mesoderm and endoderm cells occurs, converging along the A/P axis under ectoderm.

   • Ectodermal cells spread through epiboly to cover the embryo.

3. Archenteron Formation:

   • Archenteron is the cavity formed between mesoderm and dorsal endoderm, which will eventually become the gut cavity.

Neurulation

1. Chordate Development:

   • Characterized by interactions between mesoderm and ectoderm.

   • Anterior dorsal mesoderm develops into the notochord and somites.

   • Notochord induces overlying ectodermal cells to form the neural plate, neural folds, and eventually a neural tube.

Emergence of Vertebrate Features

1. Developmental Features:

   • The brain divides into fore, mid, and hindbrain in the anterior region; eye and ear placodes form; three branchial arches develop.

   • Distinct somites align along the A/P axis and the tail forms last. Organogenesis commences in this phase.

Dorsal-Ventral Axis Specification

1. Cortical Rotation:

   • Triggered by sperm entry, causing rearrangement of the cortex.

   • Causes reorganization of microtubules, plus-end away from site of sperm entry. (mt reorganization is important for cortical rotation which is important for DORSAL STRUCTURE FORMATION)

   • Causes relocation of critical vegetally localized maternal mRNAs and proteins to a site directly opposite to sperm entry.

2. Maternal mRNA Localization:

   • Key maternal mRNAs reposition opposite sperm entry, activating Wnt/β-catenin signaling in 2 of 4 cells, defining the future dorsal side.

Experiments on Dorsal Determinants

1. Experiment Details:

   • Division of the embryo into dorsal and ventral halves at the 4-cell stage shows normal development in the dorsal half and abnormal development in the ventral half, underscoring the importance of dorsal determinants.

   • Shows slight mosaic development

Wnt Pathway Activation

1. Wnt-11 mRNA:

   • Present in both dorsal and ventral cells, but signaling is activated solely in dorsal cells.

   • β-catenin stabilizes and enters nuclei of cells, thus defining organizing centers.

     • Combination of ß-catenin and VegT activity defines where the organizing centers will arise

Spemann Organizer and NKC Cells

1. Key Cell Types:

   • NKC cells from the dorsal-vegetal region at the 32-cell stage generate endoderm.

     • transplantaion of NKC into another embryo forms a double embryo with a duplicated axis (just like spemanns organizer)

       • That shows sign of regulative devlopment

   • Spemann’s Organizer cells from the equatorial region during early gastrula form the notochord

   • Both are critical for anterior-dorsal fate development.

Germ Layer Specification

• Ectoderm Development: Forms epidermis, sweat glands, hair, and nervous system (brain & spinal cord).

  • Ectodermin and Foxl1e

• Mesoderm: Gives rise to notochord, somites (muscles), and structures like the heart, kidneys.

• Endoderm: Becomes gut, lung, liver, and pancreas.

  • VegT (endoderm patterned by vegetal cells - like how vegT is vegetal and forms ENDODERM and mesoderm)

Mesoderm Induction Studies

1. Induction Mechanism:

   • Animal cells in lab made to touch vegetal tissue can induce mesoderm formation (when it was cut out before the expirement)

   • Shows that there is a mesoderm inducing signal (filter shows that it is a small, diffusible molecule)

   • Mesoderm induction occurs at the blastula stage

     • D/V: if you do the same expirement but use more ventral side cells, the mesoderm that forms will become epidermis/blood and if more dorsal side cells are used then mesoderm wil become muscle/neural tube

       • but its a gradient so if you combine some v and d cells - the d side will take over (making muscle cells instead of blood)

Mesoderm Induction Signals and Patterning

1. Signal #1: General mesoderm inducer from the vegetal region.

2. Signal #2: Signals from ventral mesoderm subdivides regional fates (e.g., blood, muscle).

3. Signal #3: Signals from Spemann’s organizer limit ventral signals, aiding somite specification.

Spemann Organizer Functionality and Nodal

• Signals from the organizer are vital for regulating and specifying tissue fates within the embryo.

• Overlap of VegT and Wnt activity results in high levels of Nodal, and specification of Spemann organizer.

  • Nodal = xnr protiens that help define the spemanns organizer

  • nodal and vegT both induce mesoderm - if vegt is gone then nodal is reduced and no mesoderm forms

    • mesoderm formation can be rescued by xnr injection though!

DV Mesoderm Patterning

• Ventral Signals

  • BMP4

  • Xwnt8

• Dorsal Signals

  • Inhibitors of ventral signals are here (inhibits BMP4 and Xwnt8) so theres low activity of them in the dorsal side

    • Inhibitors:

      • Of BMP4: Noggin, Chordin

      • Of Xwnt8: Frzb

Wnt Patterns

• Wnt before gastrulation: highest dorsally

• Wnt after gastrulation: highest posteriorly

Determined Cells

• Xenopus gastrula epidermal cells are determined

• The fate of epidermal cells is dependent on their location

  • transplanting epidermis tissue in neural area will make epidermis tissue form in the brain

More Facts

• BMP4 is key inhibitor of neural tissue (remember from N172!)

  • washing away BMP4 in ectodermal cells will allow brain tissue to form

    • also if you treat those cells with noggin (inhibits BMP4)

• Four families of secreted signaling molecules:

  • Wnts • fibroblast growth factors (FGFs) • BMPs • Nodals

• A/P Neural Patterning

  • anterior mesoderm induces head and brain.

  • posterior mesoderm induces trunk & spinal cord.

• Body Plan Steps

  • Sperm entry causes cortical rotation which defines dorsal fates

  • Mesoderm patterning (ventral = blood // dorsal = muscle) and gastrulation movements (involution/epiboly) determine the A/P axis

• Vertebrate embryos share a key developmental hallmark as chordates = a notochord

Xenopus Questions

• Which of the following is unique to Xenopus (or other amphibians) when compared to mammalian development?

  a. Early development occurs in utero with a placenta

  b. Embryos undergo discoidal cleavage on a large yolk

  c. External fertilization and external embryonic development in water

  d. Development proceeds from a blastocyst stage with an inner cell mass

  • Answer: C

• Which of the following most accurately describes a relative weakness of Xenopus as a model system?

  a. It offers a short life cycle facilitating quick breeding and genetic analysis

  b. It has a relatively long generation time making multi-generational genetic studies challenging

  c. It cannot be used to study early embryonic development because fertilization is in utero

  d. No techniques exist for gene knockdown in Xenopus embryos

  • Answer: B

• Which of the following is a major advantage of using Xenopus for developmental studies?

  a. Extremely short generation time allowing for rapid genetic screening

  b. Large externally fertilized eggs that are easy to manipulate and inject

  c. Fully transparent embryos for live imaging of internal tissues

  d. Readily available embryonic stem cell lines for cell culture

  • Answer: B

• If microtubule reorganization in a Xenopus zygote is blocked, which outcome is most

  likely?

  a. An embryo with duplicated dorsal axes (two heads)

  b. A ventralized embryo lacking normal dorsal structures

  c. No effect, since dorsal fate is determined by the blastocoel location

  d. Overproduction of neural tissue in the vegetal region

  • Answer: B

Chick Development

How Birds/Mammals Differ From Xenopus

• Epiblast = blastula

• Primitive Streak (PS) = blastopore in gastrulation

• No distinct region to form future germ layers

  • instead germ layers determined by by cell MOVEMENT

• Incomplete cleavage and garm specification happens very late

Embryo Development

• INCOMPLETE cleavage

• Blastoderm/blastodisc is a disc of cells, that sits on top of the yolk

• Dorsal side forms away from yolk and ventral side forms next to it (you vent to people you’re close to!)

Epiblast Formation

• Epiblast: embryo proper

• Hypoblast: extra-embryonic structures (e.g., yolk sac).

• Koller’s sickle: crescent shaped ridge at boundary of area pellucida and area opaca; first sign of A/P polarity

• PMZ (posterior marginal zone): epiblast cells that lie adjacent to K’s sickle, primitive streak (gastrulation) is initiated at this location.

  • Primitive streak initially a thickening that becomes a furrow (dip or indentation) ; extends half-way up the area pellucida.

  • PMZ marks posterior of embryo, anterior is where streak ends.

Gastrulation

• Cell Movement:

  • As PS moves forward, epiblast cells move into the furrow (ingression), and then they spread out in a loose mesenchymal layer (cells that break the adherin and integrin bonds to move freely)

  • Cells that move inside become the mesoderm / endoderm and cells that dont go into furrow become ectoderm

• Henson’s Node:

  • Forms the neural structures (like how Spemanns Organizer does)

  • Formed by cells at the anterior most location of PS

    • Once node is formed = PS REGRESSES (GOES BACKWARDS)

  • Some HN cells move anteriorly under the epiblast = forms the head process (like the notochord)

  • As PS regresses to the posterior side, notochord keeps forming (behind the PS // anterior to it)

• Notochord Formation:

  • Linked with somitogenesis and neural tube induction

  • Mesechymal cells on both sides of notochord = turn into somites and mesoderm

  • Notochord induces epiblast cells above it to form neural tube

  • Head fold forms anteriorly, trapping the gut (similar fold in tail happens and then ventral closure happens)

• Extra Embryonic Structures Form

  • Form from hypoblast

  • Stuctures:

    • Amnion: provides mechanical protection (You need protection to get into Omnia)

    • Allantois: site of gas exchange (O2-CO2) and recieves excretory products (Alan says FINALLY I can breathe)

    • Chorion: surrounds whole embryo and lies just beneath chick egg shell (Orion is an ALL AROUND great song - great chords)

Axis Specification

• Initially the blastodisc is a perfect circle = radially symetric (like starfish)

• After egg is laid, Kollers sickle and PMZ form at one location at the edge of disc

• Dependent on rotational forces (UNLIKE XENOPUS THERES NO DISTRIBUTION OF MATERNAL DETERMINANTS - LiKE THE DORSAL DETERMINANTS)

  • Instead of dorsal determinants - the rotational forces determine the axis

  • In the oviduct (fallopian tube), egg rotates every 6 minutes

    • causes blastoderm (the wall of cells - not refering to whole disc/ball) to tip in direction of rotation

    • THATS WHAT SPECIFIES A/P AXIS

    • Upper part forms the future posterior side of blastoderm

      • KS and PMZ formed there and PS starts there

PMZ and PS Expirement and Importance

• PMZ cells (like NKC cells) induce another PS (another AP axis) when transplanted into another embryo

  • only ONE axis develops further though. the more advanced streak inhibits the other (so there wont be two heads like in the xenopus expirement)

• Nodal is critical for the formation of the PS AND Hensons Node (just like how its important for the NKC and Spemanns Organizer on the dorsal side)

Cell Signaling Pathways

• Vg-1 (TGF-b family) and Wnt8c:

  • expressed in PMZ which induces Nodal in the future primitive streak

• Cereberus:

  • Secreted by hypoblast, INHIBITS NODAL SIGNALING.

    • PS can only go forward when hypoblast is displaced from PMZ (cause then nodal wont be inhibited)

    • Hypoblast displaced by endoblast instead which finally allows nodal to help FGF form the PS)

• Nodal:

  • Critical for PS and Hensons Node

• FGF (Fibroblast Growth Factor):

  • Kollers sickle cells secrete this (KS also promotes expression of nodal in PS)

Mouse Development

Cleavage and 2 Cell Types

• Fertilization and cleavage occur in the oviduct. First cleavage at 24 hours, then every 12 hours

• 8 cell stage: cells compact to form a solid ball = compacted morula

• Two Cell Types

  • Trophectoderm: forms JUST extra embryonic tissues, aka placenta (like hypoblast)

  • Inner Cell Mass (ICM): forms embryo (like endoblast) and some extra embryonic tissues

  • TO MAKE TRANSGENIC MICE, transfer gene in the ICM not the trophectoderm - thats where the actual embryo will be

Gastrulation

• Day 6 (E6): starts with appearance of PS at one side of the cup (thats the future posterior side) - then it moves towards the bottom of the cup (future anterior side)

• Process:

  • Epiblast cells move into PS between the ecto and endo to form the mesoderm

  • A node forms at the anterior tip of PS (same as Hensons Node) and that makes the PS start to regress

  • Mesoderm cells that are anterior (behind the PS) to the PS form the notochord (same as chick)

• D/V Sides

  • Dorsal = inner side of cup (rats! the d is inside when it came!)

  • Ventral = outer side of cup

• Gut

  • endoderm forms the gut

  • lateral surfaces come together to enclose the gut

Neurulation

• Day 8: (8 days after fertilization) neural folds begin to form at anterior dorsal side

• Day 9: turning happens to form a more recognizable mouse embryo

Axis Specification for Mammals (not bird or frog / bird is similar though)

• formation of extra embryonic stuctures (placenta) are essential for mammal embryonic development

• No clear sign of polarity and no evidence of localized maternal factors in mammal egg

• No yolk in eggs

• Zygotic gene transcription occurs at 1 cell stage

• Seperation of embryos at the 2 cell stage (like in previous expirements) result in two twins! (humans and mice)

  • Regulative development

Cell Determination and Lineage

• Cell fate determined by POSITION OF CELLS

  • if outer cell is pushed inside then that cell becomes part of ICM (not trophectoderm)

    • determination of ICM vs troph. happens after 32-cell stage

• Cdx2 expressed in troph cells

• Oct4 (pluripotency transcription factor) expressed in ICM cells

• Nanog expressed in ICM

• Gata6 expressed in primitive endoderm

Polarity and AP Specification for Mouse

• DVE: Distal Visceral Endoderm specified

  • expresses ANTAGONISTS of nodal, wnt, and BMP (lefty-1)

• EEE

  • releases inhibiting signal (BMP4) that restrict nodal to the most distal cells

  • nodal signals released from epiblast determine the anterior side and they push the vicseral endoderm to become the AVE

  • if there was no BMP4 - there would be no anterior specification

• AVE (like chick hypoblast)

  • formed by cells induced by nodal signaling from epiblast

    • aka formed by induction by Nodals from the epiblast and repression by BMP4 from the EEE.

  • expands to one side to form future anterior side

  • secretes nodal inhibitors and inhibits PS on that side

    • so it forms on the opposite side (to form posterior side)

Somite Formation

• Somites

  • blocks of mesoderm derived from the paraxial mesoderm on

    either side of notochord

  • formed in A → P order

  • Give rise to

    • bones / cartilage

    • head / neck muscles

    • dermis of skin

  • Theres a fixed number of them in each organism

• Gradients

  • Opposing gradients of fibroblast growth factor (FGF)

    + Wnt and retinoic acid (RA) maintain the Stem Zone

• Formation in Chick Embryos

  • Somitogenic stem cells form around the Hensens node.

  • Pre-somitic mesoderm (PSM): unsegmented mesoderm between the regressing node and the last formed somite.

  • As node regresses, cells at the anterior end of PSM start forming somites and differentiate into distinct structures based on their A/P position

• Rotation

  • 180 degree rotation does not alter OG timing and order of somites (even though orientation is reversed)

• Clock and wave front model

  • Wavefront of determination moving in from A to P corresponds to a threshold level of FGF/Wnt signaling.

  • A clock - periodic or oscillatory gene expression associated with Wnt, Notch and FGF signaling moving from P to A.

  • The distance moved by the wavefront per oscillation of the clock marks the region/length of PSM that will form a somite.

  • Period of oscillation = time it takes one pair of somites to form.

• Gene Expression

  • c-hairy1 expression oscillates every 90 minutes in chick embryos, ~ the time it takes one pair of somites to form

  • Wave of c-hairy1 expression is shown during formation of somites 15-17

• Notch and Delta signaling

  • Notch/Delta signaling involved in delimiting boundaries between somites.

  • Lunatic fringe – glycosyl transferase that potentiates Notch/Delta signaling and shows cyclic expression.

  • Mice deficient in Notch/Delta pathway – somites either not formed or irregularly spaced; similar rib abnormalities

  • Lunatic fringe mutations in humans cause spondylocostal dysostosis – short stature, severe spinal patterning defects.

• Hox gene expression and axial patterning

  • Mesodermal cells express Hox genes from early gastrulation.

  • Neural tube is also patterned through Hox gene expression.

  • Posterior dominance encountered in vertebrates as well.

  • Redundancy; multiple HOX clusters present in vertebrates

• Homeotic transformations in mice

  • Homeotic mutant phenotypes result from loss or gain of Hox gene expression.

  • Loss of gene function leads to cells assuming a more anterior value for example, Hoxc8 mutant mice have extra ribs

• Origin and patterning of Neural Crest cells

  • Neuronal:

    • Sympathetic and parasympathetic NS

    • Glial cells

  • Non-Neuronal:

    • Melanocytes

    • Adrenaline producing cells of the adrenal gland

    • Bone, cartilage and muscles of the face

  • Transplantation experiments suggest that NCCs acquire regional identity prior to migration based on the Hox gene expression at site of NCC origin

    • Other experiments suggest that NC cells have some plasticity and can sometimes change their fate based on local signaling at destination as well.

• Disruption of NC Migration

  • Cleft palate

  • Treacher Collins syndrome and other craniofacial defects

  • Hirschsprung Disease – defects in Notch and Hh pathways; failure of

    innervation in hindgut

  • Waardenburg Syndrome – piebaldism, heterochromia; rare autosomal

    dominant disorder of melanocyte development

• Neurula stage embryo

  • By neurula stage, a clear body plan has been established & regionalized.

  • Limb and organ (for example eyes, heart) forming regions have been

    specified; but no differentiation or elaboration of cell fates has occurred.

  • Cells within these areas retain considerable capacity for regulation

Nematode Development

Uses

• Simple development

• Rapid development time (15 hrs to hatch - 50 hrs to adult stage)

• Easy to breed / self fertilizing

• Transparent embryos (can see mutants easily)

• Small genome size (3% of humans - but still a lot of genes - 16k)

• Can track exact cell lineage (558 cells at hatching and 959 in the adult after 131 cells have died by apoptosis)

Life Cycle

• Cleavage

• Embryogenesis

• Hatching

  • after hatching the larvae are very similar to adults, they just need to undergo 4 larvae molts and thats it!

Cell Divisons

• First Division: gives one LARGE anterior AB cell and one small P1 cell

  • P1 gives rise to P2 cells and EMS (endo and meso) cells

    • P1 → P2 → P3 = makes germ cells

    • AB - P1

      • P1 → EMS and P2

        • EMS → MS and E cells // P2 → P3 and C

          • P3 → P4 (germ cells) and D

      • AB → ABa and ABb

• 2nd: AB cell divides to form ABa (anterior: neurons, epidermis + pharynx meso) and ABp (posterior: neurons, epidermis, and specialized cells)

• Fate map is FIXED and invariant (never changes)

  • Therefore, early c eligans devlopment is MOSAIC

    • So, body axes depend on asymmetric cell division and inheritance of cytoplasmic factors, not cell-cell interaction (regulative)

  • Gastrulation starts at 28-cell stage and MS cells become muscle and E cells invaginate to become the endoderm (gut) surrounded by meso and ecto

    • MS and E cells come from EMS (which came from P1)

  • Zygotic gene expression starts at 4-cell stage!!!

Axis Determination

• Determined by assemetrical cell division

  • Sperm entry controls first cleavage and determines the future posterior end

    • Sperm brings in a centriole that become a microtubule organizing center (MTOC)

  • Local cell-cell interactions regulate the determination the D/V axis

    • example: If you experimentally switch the positions of ABa and ABp you still get a normal worm - P1 progeny are also altered; the position of the EMS daughter cell with respect to the AB cells is reversed. This results in reversal of D/V axis.

    • The cell next to P2 will become ABp – in the absence of P2 signaling, the cell fate becomes ABa

    • Glp-1 and apx-1 genes are involved in cell-cell interactions that establish the A/P axis

      • ABa and ABp both express Glp1 (receptor) and P2 expresses Apx-1 (ligand) on the surface

      • Apx1 and Glp1 interact with eachother in the adjacent cell - this causes the descendant cells from ABa and ABp to respond differently to signals from the MS cell

• Mom and Pop Genes

  • Mom: More mesoderm

    • formed when endoderm induction fails

  • Pop: Posterior pharynx defect

    • extra gut causes MS cells to adopt E like fate

    • absence of pop gene makes EMS daughter cells act like they get Wnt signal cause its no longer being repressed by pop gene

• Hox Genes

  • specify positional identity on AP axis - and they are colinear except ceh-13 (needed for anterior fates in the embryo)

  • Mutations result in homeotic transformations (turning one body part into another)

MicroRNA and Timing of Development

• lin-14 and lin-4

  • lin-14 = transcription factor

  • lin-4 = expresses microrna which REPRESSES lin-14 expression

    • Dicer processes lin-4 which binds to RISC to cut up the lin-14 mRNA (gene silencing)

  • mutaions of lin-14 change the timing of nematode devlopment

  • lin-14 gain of function and lin-4 loss of function mutants have the same phenotype (cause its the same thing)

Morphogenesis

• Apoptosis Process

  • Apoptotic death causes cell to shrink, shred DNA and remain enclosed

  • Cell forms fragments and is degraded by phagocytic cells

• Apoptosis Uses

  • remove excess cells in the c eligans lineage

  • remove tissue between vertebrate digits

  • form neural tube and circuits

• Apoptosis Regulation

  • Ced-3 is the executioner caspase that fragments the DNA

  • Cascade

    • EGL1 inhibits Ced 9

      • (to allow apoptosis by activating Ced 3)

    • Ced 9 inhibits Ced 4

      • (by binding to Ced 4 it inhibits it)

    • Ced 4 activates Ced 3

• Changing Cell Shape

  • shape changes are needed for morphogenesis

  • 2 Types

    • Intracellular Mechanisms

      • through cytoskeleton (actin, myosin, etc)

    • Extracellular Mechanisms

      • cell to cell interactions

      • cell to substrate interactions

  • ECM

    • cells must interact with ECM to change shape

      • Cadherins (cell to cell bonds - adherens junctions and desmosomes)

        • they need Ca2 so tissues can dissociate by just removing Ca2

        • Tissue specific cadherins:

          • E - epithelial

          • P - placental

          • N -neural

        • They provide specificity

          • cells expressing more of a certain type of cadherin will aggregate and group together

      • Integrins (attach cell to basal membrane/ECM)

        • interact with the ECM (structural, bioactive, have proteoglycans)

      • Immunoglobulin (cell to cell bonds)

        • these DONT need Calcium (unlike the cadherins)

  • Movement

    • loss of adhesion by breaking cadherin and integrin bonds = mesenchyme state (moving freely)

    • Bottle Shape Cells - Xenopus

      • form at blastopore - actin causes this shape through apical constriction

    • Epiboly = when cells squish down (stretch out) to form a thinner sheet of cells that cover the all of the layers

    • Convergent Extention / INTERCALATION = when 2 layers of cells in a row converge into 1 row (1 line of cells instead of 2) to form a longer line

      • happens during amphibian gastrulation

Differentiation and Stem Cells

• STEM Cells

  • Give rise to two multipotent progenitor (MPP) cells: GATA1+ & Flt3+

  • Cell lineages are controlled by transcription factors

    • GATA1 and PU.1 inhibit each other, thereby defining two lineages

  • Growth factors are active in diff areas

    • G-CSF + GM-CSF + IL-3 → neutrophils

    • M-CSF + GM-CSF + IL-3 → macrophages

• Embryonic stem (ES) cells

  • Mouse ES cells are totipotent

  • Mouse epiblast stem cells (EpiSCs) are multipotent

  • Human ES cells are multipotent

  • All three SC’s express four transcription factors that maintain the undifferentiated state

• Somatic cell nuclear transfer (SCNT)

  • Nuclei of adult and baby xenopus cells into a xenopus EGG with no nucleus results in a tadpole devloping!

  • can reprogram many cells in many organisms, but is inefficient and often leads to incomplete or mutant development

    • only efficient in xenopus cells (not sheep, mice, etc)

• Cell Transdifferentiation

  • Human liver cell can transform into muscle cell when its fused with a mouse muscle cell

  • This is possible in some species but rarely in others

  • THIS HAPPENS NATURALLY IN NEMATODE DEVELOPMENT

• Induced Pluripotent Stem Cells

  • Oct4, Sox2, Klf4 and c-Myc (these genes can reprogram: Yamanka study)

  • can help with study anbd replacement of neurons

Review Questions:

• Localization of dorsal determinants at the four-cell stage of the Xenopus embryo is an example of …

  • A. mosaic development

  • B. regulative development

  • C. random development

  • D. preformation

    • Answer: A

• Transplantation of Spemann’s organizer from one early Xenopus gastrula into the ventral region of another early Xenopus gastrula results in …

  • A. immediate death of the gastrula

  • B. second head and D/V axis

  • C. second head and A/P axis

  • D. second tail and D/V axis

    • Answer: B