Developemental Biology

• Oogenesis (Bildung der Eizelle)

• fertilization (fusion of Sperm pronucleus and Egg pronucleus to form zygote)

• cleavage stages (cell division to form Blastula)

• Gastrulation

• Neurulation

• Organogenesis (a tadpole is formed)

• metamorphosis (Xenopus = African clawed frog)

In amphibians the Animal and vegetal pole form prior to fertilization

Animal pole high cell division, dark color (less visible in nature? protection from sunlight?)

vegetal pole low cell division, yolk containing

this polarity will be impotant for the developement of the organism

the sperm entry can only occur at the animal pole

Formation triggered by sperm entry (fertilization)

sperm entry point = ventral

opposite side = dorsal

1. cortical rotation towards sperm entry point forming grey cresent at dorsal side

2. ß-catenin (from dorsal) plus Nodal (from vegetal pole) overlap and form Nieuwkoop center which leads to Gastrula organizer (Spemann/Mangold experiments)

3. Microtubule arrays form around vegetal pole which makes membrane molecule transport easier (minus end at sperm entry point plus end at dorsal) e.g. WNT signal molecules transported via these microtubules

Transplantation of a second blastoporus lip to an developeing embryo, that leads to a second invagination and to the formation of a whole second animal (second axis) (Neural tube, gut lumen, notochord, somite all developed twice)

The vegetal cells acting on the adjacent animal pole cells at the equatorial region

the mesoderm has a Ventral and dorsal side which is induced by a BMP and Nodal gradient

Nodal + ß-Catenin create Organizer at Dorsal side here BMP antagonists prevent Ventral formation and mesoderm formation but establish Notochord (Urwirbelsäule), Somiites (Wirbel), Neural tube / Plate [together with Anti WNTs e.g. frzb]

1. fertilized egg

2. Cleavage (Zygotic nucleus undergoes 12 rapid nuclear divisions without cell formation)

   —> forming the syncytial blastoderm : One single cell, multiple nuclei!

3. Cellularization : - Membrane starts to fold inwards between nuclei

   —> Eventually, formation of a cellular blastoderm : One nucleus per cell!

4. Gastrulation (the Midblastula transition induces the Gastrulation: Embryo developement / formation of the three germ layers)

5. Hatching -> Larva -> Pupa -> Metamorphosis

In Drosophila Oogenesis the stem cells divides to 16 cells, they are surrounded by the egg chamber follicle cells, the follicle cells already have a anterior / posterior polarity at the posterior follicle cells 1 of the 16 cells becomes the oocyte the rest 15 become nurse cells (feeding oocyte). inside the Oocyte microtubuli are formed their plus end on posterior side and minus end towards anterior side where the nurse cells are. Along these microtubuli scaffolds mRNAs are transported

—> Bicoid (via Dynein) to minus end

—> Oskar (via Kinesin) to plus end prolocalized there to trap and anchor nanos

Bicoid and Nanos protein (translated at fertilization) diffuse trough syncitial blastoderm and form concentration gradients where they act as inhibitors for hunchback and caudal

—> Anterior = Bicoid inhibiting caudal

—> Posterior = Nanos inhibiting hunchback

Bicoid and Nanos are maternal gradients they are at top of the hierarchy of signals. Addition of Bicoid mRNA to bicoid lacking embryo mutants leads to head developement where added (head in middle) adding it to wild type at posterior side can create two head mutant.

1. Maternal Gradients (bicoid / nanos)

2. Gap genes (Giant, Krüppel, knirps, tailless)

3. pair rule genes (e.g. even skipped —> stripe where the gap gene pattern fits)

4. segment polarity genes [engrailed / wingless] (activated in 14 stripes, equipping segments with polarity signal transducer (WNT and HedgeHog Signaling), syncitial phase over: cells form, A-P axis of the cells maintained by the formed receptors e.g. frizzled and autoregulatory circuits)

5. homeotic genes (equip segments with identity e.g. antennapedia can form legs for antenna or Bithorax forms double wing pairs), Highly conserved gene clusters, homeotic genes are TF with a typical homedomain (Homeobox)

Its a hierarchy because the gradient or pattern of signals before creates the foundation for the next signals. Bicoid and hunchback (caudal) gradient help determine gap gene pattern. The differential gap gene expression follows two principles:

1.Regulation by maternal gradients Bicoid, Hunchback, Caudal directly regulate gap gene transcription (binding site number, affinity, clustering matter)

2. Cross-regulation!! Gap genes encode for transcriptional regulators (mostly repressors) that affect each-others expression

Key event: Oocyte nucleus associates with the microtubuli network and is transported the minus end. The location of the nucleus defines the dorsal pole

gurken mRNA is anchored to nucleus -> Gurken protein translated locally and secreted dorsally

—> gurken mutant is ventralized

Dorsal Protein:

• Dorsal is another key protein involved in dorsal-ventral patterning. It is distributed uniformly in the cytoplasm of the early embryo. Dorsal protein is initially present in an inactive form.

The follicular epithelium and the extracellular matrix (ECM) help forming signaling gradients in the follicle cells pipe protein is transported (pipe activates the Spätzle precourser by cleaving it into active state) in the ECM Spätzle is transported a signal molecule that acts on the Toll receptor, downstream that signal activates Dorsal protein which is the signal for ventral genes to be expressed. —> pipe mutant is dorsalised

Dorsal: represses the expression of genes that promote dorsal cell fates.

The Dorsal morphogen gradient determines ME: Mesoderm VNE: Ventral Neurogenic Ectoderm DE: Dorsal Ectoderm AS: Amnioserosa by a threshold dependent regulation of the dorsal targets

The antigradient consisting of the BMP / TGF superfamily ligand dpp !

Sog acts as an inhibitor to dpp and shapes the gradient to the dorsal top has high dpp activity and the sides where dorsal ectoderm should be formed have lower activity and the middle to bottom sides where the Ventral Neurogenic Ectoderm should be formed sog and brinker inhibit the dpp completely by binding the dpp (=BMP) over and over again so the BMP gradient form over time until all BMP can bind to the receptors and where less BMP is inhibited it can bind more —> AS Amnioserosa

Bicoid gradient [source and sink = diffussion]: Anterior - Posterior developement —> Bicoid protein TF for anterior developement

Dorsal gradient [nuclear translocation]: Dorsal - Ventral developement, Dorsal protein uniformly present in cytoplamn of early embryo activated by Toll receptor signal pathway with ligand Spätzle (spätzle activation in ECM by pipe (transported in follicle cells)) —> Toll receptors on ventral side, Dorsal gradient from ventral = high to dorsal = low, Dorsal represses genes that promote dorsal cell fate when it is translocated into nucleus

BMP gradient [time resolving]: e.g. dpp in dorsal - ventral developement where dpp high activity on dorsal side and formed by anti-gradient of sog binding it along the V-D axis so it cant bind to the BMP receptor

Animal cap cells when dissociated form neurons, when still intact they form epidermis and when BMP is added they form epidermis

—> Chrdin, Noggin important for neural induction by repressing BMP, also FGF induces neural fate directly when present in low conc.

1. Formation of the neural folds (at edges of neural plate / epidermis)

2. Elevation of the neural folds (middle anchored to notochord)

3. closing to form neural tube

Dorsal side: the notochord (Chorda dorsalis) forms the floor plate (organizing center)

—> Sonic hedgehog (Shh)

Ventral side: the surface ectoderm (after evelation of neural folds) forms a ventral organizing center the roof plate

—> BMPs, dorsalin

Important because Different neuron types differentiate along the dorso-ventral axis of the spinal cord

—> where notochord motor neurons form (Shh gradient)

Hedgehog pathway: without Hh: Gli/Ci [TF] Repressor with Hh: Gli/Ci [TF] Activator

1. global organization

WNT signalling = formation of anterior structure vs Frzb that binds WNT so it cant interact with Frizzled rezeptor —> Competition mechanism

Absence of Wnt activity required for forebrain (kranial), increasing Wnt conc. specify more caudal CNS domains (forbrain -> midbrain -> Hindbrain -> spinal chord)

1. regional organization

anterior neural border, Zona limitans and midbrain-hindbrain boundary —> Functions of the MHB • midbrain Patterning • anterior hindbrain patterning • control of neuronal differentiation • control of proliferation • morphogenesis • establishes polarity in the Tectum • axon guidance of the retinotectal projektion

1. Segmentation of the Rhombencephalon

Each rhombomere (segment with scrict cell boundaries) will give rise to a unique set of motor neurons that control different muscles in the head