dev bio module 8: ectoderm

**watch and add last 20 min of 4/7 lecture

neurulation

the formation of the neural tube, can overlap with when gastrulation starts

ectoderm includes:

surface ectoderm, neural crest, neural plate/neural tube

DPP

• homolog to BMP

• is inhibited by Dorsal on the ventral side in drosophila

• specifies surface ectoderm in drosophila

surface ectoderm derivatives

mouth epithelium (tooth enamel), lens, cornea, epidermis, hair, nails, sebaceous glands, olfactory epithelium

• high BMP (=DPP homolog)

neural crest derivatives

facial cartilage, dentin, pigment cells, adrenal medulla, peripheral NS (nerves, neurons outside of CNS)

neural plate derivatives

brain, spinal cord, motor neurons, retina

• no BMP

stage 1: Early neurula

neural folds = border between surface ectoderm and neural plate

stage 2 neurula

• neuropores

• neural groove: canal folding into itself

BMP

Bone Morphogenic Signaling

• paracrine signal: signals to other cells (not self)

• cell proliferation

• cell migration

• induces surface ectoderm formation in vertebrates

BMP signaling

• part of TGFb family of receptors (transforming growth factor beta)

• 1. BMP ligan binds to TGFb receptors

• 2. receptors dimerize

• 3. causes SMAD to become phosphorylated —> turns ON

• 4. ON SMAD enters nucleus and starts transcription (TF)

how to inhibit BMP signaling?

• inhibit SMAD phosphorylation

• receptor mutation

• overexpress TGFb ligand

• mutate binding seq of SMAD

Noggin

inhibitor of TGFb/sequesters BMP —> inhibits BMP signaling pathway

Chordin

inhibitor of TGFb/sequesters BMP —> inhibits BMP signaling pathway

SOX TFs

they can…

• activate neural plate gene programs

• or inhibit BMP signaling

  • SOX represses the expression of ligands or receptors - does not interact directly with BMP

external neural plate —> neurulation —→ __

internal neural tube

Primary neurulation

neural plate invaginates and forms neural tube

• formation of the anterior of the tube = rostral side

secondary neurulation

ingression:

• cells in ectoderm ingress

• they detach and undergo E→M transition

• they condense into a tube with no gap

• cavitation

posterior of the tube formation = caudal side

junctional neurulation

formation of the middle of the neural tube

Neurulation 1: ELONGATION

growth in the anterior-posterior axis directions

• 3 ectoderms:

  • surface ectoderm

  • progenitors of the neural crest

  • neural plate

    • Axial elongation: cells in the neural plate are dividing in A-P direction

Neurulation 2: FOLDING

• medial hinges (on top of notochord): stiff rod - axis of folding

• embryo grows towards midline

• notochord (mesodermal derivative) induces invagination:

  • notochord signals neural plate —> cell differentiation in neural plate—> formation of medial hinge

Neurulation 3: ELEVATION

• elevation of either sides of the neural plate,

• neural crest progenitors + surface ectoderms move towards each other

• notochord dragged towards ventral side

• formation of the neural groove

Neurulation 4: CONVERGENCE

• formation of 2 new hinges:

  • 1. medial hinge

  • 2. dorso-lateral hinge points

Neurulation 5: CLOSURE/FUSION

• surface ectoderm is connected

• neural crest cells are located most dorsally in the tube —> will migrate

neural crest cells vs neural crest

neural crest = transient structure

neural crest cells = permanent structure

regulation of hinge point formation

pre-neurulation

• precursors of notochord activate noggin and chordin

• chordin/noggin inhibit BMP in neural plate area (BMP is expressed in surface ectoderm

neurulation

• notochord expresses shh —> shh inhibits noggin

noggin presence = bend (expressed in dorso-lateral hinge points and in neural plate (pre-neurulation)

high BMP presence = prevents bending (noggin needed to inhibit BMP in hinge areas)

for hinges to form…

apical side of cells shrink smaller than basal side of cells

defects in neural tube closure

• exencephaly: brain expands outside of skull cavity

• anencephaly: most brain does not develop

• spina bifidia: gap in spine vertebrae

• paralysis

Hedgehog signals

• dessert Hh —> spermatogenesis

• Indian Hh —> gut, cartilage, bone

• Sonic Hh —> morphogen

sonic Hh

• specification of motor neurons NT

• vertebrae formation

• feather formation

• hair

• patterning of the hand

shh mutation

• cyclops: born with one eye

• corn lily: alkaloid

SHh secretion

nucleus (mRNA) —> rough ER (translation) —> membrane (secretion)

1. translation

2. autoprotylitic cleavage (auto terminal exposed)

3. modify: adds cholesterol and palminic acid

4. transported to membrane vesicles

5. secretion methods (5)

Shh secretion methods

1. multimers formation

2. complex with lipoproteins

3. exovesicle formation

4. detaining from membrnae as monomers —→ needs protein channel

5. cytonemes (actin)

Shh secretion signaling

Shh OFF

• “patched” receptor inhibits “smoothened” receptor which internalizes

• Gli connected to microtubule

• PKA ON cleaves and phosphorylated Gli

• half Gli enters nucleus and inhibits transcription

Shh ON

• Shh binds patched receptor and deactivates it

• smoothened receptor can now travel up to top of cilium

• smoothened activates Gli and inhibits PKA

• whole Gli enters nucleus and activates transcription

Cadherins

Ca2+ dependent addition molecules, transmembrane proteins that anchor to the cytoskeleton

• E cadherins = early embryo + epithelium

• N cadherins = neural tissue

E cadherins

attraction towards the same type —> closure

fusion happens at different points (mouse @ points 1,2,3

• Zipper closure

• neuropores: normal gaps where neural tube is not yet closed

Dorso-ventral regionalization of neural tube 1

newly formed Neural Tube

• 2 main signaling centers: epidermis (BMP4,7), notochord (shh)

• shh from floor plate in gradient manner vs BMP from roof plate in gradient manner

DV regionalization of NT 2

formation of secondary signalling centers

• BMP and shh = paracrine , gradient

• BMP: feedback loop (autocrine)

DV regionalization of NT 3

• dorsal interneurons: processors and integrators of info from sensory neurons

• ventral motor neurons: sends axons from spinal cord to muscles, controls voluntary/reflex movements

AP regionalization of NT

• very dependent on Hox genes

3 primary brain vesicles

1. prosencephalon

2. mesencephalon (midbrain)

3. rhombencephalon (hindbrain)

5 secondary brain vesicles

1. telencephalon: cerebral hemispheres

2. diencephalon: eye cap, retina

3. mesencephalon: midbrain

4. metencephalon: pons, cerebellum

5. myencephelon: medulla oblongata

4 flexures

flexures: curvatures points in the anterior NT

1. sagittal flexure: between the cerebral hemispheres from dorsal view

2. cephalic flexure

3. pontine flexure

4. cervical flexure

Neural crest

• comes from neural crest progenitor cells next to surface ectoderm that ingresses and delaminates

• ectodermal derivative

• transient

• EMT epithelial to mesenchymal transition + extensive migration

  • A→P

  • D→V

neural crest as 4th germ layer?

gives rise to

• peripheral nervous system PNS,

• endocrine and paraendocrine derivatives,

• pigment cells,

• facial cartilage and bones,

• connective tissue

specification of neural crest cells

• depends on the expression of the same morphogens (BMP, Wnt) at different concentrations and for diff periods of time

not variations of BMP or wnt - its the same morphogen

cranial placodes

• they are thickenings in the surface ectoderm/border of surface ectoderms

• sensory organs

  • 1. olfactory: olfactory epithelium

  • 2. lens: eye lens

  • 3. trigeminal: sensory neurons of trigeminal nerve

  • 4. otic: inner ear (cochlea, vestibular apparatus)

  • 5. epibranchial: nerves (vagus, facial, etc)

Cranial NCC

• craniofacial mesenchyme → bone, cartilage, pigment cells, glia, connective tissue

• pharyngeal arches and pouches → thymus, odontoblasts, middle ear and jaw bones

cardiac NCC

• muscular and connective wall of large arteries of heart

• septum that separates pulmonary circulation for aorta

• melanocytes, neurons, cartilage, connective tissue

trunk NCC

1. ventrolateral migration: sensory neurons of dorsal root ganglia + sympathetic ganglia, adrenal medulla, nerve clusters around the aorta

2. dorsolateral migration → pigment cells

vagal & sacral NCC

• parasympathetic ganglia of the gut

early inter-species NCC transplant expts

• created hybrid Quck or Duail skull structures

• showed NCC autonomous properties - not environment dependent - the NCC have some cell pre-determined cell fate

how the test if NCC can be multipotent?

• fluorescing every step of NCC differentiation (single cell resolution)

• reporter line: multiple cassettes with diff color reporters

• can perform tissue specific deletions with flox + cre driver lines (LoF)

  • 1. lineage tracing

  • 2. conditional gene deletion: creating mutation in the cells that express cre → cut at lox sites → stop codons → color expressed