Chapter 13- The neural crest Ectoderm-skin/nerves Endoderm- Gut and associated organs Mesoderm-Blood, heart, kidney, bones Recall lineages Recall- Ectoderm has three fates Epidermis (skin) Brain and spinal chord Peripheral neurons, facial cartilage 2.Neural crest cells 3.Neural tube 1.Epidermis Fig This process is called neurulation The neural crest

Neural crest cell fate depends largely on where they migrate Potential cell fates include- 1. neurons and glia 2. medulla of adrenal gland (produces epinephrine) 3. Pigment cells of epidermis 4. Skeletal/connective tissue of head 1. Cranial- cartiledge, bone, neurons, glia of face 4. Trunk- melanocytes (produce pigment); sensory and sympathetic neurons, medulla 3. Vagal- parasympathetic ganglia 2. Cardiac Neural crest- four functional domains The neural crest The neural crest is a transient structure Fig. 13.1

A quick review of nerve nomenclature 1. Autonomic nervous system 2. Somatic nervous system- Sensory pathways- conduct info to brain-, spinal cord -“involuntary controlled muscles”- - CNS sends signals to smooth muscles of heart, blood vessels, iris, pancreas liver, digestive tract, kidney 1.Parasympathetic- -homeostasis of body systems, originate from hindbrain 2. Sympathetic-fright and flight reactions- originate form spinal chord Figure not in text -“voluntary controlled organs”- - CNS sends signals to striated muscles communication between various parts of the body (e.g. thallumus, cerebellum) with muscles

A. Start with the Trunk Neural crest The neural crest Two major paths taken Epidermis This is a somite Path 1-cells travel under epidermis, become melanocytes, colonize hair and skin follicles Path 2-cells to side of neural tube and through anterior sclerotome to become sympathetic and sensory neurons Note – Sclerotome will become vertebral cartilage Fig SclerotomeNeural tubeNotochord

How do these neural crest cells know where to migrate? 1. Epidermis secrete BMP-4 and BMP-7 - BMP-4 and –7 induce neural crest cells to produce slug and RhoB - Slug dissociates cell-cell tight junctions 2. N- cadherin expression is also lost then regained once reaching final destination 3. Ephrin proteins in extracellular matrix guide cells Neural crest cells have Eph receptors Trunk sclerotome express Eph ligand Binding of Eph receptor to Eph ligand interferes with migration Thus, Eph proteins tell neural crest cells where not to go Ephrinin sclerotome Neural Crest cells Fig Stem cell factor allows continued proliferation 5. Other chemotactic and maintenance factors The neural crest

Trunk neural crest cells are pluipotent (can become many cell types) However, it may be that only certain populations of cells are pluripotent Neurogenin Sensory neuron Mash-1 Sympathetic and parasympathetic neurons Trunk neural crest cell Some transcription factors have been identified that dictate cell fate Final cell fate is determined by final environment Fig Fate of a trunk neural crest cell is influenced by FGF2 and glucocorticoids

B. The Cranial neural crest The neural crest Like the trunk neural crest cells, these can produce glia, neurons and melanocytes But, only cranial neural crest cells can produce cartilage and bone Recall – the neural tube subdives into forebrain, midbrain and hindbrain The hind brain then further subdivides into rhombomeres Each rhombomere is a territory, each produces ganglia, but each has a distinct fate Rhombomeres sit behind the pharyngeal arches Fig Pharyngeal arches Rhombomeres

Three paths for cranial neural crest cells 1. Rhombomere 1+2- to 1 st Ph. Arch Rhombomeres in hind brain of neural tube 2. Rhombo. 4- to 2 nd Ph. Arch 3. Rhombo. 6 to 3 rd and 4 th Ph. Arch Rhombomeres 3 and 5 do not migrate through arches Pharyngeal arches Fig Fate map of pharyngial arches contributions to face formation The neural crest

What determines distinct fates of cranial neural crest cells? Answer- The combination of hox genes 1. Hoxa-2 KO- neural crest cells of 2 nd Ph. Arch transformed into 1 st Ph. Arch structures Evidence 2. Hoxa-1 and Hoxb-1 double KO- no rhombomere 4 migration 3. Retinoic acid induces more anterior expression of certain Hox genes- - rhombomeres 2 and 3 assume role of rhombomeres 4 and 5 + retinoic acid WT Fig No ear The neural crest

How is neuronal diversity achieved?? 2. Notch-delta specifies neural fate (not epidermal or glial) 1. Blocking BMP signal allows formation of dorsal neural tube (recall chapter 12) 3. Initial location determines neuronal type 4. Migration route further dictates specificity 5. Specific connection made with target organs or other neurons 3 parts described 1. Pathway selection- axons travel along a given route 2. Target selection- axons reach a target, then bind to specific cells 3. Address selection- axons now refine interactions- bind to only a subset of possible targets 5 ways-

A. Hypotheses for pathway selection- 1.Cell adhesions- Growth cone can adhere to certain cells, but not others 2. Physical barriers- Growth cone can adhere to certain cells, but not others Laminin – a glycoprotein which appears to pave the road for several axonal migrations N-CAM 3. Labeled pathway hypothesis- in insects, a neuron can precisely follow the path of a prior neuron Kallmann syndrome- an infertile man with lack of smell Reason- a single protien directs migration of both olfactory axons and hormone-secreting nerve cells 4. Repulsion- a.Ephrin (recall Fig 13.4) – Growth cones contain Eph receptors- binding prevents migration into undesirable areas b.Semaphorin proteins- important in directing axon turns

5. Diffusible molecules- a. Netrin-1 and –2 are chemotactic Fig Netrins are homologues of the UNC-6 protein in C. Elegans Loss of Unc-6 prevents migration of both sensory (to ventral) and motor (to dorsal) neurons Fig Unc-6 -/- WT Sensory Neuron Motor Neuron b. Slit proteins are repulsive Hypotheses for pathway selection- (Cont.) Neutrin producing cell Rat dorsal spine explant Neuron Outgrowth

B. Hypotheses for target selection- Target cells secrete short-range chemotactic or chemorepulsive factors Example- NT-3 attracts axons 0 min2 min 6 min10 min Fig C. Hypotheses for address selection - Growth cone makes contact with a cell, acetylcholine receptors cluster on target cell surface, and a synapse is formed Additional axons synapse target cell, but eventually only one/cell remains Fig