Have Competence: becoming neuroblasts Chapter 9. Ectoderm Neurulation: instructing the ectoderm to form neural tube, the rudiment of central nervous system Four stages that pluripotent ectodermal cells have to pass to become Neural precursor cells/Neuroblasts and then Neurons Have Competence: becoming neuroblasts Specification: stay or leave from neuroblast characteristics (intermediate; reversible) Commitment (determination): entering neural differentiation pathway (neuronal subtype has been selected) Differentiation: exit cell cycle and express neuronal genes devbio8e-chptopener-12.jpg

Figure 9.1 Major derivatives of the ectoderm germ layer DevBio9e-Fig-09-01-0.jpg

Figure 9.2 Gastrulation and neurulation in a chick embryo (Part 1) DevBio9e-Fig-09-02-1R.jpg

Figure 9.3 Three views of neurulation in an amphibian embryo, showing early, middle, and late neurulae in each case DevBio9e-Fig-09-03-0.jpg

Formation of the Neural Tube Primary neurulation: generating neural tube at the neural plate (anterior) Forms brain and spinal cord inside, epidermis at the outside, and neural crest cells in between Neural plate: formation -> shaping -> bending -> closure Secondary neurulation: forming the neural tube from the mesenchymal cells (posterior)

Primary Neurulation 1. Formation of the neural plate Signals from dorsal mesoderm elongate anterior-posterior axis into columnar cells (~50% ectodermal cells) Convergent extension movement (intercalation into less layers) 2. Bending of the neural plate Medial hinge point (MHP): cells at the midline of the neural plate; anchor to the notochord and form a hinge Dorsal hinge point (DHP): connection between neural plate cells and other ectoderm; wedging for the neural tube closure, which is dependent on intrinsic cytoskeletal rearrangement and external power supported by surface ectoderm

Primary Neurulation 3. Closure of the neural tube Junctional region between both side of neural plate becomes the source of the neural crest cells Chick: zipping initiates at the primitive midbrain region Human: three sites of neural tube closure; defects 1/1000 births spina bifida: a failure in the closure at the posterior neuropore anencephaly: failures in the closure at the anterior neuropore craniorachischisis: failure in the whole neural tube closure Mediated by adhesion molecules: transition from E-cadherin to N-cadherin and N-CAM Critical genes: Pax3, Shh, openbrain Dietary factors: cholesterol (DHA), folic acid (vitamin B12) Other factors: socioeconomic situation, seasonal variation, teratogens from the food (fumonisin, a fungal material)

Figure 9.6 Expression of N- and E-cadherin adhesion proteins during neurulation in Xenopus DevBio9e-Fig-09-06-0.jpg

Differentiation of the Neural Tube : Anterior-Posterior Axis Form brain and spinal cord structures in anterior-posterior axis from 3 primary cavity and then 5 secondary vesicles Rhombomere: the primitive hindbrain regional unit in periodic swelling structures; divide different types of cranial nerves r2 ganglia: form 5th (trigeminal) cranial nerve r4 ganglia: form 7th (facial) and 8th (vestibulo-acoustic) cranial nerve r6 ganglia: form 9th (glossopharyngeal) cranial nerve Ganglia: clusters of neuronal cell bodies Cerebrospinal fluid in ventricles: inflate brain size by osmotic gradient generation; Na+/K+ ATPase plays role by setting up osmotic gradient Occlusion: separate brain and spinal cord

Figure 9.9 Early human brain development (Part 2) DevBio9e-Fig-09-09-2R.jpg

The vertebrate nervous system Most of the vertebrate CNS comes from the neural plate. Specification of vertebrate neuronal precursors also involves lateral inhibition. Delta activates Notch which inhibits synthesis of neurogenin (related to the achaete-scute proteins). The cell expressing neurogenin then expresses neuroD (a transcription factor required for neuronal differentiation). 2

Differentiation of the Neural Tube : Dorsal-Ventral Axis Differential structures within brain Hypothalamus at the ventral, while cortex at the dorsal Differential neuronal distribution in spinal cord: Dorsal: sensory neurons Ventral: motor neurons Intermediate zone: interneurons

Figure 9.13 Dorsal-ventral specification of the neural tube Graded cues along dorso-ventral spinal cord Notocord at the ventral side of neural tube: secrete Shh to induce floor plate cells, which in turn express Shh Surface ectoderm at the dorsal side of neural tube: secrete BMPs to let roof plate cells, in turn, express TGFb family proteins (BMP4, BMP7, BMP5, Dorsalin, Activin) Differential subsets of transcription factors dictate the fate of neural precursor cells Ex) Nkx2.2: V0; Nkx6.1+Pax6: motor neuron; Pax6: V2 interneuron; Dbx2: V1 interneuron; Dbx1: D2 interneuron; Pax7: D1 interneuron DevBio9e-Fig-09-13-1R.jpg

Reciprocal suppression for boundary formation

Figure 9.13 Dorsal-ventral specification of the neural tube (Part 4) Shh/dorsalin/Hb9 (motor neuron) Nkx6.1/Pax6/Pax7 DevBio9e-Fig-09-13-4R.jpg

Figure 9.14 Cascade of inductions initiated by the notochord in the ventral neural tube (Part 1) DevBio9e-Fig-09-14-1R.jpg

Differentiation of neurons The nervous system 1) is the most complex organ system in the animal embryo. 2) provides communication via a network of varied neurons. 3) is connected by a process of action potential propagation and neurotransmitter release. Supporting tissues (glia) include Schwann cells (surround peripheral neuron axons), oligodendrites and astrocytes (surround central neurons). In human brain: 1011 neurons and 1012 glia From common ancestor, ventricular (ependymal) cells in ventricular zone

Figure 9.15 Diagram of a motor neuron Neurons Dendrites One neuron accommodates about 100,000 synapses with about 10,000 other neurons Axons Axons outgrow from cell body in variety of length led by growth cone Axons navigate to their targets in recognizing numerous attractive or repulsive cues Myelination of axons help electrical conductance: Oligodendrocytes (CNS) or Schwann cell (PNS) Myelination defective mice: trembler (PNS: PMP20), jimpy (CNS: myelin proteolipid protein) Secrete neurotransmitters DevBio9e-Fig-09-15-0.jpg

Santiago Ramón y Cajal (1852 –1934) Drawing of the neural circuitry of the rodent hippocampus Drawing of a section through the optic tectum of a sparrow Structure of the Mammalian Retina Drawing of the cells of the chick cerebellum

Neural stem cells in the germinal epithelium Neurons in brain: connected in network in structures of cortices (layers) and nuclei (clusters) Germinal neuroepithelium contains rapidly dividing neural progenitor/stem cells, whose nuclei locate at the certain level according to their cell cycle status Mitosis occurs at the luminal side after moving downward from the outside edge (cortical) Stem cells remain at germinal neuroepithelium through the adhesion, while differentiating neurons leave after cell division Symmetric horizontal cell division generates two stem cells at the luminal surface Asymmetric cell division: One having more Par3 and high notch activity remains as stem cells. Other having less Par3 and less Notch, high Delta become a neuroblast(progenitor) and primed for neuronal differentiation devbio8e-fig-12-15-1.jpg

Figure 9.19 Differentiation of the walls of the neural tube (Part 1) DevBio9e-Fig-09-19-1R.jpg White layer Gray matter

Figure 9.19 Differentiation of the walls of the neural tube (Part 2) DevBio9e-Fig-09-19-2R.jpg

Figure 9.19 Differentiation of the walls of the neural tube (Part 3) DevBio9e-Fig-09-19-3R.jpg

Neocortex formation Migration of neuroblasts along radial glia from mantle zone to marginal zone results in the formation of neocortex. Neocortex subdivided into six layers in differential functional properties – ex. thalamic inputs to layer IV, while outputs from layer VI.) Neocortx subdivided into about 40 functional domains – visual cortex; auditory cortex Inside-out layer formation (early-born neurons form the layer closest to the ventricle. Subsequent neuron travel greater distance to form the more superficial layers of the cortex) Layer-specificity is instructed during cell division Foxg1 (BF1) suppress the layer 1 neural fate

Determination of cortical laminar identity in the mouse cerebrum

Commitment occurs during S phase Figure 9.23 Determination of cortical laminar identity in the ferret cerebrum (Part 2) DevBio9e-Fig-09-23-2R.jpg Commitment occurs during S phase

Figure 9.24 Cortical neurons are generated from three types of neural precursor cells: radial glia cells, short neural precursors, and intermediate progenitor cells DevBio9e-Fig-09-24-0.jpg

Adult neural stem cells BrdU labeling showed the existence of proliferating cells in adult brain Granular cell l ayer of hippocampus and subventricular zone for olfactory bulb (~0.3% of ventricle wall population) Respond to Shh for a long time (about a year in mice)

Figure 9.25 Evidence of adult neural stem cells DevBio9e-Fig-09-25-0.jpg New born neuron / Synaptophysin BrdU / Neurons / Glia

Human cerebral cortical map

Figure 9.27 Retention of fetal neuronal growth rate in humans DevBio9e-Fig-09-27-0.jpg

Figure 9.28 Dorsal view of the human brain showing the progression of myelination (“white matter”) over the cortical surface during adolescence DevBio9e-Fig-09-28-0.jpg

Development of the vertebrate eye General Developed from optic vesicle and lens placode Optic vesicle: differentiate into retina, pigment epithelium, and optic stalk (optic nerve) Key early factors for optic vesicle formation: Six3, Pax6, and Rx1 Evolutionary conserved functions of the early determinants External factors control the expression of the key factors: e.g. Shh suppresses expression of Pax6 Environmental adaptation of development: Eyeless cave fish Astyanax mexicanus have impaired expression of Pax6 because of the elevated action of Shh in the optic field Axial patterning Dorsal-ventral & Proximal-distal axes Dorsal BMP and ventral Shh: result in the grated expression of key factors. e. g. optic stalk: Pax2 and Vax1 by Shh; PE: Mitf by BMP; retina: Rx Anterior-posterior axis Otx2 at the anterior head region -> Six3 and Pax6 Retinal differentiation Sensory neurons: rod and cone photoreceptors Interneurons: bipolar cells, horizontal cells, amacrine cells Loss of competence of retinoblasts along developmental stages Lens and cornea Lens is differentiated from lens placode, but cornea is originated from neural crest cells Express crystallins: by Sox2, Pax6, and L-Maf

Figure 9.29 Development of the vertebrate (Part 1) DevBio9e-Fig-09-29-1R.jpg

Figure 9.29 Development of the vertebrate (Part 2) DevBio9e-Fig-09-29-2R.jpg

Figure 9.30 Dynamic formation of the eye field in the anterior neural plate (Part 1) DevBio9e-Fig-09-30-1R.jpg

Figure 9.30 Dynamic formation of the eye field in the anterior neural plate (Part 2) DevBio9e-Fig-09-30-2R.jpg

Figure 9.31 Sonic hedgehog separates the eye field into bilateral fields DevBio9e-Fig-09-31-0.jpg

Figure 9.32 Surface-dwelling (A) and cave-dwelling (B) Mexican tetras (Astyanax mexicanus) DevBio9e-Fig-09-32-0.jpg

Figure 9.33 Expression of Rx genes in vertebrate retina development (Part 1) DevBio9e-Fig-09-33-1R.jpg

Figure 9.33 Expression of Rx genes in vertebrate retina development (Part 2) DevBio9e-Fig-09-33-2R.jpg

Figure 9.34 Retinal neurons sort out into functional layers during development DevBio9e-Fig-09-34-0.jpg

Figure 9.36 Differentiation of the lens and anterior portion of the mouse eye DevBio9e-Fig-09-36-0.jpg

Figure 9.36 Differentiation of the lens and anterior portion of the mouse eye (Part 1) DevBio9e-Fig-09-36-1R.jpg

Figure 9.36 Differentiation of the lens and anterior portion of the mouse eye (Part 2) DevBio9e-Fig-09-36-2R.jpg

Figure 9.36 Differentiation of the lens and anterior portion of the mouse eye (Part 3) DevBio9e-Fig-09-36-3R.jpg

Epidermis and the origin of cutaneous structures Skin Protective role Consistently renewable Dermis (inner) and epidermis (outer) Epidermis Two layers: periderm and basal layer (or stratum germinativum) Epidermal stem cells: in the basal layer; asymmetrically divide to generate new epidermal cells, keratinocytes TGFa: autocrine factor produced in the basal cells to stimulate division Keratinocyte growth factor (KGF) and FGF7: migration and differentiation Cutaneous appendages Hairs, scales, feathers Requires a series of reciprocal inductive events between dermal mesenchyme and the ectodermal epithelium to form placode, a precursor of hair follicles Wnt signaling is critical for the follicle development Hair follicular stem cells: at the bulge of hair follicle; enable the cycling of hair

Figure 9.37 Layers of the human epidermis DevBio9e-Fig-09-37-0.jpg

Figure 9.38 Early development of the hair follicle and hair shaft DevBio9e-Fig-09-38-0.jpg

Figure 9.39 Patterning of hair follicle placodes by Wnt10 and Dickkopf DevBio9e-Fig-09-39-0.jpg

Figure 9.39 Patterning of hair follicle placodes by Wnt10 and Dickkopf (Part 3) DevBio9e-Fig-09-39-3R.jpg

Figure 9.42 Model of follicle stem cell migration and differentiation DevBio9e-Fig-09-42-0.jpg