Chapter 12- CNS and epidermis Ectoderm-skin/nerves Endoderm- Gut and associated organs Mesoderm-Blood, heart, kidney, bones Recall lineages Now we focus on individual lineages 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

Dorsal ectoderm becomes neural ectoderm to become neural plate to become neural tube Neural plate Neural crest epidermis Primary Neurulation 1. Folding 2. elevation 3. convergence 4. closure Two types of neurulation 1.Primary- “pinching off” 2.Secondary – hollow out a cord Both are used in many creatures Fig Amphibian embryo Fig Neural tube

1. Folding 2. Elevation3. Convergence 4. Closure A few details at each step in primary neurulation Mesoderm signals ectodermal cells to form neural plate Hinge cells (called medial hinge point cells) attached to notochord Cell shape and cells movement contribute to elevation and Folds adhere to each other Failure of complete closure results in neural tube defects Spina bifida – posterior tube fails to close at human day 27 anacephaly – anterior tube fails to close- brain development ceases Fig % of spina bifida preventable with 0.4mg/day vitamin B 12

Secondary neurulation A cord is first made, then hollowed out Example- posterior end of chick Note- rest of chick uses primary neurulation Further neural tube differentiation 1. Anterior-posterior axis Anterior portion of neural tube forms three vesicles: 1. Forebrain 2. Midbrain 3. Hindbrain Brain volume increases 30-fold between days 3 and 5 of development

Brain development is complex and laden with nomenclature Fig human brain development

2. Dorsal-ventral axis Notochord (then hinge cells) secretes sonic hedgehog to signal ventral portion of neural tube to become motor neurons Retinoic acid also plays a role Epidermis (then roof plate) secretes TGF-  family proteins (BMP-4 and –7, dorsalin, activin) to signal dorsal portion of neural tube to become sensory neurons Roof plate Hinge cells Fig chick neural tube

Neuronal types Brains consists of neurons (nerve cells) and glia (support cells) The long-held belief that neurons were fully determined at birth is incorrect- Evidence for neuronal stem cells exists Cells lining neural tube can give rise to neurons or glia cells Fig A motor neuron Dendrites- connect to other neurons Cortical neurons connect to 10,000 other neural cells during 1 st year post birth!! At birth, very few dendrites are present on cortical neurons Axon Input axons from other neurons Axons are part of the cell body that can extend several feet Growth cone explores and moves into new regions of body Growth cone

Nerve cells are protected to facilitate electrical signal conduction by: By myelin sheath produced by oligodendrocytes In central nervous systemIn peripheral nervous system By myelination from Schwann cells Vertebrate eye development Fig the Pax 6 gene Pax6 gene encodes protein that directs eye development Neural-tube specific enhancer Pax gene expression

Fig Recall chapter 5- introduce DNA containing pax6 cDNA under control of an inducible promoter + a tissue-specific enhancer Observe additional eyes Pax6 mutants lack eyes in flies, mice and humans Sonic hedgehog dictates formation of two eyes Mutants produce one eye (cyclopia) Fig a cyclopic lamb

Eye development requires the specification of numerous tissues Fig Eye lens development forms by: 1. Lens vessicle folds onto itself to form ring Fig Interior cells elongate across cavity to produce crystallin lens fibers 3. Cells enucleate

A few words about epidermis (skin) development Recall: Epidermis becomes two layers, a periderm (which is shed) and a basal layer that gives rise to skin cells Epidermis Basal layer Periderm Spinous layer Termed “Malpighian layer” (Shed) Granular cellsKeratinocytes

Keratinocytes (continually shed) Spinous layer Basal layer Granular layer Malpighian layer Cells differentiate and migrate toward surface Fig TGF-  and FGF7 are important factors in skin development Feather, hair and scales are formed by epithelial- mesenchymal interactions between epidermis and mesoderm