gastrulation, After the embryo begins neurulation: formation of the neural tube
Head
12.4
apical
basal
basal
apical
Neurulation: mechanical and molecular aspects
Presumptive neural plate cells stop expressing Ecadherin, begin to express Ncadherin
Median hinge plate cells undergo convergent extension, forcing the plate to buckle
Required for folding of neural plate
12.3
Microtubules and actin allow shape changes of cells
As the folds converge, the prospective neural crest is placed dorsal to the neural tube
Neural cells crawl under epidermal cells
Prospective neural plate cells have already been determined by the cells of the organizer (secrete noggin and chordin) during gastrulation
12.4
12.4
Anterior/Posterior organization of the neural tube
12.10
Regions of the brain can be further subdivided One important set of subdivisions are the Rhombomeres, units of the myelencephalon
Cells do not migrate between Rhombomeres
Each rhombomere has different pattens of gene expression
How are A/P differences established? Gradients of morphogens and signaling molecules
Chordin, noggin, FrzB, retinoic acid etc.
Hox genes
Doral/Ventral organization of the neural tube
Ventral Signals
The notochord can induce an ectopic floorpate and motor neurons The floorplate alone The floorplate alone can also induce can induce floorplate motor neurons and motor neurons
Shh is the morphogen responsible for ventral patterning
Establishment of different classes of neurons depends on Shh concentration.
12.14
Dorsalizing Signals
Overlying epidermis
1. Explant intermediate region of neural tube: no dorsal markers or neural crest produced 2. Explant intermediate neural tube with dorsal epidermis:
Overlying epidermis
dorsal markers, neural crest and dorsal neurons produced 3. Remove roofplate of neural tube, culture the rest:
remove
no dorsal neural tissue
The spinal cord is organized by an interplay of dorsalizing and ventralizing molecules
GDF-7
Dorsalizing molecules: BMPs in ectoderm and roof plate GDF-7 in roof plate Dorsalin, Activin in dorsal NT
12.13
Formation of the neural crest
Initially lateral to the neural plate
Rises as the folds rise
12.3
Becomes dorsal to the neural tube
Neural crest lies between neural plate and epidermis
Become mesenchymal and migrate away
12.3
3 main kinds of neural crest: cranial, cardiac, trunk
Spinal cord
Somites
13.1
Each type makes specific and derivatives has a stereotyped pathway of migration
Neural crest cells have many different derivatives Cranial Cranial nerves; cartilage, connective tissue Parts of eyes, ears, pharyngeal arches Trunk Dorsal root ganglion (ganglion of sensory neurons) Autonomic nervous system Adrenal medulla Pigment cells Cardiac Septum of heart Connective tissue of arteries
The neural crest are migratory cells that have multiple fates
radiolabeled donor host
Melanocytes
Neural tube Dorsal root ganglion
Sympathetic ganglion
Trunk neural crest
Spinal cord
Somites
Migratory pathways of the trunk neural crest
Path 1: ventrally through the sclerotome of the somites --populate DRG and other neuronal ganglia
Path 2: dorso-laterally between epidermis and somite --populate skin (melanocytes)
Neural crest cells migrate through the anterior region of each sclerotome
Migrating neural crest cells (stained with an antibody, HNK-1)
Neural tube
Anterior sclerotome Posterior sclerotome
Section through posterior sclerotome
Section through anterior sclerotome
Exclusion from posterior region depends on molecules called Ephrins
13.2
Cranial neural crest
Cranial nerual crest originate in the rhombomeres and migrate into the pharyngeal arches
Derivatives of cranial neural crest
Altering Hox gene expression alters specification of neural crest cell fates
normal normal + Retinoic acid
Cranial nerve VII normally develops at R4 hoxb-1
Day 8....