Lecture 9: Patterning tissues: cell specification and connection in the nervous system Flashcards
What is induction of the neural plate followed by?
Following induction the neural plate undergoes morphogenetic movements that lead to the formation of the neural tube – this closure of the neural tube is known as neurulation. At about the same time the neural tissue is also acquiring dorso-ventral pattern – this is manifest by the emergence of different cell types along the DV axis of this structure – which is defined ventrally by the position of the notochord and dorsally by the epidermis. An early indication of this patterning is the appearance of the dorsal most cell type –known as the neural crest – this is a migratory cell population which gives rise the peripheral nervous system and other cells (slide 2)
Neural crest derivatives
Neural crest derivatives :
- entire peripheral nervous system (sensory & autonomic ganglia, e.g. dorsal root ganglia, enteric/gut ganglia)
- endocrine chromaffin cells of adrenal medulla
- smooth muscles of the aorta
- melanocytes
- cranial regions cartilage/bone
-send sensory cells out to the periphery, sympathetic chain ganglia, chromaffin cells of the adrenal medulla (which release adrenalin, noradrenalin and enkephalin; fight or flight response) , and ganglia (groups of nerve cells) which innervate the gut…. It is often said that you have more neurons in your gut than in your brain…
(slide 3)
Neurulation in the human embryo
In humans neurulation this takes place over a number of days. As we have seen the neural plate is established by 19 days of human development and by the 20th day the edges of the neural plate begin to elevate and by 32 days the neural tube is completely formed. This process actually takes place over 4 key days… Lateral edges of the neural folds first begin to fuse at the level of the forming somites on day 22, leaving cranial and caudal neuropores open at each end. The neural tube increases in length as it zippers up and the neuropores at either end become smaller. Cranial neuropore closes first,on day 24 and posterior/caudal neuropore on day 26
(4)
Distinct cell types form in specific positions within the neural tube
- The nervous system consists of neurons
and their supporting cells, called glia. - Neurons are born first in the neural tube
- Different types of neurons form in
specific positions within the neural tube
C = Commissural interneurons
M = Motor neurons
V3 = medial interneurons - Specialised non-neural cells are also
present ventrally (floor plate cells) and
dorsally (roof plate cells)
-Commissural interneurons form dorsally adjacent a midline structure called the roof plate, while other classes of interneurons form in ventral regions, such as those called V3 adjacent to the ventrally located floor plate, along with motor neurons (M) which receive inputs from sensory nerves (DRGs) and themselves send axons out to innervate the muscles. (5)
What do signals from the notochord do?
-Signals from the notochord pattern the neural tube
-Signals from the notochord are
sufficient and necessary to induce
floor plate and ventral neurons
- The floor plate can also pattern
the neural tube
-The process of cell type specification has perhaps been best studied and elucidated in the developing spinal cord. Here grafting experiments have shown that pattern is imposed on the neural tube by signals from the ventral mesoderm. In these experiments the ventrally located notochord is either transplanted or removed. An additional notochord results in induction of an ectopic floor plate and ectopic ventral cells types such as motor neurons, while removal of the notochord leads to the absence of ventral cell types in the neural tube. Signals from the notochord are therefore sufficient and necessary for the induction of the floor plate and specification of ventral neurons. Grafts of the floor plate can also specify ventral neurons, raising the possibility that signals from the notochord first induce the floor plate and that this tissue subsequently specifies ventral neurons, such as motor neurons.
(6)
Sonic Hedgehog
-The protein Sonic hedgehog (Shh) is expressed by the notochord & floor plate
-Failure to express Sonic hedgehog leads to absence of the ventral midline (Single median eye, Cyclops)
-Sonic hedgehog specifies ventral cell types
-Different concentrations of Shh specify different types of ventral neurons eg a Shh concentration required to induce MN is x3 lower than that required for FP induction.
-Shh controls a combinatorial code of transcription factors that determine neuronal cell type
->a whole cohort of transcription factors show differential responses to Shh signalling … genes such as Nkx6.1 are induced by high levels of Shh and are therefore present ventrally, while genes such as Pax7 are repressed, even by low levels of Shh and are therefore restricted to a dorsal domain. Pairs of these TFs have also been shown to be mutually inhibitory, Pax6 inhibits Nkx2.2 and this helps define boundaries of gene expression across the neural tube. Specific combinations of transcription factors, for example Nkx.6.1 + Pax6 generate particular cell types … in this case motor neurons
(slides 7-10)
What do Opposing gradients of dorsal TGFβ and ventral Shh signalling do?
-Opposing gradients of dorsal TGFβ and ventral Shh signalling specify cell types in the neural tube
-Dorsal cell fates are imposed by dorsal signals of the TGFb family…. These oppose the effects of shh and promote dorsal cell types… such as the commissural interneurons
-Outcome: functional circuits
-Dorsal interneurons receive input from sensory neurons in the periphery and modulate the activity of motor neurons which innervate muscles. Local functional circuit: knee jerk reflex, involves direct feedback from sensory neurons in the DRGs to motorneurons in the ventral horn…
(11-12, look over!!!)
Insect nervous system
-similar subdivisions
look side 13
What does lateral inhibition ensure?
-LATERAL INHIBITION ensures that cells do not differentiate simultaneously
-proneural cluster: Group of cells in the
neuroepithelium in direct contact with each other
-lateral inhibition: Central cell inhibits neighbouring cells
from differentiating
-neuronal precursor: Central cell differentiates into a neuron. Neighbouring cells may undergo further
divisions (14)
Neurogenesis in the frog embryo
-Expression of the proneural transcription factor Neurogenin related-1 (Ngnr-1) defines the regions within which neurons can form. Forming neurons express the signalling molecule, Delta-1
(look diagram!! slide 15)
What is the molecular basis of lateral inhibition?
-proneural cluster: Proneural gene Ngnr-1 defines
cells that can form neurons
-lateral inhibition: Ngnr-1 induces expression of
Delta-1, but one cell expresses this gene
quicker than its neighbours
-neuronal precursor: Delta-1 presented on the surface of the central cell inhibits the differentiation of neighbouring
cells by signalling through the Notch receptor (16)
How does Notch signalling inhibit differentiation?
-relies on feedback loop:
- At first all cells express Neurogenin, Notch receptors and low levels of Delta-1
- Notch signalling inhibits neurogenin, which normally induces Delta-1
- When one cell comes to express slightly more Delta-1, this increases Notch signalling in neighbouring cells
- Cells with high Notch signalling, will have less Neurogenin and less Delta-1
- The central cell will therefore receive less inhibition and will have increasing levels of Neurogenin and Delta-1.
- Neurogenin also induces neuronal differentiation genes, such as NeuroD
(17)
Experiment injecting Delta-1 mRNA and dominant negative Delta-1 mRNA (prediction and results)
Predictions:
1) If every cell expressed Delta-1, all the cells would inhibit each other and there would be no or fewer neurons
Result: increase delta-1, neurons missing on injection site
2) If Delta-1 is blocked then many more cells would be free to differentiate into neurons
Results: decrease Delta-1, more neurons on injection site
(18)
A gene hierarchy leading to neuronal differentiation
-The proneural gene neurogenin not only induces delta-1, but it also promotes expression of neuronal differentiation genes, such as NeuroD
-Ngnr-1 -> Neuro D -> Structural neuronal differentiation genes
-Can even express NeuroD outside the nervous system… in the epidermis and get neurons… master regulatory gene..
(diagram slide 19 look!!)
Anatomy of a growing neuron
Growing nerves have a characteristic morphology, consisting of the cell body, where the nucleus of the cell is located, a major extending process called an axon, and at the tip of the growing axon, a unique, specialised and highly dynamic structure, the growth cone. The growth cone, first described by the Spanish anatomist Ramon y Cajal, explores the environment through which it travels by fine processes called filopodia, these structures detect molecules presented on cell surfaces, by the extra cellular matrix and diffusing through the extra-cellular space, and they need sensory abilities to detect these molecules.
Growth cones also need to be able to move in response to molecular cues, sometimes rapidly (e.g. 250 um per hour) and to navigate around obstacles and to do this they need motor abilities. Finally, growth cones need a stopping mechanism which allows them to stop growing and make connections with target cells. (21)
