Cellular Events in Development Flashcards
What are the three important parameters in brain organization?
1) Neuron Density
2) The formation of axonal pathways (and dendritic trees)
3) the pattern of synaptic contacts (synaptogenesis)
Neuron Density: When do neurons proliferate?
With the exception of cells in the olfactory epithelium and in the hippocampus, all neuron cell division occurs before birth.
Neuron Density: period of over production
What is the period of over production? When and why does it occur?
By 24 weeks of gestation, neurogenesis produces 1.5 to 2 times more neurons that are present in the mature human brain (there are 100 billion in the mature human brain.
This occurs because it is followed by a period of programmed cell death that follows
Neuron Density: Programmed Cell death
What is programmed cell death? When does it occur? What is its function?
Subsequent to the period of over production, there is a selective genetically programmed cell death (also called apoptosis) of neurons in many areas of the brain.
The main purpose of developmental cell death appears to be to ensure that neurons form the appropriate connections with their targets.
Neuron Density: Programmed Cell death
What determined which cells die and which cells survive?
Neurons that form the appropriate connections, meaning that they make contact with the appropriate target, will succeed in gaining access to target-derived neurotrophic factors that maintain the metabolism of a cell or its process and will survive.
Neurons that form inappropriate connections do not get access to target-derived neurotrophic factors and consequently undergo cell death.
Neuron Density: The mature human brain
How many cells are there in the mature human brain and when are they developed?
About 100 billion neurons
All are produced having undergone their last cell division before birth with the exception of cells in the olfactory system and in the hippocampus that preserve the ability to divide.
Neuron Density: The mature human brain
Why can’t neurons be added after birth?
After birth, precursor cells disappear, and few, if any new neurons can be added to replace those lost by age or injury in most brain regions.
Formation of Axonal Pathways: What are the preliminary steps in axonal pathway formation?
After neuroblasts complete their final cell division and migrate to their final location, they begin to extend a single axon. As the axon grows, it may send out branches, each with its own growth cone.
Once the first or pioneer growth cone traverses a route, it may establish a pathway that is later used by other growth cones.
Formation of Axonal Pathways: Axon Growth Cone
Definition
It is a distal spade-shaped elaboration of the axon capable of PATHFINDING by driving through and navigating amongst fields of developing nervous or mesenchymal tissue to reach distant targets.
Formation of Axonal Pathways: Axon Growth Cone
What allows growth cones to function?
The are capable of pathfinding because the posses receptors that communicate with the intracellular and extracellular environments, and are able to recognize and interpret molecular cues from cells in their path and from within the cell into signals that regulate the cytoskeleton, receptors, and channels on the cell surface, or gene expression.
Formation of Axonal Pathways: Axon Growth Cone
What happens to growth cones when they reach their targets?
Upon reaching their targets, growth cones undergo biochemical and morphological changes to become presynaptic terminals.
**The growth cone is the precursor of the presynaptic terminal
Formation of Axonal Pathways: Molecules guide axons
Where do the molecular cues come from? What effects can the signaling molecules have?
The pathways along which axons grow provide molecular cues to guide axons to their targets. The molecular cues can be in the extracellular matrix, soluble, or cell membrane bound.
The signals can tell the axon to continue or can deter it, they can be attractive or repulsive.
Formation of Axonal Pathways: Molecules guide axons
What kind of cue does each signal source give?
The molecular cues can be in the extracellular matrix, and might have once been released by neighboring cells. These cues are ADHESION and RECOGNITION molecules.
Molecular cues may be soluble. These are CHEMOATTRACTANT or CHEMOREPULSIVE signals.
Molecular cues can also be cell membrane bound.
Formation of Axonal Pathways: Molecules guide axons
What is the role of chemoattractant molecules?
Chemoattractant molecules may be TROPIC factors (which guide the cell toward the target) or TROPHIC factors, which promote cell metabolism.
Formation of Axonal Pathways: Molecules guide axons
What is the role of chemorepulsive molecules?
Chemorepulsive molecules discourage axon growth along a particular pathway.
