Flashcards in Chapter 22 - Early Brain Development Deck (25):
What is invagination in terms of development?
Invagination is the morphogenetic processes by which an embryo takes form, and is the initial step of gastrulation, the massive reorganization of the embryo from a simple spherical ball of cells, the blastula, into a multi-layered organism, with differentiated germ layers: endoderm, mesoderm, and ectoderm. More localized invaginations also occur later in embryonic development,
By the time invagination is complete, the early embryo consists of three layers of cells called the germ layers. What are the different germ layers?
1. An outer ectoderm
2. A middle mesoderm
3. An inner endoderm
What is the notochord?
The notochord is a distinct cylinder of mesodermal cells that condenses at the midline as the mesoderm invaginate and extends from mid-anterior to the posterior aspect of the embryo. The formation of the notochord at the midline of the gastrulating embryo is a central event forms eh development of the nervous system.
What causes ectodermal cells to differentiate into neuroectodermal precursor cells?
Along with cells that define the primitive pit, the notochord sends inductive signals to the overlying ectoderm that cause a subset of cells to differentiate.
What is neurulation?
The process of differentiation in the ectoderm, where neuroectodermal precursor cells are created.
What happens to the ectoderm once differentiation has begun?
The midline ectoderm that contain the cells that differentiate into neuroectodermal precursor cells thickens into a distinct columnar epithelium called the neural plate.
The midline ectoderm that contain the cells that differentiate into neuroectodermal precursor cells thickens into a distinct columnar epithelium called the neural plate. And then what happens?
The lateral margins of the neural plate then fold inward, transforming the neural plate into a tube. This cells within this neural tube subsequently gives rise to the brain, spinal cord, and most of the peripheral nervous system.
Classic experiments wanted in the first half of the twentieth century wanted to gain knowledge about how the neural stem cells knew what to differentiate into. How were these questions researched?
It was quite clear the these instructions to differentiate came from neighbouring cells or tissues. The classic experiments tried removing or transferring embryonic tissues to assess the capacity of ectodermal, mesodermal and endodermal cells to form organs composed to differentiated cell types. Cells that are moved either acquire the identity of the new region, or they retain an identity that reflects the original position.
Mention some of induction factors.
1. Retinoid acid (RA)
2. Fibroblast growth factor (FGF)
3. Bone morphogenetic proteins (GMPs)
4. Endogenous antagonists (Noggin and Chordin)
6. Sonic hedgehog (Shh)
What is reninoic acid (RA)?
An inductive signal that activates a unique class of transcription factors, the retinoid receptors. This can cause the expression or repression of a gene. Excess retinoid signalling can cause severe birth defects, including incomplete neural tube closure and other disruptions of early brain morphogenesis.
What is fibroblast growth factor (FGF)?
The Fibroblast growth factor family of peptide hormones is among the largest set of peptide hormones. FGFs from the pre-somatic mesoderm regulate spinal cord neurogenesis.
What is bone morphogenetic proteins (BMPs)?
In mesodermal cells, the BMPs elicit osteogenesis (bone formation). When ectodermal cells are exposed to BMPs, they assume an epidermal fate, forming structures associated with the skin.
What are endogenous antagonists, like Noggin and Chordin?
Noggin and Chordin are two members of the broad class of endogenous antagonists that modulate singling. These antagonists can bind directly to BMPs, preventing their binding to BMP receptors. BMPs normally cause ectoderm to become skin - Noggin and Chordin prevents that.
What is Wnt?
Wnt is a family of secreted inductive signals. They're involved in lengthening the neural plate and neural tube, and influences cell proliferation, adhesion, and differentiation after the initial morphogenesis of the nervous system.
What is Sonic hedgehog (Shh)?
Sonic hedgehog is another peptide hormone that is essential for induction in the developing nervous system. It is thought to be particularly important for closing the neural tube, and establishing the identity of motor neurons in the ventral portion of the spinal cord and hindbrain.
Neurogenesis begins after the initial patterning of the neural tube is complete. At this time, precursor cells in various brain regions have distinct signatures of gene expression that assign basic identities. These precursor cells are located in ...
the ventricular zone: the innermost cell layer surrounding the lumen of the neural tube.
New brain cells in the developing brain arise from divisions of cells in the ventricular zone. Both new precursor cells and new stem cells are created. How do these differ when they are created?
New stem cells arise from symmetric visions of neuroectodermal cells. These cells divide relatively slowly and can renew themselves indefinitely.
Postmitotic neurons, in contrast, are generated from cells that divide asymmetrically: one of the two daughter cells becomes a post mitotic neuroblast while the other re-enters the cell cycle to give rise to yet another post mitotic progeny via asymmetric division.
How are neurons of the six cortical layers generated in development?
In the cerebral cortex, most neurons of the six cortical layers are generated in an inside-out manner.
Where in the cortex will you find the oldest cells?
The firstborn cells are eventually located in the deepest layers.
Where in the cortex will you find the youngest cells?
Later generations of neurons migrate radially from the site of their final division in the ventricular zone, traveling through the older cells and coming to lie superficial to them.
What do we know about how a neural stem cells "decides" whether to divide into another neural stem cell, or a post mitotic neuron?
Interactions between a family of cell surface ligands, the Deltas, and their cell surface receptors, the Notches, are key regulators of stem cell decisions to generate either additional stem cells or post mitotic neurons.
True or false: Signalling via Delta ligands binding to Notch receptors happens only between cells that are next to one another.
How do Delta ligands affect the Notch receptors?
Delta binding to Notch leads to the cleavage of the intracellular domain of the receptor, liberating a protein fragment of the Notch receptor into the cytoplasm, form whence it is transported into the nucleus. Once inside the nucleus, the receptor fragment causes the transcription of a number of genes. Among these is a signalling factor that leads to down regulation of Delta in most neighbouring cells, but upregulation in one or a few of the neighbouring cells. Cells whose Delta levels have been diminished remain as neural stem cells.
What is gastrulation?
Gastrulation is the process of cell movement in which the endoderm is formed.