17 Embryonic development (CNS) Flashcards
What is the embryological origin of neural and glial cells?
Neuroectoderm
What is the origin of basal (motor) plate?
The ventral (‘lower’) side of the neural tube
So ectoderm
What is the origin of the alar (sensory) plate?
The dorsal (‘upper’) side of the neural tube
Ectoderm
What are neural crest cells?
- Cells released from the dorsal lip when the neural tube is closing
- Neural tube cells undergo epithelial-to-mesenchymal transition (EMT), delaminate and migrate to the periphery
- These cells are essentially a sea of stem cells that can then migrate and differentiate to form a wide range of cells in the periphery
- The PNS is derived from neural crest cells
What are the derivatives of NCCs?
Neural aspects e.g. ganglia
Melanocytes
Enteric nervous system
Can mature neurons undergo mitosis?
No
Where are the sites of adult neurogenesis?
Sub-ventricular zone and dentate gyrus.
What are the first three regions of the developing brain that form?
Forebrain = prosencephalon
Midbrain = mesencephalon
Hindbrain = rhombencephalon
What are the 5 secondary vesicles the 3 primary vesicles? What adult structures do these eventually develop into?
What are the different steps of CNS development? (Brief)
- Neural induction
- Neurogenesis
- Cell differentiation
- Differentiation of connections
- Specialisation within the CNS
- Early spontaneous activity within the CNS
- Sensory connectivity patterns
- Plasticity
What is neural induction? (Brief)
- This is where the CNS is developed from the neural plate
- The plate closes to form the neural tube
- This tube is patterned in a longitudinal and ventral/dorsal pattern
- Longitudinal patterning by Hox genes
- Ventral/dorsal patterning by interactions from the surrounding mesoderm
- The rostral end of the neural tube forms the telencephalic vesicles, including the cerebral cortex (largest part of the brain)
- This process is linked to and occurs shortly after gastrulation
What occurs prior to neural induction?
- Fertilised egg divides and then invades the endometrium of the uterus
- The placenta, bilaminar and eventually trilaminar disc is formed
- Trilaminar disc is formed during gastrulation
What is gastrulation?
- This is the conversion of the bilaminar disc (made up of hypoblast and epiblast) into the trilaminar disc (mesoderm, endoderm and ectoderm)
- Cells migrate between the two primitive layers via the primitive streak - the cells that migrate become the mesoderm
- Some of the primitive mesodermal cells migrate below the hypoblast to form the neural plate
What is the neural plate and how does it develop?
- The neural plate is a thickening made up of ectoderm, and lies opposing the primitive streak (formed from migrating primitive mesodermal cells)
- Neural plate develops after the inducing effect of the primitive streak and is the basis of the nervous system
- Formation of nervous tissue involved complex interactions between mesoderm and ectoderm, mediated by:
- Shh and noggin
- BMPs, wnt, FGFs
What germ layer forms the nervous tissue?
Ectoderm
How is the eye formed?
- As an outgrowth of the CNS (specifically the diencephalon)
- This includes the retina, optic nerve and tract
- This means that the eyes are part of the CNS
- A cup is formed with inner and outer layers
- Inner layer gives rise to the neuroretina
- Outer layer gives rise to the pigment epithelium
- [EXTRA] Retinal detachment occurs directly at this embryonic boundary
- The subarachnoid space extends to the optic disc
- [CLINICAL] This means that intercranial pressure can cause the optic disc to protrude into the eye, so can be measured by looking at the papilla using an ophthalmoscope
How does the ventricular system develop?
- These are formed from dilation of the space within the neural tube
- Start simple, but become more and more complex until their characteristic shape is obtained
- Eventually forms two lateral ventricles, the third ventricle, the cerebral aqueduct and the fourth ventricle
What are the different structures within the embryonic forebrain, midbrain and hindbrain?
- Forebrain
- Telencephalon (cerebral hemispheres)
- Diencephalon (anterior forebrain structures, including thalamus, hypothalamus, posterior pituitary, pineal gland)
- Neural retina
- Lens
- (Lateral ventricle and 3rd ventricle)
- Midbrain
- Mesencephalon (all midbrain structures, e.g. colliculi, tegmentum, cerebral peduncles)
- (Cerebral aqueduct)
- Hindbrain
- Metencephalon (pons and cerebellum)
- Meyelencephalon (medulla)
- (4th ventricle)
What is the signalling centre for ventro-dorsal patterning? What is established by this patterning?
- Notochord - this is a signalling centre outside of the CNS, and is of mesodermal origin, using Shh as a signalling molecule
- Notochord induces the floor plate, which induces further differentiation along the neural tube
- Patterning also allows the establishment of motor neuron pools (somatic motor and visceral)
What is the roof plate (RP)?
- Thickening on the dorsal side of the neural tube
- Expresses BMP4 to effect development of neurons
- Specifically induces the formation of commissural interneurons (decussating interneurons)
- Dorsal side contains sensory (somatic and visceral) nerve fibres
Is embryonic patterning of the spinal cord retained in the adult?
Yes, roughly
How are neural connections established?
- If there are lots of projections extending from one region of the brain to another, they have to interconnect in a topographic way/organised manner
- Some rely upon molecular cues to reach their final destination, whereas a small proportion are able to find their way using neuronal activity
- The axons and cells extend long neurites that are able to navigate within the environment through the use of growth cones
- Connections are usually established when the distance between cells is minimal
- The growth cone exists on the tip of the advancing neuron, using filopodia to explore the immediate environment (look like flickering fingers)
- There is actin polymerisation occurring within these filopodia (can be visualised using GFP)
- There is local attraction and repulsion, the insertion of new cytoskeleton and autonomous action
- Growth cones respond to various positive and negative cues (e.g. contact, distance, diffusible chemicals)
How are axons thought to be guided to their correct connections?
- Using molecular cues - growth cones are either attracted or repelled by these
- These cues can be local (substrate-bound) or long-range (diffusible)
- A given molecule can be both attractive and repulsive, depending on the interaction
