2.1 Embryology (Pt 1) Flashcards
What is the process that forms the basic germ layers and when does this occur?
List the 3 layers and what these are collectively known as
GASTRULATION: occurs 18 Days following conception
Blastula reaches uterine cavity and implants, Blastula cells differentiate into 3 layers:
- Endoderm
- Mesoderm
- Ectoderm
Collectively known as a Trilaminar disc
Ovum + Sperm forms what?
ZYGOTE
What is the timescale that defines an embryo vs a foetus?
Embryo = first 8 weeks
Foetus = 8 weeks till birth
Label the image below and state 4 thing each germ layer becomes
Following formation of the 3 germ layers, what occurs next?
Briefly explain how this occurs?
Neurulation:
1) Thickening of ectoderm: development of neural plate
2) formation of the neural groove: at 20 days
3) formation of neural folds
4) completion of neural Tube
List the 5 phases of brain development
1) Neural plate induction
2) Neural proliferation
3) Migration & Aggregation
4) Axon growth & Synapse formation
5) Apoptosis & Synapse rearrangement
What is neurulation and when does it begin?
A process of folding and transformation of neural plate into neural tube which begins in 3rd week
How do we get development of the neural plate?
The notochord induces the formation of the neural plate in overlying ectoderm (which goes on to form the neural fold)
As both the skin and neural tube develop from the ectoderm what triggers differential development?
The ectoderm develops into skin due to exposure to proteins such as BMP-4 (from endoderm & mesoderm)
Primitive node secretes BMP-4 anatagonists (eg. noggin, chordin, follistatin)
This allows a certain region of ectoderm to develop into nerve tissue
What triggers ‘neural plate induction’?
What 4 things are formed after neural plate induction?
Triggered by BMP-4 antagonists which allow a certain region of ectoderm to develop into nerve tissue
Results in formation of:
1) Primitive node
2) Notochord
3) Primitive streak
4) NEURAL PLATE
What structure induces neurulation?
The Notochord: thickening of mesodermal cells
How does the neural plate develop?
The notochord signals the overlying ectoderm to thicken and develop to form the neuroectoderm
This then forms the neural plate
Describe the process of neurulation
1) The notochord induces the formation of the neural plate in overlying ectoderm at the cranial pole
2) It forms and the neural groove and neural folds (as it has to accommodate more cell)
3) The groove deepens (becoming the neural trench), and neural crest cells begin to detach (these will go on to form a variety of structures).
4) Eventually the neural folds fuse at the midline to form the neural tube (first fuse in the centre then towards the cranial and caudal ends)
Why is the neural groove wider at the cranial and caudal ends?
This is where cranial (anterior) and caudal (posterior) neuropores will form
What process accompanies organisation and closure of the neural tube?
Why is this useful clinically
Somitogenesis: mesoderm starts to organise to form somites
Somites help us to identify foetal age
At day 22, there are 4 occipital somites that form a rough guide allowing development to continue craninally and caudally
Once the neural tube structure is formed, how does development continue?
The walls thicken to form various parts of brain and spinal cord and
The cells in the walls of the cranial neural tube give rise to neurones and glial cells
The lumen forms ventricular system and central canal
From which part of the neural tube does the brain vs the spinal cord develop?
Cranial 2/3rd (up to 4th pair of somites) develops into future brain
Caudal 1/3rd (distal to 4th pair of somites) develops into future spinal cord
When does each neuropore close?
Cranial neuropore - day 25
Caudal neuropore - day 27
What happens if the cranial vs caudal neuropores fails to close?
How can this be diagnosed?
If Cranial fails to close there will be failure of development of the brain, skull and scalp
If Caudal fails to close there will be failure of development of the spinal cord
Diagnosis: elevated Alpha-fetoprotien in maternal blood
The cephalic end of the neural tube forms 3 primary vesicles, List these and state which part of the adult brain each becomes?
1) Prosencephalon = forebrain
2) Mesencephalon = midbrain
3) Rhombencephalon = hindbrain
What does the Prosencephalon differentiate into?
What ‘wall’ and ‘cavity’ of the brain does each specially become?
Telencephalon
- wall: cerebral hemispheres
- cavity: lateral ventricles
Diencephalon
- wall: thalamus, hypothalamus and epithalamus
- cavity: third ventricle
What does the Rhombencephalon differentiate into?
What ‘wall’ and ‘cavity’ of the brain does each specially become?
Metencephalon
- wall: pons and cerebellum
- cavity: upper part of 4th ventricle
Myencephalon
- wall: medulla oblongata
- cavity: lower part of the 4th ventricle
What cavity does the Mesencephalon become?
Aqueduct
What are the 5 secondary vesicles?
Study image attached
Telencephalon
Diencephalon
Mesencephalon
Metencephalon
Myencephalon
During the 4th week we get formation of 2 brain flexures, what are these?
Where is each located and when is this completed?
Folding of embryo forms 2 brain flexures in the neuraltube:
1) Cephalic flexure: in mesencephalon region.
2) Cervical flexure: at junction of Rhombencephalon & spinal cord.
Completed by the 5th Week
What structure connects the lateral ventricles to the third ventricle?
Foreman monro
What structure connects 3rd ventricle to the 4th ventricle?
Aqueduct of Sylvius
If there is If there is a blockage of any cavity within the brain what may occur?
CSF will accumulate which can lead to increased intercranial pressure and compression of surrounding structures.
This can lead to hydrocephalus.
Label the image of the 2o vessicles and the cavities each forms
What 2 things occur during the migration and aggregation stage? (stage 3)
What day does each occu?
1) Formation of neurons (grey and white matter) and Glial cell: by day 26
2) Development and migration of Neural Crest Cells: during the 3rd week
Explain the process of how neuron (grey and white matter) and glial cells are formed?
Tube differentiates into two concentric rings by day 26:
- Mantle layer (grey) unmylinated
- Marginal layer (white) myelinated
Occurs in 3 waves:
- 1st wave: neurons that migrate to form mantle layer
- 2nd wave: glioblasts give rise to oligodendrocytes and astrocytes (oligodendrocytes mylinate marginal layer)
- 3rd wave: ependymal cells form and remain attached to membrane of tube (line the cavity)
What are Neural Crest cells and how do they form?
What NS do these cells give rise to?
Neural crest cells are of Neuro-ectodermal origin:
They form at neural folds and dissociate once the tube is formed
They migrate into the pharyngeal arches and facial region
They give rise to cells that form most of PNS and ANS:
(cranial, spinal, and autonomic ganglia)
What drives the formation of axons, nerves and synaptogenesis?
What is this known as?
As the spinal nerve emerges from the neural tube they extend into the peripheries.
This process is driven by the chemical changes in the surroundings. These are called chemical cues eg neurotrophins
These cause chemotaxis which allows the axon to grow outwards, this is known as ‘the growth cone’.
How are axons prevented from growing in the wrong direction?
If the nerve grows in the wrong direction, neurotrophins are released causing chemotaxis to occur in the correct direction
Give 2 factors that drive apoptosis and synapse elimination?
1) Activity at the connections as they develop
* The more active a cell is, the less likely it will be eliminated
2) Axonal competition will drive the elimination of some synapses
* muscles: poly-innervated ➞ mono-innervated
Give 3 functions of Neurotrophins
1) promote growth and survival
2) guide axons
3) stimulate synaptogenesis
involved in both ‘formation of axons’ and ‘synaps rearrangment’
What 2 things cause the neural tube to change in shape? (to better resemble our spinal cord, butterfly shape)
What ‘plate’ does each eventually become?
1) BMPs from skin induce expression of BMP’s in roof plate (dorsal side)
* becomes the alar plate
2) chemical substances induce changes in the floor plate (ventral side)
* becomes the basal plate
What is the roof plate?
What is the floor plate?
Signaling center: secretes BMPs and Wnts
Signaling center: secretes Shh
What nerve fibres do the basal and alar plates go on to become?
1) Alar plate: cells here send sensory projections to brain (afferent neurons) and form the dorsal (posterior) horn of the spinal cord
2) Basal plate: responsible for motor neuron development (efferent neurons) and form the ventral (anterior) horn of the spinal cord.
What structure of the midbrain do the basal and alar plates eventually become?
ALAR PLATE (neuroblasts migrate towards roof) forms the:
- Tectum
- Superior and Inferior colliculi
BASAL PLATE forms the:
- Tegmentum
- Efferent Nuclei
- Red and Reticular Nuclei
- Substantia Nigra
- Crus Cerebri
What structure of the hindbrain do the alar plates eventually become?
- 4th Ventricle
- Pons
- cerebellum
What is the Pons?
Passageway for fibres from cerebral and cerebellar cortices to spinal cord
Which vessicle does the chorid plexus cells develop from?
Telencephalon
At what week are the spinal cord and vertebral column the same length?
What subsequently happens?
Week 12: Spinal cord and vertebral column same length
Later the vertebral colum will thicken, elongate and exceed length of spinal cord
Where does the spinal cord end in adults vs children
In adults: L1
In children: L3/L4
What structure marks the end of the spinal cord?
What is the gathering of spinal nerves that extend beyond the cord?
Conus medularis
Cauda Equina
List the 10 structures passed in a lumbar puncture from superficial to deep
Incl the 3 ligaments
- Skin
- Superficial fascia
- Thoracolumbar fascia
- Supraspinous ligament
- Interspinous ligament
- Ligamentum Flavum
- Epidural fat
- Dura mater
- Arachnoid mater
- Subarachnoid space
ligaments shown on image
What gives rise to dorsal route ganglion?
Migrating neural crest cells
(these cells associate themselves with the growing spinal nerves)
