2. Embryology Flashcards
(44 cards)
define gastrulation
Gastrulation is the process whereby the outer layer of the bilayer germ disc (ectoderm) invaginates at the primitive streak to produce a three layered germ disc
what are the 3 layers of the germ disk after gastrulation>
ectoderm, mesoderm, endoderm
what does the ectoderm become?
Becomes skin and neural tissue as well as neural crest
what does the mesoderm become?
Heart, vessels, muscles etc.
The notochord is a crucial structure composed of endoderm
what is the function of the notochord?
It inducts the overlying ectoderm to invaginate and form the neural tube
what does the endoderm become?
Gut including accessory organs
describe the process of neurulation
Neurulation is the process of formation of the neural tube
o Induced by the notochord
o Elevation of the neural folds
o Fusion of the folds in the midline at mid cervical level with reconstitution of the surface ectoderm
o Neural tube zips up rostrally and caudally
what happens to neural crest cells in neurulation?
When the folds fuse, neural crest cells detach and migrate their ultimate destinations
what happens if there is failure of the neural tube to zip rostrally or caudally?
If this process fails in the rostral direction can result in anencephaly
Failure in the caudal direction can lead to spina bifida
what are the 3 Primary brain vesicles?
The rostral neural tube displays a number of swellings which will become the major parts of the adult brain
- Forebrain (Prosencephalon)
- Midbrain (mesencephalon)
- Hindbrain (rhombencephalon)
What are the 5 secondary brain vesicles and what do they form from?
- Prosencephalon forms the telencephalon and the diencephalon
- Mesencephalon form the mesencephalon
- Rhombencephalon form the metencephalon and the myelencephalon
What are the mature derivatives of the secondary brain vesicles?
- Telencephalon: cerebral hemispheres
- Diencephalon: thalamus, hypothalamus and optic nerve/retina
- Mesencephalon: midbrain
- Metencephalon: pons and cerebellum
- Myelencephalon: medulla
what is a fundamental pattern to the organisation of the nervous system?
Motor structures tend to sit anteriorly (I remember this as you ‘motor forwards)
Sensory structures tend to sit posteriorly
What causes the pattern of motor anteriorly and sensory posteriorly?
This pattern exists due to the development of the basal (floor) and alar (roof) plates in the neural tube
• The notochord inducts the ventral (anterior) portion of the neural tube to become the basal (or floor) plate
o The basal plate gives rise to motor neurones
• The alar (or roof) plate (dorsal) forms in the absence of influences from the notochord
o Gives rise to inter- and sensory neurones
How is this fundamental pattern seen in the spinal cord?
Dorsal roots are sensory whilst ventral roots are motor
The dorsal horn contains sensory neurones whilst the ventral horn contains motor neurones
The dorsal columns (a sensory tract) sit posteriorly whilst the corticospinal tract (motor) sits anteriorly
How is this fundamental pattern seen in the medulla?
The lemnisci (sensory) sit posterior to the pyramids of the medulla (motor) (contain corticospinal tracts)
How is this fundamental pattern seen in the midbrain?
The colliculi (sensory) sit posterior to the cerebral peduncles (motor)
How is this fundamental pattern seen in the cerebral cortex?
The primary sensory cortex (sensory) sits posterior to the primary motor cortex (motor)
Why are there flexures in the CNS?
Growth & development at cranial NT exceeds available space linearly, so….it must fold up
What are the 2 flexures of the CNS?
- Cervical flexure (spinal cord- hindbrain)
- Cephalic flexure (midbrain region)
what leads to the development of the cauda equina?
o Initially, there is a one to one correspondence between cord levels and vertebral levels (the cord fills the entire vertebral canal)
o but, the spine grows faster than the spinal cord, particularly at the lumbar levels
o so, the lower portions of the cord are stretched, drawing out the cauda equina
How do neural tube defects predispose to hydrocephalus?
This may be caused by tethering of the cord at the site of the defect
Thus, as the spine grows the cord cannot move within the vertebral canal, resulting in the brainstem (containing the fourth ventricle) being pulled down through foramen magnum and becoming occluded
What do neural tube defects result from?
Failure of the neutral tube to close
- can occur cranially or caudally
What do cranial neural tube defects result in?
Anencephaly
