Development of the central nervous system Flashcards
What is gastrulation?
- Process whereby outer layer of bilaminar disc invaginates at primitive streak to produce 3 cell layers
- Endoderm, mesoderm, ectoderm
What are the 2 layers of the embryonic disc?
- Epiblast
- Hypoblast
From where does the nervous system derive?
- Ectoderm
- Becomes skin and neural tissue, as well as neural crest
Which key structure develops in the midline of the mesoderm?
- Notochord
- Grows towards where head will be
- Instructs overlying ectoderm to invaginate and form neural tube
- Via chemical signals
What is neurulation?
- Process of formation of neural tube
- Induced by notochord
Outline the process of neurulation
- Day 22
- Elevation of neural folds
- Surface over top of invagination needs to be sealed to create a fully formed neural tube
- Part that fuses first corresponds to cervical level of adult spine
- Fusion then proceeds cranially and caudally
- When folds fuse, neural crest cells detach and migrate to their ultimate destinations
What are the primary brain vesicles?
- Rostral neural tube displays 3 swellings which go on to form major parts of the adult brain
- Prosencephalon - forms forebrain
- Mesencephalon - becomes midbrain
- Rhombencephalon - becomes hindbrain
What developments have occurred in the embryo by day 40 of development?
- Cerebral vesicles have become very elaborate
- Very prominent swellings and subdivisions of basic forebrain, midbrain and hindbrain have formed
- Special sense organs have started to develop
- PNS has started to develop
- Sympathetic chain has formed
- Cranial nerves have formed
What has happened to the forebrain by day 43 of development?
Forebrain is divided into telencephalon and diencephalon
- Telencephalon forms cerebral cortex, basal ganglia, hippocampus
- Diencephalon forms thalamus, hypothalamus and optic nerves
Outline the fundamental relationship between the sensory and motor systems
- Motor structures tend to sit anteriorly
- Sensory structures tend to sit posteriorly
- Due to development of basal and alar plates in neural tube
- Pattern seen at all levels of nervous system
Outline the development of cauda equina
- Initially (at 60 days) spinal cord and vertebral column are same length
- However, spine grows faster than spinal cord (particularly at lumbar levels)
- Lower portions of cord are stretched, creating cauda equina
Why do neural tube defects predispose to hydrocephalus?
- May be caused by tethering of cord at site of defect
- As spine grows, cord cannot move within vertebral canal
- Results in brainstem being pulled through foramen magnum and becoming occluded
Outline the developmental basis of neural tube defects
- Failure of neural tube to zip up
- Failure to close in cranial direction involves the brain
- Failure to close in caudal direction caudal direction involves spine/spinal cord
- Failure of development of posterior vertebral arches at one or more levels
List the different neural tube disorders from most to least severe
- Craniorachischisis
- Anencephaly
- Myelocoele
- Myelomeningocoele
- Meningocoele
- Spina bifida occulta
Outline craniorachischisis
- Entire neural tube remains open
- Hence failure of both brain and spinal cord to form
- Incompatible with life
Outline anencephaly
- Cranial neural tube fails to close
- Hence failure of brain to form
- Child may be born alive but does not live for long
Outline meningomyelocoele
- A CSF-filled cyst containing spinal cord
- Transilluminates relatively poorly
- Children may have neurological deficits - roots are stretched and damaged
- Repair is necessary
Outline meningocoele
- Presence of a CSF filled cyst
- Cord is sited within vertebral canal
- Transilluminates brilliantly
- Children tend to have a good neurological prognosis
- Cyst requires repair as it predisposes child to infectiom
Outline spina bifida occulta
- Only anomaly is lack of posterior vertebral arch
- May manifest a sign such as a tuft of hair or a large naevus over defect
- Not associated with significant neurological problems
- Occurs in about 10% of the population
How are neural tube defects prevented?
- Folic acid taken from 3 months before conception and until week 12 of pregnancy
- Unknown reason why folate helps
What is the neural crest?
- A highly specialised population of cells that are a vertebrate innovation
- Multifunctional, contributing to a wide range of tissues
What forms the neural crest?
- Derived from point at which neural folds fuse
- When surface of ectoderm is reconstituted
- Neural crest cells become detached from ectoderm and then migrate to distant targets
What are the derivatives of the neural crest?
- Sensory neurones in the PNS
- Post-synaptic autonomic neurones
- Enteric neurones
- Schwann cells
- Adrenal medulla cells
- Melanocytes
- Arachnoid and pia mater of meninges
Which tissues receive a significant contribution from the neural crest?
- Thymus
- Thyroid
- Parts of the heart e.g. spiral septum
- Teeth
Which disorders occur due to defects in the neural crest?
- Di George syndrome
- Hirschsprung’s disease
How does Di George syndrome manifest?
- Immunodeficiency
- Facial anomalies
- Heart anomalies
- Hypocalcaemia
How does Hirschsprung’s disease manifest?
- Lack of enteric neurones in sections of large intestine
- Leads to hypomobility and constipation
What has happened to the hindbrain by day 43 of development?
Hindbrain is divided into metencephalon and myelencephalon
- Metencephalon gives rise to pons and cerebellum
- Myelencephalon gives rise to medulla
