Nervous system development

Developmental stages in utero

1. Preembryonic period: conception -2nd week

2. Embryonic period: 3rd - 8th week

3. fetal period: 9th - 38th week (birth)

Preembryonic period

• ~3 days after fetilisation

  • morula (sphere of cells)

  • enters the uterus

• cavity opens in sphere

• now called blastocyst

Preembryonic period cont.

• blastocyst implants into endometrium (lining of uterus) at end of 1st week

  2nd week

• blastocyst completely implanted

• inner cell mass develops into embryonic disc

Embryonic disc initially divides into

• Ectoderm

• Endoderm

2 key phases of embryonic period

• neural tube formation

• when ends of tube close, brain formation begins

Neural tube formation 1

Embryonic disc becomes trilaminar

• ectoderm

• mesoderm

• endoderm

Ectoderm

• thickens and forms neural plate

• gives rise to nervous system

Mesoderm

gives rise to cardiovascular, musculoskeletal, urinary and reproductive systems

Endoderm

gives rise to respiratory and gastrointestinal system

Neural tube formation 2

• About day 18 neural plate forms neural groove

• edges of neural plate fold toward each other until they meet and form neural tube

Neural tube

• will form brain and spinal cord

• 2 open ends = neuropores, rostral and caudal

Rostral end of neural tube develops into

• The brain

• closes about day 25

Caudal end of neural tube develops into

• The spinal cord

• closes about day 27

Neural crest

• cells beside neural tube separate from tube & remaining ectoderm to form neural crest

• will form most of PNS

Brain formation during week 4 (after neuropore closure)

enlarged region develops

• hindbrain (rhombencephalon)

• midbrain (mesencephalon)

• forebrain (prosencephalon)

Central canal of neural tube becomes ventricular system

choroid plexus develops for production of CSF

Rhombencephalon divides into

• myelencephalon (medulla)

• metencephalon (pons and cerebellum)

Mesencephalon

Midbrain

Prosencephalon divides into

• diencephalon (inc. thalamus & hypothalamus)

• telencephalon (inc. cerebral hemispheres)

Fetal period

• nervous system develops more fully

• 4th month - few years after birth: myelination

• 20 weeks: brain begins to convolute from smooth cortex

• 24 weeks: gyri and sulci formed

After birth

• most rapid brain growth (0-2 years)

• myelination continues during first few years

• pruning (removing synapses that are no longer helpful)

Fluid-filled cavity of neural tube evolves into

Ventricular system

Ventricular system

Contains CSF

Provides buoyancy and protection to the brain

• clear colourless fluid

• circulates through nervous system

• continually produced and reabsorbed within the brain

• replaced up to 4x/day

• produced by choroid plexus

Cerebral ventricles

Interconnected system of 4 cavities:

• 2 lateral ventricles

  • 1 in each hemisphere

  • largest, courses through all 4 lobes

• 3rd ventricle

  • behind lateral ventricles, top of brainstem

• 4th ventricle

  • between cerebellum & pons

Lateral and 3rd ventricle connected by

intra-ventricular foramen

3rd and 4th ventricle connected by

cerebral acqueduct

Flow of CSF

Flows in 1 direction over brain surfaces

• lateral ventricles >

• 3rd ventricle >

• 4th ventricle >

• foramina (openings) >

• subarachnoid space >

• circulates around brain and spinal cord >

• reabsorbed through arachnoid villi/granulations (project into venous sinuses)

Causes of congenital abnormalities

• genetic factors affecting development including chromosomal abnormalities

• environmental teratogens (e.g. drugs, chemicals) that cause developmental defects

Abnormal embryonic development

• anencephaly

• spina bifida

• hydrocephalus

• chromosomal abnormalities - Down Syndrome and Fragile X Syndrome

Critical period for brain development

• 3 -16 weeks gestation

• particularly when neural tube and neural crest form

Anencephaly

• defective fusion of neural tube

• cranial (rostral) end of neural tube remains open and forebrain doesn’t develop

• most die before birth, almost none survive more than 1 week after birth

• causes: e.g. maternal nutritional deficiencies (folic acid), chromosomal abnormalities

Spina bifida

• defective fusion of neural tube

• caudal end of neural tube remains open > results in malformation of lower spinal cord

• variable severity: asymptomatic (occulta - no visible sign) > paralysis of lower limbs

• folic acid supplements reduce incidence of neural tube defects by about 70%

Hydrocephalus

• enlargement of ventricles due to CSF accumulation

• raised intracranial pressure > progressive brain damage

  Types:

• obstructive hydrocephalus

• communicating hydrocephalus

Obstructive hydrocephalus

Occlusion of CSF drainage channel > dilation above blockage

Communicating hydrocephalus

Inadequate reabsorption of CSF or excessive CSF production > dilation of entire ventricular system

Hydrocephalus treatment

shunt (one-way valve) in ventricle, excessive CSF drained to abdominal area, needs frequent replacement

Down Syndrome (trisomy 21)

Extra copy of chromosome 21

Characteristics may include:

• cognitive impairment

• small, flattened skull

• short and flat nose

• short fingers

• small oral cavity, large tongue, low tone of tongue and lips - language development or speech production can be affected

Fragile X Syndrome

X-linked recessive syndrome

expressed much more frequently in males

characteristics include:

• cognitive impairment

• long and narrow face with large ears

• high-arched palate - can affect speech and language development