EARLY DEVELOPMENT OF THE CENTRAL NERVOUS SYSTEM
Steps in early development of neural tube from neuroectoderm
Neuroectoderm – embryonic ectoderm
Day 19
- Neural plate formation
- Neural groove forms
Week 3
- Neural folds
- Folds fuse to form neural tube
- During fold fusion, cell proliferation between surface ectoderm and neuroectoderm – forms 2 bands = neural crest cells
- Neural crest cells migrate and differentiate à form DRG and autonomic ganglia
o DRG cells give rise to fibers that enter alar plate
o Neural crest cell strips on either side of s/c break up into somites – for every somite you have a chunk of neural crest cells = segmentation
How three layers arise in neural tube
- Neural tube initially = pseudostratified epithelium with pluripotent ependymal cells that divide in ventricular region and extend to pial region of tube
- Interkinetic nuclear migration – nucleus migrates to pial surface during DNA replication and then divides in ventricular region
- Neurons then break contact with ventricular surface and migrate towards pial surface to form layers and send out axons
Organization of neural tube into alar and basal plates & basic function derived from each plate
Week 5
- 3-layered tube with slit-shaped lumen is formed
- Layers inside to outside
o Ependymal layer – where cell division occurs (form neuroblasts)
o Mantle zone – where newly born neurons live
o Marginal zone – where axons of new neurons project
- Growth of lumen – now diamond shape lined by longitudinal furrows (sulcus limitans)
- Sulcus limitans divides mantle layer into alar and basal plates
o Basal plate gives rise to all spinal cord and brain stem MN that project out of CNS = ventral s/c
o Alar plate yields neurons involved with sensory processing and relay centers = dorsal s/c
Development of cephalic portion of neural tube (including primary vesicles, divisions, flexures)
- Neural fold fusion starts in center of neural plate and grows in R/C direction
- Anterior and posterior ends are open (neuropores)
- Before anterior neuropore closes, there are 3 swellings and 2 constrictions in rostral end of tube which are the primary brain vesicles that also divide into 5 vesicles by week 5
o Rhombencephalon (hindbrain) – divides into
§ Metencephalon
§ Myelencephalon
§ 4th ventricle
o Mesencephalon (midbrain) – with cerebral aqueduct
o Prosencephalon (forebrain) – divides into
§ Telencephalon w/lateral ventricles
§ Diencephalon w/3rd ventricle
Week 3 and 4
- Cervical flexure – at s/c and rhombencephalon jxn
- Mesencephalic flexure – at level fo mesencephalon
Week 5
- Pontine flexure – at jxn of myelencephalon and metencephalon
- Opens lateral walls of neural tube out like a book
- Causes sulcus limitans to lie in floor of fourth ventricle and alar plate to be lateral rather than dorsal to ventricle basal plate
How adult pattern of brainstem nuclei is derived from original alar and basal plate neuroblasts
Three things that happen to neuroblasts of alar and basal plates
- Organize into discrete CN nuclei and not columns (as in s/c)
- Some neuroblasts migrate away from lumen and become important relay nuclei in the brainstem
o Because they are relay nuclei, they will be alar plate neuroblasts
- They will migrate away from lumen and become part of marginal layer to form the reticular formation
Alar plate neuroblasts = dorsal s/c – sensory and relay functions
Basal plate neural blasts = ventral s/c – motor
- Neurons in basal plate give rise to axons that innervate muscles derived from adjacent somites
- Neurons that are part of the visceral motor system in the basal plate also go out and innervate structures derived from adjacent somites
In the brainstem
- In myelencephalon, alar plate is lateral and basal plate is medial
- Basal plate –forms tongue and extrinsic eye muscles
o Nuclei medial to lateral
§ Hypoglossal
§ Abducens
§ Oculomotor
§ Trochlear
o Dorsal motor n. of X in between basal and alar plates
o Metencephalon – salivatory nuclei
o Mesencephalon – Edinger-Westfall n.
o Branchiomeric cranial n – n. ambiguous
- Alar plate
o Met – n. solitarious
o Mes – sensory and motor n. of V
o Mye – spinal n. of V
o Mes – mesencephalic n. of V
o Laterally in met and mye – cochlear and vestibular n.
Four evaginations of prosencephalon and structures that arise from each
Optic vesicles
- First before ant neuropore closes
- Form optic nerve and retina
Midline evagination in floor of forebrain – forms posterior pituitary
Midline evagination in caudal part of roof of forebrain – forms pineal gland
Paired telencephalic eaginations rostral to optic vesicles – form cerebral hemispheres that cover diencephalons completely
Steps that occur during development of cerebral hemispheres in terms of gross changes
Cerebral/telencephalic vesicles
- Consist of floor fused with diencephalons by thin roof plate (future choroid plexus of lateral ventricles)
- Entire telencephalon formed from alar plate
Cerebral cortex formation
- Marginal layer of 3-layer tube invaded by neuroblasts that arrange on outer surface to develop into layered cortical grey matter
- Inside-out laminar development
o Deepest cells of cortex form earliest
o Later-forming neurons migrate to more superficial areas
- Mantle neuroblasts don’t migrate and become corpus striatum (cells of basal ganglia)
Internal capsule
- In telencephalon
- Fiber layer between caudate nucleus and lentiform nucleus
- Forms by axonal growth from cortex through corpus stiratum to diencephalons and brainstem
- Caudally forms between thalamus medially and lentiform nucleus laterally
Temporal lobe formation
- Inferior/temporal horn of lateral ventricles results from forward growth of temporal lobe
- Choroid plexus, cerebral cortex, corpus striatum grown in C-shape around third ventricle
- Spatial arrangement of above 3 strucures is upside down because of C-shaped curvature of the horn
Corpus callosum
- Initially a small commissure in dorsal lamina terminalis (most rostral midline structure of neural tube)
- Caudal expansion with cerebral hemisphere growth
- Hippocampus regresses and cerebral fissure forms