Embryology-Nervous System

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Neurulation:

Notochord induces columnar ectodermal cells to become pseudostratified neuroepithelial cells & form a spoon-shaped plate called neural plate to forms primordium of the brain (earliest recognisable stage of development).

Caudal primordium rise to neural tube

Lateral edges of neural plate form neural folds while midline of neural plate forms a neural groove

When neural fold meet at the midline & fuse, it encloses a neural canal but still have neuropores to supply the nutrient

All this processes called Neurulation

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Spinal Cord:

Start from neuroepithelium in the neural tube differentiates, thickens and appears layered.

The layer closest to the lumen (central canal) is called Ventricular or Neuroepithelium layer and become Ependymal layer

Ependymal layer is surrounded by Intermediate or Mantle layer which contains cell bodies of neuroblasts and presumptive glia cells.

Mantle layer becomes the grey matter and migrate to form marginal layer of white matter of the spinal cord later.

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Neural Crest

During fusion of neural folds, a population of specialised cells derived from neuroepithelium develops along the lateral side of neural folds between neural & ectoderm layers

Neural crest migrates to dorsal position & extend along neural tube & become dorsal root ganglion, autonomic ganglion & glial cells

Peripherally growing processes from marginal layer join the fibres of the ventral root to form the trunk of the spinal nerves

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Brain Development:

Anterior two-thirds of the neural tube develop into the brain

First stage of brain division is divided into 3 parts:

Prosencephalon (fore-brain)

Mesencephalon (mid-brain)

Rhombencephalon (hind-brain)

Higher vertebrates, brain is divided into 5 parts

Prosencephalon (fore-brain): Telencephalon & Diencephalon

Mesencephalon (mid-brain)

Rhombencephalon (hind-brain): Metencephalon & Myelencephalon

Brain Flexures:

Differential growth of the 5 parts of the brain gives rise to 3 flexures:

Ventral cranial flexure (cephalic flexure) occurs in the mid-brain (Mesencephalon) after the head folding

Cervical flexure occurs between hind-brain (Rhombencephalon) & spinal cord

Dorsal flexure (pontine flexure) occurs between Metencephalon & Myelencephalon of the hind brain (Rhombencephalon)

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Rhombencephalon:

After closure of neuropores & formation of pontine (dorsal) flexure, lateral walls of Rhombencephalon splay apart dorsally & stretch to the roof plate forming diamond -shaped structure overlying a large central space called fourth ventricle

Inside of the fourth ventricle has Choroid plexus which produces cerebrospinal fluid

Rostral to pontine flexure = Metencephalon

Caudal to pontine flexure = Myelencephalon

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Myelencephalon:

It consists of lateral walls each with dorsal alar & ventral basal plates separated by sulcus limitans, roof & floor of fourth ventricle

It develops into Medulla oblongata which has similar formation to the spinal cord but unlikely the same. – Alar plate located laterally to basal plate compared to spinal cord (alar located dorsally to basal plate). – Roof plate consists of a single layer of ependymal cells covered by pia mater layer

Medulla oblongata serves a relay centre for neurological signals from/to spinal cord to/from the brain = Vital centre (heart rate, blood pressure, respiration) (CN 9, 10, 11 & 12)

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Metencephalon

Cranial part of the Rhombencephalon

It differs from Myelencephalon by forming 2 specialised structures (cerebellum & pon): – Pon serves as a pathway for nerve fibers between cerebrum and cerebellum (cranial medulla oblongata in order to deal primarily with sleep, respiration, swallowing, bladder control, hearing, equilibrium, taste, eye movement, facial expressions, facial sensation & posture (CN 5, 6, 7 & 8).Cranial part of the Rhombencephalon • It differs from Myelencephalon by forming 2 specialised structures (cerebellum & pon) – Pon serves as a pathway for nerve fibers between cerebrum and cerebellum (cranial medulla oblongata in order to deal primarily with sleep, respiration, swallowing, bladder control, hearing, equilibrium, taste, eye movement, facial expressions, facial sensation & posture (CN 5, 6, 7 & 8) – Cerebellum plays an important role in motor control, posture, auditory & visual reflexes – Cerebellum plays an important role in motor control, posture, auditory & visual reflexes

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Mesencephalon

Undergoes fewer developmental changes than other part of the brain.

Due to medial expansion of alar & basal plates into the roof & floor, neural canal is reduced in size to from Aqueduct

Forms 4 aggregations of neuroblasts (paired rostral & caudal colliculi) associated with visual & auditory functions, respectively

Also, it serves as pathways for nerve tracts descending from cerebrum to pon & spinal cord (CN 3, 4)

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Prosencephalon (fore-brain)

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Diencephalon

As basal plates are absent from the fore-brain, Diencephalon is formed from left & right alar plates & roof plate.

A cavity in diencephalon is third ventricle

Consists of:

Epithalamus (dorsally) including pineal gland (epiphysis) produces melatonin & serotonin hormones: 24 hour rhythms

Thalamus: 3 swelling developments occur to form dorsal, middle & ventral masses on each side & then fuse form one structure called Thalamus. – Acts as a centre for relaying sensory impulses to regulate visceral function, sleep, digestion, body temperature

Hypothalamus (ventrally): responsible for metabolic process including body temperature, hunger associated with pituitary gland (hypophysis) produces hormones • Optic cups will give rise to the retina of the eye

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Telencephalon

Develop into cerebral hemispheres & olfactory bulbs

Has a paired cavities in Telencephalon called lateral ventricles & communicate with third ventricle through interventricular foramina

Expansion of dorsal, caudal & ventral directions creates c-shaped appearance

Final form, L & R cerebral hemispheres located over diencephalon, mesencephalon & rostral area of hind brain separated by longitudinal fissure

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Medial walls of hemispheres increase in thickness forming hippocampi associate with memory

Surface of hemispheres forms small elongated elevations (gyrus, gyri) which are separated by shallow grooves called sulcus, sulci

Cerebrum is divided into 5 lobes (frontal, parietal, occipital, temporal & olfactory lobes)

The largest & most important nerve bundle which connects non-olfactory areas of each hemisphere is corpus callosum

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Meninges:

In the embryonic and early fetal period, brain and spinal cord are surrounded by two mesenchymal layers: ectomeninx (from axial mesoderm) & endomeninx (from neural crest)

Ectomeninx forms dura matter while endomeninx subdivided into pia mater and arachnoid mater

Subarachnoid space

No epidural space is found in cranium because dura matter is fused with periosteum of the cranium