Neuro II embryo

Nervous System Development

First Week

• Fertilization

• Cleavage: Mitotic divisions

  • Zygote: Fertilized oocyte

  • Morula: Early cleavage stage of zygote

  • Blastocyst

• Implantation

Second Week

• Bilaminar Embryonic Germ Disk

  • Epiblast

  • Hypoblast

Timeline of Events

• Embryonic Period: Organogenesis

  • Duration: 3rd - 8th week

  • Major body features become recognizable

• Gastrulation

  • Occurs during the 3rd week

  • Formation of 3 germ layers: endoderm, mesoderm, ectoderm

Gastrulation

• Processes Involved

  • Epiblasts → Primitive streak → Mesenchyme (embryonic connective tissue) → Fibroblasts, osteoblasts, chondroblasts

  • Primitive streak degenerates after 4th week

Persistence of Primitive Streak

• Leads to the formation of sacrococcygeal teratoma

• Incidence: 1 in 35,000 births

• Most common tumor in newborn infants

Embryonic Germ Layers

• Endoderm: Forms epithelial lining of the respiratory and GI tract

• Mesoderm: Forms cranium, muscles, urogenital system, heart, spleen, blood, bone

• Ectoderm: Forms skin, hair, nervous system

Notochord

• Mesodermal cells migrate through the primitive streak and condense in the midline to form the notochord

• Notochord formation triggers the development of the neural plate from the ectoderm

Formation of Nervous System

1. Neurulation: Formation of neural plate from ectoderm

2. Neural plate folds along the neural groove to form the neural tube

Neurulation

• Neural tube closure occurs, and the embryo begins to flex at day 22

  • Cranial end closes before the caudal end

Neural Tube Defects

• Craniorachischisis Totalis: Failure of entire neural folds to differentiate and close

• Spina Bifida: Failure of neural arches to fuse

  • Occulta: No vertebral arch but no underlying neurological involvement; occurs in 10% of normal individuals

  • Cystica: Occurs in 1 in 1000 births

    • Meningocele: Meninges protrude through vertebral arch

    • Meningomyelocele: Spinal cord and/or nerve roots protrude into the sac

Neural Crest

• Neural crest cells dissociate from the neural tube and migrate dorsally, ventrally, and cranially

Neurocristopathies

• Malformations due to defective neural crest cell development and migration

  • Genetic Causes:

    • Treacher Collins Syndrome: Underdeveloped zygomatic bones, small mandible, malformed ears

    • DiGeorge Syndrome: Craniofacial defects, immune deficiencies, cardiac abnormalities

    • Fetal Alcohol Syndrome

Neural Tube Structure

• Composed of neuroepithelial cells that form layers:

  • Ventricular Layer: Gives rise to neuroblasts that migrate to the mantle layer

  • Mantle Layer: Contains neuronal cell bodies (gray matter) of the CNS

  • Marginal Layer: Composed of nerve fibers (white matter) of the CNS

Neuroepithelial Cells

• Give rise to glioblasts which develop into astrocytes and oligodendrocytes

• After glioblast formation, neuroepithelial cells develop into ependymal cells

  • Ependymal cells and vascular mesenchyme together form the choroid plexus in each ventricle

Spinal Cord Development

• During the 4th week, the mantle layer differentiates into:

  • Alar Plate: Dorsal horn (sensory)

  • Basal Plate: Ventral horn (motor)

• Intermediate Cell Columns:

  • T1-L2: Sympathetic nervous system

  • S2-4: Parasympathetic nervous system

PNS Development

• Axonal sprouts from ventral horn cells form the ventral root

• Dorsal Root Ganglion (DRG) forms from neural crest cells

  • DRG gives rise to 2 axonal sprouts – pseudounipolar cells

    • One to dorsal column

    • Join the ventral root

Somites

• Begin formation between 19-20 days from paraxial mesoderm

  • Types: occipital, cervical, thoracic, lumbar, sacral, coccygeal

  • Form the axial skeleton

• 4th Week Differentiation:

  • Sclerotome: Tendon, cartilage, bone

  • Myotome: Muscle (epaxial, hypaxial)

  • Dermatome: Skin

Brain Development

• Proencephalon:

  • Telencephalon: Two cerebral hemispheres, commissures

  • Diencephalon:

    • Dorsally: Thalamus, epithalamus

    • Ventrally: Hypothalamus, infundibulum, posterior pituitary (neurohypophysis)

• Mesencephalon: Midbrain

• Rhombencephalon:

  • Metencephalon: Pons, cerebellum

  • Myelencephalon: Medulla oblongata

Flexures

• Cranial neural tube flexes to establish the shape of the adult brain

  • Mesencephalic: Forebrain folds under the rest

  • Cervical: Myelencephalon folds ventrally on the spinal cord

  • Pontine: Metencephalon folds back against myelencephalon

Ventricles

• Rhombencephalon cavity: IV ventricle

• Mesencephalon cavity: Cerebral aqueduct

• Diencephalon cavity: III ventricle

• Telencephalon Cavity: Lateral ventricle

Hydrocephalus

• Blockage of Cerebrospinal Fluid (CSF)

At Birth

• Myelination: Begins at the 4th month of development and continues to the 1st year

• Vertebral column and dura mater lengthen faster than the spinal cord leading to the cauda equina and filum terminale

• Thecal sac: Dura and arachnoid around the cauda equina extends to S2

• The brain is 25% of adult size at birth and increases in size due to nuclear cell growth and number of neural processes (axons)

• The cerebrum grows rapidly in the final part of fetal life, with the appearance of gyri and sulci.