Development of the Nervous System

Development of the Nervous System

Introduction

  • Presenter: Dr. Jittima Muensoongnoen

  • Institution: Cardiff University, Wales Centre for Anatomical Education

Learning Outcomes

  • Describe neurulation (the process of neural tube formation).

  • Explain the development of the neural canal into the ventricular system.

  • List and describe several neural tube defects such as:
      - Exencephaly
      - Anencephaly
      - Microcephaly
      - Spina Bifida

  • Describe the development of the neural crest and list its derivatives.

  • List and explain derivatives of the ectoderm.

  • Explain the early development of brain vesicles and their associated brain regions.

  • Explain the early development of the spinal cord and relate it to the development of other parts of the central nervous system (CNS).

  • Explain the development of neurons and glial cells.

  • By the end of the session and recommended readings, students should have comprehensive knowledge of the above topics.

Germ Layers

  • The trilaminar embryonic disc consists of three germ layers:
      - Ectoderm
      - Mesoderm
      - Endoderm

  • Orientation: Head end (anterior) and tail end (posterior).

Derivatives of Germ Layers

Ectoderm

  • Central Nervous System (CNS): Brain and spinal cord

  • Peripheral Nervous System (PNS): Nerves and ganglia

  • Sensory structures: Retina, cornea, lens, sclera

  • Epidermis (skin)

  • Hair and nails

  • Glands: anterior lobe of the pituitary, thyroid gland

Mesoderm

  • Muscles (skeletal and smooth)

  • Major blood vessels

  • Heart

  • Adrenal cortex

  • Kidneys and ureters

  • Gonads

  • Cartilage, connective tissues, and bones.

Endoderm

  • Epithelial lining of the GI tract

  • Glands: liver, pancreas (glandular cells)

  • Respiratory tract structures: lungs, trachea

  • Bladder and urethra.

Role of the Notochord

  • Provides longitudinal support for the embryo.

  • Stimulates the conversion of overlying surface ectoderm into neuroectoderm.

Neurulation

Day 17 After Fertilization

  • Neurulation involves the formation of the neural tube:
      - The overlying ectoderm is induced by the notochord to form a neural plate.
      - The ends of the neural plate develop into neural folds and a neural groove.
      - The fusion of neural folds results in the formation of the neural tube, which encompasses a cavity known as the neural canal.
      - The neural tube separates from the surface ectoderm.

Neurulation Timeline

  • Day 19 - Day 23: Neural tube fusion occurs bidirectionally.
      - Cranial and caudal ends remain open during the initial fusion, referred to as neuropores.
      - Neurulation begins on day 17 and completes by the end of week 4.
      - Anterior neuropore closes on day 24 and posterior neuropore on day 26.

  • Diagrammatic Representation:
      - Cephalic part of neural tube develops into the brain.
      - Caudal part of neural tube develops into the spinal cord.
      - The neural canal becomes the ventricular system and central canal of the spinal cord.

Neural Tube Defects

  • Exencephaly: Result of failure of closure of the anterior neuropore, leading to brain development outside the skull; considered a precursor to anencephaly (absence of brain).

  • Microcephaly: Characterized by a small brain within a reduced cranium.

  • Spina Bifida: Incomplete closure of the vertebrae usually occurs in the lumbosacral region; spinal cord, nerve roots, and meninges may protrude outside the vertebral canal.

Neural Crest Development

  • Neural Crest Cells: Located at the border of the neural plate, the closure of the neural tube disconnects neural crest from surface ectoderm.

  • They migrate to various body locations and give rise to multiple structures including:
      - Bones of the face and skull
      - Ganglia of cranial and spinal nerves
      - Schwann cells and glial cells
      - Meninges (pia & arachnoid)

Derivatives of the Ectoderm

Neuroectoderm

  • Forms the neural tube and derives parts of the CNS including the brain and spinal cord.

Surface Ectoderm

  • Forms epidermis (skin), hair, nails, and secretory cells of glands.

Development of Brain Vesicles

  • Primary Brain Vesicles:
      1. Prosencephalon (Forebrain)
      2. Mesencephalon (Midbrain)
      3. Rhombencephalon (Hindbrain)

  • Secondary Brain Vesicles:
      1. Telencephalon – develops into the cerebral cortex and basal ganglia.
      2. Diencephalon – forms the thalamus, hypothalamus, pituitary gland, pineal gland, optic stalk, and retina.
      3. Mesencephalon – remains as midbrain.
      4. Metencephalon – develops into pons and cerebellum.
      5. Myelencephalon – forms the medulla oblongata.

Cranial Nerves

  • 12 pairs of cranial nerves arise from various parts of the brainstem, crucial for various sensory and motor functions.

Development of the Spinal Cord

  • During the neural groove stage, neuroepithelial cells undergo rapid division, resulting in the formation of the neuroepithelium.

  • Post-tube closure, neuroepithelial cells give rise to:
      - Neuroblasts: form mantle zone leading to grey matter formation.
      - Marginal Zone: contains nerve fibers from neuroblasts, making up white matter.

Structural Layers of the Neural Tube

  1. Neuroepithelial Layer: The innermost layer.

  2. Mantle Layer: Composed of neuroblasts and glial cells, forms grey matter.

  3. Marginal Layer: Becomes the white matter.

Functional Zones of the Spinal Cord

  • Ventral Horn: Motor neurons (basal plates).

  • Dorsal Horn: Sensory neurons (alar plates).

  • Intermediate Zone: Contains sympathetic neurons located between the ventral and dorsal horns (present at T1-T12 and upper lumbar levels).

Histological Differentiation of Neurons

  • Neuroblasts migrate, becoming bipolar and eventually multipolar as they establish axons and dendrites:
      - Axons emerge from the basal plates, forming ventral roots.
      - Dendrites develop from alar plates, forming the incoming sensory pathways.

Histological Differentiation of Glial Cells

  • Following cessation of neuroblast production, glial cells including:
      - Protoplasmic and Fibrillar Astrocytes: Provide support and metabolic functions.
      - Oligodendrocytes: Myelinate CNS axons.
      - Microglial Cells: Appear later in development, responsible for phagocytosis.

Myelination

  • Oligodendrocytes: Arise from oligodendroglial cells, myelinate up to 50 CNS axons.

  • Schwann Cells: Myelinate PNS axons, each Schwann cell myelinating only one axon.

Brain and Brainstem Development

  • The brainstem is a continuation of the spinal cord, with complex organization differing from spinal cord structure.

  • Medulla features an everted lateral architecture with distinctive sensory and motor nuclei.

Conclusion

  • The study of the development of the nervous system encompasses various stages of cellular differentiation and structural formation, crucial for understanding neuroanatomy and associated clinical conditions.