L15-16 Embryo

DEVELOPMENT OF CNS (CENTRAL NERVOUS SYSTEM)

Introduction to CNS Development

  • The brain and nervous system develop earliest during gestation and complete their development last, extending into adolescence.
  • Brain formation initiates in the 3rd week of fetal life and continues until adolescence through a process known as myelination, where the myelin sheath of neurons is formed.

Duration of CNS Development

  • Development of the nervous system often extends beyond adolescence, sometimes lasting until the age of 30.
  • Neurogenesis, the birth and reproduction of neurons, can continue into individuals' 60s.
  • An increase in neuron count does not necessarily indicate improvement in brain functionality; the key aspect is the increase in the number of synapses, which peaks in the 30s.
  • Post-30s, mental practice can help preserve synaptic numbers longer than in those who do not engage in mental exercises.

Organogenesis

  • Organogenesis Definition:
    • The transformation of structural layers into organs, predominantly observed during the embryological period.
    • Early embryonic development showcases a double-layered body plan (germinative disc).
  • Transition from dipoblastic organisms to triploblastic organisms occurs:
    • Three layers are formed:
    • Endoderm (inner layer)
    • Mesoderm (middle layer)
    • Ectoderm (outer layer)

Differentiation of Embryonic Layers

  • As organogenesis commences, these layers differentiate into various organs, establishing foundational structures.
  • Human anatomy can be classified as a three-layered organism, beginning with the formation and structuring of the notochord leading to neural tube formation.

Gastrulation Process

  • Takes place in the 2nd week, transforming the embryo from a two-layered disc (epiblast and hypoblast) into a three-layered structure via the formation of the primitive streak and subsequently, the notochord.
  • Gastrulation Definition:
    • The process of transforming a two-layered embryonic disc into a three-layered embryonic disc.
  • Begins with the emergence of the primitive streak.
  • Key features include:
    • Proliferation of epiblast cells forming the median primitive line, which elongates over time.
    • The primitive node (Hensen node) forms at the cranial end, leading to the development of the primitive pit and groove.

Notochord Formation

  • The primitive pit migrates to form the notochord extension between the ectoderm and endoderm, establishing the foundational axis of the embryo.
  • Key functions of the notochord include:
    • Determining the body's axial skeleton
    • Forming the basis for the spinal structure
    • Gradual degeneration of the notochord leads to its persistence only as nuclei pulposus in intervertebral discs.

Neurulation

  • Neurulation Definition:
    • The process giving rise to the neural tube essential for central nervous system development.
  • Begins with the formation of the neural plate due to dorsal thickening of ectodermal cells, leading to the folding of the neural plate into neural folds.
  • Post folding, the neural tube forms and separates, giving rise to neural crest cells which migrate away to develop various cell types.

Neuroectoderm Derivatives

  • Epidermal ectoderm derivatives include:
    • Epidermis, hair, glands, nails, cornea, otocyst, gut linings, ameloblasts, and tongue covering.
  • Neural crest cells (the “4th germ layer”) derivatives include:
    • Peripheral nervous system (PNS) ganglia, myenteric and submucosal plexus of the GI tract, Schwann cells, adrenal medulla, odontoblasts, melanocytes, and connective tissue derivatives in the head and neck.

Developmental Stages of CNS

  • Second Week:
    • Germ layers form: ectoderm, mesoderm, endoderm. Ectoderm leads to the central nervous system and epidermis.
  • Third Week:
    • The neural plate develops as a thickened dorsal midline of ectoderm over the notochord, creating neural folds alongside a neural groove.
  • Neural Tube Formation:
    • Formed through the apposition and fusion of neural folds, sealing the groove to form a tubular structure.

Primary and Secondary Brain Vesicles

  • Fifth Week:
    • Formation of three primary brain vesicles:
    • Forebrain (Prosencephalon)
    • Midbrain (Mesencephalon)
    • Hindbrain (Rhombencephalon)
  • Seventh Week:
    • Expansion leads to five secondary brain vesicles:
    • Telencephalon
    • Diencephalon
    • Mesencephalon
    • Metencephalon
    • Myelencephalon

Structural Development of the Brain

  • Telencephalon: Largest brain vesicle that forms the cerebral hemispheres.
  • Diencephalon: Consists mainly of the thalamus, containing multiple neuronal groups with connections to the cerebral cortex.
  • Rhombencephalon: Differentiates into:
    • Metencephalon (which becomes the pons and cerebellum)
    • Myelencephalon (which forms the medulla oblongata).

Differentiation of Brain Centers

  • Primary sensory brain centers emerge from the tubular brain structure as expansions.
  • The centers are organized into outer cortex of nerve cells and underlying nerve fiber cores.
  • Visual and auditory centers develop automatic reflex functions, represented by superior and inferior colliculi. Motor centers are situated in the cerebellum.

Prosencephalisation

  • Characterized by expansive development of cerebral hemispheres from the rostral brain, housing higher cognitive, sensory, and motor control functions.

Developmental Overview of CNS

  • Neural tube differentiation leads to the establishment of primary and secondary brain vesicles, from which adult structures develop, including diverse brain areas such as:
    • Olfactory lobes, hippocampus, cerebrum, retina, and various thalamic structures (epithalamus, thalamus, hypothalamus).
  • Neural Tube Segmentation:
    • Hox genes influence the differentiation of rostral ends into cranial structures and regional specificity.

Spinal Cord Development

  • Developed from the neural tube, structured with varying zones (ventricular, intermediate, marginal) based on neuron type.
  • Neurogenesis: Neurons develop from neuroepithelial cells forming grey and white matter with sensory and motor neuron differentiation.
  • The formation of spinal nerves and myelination occurs, with Schwann cells and oligodendrocytes playing significant roles in nerve covering.

Developmental Malformations

  • Common malformations associated with CNS developmental processes include:
    • Anencephaly: Incomplete brain and skull development incompatible with life.
    • Spina Bifida: Underdevelopment of the spinal cord, leading to varying degrees of paralysis and deficits.
    • Meningomyelocele: A form of spina bifida where the spinal cord and meninges protrude abnormally into a cyst.
  • Embryonic alterations leading to congenital anomalies:
    • Various malformations can arise due to disruptions in normal embryological processes affecting the neural tube and associated systems.

Conclusion

  • The CNS development process is complex and involves myriad interactions and differentiations beginning in embryogenesis and continuing through notable stages that lead to mature functional structures essential for life.

Thank you for your attention! \n "Mr. Osborne, may I be excused? My brain is full."