Embryology Chapter 5 Quiz

Embryological Development and Defects

• Neuroembryology: A crash course focusing on the development of the nervous system in embryos.
• Learning Objectives: Students will define the notochord and describe its role in embryonic development and in the adult. They will also learn the process of neurulation and neural tube closure. Additionally, they will be able to describe the consequences of incomplete neural tube closure and identify primary and secondary vesicles, noting their adult structures. The significance of the cephalic flexure impacting directional terms in the nervous system will also be addressed.

Very Early Embryology

• Germ Layers: The gastrulation process establishes three primary germ layers:
  • Endoderm: forms internal organs (digestive, urinary, and respiratory tracts)
  • Mesoderm: forms musculoskeletal and circulatory systems
  • Ectoderm: develops into integumentary and nervous systems
• Notochord: A collection of cells, initially part of the primitive streak, developing into a cylinder-shaped structure which is crucial for initiating neurulation.

Major Structures

• Notochord: A solid cylinder of tissue that secretes factors guiding brain and spinal cord development.
• Neural Plate: A thickening in the ectoderm that folds inwards, ultimately becoming the neural tube and neural crest.
• Somites: Segmented mesoderm secreting factors that organize body development (rostrocaudally).

Neurulation

• The initial stages of the process involve formation of a neural plate.
• Sequential stages of neurulation are shown as diagrams (1-6).
• The diagrams show the folding and closure of the neural plate to form the neural tube.
• The process of folding and closure is further discussed in later diagrams.
• Neural crest cells migrate away from the ectoderm.
• The neural crest cells fuse together in a process called convergence.

Neuropore Defects

• Failure of closure: Incomplete closure of the neuropores (specifically the caudal neuropore) during the process of neurulation can lead to congenital defects.
• Spina Bifida: An example of a caudal neuropore defect leading to various degrees of severity, ranging from no deficit to paralysis. Severity depends on the degree of neural tissue outside the vertebral cavity.
  • Forms: Normal, Occulta, Meningocele, Myelomeningocele
• Anencephaly / Craniorachischisis : Defects related to anterior and/or complete neural tube failure.

Brain Development

• Vesicle Formation: The neural tube begins to dilate into three primary vesicles: prosencephalon, mesencephalon, and rhombencephalon.
  • The anterior neuropore closes, initiating this dilation.
• Secondary Vesicles: The primary vesicles further differentiate into secondary vesicles: telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon.
  • Each of these give rise to specific adult brain structures. Example: telencephalon to cerebral hemispheres

Flexure Formation

• Cephalic flexure: In the fifth week of gestation, a bend in the developing brain (cephalic flexure) occurs.
• This bend influences directional terms (rostral/caudal, dorsal/ventral) for the CNS.
• A second bend in the developing brain (cervical flexure) also occurs during this period.

Continued Growth and Adult Structure

• Brain Growth: Cerebral hemispheres develop and expand posterolaterally to encompass the diencephalon and rostral brain stem.
• Adult Brain Structures: Labels on diagrams identify components of the mature brain. Examples: Cerebral hemispheres, Diencephalon, Cerebellum, Midbrain, Pons, Medulla oblongata

Directional Terms and Clinical Significance

• Directional Variation: Explains how the cephalic and cervical flexures impact the application of directional terms (anterior/posterior, dorsal/ventral, superior/inferior) when discussing the CNS.
• Coronal, horizontal, mid-sagittal sections: Demonstrates various methods for viewing the brain in cross-sections.
• Clinical Significance: Summarizes and explains the need to understand embryological development to comprehend related neurological conditions and terms.

Additional Questions/Cases

• Caudal neuropore closure: Failure of caudal neuropore closure leads to Spina Bifida.
• Notochord descendants: The notochord eventually forms part of the intervertebral discs.
• Neural crest cells: Migrate to form important structures in the peripheral nervous system.