Development of the Nervous System

DEVELOPMENT OF THE NERVOUS SYSTEM

• The nervous system development involves mechanisms for both the Central Nervous System (CNS) and Peripheral Nervous System (PNS).
• The brain is the first organ to develop; defects in the CNS can lead to irreversible damage.
• Neurons do not regenerate following injury or damage.
• CNS development begins around week 3 post conception, marked by gestational age based on the last menstrual period.
• Neural tube formation comprises an anterior tube (leading to the brain) and a posterior tube, with anterior closure occurring at approximately 23-25 days.
• Failure of anterior neural tube closure can result in anencephaly, a serious congenital malformation.
• Posterior neuropore closure occurs by 25-28 days; issues caused by maternal illnesses or medications can lead to defects like meningocele or tethered cord syndrome.

DEVELOPMENTAL STAGES OF THE NERVOUS SYSTEM

Embryogenesis

• Occurs post-fertilization when the sperm and egg unite to form a zygote, which undergoes cleavage resulting in morula and blastocyst formation.
• The embryoblast differentiates into hypoblast and epiblast layers, with the epiblast giving rise to the ectoderm, the precursor of the nervous system.
• Week 1: Trophoblast (including cytotrophoblast and syncytiotrophoblast) attaches to the endometrium.

Gastrulation

• Bilaminar embryonic disc reorganizes into a trilaminar disc, forming three germ layers: ectoderm, mesoderm, and endoderm during the third week of development.

Neurulation

• Begins with the formation of the primitive streak in the epiblast, crucial for the subsequent development of the CNS.
• The neural plate forms and then folds to create the neural groove, eventually closing to form the neural tube.
• The process includes the engagement of the amniotic cavity and communication until its closure separates the neural crest cells, which later develop into the PNS.
• The notochord degenerates into the nucleus pulposus within intervertebral discs, while surrounding mesoderm cells form somites, precursors of skeletal muscles.

FORMATION OF CENTRAL NERVOUS SYSTEM

Primary and Secondary Vesicles

• CNS development progresses through the formation of primary and secondary brain vesicles.

Formation of Flexures

• Notable curvature or flexures in the neural tube contribute to the architecture of the developing brain.

NEURAL DEVELOPMENT

• Involves six critical cellular processes:
  • Neurogenesis: formation of neurons from neural stem cells.
  • Cell migration: movement of newly formed neurons to their destined locations.
  • Cell differentiation: maturation of cells into specialized neuron types.
  • Synaptogenesis: formation of synaptic connections between neurons.
  • Neuronal cell death (apoptosis): programmed death of excess neurons to refine neural circuits.
  • The sixth process is unspecified but equally important in neural network formation.

NEURULATION TIMELINE

• Day 17-18: Neural plate induction begins.
• Day 19-20: Neural plate elongation occurs.
• Day 20-21: Formation of neural folds and groove is observed.
• Day 21-22: Neural folds start to fuse.
• Day 22: Midbrain flexure forms.
• Day 25: Closure of cranial neuropore takes place.
• Day 27: Closure of caudal neuropore is achieved.
• Week 4-8: Secondary neurulation further develops the neural structures.

This quiz covers the detailed stages involved in the development of the nervous system, including embryogenesis and the Carnegie stages. It explores concepts from neurulation to the formation of the central nervous system. Ideal for students seeking to understand the intricacies of neurodevelopment.