12.2 Applications and skills

12.2.1 Models and applications of neural development

  • The external environment influences the development of neurons, in an embryo and in the final years of life.
  • Scientists use model animal systems to learn about general principles in developmental neuroscience.

Application: Spina bifida

  • Spina bifida is a congenital disorder caused by the incomplete closing of the neural tube during fetal development, resulting in incomplete fusion of the vertebral column.

12.2.1a

Figure 12.2.1a – Spina bifida in a human embryo

  • In mild cases, the condition may present simply as a dimple on the lower back. This child is unlikely to show any symptoms as he or she grows.
  • In most cases, the child is born with a large cyst containing a portion of the spinal cord that has protruded from between the unfused vertebrae.
  • The cyst can be treated surgically, but the neurological effects of spina bifida – seizures, orthopedic problems and bowel problems – cannot be cured.
  • It is one of the most common congenital birth defects in the world, affecting 0.1% of live births.

Application: Neuroplasticity and stroke

  • An ischemic stroke occurs when the supply of oxygen to a part of the brain is disrupted. Without oxygen for cellular respiration, neurons die.
  • After a stroke, a patient may suffer from partial paralysis – an indication that irreparable nerve damage has occurred.
  • Depending on the severity of the damage, neuroplasticity allows for partial recovery from injuries to neurons:
    • Neurogenesis: stem cells in the hippocampus are cued by chemical signals to migrate to the damaged site in order to develop into new neurons.
    • Synaptic modification: existing neurons develop new neural pathways in order to improve at least some of the function lost as a result of cell death.

Nature of Science: Model systems in biological research

  • Much of what biologists know about human neural development was learned from studying animals as models.
  • The choice of model organism depends on the field of study. For example, the African clawed frog, Xenopus laevis, is commonly used to study morphological changes in embryology because it has very large embryos with a transparent yolk that can be easily observed and manipulated.
  • Suitability of other animal models used commonly in neurological research are summarised in Figure 12.2.1b.

neurological research

Figure 12.2.1b – Suitability of animal models for neurological research

Skill: Annotation of embryonic tissues in Xenopus during neurulation

Figure 12.2.1c – Stages of neurulation in XenopusFigure 12.2.1c – Stages of neurulation in Xenopus 

Neural plate

• Differentiated cells of the ectoderm that will form the neural tube

Notochord

• Cells derived from the mesoderm

• Defines the axis of the embryo

• Plays a role in chemical signalling throughout embryogenesis

• Vertebrates only: develops into the bones of the spine

Archenteron

• Cavity that will eventually become the gut

Neural fold

• Formed by the invagination of the neural plate

Neural tube

• Formed by closing of neural fold

• Precursor to the central nervous system (brain and spinal cord)

Ectoderm, mesoderm and endoderm

• Layers of germ cells formed during gastrulation of the blastula

• Each layer will develop into specific body parts

12.2.1d

Figure 12.2.1d – Spina bifida scar tissue
This child has a scar from surgery to remove a cyst caused by spina bifida.

Food for thought

During pregnancy, increased intake of vitamin B9, also known as folic acid, lowers the incidence of fetal neural tube defects including spina bifida.

12.2.1eFigure 12.2.1e – Symptom of stroke
Facial paralysis and slurred speech result from neuron damage caused by an ischemic stroke.

Discussion

Which animals would be the most suitable for testing therapies using stem cells? What is the likeliest reason that chicken embryos are not used more frequently as model species?

12.2.1fFigure 12.2.1f – Xenopus laevis embryos
Embryos of the African clawed frog, Xenopus laevis, at the neurula stage

Language help

  • Spina bifida (Latin) means ‘split spine’.
  • Congenital means the disorder is present at birth. A congenital disorder may be caused by epigenetic factors, infection during pregnancy, or chromosomal abnormalities, but they are not genetically inherited.
  • A blastula is the earliest form of the embryo. It is a sphere of undifferentiated cells surrounding a fluid-filled cavity. In mammals, it is called a blastocyst.

12.2.1gFigure 12.2.1g

TOK

Cultural experiences, including the acquisition of language, result in neural pruning. Does that mean that people who speak a different language think differently? How does language affect thought?

Course link

Blood clots and aneurysms are causes of stroke. Review the process of blood clot formation in 6.1.3.