Abnormal neurulation and holoprosencephaly
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Abnormal neurulation and holoprosencephaly
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[post_date] => 2024-12-23 18:04:12
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[post_content] => Practice Passage (Question 1-6)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
In the early stages of human embryological development, the neural tube begins a process called ventral induction. During ventral induction, the neural tube divides into three primary brain vesicles, or subdivisions, which later differentiate into five vesicles, as shown in Figure 1.
Figure 1 Embryological development of the brain vesicles; attribution: Tauiris, CC ASA 3.0
The division of the prosencephalon into the telencephalon and diencephalon occurs between the 18th and 28th day of gestation. These two parts then undergo a cleavage process, during which the neural tissue divides and forms distinct regions within the telencephalon and diencephalon. In addition to cleavage, there is an inward rotation of the developing brain structures. These processes are shown in Figure 2.
Figure 2 Normal ventral induction process showing brain formation around ventricles
Cleavage and rotation are crucial for the proper alignment and organization of the neural components. Under normal conditions, they result in a telencephalon and diencephalon each dividing into two halves, with no missing components. However, incomplete cleavage and failure of rotation of the prosencephalon can result in a brain malformation known as holoprosencephaly. Different forms of holoprosencephaly are shown in Figure 3.
Figure 3 Development of holoprosencephaly with a single ventricle
In the most severe form, alobar holoprosencephaly, the telencephalon and diencephalon are fused and a single ventricle is formed. Individuals with the alobar form experience severe mental retardation, seizures, and facial malformations of the nose, lips and eyes, or, in extreme cases, cyclopia (fused eyes); in many cases alobar holoprosencephaly is fatal.
The other two forms result in limited clinical deficits, such as cognitive delay and seizures that can be treated; life expectancy in these individuals is not reduced. In semilobar holoprosencephaly, the diencephalon remains fused but the telencephalon presents a fissure posteriorly. In lobar holoprosencephaly the diencephalon may or may not be fused, but the telencephalon presents a nearly normal fissure.
[post_title] => Abnormal neurulation and holoprosencephaly
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[question] => Holoprosencephaly can result from defects in signaling from what structure that directs the patterning and differentiation of the neural tube?
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[answer] => 1
[description] => Reason for Correct Answer:
The structure referred to in the question is a temporary structure that sits below the developing neural tube and directs the formation of the neural tube.
This structure is made of mesodermal tissue.
This structure induces the neural plate to form the neural tube.
The structure is called the notochord. The notochord forms along the dorsal midline of the embryo and acts as a signaling center, secreting molecules like Sonic Hedgehog protein (Shh) that play a crucial role in inducing and patterning adjacent tissues, especially the neural plate. Through its signaling activities, the notochord guides the development of the central nervous system and provides structural support to the developing embryo, contributing significantly to proper vertebrate development. See image here:
https://upload.wikimedia.org/wikipedia/commons/0/0f/2912_Neurulation-02.jpg
In cases of holoprosencephaly, disruptions in the Shh signaling pathway or mutations in genes associated with the pathway can lead to improper division and differentiation of the prosencephalon. This failure in proper signaling can result in the fusion of brain structures, characteristic of holoprosencephaly.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Neural crest cells
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[1] => Array
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[each_answer] => B. Neural plate
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[2] => Array
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[each_answer] => C. Notochord
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[3] => Array
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[each_answer] => D. Endoderm
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[1] => Array
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[quiz_unique_key] => 1403770772
[question] => Which brain structures will be deformed in alobar holoprosencephaly?
[value] => Array
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[answer] => 2
[description] => Reason for Correct Answer:
The passage describes holoprosencephaly as a condition caused by impaired division of the prosencephalon into the diencephalon and telencephalon.
The prosencephalon forms the “forebrain,” a general descriptor of the anterior part of the brain during its early development.
https://en.m.wikipedia.org/wiki/File:Four_week_embryo_brain.svg
This region will give rise to the cerebral cortex/cerebral hemispheres, which are responsible for advanced functions, such as thinking, perceiving, and producing and understanding language.
This region also includes structures like the thalamus, which acts as a relay station for sensory information, and the hypothalamus, which plays a vital role in regulating basic biological needs like hunger, thirst, and body temperature. The hypothalamus is also crucial in controlling the endocrine system through its influence on the pituitary gland.
Note that the medulla oblongata and pons are structures in the brainstem, which is the lower part of the brain that connects to the spinal cord. These structures develop from the rhombencephalon, also known as the hindbrain.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Cerebral hemispheres and medulla oblongata
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[1] => Array
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[each_answer] => B. Cerebral hemispheres and thalamus
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[2] => Array
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[each_answer] => C. Medulla oblongata and pons
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[3] => Array
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[each_answer] => D. Medulla oblongata and thalamus
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[2] => Array
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[quiz_unique_key] => 1403770772
[question] => Which embryonic event immediately precedes neural tube development and the processes shown in Figure 2?
[value] => Array
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[answer] => 1
[description] => Reason for Correct Answer:
Neural tube formation (neurulation) occurs simultaneously with somitogenesis. Somitogenesis is the process by which somites, segmented blocks of mesoderm, form along the developing vertebrate embryo’s dorsal axis. These somites are the precursors to important structures in the body, including muscles, bones, and dermis.
Organogenesis occurs after neurulation.
Blastula formation occurs prior to gastrulation.
Gastrulation is the process that precedes neurulation. Gastrulation is a key event in early embryonic development that occurs after fertilization and precedes the formation of the neural tube. During gastrulation, the single-layered blastula undergoes complex movements and transforms into a three-layered structure called the gastrula. This process sets the stage for the formation of various organ systems, including the neural tube, which eventually gives rise to the central nervous system.
Here is the formation of the gastrula from the blastula (gastrulation):
https://commons.wikimedia.org/wiki/File:Blastula.png
)
[answers] => Array
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[0] => Array
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[each_answer] => A. Gastrulation
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[1] => Array
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[each_answer] => B. Blastulation
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[2] => Array
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[each_answer] => C. Organogenesis
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[3] => Array
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[each_answer] => D. Somitogenesis
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[quiz_unique_key] => 1403770772
[question] => A seizure that is initiated in one cerebral hemisphere may spread to the opposite side via bridges between the two hemispheres. Which structures are most likely to act as a bridge to spread a seizure in lobar holoprosencephaly?
[value] => Array
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[answer] => 2
[description] => Reason for Correct Answer:
The corpus callosum is a normal bridge between the two hemispheres.
Figure 3 shows that the corpus callosum is intact in lobar holoprosencephaly. Seizures could therefore spread via the corpus callosum.
Figure 3 also shows that the anterior portion of the brain fails to separate normally in lobar holoprosencephaly.
The frontal lobe is anterior portion of the brain. Therefore, in lobar holoprosencephaly, a seizure may spread via the frontal lobe and corpus callosum.
)
[answers] => Array
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[0] => Array
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[each_answer] => A. Temporal lobe and medulla oblongata
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[1] => Array
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[each_answer] => B. Frontal lobe and corpus callosum
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[2] => Array
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[each_answer] => C. Parietal lobe and medulla oblongata
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[3] => Array
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[each_answer] => D. Occipital lobe and corpus callosum
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[4] => Array
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[quiz_unique_key] => 1403770772
[question] => In addition to the role of signaling molecules, molecules like N-cadherin and integrins play crucial roles in the development of the central nervous system, including the prosencephalon. Which of the following is LEAST likely to be a role of N-cadherin and/or integrins?
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[answer] => 4
[description] => Reason for Correct Answer:
N-cadherin is a calcium-dependent cell–cell adhesion molecule that is crucial for the maintenance of tissue integrity; it does this by mediating strong adhesive interactions between adjacent cells in various tissues, including the nervous system.
Integrins, on the other hand, are cell-surface receptors that enable cells to adhere to and interact with the extracellular matrix (ECM). Integrins play pivotal roles in cell adhesion, migration, and signal transduction.
These molecules would thus play a vital role in processes such as neural cell migration, guiding neurons along specific pathways during development, and facilitating the formation and stability of synaptic connections in the mature brain. Integrins also contribute to synaptic plasticity by modulating the formation and remodeling of synapses in response to neural activity.
While N-cadherin and integrins are important for cell adhesion, migration, and synaptogenesis, their primary roles are not directly related to the complex process of spatial patterning and differentiation along the anterior-posterior axis, which is primarily orchestrated by signaling molecules and transcription factors such as Hox genes.
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[answers] => Array
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[0] => Array
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[each_answer] => A. Helping neural cells stick to one another, thus facilitating the formation and maintenance of tissue structures like the neural tube
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[1] => Array
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[each_answer] => B. Mediating interactions between migrating neurons and the extracellular matrix and thus helping to guide neuron movement
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[2] => Array
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[each_answer] => C. Mediating synaptogenesis by aiding the formation of synaptic contacts between neurons and target cells
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[3] => Array
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[each_answer] => D. Regulating spatial patterning and differentiation of neurons along the anterior-posterior axis, ensuring proper segmental identity and organization
)
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[5] => Array
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[quiz_unique_key] => 1325138223
[question] => Sonic Hedgehog protein (Shh) is a secreted molecule that plays a crucial role in embryonic development. Shh binds to a cell-surface protein called Ptch1; this binding activates an adjacent protein called Smo, which activates an intracellular signaling pathway. Based on this information, Smo is most likely a:
[value] => Array
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[answer] => 3
[description] => Reason for Correct Answer:
The question stem suggests that a cell surface receptor, Ptch1, activates Smo, which leads to activation of an intracellular pathway.
A secreted protein or extracellular protein would not help in relaying information from the cell’s exterior to its interior.
A transcription factor would move to the nucleus and alter gene expression; it does not directly activate an intracellular pathway.
G protein-coupled receptors are proteins that span the membrane and allow signals to pass across the membrane; they are a common type of receptor in signaling pathways. They can also be activated by other receptors adjacent in the membrane. This is the most likely identity of Smo out of the options given.
)
[answers] => Array
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[0] => Array
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[each_answer] => A. secreted glycoprotein.
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[1] => Array
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[each_answer] => B. transcription factor.
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[2] => Array
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[each_answer] => C. G protein-coupled receptor.
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[3] => Array
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[each_answer] => D. component of the extracellular matrix.
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Figure 1
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