Mechanisms of malformations (Teratogenesis)
Normal formation of the nervous system requires strict adherence to the genetic program. However, the execution of this program critically depends on constant interaction with both external and internal environmental factors of the organism.
Any significant disruption or imbalance in this fragile interaction leads to dysembriogenesis — the formation of structural anomalies. As a result, specific malformations develop, affecting organ volume, cortical relief formation, the cleavage of cerebral vesicles, and the formation of connecting pathways.
General volume and pressure disorders
One of the most well-known general disorders is hydrocephalus. This pathology is characterized by a pronounced change in the physical parameters of the cranial vault and intracranial spaces.
Key morphological and clinical signs of hydrocephalus:
- Elevated intracranial pressure: occurs due to the accumulation of excess fluid.
- Increased brain volume: the organ is physically stretched from the inside.
- Enlargement of the ventricular system: the internal cavities of the brain (ventricles) are pathologically enlarged.
- Enlargement of the skull: this sign is particularly well visualized on diagnostic images and scans.
Anomalies in cortical relief formation
Normally, the cerebral cortex has a complex folded pattern. When neuronal migration and folding are disrupted, relief pathologies (sulci and gyri) develop. There are three main types of such disorders:
- Agyria (literally — smooth brain). Characterized by the complete, absolute absence of sulci and gyri of the cerebral hemispheres. The cortical surface remains completely smooth.
- Lissencephaly. Manifests as pronounced smoothing of sulci and gyri. The pattern of the cerebral cortex is significantly simplified, although some indistinct folds may persist.
- Microgyria. With this anomaly, the number of sulci and gyri is extremely small, and those that have formed remain superficial and shallow.
Disorders of telencephalon cleavage
During embryogenesis, the telencephalon must divide into two full-fledged hemispheres. Failures at this stage are called prosencephalies. Depending on the severity of the defect, several variants are distinguished:
- Holoprosencephaly: the most severe variant, in which the telencephalon does not divide into hemispheres at all. The organ looks like a single hemisphere with a single common cavity (common ventricle).
- Alobar prosencephaly: partial cleavage occurs only in the posterior part of the brain. The frontal lobes remain fused and completely undivided.
- Prosencephaly: the telencephalon externally divides with a longitudinal fissure, however, this division is incomplete. Deep within the structure, both hemispheres remain densely connected to each other.
Commissural development disorders
For the nervous system to function normally, the hemispheres must exchange information via special nerve bundles (commissures), the main one being the corpus callosum.
The pathology known as agenesis of the corpus callosum is the complete absence of this crucial structure physically connecting the right and left hemispheres. This leads to the isolation of processes occurring in the two halves of the cortex.
Briefly: answers to topical questions
Which brain structures suffer in case of impaired neuronal migration?
When neuronal migration is impaired, islands of gray matter may remain near the ventricles and within the white matter tracts, failing to reach the cortex. Migration disorders are also associated with anomalies in cortical layering.
Which genetic syndromes cause lissencephaly?
Lissencephaly is listed among brain malformations in triploidy syndrome with a 69, XXY karyotype in males and 69, XXX in females. Band heterotopia is considered by some authors as a mild form of lissencephaly and is associated with mutations in several genes.
What are the methods for diagnosing fetal brain malformations?
The main methods for prenatal diagnosis of fetal brain malformations are ultrasound examination and amniocentesis. Ultrasound is performed at 11–14 weeks and 18–21 weeks to visualize gross CNS malformations and echo-markers of pathology. Amniocentesis is a method of prenatal diagnosis of chromosomal and biochemical disorders by puncturing the amniotic cavity followed by analysis of fetal cells.
To remember the degrees of cortical relief impairment, use the acronym ALM (from severe to mild): Agyria (Absolutely smooth) — Lissencephaly (Light smoothing) — Microgyria (Micro-gyri).
Class and exam questions
What is the primary cause of brain dysmorphogenesis?
The primary cause is the disruption of the proper interaction between the organism's genetic developmental program and environmental and internal factors.
How to distinguish hydrocephalus from other pathologies on scans?
Images in hydrocephalus clearly show enlargement of the ventricular system of the brain, as well as an overall increase in skull size due to elevated pressure.
How does holoprosencephaly differ from alobar prosencephaly?
In holoprosencephaly, the brain is not divided at all and appears as a single hemisphere with a common ventricle. In the alobar form, the posterior part of the brain is divided, but the frontal lobes remain fused.
What is agyria?
Agyria is a malformation in which the surface of the cerebral hemispheres completely lacks sulci and gyri, making the brain entirely smooth.
What else to study on this topic
- Morphological picture of the ventricular system in hydrocephalus
- Degrees of cortical smoothing: lissencephaly vs. agyria
- Anatomy of the common ventricle in holoprosencephaly
- Formation of the longitudinal fissure in various types of prosencephalies
- The connection of the corpus callosum with interhemispheric interaction
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