Difference between revisions of "2014 Group Project 7"

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# Passive cell displacement: Moving of cells in this way does not require active locomotor activity. In some parts of the developing CNS, the postmitotic neurons that only move a very short distance from the border of proliferative zone are displaced outward by newly produced cells (figure). This results in an “outside-to-inside” spatiotemporal gradient that the earliest generated neurons are located farthest away from the proliferative zone, where the youngest generated ones are located closest to the zone. This pattern can be found in the thalamus, hypothalamus, spinal cord, retina, dentate gyrus of the hippocampal formation and many regions of the brainstem.   
 
# Passive cell displacement: Moving of cells in this way does not require active locomotor activity. In some parts of the developing CNS, the postmitotic neurons that only move a very short distance from the border of proliferative zone are displaced outward by newly produced cells (figure). This results in an “outside-to-inside” spatiotemporal gradient that the earliest generated neurons are located farthest away from the proliferative zone, where the youngest generated ones are located closest to the zone. This pattern can be found in the thalamus, hypothalamus, spinal cord, retina, dentate gyrus of the hippocampal formation and many regions of the brainstem.   
 
# Active migration: This requires the active participation of the moving cell for its displacement which neurons move at a greater distance and the migrating young neuron bypasses the previously generated cell (figure). This results in an “inside-to-outside” spatiotemporal gradient. This pattern can be found in well-laminated structure including cerebral cortex and several subcortical areas.
 
# Active migration: This requires the active participation of the moving cell for its displacement which neurons move at a greater distance and the migrating young neuron bypasses the previously generated cell (figure). This results in an “inside-to-outside” spatiotemporal gradient. This pattern can be found in well-laminated structure including cerebral cortex and several subcortical areas.
[[Image:Internurons migration in cerebral cortex.jpg|frame|200x200px|Image of interneurons migration and interactions with radial glia in the developing cerebral cortex]]
 
  
  
  
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Revision as of 21:26, 22 October 2014

2014 Student Projects
2014 Student Projects: Group 1 | Group 2 | Group 3 | Group 4 | Group 5 | Group 6 | Group 7 | Group 8
The Group assessment for 2014 will be an online project on Fetal Development of a specific System.

This page is an undergraduate science embryology student and may contain inaccuracies in either description or acknowledgements.

Neural - CNS

Introduction

The Central Nervous System (CNS) is a complex network of neurons which are responsible for the sending, recieving and integration of information from all parts of the body, serving as the processing center of the bodies nervous system. The CNS controls all bodily functions (sensory and motor), consisting of 2 main organs; The Brain and Spinal Cord

Brain

The Brain is the body's control center consisting of 3 main components; Forebrain, Midbrain and Hindbrain. The forebrain functions in receiving and processing sensory information, thinking, perception, and control of motor functions as well as containing essential structures; Hypothalamus and Thalamus, which are responsible in motor control, autonomic function control and the relaying of sensory information. The Midbrain along with the Hindbrain together form the brain-stem and are both important in auditory and visual responses

Spinal Cord

The Spinal Cord is a cylindrical shaped structure composes of nerve fiber bundles which is connected to the brain via the brain-stalk formed from the Midbrain and Hindbrain, running through the spinal canal in the vertebrae (in animals) from the neck to the lower back. The spinal cord plays the important role of transmitting information from bodily organs and external stimuli to the brain and acts as a channel to send important signals to other parts of the body. The nerve bundles in the Spinal cord are divided into

1) Ascending bundles - Transmits Sensory information from the body to the brain

2) Descending bundle - Transmits Motor function information from the brain to the body

Before fetal period, nerulation occurs that ectoderm forms initial structure of the CNS and folds upon itself to form neural tube towards the end of week 3. The head portion becomes the brain, which further differentiates into forebrain, midbrain and hindbrain, while the middle portion becomes the brain stem. Around week 5, neural tube differentiates into the proencephalon (forebrain), the mesencephalon (midbrain) and the rhombencephalon (hindbrain). By week 7, the prosencephalon divides into the telencephalon and the diencephalon, while the rhombencephalon divides into the metencephalon and the myelencephalon. The formation of these 2 additional structures creates 5 primary units that will become the mature brain.

In this website, CNS development during the fetal period, the current research models and finding, historic findings and abnormalities that can occur in the fetal period is identified.

Development during fetal period

Neural-development.jpg

Timeline of human neural development [1]

Cellular process

In developing CNS, there are 4 major cellular processes, including cell proliferation, cell migration, cell differentiation and cell death. They are a cascade of events that the earlier occurring process may influence the subsequently occurring ones, but a late-occurring event cannot influence the earlier ones.

1. cell proliferation

  • This process is responsible for the formation of neurons and glia.
  • Begins around 40th embryonic day and is almost complete around the 6th month of gestation [2]
  • Location: occurs in germinal matrix that comprised of ventricular and subventricular proliferative zones of cells.
  1. Ventricular zone: This is the proliferative zone that appears first which is a pseudostratified columnar epithelium [3]. In some part if the developing CNS, this is the only proliferative zone and therefore it is assume that ventricular zone produces all of the cell types. For example, in the hippocampus, all of the neurons of the major subdivisions (areas CA1, CA2 and CA3) are derived from the ventricular zone. There is substantial movement of the nuclei as they move through the cell cycle [4]. The nuclei move between the ventricular surface and the border of the ventricular zone with the subventricular zone [5].
  2. Subventricular zone: This is the second proliferative zone that appears in some parts of the developing CNS. Most of the glia for most of the brain are produced in this zone, therefore it is important to the adult brain [6]. In some parts of the brain, there is the production of a significant number of neurons in this zone [7]. For example, the subventricular zone contributes large number of cells to the neocortex, which is the youngest structure in the brain [8]. In contrast to ventricular zone, the proliferating cells do not move through the cell cycle.

Somatosensory cortex of E20 rat.jpeg

Image of coronal sections of somatosensory cortex of E20 rat showing boundaries between the ventricular zone (VZ), inner subventricular zone (iSVZ) and outer SVZ (oSVZ) through staining [9]

2. cell migration

  • This is the process that influences the final cell position by migrating the cells produced from the two ventricular zones.
  • The postmitotic young neurons migrate from the proliferation site to their ultimate position in two different ways.
    Passive cell displacement and outside-to-inside spatiotemporal gradient
    Active cell migration and inside-to-outside spatiotemporal gradient
  1. Passive cell displacement: Moving of cells in this way does not require active locomotor activity. In some parts of the developing CNS, the postmitotic neurons that only move a very short distance from the border of proliferative zone are displaced outward by newly produced cells (figure). This results in an “outside-to-inside” spatiotemporal gradient that the earliest generated neurons are located farthest away from the proliferative zone, where the youngest generated ones are located closest to the zone. This pattern can be found in the thalamus, hypothalamus, spinal cord, retina, dentate gyrus of the hippocampal formation and many regions of the brainstem.
  2. Active migration: This requires the active participation of the moving cell for its displacement which neurons move at a greater distance and the migrating young neuron bypasses the previously generated cell (figure). This results in an “inside-to-outside” spatiotemporal gradient. This pattern can be found in well-laminated structure including cerebral cortex and several subcortical areas.


Image of interneurons migration and interactions with radial glia in the developing cerebral cortex



3. cell differentiation

  • This is the process begins after the migration of neuronal and glial cells to the final positions and it is responsible for the generation of a wide variety of cells in the adult CNS. During the differentiation, each neuron grows out its axon and dendrites.
  • Time: starts about the 25th month of gestation until adolescence
  • The axons do not grow directly to their final targets, but they transiently innervate areas and cells in two ways that the connections cannot be found in adult brain. These two types of transient connections are not mutually exclusive and can be found within a single population of cells.
  1. Divergent transient connections: one neuron innervates more cells than normal which will be eliminated by the reduction of projection area.
  2. Convergent transient connections: several neurons innervate one target neuron where only one of these neuronal connections is found in adult.


4. cell death (apoptosis)

  • This is the process where elimination of transient connections occurs and two mechanisms, axonal retraction and neuronal pruning [10], are involved.
  1. Axonal retraction: The transient connections are removed by the recession of the collaterals of the neuron’s axon or by the shrinking of the terminal arborisation of the axon.
  2. Neuronal pruning: The transient connections are removed through a selective cell death that neurons die due to the failing to establish projections.
  • Critical for appropriate brain development


Brain Development

- The human brain development begins in the 3rd gestational week with the start marked by differentiation of the neural progenitor cells

- By the end of the embryonic period in gestation week 9 (GW9), the basic structures of the brain and CNS are established as well as the major parts of the Central and Peripheral nervous systems being defined [11]

- The early fetal period (mid-gestation) is a critical period in the development of the neocortex, as well as the formation of vital cortical neurons which are vital in the brain processing information

During Fetal Period

- Extends from the ninth gestational weeks through to the end of gestation

- Gross morphology of the developing brain undergoes striking change during this time, beginning as a smooth structure and gradually developing the characteristic mature pattern of gyral and sulcal folding

- Brain development begins rostral in GW8, proceeding caudally until it is complete at GW22

- Secondary sulci emerge between GW30-35

- Formation of Tertiary sulci begins during GW36 and into the postnatal period

- Different population of neurons form grey matter structures in many regions of the brain including hindbrain and spinal column, cerebellum, midbrain structure and the neocortex

- Neurons, after production, migrate away from the proliferative regions of the VZ, the neurons that will form the neocortex migrate in an orderly fashion forming the 6 layered neocortical mantle

- The major fibre pathways make up the brain white matter

- AS development proceeds, the brain becomes larger and the primary mode of neuronal migration from the VZ changes.


Increase in size and weight

Brain ventricles and ganglia development 03.jpg

Image of brain and ventricular development [12]

Brain fissure development 02.jpg

Image of brain fissure development [12]

sulcation and gyration

Sulcation: development of sulci. Primary sulci appear as shallow grooves on the surface of the brain and become more deeply infolded. Secondary sulci are formed from the development of side branches of primary sulci [13]

Gyration: development of gyrus that occurs late during fetal development until end of the pregnancy or even later after birth [13]


Weeks Visible anatomical details
20-21 smooth, "lissencephalic" brain, wide Sylvian fissures
22-23 corpus callosum; beginning of calcarine and hippocampal fissures and of callosal sulci
24-25 start of opercularization of Sylvian fissures; calcarine fissures and cingular sulci
26 central and collateral sulci
27 marginal and precentral sulci
28 postcentral and intraparietal sulci
29 inferior frontal sulci; bright white matter, dark cortical ribbon
30-31 narrower ventricular system and subarachnoid spaces
32 superior and inferior temporal sulci
33 external occipitotemporal sulci
34-35 close to final shape of gyration
36-37 completed opercularization of Sylvian fissures; narrow pericerebral fluid spaces; dark subcortical fibres and corona radiata
38-40 dark posterior limbs of internal capsules

table obtained from <pubmed>20608424</pubmed> [14]

Spinal Cord Development

The spinal cord is formed from parts of the neural tube during embryonic and fetal development

Meninges Development

Historical Research and Findings

Historical knowledge, predating when modern research techniques were made available, in understanding and studying the Central Nervous structure of humans and other animals were gathered by various investigations by Pathologists, Anatomists, Physiologists from the early 1800’s.

Year Research and Findings
1824 Luigi Rolando first discovered a method to study Central Nervous system structures via cutting chemically hardened pieces of brain tissue into thin sections for microscopical observations
1833 Robert Remak discovers that the brain tissue is cellular. Ehrenberg discovers that it is also fibrillar
1842 Rolando’s method of observing CNS structures was perfected by Benedikt Stilling by cutting series of consecutive slices of the same tissue, this allowed the ability to trace nerve tracts and establish spacial relations
1858 Joseph von Gerlach brings forth a new process to differentiate between the different microstructures in the brain by treating the sample to a solution of ‘Carmine’.

This solution made the sample no longer appear homogenous under the lens but able to be differentiable to its components

1889 Camille Golgi comes forth with the procedure of impregnating hardened brain tissues with silver nitrate solution which resulted in the staining of nerve cells.

Possibility to trace cellular prolongations definitely to their termini now present. Ramon y Cajal announces his discoveries.

Old theory of union of nerve cells into an endless mesh-work is discarded altogether, the new theory of isolated nerve elements ‘theory of neurons’ is fully established in its place [15]

HOW DO WE REFERENCE BOOKS? A History of Science by Henry Smith Williams, M.D., LL.D. assisted by Edward H. Williams, M.D. (1904)

Current research models and findings

Current Research

Most previous studies describe overall growth of brain based upon in utero imaging studies with the use of magnetic resonance imaging (MRI) and ultrasound; however, complicated folding of the cortex in adult brain is due to different rates of regional tissue growth. In the following study, maps of local variation in tissue expansion are created for the first time in the living fetal human brain, in order to examine how structural complexity emerges in fetal brain.

Local tissue growth patterns underlying normal fetal human brain gyrification quantified in utero[16]


Recent development in fetal MRI motion correction and computational image analysis techniques were employed in this study to help with the understanding of the patterns of local tissue growth. These techniques were applied to 40 normal fetal human brains in the period of primary sulcal formation (20–28 gestational weeks). This time period covers a developmental stage from the point at which only few primary sulci have developed until the time at which most of the primary sulci have formed, but before the emergence of secondary sulci on MRI. This developmental period is also important clinically, since the clinical MRI scans are also performed at this gestational age. Therefore it is important to describe the normal growth patterns in this period in order to be able to recognise abnormalities in the formation of sulci and gyri.

Techniques mentioned previously were utilised to quantify tissue locations in order to map the tissues that were expanding with a higher or lower growth rate than the overall cerebral growth rate. It was found that relatively higher growth rates were detected in the formation of precentral and postcentral gyri, right superior temporal gyrus, and opercula whereas slower growth rates were found in the germinal matrix and ventricles. Additionally, analysis of the cortex illustrated greater volume increases in parietal and occipital regions compared to the frontal lobe. It was also found that gyrification was more active after 24 gestational weeks. These maps of the fetal brain were used to create a three-dimensional model of developmental biomarkers with which abnormal development in human brain can be compared.

The following are recent studies that use a similar model to what was described above:

Mapping Longitudinal Hemispheric Structural Asymmetries of the Human Cerebral Cortex From Birth to 2 Years of Age [17]

  • In this study longitudinal cortical hemispheric asymmetries were mapped in infants using surface-based morphometry of magnetic resonance images
  • Some of the findings of this study:

-Sexual dimorphisms of cortical asymmetries are present at birth with males having larger sizes of asymmetries.

-The left supra marginal gyrus is much more posterior compared to the right supra marginal gyrus at birth and this position difference increases for both males and females by 2 years of age.

-The right superior temporal parieto-occipital sulci are significantly larger and deeper than those in the left hemisphere while the left planum temporale is significantly larger and deeper than that in the right hemisphere at all 3 ages.

  • It was concluded in this study that early hemispheric structural asymmetries are inherent and gender related.

Sexually Dimorphic White Matter Geometry Abnormalities in Adolescent Onset Schizophrenia [18]

  • In this study, they investigate the geometry of inter-hemispheric white matter connections in patients with schizophrenia with a particular focus on sexual differences in white matter connection.
  • They find a correlation between the sex-dependent abnormality in the geometry of white matter connecting the two hemispheres and the severity of schizophrenia

Asymmetry of White Matter Pathways in Developing Human Brains [19]

  • The use of high-angular resolution diffusion imaging tractography has allowed the above article to investigate the emergence of asymmetry of white matter pathways in fetal brains typically being less than 3 years of age comparable to adult brains over 40 years of age. Furthermore, the high spatial resolution generated from the use of this imaging technique provides a detailed image in order to shed some light on the irregular spatial pattern of white matter systems and whether primary associative functions form before higher cognitive functions. This is essentially important in the application of learning difficulties at a youthful age due to improved and revised understanding on cognitive development in children.
  • It was found that the emergence of asymmetry of white matter pathways in children less than 3 years of age specifically the association of higher order cognitive functions (arcuate fasciculus) was not observed however the emergence of the ILF pathway in FA occurred. Hence asymmetry while present in prenatal development, a more resilient and sturdy asymmetry develops at a later age.
  • The study however was unable to use a wider range of age intervals for brains obtained (only up to 3 years) and hence was unable to investigate the asymmetry of white matter pathways during important periods further.

Cortical Overgrowth in Fetuses With Isolated Ventriculomegaly [20]

  • The condition fetal ventriculomegaly is characterised by dilation of lateral ventricles whilst sharing associations with other malformations. It was hypothesised that using relative brain overgrowth as marked measure of altered brain development is due to the occurrence of ventriculomegaly and to evaluate this brain overgrowth through the use of magnetic resonance imaging (MRI) of fetuses isolated with enlarged ventricles (3rd and 4th ventricles as well as cerebrospinal fluid). Furthermore, significant changes to thalamic volumes, basal ganglia and white matter did not occur between cohorts.
  • The study found that there was a sufficient increase in brain volume (overgrowth) of fetuses that had ventriculomegaly in comparison to controls. Also, larger lateral ventricle volumes were yielded with fetuses with ventriculomegaly assessed via 2-dimensional movement of the atrial diameter (ultrasound and MRi) as well as a larger total brain tissue volume. This provides support for the hypothesis in that brain overgrowth can be used as a marked measure for ventriculomegaly with results showing that overgrowth was not localised in one region but across both hemispheres and thus lead to a neural deficit in children (altered neural connectivity).
  • Hence, this study has assisted in the improved understanding of the effects of ventriculomegaly on cognition, language and behaviour in children.

Future Research

Abnormalities

Microcephaly, Macrocephaly and Hydrocephalus

Clinical photograph showing the giant occipital encephalocele associated with microcephaly and micrognathia. [21]

Microcephaly and macrocephaly refer to abnormal head size. These abnormalities are seen in less than 2% of all newborns. Learning abnormalities and neurophysiological malfunctioning associated with these abnormalities are dependent on etiology, severity and patient’s age. The most frequent cause of macrocephaly is hydrocephalus.

Microcephaly

  • Noticeable reduction in the size of brain is observed due to factors that kill the dividing cells in the ventricular germinal zone. These dividing cells give rise to brain cells (both neurons and glia).
  • Microcephaly is specifically defined as a head size more than two standard deviations below the mean for age, gender and race.There are two diagnostic types of primary and secondary microcephaly.
  • Primary Microcephaly: Abnormal development is observed in the first seven months of gestation.
  • Secondary Microcephaly: Abnormal development occurs during the last 2 months of gestation (prenatal period) in the secondary type.
  • Microcephaly is caused by various factors that prevent normal proliferation and migration of cells during CNS development. These factors are divided into physical (irradiation, raised maternal temperature), chemical (anticancer drugs) and biological (infection of uterus due to rubella, cytomegalovirus and herpes simplex virus).
  • Genetic defects and chromosomal disorders can also play a role.All of these factors result in destruction of the brain tissue (encephalopathy) with multiple areas of scarring and cyst formation.


Macrocephaly

  • In patients with macrocephaly the head is enlarged.
  • Macrocephaly is specifically defined as a head size more than two standard deviations above the mean for age, gender and race.
  • Macrocephaly is a syndrome of diverse etiologies rather than a disease and the most frequent cause is hydrocephalus.

Hydrocephalus

Magnetic Resonance image showing Arachnoid Cyst with Hydrocephalus [22]
  • Progressive enlargement of head due to accumulation of cerebrospinal fluid in ventricles is known as hydrocephalus.
  • Excessive accumulation of cerebrospinal fluid is due to an imbalance between the formation and absorption of cerebrospinal fluid (communicating hydrocephalus) or obstruction of circulation of cerebrospinal fluid (non-communicating hydrocephalus).
  • Multiple abnormalities such as brain tumours, congenital malformations and inflammatory lesions are associated with hydrocephalus.
  • In a patient with hydrocephalus, cerebrospinal fluid accumulation results in raised intracranial pressure which in turn results in enlarged ventricles and skull. Raised intracranial pressure is further associated with behavioural change, headache, papilloedema (oedema of the optic nerve) and herniation syndromes (subfalcine, uncal and cerebellar).
  • Enlargement of cranial sutures, progressive thinning of cerebral walls and lamination of cerebral cortex are all manifestations of hydrocephalus. Symptoms include significant deficits in motor skills (damage to pyramidal tracts) and cognitive functioning.
  • The extent of brain damage depends on the underlying factor and developmental stage in which damage occurs. Hydrocephalus can be treated by shunting the excess fluid from the lateral ventricles into the heart or peritoneal cavity.

Fetal Alcohol Syndrome

[23]

  • Severe alcohol consumption during pregnancy and especially at critical stages of development (i.e. just after neural tube closure) can result in fetal alcohol syndrome (FAS).
  • FAS is the most severe form of a spectrum of physical, cognitive and behavioural disabilities, collectively known as fetal alcohol spectrum disorders (FASD).
  • Mental retardation is the most serious abnormality associated with FAS. In addition, FAS is typically associated with central nervous system abnormalities, impaired sensation, impaired motor skills and lack of coordination.
  • Patients diagnosed with FAS have a small head size relative to height, and demonstrate minor abnormalities of the face, eye, heart, joints, and external genitalia. (image)
  • Ethanol in alcohol directly damages neurons by acting as an agonist for GABA receptors in the brain as well as interfering with many other receptors. Ethanol can also alter body’s metabolism by an indirect effect on neurons that modulate the secretion of hormones. In addition, it is postulated that malnutrition intensified by alcohol abuse is another cause of FAS since FAS is more common in individuals with low socioeconomic status.
  • Nutritional deficiency and alcohol abuse inhibit the metabolism of folate, choline and vitamin A which are necessary for neurodevelopment. Therefore supplementation of these three nutrients to mothers with Disorder Binge drinking or low socioeconomic status may reduce the severity of FAS.
  • Consequently, pregnant mothers need to be aware of the risk associated with consuming even small amounts of alcohol. FASD and FAS represent a serious problem for both the individuals and society but are easily preventable.
The standard facial features of individuals with FAS include microcephaly (decreased cranial size at birth), flat mid-face with short palpebral fissures, short nose with a low bridge, long smooth or flat phylum with a narrow vermilion of the upper lip [1]</ref>

Iodine deficiency

A) A three- month old infant with untreated congenital hypothyroidism. Image illustrates hypotonic posture, myxedematous facies., macroglossia, and umbilical hernia. B) Same infant- close-up of the face, showing myxedematous facies, macroglossia, and skin mottling. C) Same infant- close up showing abdominal distension and umbilical hernia. [24]
  • Iodine is required for the synthesis of thyroid hormones. Thyroid hormones play an important role in the regulation of metabolism of an organism. Additionally, thyroid hormones take part in early growth and development of most organs particularly the brain, during fetal and early post-natal development. [25]
  • The physiological role of thyroid hormones is to coordinate the time of different developmental events through specific effects on the rate of cell differentiation and gene expression during fetal and early postnatal development. Iodine deficiency may affect thyroid hormone synthesis during this critical period which will further result in hypothyroidism and brain damage. The clinical outcome is mental retardation. [26]
  • All degrees of iodine deficiency (mild: 50-99 μg/day, moderate: 20-49 μg/day and severe≤20 μg/day) affect thyroid function of both mother and the neonate as well as mental development of the child. The damage increases with the degree of deficiency; overt endemic cretinism is the most severe consequence.
  • Iodine deficiency disorders refer to complications that arise when the recommended dietary allowance of iodine is not met. These complications include thyroid function abnormalities and when iodine deficiency is severe, endemic goitre and cretinism, endemic mental retardation, decreased fertility rate, increased perinatal death and infant mortality.
  • Iodine deficiency represents the world’s greatest cause of preventable brain damage and mental retardation, resulting of a loss of 10-15 IQ points globally. Therefore it is essential that the recommended dietary requirement for iodine is met, especially during pregnancy and lactation (200–300 μg/day). [27]
  • Clinical manifestations of hypothyroidism include: Myxedematous facies (condition characterized by thickening of the skin, blunting of the senses and intellect, and laboured speech), jaundice, a puffy face and a wide posterior fontanelle with open sutures. The nasal bridge is flat. The mouth may be slightly open revealing macroglossia. Further examination would reveal bradycardia and a protuberant abdomen with a large umbilical hernia. Neurologic examination findings include hypotonia with delayed reflexes. Skin may be cool to touch and mottled in appearance due to circulatory compromise.[28]

Abnormalities associated with apoptosis and migration of cells in the fetal CNS

  • The migration of cells and cell death are critically important to fetal brain and CNS development. Abnormalities in each of these processes, programmed cell death (apoptosis) in particular, have been shown to result in severe developmental abnormalities in experimental animals. Apoptosis is critically important for appropriate brain development; certain areas in brain may experience up to 50% cell death. [29]

In an experiment conducted by kuida et. al 1996, it was observed that mice that were deficient for CPP32 (a protease responsible for apoptosis), were born at a lower frequency than expected, were smaller in size compared to the normal mice and died at an early age. Brain development is significantly affected in CPP32-deficient mice resulting in a variety of hyperplasia and disorganised cell development. [30]

On the other hand, disrupted neuronal migration can lead to an abnormality in cell position. When this happens, the neurons are said to be heterotopic. Abnormalities in neuronal migration have been studied extensively in human cerebral cortex where these defects are associated with a variety of syndromes and diseases, ranging from behavioural disorders (including some forms of schizophrenia, dyslexia and autism) to extremely severe mental retardation and failure to thrive. [31]


Neural Tube Defects

Defects which affect the either the brain or the spinal cord in which openings remain. Grastulation occurs in week 3 of embryonic development where specialized cells (dorsal side) structurally change shape leading to the formation of the neural tube. If the neural tube does not close or fuse together properly, then openings remain which lead to various neural tube defects such as Spina bifida cystica and Spina bifida occulta.

Anencephaly

  • A neural tube defect involving abnormal development of the brain and incomplete skull formation leading to high infant mortality rates with infants being stillborn or dying within a few hours after birth.
  • The failure of the neural tube to close around the 23rd and 26th day into embryonic development is characteristic of this condition. As a result, the forebrain is severely affected where the cerebrum does not grow and hence partial growth occurs only. The cerebrum has a key importance in maintaining thought, consciousness and coordination whilst having no intact cerebrum rules out the probability of the affected fetus gaining consciousness .
  • Increasing one's dietary intake of folic acid notably reduces the incidence of neural tube defects. Consuming 0.4mg of folic acid is sufficient to induce these results.
  • Since incomplete skull formation occurs, brain tissue becomes exposed due to the absence of bone covering and therefore leading to further complications.

Encephaloceles

  • Another neural tube defect in which a sac-like projections (including membrane covering) that occurs around the 3-4 week of embryonic development in which neural tube closure has not successfully occurred. The location of these protrusions varies but can be naso-frontal (nose and head), naso-ethmoidal (nose and ethmoid bone), naso-orbital (nose and eyes), meningocele (cerebrospinal fluid and membrane covering) and encephalomeningocele (meningocele with brain tissue as well).
  • These sac-like protrusions lead to a variety of abnormal effects which include cerebro-spinal fluid build-up in brain, microcephaly, hydrocephaly, mental retardation, developmental problems, uncoordinated muscle movement and seizures. Consumption of folic acid again has been shown to significantly reduce the occurrence of encephaloceles in infants.

Hydranencephaly

  • A very rare neural tube defect in which the cerebral hemispheres are destroyed (necrotic tissue) with the occupied space being replaced by cerebro-spinal fluid, cortex and white matter remnants within a membranous sac. Interestingly, this condition can also develop in the postnatal peroid due to viral infections or traumatic brain injury. The brain stem may remain intact and functioning in hydranencephalic infants.
  • Accompanied with this condition are many effects in which include seizures, hydrocephalus, increases in head circumference,impairment in vision/ body temperature regulation and cerebral palsy. Infants affected by hydranencephaly live typically short lives being no longer than 1 year of age. Lastly, there are no viable treatment options for this condition and only supportive care is available.

Iniencephaly

  • Another neural tube defect involving severe backward bending of the head (retroflexion) with subsequent spinal distortions as well. Affected infants have no neck and hence the scalp of the head is directly joined to the skin of the back (also skin of the face connected to skin of the chest). Other conditions that develop along with iniencephaly are cyclopia, cephalocele and anencephaly. The development of this condition is not solely due to one factor but from various causes (both genetic and environmental factors).
  • There are specific factors that have been shown to increase the occurrence of this condition. Malnutrition, reduced folic acid intake and elevated levels of homocysteine in blood are environmental factors that increase the risk of iniencephaly [32]. Intake of certain drugs such as anti-histamines and sulphonamide/tetracycline (antibiotics) have also increases this risk [33]. Furthermore obesity has been shown to have a significant effect on the incidence of iniencephaly with 1.7-3 fold increase [34].
  • In terms of treatment, folic acid supplementation as well as avoiding certain drugs such as those outlined above significantly reduces the occurrence of this condition.

Spina Bifida Cystica

  • A neural tube defect in which involves either the formation of a meningocele or myelomeningocele. Of the two, the meningocele, is the least debilitating in that a cyst-like structure forms when the neural tube does not close properly. The membrane (meninges) is pushed into openings of the vertebrae where spinal fluid builds up within the cyst. The myelomeningocele leads to severe problems as the portion of the nueral tube that is unfused allows the spinal cord to protrude through. As a result, neuronal tissue is highly exposed due to myeloschisis as well as infections and hence may lead to severe complications.

Spina Bifida Occulta

  • A neural tube defect that has minimal complications in comparison to other defects. Furthermore, this defect is hidden in that a tethered spinal cord may arise as well as a thicker filum terminale and diastematomyelia (spinal cord split into two). It is also important to note that no cysts form as opposed to the other more severe spina bifida defects.

References

  1. Report of the Workshop on Acute Perinatal Asphyxia in Term Infants, U.S. Department of Health and Human Services, Public Health Service, National Institutes of Health, National Institute of Child Health and Human Development, NIH Publication No. 96-3823, March 1996.
  2. <pubmed>4203033</pubmed>
  3. <pubmed>5414696</pubmed>
  4. <pubmed>7204662</pubmed>
  5. <pubmed>12764033</pubmed>
  6. <pubmed>8523077</pubmed>
  7. <pubmed>9712307</pubmed>
  8. <pubmed>1713238</pubmed>
  9. <pubmed>22272298</pubmed>
  10. <pubmed>10532616</pubmed>
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