2014 Group Project 7
|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.
- 1 Neural - CNS
- 1.1 Introduction
- 1.2 Development during fetal period
- 1.3 Historical Research and Findings
- 1.4 Current research models and findings
- 1.5 Abnormalities
- 1.6 References
Neural - CNS
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
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
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
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.
A simplified timeline of human neural development (modified) 
This simplified graph shows a timeline for major events of neural development that occur during fetal and postnatal periods. These events, which are summarized in the following table, are broadly classified by cell multiplication, cell migration, growth and differentiation and angiogenesis:
|Cell migration||* Neuronal migration is maximised during 2nd trimester while glial migration is maximised during 3rd trimester|
|Growth and differentiation||
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 
- Location: occurs in germinal matrix that comprised of ventricular and subventricular proliferative zones of cells.
- Ventricular zone: This is the proliferative zone that appears first which is a pseudostratified columnar epithelium . 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 . The nuclei move between the ventricular surface and the border of the ventricular zone with the subventricular zone .
- 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 . In some parts of the brain, there is the production of a significant number of neurons in this zone . For example, the subventricular zone contributes large number of cells to the neocortex, which is the youngest structure in the brain . In contrast to ventricular zone, the proliferating cells do not move through the cell cycle.
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: 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.
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.
- Divergent transient connections: one neuron innervates more cells than normal which will be eliminated by the reduction of projection area.
- 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 , are involved.
- 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.
- 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
- 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 
- 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 
- Formation of Secondary Sulci 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.
- The human brain contains billions of neurons which are produced by mid-gestation; during fetal development 
- Processes of Neuronal production is initiated by first increasing the size of the Neural progenitor cell population within the body, these cells are mitotic in nature and are capable of forming new cells.
- Initially (from the end of gastrulation through to embryonic day 42), the neural progenitor cell population is increased greatly when the progenitor cells divide through 'symmetrical' mode of cell division. Multiple repeats of the cell division occurs throughout this period. This 'symmetrical' division method brings about the formation of 2 new identical neural progenitor cells
- Mode of cell division changes from a symmetrical cell division type to an 'asymmetrical' cell division from the beginning of E42. This asymmetrical cell division forms 2 different cell types; one Neural progenitor & one Neuron 
- Newly formed Neural progenitor cells remains to undergo more processes of cell division whereas the newly formed neuron moves into position in the developing neocortex
Increase in size and weight
Image of brain and ventricular development 
Image of brain fissure development 
folding: sulcation and gyration
During the fifth and sixth month of gestation, the smooth cortex begins to fold by sulcation and gyration. 
Sulcation: This is the development of sulci, including primary and secondary. The formation of primary sulci involve the appearance of shallow grooves on the surface of the brain, which then become more deeply infolded, while the formation of secondary sulci is due to the development of side branches of the primary ones .
Gyration: This is the development of gyrus that occurs during late during fetal development until the end of the pregnancy or after birth . This process is the formation of tertiary sulci, which is the formation of other side branches of the secondary sulci .
|Weeks of gestation||Visible anatomical details|
|20-21||smooth, with the impression of "lissencephalic" brain; wide Sylvian fissures; visible interhemispheric fissure and parieto-occipital fissure|
|22-23||beginning of calcarine and hippocampal fissures and of callosal sulci|
|24-25||smooth cerebral cortex surface; visible shallow grooves in the central sulci, interparietal sulci and superior temporal sulci; start of opercularization of Sylvian fissures; presence of calcarine fissures and cingular sulci|
|26||presence of central and collateral sulci|
|27||presence of marginal and precentral sulci|
|28||presence of postcentral and intraparietal sulci|
|29||presence of superior and inferior frontal sulci; narrower Sylvian fissure; clear corpus callosum; bright white matter; dark cortical ribbon|
|30-31||beginning of infolding of cortex which is first apparent in the occipital lobe; narrower ventricular system and subarachnoid spaces|
|32||presence of superior and inferior temporal sulci|
|33||presence of external occipitotemporal sulci|
|34-35||close to final shape of gyration; compactly and extensively folded cortex|
|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 of the process of sulcation and gyration 
Spinal Cord Development
The spinal cord is formed from parts of the neural tube during embryonic and fetal 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 
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
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
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.
Microcephaly, Macrocephaly and Hydrocephalus
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.
Fetal Alcohol Syndrome
Abnormalities associated with apoptosis and migration of cells in the fetal CNS
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. 
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. 
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.
Spina Bifida Cystica
Spina Bifida Occulta