In one of the most remarkable advances in the human brain research the recent progress in imaging technologies delivered in the hands of neuroscientist powerful tools for non-invasive studies of the human brain. This means an opportunity to study a living human brain; its structure, its function and its dysfunction. Consider that most of what we knew up until recently of the human brain was instigated by studies of a dead brain or deduced from experiments on other animals. Non-invasive imaging means that reading a book, making a joke, being hungry, experiencing sexual desire or fear can all be associated with specific patterns of brain activity that can be recorded, characterised and, possibly, understood. Currently, imaging technology allows us to see not only distinct activation sites in the brain but also neural pathways involved in a specific functional activity. Implications of such a research are wide and deep ranging from understanding of neural networks which sustaining our body alive, to learning about the mechanisms underlying numerous human brain dysfunctions, to changing perception of ourselves. Consider, for example, that our thoughts, feelings, our conscious and mystical experiences are no longer experienced on our own, but can be observed and analysed in a scientific way. Indeed, it seems that even religious feelings which many regard as exclusively personal are no longer necessarily confined by privacy. The untouchable, "noble" connotation of the nature of brain function is disappearing.

Can functional imaging provide a Rosetta Stone for interpretation of the language of the human brain?

Despite the potential there is, however, some scepticism that surrounds interpretation of imaging studies. The question many are asking is - how can we interpret the phenomenon of the imaging signal into common neurological and philosophical terms? In other words, can we treat the imaging signal as Rosetta Stone to translate the activity of neural networks into personal experiences? It must be said that there is a broad confidence in the neurological community that we can. So much confidence, in fact, that it is now generally accepted that consciousness has a neural basis. Most of the accompanying arguments are, however, build on inferences and speculations. The Gordian knot of meaningful interpretation of the imaging signal is the magnificent structural complexity of the human brain. Non-invasive imaging allows us to visualise activity within an area of the brain. The "active" area is, however, quite broad in neuroanatomical terms and therefore can not be directly associated with any specific cell group or functionally specific circuitry. Though it is reasonable to anticipate a significant increase in images resolution, the complexity of the brain circuitry extends further. The human brain contains about 100 billion neurons (functional neural cells) and about 2500 billion glial cells (support cells). Each neuron is characterised by a specific set of neurotransmitters (chemical molecules used by neuron as means of communication), a specific set of receptors (chemical compounds used by neuron as means of receiving information) and a specific set of connections with its functional targets as well as with its neighbouring neurons. Neurons are grouped, off course, but in terms of functional specificity such groups are quite small and constituent neurons may change groups or chemical signature. Some groups of neurons play active roles in several pathways, others are characterised by tonic activation. Depicting structural organization of the human brain is the work of neuroanatomists who have been trying to establish a comprehensive and sufficiently detailed account of the human brain for more then a hundred years, employing various techniques from cytochemistry to molecular biology, to computer aided 3D reconstructions. Evidently, structural complexity of the human brain is astonishing but resolving it is the key to understanding the human brain.

Development may provide a key to resolve the enigma of the human brain organization.

How can the complexity of the brain be resolved? Pivotal help in solving this problem comes from studying the development of the brain. Indeed, it is quite reasonable to look at how does the brain generate itself in order to establish how is it build. More specifically, how does the highly complex 3D structure of the brain develops from a linear strand of DNA? (Figure 1) To answer this question, naturally, one must depict successive principal events of brain development from undifferentiated cell to functioning organ as well as understand mechanisms underlying such events.

The brain develops out of a homogenous cell layer which first becomes segmented. Segments then develop into progressively more distinct building blocks which in turn further differentiate into specific brain structures and functional cell groups. Thus, within the multifaceted variety of the adult brain structures all have compatible origins which could be followed through the steps of progressive differentiation. If we can learn the mechanisms that govern such differentiation we would be able to deal with the complexity of the adult brain structures in terms of their more comprehensive developmental precursors. Studying development, however, presents major problems on itself and history of developmental neuroscience is covered with missing teeth of unresolved contradictions as well as with glory of Nobel prises.

Four principal mechanisms directing brain development.

Because the nervous system originates as an undifferentiated flat cell layer which folds into a "neural" tube the central nervous system can be viewed in three axis (the longitudinal, the circumferential and the radial axis). It is easier to imagine it as a tube which has stretches, invaginations, bents and flexures. In development such deviations give the nervous system and the brain very complex shape. To illustrate, a section through the brain can be compared to a section through the ball of cooked spaghetti which once were spread in a straight line. During pregnancy undifferentiated cells are generated at an average rate of 250 000 every minute. Much like a dough rises to become a puffed up loaf the undifferentiated neurons divide to increase the thickness of the wall of the embryonic neural tube (the future brain) with one important difference: the neurons are generated in precise order. As far as we can deduce and currently conceive, every neuron is destined to a very specific fate and the early neural tube is best described as an extended Russian wedding ring of distinct neuronal populations. Such segmentation of the neural tube produces basic partitions of the nervous system and represents the first principal step of the developmental differentiation.

The walls of neural tube develop a sort of a scaffolding out of support cells. These cells are called "radial glia" because they extend their processes between the inner and outer surface of the epithelial wall (Figure2). The developing and multiplying neurons, thus, begin to move within a relatively confined space of radial glial scaffolding which draws first strokes to a structural blueprint of the future brain. The function of the glial scaffolding in determining the structure of the brain is pivotal because it is in effect from the time when the brain is still very simple. As nervous system develops the scaffolding becomes more complex along with increasing complexity of the brain. Important quality of the scaffolding is that the relationship of the radial glial cells to the epithelial surface remains even as the brain structure becomes very sophisticated and twisted. In other words while the brain undergoes complex 3D changes, the glial processes retain the grip on the origin in the epithelial lining thus retaining the boundaries of the original scaffolding. The glial scaffolding, thus, represents another degree of differentiation which generates elementary outlines for the developing building blocks of the future brain.

Neuronal migration is another major mechanism underlying structural differentiation of the brain. Neurons do not simply move chaotically but follow a their specific fate. This means that one neuron will stay close to where it was born while the other will immigrate to another side of the nervous system. The movement of the neurons is directed by repulsion or adhesion they experience on the way of migration. This process is based on the expression of numerous specific adhesion repulsion molecules in the developing nervous system. To illustrate the sophistication of the mechanism driven by such molecules we can turn to an example from the other bodily structure - the immune system. When one gets bitten by a mosquito the resulting oedema is a complex event. The immune cells, essential to the oedema, have to rush through the complex network of tiny capillaries to the place of the bite, stop at the exact location, endure the stream of the blood, and finally migrate through the epithelial wall of the vessel into the unknown territory of the skin. Much the same determination is demonstrated by the developing neurons moving to the place of their destiny in the rapidly changing environment of the developing nervous system. What governs such a determined effort is the pull of the relevant guiding molecules. Adhesion and repulsion molecules with authority to direct the spatial movement of the neurons determine to a great degree the position and the concentration of the migrating neurons in the developing brain. Many neurons, of course, get lost but this just means that they don't find their final destination and are not subjected to proper connection growth and function. Because later in development the rule of - use it or loose it - is critical, these neurons simply commit suicide or starve to death. However, what seems to be a failure of nature may be a useful defence mechanism that brain uses to insure itself against predicaments of early development such as ischemia, circulation problems etc. With many lost and unemployed neurons present in every area of the brain they may substitute the function of cells which found their destiny but could not function properly. The most significant result of this stage of the structural development is the increasing segregation the neuronal mass into of numerous building blocks of the eventual brain. These building blocks of undifferentiated neurons are still very comparable at this stage of development. Relatively inarticulate they are already bound to somewhat discrete fates and will further differentiate to form very distinct brain regions.

To differentiate into a more specific functional identity the neurons have to know where they are during their migration in the developing embryo. What is the mechanism of such orientation? This has been a long standing question in the developmental neuroscience and the one possible explanation can be that the position of the neuron is defined by an interaction of the expression gradients of the morphogenic molecules. Such morphogens have a direct effect of the migrating neuron and affect the development of the neuron in a concentration-dependent manner. In the basis of the mechanism lie the above mentioned adhesion/repulsion molecules. The gradient of their expression determines distribution of molecules that characterise areas of morphogenetic gathering for migrating neurons. Two variables which define the neuronal position are the concentration of the specific morphogenic molecule which is expressed in a circular gradient from the centre out and the neuronal response to the concentration of various morphogenic molecules expressed in the area. This process is crucial for the organization of neurons into a high degree of structural complexity found in the functioning brain. This morpho-defining gradient of gene expression, thus, represents the fourth degree of logic in formation of the brain structure with, up until now comparable building blocks of the nervous system undergoing fine region specific differentiation.

The brain structure then becomes radically more complex with general morphogenetic areas differentiating further to give the brain its final complexity. This process, at least in part is governed by molecular mechanism for adhesion between cells and it provides a new framework for same-cell and trans-cell interactions. As neurons differentiate functionally and chemically, they develop differential adhesive properties, and their affinity differences allow cells to sort out from one another. Adhesive molecule - mediated cell-cell interactions is thought to be critical in controlling cell sorting during embryogenesis. The formation of a myriad of neuronal connections within the vertebrate nervous system relies heavily on expression of molecular tags that match extending axon populations with synaptic target sites. The diversity of the adhesion repulsion tags in the nervous system allows for a multitude of interactions to specify neuronal connections. Specific molecules demarcate subpopulations of developing axons that interconnect within neuronal circuits.

Developmental processes determine order.

Development processes, described above, result in the order which defines the final structural organisation of the brain. Under the conditions set by the dictatorship of the modelling rules neurons have little choice but to develop into a highly complex structure of the nervous system. Thus, in the developed brain any functionally distinct constituent structure, however small, can be viewed through the prism of these rules. It is also reasonable to expect that the differentiated structures in all their complexity and diversity still hold still display remnants of their origins. To refer to an above analogy, the rules of developmental differentiation, if understood, are capable of untying the ball of spaghetti.

In conclusion, beyond a simple strain of DNA one can note at least three levels of complexity the nervous system; 2D regularity of neural epithelium, 3D assemblage of the poorly variable structural neuronal modules and finally a complex and highly diverse structure of the functional nervous system. Because the rules of development govern in a highly restrained environment many early structural elements of the brain are inarticulate in development, like the brick in the wall. Rudiments of such patterning are still seen in the adult brain but more importantly because these building blocks are easily identified in the development they inspired the idea that one can find the key for the structural differentiation of the brain. Much like Mendeleev once found a key to make sense of many chemical elements one can find a process that puts the cataphony of 100 billion neurons into a symphony of the functional brain. Clearly such key is of molecular biological nature. It is also clear that we are currently capable of resolving the governing principle of brain development. What comes afterward is the question of how does this governing biological principle enables consciousness to emerge.

Figures

Fig. 1. Cross-sections through the developing spinal cord of the human shows the orientation of processes of radial glial cells. (a) Camera Lucida drowing a coronal section through the brain of an embryo of approximately 26 days, (b) Photomicrograph of a coronal section of the brain of a fetus of about 20 weeks. After Retzius from Clara, 1940.

Fig. 2. Schematic median section through the brain of an embryo. The embryonic brain is divided by major longitudinal boundary which follows its curvature throughout its anterior-posterior extension and by multiple transverse boundaries which slice the brain like a russian wedding ring. If the brain would be stretched, the stereotypical building blocks would became evident.

Fig. 3. Cross section through the barin of a developing Wallaby shows the segmented arrangement of radial glial cells and their processes. Positive and negative areas will later become different parts of the brain.

Fig. 4. Human brains during development from 9 weeks to birth.