Pattern and cell type
specification in the vertebrate CNS (NP)
The vertebrate central nervous system (CNS) includes the brain, spinal cord, optic nerve and retina. The CNS is concerned with receiving and processing sensory information from the periphery, initiating movements and "higher functions" such as understanding developmental biology lectures etc. etc.
The peripheral nervous system (PNS) includes the peripheral ganglia - nodules of nervous tissue containing the cell bodies of sensory neurons and glial cells that lie along the long axis of the CNS. There are cranial (head) ganglia (e.g. the trigeminal and nodose ganglia), the cervical (neck) sympathetic ganglia and the spinal or dorsal root ganglia (DRGs). Each ganglion contains sensory neurons of several types - e.g. mechanosensory neurons that detect pressure on the skin, proprioceptive neurons that monitor the stretch state of muscles, nociceptive neurons (pronounced no-see-) that detect noxious chemicals (pain) etc. More about this in a future lecture.
The nervous system, unsurprisingly, is very complicated and contains a large number of different kinds of neurons and glial cells (more classes of neurons than glia, but a larger absolute number of glial cells). In the spinal cord, for example, there are several classes of motor neurons (which connect to muscle fibres), spinal sensory neurons (which connect to sensory neurons in the DRG) and various classes of interneurons, which connect motor neurons to other motor neurons, motor neurons to spinal sensory neurons and both motor and sensory neurons to the brain. Interneurons are responsible for co-ordinating muscle contractions from one segmental level of the cord to the next (e.g. wriggling movements), for controlling the precise amount of muscle extension or contraction (by integrating proprioceptive inputs via the DRG with motor neuron output) and for various reflex movements (like the stretch reflex, withdrawal reflex etc).
This section is about how some of the neuronal and glial cells in the spinal cord gain their identity - how motor neurons are formed in the ventral regions of the cord and sensory neurons in the dorsal regions. This relates directly to how cells in a Drosophila embryo "know" whether they are towards the anterior or the posterior side of the segment. You will recall that this is under the control of the segment polarity genes - notably hedgehog (hh) and its downstream signalling components (patched, smoothened, fused, suppressor of fused, cubitus interruptus). The Hedgehog protein is a diffusible morphogen that acts on cells at a distance from the cells that secrete it, inducing a graded response according to its local concentration at that point. There are vertebrate homologues of Hedgehog, called Sonic hedgehog (Shh), Desert hedgehog (Dhh), Indian hedgehog (Ihh), Tiggywinkle (Twhh) and others. Sonic hedgehog was the first to be discovered and is believed to play the crucial role in patterning the ventral spinal cord. (It crops up in many other places too, including the developing vertebrate limb). In addition, there are known vertebrate equivalents of Patched, Smoothened, Cubitus interruptus (known as Gli in vertebrates). As with the Hedgehogs, each of these vertebrate homologues is part of a small family of genes/gene products; for example there are three mouse gli genes known to date, imaginatively called gli1, gli2 and gli3.
Just after neural tube closure, the notochord is in close contact with the ventral-most part of the spinal cord. Notochord cells make Sonic hedgehog (Shh), which induces neighbouring cells in the neural tube to form floor plate. The floor plate itself starts to express Shh and it is probably this floor plate-derived Shh that subsequently induces other cell types - motor neurons and ventral interneurons - to form in the ventral cord.
To understand how Shh might pattern the ventral cord we need to know a little about how neurons are generated in the developing CNS. Immediately after neural tube closure, the walls of the tube are composed of a single-cell-thick layer of neuroepithelial cells - these are organized into a typical epithelial sheet i.e. they are polarized cells with an apical surface that faces the lumen of the cord (the hole down the middle of the cylinder) and a basal surface towards the outside of the cord, and they are firmly attached to one another by tight junctions. The neuroepithelial cells are the precursors of all the many kinds of neurons and glial cells that will eventually make up the mature CNS. At first the neuroepithelial cells divide in the plane of the epithelial sheet, generating two similar neuroepithelial cells that remain in contact with each other and the lumen of the neural tube. This serves to increase the number of neuroepithelial cells and the surface area of the neural tube (ie it grows in size). Then, at a certain point, they start to divide perpendicular to the epithelium - this generates two dissimilar cells, one of which remains in contact with the lumen (a replacement neuroepithelial cell) while the other loses contact with the lumen, migrates away towards the outer (pial) surface of the neural tube and differentiates into a neuron. The migrating cell, which does not divide further, is known as a neuronal progenitor. In general, the neuronal progenitors migrate radially so that their final resting places are approximately level with the neuroepithelial cells that gave rise to them. That is, motor neurons that reside in the ventral part of the spinal cord are generated from ventral neuroepithelial cells, while other types of neurons that lie in more dorsal parts of the cord are derived from more dorsal parts of the neuroepithelium. The layer of neuroepithelial cells continues to divide and throw off neuronal progenitors for some time, and the number of differentiated neurons continues to rise. Later, the neuroepithelial cells switch to producing glial cells instead of neurons. The neuroepithelial cell layer is known as the germinal zone of the developing CNS. Because the neuroepithelial layer lines the lumen of the spinal cord, which is continuous with the ventricles of the brain, it is also commonly known as the ventricular zone. When the full complement of neurons and glial cells have been generated, the ventricular zone regresses and the lumen of the spinal cord shrinks until it is a relatively small canal down the centre of the mature spinal cord.
An increasing amount of evidence suggests that each neuroepithelial cell in the ventricular zone "knows" what kind(s) of mature neuron it is destined to produce - its future fate - by virtue of its position in a concentration gradient of Sonic hedgehog (Shh) emanating from the floor plate. The neuroepithelial cells are said to be specified to give particular types of neurons, according to their distance from the floor plate.
Right next to the floor plate are the precursors of visceral motor neurons. These innervate visceral muscles; for example, in the cervical (neck) spinal cord they send axons to the diaphragm, while at higher levels (in the brainstem) they send axons to the heart and other internal organs. Above these (ie further dorsal) are the precursors of somatic motor neurons. These innervate skeletal muscles, either close to the spinal cord (axial muscles) or in the body wall or in the limbs. Each of these two major classes (visceral and somatic) subdivide into sub-classes. For example, at any one level of the spinal cord there will be somatic motor neurons that project to a number of different muscle blocks (ventral and dorsal body wall, vental and dorsal limb etc) and visceral motor neurons that project to different parts of the gut. Potentially, the precursors of each of these subpopulations of motor neurons could be specified by distinct threshold concentrations of Shh. Alternatively, particular sets of motor neuron subtypes could be generated sequentially from the same block of neuroepithelial cells.
In addition to these different motor neuron subclasses, there are different types of interneurons. Two different types have been defined in the ventral spinal cord by the use of antibody markers - the so-called ventral-1 (V1) and V2 interneurons. These are generated from more dorsal parts of the neuroepithelium than motor neurons, though still within the ventral half of the cord. The dorsal half of the cord is not strongly influenced by Shh; the different sensory neurons and interneurons in the dorsal cord are probably induced by an opposing gradient(s) of bone morphogenetic proteins (e.g. BMP4) - members of the extended family of signalling molecules epitomised by transforming growth factor-beta (TGFbeta). The Drosophila protein Decapentaplegic (Dpp) is related to the BMPs. In Drosophila, Hedgehog signalling is frequently associated with and antagonized by Dpp, just as BMPs antagonize Shh signalling in vertebrates.
still in preparation ....
reading:
1. Wolpert, Chapter 11 pp346-348 (Beware: in Fig 11.12 the colour codes for sensory and motor neurons have been reversed. Motor neurons are ventral cells and sensory neurons dorsal, not the other way round as depicted in this figure.)
2. Tanabe, Y and Jessell, TM (1996) Diversity and pattern in the developing spinal cord. Science 274, 1115-1123
3. Ingham, PW (1998) Transducing hedgehog: the story so far. EMBO Journal 17, 3505-3511.
4. Goodrich, L.V. and Scott, M.P. (1998) Hedgehog and patched in neural development and disease. Neuron 21, 1243-1257. (this is an in-depth review of what we do and do not know about hedgehog signalling. Don't worry if you find it heavy going. Read the final few sections on the Hh pathway and neural diseases)