Common Features of Neurons
From Program on Ontologies of Neural Structures
The first three properties identify which cell we are concerned with, defined provisionally in the manner previously described for "Neuron names by region". The question before us now is which specific criteria are necessary and sufficient to define exactly that cell and no other. We take the criteria in turn.
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Histological layer
In some regions this may seem obvious. The cell bodies forming a single layer in the cerebellum are Purkinje cells; those forming a single layer in the main olfactory bulb are mitral cells. However, other cell types may be present in that single layer: granule cells or Golgi cells in the Purkinje cell layer, granule cells in the mitral cell layer. This is even more a problem in the cortex, with scattered layers in which several cell types are found. Layer by itself therefore is a necessary, but not in general a sufficient criterion for defining a cell type.
Cell body size
Cell types characteristically have different size cell bodies, which can be generally characterized as large, medium or small. Cell size plus histological layer is often used as a specific identifier. Thus, cerebellar Purkinje cells are the large cell bodies in a single layer; olfactory bulb mitral cells are the large cell bodies in a single layer. However, this becomes unreliable in many other regions because of the overlap in cell body sizes; for example, in layer 3 of the neocortex, the sizes of pyramidal cells grade from large to medium, overlapping with the cell body sizes of several interneurons. In general, it may or may not help, even teamed with layer.
Cell body shape
The shape of the cell body often was invoked by classical histologists as the defining characteristic of a cell type. Thus for example on had the pyramidal cell of the cerebral cortex with its pyramidal-shaped cell body, or the mitral cell of the olfactory bulb with its cell body in the shape of a bishop's mitre (cone shaped cap with convex sides). Other cell body shapes include spherical and fusiform (elongated in two directions, like a spindle).
The problem with cell shape is that it tends to reflect the dendritic trunks arising from it, and may often vary; thus, a pyramidal neuron may have a spherical cell body, etc.. Cell body shape thus may be important for the historical origin of a name, but in general is not reliable as a sole basis for a defining characteristic for all members of a type.
Dendrite branching pattern
The most common basis for identification of a cell type is the morphology - the outward form - of the dendrites, the relatively short branches that extend from the cell body into the surrounding part of the brain region. For example, the cerebellar Purkinje cell has a strikingly large dendrite tree, distinctive by the fact that it is flattened into one plane, and covered with dendritic spines. The dendrites also have a characteristic branching structure, originating as stout trunks from the cell body, and terminating in many thin dendritic branches. This is why no one doubts the definition of a cell as a Purkinje cell that has these characteristics. However, it would be better to have a non-eponymous term, such as Large One-Dimensional Spiny, to make this distinctive cell type more explicit.
Other terms for characterizing dendritic branching patterns include the following; Bipolar cells have their dendrites organized into two oppositely directed tufts (although in some cases bipolar refers to a cell with a single dendrite at one pole and the axon at the other). Stellate refers to a pattern in which the dendrites emanate from the cell body in all directions, as the light from a star; this is a common shape, found in the thalamus, cortex, cerebellum, etc.. A distinctive type in cortical regions is the pyramidal neuron, with an apical dendrite arising from the apex of the pyramidal shaped cell body, and basal dendrites emanating widely from the sides of the pyramid.
The presence or absence of spines is an essential characteristic of many cell types. Pyramidal cell dendrites are covered with spines, as are Purkinje cells and olfactory granule cells. The medium spiny neuron in the neostriatum is another example. In contrast, spinal motoneuron dendrites are smooth, as are olfactory mitral cells. Most interneurons in the cerebral cortex also have smooth dendrites. These characteristics have important correlations with pharmacology and function, as we shall see.
In conclusion, dendritic morphology has characteristically been the main basis for identification of cell type. These historical roots must be respected in any attempt to be more precise about how necessary and sufficient these morphological forms are for defining every member of a cell type.
Axon and terminal branching pattern
Most cells in the nervous system give rise to a single axon, which branches locally or leaves the region to connect to distant regions. There are several general axonal branching patterns of the axons. Local branches within the region may terminate in a single layer or in multiple layers. Distant branches may go to a single region or to multiple regions, where they terminate within single or multiple layers. These general characteristics have not found their way into defining cell types.
In contrast, the morphology of an axon terminal has historically been a critical characteristic for defining some cell types, particularly interneurons whose axons branch and terminate near their cell bodies within a brain region. For example, the basket cell of the cerebellum branches and terminates around the initial axonal segment of a Purkinje cells with a large basket-shaped terminal that defines it as a basket-cell, even though its dendritic morphology classifies it as a stellate cell. Similar considerations apply to basket cells in the neocortex and hippocampus. Chandelier cells, whose axons form candelabra-like terminal branches in the neocortex, are another example.
When present, axon terminals such as these can be unambiguous identifiers of a cell type. However, this is limited for most purposes to interneurons. Cells whose axons leave the region of origin usually branch and terminate at a distance, and have to be observed by other means. Exceptions occur in the hippocampus, where dentate granule cells can be seen under the microscope to send "mossy fibers" to terminate within CA3 of the hippocampus, and CA3 pyramidal neurons send Schaffer collaterals to end by "en passant" synapses on the apical dendrites of CA1 pyramidal neurons.
Summary
These appear to be the five main ways of establishing a unique identity for a given nerve cell on the basis of histology alone. Further discussion by ontologists is needed to determine which of the criteria are necessary and which are sufficient!
