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Guidepost cells are cells which assist in the subcellular organization of both neural axon growth and migration.[1] They act as intermediate targets for long and complex axonal growths by creating short and easy pathways, leading axon growth cones towards their target area.[2][3]
| Guidepost cells | |
|---|---|
| Anatomical terminology |
In 1976, guideposts cells were identified in both grasshopper embryos and Drosophila.[4][5][6][7] Single guidepost cells, acting like "stepping-stones" for the extension of Ti1 pioneer growth cones to the CNS, were first discovered in grasshopper limb bud.[4][6] However, guidepost cells can also act as a group.[4] There is a band of epithelial cells, called floor-plate cells, present in the neural tube of Drosophila available for the binding of growing axons.[4] These studies have defined guidepost cells as non-continuous landmarks located on future paths of growing axons by providing high-affinity substrates to bind to for navigation.[2]
Guidepost cells are typically immature glial cells and neuron cells, that have yet to grown an axon.[2][4][8] They can either be labeled as short range cells or axon dependent cells.[2]
To qualify as a guidepost cell, neurons hypothesized to be influenced by a guidance cell are examined during development.[9] To test the guidance cell in question, neural axon growth and migration is first examined in the presence of the guidance cell.[9] Then, the guidance cell is destroyed to further examine neural axon growth and migration in the absence of the guidance cell.[10][9] If the neuronal axon extends towards the path in the presence of the guidance cell and loses its path in the absence of the guidance cell, it is qualified as a guidepost cell.[9] Ti1 pioneer neurons is a common example neurons that require guidepost cells to reach its final destination.[6][9] They have to come in contact with three guidepost neurons to reach the CNS: Fe1, Tr1, and Cx1.[6][9] When Cx1 is destroyed, the Ti1 pioneer is unable to reach the CNS.[6][9]
Lateral olfactory tract
The lateral olfactory tract (LOT) is the first system where guideposts cells were proposed to play a role in axonal guidance.[2] In this migrational pathway, olfactory neurons move from the nasal cavities to the mitral cells in the olfactory bulb.[2] The mitral primary axons extend and form a bundle of axons, called the LOT, towards higher olfactory centers: anterior olfactory nucleus, olfactory tubercle, piriform cortexr, entorhinal cortex, and cortical nuclei of the amygdala.[2] "Lot cells", the first neurons to appear in the telencephalon, are considered to be guideposts because they have cellular substrates to attract LOX axons.[2] To test their role in guidance, scientists ablated lot cells with a toxin called 6-OHDA.[2] As a result, LOT axons were stalled in the areas where lot cells were destroyed, which confirmed lot cells as guidepost cells.[2]
Entorhinal projections
Cajal-Retzius cells[11] are the first cells to cover the cortical sheet and hippocampal primordium, and regulate cortical lamination by Reelin.[2] In order to make connections with GABAergic neurons in different regions of the hippocampus (stratum oriens, stratum radiatum, and inner molecular layer), pioneer entorhinal neurons make synaptic contacts with Cajal-Retzius cells.[2] To test their role in guidance, scientists (Del Rio and colleagues) ablated Cajal-Retzius cells with 6-OHDA.[2] As a result, entorhinal axons did not grow in the hippocampus and ruled Cajal-Retzius cells as guidepost cells.[2]
Thalamocortical connections
Perirecular cells (or internal capsule cells) are neuronal guidepost cells located along the path of creating the internal capsule.[2] They provide a scaffold for corticothalamic and thalamocortical axons (TCAs) to send messages to the thalamus.[2] There are transcription factors associated with perirecular cells: Mash1, Lhx2, and Emx2. When guidepost cells are mutated with knock out expressions of these factors, the guidance of TCAs are defected.[2]
Corridor cells are another set of guidepost cells present for TCA guidance.[2] These GABAergic neurons migrate to form a "corridor" between proliferation zones of the medial ganglionic eminence and globus pallidus.[2] Corridor cells provide TCA growth through MGE-derived regions.[clarification needed] However, the Neurgulin1 signaling pathway needs to be activated, with the expression of ErbB4 receptors on the surface of TCAs, for the connection to occur between corridor cells and TCAs.[2]
Corpus callosum
There are subpopulations of glial cells that provide guidance cues for axonal growth.[2] The first set of cells, called the "mid-line glial zipper", regulate the midline fusion and guidance of pioneer axons to the septum towards the contralateral hemisphere.[2][7] The "glial sling" is a second set, located at the corticoseptal boundary, which provide cellular substrates for callosal axon migration across the dorsal midline.[2][7] The "glial wedge" is made up of radial fibers, secreting repellent cues to prevent axons from entering the septum and positioning them towards the corpus callosum.[2][7] The last set of glial cells, located in the induseum griseum, control the positioning of pioneer cingulate neurons in the corpus callosum region.[2]
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