Taylor JS (1991), The early development of the frog retinotectal ...

XB-ART-25210

Development 1991 Jan 01;Suppl 2:95-104.

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The early development of the frog retinotectal projection.

Taylor JS .

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The guidance of retinal ganglion cell axons has been investigated in embryos of the frog Xenopus. During the initial development of the brain a series of axon tracts are laid down forming a basic 'scaffold' or framework. Retinal axons grow through one of these tracts, the tract of the post-optic commissure (tPOC). This is the only tract that extends through the rostral part of the brain at these early stages of development. The origin and development of the tPOC has been studied using antibodies which label neurons at their earliest stages of differentiation. The first sign of the tPOC is a chain of neurons which differentiate simultaneously in the caudolateral part of the diencephalon. Axons from these neurons grow the short distance between adjacent cells interlinking the chain to form a descending tract. A series of other axon projections are then added to the tPOC, each of which is segregated into a particular subregion of the tract. Retinal axons are added to the tract approximately 18 h after its formation. They grow in the sub-pial part of the tract and always occupy the rostral-most edge. Retinal axons follow the tract to the region of the developing tectum where they leave, turn dorsally, and terminate. The reliance of retinal axons on this pre-existing pathway has been demonstrated by experimentally altering the course of the tPOC during its early development. The caudo-lateral wall of the diencephalon has been rotated through 90 degrees at a stage just before the tPOC neurons differentiate. Confirmation of the predicted alteration in the course of the tPOC has been made using immunocytochemistry. In such manipulated brains, retinal axons maintain their strong affinity for the rostral edge of the tPOC, following its altered course through the diencephalon.

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Species referenced: Xenopus
Genes referenced: b3gat1l

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	Fig. 1. (A) Ventral view of a stage 34 brain with HRP-labelled axons from the left eye leaving the optic stalk (OS) and passing through the optic chiasm (OC). Growth cones of these axons are clearly visible (arrowheads). Scale bar, 50,um. (B) Lateral view of a stage 39 brain showing the course of HRP-labelled retinal axons in the optic tract (OT). The retinal axons follow a well defined course through the diencephalon heading directly to the optic tectum (T), where the retinal axons arborise. Scale bar, 100um.
	Fig. 2. (A) Semi-thin section, cut transversely through the optic tract of a stage 35 brain, showing the HRP-labelled retinal axons (arrowhead), fasciculating with the unlabelled axons of the tPOC. The retinal axons are confined to the rostral-most edge of the tPOC. Scale bar, 10 ^m. (B) Electronmicrograph of HRP-labelled retinal axons and their growth cones in the rostral part of the tPOC in a stage 39 brain. There is no structural barrier separating the retinal axons from other components of the tPOC. The filopodia of the growth cones are in contact with other axons of the tract and with the end-feet of neuroepithelial cells at the pial margin (P). N, nucleus of neuroepithelial cell. Scale bar, 5 um.
	Fig. 3. (A) Ventral view of a stage 24 embryo showing a chain of early differentiating cells (arrows) labelled with antigoldfish GFAP. The line of cells run from the rostral end of the brain, around the optic stalks (os) and through the diencephalon. E, eye cup; R, rostral. Scale bar, 100 j.im. (B) A slightly squashed, stage 28 brain, seen in lateral view showing the early axon pathways labelled with HNK-1. The chain of cells in the lateral diencephalon (arrows) has become inter-linked with axons running from a prominent cluster of cells in the midbrain flexure (mf) around to the rostral end of the brain (R). The tract is continuous caudally with the ventrolateral tract of the hindbrain (vlt). At the mid-line, axons cross from one tPOC to the other forming the post-optic commissure (poc). Axons from the pineal join the tPOC rostral to the flexure via the dorsoventral diencephalic tract (dvdt). Further caudally, the axons of the trigeminal and dorsal sensory tracts are apparent (V) and commissural fibres (com) can be seen in the ventral part of the hindbrain. Scale bar, 100urn.
	Fig. 4. Semi-thin horizontal sections through the ventral diencephalon of stage 35 brains in which the olfactopeduncular, ascending hindbrain, and retinal components of the tPOC have been labelled with HRP. (A) Olfactory axons enter the tPOC at an oblique angle and cross through the other axons of the tPOC to reach their normal position at the caudal edge of the tract. (B) Hindbrain ascending axons lie in the central region of the tract where they have a loose distribution. (C) Retinal axons occupy the rostral edge of the tract. R, rostral; tPOC, tract of the post-optic commissure; OS, optic stalk. Scale bars, 10 um.
	Fig. 5. (A) HRP-labelled retinal axons which have grown directly towards the position of a rostrally transposed tectal rudiment in a stage 39 brain, P, pineal, T, transposed tectum. (B) HRP-labelled axons from a grafted eye, which having entered the brain (arrow) just caudal to the tectum (T), turned caudally and followed a well defined route through the hindbrain (Hb). (C) HRP-labelled axons, from a grafted eye, which have entered the brain just rostral to the tectum (arrow). Rather than terminating, these axons tipped with growth cones (arrowheads) have ignored the tectum (T) and have grown the 'wrong-way' down the optic tract. P, pineal. Scale bars, 100;um.
	Fig. 6. HNK-1 staining of the tPOC in a stage 28 brain which has had the lateral part of the diencephalon (graft indicated by arrows) rotated by 90° in an anti-clockwise direction. The stained axons and cells of the tPOC that lie within the graft can be seen to lie perpendicular to the normal course of the tPOC (arrowheads). P, pineal; dvdt, dorsolateral diencephalic tract; OS, hole left by removal of eye cup and optic stalk; MF, midbrain flexure. Scale bar, 100 um.