Polarity and laminar formation of the optic tectum in relation to retinal projection

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<jats:title>Abstract</jats:title><jats:p>The mes‐metencephalic boundary (isthmus) works as an organizer for the tectum, and the organizing molecule may be Fgf8. The region where <jats:italic>Otx2, En1</jats:italic>, and <jats:italic>Pax2</jats:italic> are expressed overlappingly may differentiate into the mesencephalon. The di‐mesencephalic and mes‐metencephalic boundaries are determined by repressive interaction of <jats:italic>Pax6</jats:italic> and <jats:italic>En1/Pax2</jats:italic> and of <jats:italic>Otx2</jats:italic> and <jats:italic>Gbx2</jats:italic>, respectively. The optic tectum is a visual center in lower vertebrates. The tectum and the retina should be regionalized and be positionally specialized for the proper retinotopic projection. Gradient of En2 plays a crucial role in rostrocaudal polarity formation of the tectum. En2 confers caudal characteristics of the retina by inducing ephrinA2 and A5, which are the repellant molecules for the growth cones of temporal retinal ganglion cells. <jats:italic>Grg4</jats:italic> antagonizes the isthmus‐related genes, and is involved in the formation of di‐mesencephalic boundary and tectal polarity formation at an early phase of development. Then, <jats:italic>Grg4</jats:italic> plays a role in tectal laminar formation by controlling the migration pathway. Migration pathway of tectal postmitotic cells changes after E5. The late migratory cells split the early migratory neurons to form laminae h–j of SGFS. <jats:italic>Grg4</jats:italic> is expressed in the ventricular layer after E5, and forces postmitotic cells to follow the late migratory pathway, though retinal fibers terminate at laminae a–f of SGFS. Misexpression of <jats:italic>Grg4</jats:italic> disrupts the lamina g, and in such tecta retinal arbors invade deep into the tectal layer, indicating that lamina g is a nonpermissive lamina for the retinal arbors. © 2004 Wiley Periodicals, Inc. J Neurobiol 59: 48–56, 2004</jats:p>

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