Postnatal development of primary visual projections in the tammar wallaby (Macropus eugenii)

Abstract

The time course and pattern of retinal innervation of primary visual areas was traced in pouch-young wallabies. Tritiated proline was injected into one eye of animals ranging in age from 1 to 72 days after birth. These results are compared to the 11 primary visual areas found in the adult wallaby, seven of which receive binocular input while four are monocular. At birth retinal ganglion cell axons have not reached any visual areas. Two to 4 days after birth, all of the axons are crossing to the contralateral optic tract. Nine to 12 days after birth axons begin to invade the contralateral lateral geniculate nucleus, the superior colliculus, and the medial terminal nucleus. Twenty to 21 days after birth, ipsilateral axons invade the lateral geniculate nucleus and superior colliculus. The contralateral projection precedes the ipsilateral projection in all binocular visual areas. By 25 days, ipsilateral and contralateral afferents share common territory in the lateral geniculate nucleus; however, afferents from each eye are initially concentrated in appropriate areas. Between 52 and 72 days, afferents to the dorsal lateral geniculate nucleus are gradually segregated into nine terminal bands. Four are contralateral while five are ipsilateral. By 72 days, the ipsilateral component to the superior colliculus is clustered beneath the contralateral projection a deeper layer. Projections to four monocular visual areas--lateral posterior nucleus, dorsal terminal nucleus, lateral terminal nucleus, and nucleus of the optic tract--are established later than binocular visual areas, except the suprachiasmatic nucleus. The suprachiasmatic nucleus is the last to be bilaterally innervated even though it is situated closest to the optic chiasm. At the light microscope level a mature pattern of visual development is emerging by 72 days, although the eyes do not open until 140 days.