Primary optic pathways in the echidna, Tachyglossus aculeatus: An experimental degeneration study

Abstract

The retinofugal axons of Tachyglossus aculeatus were found to project to two diencephalic nuclei, disparate in size and location and designated provisionally as LGNa and b, and to the pretectum and superior colliculus. The primary optic pathways of the echidna are crossed except for a few fibers which pass to the ipsilateral large diencephalic nucleus (LGNa). Further work is required before the homologies of the two diencephalic nuclei can be determined. In addition, entirely crossed projections were also found to a large number of optic nuclei including a medial terminal nucleus, four cell groups interstitial to the optic tract, and a nucleus located within the brachium of the superior colliculus which may represent a dorsal terminal nucleus. The latter nucleus may, however, actually be the nucleus of the optic tract which is displaced from its usual close association with the prctectal area. Only an inferior fasciculus of the accessory optic tract was found.In recent years the primary optic pathways of therian mammals, both marsupials and placentals, have received a great deal of attention from neuroanatomists. Experimental studies of retinal projections have been performed on representatives of the Orders Marsupialia (Bodian,

The accessory optic system: basic organization with an update on connectivity, neurochemistry, and function

“…Echidna and the duckbilled platypus show considerably more features resembling reptiles than either marsupials or eutherian mammals (Hopson, 1969;Hayhow, 1971, 1972). Campbell and Hayhow (1971) found that echidna has an AOS consisting solely of the inferior fasciculus (IF) and the MTN. In the duckbilled platypus, Campbell and Hayhow (1972) identified the MTN but were able to see only diffuse fibers representing an IF.…”

“…In the duckbilled platypus, Campbell and Hayhow (1972) identified the MTN but were able to see only diffuse fibers representing an IF. Campbell and Hayhow (1971) described an AOS in echidna (Table 1) Among marsupials, Didelphis (the opossum) has a worldwide distribution. Its AOS exhibits a well developed IF, and like many mammalian species, a typical SF divisible into SFa, SFm and SFp tracts (Lent et al, 1976;Royce et al, 1976).…”

Phylogeny through Brain Traits: Fifteen Characters Which Adumbrate Mammalian Genealogy

“…In the anterior colliculus (optic tectum) of frogs [20], our reptiles and birds [see also 23], and two monotremes [3,4], the enter ing fibers of the optic tract are in the upper most layer. We consider this state as primi tive, 0.…”

The thalamus of the monotremes: cyto- and myeloarchitecture and chemical neuroanatomy

“…The ventral lateral geniculate nucleus of the echidna is not only far less significantly displaced, but also reaches up the medial side of the dorsal thalamus to the superior colliculus. Campbell & Hayhow (1971 called the dorsal and ventral lateral geniculate nuclei of the echidna and platypus LGNb and LGNa, respectively. Since LGNb underwent retrograde degeneration after ablation of the visual cortex whereas LGNa did not (Welker & Lende 1980), LGNb was regarded as equivalent to the dorsal and…”

“…In two of the three living monotremes, the platypus and short-beaked echidna, the cytoarchitecture of the thalamus has been described as being rather monotonous, the nuclear divisions being less evident than in marsupials and eutherian mammals. The monotreme thalamus as a consequence has come to be regarded as less differentiated than that of other mammals (Ziehen 1897;Hines 1929;Abbie 1934;Lende 1964;Campbell & Hayhow 1971Welker & Lende 1980;Ulinski 1984). The thalamus of the third living monotreme species, the rare long-beaked echidna, has not been examined.…”