Ontogeny of the head of the Pacific hagfish ( Eptatretus stouti , Myxinoidea): development of the lateral line system

Abstract

The head of adult hagfishes (jawless craniates, Myxinoidea) of the family Eptatretidae displays a number of skin grooves of uncertain origin. These grooves have been homologized to the neuromast lines of other craniates, and they are innervated by two ganglionated cranial nerves that have been interpreted as lateral line nerves. The grooves do not, however, contain the compound receptors that are typical of a lateral line (i.e. neuromasts or electroreceptors), and both their development and function have remained enigmatic. To elucidate the embryonic origin of the grooves (which should develop from placodes if they are homologues of the lateral line system), embryos of Pacific hagfish were examined by means of three-dimensional reconstructions from serial sections. Because of the scarcity of specimens of embryonic hagfishes, only two embryos were reconstructed, but these reconstructions clearly show that a number of placodes and placodal derivatives (i.e. sensory ridges, receptor primordia, and cranial ganglia) occur in the head of embryonic eptatretid hagfishes. Some of these placodes correspond to the lens and epibranchial placodes of other craniates, but there are also three other placodes which represent possible homologues of lateral line placodes. The topology of the placodes in this latter group corresponds to the topology of the grooves of adult hagfishes, and we therefore reach three conclusions: (i) that an embryonic lateral line system is present in hagfishes; (ii) that the grooves of adult hagfishes in all probability derive from lateral line placodes; and (iii) that the presence of lateral line placodes is a primitive character of craniates.

Evolution and Ecology of Retinal Photoreception in Early Vertebrates

“…At this time in evolution, the eye-field region of the diencephalon bulged outwards to form lateral 'eye vesicles', which ultimately invaginated to bring the proto-retina into apposition with the proto-retinal pigment epithelium [Walls, 1942]. A primordial lens placode then developed, preventing pigmentation of the overlying skin [Wicht and Northcutt, 1995], resulting in paired lateral photoreceptive organs, which may have resembled the 'eyes' of extant hagfish.…”

Hagfish (cyclostomata, vertebrata): Searching for the ancestral developmental plan of vertebrates

“…These somite-associated cells (which seem to correspond to ventrolateral cell populations differentiating into spinal dorsal root ganglia) and dorsolateral cell populations appeared successively, as in the typical crest cell development found in amniotes. (32,33) Moreover, topographical expression patterns of other regulatory genes such as Pax6, Pax3/7 and SoxE, were similar to those expressed in gnathostome embryos. This suggests that the neural crest of the hagfish is specified as a part of the dorsoventral specification of the neurectoderm, based on the shared molecular mechanisms recognized in vertebrate model animals.…”

“…(12) For example, hagfish have lost eyes, vertebrae, and the lateral line system, although their anlagen appear during development (Fig. 1A).…”

“…(31) However, the dorsal (or lateral) edge of the neural plate at this stage already expressed a Sox9 cognate in the hagfish, a homeobox gene known to function in the specification of the neural crest in other vertebrates. (32)(33)(34)(35) Rostrally, this gene expression is carried over to the delaminated and emigrating crest-like cells, which cluster in intersomitic regions. Further rostrally, similar delaminated cells were also seen under the surface ectoderm, possibly representing the pigment cell precursors.…”

Cephalic sensorial pores in galaxiid fishes from Chile (Osmeriformes: Galaxiidae)

“…), in galaxiid fishes the cephalic pores have been poorly studied (McDowall, 1970(McDowall, , 1971Nelson, 1972Nelson, , 1994. In other fish taxa these structures are a relevant taxonomic character for species identification, determination of intra and interspecific relationships and characteristics of large taxa such as genera, family and suborders (Jakubowski, 1970;Balushkin, 1989Balushkin, , 1996Jansen & Corcoran, 1998;Webb, 1989;Wicht & Northcutt, 1995;Northcutt, 1997;Schaefer & Aquino, 2000;Voronina, 2002;Kasumyan, 2003;Tarby & Webb, 2003).…”