Development of head and trunk mesoderm in the dogfish, Scyliorhinus torazame: I. Embryology and morphology of the head cavities and related structures

Abstract

Vertebrate head segmentation has attracted the attention of comparative and evolutionary morphologists for centuries, given its importance for understanding the developmental body plan of vertebrates and its evolutionary origin. In particular, the segmentation of the mesoderm is central to the problem. The shark embryo has provided a canonical morphological scheme of the head, with its epithelialized coelomic cavities (head cavities), which have often been regarded as head somites. To understand the evolutionary significance of the head cavities, the embryonic development of the mesoderm was investigated at the morphological and histological levels in the shark, Scyliorhinus torazame. Unlike somites and some enterocoelic mesodermal components in other vertebrates, the head cavities in S. torazame appeared as irregular cyst(s) in the originally unsegmented mesenchymal head mesoderm, and not via segmentation of an undivided coelom. The mandibular cavity appeared first in the paraxial part of the mandibular mesoderm, followed by the hyoid cavity, and the premandibular cavity was the last to form. The prechordal plate was recognized as a rhomboid roof of the preoral gut, continuous with the rostral notochord, and was divided anteroposteriorly into two parts by the growth of the hypothalamic primordium. Of those, the posterior part was likely to differentiate into the premandibular cavity, and the anterior part disappeared later. The head cavities and somites in the trunk exhibited significant differences, in terms of histological appearance and timing of differentiation. The mandibular cavity developed a rostral process secondarily; its homology to the anterior cavity reported in some elasmobranch embryos is discussed.

A new heart for a new head in vertebrate cardiopharyngeal evolution

“…Cyclostomes (hagfish and lampreys 49–52 ) lack some of these muscles (for example, the cucullaris group), but like some chondrichthyans (Selachii and Holocephali) they possess an additional, seventh group of head muscles: epibranchial muscles, which are derived from anterior somites 53 . Thus, extraocular, branchiomeric, and both hypobranchial and epibranchial somite-derived muscles were integral parts of the heterogenous head musculature of early vertebrates 54–57 (Fig. 3).…”

Insect morphology in the age of phylogenomics: innovative techniques and its future role in systematics

“…Nagashima et al . ; Adachi & Kuratani ; Tokita et al . ) and will also be established in the entomological laboratory of R. Machida (University of Tsukuba) in the near future (Y Nakagaki & R Machida, pers.…”

“…This technique is not widely used in Japan, but some morphological studies were published by Japanese authors (e.g. Nagashima et al 2009;Kaji et al 2011;Adachi & Kuratani 2012;Tokita et al 2012;Matsumura et al 2013), even though only a few of them study insects, and some of them in collaboration with European researchers. Nagashima et al (2009) used free software (imageJ) for 3D reconstructions to illustrate the development of turtles.…”

Developmental expression and evolution of muscle‐specific microRNAs conserved in vertebrates

“…The cloudy catshark (Scyliorhinus torazame) embryos and tissues were collected as described by Adachi and Kuratani (2012). Embryos were staged according to Ballard et al (1993).…”