Shigeru Kuratani

Normal Embryonic Stages of the Chinese Softshelled Turtle Pelodiscus sinensis (Trionychidae)

Abstract We describe the normal embryonic development of the Chinese soft-shelled turtle, Pelodiscus sinensis. We identified 23 stages between the late neurula and hatching, based on chronology and the external morphology of embryos. The anlage of the sensory organs appeared in early stages of the pharyngula, followed by limb buds. The lateral ridge of the carapace, or the carapacial ridge, could be recognized by two weeks of incubation, and the basal structure of the shell was complete by the mid stage (24 days) of development. The hatchling was well pigmented, and the yolk had been entirely absorbed. The developmental stages of P. sinensis were comparable with those of other turtle species. The developmental pattern of P. sinensis resembled basal taxa rather than more derived groups. Some peculiarities were recognized in the development of P. sinensis, including the appearance of cutaneous papillae on the carapace instead of horny scutes. Those features are shared by Carettochelys, possibly the species taxonomically closest to the Trionychids.

Developmental Morphology of Branchiomeric Nerves in a Cat Shark, Scyliorhinus torazame, with Special Reference to Rhombomeres, Cephalic Mesoderm, and Distribution Patterns of Cephalic Crest Cells

Abstract Peripheral nerve development was studied in the cat shark, Scyliorhinus torazame, using whole-mount and sectioned embryos. Nerve fibers were immunohistochemically stained using a monoclonal antibody against acetylated tubulin and, in early embryos, cephalic crest cells were observed by scanning electron microscopy. The initial distribution patterns of crest cells were identical to the typical vertebrate embryonic pattern, in that three crest cell populations were associated with even-numbered rhombomeres, prefiguring the pattern of the branchiomeric nerve roots. In older pharyngula, however, the trigeminal and postotic branchiomeric nerve roots were found to have shifted caudally along the neuraxis: the trigeminal nerve root finally arose from r3, while the glossopharyngeal nerve root arose from the presumptive region r7 of the hindbrain. The shift apparently takes place between the root fibers and the dorsolateral fasciculus. From observing the topographical relationships between the peripheral nerves and other epithelial structures (for example, the otocyst and the mesodermal head cavities - the anlagen of extrinsic ocular muscles), it was assumed that the shift was the result of an epigenetic effect caused by allometric growth of the otocyst, the mandibular cavity, and the spiracle epithelia anchoring the mandibular branch of the trigeminal nerve. It was concluded that the deviated morphological pattern of elasmobranch cranial nerves is a secondary phenomenon caused by the well-developed head cavities. In those animals whose head cavities are degenerated, the original pattern of the cranial nerve-rhombomere assignment, which is intact in lamprey embryos, is retained.

Morphological Characteristics of the Developing Cranial Nerves and Mesodermal Head Cavities in Sturgeon Embryos from Early Pharyngula to Late Larval Stages

Abstract As sturgeons are considered to represent a basal group of Osteichthyes, it is necessary to evaluate their developmental features to understand the evolution, not only of bony fishes, but also of tetrapods in general. Using Besters, commercially established hybrid sturgeons, the neural crest cell distribution pattern, mesodermal epithelium, and peripheral nerves were observed based on whole-mount immunostained and -sectioned embryos, from the pre-hatching embryonic stage to a late swimming larval stage. At the early pharyngula stage, the hindbrain exhibits at least six rhombomeres. These have a typical arrangement of neuroepithelial cells, and segmentally distributed cephalic crest cell populations associated with even-numbered rhombomeres medially, and single pharyngeal arches laterally. The head cavities first arise as a pair of epithelial primordia in the prechordal region. Secondarily, the cavity is subdivided mediolaterally into the premandibular and mandibular cavities. These mesodermal components never affect the segmental pattern of cranial nerve roots as seen in the shark embryo (Kuratani and Horigome, 2000), probably due to the early degeneration of the cavities. The hyoid cavity never appears. As observed in several teleosts, the newly hatched Bester larva possesses extensive neurites in the epidermis, originating from both trigeminal placodes and Rohon-Beard cells. This neurite network diminishes during development, in concordance with the appearance of lateral line nerves. All the epibranchial placodes are seen as focal, HNK-1-positive epidermal thickenings and give rise to inferior ganglia of the branchiomeric nerves. Metameric morphology of the branchiomeric nerve innervation is secondarily disturbed through modification of the head region, involving the expansion of the operculum and modification of the jaw.

Compartments in the lamprey embryonic brain as revealed by regulatory gene expression and the distribution of reticulospinal neurons

The vertebrate neural tube consists of a series of neuromeres along its anteroposterior axis. Between amphioxus that possesses no neuromeres and gnathostomes, the lamprey occupies a critical position in the phylogeny for the origin of the segmented brain. To clarify the rhombomeric configuration of the Japanese lamprey, Lampetra japonica, we injected rhodamine- and fluorescein-labeled dextrans into the larval spinal cord, and retrogradely labeled the reticulospinal neurons. We also isolated prosomere marker genes from the embryonic cDNA library of L. japonica, and performed in situ hybridization on the embryonic brain. Of the genes examined, LjOtxA, LjPax6, LjPax2/5/8, LjDlx1/6, and LjTTF-1 were expressed in clearly demarcated polygonal domains. In the telencephalon, LjDlx1/6, LjPax6, and a putative paralogue of LjEmx were expressed in different domains; the LjEmx paralogue was expressed in the dorsal region, and LjDlx1/6 and LjPax6 in a complimentary fashion of the middle part. These expression patterns implied existence of a tripartite configuration of the lamprey telencephalon similar to that in gnathostomes. All these evidences strongly suggest that the segmental and compartmental architecture of the vertebrate brain was already established before the divergence of agnathans and gnathostomes.

Evolution of the vertebrate jaw: homology and developmental constraints

Abstract In embryonic development of the vertebrate head, neural crest-derived ectomesenchyme contributes to a wide range of tissue types including oro-pharyngeal and ethmoidal cartilages. The evolution of the jaw, therefore, can be viewed as a change of developmental program for specification of the crest cells. Along the anteroposterior axis of the neural crest of amniote embryos, a series of homeobox genes are expressed in a nested pattern, and the jaw-forming mandibular arch receives crest cells expressing no Hox genes and midbrain-derived crest cells that express Otx2. Cognates of these regulatory genes are present in the lamprey, and are expressed in the comparable cell lineages of the embryo. Evolution of the jaw cannot be explained from such shared developmental mechanisms, but rather noncomparable elements have to be sought, if the jaw is truly an evolutionary novelty. By precise comparative morphology and gene expression analyses, a possibility was inferred that ammocoete lips may not be identical to gnathostome jaws.

Development of the Chondrocranium of the Loggerhead Turtle, Caretta caretta

Abstract To better understand the evolution and development of the amniote cranium, it is necessary to examine non-avian, non-mammalian embryos. Herein, development of the chondrocranium in the loggerhead turtle, Caretta caretta, is described based on whole-mount and sectioned specimens. Primitive characteristics were found to be established rather early in development; at these stages, the cranium resembled not only the early amniote chondrocrania, but also those of anamniote embryos. Several characteristics were noted in the late chondrocranium that represent shared, derived characteristics of chelonians. In particular, one amniote-specific characteristic, the tropibasic trabecula, appeared at an intermediate stage of development. In addition, developmental changes in the orbital cartilage were reemphasized, and the morphological significance of the crista sellaris was discussed in terms of the basic architecture of the vertebrate neurocranium.

The neural crest and origin of the neurocranium in vertebrates

Cranium of jawed vertebrates is composed of dorsal moiety that encapsulates the brain, or the neurocranium, and the is called the neurocranium, and the ventral moiety, the viscerocranium, that supports the pharynx. In modern jawed vertebrates (crown gnathostomes), the viscerocranium is predominantly of neural crest origin, and for the neurocranium, the rostral part is derived from neural crest cells, whereas the posterior part from the mesoderm. In the cyclostome cranium, the mesoderm/neural crest boundary of the neurocranium used to be enigmatic, let alone the morphological comparison of neurocranial between two cyclostome groups, lampreys and hagfishes. By examining the hagfish development it has become clear that cyclostomes share a common craniofacial embryonic pattern that is not shared by modern gnathostomes, and cyclostome cranium can be compared among the group as developmental modular units with comparable mesoderm/neural crest boundary within the neuroranium. Also, the dual origin of the jawed vertebrate neurocranium has now turned out to represent a derived condition, and ancestrally, the neurocranium would likely have been predominantly of mesodermal origin. Enlargement of the forebrain and reorganization of the oral apparatus seem to have led to the involvement of the neural crest in the rostral neurocranium.