Studies of Temporal and Spatial Peculiarities of Somitogenesis in Fish Embryos

New data were obtained corroborating that somitogenesis is a rhythmic process, in which the time of somite formation is strictly constant. This constant ( _s ) can be considered as a natural unit of developmental biological clock characterizing rhythmic processes. The constant _s can be determined with an exceptional accuracy that has no analogs among the known biological processes. This fact alone suggests that the accuracy of developmental clock is very high. In addition to the constancy of _s , all forming somites have equal linear size along the notochord axis. In the process with strictly constant temporal and spatial factors, time plays the leading role in triggering the formation of new somite. This became clear in studies of twin embryos. Both embryos had the same number of somites but they were shorter than in the normal embryos. Also, we measured the length of head and both segmented and unsegmented caudal parts of the trunk. Combined with the published data on somitogenesis, our results suggest that the previously proposed scheme for the role of developmental clock in embryogenesis predicts: (1) a possible loss of some embryonic stages without serious consequences for subsequent development and (2) periodic switching on/off any embryonic processes (at the molecular, cellular, or supercellular level) with intervals multiple to _s .     

Hypophysis: a new scheme of ontogenetic development

It is usually accepted that hypophysis originates from two ecotodermal primordiums that give rise to its two main parts. The transformation of initial primordiums into definitive hypophysis is regarded as a result of mutual inductive influences between hypothalamus and Rathke's pouch. This scheme was remaining unchanged during many decades. Based upon the arguments that Spemann's organizer, prechordal mesoderm (PCM) and hypophysis are consecutive derivatives that replace each other during ontogeny, the author proposes that the two novel stages preceding the former starting point should be added to traditional scheme of pituitary development. Besides, according to new scheme the PCM (successor of Spemann's organizer) plays the main role in hypophysis development by active stimulation of morphogenesis in adjacent tissues. This leads to formation of Rathke's pouch and infundibulum, to their approaching each other, and to reprogrammin of Rathke's pouch ectoderm into hormonal cells of adenohypophysis. According to new hypothesis role of Rathke's pouch is accessorial and consists in provision of cell material.     

The studies of temporal and spatial characteristics of somitogenesis in fish embryos

New data were obtained corroborating that somitogenesis is a rhythmic process, in which the time of somite formation is strictly constant. This constant (tau s) can be considered as a natural unit of developmental "biological clock" characterizing rhythmic processes. The constant tau s can be determined with an exceptional accuracy that has no analogs among the known biological processes. This fact alone suggests that the accuracy of developmental clock is very high. In addition to the constancy of tau s, all forming somites have equal linear size along the notochord axis. In the process with strictly constant temporal and spatial factors, time plays the leading role in triggering the formation of new somite. This became clear in studies of twin embryos. Both embryos had the same number of somites but they were shorter than in the normal embryos. Also, we measured the length of head and both segmented and unsegmented caudal parts of the trunk. Combined with the published data on somitogenesis, our results suggest that the previously proposed scheme for the role of developmental clock in embryogenesis predicts: (1) a possible loss of some embryonic stages without serious consequences for subsequent development and (2) periodic switching on/off any embryonic processes (at the molecular, cellular, or supercellular level) with intervals multiple to tau s.     

Spemann's organizer--it's origin and derivatives (cellular-tissue and molecular-genetic aspects)

In 1924 H. Spemann and H. Mangold discovered that a piece of the dorsal lip of a blastopore from Triturus cristatus, after transplantation to the ventral side of another embryo, was able to cause the neighbouring tissues to change their fate and participate in the formation of a new embryo. The dorsal lip was termed "the organizer". Since then, for as long as 75 years, attempts have been made to establish the intimate mechanisms of the organizer activity. However, no real advance was achieved in their understanding. Within the last 15 years, genetic and molecular techniques have been vastly improved, to help in tracing the fate of many cell lineages, and in compiling more exactly the fate maps for different parts of the embryo. Using these data, I have attempted to trace the fate of Spemann's organizer after the early gastrula stage. Analysis of data on inductive abilities of the organizer cells, on the use of markers, and on the observation of expression of specific genes allowed to conclude that Spemann's organizer in amphibia and its homologues in other vertebrates too are heterogeneous: they are composed of distinct cell populations able to induce primarity the development of either the head or trunk parts of the embryo. These population, determined to become the head of the trunk organizers still at the blastula stage, may be located either in the single continuous cell layer (as in amphibia and birds) or separated among different tissue germs (as in mammals). When the dorsal-ventral orientation of the embryo is established and the organizer is switched on the very early invaginating cells of the dorsal blastopore lip (in the case of amphibia) move in advance of the entire invaginating mesoderm and by the end of gastrulation occupy the place just in front of the notochord. It is supposed that the early dorsal lip and the prechordal mesoderm (PCM) are one and the same cell population, i.e. during gastrulation Spemann's organizer transfers from the lip of blastopore to the prechordal zone. The PCM seems to play an exclusive role in the formation of a head in vertebrate, because some mutations in genes expressed in the PCM result in the entire head deletion. It is supposed that spreading of differentiating signals from the PCM occurs along the main body axis in both caudal and rostral directions. After the main body plan formation the PCM is replaced by adenohypophysis. This conclusion is drawn not only from the same topology of both these structures, but also from the similarities of a set of specific genetical markers expressed in these, that makes it possible to suppose the existence of deep connections and succession between them. The adenohypophysis seems to arise directly from the PCM, or cells of the ectoderm influenced by the PCM may be subsequently transformed into humoral cells of adenohypophysis. In this interpretation, adenohypophysis and the much earlier established PCM may be considered as derivatives of Spemann's organizer. This inference is supported by the fact that all the three above structures first originate in vertebrates only.     

The biological clock in vertebrate embryogenesis as a mechanism of general control over the developmental organism

The problem of understanding the role of the time factor in embryogenesis is still at the conceptual stage. At the same time, a number of rhythmic processes described in the embryogenesis of animals point to the involvement of a biological clock in this period of ontogenesis. Most of them (biochemical, biophysical, cytological) have been identified during the process of cleavage and have a duration equal to that of a single cleavage division τ₀.     

The peculiarities of larval development of the European smelt Osmerus eperlanus eperlanus (L.)

The larval development of the European smelt Osmerus eperlanus eperlanus from hatching to full resorption of the vitelline sac has been described. On the basis of the morphological and dimensioning characteristics, three stages almost equal in the duration have been distinguished during present period. The duration of the larval stages was measured in relative units τ_s. Two palingeneses were discovered during larval development of the smelt: (1) the presence of the ciliary epithelium in the intestine and (2) plasmatic circulation of the blood without blood cell formation. These features are evidence of the antiquity of the origin of smelt and place them at the base of the suborder Salmonoidei.