The model of “three signals” was proposed later with the aim of taking into account a greater number of experimental facts that had been discovered (Slack, 1983 Smith, 1989 Christian and Moon, 1993a). This point of view about the existence of two basic inducers and about the possibility to obtain very different types of tissue and cellular differentiation by mixing two inductors in various proportions retains support today ( Saxen, 1997).
It was supposed that both inducers may act jointly in various concentrations and form the complete spectrum of CNS derivatives ( Saxen and Toivonen, 1962). Subsequent efforts in the 40′s–60′s led to the formation of the “double-gradient hypothesis” according to which there are two basic inducing structures “neural” and “mesodermal”. These proved that different tissues from adult organisms of various species of animals had unequal inducing effects giving rise to different regions of the nervous system. In parallel with this initial period of chemical studies, investigations into the tissue specifity of inducing agents had begun. However, the studies reached a deadlock on account of weakness in the available microchemical methods which existed at that time (reviews: see Saxen and Toivonen, 1962 Saxen, 1997). During the 10 following years the leading scientists of the time studied a tremendous number of different substances in order to ascertain the chemical nature of inducers. Since Spemann's group ( Bautzmann et al., 1932) showed that dead tissues can also act as inducers and that the inductive signals can be transferred in cell-free conditions, it has inferred that inducing signals are of a chemical nature. He soon after demonstrated that the fragment of dorsal lip taken from an early gastrula induced anterior neural structures, whereas the same region from a late gastrula led to the formation of caudal neural structures and that's why Spemann is credited with distinguishing between a “head” and “trunk” organizer respectively ( Spemann, 1938 Hamburger, 1988). This latter process was initially called the primary embryonic induction and is now more properly called neural induction.Ī certain degree of regional differences among induced neural structures had already been observed by Spemann himself: some of them represented more cranial derivatives of CNS, others were more caudal ones. The formation of a neural plate which gives rise to the central nervous system (CNS) was considered to be a result of an inductive influence from the underlying mesoderm (organizer property). Today researchers usually call it the Spemann's organizer. The dorsal lip, the influence of which causes the development of a new embryo, had been termed the organizer. The participation of a dorsal blastopore lip in the programme of development of a new embryo may be regarded in three significant aspects: 1/Formation of axial structures and establishment of the plane of bilateral symmetry 2/Redetermination of fate for the party of host cells and their inclusion in the structures induced by the transplant 3/Induction of the nervous system. In another experiment, when the transplant was taken from a lightly pigmented embryo of Triturus cristatus and transferred to a heavily of pigmented Triturus vulgaris, it was shown that the secondary neural structures originated mainly from cells of the recipient. As a result, a secondary complex of axial organs including head structures was formed at the place of a transplant. Recent studies into the history of this discovery ( Sander, 1993) have detected that the decisive experiment had been made by Hilda Mangold in 1921 when she transplanted a piece of dorsal lip of the ring-like blastopore of a Triturus embryo to the ventral side of another embryo at the same stage. They had observed that after transplantation of a dorsal blastopore lip of the early gastrula to the ventral side of another embryo a secondary embryonic axis developed on this side.
Mangold discovered the so-called “organizer” in amphibian embryos ( Spemann and Mangold, 1924). Sometimes it seems that to the end of XX century the embryologists are further from the original goal to explain how the development of an organism is accomplished than it seemed at the beginning of the century when H. Up to now researches are far from understanding fully the mechanisms which realize the embryonic development of vertebrate animals. The researches of many commentators have already thrown much darkness on the subject, and it is probable that, if they continue, we shall soon know nothing at all about it.
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