One hundred years of positional information.

One mechanism by which spatial patterns of cell differentiation could be specified during embryonic development and regeneration is based on positional information. Cells acquire a positional value with respect to boundaries and then interpret this in terms of a programme determined by their genetic constitution and developmental history. The signals and the molecular basis of such a system have both been rather well conserved. Recent work has shown that cells can respond to quite small differences in the concentrations of molecules whose concentration could provide positional information.

Overexpression of BMP-2 and BMP-4 alters the size and shape of developing skeletal elements in the chick limb.

Bone morphogenetic proteins are members of the transforming growth factor beta (TGF beta) superfamily which are involved in a range of developmental processes including modelling of the skeleton. We show here that Bmp-2 is expressed in mesenchyme surrounding early cartilage condensations in the developing chick limb, and that Bmp-4 is expressed in the perichondrium of developing cartilage elements. To investigate their roles during cartilage development, BMP-2 and BMP-4 were expressed ectopically in developing chick limbs using retroviral vectors. Over-expression of BMP-2 or BMP-4 led to a dramatic increase in the volume of cartilage elements, altered their shapes and led to joint fusions. This increase in volume appeared to result from an increase in the amount of matrix and in the number of chondrocytes. The latter did not appear to be due to increased proliferation of chondrocytes, suggesting that it may result from increased recruitment of precursors. BMP-2 and BMP-4 also delayed hypertrophy of chondrocytes and formation of the osteogenic periosteum. These data provide insights into how BMP-2 and BMP-4 may model and control the growth of skeletal elements during normal embryonic development, suggesting roles for both molecules in recruiting non-chondrogenic precursors to chondrogenic fate.

Debatable issues. Interview by Alain Ghysen..

This paper reports a discussion between Antonio Garcia-Bellido and Lewis Wolpert about a number of questions raised by Alain Ghysen. The questions follow, in reverse order, the subjects dealt with in this issue: first the principles (are there unifying principles of development?), then questions dealing with evolution (why are patterns conserved?) and with the homeotic genes (what is their function?), then the cell biology of development (who is controlling actual morphogenesis?), and the generation and evolution of patterns (what makes development so reproducible and how does it change from one species to another?) and finally about the genetics of cell determination and specification (how does a cell measure its position?). Obviously the discussion did not provide any firm answers to any of these questions. Perhaps more importantly, it provides a vivid picture of two contrasting ways of thinking about developmental problems.

The formation of the feather pattern in chick skin after a proportion of cells have been killed by X-irradiation.

The formation of periodic patterns is of fundamental importance in embryonic development. One of the simplest and most frequently observed patterns is the maintenance of a minimum distance between neighbouring elements, for example between teeth, hair, feathers, digits etc. Theoretical models describing these phenomena have been proposed for feather patterning. However, there has been no detailed quantitative analysis of the relationship between cell population density and feather spacing. To define the relation between these quantities and specifically to test the prediction of a mathematical model, we have examined the formation of the feather pattern after varying proportions of the dermal cells have been killed by X-irradiation. It is known that the development of a feather primordium is normally associated with an increase in cell population density in the dermis. Using X-ray irradiation of the skin in vivo and in vitro, we show that the relation between cell population density and spacing of feather primordia indicates the importance of a threshold number of cells for feather patterning. Moreover, there is a prima facie case for supposing that X-rays act on feather spacing system, reducing the ability of dermal cells to prevent spreading of the pattern. Thus, X-irradiation may have a secondary effect on the spacing of primordia rather than, or as well as, affecting the mechanisms that determine their primary positions.

What is evolutionary developmental biology?

All changes in animal form and function during evolution are due to changes in their DNA. Such changes determine which proteins are made, and where and when, during embryonic development. These proteins thus control the behaviour of the cells of the embryo. In evolution, changes in organs usually involve modification of the development of existing structures--tinkering with what is already there. Good examples are the evolution of the jaws from the pharyngeal arches of jawless ancestors, and the incus and stapes of the middle ear from bones originally at the joint between upper and lower jaws. However, it is possible that new structures could develop, as has been suggested for the digits of the vertebrate limb, but the developmental mechanisms would still be similar. It is striking how conserved developmental mechanisms are in pattern formation, both with respect to the genes involved and the intercellular signals. For example, many systems use the same positional information but interpret it differently. One of the ways the developmental programmes have been changed is by gene duplication, which allows one of the two genes to diverge and take on new functions--Hox genes are an example. Another mechanism for change involves the relative growth rates of parts of a structure.

Anesthesia for glucagonoma resection.

Anesthetic experience with three cases of the resection of glucagonoma, a rare tumor of alpha cells of pancreatic islets, is presented. Marked increases of blood glucagon and glucose levels, with the potential for clinically significant metabolic and myocardial dysfunction, did not occur during anesthesia and surgery. Associated tumors of other endocrine cell types also were absent in the three study patients. Strategies for anticipating and managing other perioperative problems associated with glucagonoma also are discussed.

The social responsibility of scientists: moonshine and morals.

KIE: Two historical cases are used to explore the nature of the scientist's obligations to society on technological issues. The physicist Leo Szilard is praised as a moral scientist and a moral citizen for contributing to the development of the atomic bomb in the Manhattan Project and then arguing against its testing when the danger that Germany might use the bomb against the United States subsided. On the other hand, the scientists, including physicians, who promoted the views of the eugenics movement in Nazi Germany were immoral in not considering the social implications of their scientific conclusions. Wolpert maintains that, while there are no areas that should not be subject to research, the scientist's obligations are to make the reliability of the research clear and to inform the public about its possible ramifications.^ieng

The public's belief about biology.

This short review is concerned with a topic that has been neglected and is still very poorly understood: what the general public think and believe about biology (including health and medicine, and bioethics), and, in particular, about biotechnology.

Stem cells: a problem in asymmetry.

The special property of stem cells is that their development is asymmetric. They give rise both to cells that are identical to themselves and to cells that are different. The mechanism that provides this asymmetry may be intrinsic or extrinsic. Such mechanisms are considered within the context of other systems where asymmetric development occurs. The specification of mating types in yeast provides a clear example of a stem cell system generated intrinsically. In fission yeast it appears that the asymmetry is due to chromosomal differences: this is the only known mechanism for intrinsic asymmetry. While there is good evidence for intrinsic asymmetry in both plants and invertebrates--particularly the nematode--the mechanism is not known. In insects and vertebrates there is no well established example of intrinsic asymmetry if one excludes asymmetric cytoplasmic localization during cleavage of the egg. Asymmetry is thus due to environmental influences. Stem cell systems are usually well structured and the cell's behaviour seems to be position-dependent. This is well established for the stem cells of hydra. By contrast it is claimed that the mammalian haemopoietic system is generated by an intrinsic, asymmetric, probabilistic mechanism--the validity of this view is questioned.

Positional information revisited.

Positional information has been suggested to play a central role in pattern formation during development. The strong version of positional information states that there is a cell parameter, positional value, which is related to position as in a coordinate system and which determines cell differentiation. A weaker version merely emphasises position as a key determinant in cell development and differentiation. There is evidence for boundaries and orthogonal axes playing an important role in positional systems. A positional signal is distinguished from an inductive interaction because the former specifies multiple states, confers polarity, and can act over a long range. A gradient in a diffusible morphogen is just one way of specifying position. There is now good evidence in several systems for substances which may be the morphogen for positional signalling. The product of the bicoid gene in early Drosophila development is the best prospect. Retinoic acid is unique in its ability to alter positional value and may also be a morphogen. The best evidence for positional value, a concept fundamental to positional information, remains a biological assay based on grafting. The idea of positional value uncouples differentiation and position, and allows considerable freedom for patterning. It is not clear whether positional value or differentiation involves a combinatorial mechanism. Interpretation of positional information remains a central problem. There is good evidence that cells can respond differentially to less than a two-fold change in concentration of a chemical signal. It may be that interpretation involves listing the sites at which a particular class of cell differentiation will occur. The problem is made less severe when blocks of cells are specified together as in mechanisms based on an isomorphic prepattern. Isomorphic prepatterns could establish repeated structures which are equivalent and which are then made non-equivalent by positional information. This would enable local differences to develop. The combination of these two mechanisms may be wide-spread. There is evidence that positional signals within a single animal and in related animals are conserved. It is not clear just how wide this conservation is, but it is at phylotypic stages, rather than in eggs, that similarity might be expected. It is nevertheless impressive that the polar coordinate model can be applied to regulation in systems as diverse as insects, vertebrates and protozoa. The molecular basis of positional signalling is just becoming accessible; the molecular basis of positional value is still awaited. A brief personal history of positional information is provided in an appendix.