The interpretation of positional information can provide the basis for pattern formation in limb morphogenesis. The gradient in positional information along the antero-posterior axis, which is specified with respect to a localised boundary region, can be modified by grafting this region to successive positions along the axis. The pattern of digits obtained is consistent with a model based on diffusion of a labile morphogen and is thus similar to models proposed for the development of pattern in invertebrates.

Animals↗

PubMed1975

Morphogenesis of the vertebrate limb.

Pattern formation in both the developing and the regenerating vertebrate limb can be understood in terms of the assignment of positional values to cells. The positional value may then control the selection of a course of cytodifferentiation as, say, muscle or cartilage. It is proposed that the proximodistal coordinate of positional value is extablished by a mechanism based on autonomous change with time in a 'progress zone' at the tip of the limb bud. The anteroposterior coordinate seems to be specified by a positional signal from an orgainzing region. Only cells in the progress zone respond to this signal. The change of character in the progress zone may be linked to cell division. It seems that each skeletal rudiment may initially be the same length, corresponding to the amount of tissue that leaves the progress zone during one cell cycle. The lengths on the skeletal elements are already specified shortly after exit from the progress zone, and are capable of very little regulation. The positional value may determine the pregramme of later growth. The relevance of the progress zone model to epimorphic regulation, and amphibian limb regeneration in particular, is discussed.

Animals↗

PubMed1975

Control processes in development: Pattern formation in chick limb morphogenesis.

Explore the source record for details and available documents.

Animals↗

PubMed1975

Towards a theory of development.

A theory of development would effectively enable one to compute the adult organism from the genetic information in the egg. The problem may be approached by viewing the egg as containing a program for development, and considering the logical nature of the program by treating cells as automata and ignoring the details of molecular mechanisms. It is suggested that development is essentially a simple process, the cells having a limited repertoire of overt activities and interacting with each other by means of simple signals, and that general principles may be discerned. The complexity lies in the specification of the internal state which may be described in terms of a gene-switching network. Pattern formation is a central feature in development; it is the process whereby states are assigned to the cells according to their position. such that the appropriate type of cytodifferentiation is selected from the repertoire. The morphogenesis of the chick limb is briefly discussed. Genetic networks that account for such features as memory, competence and interpretation of positional information are given. The question of how these component parts are organized into a complete control system for development is posed as a problem for future study.

Aging↗

PubMed1975

Positional information and pattern regulation in Hydra. Dynamics of regions at the boundary.

The dynamics of boundary regions have been investigated mainly by axial grafting and with the emphasis on the head end. The time to resist inhibition of head-end formation and the ability to inhibit head-end formation have been assayed under a variety of conditions. The times increase with distance from the head end. The times required by a boundary region to aquire the inhibitory properties of a head end are longer than those required to aquire resistance to inhibition. Determination of a head end is faster at a cut surbface and at higher temperatures. The results are discussed in terms of a model involving two gradients. Some anomalous results are reported.

Animals↗

PubMed1975

Positional information and pattern formation in Hydra. Dynamics of regions away from the boundary.

The dynamics of regions away from the head-end boundary have been investigated by a variety of assays, and the changes appear to be slow. If, for example, a head end is grafted onto a peduncle to give H/56F, region 5 does not become a region 1 with 72 h. Such results support a gradient type model, and exclude a head end inducing a region 1 in the tissue adjacent to it. The changes can be interpreted if the positional value were effectively slowly diffusible: about 10 times more slowly than the positional signal. The results also have implications for studies on polarity reversal.

Animals↗

PubMed1975

Polarity reversal in hydra by oligomycin.

Intact hydra treated for 24 h with oligomycin gradually lose their head structures and the distal ends form feet. Grafting experiments show that the distal ends of treated animals induce proximal structures.

Animals↗

PubMed1975

Positional information and pattern regulation in hydra: enzyme profiles.

Certain key enzymes of alternative pathways of glucose metabolism, of amino acid metabolism and of redox systems have been measured in hydra and this profile compared with mammalian differentiated tissues with a view to locating pathways of specific importance in hydra. There was a marked constant proportionality in the major part of the enzymes investigated, the profile suggested a metabolic pattern geared to utilization of amino acids as a carbon source for biosynthesis and energy production and to the production and conservation of pyruvate. The importance of conversion to ionized forms was noted. The most notable specific proportion changes were the exceptionally low lactate dehydrogenase, malic enzyme and the relatively high citrate synthase. The proximal-distal gradients in hydra were examined and these gradients suggested a switch to a more anaerobic type of metabolism and an elevation of the pentose phosphate pathway as the basal region was approached. Measurements of the formation of 14CO2 from specifically labelled glucose provided additional evidence for the functional activity and polarity of the pentose phosphate pathway in hydra. The effect of oligomycin, which can reverse polarity in hydra, had a significant effect on gradients of enzymes eliminating all except that observed for G6P dehydrogenase. The profile suggested a movement towards a more anaerobic type of metabolism, in keeping with the known biochemical action of this inhibitor. It is suggested that redox states and/or phosphorylation states may be featured in the positional information of cells in hydra.

Alanine Transaminase↗

PubMed1974