Lewis Wolpert

Positional information and pattern formation

The concept of positional information proposes that cells acquire positional values as in a coordinate system, which they interpret by developing in particular ways to give rise to spatial patterns. Some of the best evidence for positional information comes from regeneration experiments, and the patterning of the leg and antenna in Drosophila, and the vertebrate limb. Central problems are how positional information is set up, how it is recorded, and then how it is interpreted by the cells. A number of models have been proposed for the setting up of positional gradients, and most are based on diffusion of a morphogen and its interactions with extracellular molecules; however, diffusion may not be reliable mechanism. There are also mechanisms based on timing. There is no good evidence for the quantitative aspects of any of the proposed gradients and details how they are set up. The way in which a signaling gradient regulates differential gene expression in a concentration-dependent manner also raises several technical and quite difficult issues. A key feature of positional information being the basis for pattern formation is that there is no prepattern in the embryo.

CELLULAR MOVEMENT AND CONTACT IN SEA URCHIN MORPHOGENESIS

V. Blastula formation . . . . . . . . . . . VI. Changes in shape of the blastula and the gastrula ectoderm . . . ( I ) Llorphopenetic models and their application. . . . . (2) Structural and biochemical basis for the differences in cell contact (3) Dynamic properties of the cell sheets . . . . , . VII. Arm formation . . . . . . . . . . . \-111. Morphogenesis of the mesenchyme, skeleton and archenteron . . ( I ) General features and morphogenetic models. . . . . (2) The primary mesenchyme and the basis of its pattern . . . (3) Development of the skeleton . . . . . . . (4) Gastrulation . . . . . . . . . . (j) The secondary mesenchyme. . . . . . . . (6) Pseudopodal activity, contact paralysis and contact inhibition . . , IX. Early morphogenesis of the archenteron after its invagination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . * . . X. Further morphogenesis of the archenteron and mouth formation . ( I ) Subdivision of the coelom and mouth formation . . . (2) Development of the peristaltic network around the oesophagus

Chapter 6 Positional Information and Pattern Formation

Publisher Summary This chapter discusses that within the conceptual framework of positional information, a new and a simple way of looking at pattern formation may be obtained. In pressing the possibility of universality, the chapter deliberately takes an extreme stand, but at least it serves to counterbalance the special-substance inductive view of pattern formation. Also, in order to show its possible relevance to pattern formation and even cell movement, procrustean view of the data is taken. One of the virtues of the positional information mechanism of pattern formation is that, with the same system for positional information one can generate an enormous number of different patterns, by changing the cells rules for interpretation. Since interpretation will be gene determined, there is little difficulty in seeing how this can be achieved. In fact, the concept of positional information makes excellent use of a central feature of development, that all the cells carry the same genetic 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.

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.

Thresholds in development

Abstract The interpretation of gradients in positional information is considered in terms of thresholds in cell responses, giving rise to cell states which are discrete and persistent. Equilibrium models based on co-operative binding of control molecules do not show true thresholds of discontinuity, though with a very high degree of co-operativity they could mimic them; in any case they do not provide the cells with any memory of a transient signal. A simple kinetic model based upon positive feedback can account both for memory and for discontinuities in the pattern of cell states. The model is an example of a bistable control circuit, and transitions from one state to another may be brought about not only by morphogenetic signals, but also by disturbances in the parameters determining the kinetics of the system. This might explain some aspects of transdetermination in insects. An attempt is made to analyse the precision with which a spatial gradient of a diffusible morphogen could be interpreted by a kinetic threshold mechanism, in terms of the length of the field, the steepness of the concentration gradient, and the intrinsic random variability of cells. It is concluded that it would be possible to specify as many as 30 distinct cell states in a positional field 1 mm long with a concentration span of 103. Mechanisms for reducing the positional error are considered.

Cell shape and cartilage differentiation of early chick limb bud cells in culture.

The onset of chondrogenesis in te embryonic chick is preceded by a prechondrogenic condensation of the prospective cartilage cells. Similarly, in culture, the chondrogenic phenotype is only expressed by limb mesoderm cells plated at densities above confluence. This has led to the proposal that chondrogenic differentiation requires immediate histogenic interactions prior to overt chondrogenesis (Solursh et al., 1978, In Vitro 14, 51-61; and, 1980, Dev. Biol. 78, 141-150). It is noticeable that in both the in vitro situations cells secreting a cartilaginous matrix are rounded in shape. Recently, Glowacki, Trepman and Folkman (personal communication) have shown a dependence of phenotypic expression by mature chondrocytes on cell shape. Cells maintained in a rounded configuration by culturing on a semi-adhesive substratum (poly(HEMA)) synthesised more sulphur-containing extracellular matrix than cells allowed to flatten on normal tissue culture plastic. We have investigated whether there is a similar shape-dependent relationship in the differentiation of chick embryonic mesoblasts into actively secreting chondroblasts. Our results show that a rounded cell shape is conductive to the synthesis of a sulphated matrix.

Specifying Positional Information in the Embryo: Looking Beyond Morphogens

Concentration gradients of small diffusible molecules called morphogens are key regulators of development, specifying position during pattern formation in the embryo. It is now becoming clear that additional or alternative mechanisms involving interactions among cells are also crucial for positional specification.

Positional information and patterning revisited

The concept of positional information proposes that cells acquire positional values as in a coordinate system, which they interpret by developing in particular ways to give rise to spatial patterns. Some of the best evidence for positional information comes from regeneration experiments, and the patterning of the leg and antenna in Drosophila and the vertebrate limb. Central problems are how positional information is set up, how it is recorded, and then how it is interpreted by the cells. A number of models have been proposed for the setting up of positional gradients, and most are based on diffusion of a morphogen and its interactions with extracellular molecules. It is argued that diffusion may not be reliable mechanism. There are also mechanisms based on timing. There is no good evidence for the quantitative aspects of any of the gradients and details how they are set up. The way in which a signalling gradient regulates differential gene expression in a concentration-dependent manner also raises several mechanistic issues.

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.