The control of size and shape in skeletal morphogenesis

A workshop sponsored by the European Science Foundation, Fitzwilliam College, Cambridge (U.K.), 3-5 September 1998

This workshop, organized by Michael Akam (University of Cambridge) brought together developmental biologists, paleontologists, morphometricians, and quantitative geneticists to discuss how size and shape are determined by cellular and developmental processes, and how they can be analyzed to shed further light on those processes. The focus on skeletal characters (both of the vertebrate endoskeleton and the insect exoskeleton) was mostly set by the abundance of information on development in these systems. This information encompasses the whole spectrum from the cellular and molecular level to whole-organism integration.

The first session emphasized studies at the cellular level. Ernst Hafen (University of Zurich, Switzerland) presented studies of the control of cell size, or more properly, cell growth. His results from the compound eye of Drosophila indicate that cell growth is regulated in a cell-autonomous manner, and independently from the cell cycle (cell division). However, there are many opportunities for cross-talk between the pathways regulating cell growth and division, resulting in a complex network of interactions that can regulate cell size and number in an organ. Maria Leptin (University of Cologne, Germany) reported her work on cell shape in Drosophila gastrulation. The movement of cells into the interior of the embryo during gastrulation occurs by marked cell shape changes that occur in a precisely timed and localized manner. The molecules that perform the movements (including actin and myosin) are widely distributed across the embryo, but only become active in specific locations when triggered by particular signals. Martin Raff (University College London) spoke on the control of cell number. Cell number is controlled jointly by both cell division and cell death, but the role of cell death has often been underestimated (there can be remarkably large mortality rates under natural circumstances). Both these processes depend on external signals, which either stimulate cell division or prevent cell death, and normally ensure that cells don't grow in the wrong place or at the wrong place. In the last talk of this session, Denis Duboule (University of Geneva, Switzerland) described research on the patterning of the whole body and the limbs by Hox genes in mice.

The second session considered how growth is coordinated to build functional forms. First, Juan Hurle (University of Cantabria, Spain) explained the way digit formation is regulated by a network of signaling interactions involving, among other players TGFs and BMPs. These factors establish where cell condensations form and develop into cartilage and ultimately bone, or here cell death occurs to form the interdigital regions. Steve Cohen (EMBL) reported on axis formation in Drosophila leg development, where a well-studied cascade of signaling interactions establishes the anterior-posterior, dorsal-ventral, and proximal-distal polarities. Marty Cohn (University of Reading, U.K.) reviewed limb specification in vertebrates and presented his work on the developmental basis of limb loss in snakes. Interestingly, the failure to form fore- and hindlimbs has two different causes in pythons: changes in Hox gene expression (relative to vertebrates that have legs, linked to the extension of the thoracic region in snakes) are responsible for the absence of forelimbs, whereas the hindlimb buds are initiated normally, but fail to develop into legs. Mike Coates (University College London) showed how developmental data can be integrated with information from fossils and from phylogenetic analyses to set up and test evolutionary models. David Stern (Cambridge University) reviewed what is known of the mechanisms that regulate the growth of organs in insects, and thus establish their shape (relative sizes of organs) and its dependence on overall size (allometry). He suggested that a mechanism measuring overall body size and signaling via hormones are most likely to be responsible.

The third session started with a review of early vertebrate phylogeny by Philippe Janvier (MNHN, Paris). He outlined the relationships among the recent hagfishes and lampreys, fossil primitive jawless vertebrates ("ostracoderms") and the jawed vertebrates, and presented intriguing evidence that knockout mice for the Otx1 gene display abnormalities in a set of characters that appeared together in the fossil record of early vertebrates. The suggestion that this reflects an evolutionary leap certainly warrants further testing from paleontological, phylogenetic, and developmental perspectives.

The rest of the session dealt with morphometrics and quantitative genetics. First Paul O'Higgins (University College London) presented an outline of the landmark-based morphometric methods. His examples concerned skull growth in monkeys and growth of the C4 vertebra in mice and rats. He made some cautionary remarks about isolated analyses of individual shape variables (e.g. principal components from a Procrustes analysis), and emphasized that they should be interpreted jointly. That said, his examples showed some rather stunning examples of the power of the landmark-based methods for allometric analyses. In my own talk, I reviewed the biomathematical foundations of allometry and the classical methods of multivariate allometry. I emphasized the potential for combining this morphometric approach with experiments using a study of transgenic mice as the primary example (Shea et al., Genet. Res., 1990). Hazel Smith (University College London) reported on her work in collaboration with Linda Partridge (also UCL) on quantitative genetics of body size in Drosophila. These studies range from large-scale studies of continent-wide latitudinal gradients to detailed experiments using artificial selection in the lab. Much of the natural genetic variation can be ascribed to relatively small regions of the genome, and it may even be possible to pinpoint particular genes. Another intriguing result is that the developmental mechanism (change in cell size or number) responsible for the response to selection on overall wing size appears to depend on the direction of selection. Sandro Cavicchi (University of Bologna, Italy) expanded on this theme by reporting on the results of his long-term studies in lab populations of Drosophila. He found that the division of the Drosophila wing into developmental subunits (the anterior and posterior compartments, and the different intervein regions) is also reflected in the results of selection studies, and concluded that these developmental units are also evolutionary units. The overview of his work also demonstrated how a variety of morphometric techniques, applied to the same system, can highlight different aspects of the variation in the data. The final talk, by Michel Baylac (MNHN, Paris, France) provided a variety of applications of landmark morphometrics to variation in insect wings. The examples included interspecific variation in the honey bee and related species, allometry in Drosophila simulans, and left-right asymmetry in flies. Overall, this session provided a good overview of most of the spectrum of morphometric methods currently in use, and a wide range of applications to biological problems concerning development in one way or another.

The rest of the workshop was devoted to discussion, during sessions as well as over meals. I must admit that I was pleasantly surprised by the positive response of the developmental biologists to the morphometric studies. Evidently, they realized that there is much to be gained from a quantitative approach to the study of size and shape for a more thorough understanding of developmental processes. Moreover, the meeting has been at a time when some among them are studying problems where experiments producing an all-or-none response are hard to design. The quantitative study of outcomes of such experiments with morphometric methods seems to be a promising field in the future. Another major strength of morphometric techniques lies in exploratory studies, where they have the power to find even subtle effects (this power may even be a bit of a difficulty, because the methods can also pick up spurious effects, and careful experimental controls should therefore be applied in such studies).

In conclusion, I think it will be worthwhile for us morphometricians to learn developmental biology, and for developmental biologists to learn morphometrics. Bridging the gap between these two disciplines will open up a vast and unexplored field of study.

Christian Peter Klingenberg, Department of Zoology, Duke University, Durham, North Carolina 27708-0325, USA. E-mail: