Limb development: an international model for vertebrate pattern formation

Limb development is an excellent model for studying how patterns of differentiated cells and tissues are generated in vertebrate embryos. The cell interactions that mediate patterning have been discovered and, more recently, some of the molecules involved in these interactions have been identified. This has provided a direct link to genetics and thus to genes that cause human congenital limb defects.     

Retinoids and vertebrate limb pattern formation

It has long been suggested that pattern formation depends in part on signalling molecules known as 'morphogens', diffusible substances that determine cell fate in a concentration-dependent way. Retinoic acid, a small hydrophobic molecule that binds to nuclear receptors, is a candidate morphogen for specifying the anteroposterior pattern of vertebrate limbs.     

The role of function in the formation of the skull

Partial resection of the mandible, resection of the chewing muscles, amputation of extremities and ligature of the common carotid artery were performed in animals different in chewing type (dogs, sheep, rabbits and rats). These studies revealed a close relation between shape and function. It was shown that changes in function have a decisive effect on the development and shaping of the skull bones and musculature.     

Activator-inhibitor dynamics of vertebrate limb pattern formation

The development of the vertebrate limb depends on an interplay of cellular differentiation, pattern formation, and tissue morphogenesis on multiple spatial and temporal scales. While numerous gene products have been described that participate in, and influence, the generation of the limb skeletal pattern, an understanding of the most salient feature of the developing limb--its quasiperiodic arrangement of bones, requires additional organizational principles. We review several such principles, drawing on concepts of physics and chemical dynamics along with molecular genetics and cell biology. First, a "core mechanism" for precartilage mesenchymal condensation is described, based on positive autoregulation of the morphogen transforming growth factor (TGF)-beta, induction of the extracellular matrix (ECM) protein fibronectin, and focal accumulation of cells via haptotaxis. This core mechanism is shown to be part of a local autoactivation-lateral inhibition (LALI) system that ensures that the condensations will be regularly spaced. Next, a "bare-bones" model for limb development is described in which the LALI-core mechanism is placed in a growing geometric framework with predifferentiated "apical," differentiating "active," and irreversibly differentiated "frozen" zones defined by distance from an apical source of a fibroblast growth factor (FGF)-type morphogen. This model is shown to account for classic features of the developing limb, including the proximodistal (PD) emergence over time of increasing numbers of bones. We review earlier and recent work suggesting that the inhibitory component of the LALI system for condensation may not be a diffusible morphogen, and propose an alternative mechanism for lateral inhibition, based on synchronization of oscillations of a Hes mediator of the Notch signaling pathway. Finally, we discuss how viewing development as an interplay between molecular-genetic and dynamic physical processes can provide new insight into the origin of congenital anomalies.     

The role of Wnt genes in vertebrate development.

Over the past decade, many potential candidates for molecules involved in pattern formation in the vertebrate embryo have been identified. Manipulation of the expression of some of these factors has generated fascinating results that have allowed investigators to address their roles in embryogenesis. One such family consists of a group of putative cell signaling molecules related to the proto-oncogene Wnt-1. An accumulating body of evidence suggests that the Wnt-family plays a major role in several aspects of vertebrate development.     

Evolution of developmental pattern for vertebrate dentitions: an oro-pharyngeal specific mechanism

Classically the oral dentition with teeth regulated into a successional iterative order was thought to have evolved from the superficial skin denticles migrating into the mouth at the stage when jaws evolved. The canonical view is that the initiation of a pattern order for teeth at the mouth margin required development of a sub-epithelial, permanent dental lamina. This provided regulated tooth production in advance of functional need, as exemplified by the Chondrichthyes. It had been assumed that teeth in the Osteichthyes form in this way as in tetrapods. However, this has been shown not to be true for many osteichthyan fish where a dental lamina of this kind does not form, but teeth are regularly patterned and replaced. We question the evolutionary origin of pattern information for the dentition driven by new morphological data on spatial initiation of skin denticles in the catshark. We review recent gene expression data for spatio-temporal order of tooth initiation for Scyliorhinus canicula, selected teleosts in both oral and pharyngeal dentitions, and Neoceratodus forsteri. Although denticles in the chondrichthyan skin appear not to follow a strict pattern order in space and time, tooth replacement in a functional system occurs with precise timing and spatial order. We suggest that the patterning mechanism observed for the oral and pharyngeal dentition is unique to the vertebrate oro-pharynx and independent of the skin system. Therefore, co-option of a successional iterative pattern occurred in evolution not from the skin but from mechanisms existing in the oro-pharynx of now extinct agnathans.     

The role of Hox genes during vertebrate limb development

The potential role of Hox genes during vertebrate limb development was brought into focus by gene expression analyses in mice (P Dolle, JC Izpisua-Belmonte, H Falkenstein, A Renucci, D Duboule, Nature 1989, 342:767-772), at a time when limb growth and patterning were thought to depend upon two distinct and rather independent systems of coordinates; one for the anterior-to-posterior axis and the other for the proximal-to-distal axis (see D Duboule, P Dolle, EMBO J 1989, 8:1497-1505). Over the past years, the function and regulation of these genes have been addressed using both gain-of-function and loss-of-function approaches in chick and mice. The use of multiple mutations either in cis-configuration in trans-configuration or in cis/trans configurations, has confirmed that Hox genes are essential for proper limb development, where they participate in both the growth and organization of the structures. Even though their molecular mechanisms of action remain somewhat elusive, the results of these extensive genetic analyses confirm that, during the development of the limbs, the various axes cannot be considered in isolation from each other and that a more holistic view of limb development should prevail over a simple cartesian, chess grid-like approach of these complex structures. With this in mind, the functional input of Hox genes during limb growth and development can now be re-assessed.     

Dynamical mechanisms for skeletal pattern formation in the vertebrate limb

We describe a 'reactor-diffusion' mechanism for precartilage condensation based on recent experiments on chondrogenesis in the early vertebrate limb and additional hypotheses. Cellular differentiation of mesenchymal cells into subtypes with different fibroblast growth factor (FGF) receptors occurs in the presence of spatio-temporal variations of FGFs and transforming growth factor-betas (TGF-betas). One class of differentiated cells produces elevated quantities of the extracellular matrix protein fibronectin, which initiates adhesion-mediated preskeletal mesenchymal condensation. The same class of cells also produces an FGF-dependent laterally acting inhibitor that keeps condensations from expanding beyond a critical size. We show that this 'reactor-diffusion' mechanism leads naturally to patterning consistent with skeletal form, and describe simulations of spatio-temporal distribution of these differentiated cell types and the TGF-beta and inhibitor concentrations in the developing limb bud.     

Evolution of crystallins for a role in the vertebrate eye lens

The camera eye lens of vertebrates is a classic example of the re‐engineering of existing protein components to fashion a new device. The bulk of the lens is formed from proteins belonging to two superfamilies, the α ‐crystallins and the β γ ‐crystallins. Tracing their ancestry may throw light on the origin of the optics of the lens. The α ‐crystallins belong to the ubiquitous small heat shock proteins family that plays a protective role in cellular homeostasis. They form enormous polydisperse oligomers that challenge modern biophysical methods to uncover the molecular basis of their assembly structure and chaperone‐like protein binding function. It is argued that a molecular phenotype of a dynamic assembly suits a chaperone function as well as a structural role in the eye lens where the constraint of preventing protein condensation is paramount. The main cellular partners of α ‐crystallins, the β ‐ and γ ‐crystallins, have largely been lost from the animal kingdom but the superfamily is hugely expanded in the vertebrate eye lens. Their structures show how a simple Greek key motif can evolve rapidly to form a complex array of monomers and oligomers. Apart from remaining transparent, a major role of the partnership of α ‐crystallins with β ‐ and γ ‐crystallins in the lens is to form a refractive index gradient. Here, we show some of the structural and genetic features of these two protein superfamilies that enable the rapid creation of different assembly states, to match the rapidly changing optical needs among the various vertebrates.     

Roles of cell-extrinsic growth factors in vertebrate eye pattern formation and retinogenesis

Formation of the vertebrate visual system involves complex interplays of cell-extrinsic cues and cell-intrinsic determinants. Studies in several vertebrate species demonstrate that multiple classes of signaling molecules participate in pattern formation of the eye and neurogenesis of the retina. Certain signals, such as hedgehog, BMP, and FGF molecules, are repeatedly deployed at varying concentration thresholds and in different cellular contexts. Accumulating evidence reveals a striking conservation of molecular mechanisms regulating the neurogenic process between Drosophila and vertebrate retinas. The remaining challenge is to understand how these well-characterized signaling pathways are activated and integrated to impact eye morphogenesis and retinal progenitor cell fate determination.     

The morphological pattern of the vertebrate brain.

Bergquist and K?llén as well as Puelles and collaborators have presented models of the developing vertebrate brain, the basic units of which are formed by intersection of transversely oriented neuromeres and longitudinally arranged zones. These units represent initially discrete, developmentally independent compartments, but during later development some (many?) are invaded by sizable numbers of neuroblasts generated in adjacent units. No consensus exists with regard to the number and arrangement of the units involved in the formation of the telencephalon and hypothalamus.     

Evolution of danio pigment pattern development

Pigment patterns of danio fishes are emerging as a useful system for studying the evolution of developmental mechanisms underlying adult form. Different closely related species within the genera Danio and Devario exhibit a range of pigment patterns including horizontal stripes, vertical bars, and others. In this review, I summarize recent work identifying the genetic and cellular bases for adult pigment pattern formation in the zebrafish Danio rerio, as well as studies of how these mechanisms have evolved in other danios. Together, these analyses highlight the importance of latent precursors at post-embrynoic stages, as well as interactions within and among pigment cell classes, for both pigment pattern development and evolution.     

Pattern formation in epithelial development: the vertebrate limb and feather bud spacing.

The ectoderm of the vertebrate limb and feather bud are epithelia that provide good models for epithelial patterning in vertebrate development. At the tip of chick and mouse limb buds is a thickening, the apical ectodermal ridge, which is essential for limb bud outgrowth. The signal from the ridge to the underlying mesoderm involves fibroblast growth factors. The non-ridge ectoderm specifies the dorsoventral pattern of the bud and Wnt7a is a dorsalizing signal. The development of the ridge involves an interaction between dorsal cells that express radical fringe and those that do not. There are striking similarities between the signals and genes involved in patterning the limb ectoderm and the epithelia of the Drosophila imaginal disc that gives rise to the wing. The spacing of feather buds involves signals from the epidermis to the underlying mesenchyme, which again include Wnt7a and fibroblast growth factors.     

The role of neurexins and neuroligins in the formation, maturation, and function of vertebrate synapses

Neurexins (NXs) and neuroligins (NLs) are transsynaptically interacting cell adhesion proteins that play a key role in the formation, maturation, activity-dependent validation, and maintenance of synapses. As complex alternative splicing processes in nerve cells generate a large number of NX and NLs variants, it has been proposed that a combinatorial interaction code generated by these variants may determine synapse identity and network connectivity during brain development. The functional importance of NXs and NLs is exemplified by the fact that mutations in NX and NL genes are associated with several neuropsychiatric disorders, most notably with autism. Accordingly, major research efforts have focused on the molecular mechanisms by which NXs and NLs operate at synapses. In this review, we summarize recent progress in this field and discuss emerging topics, such as the role of alternative interaction partners of NXs and NLs in synapse formation and function, and their relevance for synaptic plasticity in the mature brain. The novel findings highlight the fundamental importance of NX-NL interactions in a wide range of synaptic functions.