Biomedical subjects

Philippa Francis-West

Publications and source records attributed to Philippa Francis-West.

11 recordsLinked to original sources

PubMed2007

Tbx1 regulation of myogenic differentiation in the limb and cranial mesoderm.

The T-box transcription factor Tbx1 has been implicated in DiGeorge syndrome, the most frequent syndrome due to a chromosomal deletion. Gene inactivation of Tbx1 in mice results in craniofacial and branchial arch defects, including myogenic defects in the first and second branchial arches. A T-box binding site has been identified in the Xenopus Myf5 promoter, and in other species, T-box genes have been implicated in myogenic fate. Here we analyze Tbx1 expression in the developing chick embryo relating its expression to the onset of myogenic differentiation and cellular fate within the craniofacial mesoderm. We show that Tbx1 is expressed before capsulin, the first known marker of branchial arch 1 and 2 muscles. We also show that, as in the mouse, Tbx1 is expressed in endothelial cells, another mesodermal derivative, and, therefore, Tbx1 alone cannot specify the myogenic lineage. In addition, Tbx1 expression was identified in both chick and mouse limb myogenic cells, initially being restricted to the dorsal muscle mass, but in contrast, to the head, here Tbx1 is expressed after the onset of myogenic commitment. Functional studies revealed that loss of Tbx1 function reduces the number of myocytes in the head and limb, whereas increasing Tbx1 activity has the converse effect. Finally, analysis of the Tbx1-mesoderm-specific knockout mouse demonstrated the cell autonomous requirement for Tbx1 during myocyte development in the cranial mesoderm.

Animals↗

PubMed2006

The differentiation and morphogenesis of craniofacial muscles.

Unraveling the complex tissue interactions necessary to generate the structural and functional diversity present among craniofacial muscles is challenging. These muscles initiate their development within a mesenchymal population bounded by the brain, pharyngeal endoderm, surface ectoderm, and neural crest cells. This set of spatial relations, and in particular the segmental properties of these adjacent tissues, are unique to the head. Additionally, the lack of early epithelialization in head mesoderm necessitates strategies for generating discrete myogenic foci that may differ from those operating in the trunk. Molecular data indeed indicate dissimilar methods of regulation, yet transplantation studies suggest that some head and trunk myogenic populations are interchangeable. The first goal of this review is to present key features of these diversities, identifying and comparing tissue and molecular interactions regulating myogenesis in the head and trunk. Our second focus is on the diverse morphogenetic movements exhibited by craniofacial muscles. Precursors of tongue muscles partly mimic migrations of appendicular myoblasts, whereas myoblasts destined to form extraocular muscles condense within paraxial mesoderm, then as large cohorts they cross the mesoderm:neural crest interface en route to periocular regions. Branchial muscle precursors exhibit yet another strategy, establishing contacts with neural crest populations before branchial arch formation and maintaining these relations through subsequent stages of morphogenesis. With many of the prerequisite stepping-stones in our knowledge of craniofacial myogenesis now in place, discovering the cellular and molecular interactions necessary to initiate and sustain the differentiation and morphogenesis of these neglected craniofacial muscles is now an attainable goal.

Animals↗

PubMed2006

Developmental expression of the chick four-jointed homologue.

Four-jointed is a type II transmembrane protein that is thought to be cleaved to give rise to a secreted protein. In Drosophila, four-jointed controls outgrowth, vein patterning, and bristle polarity in the developing limb together with the polarity of the ommatidia in the developing eye. In Drosophila and mice, Fj is regulated by notch signaling. Here, we have determined the expression of the chick four-jointed (fjx) homologue during embryonic development. We show that fjx is expressed in the limb bud; facial primordia; the proliferating zone of the lens, feather buds, the neural tube; and neural crest derivatives such as the dorsal root ganglia. Analysis of the fjx expression in the developing limb bud showed that initially fjx is expressed throughout the limb bud, but as the limb develops, highest levels of fjx transcripts are found distally. However, by stage 27, fjx expression is predominantly found in the central core of the limb bud. Finally, fjx expression becomes confined to the developing tendons, ligaments, articular cartilage, and arteries but not the veins. Comparison with scleraxis (scx), a marker of tendons and ligaments, revealed that they are coexpressed in the majority of tendons but that fjx is expressed after scx, when the tendons have begun to differentiate. These data suggest that fjx has two roles during limb development: the first controlling outgrowth and the second tissue differentiation.

Amino Acid Sequence↗

PubMed2005

Expression and function of Bapx1 during chick limb development.

The homeobox-containing transcription factor Bapx1 (also known as Nkx3.2) is crucial for development of the axial skeleton and parts of the chondrocranium. Here we describe the detailed expression of Bapx1 during chick limb development and show that in contrast to its expression in the axial skeleton, Bapx1 is expressed after the commitment to chondrogenesis. Bapx1 is initially expressed throughout the developing skeletal elements prior to the overt differentiation of the distinct chondrogenic layers. Once distinct layers (proliferating, prehypertrophic and hypertrophic) have formed, Bapx1 expression is restricted to the proliferating chondrocytes. Bapx1 transcripts are excluded from the articular cartilage. A second homeobox-containing transcription factor, Barx1, is expressed in a complementary fashion in the developing joint and articular cartilage. Interestingly, in vitro functional analyses showed that Bapx1 overexpression in micromass cultures increased both matrix production and nodule number suggesting that Bapx1 is sufficient to promote chondrogenesis in the limb. In contrast, Barx1 had the opposite effect on nodule number suggesting that it has an inhibitory effect on chondrogenic initiation consistent with its expression in the developing joint. A slight increase in matrix levels was also observed consistent with its expression in the articular chondrocytes. Finally, we show that Bapx1 is also expressed in the soft tissues such as the developing tendons, muscle sheaths and surrounding mesenchyme, and therefore may have additional as yet uncharacterized roles in limb morphogenesis.

Amino Acid Sequence↗

PubMed2003

Development of synovial joints.

Synovial joints arise through two main processes. In long bone elements, cartilaginous differentiation occurs across the locations of the prospective joints that then segment secondarily. This process occurs through the development of a noncartilaginous region known as the interzone. The interzone becomes an important signaling center to the opposing elements, which can regulate growth through such factors as GDF-5. The interzone also expresses bone morphogenetic proteins (BMPs) and their antagonists, such as noggin. Overexpression of BMPs, or the loss of noggin leads to joint fusions. The interzone also expresses Wnt-14, which appears to be specific for this region in the developing anlagen, and regulates its nonchondrogenic nature. Cavitation of the joint follows, driven by selective high-level synthesis of hyaluronan by interzone cells and presumptive synovial cells. In addition, as the interzone disperses during cavity enlargement, data are now accruing that suggest that both the synovium and articular cartilage develop from this population. Finally, the development of articular cartilage progresses through appositional growth driven by a progenitor/stem cell subpopulation that resides in the articular surface. The individual elements of the skeleton are connected together at regions termed joints or articulations. Classically, there are three broad categories of joints: immovable joints (syntharthroses); mixed articulations, in which the range of movement is limited (amphiarthroses); and the movable, or synovial, joints (diarthroses). This review concentrates on the development of the synovial joints.

Animals↗

PubMed2003

The chondrocyte.

The chondrocyte is the resident cell of cartilage that is a prominent tissue in the embryo acting as a template for the development of skeletal elements. In the adult, the distribution of permanent cartilage is much more restricted and is necessary for mechanical support, growth and movement. The cell is isolated within a voluminous extracellular matrix (ECM) that is neither vascularised nor innervated. As a result, nutrient/waste exchange occurs through diffusion and, consequently, under normal and pathological conditions, the cell is unique in its ability to exist in a low oxygen tension environment. Partly as a result of these properties, the tissue has a low reparative potential that, in the case of articular cartilage, predisposes the tissue to degenerative conditions such as arthritis that is a significant clinical problem. Cellfacts. Cytoplasmically isolated. High matrix/cell volume ratio. Do not divide after skeletal maturity unless during pathology. Major contributor to growth of the body. Most energy requirements obtained through glycolysis.

Animals↗

PubMed2003

Wnt signalling regulates myogenic differentiation in the developing avian wing.

The limb musculature arises by delamination of premyogenic cells from the lateral dermomyotome. Initially the cells express Pax3 but, upon entering the limb bud, they switch on the expression of MyoD and Myf5 and undergo terminal differentiation into slow or fast fibres, which have distinct contractile properties that determine how a muscle will function. In the chick, the premyogenic cells express the Wnt antagonist Sfrp2, which is downregulated as the cells differentiate, suggesting that Wnts might regulate myogenic differentiation. Here, we have investigated the role of Wnt signalling during myogenic differentiation in the developing chick wing bud by gain- and loss-of-function studies in vitro and in vivo. We show that Wnt signalling changes the number of fast and/or slow fibres. For example, in vivo, Wnt11 decreases and increases the number of slow and fast fibres, respectively, whereas overexpression of Wnt5a or a dominant-negative Wnt11 protein have the opposite effect. The latter shows that endogenous Wnt11 signalling determines the number of fast and slow myocytes. The distinct effects of Wnt5a and Wnt11 are consistent with their different expression patterns, which correlate with the ultimate distribution of slow and fast fibres in the wing. Overexpression of activated calmodulin kinase II mimics the effect of Wnt5a, suggesting that it uses this pathway. Finally, we show that overexpression of the Wnt antagonist Sfrp2 and DeltaLef1 reduces the number of myocytes. In Sfrp2-infected limbs, the number of Pax3 expressing cells was increased, suggesting that Sfrp2 blocks myogenic differentiation. Therefore, Wnt signalling modulates both the number of terminally differentiated myogenic cells and the intricate slow/fast patterning of the limb musculature.

Animals↗

PubMed2003

Secondary chondrocyte-derived Ihh stimulates proliferation of periosteal cells during chick development.

The development of the skull is characterised by its dependence upon epigenetic influences. One of the most important of these is secondary chondrogenesis, which occurs following ossification within certain membrane bone periostea, as a result of biomechanical articulation. We have studied the genesis, character and function of the secondary chondrocytes of the quadratojugal of the chick between embryonic days 11 and 14. Analysis of gene expression revealed that secondary chondrocytes formed coincident with Sox9 upregulation from a precursor population expressing Cbfa1/Runx2: a reversal of the normal sequence. Such secondary chondrocytes rapidly acquired a phenotype that is a compound of prehypertrophic and hypertrophic chondrocytes, exited from the cell cycle and upregulated Ihh. Pulse and pulse/chase experiments with BrdU confirmed the germinal region as the highly proliferative source of the secondary chondrocytes, which formed by division of chondrocyte-committed precursors. By blocking Hh signalling in explant cultures we show that the enhanced proliferation of the germinal region surrounding the secondary chondrocytes derives from this Ihh source. Additionally, in vitro studies on membrane bone periosteal cells (non-germinal region) demonstrated that these cells can also respond to Ihh, and do so both by enhanced proliferation and precocious osteogenesis. Despite the pro-osteogenic effects of Ihh on periosteal cell differentiation, mechanical articulation of the quadratojugal/quadrate joint in explant culture revealed a negative role for articulation in the regulation of osteocalcin by germinal region descendants. Thus, the mechanical stimulus that is the spur to secondary chondrocyte formation appears able to override the osteogenic influence of Ihh on the periosteum, but does not interfere with the cell cycle-promoting component of Hh signalling.

Animals↗

PubMed2002

Wnt regulation of chondrocyte differentiation.

The Wnt family of growth factors are important regulators of several developmental processes including skeletogenesis. To further investigate the role of Wnts we analysed their expression in the developing chick limb and performed functional analyses in vivo and in vitro. We found that Wnt5b and Wnt11 are restricted within the prehypertrophic chondrocytes of the cartilage elements, Wnt5a is found in the joints and perichondrium, while Wnt4 is expressed in the developing joints and, in some bones, a subset of the hypertrophic chondrocytes. These Wnts mediate distinct effects on the initiation of chondrogenesis and differentiation of chondrocytes in vitro and in vivo. Wnt4 blocks the initiation of chondrogenesis and accelerates terminal chondrocyte differentiation in vitro. In contrast, Wnt5a and Wnt5b promote early chondrogenesis in vitro while inhibiting terminal differentiation in vivo. As Wnt5b and Wnt11 expression overlaps with and appears after Indian hedgehog (Ihh), we also compared their effects with Ihh to see if they mediate aspects of Ihh signalling. This showed that Ihh and Wnt5b and Wnt11 control chondrogenesis in parallel pathways.

Animals↗

PubMed2002

Wnt signalling during limb development.

Wnts control a number of processes during limb development--from initiating outgrowth and controlling patterning, to regulating cell differentiation in a number of tissues. Interactions of Wnt signalling pathway components with those of other signalling pathways have revealed new mechanisms of modulating Wnt signalling, which may explain how different responses to Wnt signalling are elicited in different cells. Given the number of Wnts that are expressed in the limb and their ability to induce differential responses, the challenge will be to dissect precisely how Wnt signalling is regulated and how it controls limb development at a cellular level, together with the other signalling pathways, to produce the functional limb capable of coordinated precise movements.

Animals↗

PubMedDate not supplied

Mechanical regulation of secondary chondrogenesis.

The development of the skull is characterised by its dependence upon epigenetic influences. One of the most important of these is secondary chondrogenesis, which occurs following ossification within certain membrane bone periostea, as a result of biomechanical articulation. We have studied the genesis, character and function of the secondary chondrocytes of the quadratojugal of the chick between embryonic days 11 and 14. Analysis of gene expression revealed that secondary chondrocytes formed coincident with Sox9 upregulation from a precursor population expressing Cbfa1/Runx2: a reversal of the normal sequence. Such secondary chondrocytes rapidly acquired a phenotype that is a compound of prehypertrophic and hypertrophic chondrocytes, exited from the cell cycle and upregulated Ihh. Pulse and pulse/chase experiments with BrdU confirmed the germinal region as the highly proliferative source of the secondary chondrocytes, which formed by division of chondrocyte-committed precursors. By blocking Hh signalling in explant cultures we show that the enhanced proliferation of the germinal region surrounding the secondary chondrocytes derives from this Ihh source. Additionally, in vitro studies on membrane bone periosteal cells (nongerminal region) demonstrated that these cells can also respond to Ihh, and do so both by enhanced proliferation and precocious osteogenesis. Despite the pro-osteogenic effects of Ihh on periosteal cell differentiation, mechanical articulation of the quadratojugal/quadrate joint in explant culture revealed a negative role for articulation in the regulation of osteocalcin by germinal region descendants. Thus, the mechanical stimulus that is the spur to secondary chondrocyte formation appears able to override the osteogenic influence of Ihh on the periosteum, but does not interfere with the cell cycle-promoting component of Hh signalling.

Animals↗