Darrell J.R. Evans

The surface of articular cartilage contains a progenitor cell population

It is becoming increasingly apparent that articular cartilage growth is achieved by apposition from the articular surface. For such a mechanism to occur, a population of stem/progenitor cells must reside within the articular cartilage to provide transit amplifying progeny for growth. Here, we report on the isolation of an articular cartilage progenitor cell from the surface zone of articular cartilage using differential adhesion to fibronectin. This population of cells exhibits high affinity for fibronectin, possesses a high colony-forming efficiency and expresses the cell fate selector gene Notch 1. Inhibition of Notch signalling abolishes colony forming ability whilst activated Notch rescues this inhibition. The progenitor population also exhibits phenotypic plasticity in its differentiation pathway in an embryonic chick tracking system, such that chondroprogenitors can engraft into a variety of connective tissue types including bone, tendon and perimysium. The identification of a chondrocyte subpopulation with progenitor-like characteristics will allow for advances in our understanding of both cartilage growth and maintenance as well as provide novel solutions to articular cartilage repair.

Near-peer teaching in anatomy: an approach for deeper learning.

Peer teaching has been recognized as a valuable and effective approach for learning and has been incorporated into medical, dental, and healthcare courses using a variety of approaches. The success of peer teaching is thought to be related to the ability of peer tutors and tutees to communicate more effectively, thereby improving the learning environment. Near‐peer teaching involves more experienced students acting as tutors who are ideally placed to pass on their knowledge and experience. The advantage of using near‐peer teachers is the opportunity for the teacher to reinforce and expand their own learning and develop essential teaching skills. This study describes the design and implementation of a program for fourth year medical students to teach anatomy to first‐ and second‐year medical students and evaluates the perceptions of the near‐peer teachers on the usefulness of the program, particularly in relation to their own learning. Feedback from participants suggests that the program fulfills its aims of providing an effective environment for developing deeper learning in anatomy through teaching. Participants recognize that the program also equips them with more advanced teaching skills that will be required as they move nearer toward taking on supervisory and teaching duties. The program has also provided the school with an additional valuable and appropriate resource for teaching anatomy to first‐ and second‐year students, who themselves view the inclusion of near‐peer teachers as a positive element in their learning. Anat Sci Educ 2:227–233, 2009.

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 ΔLef1 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.

Spatial relations between avian craniofacial neural crest and paraxial mesoderm cells.

Fate maps based on quail–chick grafting of avian cephalic neural crest precursors and paraxial mesoderm cells have identified the majority of derivatives from each population but have not unequivocally resolved the precise locations of and population dynamics at the interface between them. The relation between these two mesenchymal tissues is especially critical for the development of skeletal muscles, because crest cells play an essential role in their differentiation and subsequent spatial organization. It is not known whether myogenic mesoderm and skeletogenic neural crest cells establish permanent relations while en route to their final destinations, or later at the sites where musculoskeletal morphogenesis is completed. We applied β‐galactosidase‐encoding, replication‐incompetent retroviruses to paraxial mesoderm, to crest progenitors, or at the interface between mesodermal and overlying neural crest as both were en route to branchial or periocular regions in chick embryos. With respect to skeletal structures, the results identify the avian neural crest:mesoderm boundary at the junction of the supraorbital and calvarial regions of the frontal bone, lateral to the hypophyseal foramen, and rostral to laryngeal cartilages. Therefore, in the chick embryo, most of the frontal and the entire parietal bone are of mesodermal, not neural crest, origin. Within paraxial mesoderm, the progenitors of each lineage display different behaviors. Chondrogenic cells are relatively stationary and intramembranous osteogenic cells move only in transverse planes around the brain. Angioblasts migrate invasively in all directions. Extraocular muscle precursors form tightly aggregated masses that en masse cross the crest:mesoderm interface to enter periocular territories, while branchial myogenic lineages shift ventrally coincidental with the movements of corresponding neural crest cells. En route to the branchial arches, myogenic mesoderm cells do not maintain constant, nearest‐neighbor relations with adjacent, overlying neural crest cells. Thus, progenitors of individual muscles do not establish stable, permanent relations with their connective tissues until both populations reach the sites of their morphogenesis within branchial arches or orbital regions. Developmental Dynamics 235:1310–1325, 2006.

Craniofacial development: the tissue and molecular interactions that control development of the head

The molecular cascades that control craniofacial development have until recently been little understood. The paucity of data that exists has in part been due to the complexity of the head, which is the most intricate regions of the body. However, the generation of mouse mutants and the identification of gene mutations that cause human craniofacial syndromes, together with classical embryological approaches in other species, have given significant insight into how the head develops. These studies have emphasized how unique the head actually is, with each individual part governed by a distinct set of signalling interactions, again demonstrating the complexity of this region of the body. This review discussed the tissue and molecular interactions that control each region of the head. The processes that control neural tube closure together with correct development of the skull, midline patterning, neural crest generation and migration, outgrowth, patterning, and differentiation of the facial primordia and the branchial arches are thus discussed. Defects in these processes result in a number of human syndromes such as exencephaly, holoprosencephaly, musculoskeletal dysplasias, first arch syndromes such as Riegers and Treacher-Collins syndrome, and neural crest dysplasias such as DiGeorge syndrome. Our current knowledge of the genes responsible for these human syndromes together with how the head develops, is rapidly advancing so that we will soon understand the complex set of molecular and tissue interactions that build a head.

Molecular and cellular mechanisms involved in the generation of fiber diversity during myogenesis.

Skeletal muscles have a characteristic proportion and distribution of fiber types, a pattern which is set up early in development. It is becoming clear that different mechanisms produce this pattern during early and late stages of myogenesis. In addition, there are significant differences between the formation of muscles in head and those found in rest of the body. Early fiber type differentiation is dependent upon an interplay between patterning systems which include the Wnt and Hox gene families and different myoblast populations. During later stages, innervation, hormones, and functional demand increasingly act to determine fiber type, but individual muscles still retain an intrinsic commitment to form particular fiber types. Head muscle is the only muscle not derived from the somites and follows a different development pathway which leads to the formation of particular fiber types not found elsewhere. This review discusses the formation of fiber types in both head and other muscles using results from both chick and mammalian systems.

Developmental stages of the Japanese quail

Developmental biology research has used various avian species as model organisms for studying morphogenesis, with the chick embryo being used by the majority of groups. The focus on the chick embryo led Hamburger and Hamilton to develop their definitive staging series nearly 60 years ago and this series is still the mainstay of all laboratories working with avian embryos. The focus on the chick embryo has somewhat overshadowed the importance of another avian embryo that has proved to be equally powerful, the Japanese quail. Since the late 1960s, chimeras have been produced using chick and quail embryos and this technique has revolutionized the approach taken to the investigation of the cellular and molecular interactions that occur during development. Reviews of the literature demonstrate that many research groups are using the quail embryo in a number of established and new ways, and this species has become a primary animal model in developmental biology. Some staging of quail has been performed but this has been incomplete and variations in descriptions, stages and incubation timings mean that comparisons with the chick are not always easily made. There appears to be general agreement that, at the early stages of embryogenesis, there is little developmental difference between chick and quail embryos, although the basis for this has not been established experimentally. The accelerated ontogeny of quail embryos at mid to late stages of development means that registration with the chick is lost. We have therefore developed a definitive developmental stage series for Japanese quail so that differences are fully characterized, misconceptions or assumptions are avoided, and the results of comparative studies are not distorted.

Can medical students teach? A near-peer-led teaching program for year 1 students.

The General Medical Council states that United Kingdom graduates must function effectively as educators. There is a growing body of evidence showing that medical students can be included as teachers within a medical curriculum. Our aim was to design and implement a near-peer-led teaching program in an undergraduate medical curriculum and assess its acceptability among year 1 students. Students received six tutorials focusing on aspects of cardiac, respiratory, and blood physiology. Tutorials ran alongside standard module teaching. Students were taught in groups of ~30 students/group, and an active teaching approach was used in sessions where possible. Using anonymous evaluations, student feedback was collected for the program overall and for each tutorial. The program was voluntary and open to all first-year students, and 94 (of 138) medical students from year 1 at Brighton and Sussex Medical School were recruited to the study. The tutorial program was popular among students and was well attended throughout. Individual tutorial and overall program quantitative and qualitative feedback showed that students found the tutorials very useful in consolidating material taught within the module. Students found the small group and active teaching style of the near-peer tutors very useful to facilitating their learning experience. The end-of-module written examination scores suggest that the tutorials may have had a positive effect on student outcome compared with previous student attainment. In conclusion, the present study shows that a near-peer tutorial program can be successfully integrated into a teaching curriculum. The feedback demonstrates that year 1 students are both receptive and find the additional teaching of benefit.

Differential response of fetal and neonatal myoblasts to topographical guidance cues in vitro

Abstract Fusion of mononucleated myoblasts into parallel arrays of mutinucleated myotubes is an essential step in skeletal myogenesis. The formation of such a highly ordered structure requires myoblasts to come together, orient and align in the correct location prior to fusion. We report here that fetal and neonatal myoblasts can use topographical features as strong guidance cues in vitro. Myoblasts were cultured on multiple grooved substrata of varying dimensions, and the axial orientations of individual cells were recorded. Both fetal and neonatal myoblasts aligned parallel with the direction of deep grooves (2.3–6.0 µm), which is correlated well with the location of myoblasts in similar sized grooves during secondary myogenesis. Fetal myoblasts also responded to shallower grooves (0.04–0.14 µm) by aligning parallel or perpendicular to the direction of the grooves, indicating the ability of these cells to respond to fine elements normally encountered within the developing muscle architecture. In contrast, neonatal myoblasts failed to respond to shallow grooves, adding to the suggestion that fetal and neonatal myoblasts may represent separate populations of myoblasts. Overall, the results demonstrate that myoblasts respond to large and small features of the physical topography in vitro and indicate that structural elements in the microenvironment of the muscle may play a critical role in myoblast spatial organization during myogenesis.

A dual fate of the hindlimb muscle mass: cloacal/perineal musculature develops from leg muscle cells.

The cloaca serves as a common opening to the urinary and digestive systems. In most mammals, the cloaca is present only during embryogenesis, after which it undergoes a series of septation events leading to the formation of the anal canal and parts of the urogenital tract. During embryogenesis it is surrounded by skeletal muscle. The origin and the mechanisms regulating the development of these muscles have never been determined. Here, we show that the cloacal muscles of the chick originate from somites 30-34, which overlap the domain that gives rise to leg muscles (somites 26-33). Using molecular and cell labelling protocols, we have determined the aetiology of cloacal muscles. Surprisingly, we found that chick cloacal myoblasts first migrate into the developing leg bud and then extend out of the ventral muscle mass towards the cloacal tubercle. The development of homologous cloacal/perineal muscles was also examined in the mouse. Concordant with the results in birds, we found that perineal muscles in mammals also develop from the ventral muscle mass of the hindlimb. We provide genetic evidence that the perineal muscles are migratory, like limb muscles, by showing that they are absent in metd/d mutants. Using experimental embryological procedures (in chick) and genetic models (in chick and mouse), we show that the development of the cloacal musculature is dependent on proximal leg field formation. Thus, we have discovered a novel developmental mechanism in vertebrates whereby muscle cells first migrate from axially located somites to the pelvic limb, then extend towards the midline and only then differentiate into the single cloacal/perineal muscles.