Regulation of scapula development

Covariance in human limb joint articular morphology

OBJECTIVES

Limb synovial joints exhibit complex shapes that must accommodate often-antagonistic demands of function, mobility, and stability. These demands presumably dictate coordination among joint articular shapes, but the structure of morphological covariance within and among joints is unknown. This study analyzes the human shoulder, elbow, hip, and knee to determine how articular covariance is structured in relation to joint structure, accessory cartilage, and function.

MATERIALS AND METHODS

Surface models were created from the CT scans of 200 modern skeletons from the University of Tennessee Donated Skeletal Collection. Three-dimensional landmarks were collected on the shoulder, elbow, hip, and knee joints. Two-block partial least squares were conducted to determine associations between surfaces of conarticular shapes, functionally analogous articulations, and articulations belonging to the same bone.

RESULTS

Except for the components of the shoulder, all conarticular pairs exhibit covariance, though the strength of these relationships appears unrelated to the amount of accessory cartilage in the joint. Only the analogous articulations of the humerus and femur exhibit significant covariance, but it is unlikely that this pattern is due to function alone. Stronger covariance within the lower limb than the upper limb is consistent broader primate patterns of within-limb integration.

DISCUSSION

With the exception of the elbow, complementary joint function does not appear to promote strong covariance between articulations. Analogous humeral and femoral surfaces are also serially homologous, which may result in the articular associations observed between these bones. Broadly, these patterns highlight the indirect relationship between joint congruence and covariance.

Effect of Scapular Stabilizer Muscles Strength on Scapular Position

BACKGROUND

Maintaining scapular mechanics is important for upper extremity functionality and posture. Determining the extent to which the scapular stabilizer muscles affect the scapular position may guide the creation of an exercise program for people with scapular dyskinesis.

HYPOTHESIS

The serratus anterior (SA), upper trapezius (UT), middle trapezius (MT), and lower trapezius (LT) muscles play different roles on scapular position when humeral elevation increase.

STUDY DESIGN

Cross-sectional study.

LEVEL OF EVIDENCE

Level 4.

METHODS

A total of 70 women aged 40 to 65 years (mean age, 49 ± 7 years) who met the inclusion criteria were included in the study. Isometric muscle strength of the SA, UT, MT, and LT was evaluated with a handheld dynamometer. For assessment of scapular position, the lateral scapular slide test (LSST) was used. Multiple stepwise regression analysis was used to evaluate scapular parameters.

RESULTS

There were positive and statistically significant correlations between the isometric muscle strength of the SA, UT, MT, and LT muscles and the values at different humerus positions in the LSST (P < 0.05). The UT and SA muscles greatly affected the changes in the position of the inferior region of the scapula (R2 > 24.5%). The LT (11.3%) in neutral position, MT (25.4%) with arm abducted at 45°, and SA (34.5%) with arm abducted 90° had a major effect on the changes in the mediolateral position of the scapula.

CONCLUSION

While the LT muscle affects the mediolateral position of the scapula to a large extent, the strength of the MT and SA muscles becomes effective as the shoulder elevation increases. SA and UT muscle strength have a greater effect on the position of the inferior region of the scapula.

CLINICAL RELEVANCE

Dyskinesis can be observed at different levels of the scapula; therefore, it is important to determine at which level the dyskinesis is more prominent for each individual and consequently to form a personalized exercise program to increase function and control dyskinesis.

Genetics of scapula and pelvis development: An evolutionary perspective

In tetrapods, the scapular and pelvic girdles perform the important function of anchoring the limbs to the trunk of the body and facilitating the movement of each appendage. This shared function, however, is one of relatively few similarities between the scapula and pelvis, which have significantly different morphologies, evolutionary histories, embryonic origins, and underlying genetic pathways. The scapula evolved in jawless fish prior to the pelvis, and its embryonic development is unique among bones in that it is derived from multiple progenitor cell populations, including the dermomyotome, somatopleure, and neural crest. Conversely, the pelvis evolved several million years later in jawed fish, and it develops from an embryonic somatopleuric cell population. The genetic networks controlling the formation of the pelvis and scapula also share similarities and differences, with a number of genes shaping only one or the other, while other gene products such as PBX transcription factors act as hierarchical developmental regulators of both girdle structures. Here, we provide a detailed review of the cellular processes and genetic networks underlying pelvis and scapula formation in tetrapods, while also highlighting unanswered questions about girdle evolution and development.

Problems in Fish-to-Tetrapod Transition: Genetic Expeditions Into Old Specimens

The fish-to-tetrapod transition is one of the fundamental problems in evolutionary biology. A significant amount of paleontological data has revealed the morphological trajectories of skeletons, such as those of the skull, vertebrae, and appendages in vertebrate history. Shifts in bone differentiation, from dermal to endochondral bones, are key to explaining skeletal transformations during the transition from water to land. However, the genetic underpinnings underlying the evolution of dermal and endochondral bones are largely missing. Recent genetic approaches utilizing model organisms—zebrafish, frogs, chickens, and mice—reveal the molecular mechanisms underlying vertebrate skeletal development and provide new insights for how the skeletal system has evolved. Currently, our experimental horizons to test evolutionary hypotheses are being expanded to non-model organisms with state-of-the-art techniques in molecular biology and imaging. An integration of functional genomics, developmental genetics, and high-resolution CT scanning into evolutionary inquiries allows us to reevaluate our understanding of old specimens. Here, we summarize the current perspectives in genetic programs underlying the development and evolution of the dermal skull roof, shoulder girdle, and appendages. The ratio shifts of dermal and endochondral bones, and its underlying mechanisms, during the fish-to-tetrapod transition are particularly emphasized. Recent studies have suggested the novel cell origins of dermal bones, and the interchangeability between dermal and endochondral bones, obscuring the ontogenetic distinction of these two types of bones. Assimilation of ontogenetic knowledge of dermal and endochondral bones from different structures demands revisions of the prevalent consensus in the evolutionary mechanisms of vertebrate skeletal shifts.

Enigma of scapular foramen and tunnels: an untold story

PURPOSE

The study was undertaken to make a qualitative and quantitative assessment of unnamed foramen and tunnels in adult human scapulae with aid of plain and contrast radiographs.

MATERIALS AND METHODS

A total of 120 dry bones, 60 each of the right and the left side were included in the study. Distribution of these foramina and tunnels was noted for their number, side, location, course and communication. Their morphometry was done using Vernier's caliper.

RESULTS

Incidence of scapular foramina was 7.5% (R > L), whereas scapular tunnels were seen in 15.8% cases. Incidence of the sinuous, curved, and straight tunnels was found to be 50, 39, and 10.7% respectively. Left-sided tunnels were longer than the right ones. Plain and contrast radiographs were taken to confirm the findings.

CONCLUSION

Anatomy literature describes only two scapular foramina, namely, nutrient foramen and suprascapular foramen/notch in a great zeal; occurrence of such anonymous foramina is hardly discussed. Through this study, there is an endeavor towards unfolding the mystery of scapular foramina in terms of their morphometry and distribution, the knowledge of which will aid clinicians, forensic experts, and surgeons in better diagnosis and management of clinical cases.

The hypaxial origin of the epaxially located rhomboid muscles

In vertebrates, skeletal muscles of the body are made up of epaxial and hypaxial muscles based on their innervation and relative position to the vertebral column. The epaxial muscles are innervated by the dorsal branches of the spinal nerves and comprise the intrinsic (deep) back muscles, while the hypaxial muscles are innervated by the ventral branches of the spinal nerves including the plexus and consist of a heterogeneous group of intercostal, abdominal, and limb as well as girdle muscles. The canonical view holds that the epaxial muscles are derived from the medial halves of the somites, whereas the hypaxial muscles are all derived from the lateral somitic halves. The rhomboid muscles are situated dorsal to the vertebral column and therefore in the domain typically occupied by epaxial muscles. However, they are innervated by a ventral branch of the brachial plexus called the N. dorsalis scapulae. Due to the apparent inappropriate position of the muscle in relation to its innervation we investigated its origin to help clarify this issue. To study the embryonic origin of the rhomboid muscles, we followed derivatives of the medial and lateral somite halves using quail-chick chimeras. Our results showed that the rhomboid muscles are made up of cells derived mainly from the lateral portion of the somite. Therefore the rhomboid muscles which lie within the epaxial domain of the body, originate from the hypaxial domain of the somites. However their connective tissue is derived from both medial and lateral somites.

Evolution of the hominoid scapula and its implications for earliest hominid locomotion

OBJECTIVES

The higher primate scapula has been subject to many explanations of the putative "adaptive value" of its individual traits. However, the shift from the bone's position in above branch quadrupeds to its more posterolateral position in recent hominoids obviously required fundamental changes to its general form. We hypothesize that most features argued to be individually adaptive are more likely secondary consequences of changes in its fundamental bauplan, a view more consistent with modern developmental biology.

MATERIALS AND METHODS

We tested this hypothesis with scapular metrics and angles from a broad anthropoid sample.

RESULTS

Our results support our hypothesis. Contrary to earlier predictions, vertebral border length differs little relative to body size in anthropoids, inferior angle position primarily reflects mediolateral scapular breadth, and supraspinous and infraspinous fossa sizes largely reflect scapular spine orientation. Suspensory taxa have cranially oriented glenoids, whereas slow clamberers and humans do not. Australopithecus most closely resembles the latter.

DISCUSSION

Most scapular features can be explained by only two primary changes: (1) reduction in mediolateral breadth and (2) change in the glenoid position relative to the vertebral border with increased reliance on suspension, which led to a more cranially angled scapular spine. Virtually all other scapular traits appear to be byproducts of these two changes. Based on fossil morphology, hominids¹ were derived from a last common ancestor primarily adapted for clambering and not for suspension. Scapular form in early hominids such as Australopithecus is therefore primitive and largely reflects the genus's general clambering heritage.

Mechanisms of myogenic specification and patterning

Mesodermal somites are initially composed of columnar cells arranged as a pseudostratified epithelium that undergoes sequential and spatially restricted changes to generate the sclerotome and dermomyotome, intermediate structures that develop into vertebrae, striated muscles of the body and limbs, dermis, smooth muscle, and endothelial cells. Regional cues were elucidated that impart differential traits upon the originally multipotent progenitors. How do somite cells and their intermediate progenitors interpret these extrinsic cues and translate them into various levels and/or modalities of intracellular signaling that lead to differential gene expression profiles remains a significant challenge. So is the understanding of how differential fate specification relates to complex cellular migrations prefiguring the formation of body muscles and vertebrae. Research in the past years has largely transited from a descriptive phase in which the lineages of distinct somite-derived progenitors and their cellular movements were traced to a more mechanistic understanding of the local function of genes and regulatory networks underlying lineage segregation and tissue organization. In this chapter, we focus on some major advances addressing the segregation of lineages from the dermomyotome, while discussing both cellular as well as molecular mechanisms, where possible.

Heterochrony and developmental timing mechanisms: changing ontogenies in evolution

Heterochrony, or a change in developmental timing, is an important mechanism of evolutionary change. Historically the concept of heterochrony has focused alternatively on changes in size and shape or changes in developmental sequence, but most have focused on the pattern of change. Few studies have examined changes in the mechanisms that embryos use to actually measure time during development. Recently, evolutionary studies focused on changes in distinct timekeeping mechanisms have appeared, and this review examines two such case studies: the evolution of increased segment number in snakes and the extreme rostral to caudal gradient of developmental maturation in marsupials. In both examples, heterochronic modifications of the somite clock have been important drivers of evolutionary change.

Temporal sequence in the formation of midline dermis and dorsal vertebral elements in avian embryos

Somites compartmentalize into a dorsal epithelial dermomyotome and a ventral mesenchymal sclerotome. While sclerotomes give rise to vertebrae and intervertebral discs, dermomyotomes contribute to skeletal muscle and epaxial dermis. Bone morphogenetic protein (BMP)-signals from the lateral mesoderm induce the lateral portion of the dermomyotome to form chondrogenic precursor cells, forming the cartilage of the scapula blade. The fact that BMPs are expressed in the roof plate of the neural tube where they induce cartilage formation led to the question why cells migrating from the medial part of the dermomyotome do not undergo chondrogenic differentiation and do not contribute to the dorsal part of the vertebrae. In the present study, we traced dermomyotomal derivatives by using the quail-chick marker technique. Our study reveals a temporal sequence in the formation of the vertebral cartilage and the midline dermis. The dorsal mesenchyme overlying the roof plate of the neural tube is formed prior to the de-epithelialization of the dermomyotome. Dermomyotomal cells start to migrate medially into the sub-ectodermal space to form the midline dermis after chondrogenesis of the dorsal mesenchyme has occurred. This time delay between chondrogenesis of the dorsal vertebra and dermal formation allows an undisturbed development of these two tissue components within a narrow region of the embryo.

Detection of genes regulated by Lmx1b during limb dorsalization

Lmx1b is a homeodomain transcription factor that regulates dorsal identity during limb development. Lmx1b knockout (KO) mice develop distal ventral-ventral limbs. Although induction of Lmx1b is linked to Wnt7a expression in the dorsal limb ectoderm, the downstream targets of Lmx1b that accomplish limb dorsalization are unknown. To identify genes targeted by Lmx1b, we compared gene arrays from Lmx1b KO and wild type mouse limbs during limb dorsalization, i.e., 11.5, 12.5, and 13.5 days post coitum. We identified 54 target genes that were differentially expressed in all three stages. Several skeletal targets, including Emx2, Matrilin1 and Matrilin4, demonstrated a loss of scapular expression in the Lmx1b KO mice, supporting a role for Lmx1b in scapula development. Furthermore, the relative abundance of extracellular matrix-related soft tissue targets regulated by Lmx1b, such as collagens and proteoglycans, suggests a mechanism that includes changes in the extracellular matrix composition to accomplish limb dorsalization. Our study provides the most comprehensive characterization of genes regulated by Lmx1b during limb development to-date and provides targets for further investigation.

An eye on the head: the development and evolution of craniofacial muscles

Skeletal muscles exert diverse functions, enabling both crushing with great force and movement with exquisite precision. A remarkably distinct repertoire of genes and ontological features characterise this tissue, and recent evidence has shown that skeletal muscles of the head, the craniofacial muscles, are evolutionarily, morphologically and molecularly distinct from those of the trunk. Here, we review the molecular basis of craniofacial muscle development and discuss how this process is different to trunk and limb muscle development. Through evolutionary comparisons of primitive chordates (such as amphioxus) and jawless vertebrates (such as lampreys) with jawed vertebrates, we also provide some clues as to how this dichotomy arose.

3D reconstructions of quail-chick chimeras provide a new fate map of the avian scapula

Limbed vertebrates have functionally integrated postcranial axial and appendicular systems derived from two distinct populations of embryonic mesoderm. The axial skeletal elements arise from the paraxial somites, the appendicular skeleton and sternum arise from the somatic lateral plate mesoderm, and all of the muscles for both systems arise from the somites. Recent studies in amniotes demonstrate that the scapula has a mixed mesodermal origin. Here we determine the relative contribution of somitic and lateral plate mesoderm to the avian scapula from quail-chick chimeras. We generate 3D reconstructions of the grafted tissue in the host revealing a very different distribution of somitic cells in the scapula than previously reported. This novel 3D visualization of the cryptic border between somitic and lateral plate populations reveals the dynamics of musculoskeletal morphogenesis and demonstrates the importance of 3D visualization of chimera data. Reconstructions of chimeras make clear three significant contrasts with existing models of scapular development. First, the majority of the avian scapula is lateral plate derived and the somitic contribution to the scapular blade is significantly smaller than in previous models. Second, the segmentation of the somitic component of the blade is partially lost; and third, there are striking differences in growth rates between different tissues derived from the same somites that contribute to the structures of the cervical thoracic transition, including the scapula. These data call for the reassessment of theories on the development, homology, and evolution of the vertebrate scapula.

Sequential and coordinated actions of c-Myc and N-Myc control appendicular skeletal development

Background

During limb development, chondrocytes and osteoblasts emerge from condensations of limb bud mesenchyme. These cells then proliferate and differentiate in separate but adjacent compartments and function cooperatively to promote bone growth through the process of endochondral ossification. While many aspects of limb skeletal formation are understood, little is known about the mechanisms that link the development of undifferentiated limb bud mesenchyme with formation of the precartilaginous condensation and subsequent proliferative expansion of chondrocyte and osteoblast lineages. The aim of this study was to gain insight into these processes by examining the roles of c-Myc and N-Myc in morphogenesis of the limb skeleton.

Methodology/Principal Findings

To investigate c-Myc function in skeletal development, we characterized mice in which floxed c-Myc alleles were deleted in undifferentiated limb bud mesenchyme with Prx1-Cre, in chondro-osteoprogenitors with Sox9-Cre and in osteoblasts with Osx1-Cre. We show that c-Myc promotes the proliferative expansion of both chondrocytes and osteoblasts and as a consequence controls the process of endochondral growth and ossification and determines bone size. The control of proliferation by c-Myc was related to its effects on global gene transcription, as phosphorylation of the C-Terminal Domain (pCTD) of RNA Polymerase II, a marker of general transcription initiation, was tightly coupled to cell proliferation of growth plate chondrocytes where c-Myc is expressed and severely downregulated in the absence of c-Myc. Finally, we show that combined deletion of N-Myc and c-Myc in early limb bud mesenchyme gives rise to a severely hypoplastic limb skeleton that exhibits features characteristic of individual c-Myc and N-Myc mutants.

Conclusions/Significance

Our results show that N-Myc and c-Myc act sequentially during limb development to coordinate the expansion of key progenitor populations responsible for forming the limb skeleton.

Control of pelvic girdle development by genes of the Pbx family and Emx2

Genes expressed in the somatopleuric mesoderm, the embryonic domain giving rise to the vertebrate pelvis, appear important for pelvic girdle formation. Among such genes, Pbx family members and Emx2 were found to genetically interact in hindlimb and pectoral girdle formation. Here, we generated compound mutant embryos carrying combinations of mutated alleles for Pbx1, Pbx2, and Pbx3, as well as Pbx1 and Emx2, to examine potential genetic interactions during pelvic development. Indeed, Pbx genes share overlapping functions and Pbx1 and Emx2 genetically interact in pelvic formation. We show that, in compound Pbx1;Pbx2 and Pbx1;Emx2 mutants, pelvic mesenchymal condensation is markedly perturbed, indicative of an upstream control by these homeoproteins. We establish that expression of Tbx15, Prrx1, and Pax1, among other genes involved in the specification and development of select pelvic structures, is altered in our compound mutants. Lastly, we identify potential Pbx1-Emx2-regulated enhancers for Tbx15, Prrx1, and Pax1, using bioinformatics analyses.

Pbx homeodomain proteins: TALEnted regulators of limb patterning and outgrowth

Limb development has long provided an excellent model for understanding the genetic principles driving embryogenesis. Studies utilizing chick and mouse have led to new insights into limb patterning and morphogenesis. Recent research has centered on the regulatory networks underlying limb development. Here, we discuss the hierarchical, overlapping, and iterative roles of Pbx family members in appendicular development that have emerged from genetic analyses in the mouse. Pbx genes are essential in determining limb bud positioning, early bud formation, limb axes establishment and coordination, and patterning and morphogenesis of most elements of the limb and girdle. Pbx proteins directly regulate critical effectors of limb and girdle development, including morphogen-encoding genes like Shh in limb posterior mesoderm, and transcription factor-encoding genes like Alx1 in pre-scapular domains. Interestingly, at least in limb buds, Pbx appear to act not only as Hox cofactors, but also in the upstream control of 5' HoxA/D gene expression.

Scapula development is governed by genetic interactions of Pbx1 with its family members and with Emx2 via their cooperative control of Alx1

The genetic pathways underlying shoulder blade development are largely unknown, as gene networks controlling limb morphogenesis have limited influence on scapula formation. Analysis of mouse mutants for Pbx and Emx2 genes has suggested their potential roles in girdle development. In this study, by generating compound mutant mice, we examined the genetic control of scapula development by Pbx genes and their functional relationship with Emx2. Analyses of Pbx and Pbx1;Emx2 compound mutants revealed that Pbx genes share overlapping functions in shoulder development and that Pbx1 genetically interacts with Emx2 in this process. Here, we provide a biochemical basis for Pbx1;Emx2 genetic interaction by showing that Pbx1 and Emx2 can bind specific DNA sequences as heterodimers. Moreover, the expression of genes crucial for scapula development is altered in these mutants, indicating that Pbx genes act upstream of essential pathways for scapula formation. In particular, expression of Alx1, an effector of scapula blade patterning, is absent in all compound mutants. We demonstrate that Pbx1 and Emx2 bind in vivo to a conserved sequence upstream of Alx1 and cooperatively activate its transcription via this potential regulatory element. Our results establish an essential role for Pbx1 in genetic interactions with its family members and with Emx2 and delineate novel regulatory networks in shoulder girdle development.

4D-analysis of early pelvic girdle development in the mouse (Mus musculus)

The formation of limb girdles is a key-novelty in vertebrate evolution. Although the knowledge of pattern formation, genetic, and molecular analysis of limb development has prodigiously grown over the past four decades, the morphogenesis of the pelvic element, joining the appendicular with the axial skeleton has poorly been investigated. Because of their heterochrony in development and evolution, axial and appendicular skeletal elements have seldom been seen as a cojoined morphological complex. The present study examines the pelvis morphogenesis in the mouse (Mus musculus), with special focuses on the axio-appendicular linkage, the formation and number of elements, and the joint formation. Serial histological sections of specimens from Theiler stages (TH) 18-25 (Theiler, 1972) were examined using bright field microscopy. 3D-models of the growing pelvis were reconstructed from these serial sections. The generated 3D-models were subsequently integrated into a computer-animated 4D-visualization illustrating the complex developmental dynamics of the mammalian pelvis morphogenesis. The findings demonstrate that the pelvic element forms from a single mesenchymal condensation in close vicinity to the appendicular skeleton. From the early start of development the pelvic element is limb-associated, and quite lately connects to the axial skeleton. Additionally, the 4D-visualization of the entire developmental process reveals a yet unnoticed reorientation of the mouse pelvic element from an initial posteriorly oblique developmental position to a ventrally oblique definitive position.

T-box protein Tbx18 interacts with the paired box protein Pax3 in the development of the paraxial mesoderm

The compartmentalization of somites along their anterior-posterior axis is crucial to the segmental organization of the vertebral column. Anterior-posterior somite polarity is generated in the anterior presomitic mesoderm by Mesp2 and Delta/Notch signaling and is further maintained by two transcriptional regulators, Uncx4.1 and Tbx18, acting in the posterior and anterior somite compartment, respectively. Here, we report that the paired box transcription factor Pax3 cooperates with the T-box protein Tbx18 in maintaining anterior somite half identity. Our findings that both genes are co-expressed in the anterior presomitic mesoderm and in early somites, that Pax3 and Tbx18 proteins physically interact, and that the loss of Pax3 gene function enhances the vertebral defects (i.e. the gain of vertebral elements derived from posterior somite halves in Tbx18 mutant mice) suggests that the two proteins cooperatively regulate the gene expression program necessary for maintaining anterior-posterior somite polarity. Genetic interaction of Pax3 with Tbx18 and the closely related T-box gene Tbx15 was also observed in the development of the scapula blade, indicating an additional cooperative function for these genes in the paraxial mesoderm.

Amniote somite derivatives

Somites are segments of paraxial mesoderm that give rise to a multitude of tissues in the vertebrate embryo. Many decades of intensive research have provided a wealth of data on the complex molecular interactions leading to the formation of various somitic derivatives. In this review, we focus on the crucial role of the somites in building the body wall and limbs of amniote embryos. We give an overview on the current knowledge on the specification and differentiation of somitic cell lineages leading to the development of the vertebral column, skeletal muscle, connective tissue, meninges, and vessel endothelium, and highlight the importance of the somites in establishing the metameric pattern of the vertebrate body.