Suprascapular notch morphology in the pediatric population: a computed tomography study.

Suprascapular notch is characterized by variable morphology. However, its development is not well studied. We hypothesize that it proceeds postnatally. Thus, the aim of this research was to characterize the morphology of the suprascapular notch in a pediatric population based on computed tomography. A retrospective analysis was performed of 291 chest computed tomography examinations of patients under 18 years old taken following other clinical indications. The inclusion criteria were as follows: both scapulae encompassed in a field of view; no artifacts; no pathologies concerning the scapulae. Based on visual assessment and measurements, the suprascapular notch was classified according to a fivefold classification (type I, deeper than wider; type II, equally deep and wide; type III, wider than deeper; type IV, bony foramen; type V, discreet notch). In all, 173 examinations were included (60 females and 113 males). The most common suprascapular notch types were discreet notch (type V, 225 scapulae; 65.0 %) and type III (114 scapulae; 32.9 %). Children with type V suprascapular notch were significantly younger than children with other types (26.1 ± 42.4 months vs. 111.2 ± 66.7 months; p < 0.05). In types I-III, a positive correlation was found between age and dimensions of the suprascapular notch (p < 0.05). This study provides the first description of the suprascapular notch in a pediatric population based on computed tomography. It confirms that morphology of the suprascapular notch undergoes postnatal development.

Bone age estimation based on multislice computed tomography study of the scapula.

Progress in medical imaging has opened new areas of research in forensic anthropology, especially in the context of the study of bone age assessment. The study of bone age has become a useful tool for age estimation at death or age of young adult migrants in an anthropological context. We retrospectively evaluated multislice computed tomography (MSCT) explorations focused on scapulae of 232 individuals (123 males; 109 females) aged between 8 and 30 years old. Computed tomography (CT) scans were viewed in axial and multiplanar reconstructed images using OsiriX 5.9 (64 bit)®. The ossification centers of the scapula studied were as follows: acromial, sub-coracoid, glenoid, coracoid, coracoid apex, and inferior angle epiphyses. Fusion status was scored based on a five-stage system (stage 1: no ossification, stage 2: visualization of an ossification center, stage 3: partial ossification, stage 4: full ossification associated to an epiphyseal scar, and stage 5: full ossification without epiphyseal scar). Intra-observer variability was excellent, and inter-observer variability was good, demonstrating the reliability of this MSCT staging system. The fusion of scapular ossification centers was statistically associated with age (p < 0.001) but not with sex (p > 0.05). In conclusion, MSCT of the scapula is an efficient method for age assessment, which is complementary to preexisting methods particularly for specifying the 18-year threshold. Further studies with larger groups are needed to support our results.

Growth of the pectoral girdle in a sample of juveniles from the kellis 2 cemetery, Dakhleh Oasis, Egypt.

OBJECTIVES: This study investigates growth patterns in the scapula and clavicle in a cross-sectional juvenile skeletal sample ranging from 20 weeks gestation to 8.5 years of age from the Kellis 2 cemetery, Dakhleh Oasis, Egypt. The primary goal is to quantify growth patterns and growth velocities in the scapula and clavicle to better understand the development of the pectoral girdle. METHODS: A series of low-order polynomial regression models was used to examine growth curves in clavicle diaphyseal length, scapular height, and scapular width. Incremental growth and relative percent increase were examined among successive age groups as a proxy measure of growth velocity. Scapular body proportions were assessed with the scapular index and compared across age groups using a Kruskal-Wallis test with post-hoc tests. RESULTS: A third-order polynomial best describes growth in clavicle diaphyseal length and scapular height, and a second-order polynomial best describes growth in scapular width. Growth velocity patterns are similar among clavicle diaphyseal length, scapular height, and scapular width particularly from birth until the end of early childhood. Clavicle diaphyseal length decelerates during middle childhood while scapular height and width accelerate during this time. With increasing age, the scapular body proportionately increases more in height than in width. The relatively narrow scapular body characteristic of adult scapulae is first evident during early childhood. CONCLUSIONS: Changes in scapular body shape during ontogeny may be a reflection of the greater alterations taking place in the integrated morphology of the pectoral girdle during the biomechanical shift from crawling to bipedalism. Am. J. Hum. Biol. 28:636-645, 2016. © 2016 Wiley Periodicals, Inc.

Normal Skeletal Maturation and Imaging Pitfalls in the Pediatric Shoulder.

A growing number of magnetic resonance (MR) imaging studies of the shoulder are being performed as a result of greater and earlier participation of children and adolescents in competitive sports such as softball and baseball. However, scant information is available regarding the MR imaging features of the normal sequential development of the shoulder. The authors discuss the radiographic and MR imaging appearances of the normal musculoskeletal maturation patterns of the shoulder, with emphasis on (a) development of secondary ossification centers of the glenoid (including the subcoracoid and peripheral glenoid ossification centers); (b) development of preossification and secondary ossification centers of the humeral head and the variable appearance and number of the secondary ossification centers of the distal acromion, with emphasis on the formation of the os acromiale; (c) development of the growth plates, glenoid bone plates, glenoid bare area, and proximal humeral metaphyseal stripe; and (d) marrow signal alterations in the distal humerus, acromion, and clavicle. In addition, the authors discuss various imaging interpretation pitfalls inherent to the normal skeletal maturation of the shoulder, examining clues that may help distinguish normal development from true disease (eg, osteochondral lesions, labral tears, abscesses, fractures, infection, tendon disease, acromioclavicular widening, and os acromiale). Familiarity with the timing, location, and appearance of maturation patterns in the pediatric shoulder is crucial for correct image interpretation.

Skeletal remodelling suggests the turtle's shell is not an evolutionary straitjacket.

Recent efforts to decipher the enigma of the turtle's shell revealed that distantly related turtle species deploy diverse processes during shell development. Even so, extant species share in common a shoulder blade (scapula) that is encapsulated within the shell. Thus, evolutionary change in the correlated development of the shell and scapula probably underpins the evolution of highly derived shell morphologies. To address this expectation, we conducted one of the most phylogenetically comprehensive surveys of turtle development, focusing on scapula growth and differentiation in embryos, hatchlings and adults of 13 species. We report, to our knowledge, the first description of secondary differentiation owing to skeletal remodelling of the tetrapod scapula in turtles with the most structurally derived shell phenotypes. Remodelling and secondary differentiation late in embryogenesis of box turtles (Emys and Terrapene) yielded a novel skeletal segment (i.e. the suprascapula) of high functional value to their complex shell-closing system. Remarkably, our analyses suggest that, in soft-shelled turtles (Trionychidae) with extremely flattened shells, a similar transformation is linked to truncated scapula growth. Skeletal remodelling, as a form of developmental plasticity, might enable the seemingly constrained turtle body plan to diversify, suggesting the shell is not an evolutionary straitjacket.

Skeletal development of the glenoid and glenoid-coracoid interface in the pediatric population: MRI features.

OBJECTIVE: To assess the MRI appearance of normal skeletal development of the glenoid and glenoid-coracoid interface in the pediatric population. To the best of our knowledge, this has not yet been studied in detail in the literature. MATERIALS AND METHODS: An IRB-approved, HIPAA-compliant retrospective review of 105 consecutive shoulder MRI studies in children, ages 2 months to 18 years was performed. The morphology, MR signal, and development of the following were assessed: (1) scapular-coracoid bipolar growth plate, (2) glenoid and glenoid-coracoid interface secondary ossification centers, (3) glenoid advancing osseous surface. RESULTS: The glenoid and glenoid-coracoid interface were identified in infancy as a contiguous, cartilaginous mass. A subcoracoid secondary ossification center in the superior glenoid was identified and fused in all by age 12 and 16, respectively. In ten studies, additional secondary ossification centers were identified in the inferior two-thirds of the glenoid. The initial concavity of the glenoid osseous surface gradually transformed to convexity, matching the convex glenoid articular surface. The glenoid growth plate fused by 16 years of age. Our study, based on MRI, demonstrated a similar pattern of development of the glenoid and glenoid coracoid interface to previously reported anatomic and radiographic studies, except for an earlier development and fusion of the secondary ossification centers of the inferior glenoid. CONCLUSIONS: The pattern of skeletal development of the glenoid and glenoid-coracoid interface follows a chronological order, which can serve as a guideline when interpreting MRI studies in children.

Hypophosphatemic rickets is associated with disruption of mineral orientation at the nanoscale in the flat scapula bones of rachitic mice with development.

Metabolic bone disorders such as rickets are associated with altered in vivo muscular force distributions on the skeletal system. During development, these altered forces can potentially affect the spatial and temporal dynamics of mineralised tissue formation, but the exact mechanisms are not known. Here we have used a murine model of hypophosphatemic rickets (Hpr) to study the development of the mineralised nanostructure in the intramembranously ossifying scapulae (shoulder bone). Using position-resolved scanning small angle X-ray scattering (SAXS), we quantified the degree and direction of mineral nanocrystallite alignment over the width of the scapulae, from the load bearing lateral border (LB) regions to the intermediate infraspinous fossa (IF) tissue. These measurements revealed a significant (p<0.05) increase in mineral nanocrystallite alignment in the LB when compared to the IF region, with increased tissue maturation in wild-type mice; this was absent in mice with rickets. The crystallites were more closely aligned to the macroscopic bone boundary in the LB when compared to the IF region in both wild type and Hpr mice, but the degree of alignment was reduced in Hpr mice. These findings are consistent with a correlation between the nanocrystallites within fibrils and in vivo muscular forces. Thus our results indicate a relevant mechanism for the observed increased macroscopic deformability in rickets, via a significant alteration in the mineral particle alignment, which is mediated by an altered spatial distribution of muscle forces.

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.

The developmental reduction of the marsupial coracoid: a case study in Monodelphis domestica.

During their embryogenesis, marsupials develop a unique structure, the shoulder arch, which provides the structural and muscle-attachment support necessary for the newborn's crawl to the teat. One of the most pronounced and important aspects of the shoulder arch is an enlarged coracoid. After marsupial newborns reach the teat, the shoulder arch is remodeled and the coracoid is reduced to a small process on the scapula. Although an understanding of marsupial coracoid reduction has the potential to provide insights into both, marsupial evolution and the origin of mammals, little is known about the morphological and cellular processes controlling this process. To remedy this situation, this study examined the morphological and cellular mechanisms behind coracoid reduction in the gray short-tailed opossum, Monodelphis domestica. A quantitative, morphometric study of shoulder girdle development revealed that the coracoid is reduced in size relative to other aspects of the shoulder girdle by growing at a slower rate. Using a series of molecular assays for cell death, no evidence was found for programmed cell death playing a role in the reduction of coracoid size in marsupials (in contrast to hypotheses of previous researchers). Although it is likely the case that coracoid growth is reduced through a relatively lower rate of cellular proliferation, differences in proliferative rates in the coracoid and scapula were not great enough to be quantified using standard molecular assays.

Músculo romboides supraescapular de la Llama (Lama glamá) / Rhomboideus suprascapularis muscle of the llama (Lama glama)

El músculo romboides (m. rhomboideus) forma parte de la sinsarcosis que une la cintura del miembro torácico con el esqueleto axil en los mamíferos domésticos. En estas especies, está integrado por los músculos romboides cervical (m. rhomboideus cervicis) y romboides torácico (m. rhomboideus thoracis), siendo imposible establecer el límite entre ellos, a diferencia de lo que sucede en el Hombre. Los carnívoros en general, el cerdo y el conejo presentan, además de las partes mencionadas, el músculo romboides de la cabeza (m. rhomboideus capitis). En la llama, la porción cefálica está ausente y la cervical pobremente desarrollada. Los autores proponen sumar a las porciones cervical y torácica del músculo romboides de este camélido sudamericano, al músculo romboides supraescapular (m. rhomboideus suprsacapularis), descrito por Lesbre (1903), en el camello y el dromedario. En este trabajo se establecen las inserciones, dimensiones e inervación del músculo romboides supraescapular de la llama, elementos que permiten definirlo como otra porción del complejo muscular romboideo en dicha especie. Además, se postula su acción como elevador de la escápula, dirigiéndola craneal y dorsalmente.

The rhomboideus muscle of the domestic mammals is part of the muscular set that joins the scapular waist to the axial skeleton. In these animals, the forenamed muscle has a cervical portion (m. rhomboideus cervicicis), and a thoracic portion (m. rhomboideus thoracis). Unlike in the man, these parts cannot be separated. In addition, carnivores, pigs and rabbits also have a cephalic portion (m. rhomboideus capitis). In the llamas, the cephalic portion is absent, and the cervical part is poorly developed. The authors propose to add a suprascapular portion (m. rhomboideus suparsacapularis), first described by Lesbre, 1903 in camels, to the Rhomboideus muscular complex of the llama. In this study, the authors describe the length, insertions, and inervation of the Rhomboideus Suprascapular muscle of the llama in order to define it as another portion of the Rhomboideus muscular complex. Moreover, the forenamed muscle is proposed as a scapular elevator.

A comparison of the ontogeny of shape variation in the anthropoid scapula: functional and phylogenetic signal.

This article compares ontogenetic shape variation in the scapula of 17 anthropoid species using three-dimensional landmark-based geometric morphometrics. These data are used to investigate functional and phylogenetic signal in the major components of scapular variation and to evaluate the degree to which postnatal growth contributes to interspecific differences in shape. Results indicate that the shape of the infant and adult scapula is primarily associated with positional behavior (e.g., orthograde suspensory nonquadrupeds versus pronograde quadrupeds), but within this functional structure there is phylogenetic signal, particularly at infant stages. Growth most closely correlates with infant/adult shape and locomotor function. These results suggest that the shape of the infant scapula drives the pattern of postnatal scapular growth and adult morphology. As such, variation in postnatal growth is not the primary source of interspecific variation in adult shape. Instead, interspecific differences in scapular morphology are hypothesized to be the result of selection for variation in embryonic developmental processes that affect shape.

Evaluation of shoulder balance in the normal adolescent population and its correlation with radiological parameters.

A descriptive clinical study in healthy adolescents was done to evaluate the clinical shoulder balance and analyze the correlation between clinical and radiological parameters which are currently used to evaluate shoulder balance. In addition to trunk shift and rib hump, shoulder balance is one of the criteria that are used to evaluate the outcomes in spinal deformity surgery. Several methods have been proposed to evaluate the shoulder balance in scoliotic patients; however, there is no uniformity to these methods in the current literature. Patients who applied to pediatric clinic without musculoskeletal pathology formed the patient population. Volunteers were asked to fill out a questionnaire assessing shoulder balance perception and had their clinical photograph taken simultaneously with a P-A chest X-ray. The clinical shoulder balance was evaluated through analysis of the clinical photograph. The X-rays were used to evaluate the radiological shoulder balance. The evaluated parameters included coracoid height difference (CHD), clavicular angle (CA), the clavicle-rib cage intersection difference (CRID), clavicular tilt angle difference (CTAD), and T1-tilt. The study group was composed of 48 male and 43 female patients with an average age of 13.6 +/- 2.1 (10-18) years. In the questionnaire, all patients stated that their shoulders were level. The digital photographs revealed that only 17(18.7%) adolescents had absolutely level shoulders. The average height difference between shoulders was 7.5 +/- 5.8 mm. The average CHD was 6.9 +/- 5.8 mm, average CA was 2.2 +/- 1.7 degrees , average CRID was 4.8 +/- 3.6 mm, average CTAD was 4 +/- 3.2 degrees , and average T1-tilt was 1.3 +/- 1.4 degrees . CHD, CA, and CRID demonstrated high correlation with clinical pictures, whereas CTAD demonstrated moderate and T1-tilt demonstrated only mild correlation. The radiological parameters used to evaluate the shoulder balance correlate with the clinical appearance. Contrary to popular belief, shoulder balance in healthy adolescents often does not exist.

Function, ontogeny and canalization of shape variance in the primate scapula.

Primates have shoulders adapted to a wide range of locomotor functions from terrestrial pronograde quadrupedalism to highly arboreal suspensory behaviours. The shape of the scapula tightly follows these functional differences. Previous analyses of primate postcrania, including the scapula, indicate that quadrupedal monkeys are less variable than non-quadrupeds. It was previously suggested that this difference was due to a relationship between the strength of stabilizing selection and the functional demands of the upper limb. Here it is shown that intraspecific scapular shape variance is highly correlated with the degree of committed quadrupedalism. Primates that engage in frequent suspensory behaviours (e.g. apes and ateline monkeys) average twice the amount of shape variance as quadrupeds (e.g. Old World monkeys and Saimiri). Because this difference in intraspecific shape variance is apparent in infants and does not increase or decrease appreciably over ontogeny, it is not likely that differences in postnatal growth, neuromuscular control or environmental factors such as habitat structure/composition are the primary contributors to differences in adult shape variance. Instead variance in embryonic factors that affect the shape/size of the scapula or epigenetic factors associated with muscle attachments are more likely candidates. In particular, the heterogeneous functional demands of the non-quadrupedal shoulder probably reduce the stringency of stabilizing selection, resulting in the persistence into adulthood of increased amounts of embryonically generated scapular shape variance.

Transformation of the pectoral girdle in the evolutionary origin of frogs: insights from the primitive anuran Discoglossus.

Using cleared-and-stained whole mounts and computer-aided three-dimensional reconstructions made from serial histological sections, we studied the development of the pectoral girdle in Discoglossus pictus, an extant member of an ancient frog lineage, represented for example by Eodiscoglossus from the Middle Jurassic to Early Cretaceous periods in Europe. Basic developmental features were compared with those of extinct Temnospondyli, considered to be the most probable anuran ancestors, and with Triadobatrachus, an early Triassic proanuran. In the endochondral girdle, the separate scapula and coracoid of Discoglossus and other anurans (completed by suprascapular and procoracoid cartilages) evolved from the compact scapulocoracoid of temnospondyls by paedomorphosis. In parallel, the dermal ossifications of the girdle were reduced to a small clavicle and cleithrum. The overall reduction in ossification of the anuran pectoral girdle supports the hypothesis of a paedomorphic origin for Anura. The almost simultaneous appearance of dermal and endochondral ossifications may be explained by the accumulation of developmental events during a short, distinct metamorphosis (which did not occur in neotenic temnospondyls living permanently in water). The sternal elements seem to be neomorphs for the most part, which help to cushion the shock of landing in jumping anurans but which also evolved as functional substitutes (insertion area for the pectoralis muscles) of the temnospondyl interclavicle.

Cross-population analysis of the growth of long bones and the os coxae of three Early Medieval Austrian populations.

Inter-population variability in long-bone and pelvic-bone growth during the Early Medieval period is examined. The materials comprise four archaeological populations: two Slavonic (Gars-Thunau, Zwentendorf, Austria, 10th-century AD), one Avar (Zwölfaxing, Austria, 8th-century AD), and one Anglo-Saxon (Raunds, England, 10th-century AD). Bone measurements are analyzed against dental age estimates in order to assess inter-population differences in growth rates for long-bone and os coxae bone dimensions. Growth curves of the upper and lower extremities of additional archaeological populations and a modern North-American population are also assessed. The expectation was that the greatest differences in growth patterns would be found between the Anglo-Saxon and the Austrian samples, due to their distinct genetic and biocultural background. Minimal differences were expected between the two Slavonic populations, as these were approximately contemporaneous, recovered from geographically close locations, and shared relatively similar archaeological contexts. Growth curves were estimated for each bone dimension by fitting least-squares fourth-order polynomials (which allowed testing of population differences by analysis of covariance), and iteratively estimating Gompertz growth curves. The results showed differences between bones in the extent of inter-population variability, with diaphyseal long-bone growth showing equivalent patterns across the four populations, but significant differences between populations in the growth patterns of distal diaphyseal dimensions of the femur and humerus and the dimensions of the ilium. Varying growth patterns are therefore associated with inter-population differences in absolute dimensions in relation to age as well as variations in growth velocities. Inter-population variability in growth curves in the case of femoral and humeral dimensions were most pronounced during infancy (0-2 years). The most consistent differences in bone growth and related dimensions are between Zwölfaxing and the other samples. No significant differences in growth were detected between the Anglo-Saxon and the Austrian populations.

Growth of the pectoral girdle of the Leopard frog, Rana pipiens (Anura: Ranidae).

This article describes the growth of the anuran pectoral girdle of Rana pipiens and compares skeletal development of the shoulder to that of long bones. The pectoral girdle chondrifies as two halves, each adjacent to a developing humerus. In each, the scapula and coracoid form as single foci of condensed chondrocytes that fuse, creating a cartilaginous glenoid bridge articulating with the humerus. Based on histological sections, both the dermal clavicle and cleithrum begin to ossify at approximately the same time as the periosteum forms around the endochondral bones. The dermal and endochondral bones of the girdle form immobile joints with neighboring girdle elements; however, the cellular organization and growth pattern of the scapula and coracoid closely resemble those of a long bone. Similar to a long bone epiphysis, distal margins of both endochondral elements have zones of hyaline, stratified, and hypertrophic cartilages. As a result, fused elements of the girdle can grow without altering the glenoid articulation with the humerus. Comparisons of anuran long bone and pectoral girdle growth suggest that different bones can have similar histology and development regardless of adult morphology.

Modularity and integration in the hominoid scapula.

In this paper, several hypotheses of morphological integration within the hominoid (ape) scapula are tested. In particular, whether the scapula represents a set of developmental tissues sharing tight correlations between constituent parts (i.e., highly integrated) or is more modularly organized (i.e., covariation is greater within regions than between) is tested. Whether the patterns of integration in the scapula have changed over phylogenetic time or in response to selective forces is also examined. Results from two different analyses (matrix correlations and edge deviance) indicate traits comprising the blade and acromion, and to a weaker degree the glenoid, correlate highly with each other. The coracoid exhibits more independence from other parts of the scapula, perhaps reflecting its distinct evolutionary developmental history. Overall, similarity in species-specific patterns of correlation was high between all taxa. Correlation matrix similarity was significantly correlated with functional similarity and morphological distance, but not with phylogenetic distance. These results are congruent with other studies of integration that suggest correlation patterns remain stable over evolutionary time. There are changes associated with phylogeny, but the tight link between functional similarity and phylogenetic distance at this level of comparison presents possible challenges to interpretation. Overall similarities in the pattern of integration in all taxa might be better interpreted as relative strengthening or weakening of trait correlations rather than broadscale changes in the pattern of relationship between developmental regions. Larger sample sizes with greater taxonomic/functional breadth, and finer scale analyses of patterns of correlation are needed to test these hypotheses further.

Ontogenesis of the scapula in marsupial mammals, with special emphasis on perinatal stages of didelphids and remarks on the origin of the therian scapula.

The development of the scapula was studied in embryonic and postnatal specimens of Monodelphis domestica and perinatal specimens of Philander opossum, Caluromys philander, and Sminthopsis virginiae using histological sections and 3D reconstructions. Additionally, macerated skeletons of postnatal M. domestica were examined. This study focused on the detachment of the scapulocoracoid from the sternum and on the acquisition of a supraspinous fossa, a supraspinatus muscle, and a scapular spine, all these events associated with the origin of the therian shoulder girdle. In none of the specimens is there a continuity of the cartilaginous scapulocoracoid with the sternum, even though the structures are in close proximity, especially in S. virginiae. At birth, the first rib laterally presents a pronounced boss that probably contacts the humerus during certain movements. Only the acromial portion of the scapular spine, which originates from the anterior margin of the scapular blade, is preformed in cartilage. The other portion is formed by appositional bone ("Zuwachsknochen"), which expands from the perichondral ossification of the scapula into an intermuscular aponeurosis between the supra- and infraspinous muscles. This intermuscular aponeurosis inserts more or less in the middle of the lateral surface of the developing scapula. Thus, the floor of the supraspinous fossa is present from the beginning of scapular development, simultaneously with the infraspinous fossa. The homology of the therian spine with the anterior border of the sauropsid and monotreme scapula is questioned. We consider the dorsal portion (as opposed to the ventral or acromial portion) of the scapular spine a neomorphic structure of therian mammals.

Distribution of cartilage molecules in the developing mouse joint.

This study describes the precise spatial and temporal patterns of protein distribution for aggrecan, fibromodulin, cartilage oligomeric matrix protein (COMP) and cartilage matrix protein (CMP) in the developing mouse limb with particular attention to those cells destined to form articular chondrocytes in comparison to those cells destined to form a mineralized tissue and become replaced by bone. Mouse glenohumeral joints from fetal mice (12-18 days post coitus (dpc) to the young adult (37 days after birth) were immunostained with antibodies specific for these molecules. Aggrecan staining defined the general chondrocytic phenotype, whether articular or transient. Fibromodulin was associated with prechondrocytic mesenchymal cells in the interzone prior to joint cavitation and with the mesenchymal cells of the perichondrium or the periosteum encapsulating the joint elements of the maturing and young adult limb. Staining was most intense around developing articular chondrocytes and much less abundant or absent in those differentiating cells along the anlage. CMP showed an almost reciprocal staining pattern to fibromodulin and was not detected in the matrix surrounding articular chondrocytes. COMP was not detected in the cells at the articular surface prior to cavitation but by 18 dpc, as coordinated movement of the mouse forelimb intensifies, staining for COMP was most intense around the maturing articular chondrocytes. These results show that the cells that differentiate into articular chondrocytes elaborate an extracellular matrix distinct from those cells that are destined to form bone. Fibromodulin may function in the early genesis of articular cartilage and COMP may be associated with elaboration of a weight-bearing chondrocyte matrix.