Pthlha and mechanical force control early patterning of growth zones in the zebrafish craniofacial skeleton

To squeeze or not: Regulation of cell size by mechanical forces in development and human diseases

Physical constraints, such as compression, shear stress, stretching and tension play major roles during development and tissue homeostasis. Mechanics directly impact physiology, and their alteration is also recognized as having an active role in driving human diseases. Recently, growing evidence has accumulated on how mechanical forces are translated into a wide panel of biological responses, including metabolism and changes in cell morphology. The aim of this review is to summarize and discuss our knowledge on the impact of mechanical forces on cell size regulation. Other biological consequences of mechanical forces will not be covered by this review. Moreover, wherever possible, we also discuss mechanosensors and molecular and cellular signaling pathways upstream of cell size regulation. We finally highlight the relevance of mechanical forces acting on cell size in physiology and human diseases.

Common features of cartilage maturation are not conserved in an amphibian model

BACKGROUND

Mouse, chick, and zebrafish undergo a highly conserved program of cartilage maturation during endochondral ossification (bone formation via a cartilage template). Standard histological and molecular features of cartilage maturation are chondrocyte hypertrophy, downregulation of the chondrogenic markers Sox9 and Col2a1, and upregulation of Col10a1. We tested whether cartilage maturation is conserved in an amphibian, the western clawed frog Xenopus tropicalis, using in situ hybridization for standard markers and a novel laser-capture microdissection RNAseq data set. We also functionally tested whether thyroid hormone drives cartilage maturation in X tropicalis, as it does in other vertebrates.

RESULTS

The developing frog humerus mostly followed the standard progression of cartilage maturation. Chondrocytes gradually became hypertrophic as col2a1 and sox9 were eventually down-regulated, but col10a1 was not up-regulated. However, the expression levels of several genes associated with the early formation of cartilage, such as acan, sox5, and col9a2, remained highly expressed even as humeral chondrocytes matured. Greater deviances were observed in head cartilages, including the ceratohyal, which underwent hypertrophy within hours of becoming cartilaginous, maintained relatively high levels of col2a1 and sox9, and lacked col10a1 expression. Interestingly, treating frog larvae with thyroid hormone antagonists did not specifically reduce head cartilage hypertrophy, resulting rather in a global developmental delay.

CONCLUSION

These data reveal that basic cartilage maturation features in the head, and to a lesser extent in the limb, are not conserved in X tropicalis. Future work revealing how frogs deviate from the standard cartilage maturation program might shed light on both evolutionary and health studies.

Mechanical force regulates Sox9 expression at the developing enthesis

Entheses transmit force from tendons and ligaments to the skeleton. Regional organization of enthesis extracellular matrix (ECM) generates differences in stiffness required for force transmission. Two key transcription factors co-expressed in entheseal tenocytes, scleraxis (Scx) and Sox9, directly control production of enthesis ECM components. Formation of embryonic craniofacial entheses in zebrafish coincides with onset of jaw movements, possibly in response to the force of muscle contraction. We show dynamic changes in scxa and sox9a mRNA levels in subsets of entheseal tenocytes that correlate with their roles in force transmission. We also show that transcription of a direct target of Scxa, Col1a, in enthesis ECM is regulated by the ratio of scxa to sox9a expression. Eliminating muscle contraction by paralyzing embryos during early stages of musculoskeletal differentiation alters relative levels of scxa and sox9a in entheses, primarily owing to increased sox9a expression. Force-dependent TGF-β (TGFβ) signaling is required to maintain this balance of scxa and sox9a expression. Thus, force from muscle contraction helps establish a balance of transcription factor expression that controls specialized ECM organization at the tendon enthesis and its ability to transmit force.

Cellular basis of differential endochondral growth in Lake Malawi cichlids

BACKGROUND

The shape and size of skeletal elements is determined by embryonic patterning mechanisms as well as localized growth and remodeling during post-embryonic development. Differential growth between endochondral growth plates underlies many aspects of morphological diversity in tetrapods but has not been investigated in ray-finned fishes. We examined endochondral growth rates in the craniofacial skeletons of two cichlid species from Lake Malawi that acquire species-specific morphological differences during postembryonic development and quantified cellular mechanisms underlying differential growth both within and between species.

RESULTS

Cichlid endochondral growth rates vary greatly (50%-60%) between different growth zones within a species, between different stages for the same growth zone, and between homologous growth zones in different species. Differences in cell proliferation and/or cell enlargement underlie much of this differential growth, albeit in different proportions. Strikingly, differences in extracellular matrix production do not correlate with growth rate differences.

CONCLUSIONS

Differential endochondral growth drives many aspects of craniofacial morphological diversity in cichlids. Cellular proliferation and enlargement, but not extracellular matrix deposition, underlie this differential growth and this appears conserved in Osteichthyes. Cell enlargement is observed in some but not all cichlid growth zones and the degree to which it occurs resembles slower growing mammalian growth plates.

Mechanical control of tissue shape: Cell-extrinsic and -intrinsic mechanisms join forces to regulate morphogenesis

During vertebrate development, cells must proliferate, move, and differentiate to form complex shapes. Elucidating the mechanisms underlying the molecular and cellular processes involved in tissue morphogenesis is essential to understanding developmental programmes. Mechanical stimuli act as a major contributor of morphogenetic processes and impact on cell behaviours to regulate tissue shape and size. Specifically, cell extrinsic physical forces are translated into biochemical signals within cells, through the process of mechanotransduction, activating multiple mechanosensitive pathways and defining cell behaviours. Physical forces generated by tissue mechanics and the extracellular matrix are crucial to orchestrate tissue patterning and cell fate specification. At the cell scale, the actomyosin network generates the cellular tension behind the tissue mechanics involved in building tissue. Thus, understanding the role of physical forces during morphogenetic processes requires the consideration of the contribution of cell intrinsic and cell extrinsic influences. The recent development of multidisciplinary approaches, as well as major advances in genetics, microscopy, and force-probing tools, have been key to push this field forward. With this review, we aim to discuss recent work on how tissue shape can be controlled by mechanical forces by focusing specifically on vertebrate organogenesis. We consider the influences of mechanical forces by discussing the cell-intrinsic forces (such as cell tension and proliferation) and cell-extrinsic forces (such as substrate stiffness and flow forces). We review recently described processes supporting the role of intratissue force generation and propagation in the context of shape emergence. Lastly, we discuss the emerging role of tissue-scale changes in tissue material properties, extrinsic forces, and shear stress on shape establishment.

Zebrafish endochondral growth zones as they relate to human bone size, shape and disease

Research on the genetic mechanisms underlying human skeletal development and disease have largely relied on studies in mice. However, recently the zebrafish has emerged as a popular model for skeletal research. Despite anatomical differences such as a lack of long bones in their limbs and no hematopoietic bone marrow, both the cell types in cartilage and bone as well as the genetic pathways that regulate their development are remarkably conserved between teleost fish and humans. Here we review recent studies that highlight this conservation, focusing specifically on the cartilaginous growth zones (GZs) of endochondral bones. GZs can be unidirectional such as the growth plates (GPs) of long bones in tetrapod limbs or bidirectional, such as in the synchondroses of the mammalian skull base. In addition to endochondral growth, GZs play key roles in cartilage maturation and replacement by bone. Recent studies in zebrafish suggest key roles for cartilage polarity in GZ function, surprisingly early establishment of signaling systems that regulate cartilage during embryonic development, and important roles for cartilage proliferation rather than hypertrophy in bone size. Despite anatomical differences, there are now many zebrafish models for human skeletal disorders including mutations in genes that cause defects in cartilage associated with endochondral GZs. These point to conserved developmental mechanisms, some of which operate both in cranial GZs and limb GPs, as well as others that act earlier or in parallel to known GP regulators. Experimental advantages of zebrafish for genetic screens, high resolution live imaging and drug screens, set the stage for many novel insights into causes and potential therapies for human endochondral bone diseases.