Editorial highlights

Every organism is a model organism for understanding development, evolution, disease, and regeneration, and we have only begun to scratch the surface of the interdisciplinary genetic, molecular, cellular, and developmental mechanisms that regulate these biological processes. These “Highlights” denote exciting advances recently reported in Developmental Dynamics that illustrate the complex dynamics of developmental biology.

Mechanical Forces in Development “Exploring the Role of Mechanical Forces on Tendon Development Using in vivo Model: A Scoping Review,” by Yuna Usami, Hirotaka Iijima, Takanori Kokubun; DevDyn 253:6, pp. 550-565. https://doi.org/10.1002/dvdy.673. Tendons comprise connective tissue that transmits muscle contraction forces to bones and drives joint movement throughout life. Interestingly, however, mechanical forces also regulate and control key cellular and molecular responses during musculoskeletal tissue development, differentiation, and growth. Scleraxis (Scx), a basic helix–loop–helix (bHLH) transcription factor, is the most representative marker of tendon development and most studies have focused on the loss of muscle, muscle dysfunction, and weight-bearing regulation, but few investigated the effect of increased mechanical force. This review summarizes our current knowledge about animal models and approaches for modulating mechanical forces on tendon development; defines the role of mechanical force through the activity of Scx and other tendon development-associated factors; and raises important questions and directions for the field to address in the future.

Zebrafish Fin Development “wnt10a is required for zebrafish median fin fold maintenance and adult unpaired fin metamorphosis” by Erica Benard, Ismail Küçükaylak, Julia Hatzold, Kilian Berendes, Thomas J. Carney, Filippo Beleggia, and Matthias Hammerschmidt; DevDyn 253:6, pp. 566-592. https://doi.org/10.1002/dvdy.672.

Wnt signaling regulates critical cell-to-cell interactions in multiple developmental processes during embryogenesis and in the homeostasis of adult tissues. Mutations in human WNT10A are associated with odonto-ectodermal dysplasia syndromes, which are primarily characterized by severe oligodontia of permanent teeth, and skin anomalies. In this study, the authors generate wnt10a mutant zebrafish embryos, which display impaired tooth development and a collapsing median fin fold (MFF), making them a good model for odonto-onycho-dermal dysplasia. Focusing on the MFF, dlx2a activity was found to be decreased in the distal-most cells, together with perturbed expression of col1a1a and other extracellular matrix proteins encoding genes. Consequently, positioning of actinotrichia within the cleft of distal MFF cells become compromised, coinciding with actinotrichia shrinkage and MFF collapse. Rescue experiments demonstrate that wnt10a is essential for MFF maintenance, both during embryogenesis and later metamorphosis. The mechanism is strikingly similar to the proposed molecular etiology and cellular pathogenesis mechanisms underlying the teeth defects caused by Wnt10 loss-of-function in fish and mammals.

Meiosis and Spermatogenesis in Mammals “Elevated Id2 Expression Causes Defective Meiosis and Spermatogenesis in Mice” by Zhen He, Rong-Ge Yan, Qin-Bang Shang, and Qi-En Yang; DevDyn 253:6, pp. 593-605. https://doi.org/10.1002/dvdy.676.

Meiosis during germ cell development generates genetic diversity via recombination and redistribution of homologous chromosomes. In the male germline, spermatogonia are the most primitive germ cells, and they arise from gonocytes (or prespermatogonia) shortly after birth. Spermatogonia are capable of entering meiosis prophase I after several rounds of division, but defects in their development and errors in meiosis often lead to infertility or aneuploidy in humans. Although meiotic progression can be driven by cyclin-CDK (cyclin-dependent kinase) complexes, the proteins that govern this process remain to be fully elucidated. Inhibitors of DNA binding/differentiation (ID proteins) are master regulators of cell cycle progression and cell differentiation, and gene expression analyses of spermatogenic cells identified ID2 as a potentially important regulator of meiosis in mice. ID2 is expressed in spermatogonia, spermatocytes, and Sertoli. In this study, the authors generated conditional Id2 knockout and transgenic Id2 overexpressing mouse lines to dissect the function of ID2 in spermatogenesis. Although a germ cell-specific knockout of Id2 did not affect spermatogenesis, most likely due to functional compensation with other Id proteins, in contrast, overexpression of Id2 in germ cells led to decreased testis size and severe meiotic defects. Thus, ID2 is functionally important for zygotene to pachytene transition during meiosis in male germ cells during mammalian development.