In the magical world of human development, limb buds emerge by the end of the fourth post-conception week (PCW) and transform into the arms and legs we all know and love during the first trimester. This incredible process starts with the condensation of the skeletal system cells and the formation of tendon, fibrous and smooth muscle populations. Meanwhile, skeletal muscle progenitor (SkMP) cells migrate to the limb field from the paraxial mesoderm, coming together to form striated muscle. The mesoderm is enveloped by a thin layer of ectoderm, with a special subset called the apical ectodermal ridge steering the ship and setting the stage for limb axes through fibroblast growth factor (FGF) signalling. The development of limbs continues in a proximal–distal manner, controlled by a complex system of gene expression programmes, with small deviations leading to significant changes in structure and function. In fact, about 1 in 500 humans are born with congenital limb malformations. The study of model organisms such as the mouse and chick provide invaluable insights, but how this knowledge translates to human development and disease remains a mystery. Enter our hero: single-cell transcriptomic RNA sequencing (scRNA-seq) and spatial transcriptomic sequencing. These innovative techniques allow us to shed new light on limb development, tracking 67 distinct cell clusters across four first trimester timepoints. Thanks to VisiumStitcher, we can even map the cells to an entire fetal hindlimb section at PCW8. This spatial transcriptomic data also provides insights into key patterning events in the developing limb, with a special focus on genes associated with limb malformation. Plus, we’ve compared limb development across species, revealing striking similarities between our classic model organisms and humans. Still not impressed? You can access all our groundbreaking data for free and witness the wonders of limb development for yourself at https://developmental.cellatlas.io/embryonic-limb.
If that’s not enough to excite you, hold onto your seat because we’re about to dive into the cellular heterogeneity of the developing limb in space and time. We’ve collected single-cell embryonic limb profiles from PCW5 to PCW9, analysing 125,955 single cells to identify an incredible 67 distinct cell clusters, including mesenchymal, chondrocyte, osteoblast, fibroblast, smooth muscle, muscle, haematopoietic, endothelial, neural crest-derived, and epithelial cells. If you’re now screaming “Read More!” in your head, you’re in luck. You can satisfy your thirst for knowledge by checking out the complete study here.