{"doi":"10.1242/dev.201308","title":"In preprints: new insights into proximodistal limb patterning and differentiation","abstract":"In recent years, multiple theories have been proposed to explain how the proximodistal (PD) axis of the tetrapod limb (i.e. from shoulder to digit tips) is patterned. They can be grouped in two broad categories based on the order of specification: progressive and early patterned. The common ground in both categories is that the ∼250 µm region under the distal epithelium of the limb bud is considered undifferentiated, such that the very early bud is fully undifferentiated and, as the limb grows out, the cells that remain in the proximal side of that limit start to differentiate (Tabin and Wolpert, 2007). In progressive models, this fate is progressively specified, first stylopod (arm), then zeugopod (forearm), then autopod (hand). This process is often divided into two phases, a first one driven by diffusible signals, either proximal and distal or only distal (Berenguer and Duester, 2021; Delgado et al., 2020; Mercader et al., 2000; Roselló-Díez et al., 2011), followed by a second phase driven by an intrinsic timer (Roselló-Díez et al., 2014; Saiz-Lopez et al., 2015), mediated in part by the progressive degradation of MEIS transcription factors (Delgado et al., 2020). In early specification models, by contrast, the proximal and distal regions are formed early on, with the intermediate segment arising either at the same time as the other two (Dudley et al., 2002), or slightly later, as a consequence of the interaction between proximal and distal domains (Mariani et al., 2008). The latter model bears significant similarity to the classic ‘distal first’ (aka intercalary) model of limb regeneration, which was conceived by Gardiner and Bryant in the 1990s following the observation that the undifferentiated early blastema (the adult equivalent of the limb bud) expresses Hox genes associated with both proximal (HoxA9) and distal (HoxA13) locations (Gardiner et al., 1993). However, grafting studies on regenerating axolotl from the Tanaka lab found that early blastema cells are not committed to an autopod fate (Roensch et al., 2013), suggesting that patterning of regenerating limb segments occurs through progressive specification, rather than intercalation.The waters were again muddied when it was shown that, under the right conditions, early limb cells show plasticity and adaptability to the environment into which they are grafted (Dudley et al., 2002; McCusker and Gardiner, 2013; Roselló-Díez et al., 2011; Roselló-Díez and Torres, 2011), making it impossible to distinguish between progressive versus a two-signal/distal-first mechanism. Luckily, the advent of single-cell sequencing strategies, as performed on developing mouse limb buds in a recent preprint by Markman et al. (2022), now provides tools to tackle this question from a new angle. Their findings put forward a completely new differentiation model that integrates progressive, early specification and intercalary modes of PD limb patterning.To better understand how the PD forelimb axis is specified in the mammalian embryo, Markman et al. (2022) started by using single-cell RNA-seq to characterise the molecular signature of each limb cell across five stages of mouse limb development spanning embryonic day (E) 10.5 to 14.5. The initial analysis identified robust and homogenous groups of cells referred to as ‘metacells’ (Baran et al., 2019). Clustering of these metacells led to the identification of six chondrocyte populations, three connective-tissue fibroblast subgroups and three progenitor cell states (P1-P3).Further analysis of the progenitor states revealed that the P2 population exhibits a ‘proximal’ gene expression signature, P3 has an ‘autopod’ signature and P1 shows high expression of limb patterning genes, but lacks a spatial signature, meaning that, surprisingly, a zeugopod (forearm) progenitor was not identified. This observation is consistent with the intercalary model of specification. Interestingly, both P1 and P2 cells are most abundant in early limb buds, and decre","journal":"Development","year":2022,"id":295910,"datarank":0.0,"base_score":0.0,"endowment":0.0,"self_citation_contribution":0.0,"citation_network_contribution":0.0,"self_endowment_contribution":0.0,"citer_contribution":0.0,"corpus_percentile":null,"corpus_rank":null,"citation_count":1,"citer_count":0,"citers_with_citation_signal":0,"citers_with_endowment":0,"datacite_reuse_total":0,"is_dataset":false,"is_dataset_confidence":0.9526,"is_data_producer":false,"deposit_databanks":null,"is_oa":true,"file_count":0,"downloads":0,"has_version_chain":false,"published_date":"2022-01-01","fair_score":null,"fair_percentile":null,"algorithm_id":"datarank_citation_only_1hop_v6","ranking_scope":"data_only","authors":[{"id":372253,"name":"Alberto Roselló‐Díez","orcid":"0000-0002-5550-9846","position":1,"is_corresponding":false},{"id":662307,"name":"Catherine D McCusker","orcid":"0000-0003-0127-433X","position":0,"is_corresponding":true}],"reference_count":18,"raw_metadata":null,"created_at":"2026-07-19T00:31:12.531553Z","pmid":"36200555","pmcid":null,"fwci":null,"citation_percentile":null,"influential_citations":0,"oa_status":null,"license":null,"views":0,"total_file_size_bytes":0,"version_count":0,"fair_f":null,"fair_a":null,"fair_i":null,"fair_r":null,"fair_zscore":null,"fair_rationale":null,"fair_model":null,"fair_agent_version":null,"fair_fulltext_source":null,"fair_has_llm":null,"fair_computed_at":null,"clinical_trials":[],"software_tools":[],"db_accessions":[],"linked_datasets":[],"topics":[]}