Alcam-a and Pdgfr-α are essential for the development of sclerotome-derived stromal cells that support hematopoiesis

The molecular and cellular hematopoietic stem cell specification niche

Hematopoietic stem cells (HSCs) are a population of tissue-specific stem cells that reside in the bone marrow of adult mammals, where they self-renew and continuously regenerate the adult hematopoietic lineages over the life of the individual. Prominence as a stem cell model and clinical usefulness have driven interest in understanding the physiologic processes that lead to the specification of HSCs during embryonic development. High-efficiency directed differentiation of HSCs by the instruction of defined progenitor cells using sequentially defined instructive molecules and conditions remains impossible, indicating that comprehensive knowledge of the complete set of precursor intermediate identities and required inductive inputs remains incompletely understood. Recently, interest in the molecular and cellular microenvironment where HSCs are specified from endothelial precursors-the "specification niche"-has increased. Here we review recent progress in understanding these niche spaces across vertebrate phyla, as well as how a better characterization of the origin and molecular phenotypes of the niche cell populations has helped inform and complicate previous understanding of signaling required for HSC emergence and maturation.

Isolation, identification and osteogenic capability analysis of mesenchymal stem cells derived from different layers of human maxillary sinus membrane

AIM

To discover the populations of mesenchymal stem cells (MSCs) derived from different layers of human maxillary sinus membrane (hMSM) and evaluate their osteogenic capability.

MATERIALS AND METHODS

hMSM was isolated into a monolayer using the combined method of physical separation and enzymatic digestion. The localization of MSCs in hMSM was performed by immunohistological staining and other techniques. Lamina propria layer-derived MSCs (LMSCs) and periosteum layer-derived MSCs (PMSCs) from hMSM were expanded using the explant cell culture method and identified by multilineage differentiation assays, colony formation assay, flow cytometry and so on. The biological characteristics of LMSCs and PMSCs were compared using RNA sequencing, reverse transcription and quantitative polymerase chain reaction, immunofluorescence staining, transwell assay, western blotting and so forth.

RESULTS

LMSCs and PMSCs from hMSMs were both CD73-, CD90- and CD105-positive, and CD34-, CD45- and HLA-DR-negative. LMSCs and PMSCs were identified as CD171+/CD90+ and CD171-/CD90+, respectively. LMSCs displayed stronger proliferation capability than PMSCs, and PMSCs presented stronger osteogenic differentiation capability than LMSCs. Moreover, PMSCs could recruit and promote osteogenic differentiation of LMSCs.

CONCLUSIONS

This study identified and isolated two different types of MSCs from hMSMs. Both MSCs served as good potential candidates for bone regeneration.

Unveiling heterogeneity in MSCs: exploring marker-based strategies for defining MSC subpopulations

Mesenchymal stem/stromal cells (MSCs) represent a heterogeneous cell population distributed throughout various tissues, demonstrating remarkable adaptability to microenvironmental cues and holding immense promise for disease treatment. However, the inherent diversity within MSCs often leads to variability in therapeutic outcomes, posing challenges for clinical applications. To address this heterogeneity, purification of MSC subpopulations through marker-based isolation has emerged as a promising approach to ensure consistent therapeutic efficacy. In this review, we discussed the reported markers of MSCs, encompassing those developed through candidate marker strategies and high-throughput approaches, with the aim of explore viable strategies for addressing the heterogeneity of MSCs and illuminate prospective research directions in this field.

Tracking early mammalian organogenesis - prediction and validation of differentiation trajectories at whole organism scale

Early organogenesis represents a key step in animal development, during which pluripotent cells diversify to initiate organ formation. Here, we sampled 300,000 single-cell transcriptomes from mouse embryos between E8.5 and E9.5 in 6-h intervals and combined this new dataset with our previous atlas (E6.5-E8.5) to produce a densely sampled timecourse of >400,000 cells from early gastrulation to organogenesis. Computational lineage reconstruction identified complex waves of blood and endothelial development, including a new programme for somite-derived endothelium. We also dissected the E7.5 primitive streak into four adjacent regions, performed scRNA-seq and predicted cell fates computationally. Finally, we defined developmental state/fate relationships by combining orthotopic grafting, microscopic analysis and scRNA-seq to transcriptionally determine cell fates of grafted primitive streak regions after 24 h of in vitro embryo culture. Experimentally determined fate outcomes were in good agreement with computationally predicted fates, demonstrating how classical grafting experiments can be revisited to establish high-resolution cell state/fate relationships. Such interdisciplinary approaches will benefit future studies in developmental biology and guide the in vitro production of cells for organ regeneration and repair.

Single-cell analysis reveals distinct fibroblast plasticity during tenocyte regeneration in zebrafish

Despite their importance in tissue maintenance and repair, fibroblast diversity and plasticity remain poorly understood. Using single-cell RNA sequencing, we uncover distinct sclerotome-derived fibroblast populations in zebrafish, including progenitor-like perivascular/interstitial fibroblasts, and specialized fibroblasts such as tenocytes. To determine fibroblast plasticity in vivo, we develop a laser-induced tendon ablation and regeneration model. Lineage tracing reveals that laser-ablated tenocytes are quickly regenerated by preexisting fibroblasts. By combining single-cell clonal analysis and live imaging, we demonstrate that perivascular/interstitial fibroblasts actively migrate to the injury site, where they proliferate and give rise to new tenocytes. By contrast, perivascular fibroblast-derived pericytes or specialized fibroblasts, including tenocytes, exhibit no regenerative plasticity. Active Hedgehog (Hh) signaling is required for the proliferation of activated fibroblasts to ensure efficient tenocyte regeneration. Together, our work highlights the functional diversity of fibroblasts and establishes perivascular/interstitial fibroblasts as tenocyte progenitors that promote tendon regeneration in a Hh signaling-dependent manner.

Dermomyotome-derived endothelial cells migrate to the dorsal aorta to support hematopoietic stem cell emergence

Development of the dorsal aorta is a key step in the establishment of the adult blood-forming system, since hematopoietic stem and progenitor cells (HSPCs) arise from ventral aortic endothelium in all vertebrate animals studied. Work in zebrafish has demonstrated that arterial and venous endothelial precursors arise from distinct subsets of lateral plate mesoderm. Here, we profile the transcriptome of the earliest detectable endothelial cells (ECs) during zebrafish embryogenesis to demonstrate that tissue-specific EC programs initiate much earlier than previously appreciated, by the end of gastrulation. Classic studies in the chick embryo showed that paraxial mesoderm generates a subset of somite-derived endothelial cells (SDECs) that incorporate into the dorsal aorta to replace HSPCs as they exit the aorta and enter circulation. We describe a conserved program in the zebrafish, where a rare population of endothelial precursors delaminates from the dermomyotome to incorporate exclusively into the developing dorsal aorta. Although SDECs lack hematopoietic potential, they act as a local niche to support the emergence of HSPCs from neighboring hemogenic endothelium. Thus, at least three subsets of ECs contribute to the developing dorsal aorta: vascular ECs, hemogenic ECs, and SDECs. Taken together, our findings indicate that the distinct spatial origins of endothelial precursors dictate different cellular potentials within the developing dorsal aorta.