骨骼干细胞(SSCs)是一群能够自我更新并分化产生软骨细胞、成骨细胞和基质细胞的组织特异性干细胞,在骨骼发育和损伤修复过程中发挥关键作用.过去SSCs被定义为特征不一致的骨髓源性细胞.然而,最近的谱系示踪实验表明SSCs不仅存在于骨髓中,还存在于骨膜和生长板保留区中,且越来越多研究表明SSCs是高度异质性的.小鼠和人类的SSCs及其分化等级相继被鉴定,进一步明确了SSCs存在的证据.本文系统地介绍了SSCs的定义、分化等级、组织分布及其临床应用,并总结了近年来SSCs在骨修复和骨科疾病领域的研究进展和应用前景,以期加深对SSCs多样性和应用的理解.
Background: Skeletal stem cells (SSCs) are a group of tissue specific stem cells that can self-renew and differentiate into chondrocytes, osteoblasts and stromal cells, and therefore play a key role in bone development and injury repair. In the past, SSCs were defined as bone marrow derived cells with high heterogeneity. However, recent lineage tracing experiments have shown that SSCs exist not only in bone marrow, but also in periosteum and growth plate. The hierarchy of SSCs in mice and humans has been identified successively, further clarifying the evidence for the existence of SSCs. Mouse SSCs were isolated from the long bones of neonatal mice and were defined by CD45–Ter119–Tie2–ITGAV+C200+. Human SSCs were defined as a group of cells by CD146–PDPN+CD73+CD164+. The hierarchy of SSCs can be defined according to the changes of distinctive immunophenotypic markers on the cell surface. SSCs are at the top of hierarchy in the skeletal system and can further differentiate into bone, cartilage and stromal progenitors.
Progress: Several remarkable progress of SSCs has been obtained in recent years. In growth plate resting zone, cells expressing parathyroid hormone related protein (PTHrP) are mainly chondrocytes that can further differentiate into hypertrophic chondrocytes and mesenchymal stem cells (MSCs). SSCs in long bones of postnatal mice can be divided into two subpopulations: early osteochondral skeletal stem cells (ocSSCs) labeled with CD45–Ter119–Tie2–Thy1–6C3–CD105–CD51+ and perivascular skeletal stem cells (pvSSCs) labeled with CD45–CD31–Pdgfrα+Sca1+CD24+. ocSSCs are mainly involved in bone development and fracture repair and can differentiate into bone, cartilage and stromal cells in vivo, whereas pvSSCs are mainly distributed in bone marrow and participate in shaping the niche of hematopoietic stem cells. MSCs are considered as the main source of stem cells in bone marrow. Leptin receptor (LepR) is a marker that is highly enriched in bone marrow MSCs. LepR+ cells gradually increase after birth and reach a peak level at 3 months. LepR+ cells can differentiate into bone, cartilage and adipocytes and account for 94% of bone marrow CFU-Fs, and are thus the main source of bone forming SSCs in adult mice bone marrow. Through adolescence, skeletal progenitors dominating bone formation undergo transition from chondrocytes to LepR+ MSCs. Aggrecan+ chondrocytes regulate bone lengthening and LepR+ control bone thickening. Most of the LepR+ adult skeletal progenitors are derived from Aggrecan+ chondrocytes, which explains how bone development transitions from rapid longitudinal growth to slow bone remodeling after adolescence. Periosteum containing SSCs participate in intramembranous osteogenesis, thickening of bone cortex and fracture repair. One of the important markers of periosteal SSCs is cathepsin K (Ctsk), which can label periosteum of long bone and skull. Ctsk+ cells isolated from periosteum display clonal multipotency and self-renewal, and can differentiate into osteoblasts, chondrocytes and adipocytes in vitro. Ctsk+ periosteal stem cells can differentiate into osteoblasts in the cortex but does not differentiate into osteoblasts in bone marrow. However, Ctsk+ cells will acquire endochondral bone formation capacity in the repair of fracture.
Perspective: The hierarchy and spatial distribution of SSCs have been preliminarily studied, and several cell surface markers were confirmed. However, the biological function of SSCs needs to be further clarified. MSCs have been widely used in treatment due to their multipotency and self-renewal. SSCs can only differentiate into osteoblasts, chondrocytes and stromal cells, possessing large potential in treating skeletal diseases such as osteoporosis and fracture nonunion. The feasibility of SSCs combined with composite biological scaffold in tissue engineering still needs to be explored. It has been proved that SSCs are involved in cartilage repair process, which may benefit osteoarthritis patients in the future and reduce artificial joint replacement. Further studies clarifying the regulatory mechanism of SSCs at molecular level will help provide new strategies for stem cell therapy and skeletal diseases.
