Oral Presentation ANZBMS-MEPSA-ANZORS 2022

Tissue-resident skeletal stem and progenitor cells (SSPCs) support homeostasis and regeneration. The periosteum is critical for fracture healing and mouse periosteum is enriched for SSPCs. Recently human SSCs have been characterized in fetal tissue, but adult human SSCs remain elusive. The aim of this study was to evaluate hSSPC populations in different tissue compartments.

We compared marker expression in human skeletal tissues with a 21-colour flow cytometry panel. Femoral heads were used to isolate matched tissue from 21 patients; specifically bone marrow (BM), trabecular endosteum (or bone-associated cells), femoral neck periosteum, and articular chondrocytes. Markers enriched in the periosteum included CD90, CD34, CD51, and CD26; while CD24 was primarily in the endosteum/BM. Cartilage showed high expression for markers including ALP, CD105, and PDPN and was the only tissue that showed a clear hSSC population as defined previously (1).

Sorted Lin- cells from the periosteum and cartilage formed more fibroblastic colonies (CFU-Fs) than endosteum; while BM Lin- cells did not form colonies. CD90 is one of the SSPC markers that significantly enriched in the periosteum (20.7%), compared with cartilage (11.8%), and endosteum (2.0%). Histologically, CD90 expression was restricted to perivascular cells in the periosteum. After sorting, CD90+ but not CD90- cells from the endosteum and BM formed CFU-Fs. Similar CFU-Fs formed from CD90+ and CD90- cartilage.

We subdivided the CD90+ periosteal population with CD73, CD34, and CD26. CD90+CD34+, the majority of which resided on the periosteum layer, showed the highest CFU-Fs. Single CD90+CD34+ colonies reached confluence faster than any other populations sorted, and 100% showed tri-lineage (osteogenic, chondrogenic, adipogenic) differentiation. Surprisingly, colonies from CD90-CD73+ and CD90+CD34- formed adipocytes spontaneously.

Using cell surface markers, we found that adult hSSC populations vary in different skeletal compartments (Figure). In the periosteum, CD90+CD34+ cells show characteristics of hSSCs in vitro.