During the last fiscal year, the following advances were made: 1) Biological activity of stem cells The Section continues to try to bring some clarity to the mesenchymal stem cell field. The concept of a post-natal mesenchymal stem cell (MSC) originated from studies focused on bone marrow stromal cells (BMSCs), which are non-hematopoietic adherent cells, a subset of which are skeletal stem cells (SSCs), able to form cartilage, bone, hematopoiesis-supportive stroma, and marrow adipocytes based on rigorous clonal and differentiation assays. Subsequently, it was speculated that BMSCs could form other mesodermal derivatives and even cell types from other germ layers. Based on BMSC surface markers, representative of fibroblastic cells, and imprecise differentiation assays, it was further imagined that MSCs are ubiquitous and equipotent. However, MSCs do not have a common embryonic origin and are not a lineage, but recent studies indicate that they are tissue-specific stem/progenitor cells. These cells share cell surface features owing to their fibroblastic nature, but they are not identical. They display different differentiation capacities based on their tissue origin, but do not trans-differentiate outside of their lineage, based on rigorous assays. For these reasons, the MSC term should be abandoned. Tissue-specific stem/progenitor cells provide the opportunity to devise methods for tissue regeneration by the cells themselves (tissue engineering). Their use in other forms of regenerative medicine based on paracrine, immunosuppressive, and immunomodulatory effects is far less clear (Robey, Faculty 1000, 2017). Human umbilical cord blood (CB) has attracted much attention as a reservoir for functional hematopoietic stem and progenitor cells, and recently, as a source of blood-borne fibroblasts (CB-BFs, also called cord blood-derived MSC). In a previous study, it was demonstrated that bone marrow stromal cell (BMSC) and CB-BF pellet cultures make cartilage in vitro. In addition, BMSC and CB-BF pellets remodeled into miniature bone/marrow organoids in vivo, suggesting that CB-BFs can support the Hematopoietic Stem Cell (HSC) niche. Compared with BM-ossicles, CB-ossicles showed a higher proportion of red marrow vs. yellow marrow. Marrow cavities from CB- and BM-ossicles included donor-derived CD146-expressing osteoprogenitors and host-derived mature hematopoietic cells, clonogenic lineage-committed progenitors, and HSCs. Furthermore, human CD34+ cells transplanted into ossicle-bearing mice engrafted and maintained human HSCs in the niche. These data indicate that CB-BFs are able to recapitulate the conditions by which the bone marrow microenvironment is formed and establish complete HSC niches, functionally supportive of hematopoietic tissue (Pievani et al, Dev, 2017). 2) Role in disease With regards to fibrous dysplasia of bone (FD), often in conjuction with the McCune-Albright Syndrome (over-active secretion of some hormones), a rare opportunity presented itself to study radiographs and tissue samples (removed during corrective orthopaedic surgery) from a single patient with FD/MAS. These radiographs and samples were collected at different times following intermittent treatment with intravenously administered pamidronate. In children, such treatment results in the formation of sclerotic bands (zebra lines) due to binding of the bisphosphonate to a mineralized surface and subsequent temporary decrease in bone resorption at the time of treatment. By way of contact microradiography, back-scattered electron microscopy and histological analysis, it was found that zebra lines formed only where the bone was normal, abruptly stopped at the interface between normal and FD bone, and were completely absent in FD bone, due to the inability of bisphosphonate to bind to unmineralized osteoid, a prominent feature of FD lesions. These results suggest that lack of zebra lines is indicative of the presence of FD bone, which may be useful in evaluating the initiation or expansion of FD lesions, and support the concept that bisphosphonates may be ineffective in FD due to their lack of binding to FD osteoid (Corsi et al, Skel Radiol, 2017). 3) Stem cells in tissue engineering and regenerative medicine Previously, a process was established for the ex vivo expansion of BMSCs under conditions that would maintain their biological properties. Human BMSCs are being manufactured around the world using many different methods (and for many different clinical applications), but little is known about the spectrum of manufacturing methods used, and their effects on BMSC characteristics and function. Eight centers (including one at the NIH CC) using or developing Good Manufacturing Practices (GMP) were surveyed as to their production methods. Among the 8 centers, all used bone marrow aspirates as the starting material, but no two centers used the same manufacturing methods. Two to four BMSC lots from each center were compared using global gene expression analysis (Agilent Chip Whole Human Genome). Among the 24 BMSC lots from the 8 centers, intra-center transcriptome variability was low and similar among centers. Principal component analysis and unsupervised hierarchical clustering analysis separated all the lots from all centers into five distinct clusters. Sufficient numbers of BMSCs from 6 of the 8 centers were available for further testing of their ability to form bone and support hematopoiesis (defining features of BMSCs) by in vivo transplantation. Cells from all 6 centers tested formed bone, but the quantity formed was highly variable, and BMSCs from only three centers supported hematopoiesis. These results show that differences in manufacturing resulted in variable BMSC characteristics, including their ability to form bone and support hematopoiesis. Although the number of centers and samples evaluated is small, the data point to a need for establishing rigorous criteria by which to establish SSCs/BMSCs in scaled-up culture conditions in order to maintain their biological properties. (Liu* and de Castro* et al, Scientific Rep, 2017, *co-first authors),
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