| Our concept of adipose tissue has evolved a great deal over the past century. Initially thought only to provide insulation and mechanical support, the definition of adipose tissue was later revised to include a role in systemic metabolism. Genetic approaches in recent years have revealed its endocrine function, as well as the inter-depot, molecular-level diversity within what is now known as the adipose organ. Because much of this research was driven by the global spread of obesity, medullary adipose tissue, the ostensibly isolated depot within bone marrow, has received comparatively little attention. It is widely hypothesized and commonly accepted that medullary adipocytes derive from multipotent progenitor cells of the marrow, such as human mesenchymal stem cells (hMSCs); however, molecular-level tracking of this development is lacking. In order to conduct a comparative study between the two cell populations, methods were adapted both for hMSC adipogenesis and for enrichment of medullary adipocytes from whole marrow aspirate. Using PCR, a high degree of similarity was determined to exist between hMSC-derived adipocytes and medullary adipocytes, as shown through transcription factor upregulation and the expression of lipid metabolism-related genes, adipokines, and other bioactive macromolecules. Furthermore, there was no detection in either population of UCP-1, demonstrating that neither population developed characteristics of, nor a substantial subpopulation of, brown adipocytes. Having supported the hypothesis that medullary adipocytes can derive from hMSCs, this culture system was then used as an in vitro model for the development of mature adipocytes from stem cells. With this tool, we investigated temporal protein expression by monitoring the occurrence and localization of PPARgamma and C/EBPalpha, the major players in transcription, as well as two well-characterized lipid droplet-associated proteins, adipophilin and perilipin. We finally approached the subject of function with an exploration of the expression and secretion of leptin and adiponectin. In this study, we have charted a lineage map of medullary adipocyte development. This map offers a novel perspective on the various morphologic and phenotypic modulations within this lineage and provides for us a means to conduct future experimentation to refine our understanding of medullary adipocyte origin, phenotype, and function. |
