Proposals submitted to the FDA for MSC-based products are undergoing a rapid expansion that is characterized by increased variability in donor and tissue sources, manufacturing processes, proposed functional mechanisms, and characterization methods. Here we discuss the diversity in MSC-based clinical trial product proposals and highlight potential challenges for clinical translation.

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Quick, Quick, Slow The slow muscles of postural stability and the fast muscles of running and jumping are driven by motor neurons that are differentiated by fast and slow biophysical properties. By retrograde labeling of mouse and chick muscle fibers, Müller et al. (p. 1264 ) characterized the developmental distinctions between fast and slow motor neurons. A transmembrane protein, when over- or underexpressed, was discovered to drive specification of the motor neurons and a downstream effector specified some, but not all, of the biophysical attributes.

Pregnancy is a state of high metabolic demand. Fasting diverts metabolism to fatty acid oxidation, and the fasted response occurs much more rapidly in pregnant women than in the non-pregnant state. The product of the imprinted Delta-like homologue 1 gene (DLK1) is an endocrine signaling molecule that reaches a high concentration in the maternal circulation during late pregnancy. By utilising murine models with deleted Dlk1 we show that the fetus is the source of maternal circulating DLK1. In the absence of fetally-derived DLK1, the maternal fasting response is impaired. Furthermore, we found that maternal circulating DLK1 levels predict embryonic mass in mice and can differentiate healthy small for gestational age (SGA) from pathologically small infants in a human cohort. Therefore measurement of DLK1 in maternal blood may be a valuable method for diagnosing human disorders associated with impaired DLK1 expression, and to predict poor intrauterine growth and complications of pregnancy.