TABLE 3.

Adipogenesis or lipogenesis? Reliable preadipocyte and adipocyte selective transcripts to assess in vitro adipogenesis

Molecular Marker (mRNA levels)	Preadipocyte (in vitro)	Adipocyte (in vitro)
Stem cell/adipose progenitor genes:
CD34	+ (expression may be lost with prolonged culture)	−
Sca1	++ (expression may be lost with prolonged culture)	−
Pdgfrβ (CD140b)	++	−
Pdgfrα (CD140a)	+++	−
Pref1	+++	−
Transcription Factors:
C/EBPα	+	+ + +
Pparγ1	+	+ + +
Pparγ2	−	+ + +
Lipid/glucose metabolism:
Fabp4	+	+ + + + +
Glut4	−	+ + +
LpL	+	+ + +
Adipokines:
Adipsin	−	+ + + + +
Adiponectin	−	+ + + + +

The ability to store excess energy in the form of lipid is an ancient and fundamental requirement for growth and survival. Virtually all species have developed mechanisms for fat storage. Caenorhabditis elegans stores lipid in intestinal cells while Drosophila maintains a “fat body” which shares similarities to the mammalian liver. In vertebrates, many cell types can accumulate lipid; however, the evolution of adipocytes provided a specialized and safe compartment for this purpose. Furthermore, the identification of leptin, adiponectin, and other adipocyte-derived hormones and cytokines revealed the endocrine function of adipocytes. As such, an adipocyte must be viewed as being more than a mere lipid-accumulating cell. Studies of adipogenesis often rely on lipid-accumulation as a primary read-out for cellular differentiation. Our current knowledge of adipocytes demands a more precise definition of terminally differentiated adipocytes. The extent of adipocyte differentiation in vitro should be determined through quantitative measurements in the expression of preadipocyte-selective transcripts (downregulated during differentiation) and adipocyte-selective transcripts (activated during adipogenesis).