Excessive expansion of adipose tissue and consequent obesity is a major risk factor for type 2 diabetes, which can further lead to stroke, hypertension and cardiovascular disease. In this light, there is a pressing need to identify factors and signaling events that regulate adipocyte differentiation and growth. Ideally, these factors and events are studied under controlled conditions in a precisely defined chemical environment. Here, we describe a microfluidic reactor-based adipose tissue model that mimics the contacting arrangement of adipocytes and capillaries. A key feature of the designed reactor is that it affords selective localization of micro-scale collagen gels, which serve as three-dimensional (3D) scaffolds for cell culture. The selective localization is accomplished by fabricating regions of high (convective) transport resistance as cell culture compartments. The culture compartment is lined by variably spaced, micro-fabricated posts. The posts define diffusion pores that connect the culture compartment to flanking medium channels. In this study, the culture compartments were seeded with 3T3-L1 preadipocytes, which have been shown to undergo consistent and uniform differentiation into adipocytes in conventional (static), Petri dish cultures. Reproducible positioning of cells in the culture compartment was obtained by first infusing a preadipocyte suspension in a collagen pre-polymer solution, and then initiating the cross-linking through a temperature and pH shift. Following localization and entrapment within the collagen gel matrix, the preadipocytes achieved their characteristic fibroblastic morphology, and proliferated at rates comparable to those observed in conventional Petri dish cultures. Long-term cell culture for up to two weeks was achieved without nutrient limitations and loss of viability by maintaining a continuous flow of medium. Morphological analysis and Oil Red O staining indicated that preadipocytes induced with a standard adipogenic cocktail underwent differentiation and formed microscopic intracellular lipid droplets. Leveraging the convection-free device design, stable diffusion gradients were generated across the culture compartment by introducing parallel media streams with different chemical compositions into each flow channel. A fluorescent dye (FITC) was used to verify and characterize the kinetics of gradient formation. The gradient feature was utilized to induce asymmetrical differentiation, where preadipocytes were exposed to the adipogenic cocktail from only one of the two flow channels. Lipid droplet formation was observed only for the preadipocytes on the induced side of the culture compartment, thus confirming spatially selective differentiation. Taken together, our results establish the proof-of-principle of a novel micro-fluidic adipose reactor that affords long-term and spatially defined co-culture of adipocytes and preadipocytes. Prospectively, the spatially defined co-culture could be used to conduct detailed studies on the recruitment of locally resident preadipocytes by mature and enlarged adipocytes.