Hypertrophy and/or Hyperplasia: Dynamics of Adipose Tissue Growth

Abstract

Adipose tissue grows by two mechanisms: hyperplasia (cell number increase) and hypertrophy (cell size increase). Genetics and diet affect the relative contributions of these two mechanisms to the growth of adipose tissue in obesity. In this study, the size distributions of epididymal adipose cells from two mouse strains, obesity-resistant FVB/N and obesity-prone C57BL/6, were measured after 2, 4, and 12 weeks under regular and high-fat feeding conditions. The total cell number in the epididymal fat pad was estimated from the fat pad mass and the normalized cell-size distribution. The cell number and volume-weighted mean cell size increase as a function of fat pad mass. To address adipose tissue growth precisely, we developed a mathematical model describing the evolution of the adipose cell-size distributions as a function of the increasing fat pad mass, instead of the increasing chronological time. Our model describes the recruitment of new adipose cells and their subsequent development in different strains, and with different diet regimens, with common mechanisms, but with diet- and genetics-dependent model parameters. Compared to the FVB/N strain, the C57BL/6 strain has greater recruitment of small adipose cells. Hyperplasia is enhanced by high-fat diet in a strain-dependent way, suggesting a synergistic interaction between genetics and diet. Moreover, high-fat feeding increases the rate of adipose cell size growth, independent of strain, reflecting the increase in calories requiring storage. Additionally, high-fat diet leads to a dramatic spreading of the size distribution of adipose cells in both strains; this implies an increase in size fluctuations of adipose cells through lipid turnover.

Author summary

Obesity is an enlargement of adipose tissue to store excess energy intake. Hyperplasia (cell number increase) and hypertrophy (cell size increase) are two possible growth mechanisms. The in vivo dynamic change of fat tissue cannot be monitored in real time due to current technical limitations. However, we can measure cell-size distributions of fat cells in individual animals. Our fundamental goal is to extract dynamic features of tissue remodeling from snapshots of cell-size distributions. We develop a mathematical model that interpolates between the cell-size distribution measurements and predicts the continuous change of the cell-size distribution with respect to fat pad mass increase. Our adipose tissue growth model includes three essential components: new cell recruitment, size-dependent cell growth, and cell-size fluctuations. In particular, we compared the adipose tissue growth of obesity-prone and obesity-resistant mice under a standard or a high-fat diet to examine the genetic and diet effect on adipose tissue growth. By applying our model to these different conditions, we found that the size increase of fat cells is dependent on diet. On the other hand, the diet-induced number increase of fat cells is dependent on strain, suggesting a synergy between genetics and diet.