The adipose organ of obesity-prone C57BL/6J mice is composed of mixed white and brown adipocytes

Abstract

White and brown adipocytes are believed to occupy different sites in the body. We studied the anatomical features and quantitative histology of the fat depots in obesity and type 2 diabetes-prone C57BL/6J mice acclimated to warm or cold temperatures. Most of the fat tissue was contained in depots with discrete anatomical features, and most depots contained both white and brown adipocytes. Quantitative analysis showed that cold acclimation induced an increase in brown adipocytes and an almost equal reduction in white adipocytes; however, there were no significant differences in total adipocyte count or any signs of apoptosis or mitosis, in line with the hypothesis of the direct transformation of white into brown adipocytes. The brown adipocyte increase was accompanied by enhanced density of noradrenergic parenchymal nerve fibers, with a significant correlation between the density of these fibers and the number of brown adipocytes. Comparison with data from obesity-resistant Sv129 mice disclosed a significantly different brown adipocyte content in C57BL/6J mice, suggesting that this feature could underpin the propensity of the latter strain to develop obesity. However, the greater C57BL/6J browning capacity can hopefully be harnessed to curb obesity and type 2 diabetes in patients with constitutively low amounts of brown adipose tissue.

Fig. 1.

Multidepot adipose organ of adult C57BL/6J female mice kept at 28°C (left) or 6°C (right) for 10 days. Anatomical dissection under a surgical microscope. Each depot was placed on a mouse template indicating approximately its original anatomical position. Kidneys (K) were dissected together with the depots. The organ is made up of the two subcutaneous depots (A) anterior [1-deep cervical portion, 2-superficial cervical portion, 3-interscapular portion (interscapular BAT), 4-subscapular portion, 5-axillo-thoracic portion] and (B) posterior (1-inguinal-dorsolumbar portion, 2-gluteal portion), and the visceral depots (C) mediastinal (including the aortic arch and its thoracic portion), (D) mesenteric, (E) retroperitoneal, and (F) abdominopelvic (1-interrenal portion, 2-periovarian portion, 3-parametrial portion, 4-perivesical portion). Scale bar for both: 2 cm.

Fig. 2.

UCP1 immunostaining. (A) Representative picture of interrenal fat showing that the tissue portion closest to the aorta is predominantly composed of UCP1-positive multilocular adipocytes, whereas the peripheral portion is predominantly composed of UCP1-negative unilocular adipocytes (black arrow). (B) Enlargement of framed area in A. Scale bar: 70 µm (A); 30 µm (B).

Fig. 3.

(A) Interscapular BAT. UCP1 immunostaining of a C57BL/6J mouse maintained at 6°C for 10 days shows three different cell types: UCP1-negative unilocular adipocytes (*), UCP1-negative multilocular adipocytes (black arrows), and UCP1-positive multilocular adipocytes (white arrowheads). Scale bar: 10 µm. (B) Total number of adipocytes contained in the adipose organ of C57BL/6J mice maintained at 28°C or 6°C for 10 days. The overall number of adipocytes in the organ was not significantly changed in either experimental condition (P = 0.14). In the cold-acclimated group, the total number of UCP1-positive multilocular adipocytes increased significantly (P = 0.01). Unilocular adipocytes decreased significantly by the same amount (P = 0.05). UCP1-negative multilocular adipocytes were almost unchanged. Data are shown as mean ± SEM.

Fig. 4.

Number of unilocular, UCP1-negative multilocular and UCP1-positive multilocular adipocytes in different depots of the adipose organ of C57BL/6J mice maintained at 28°C or 6°C for 10 days. In the cold-acclimated group, the number of UCP1-positive multilocular adipocytes increased significantly in the anterior subcutaneous and in the abdominopelvic depot. Numbers above columns indicate the average density of TH-positive parenchymal fibers in each depot (fiber number/100 adipocytes). Data are mean ± SEM. *P < 0.05, **P < 0.01.

Fig. 5.

Number of UCP1-positive multilocular adipocytes at 28°C and after cold exposure. (A) C57BL/6J and (B) Sv129 mice: anterior subcutaneous depot (composed of superficial cervical, deep cervical, interscapular, axillo-thoracic, and subscapular portions); posterior subcutaneous depot (composed of inguinal-dorsolumbar and gluteal portions); mediastinal depot, mesenteric depot, retroperitoneal depot, and abdominopelvic depot (composed of interrenal, periovarian, perivesical, parametrial, and periovarian portions). The data on C57BL/6J mice are the same presented in supplementary Table II. Sv129 data were obtained from a previous work (26). Mean ± SEM. *P < 0.05, **P < 0.01.

Fig. 6.

TH-positive (brown) parenchymal nerve fibers in three different areas of the adipose organ of C57BL/6J mice: (A) unilocular area, (B) mixed area, and (C) multilocular area. Arrows indicate some TH-positive parenchymal nerve fibers. Scale bar: 15 µm for all. (D) Mean density of TH-positive parenchymal nerve fibers in the whole adipose organ of 28°C or 6°C mice. Data are shown as mean ± SEM. **P < 0.01.

Fig. 7.

Correlation between density of TH-positive parenchymal fibers and proportion of UCP1-positive multilocular adipocytes in different adipose depots of C57BL/6J (A) and Sv129 (B) mice maintained at 28°C or 6°C for 10 days. Sv129 data from a previous work (26) are in italics. Linear correlations were calculated using Spearman's nonparametric test; r = Spearman coefficient; P = probability.