Inferior vena cava branch variations in C57BL/6 mice have an impact on thrombus size in an IVC ligation (stasis) model

Background: Animal models of venous thrombosis (VT) are critical tools for those investigating the VT mechanism. Recently, inferior vena cava (IVC) branches have been subject to debate, causing controversy in the field.

Objectives: To understand how the variability of IVC branches, in commonly used C57BL/6 mice, have an impact on thrombus formation in the IVC ligation model.

Methods: C57BL/6 male mice (n = 46), 20-25 g, were subjected to the IVC ligation model with various interruptions of the IVC branches. Control animals (n = 50) had all branches interrupted. Two days after IVC ligation, thrombus weight (TW), as a parameter of thrombus size, was assessed.

Results: We found four different anatomical patterns. Side branches were more prevalent on the mouse's right side (34%) compared with the left (20%). In mice where side branches were absent (21%), back branches appeared larger. Also, 25% of mice had both side branches. Controls that had all IVC branches interrupted had the most consistent and largest TW (32.6 mg to 34.7 mg) while groups that had no back branches interrupted had the smallest TW (3.6-9.7 mg), a 4 to 9-fold decrease. All groups with open back branches had significantly smaller TW (P < 0.05) than controls.

Conclusions: Variations in TW were observed based on different branch interruption patterns, compared with the fully ligated controls. Having two back branches was the most consistent anatomy and open back branches had the largest negative impact on thrombus size. This work confirms that the IVC branches significantly affect thrombus burden in C57BL/6 mice and further studies should be conducted in order to standardize this and other animal models of VT.