Whole slide image (WSI) of mouse testicular cross-section contains hundreds of seminiferous tubules. Meanwhile, each seminiferous tubule also contains different types of germ cells among different histological regions. These factors make it a challenge to segment distinct germ cells and regions on mouse testicular cross-section. Automated segmentation of different germ cells and regions is the first step to develop a computerized spermatogenesis staging system. In this paper, a set of 28 H&E stained WSIs of mouse testicular cross-section and 209 Stage VI-VIII tubules images were studied to develop an automated multi-task segmentation model. A deep residual network (ResNet) is first presented for seminiferous tubule segmentation from mouse testicular cross-section. According to the types and distribution of germ cells in the tubules, we then present the other deep ResNet for multi-cell (spermatid, spermatocyte, and spermatogonia) segmentation and a fully convolutional network (FCN) for multi-region (elongated spermatid, round spermatid, and spermatogonial & spermatocyte regions) segmentation. To our knowledge, this is the first time to develop a computerized model for analyzing histopathological image of mouse testis. Three segmentation models presented in this paper show good segmentation performance and obtain the pixel accuracy of 94.40%, 91.26%, 93.47% for three segmentation tasks, respectively, which lays a solid foundation for the establishment of mouse spermatogenesis staging system.