Coronal section of a juvenile male zebrafish, 15 mm (6 weeks); H&E staining testis location, coronal view

The testis is a paired organ, located bilaterally between the abdominal wall [AW] and the swim bladder. At the level of this section, the testis is further bounded by the liver and mesenchymal tissue (mainly adipose tissue); dorsally it is adjacent to the kidney and ventrally to the intestines (not visible on this level).

This image further shows: the abdominal wall musculature, the head kidney, the opercles and gills, the pharyngeal cavity and epithelium, and the pharyngeal musculature.

Axial section of an adult male zebrafish; H&E staining testis location, axial view

This axial section of an adult male zebrafish is from the rostral part of the abdominal cavity, where the testis is in the upper part, between swim bladder and abdominal wall. The area of testis will show largely similar in more caudal sections.
The abdominal organs shown in this image are liver, pancreas, spleen, intestinal loops.
Major blood vessels in this section are dorsal aorta and posterior cardinal vein, embedded in the kidney. Intestinal arteries, intestinal veins and epigastric vein, partly embedded in the liver.

The horizontal skeletogenous septum separates the epaxialis and hypaxialis muscles, which are inserted to this septum and to the spine , and which are also bound to the skin. In the midline, the epaxial muscles insert to the vertical skeletogonous septum. Although organized in myotomes which are oriented perpendicular to the length axis of the fish, these muscles appear cross-sectioned due to their waved course. The hypaxialis muscles includes the dorsal supracarinalis muscles, the hypaxialis muscles include the ventral infracarinalis muscles. The lateralis superficialis muscles overlay the peripheral part of the horizontal septum. Pleural rib structures can be observed within the hypaxialis muscles.