Coronal section of a juvenile male zebrafish, total body length 11 mm; age 6w; H&E staining 1. undifferentiated gonad

The undifferentiated gonad is an elongated structure which diametrically only spans a few cells; the organ is composed of mainly undifferentiated gonocytes. In this image, it is bounded by liver and its associated peritoneal tissue on the medial side, and lateral by mesenchyme. Adjacent to the liver, part of the swim bladder is visible.

Coronal section of a juvenile male zebrafish, total body length 11 mm; age 4w; H&E staining 2. early development

An early feature of male differentiation of the gonad is ingrowth of stroma. Other features, which are not present on this image are appearance of a clustered organisation (spermatocysts) and lumen formation. This gonad mainly contains undifferentiated gonocytes, but early stages of differentiation are also present.

A small part of the liver is visible.

Coronal section of a juvenile male zebrafish, total body length 11 mm; age 5w; H&E staining 3. early development

This gonad mainly contains early stages of differentiation, which are characterized by the increased nuclear basophilic staining and decreased size, compared to undifferentiated gonocytes. The hole in this image probably indicates early lumen formation; early ingrowth of stroma is also visible.

Adjacent structures in this image are: liver on the medial side and mesenchyme on the lateral side.

Coronal section of a juvenile male zebrafish, total body length 12 mm; age 5w; H&E staining 4. early development

Early features of male differentiation of the gonad are the appearance of a clustered organisation (spermatocysts) and ingrowth of stroma.

This gonad contains early stages of differentiation, as well as undifferentiated gonocytes. A vessel can be observed, and adjacent structures in this image are: liver, pancreas, and peritoneal cells.

Coronal section of a juvenile male zebrafish, total body length 12 mm; age 5w; H&E staining 5. early development

This gonad shows the combined presence of clustered organisation (spermatocyst) and lumen formation, as indications of early male differentiation.
Early stages of differentiation are predominant, but undifferentiated gonocytes are also abundant.

Adjacent structures in this image are: liver and eosinophilic peritoneal cells in mesenchyme.

Coronal section of a juvenile male zebrafish, total body length 12 mm; age 6w; H&E staining 6. intermediate development

This gonad shows the combined presence of clustered organisation (spermatocysts) and lumen formation as indications of early male differentiation. A prominent blood vessel filled with erythrocytes is present in the stroma.
The spematogenic epithelium contains mainly early and intermediate stages of differentiation, up to early spermatids; clusters of undifferentiated gonocytes are also present.

Adjacent structures in this image are: liver and abdominal wall.

Coronal section of a juvenile male zebrafish, total body length 12 mm; age 6w; H&E staining 7. advanced development

This gonad can be identified as a testis by the presence of well-developed lumina, which are now filled with sperm, and by the clustered organisation (spermatocysts). Clusters of spermatogonia are also present.

Adjacent structures in this image are: clustered peritoneal cells.