Leydig's organ

Leydig's organ is a hemopoietic (blood-forming) organ found exclusively in elasmobranchs — the group of cartilaginous fish that includes sharks, rays, and skates. It is embedded along the upper and lower walls of the esophagus and produces red blood cells, functioning similarly to bone marrow in other vertebrates.

Elasmobranchs have multiple hemopoietic sites including Leydig's organ, the spleen, and specialized tissue around the gonads. Leydig's organ is notable for its location along the esophagus and its substantial contribution to red blood cell production. The relative importance of each site varies among elasmobranch species.

No — these are completely unrelated structures that share a name because both were described by the 19th-century German anatomist Franz von Leydig. Leydig cells in the testes produce testosterone in mammals, whereas Leydig's organ in elasmobranchs is an esophageal hemopoietic organ with no reproductive function.