Although Hees et al. [8, 11] and Hundeiker [12, 13] have already described the presence of centripetal, centrifugal and knot-like vessels, they do not discuss the types of intra-testicular arterial vasculature. In contrast, the present study describes the morphological variations of the arteries inside the testis in a detailed way. According to this classification, the arterial network of the mediastinum testis is formed mostly by type II intra-testicular arteries, which run more longitudinally than type I intra-testicular arteries and have almost twice as many knot-like structures. The centrifugal branches of the type II intra-testicular arterial vasculature are short but are divided like type I into between one and four generations. Our findings indicate that type III intra-testicular arteries, with centrifugal branches arising half way along the course of the centripetal arteries, are typically complementary to type II.
Similar to bulls, the centripetal arteries of dogs, stallions, buffalo and rams run straight from the arterial network of the tunica albuginea to the mediastinal region, where they form centrifugal, recurrent branches, running peripherally, which do not reach the surface of the testis [2, 14–16]. However, in the common tree shrew (Tupalia glis), the testicular artery emerges from the pampiniform plexus and approaches the gonad at the rostro-medial pole. It then courses caudally along the medial border, then rostrally along the lateral border, and penetrates into the parenchyma near the rostral pole. While curving on the lateral border of the testis, it gives off 4–5 parenchymal branches penetrating perpendicularly into the testicular tissue .
Significant differences exist with regard to human and bovine intra-testicular arterial topography [17, 18]. Presumably this is based on differences in mediastinum testis topography present in the two species: central in bovine, peripheral in human. In humans, the intra-testicular arteries originate and run in straight lines centripetally from the tunica albuginea arterial network, and centrifugally from the mediastinum testis network. The centripetal arteries come off the tunica albuginea and run straight towards the mediastinum testis. The centrifugal arteries, running parallel to the centripetal ones, lead off from the mediastinum testis and head towards the tunica albuginea. As a consequence, the blood supply of the testicular tissue is provided by two relatively independent sources.
In bulls, the intra-testicular arteries come off the tunica albuginea arterial network (rami tunicales majores and minores) perpendicularly and pass towards the mediastinum testis, running centripetally. Some of them reach the mediastinum testis and form screw-like helicoids or knot-like vascular conglomerations. Each conglomeration is an originating structure for between 3 and 20 centrifugal arteries parallel to the centripetal ones. The centrifugal arteries do not anastomose with the arterial network of the tunica albuginea. As a consequence, the blood supply of the testicular tissue is provided by one consequent system of arterial vessels [3, 8, 11–13].
In the bull gonad, the highly twisted arrangement of alternating segments straight from the knot-like shape arteries may facilitate the passage of blood and lymph from the centrally located mediastinum to the peripheral part of the organ. No such arrangements were observed in the gonads of a man with an eccentrically located mediastinum [17, 18].
Godinho et al.  suggest that the convolutions in the capsular and intra-testicular arteries in ruminants may increase the effective surface area of the arteries to allow exchange of heat and metabolites with the extensive network of juxtaposed centrifugal and capsular veins. Dhinjgra  proposes that the coilings of the arteries may also act as a pulse eliminator and a damping mechanism for arterial blood flow. Additionally, Hess et al.  propose that the functional significance of the knot-like vascular structures in mediastinum is associated with their location, i.e., where the pulse wave passes the mediastinum. These pulse waves, together with the contractility-flexible fibers of the mediastinum, may support the drainage of lymph towards the surface of the testis. Moreover, arterial conglomerates may facilitate the passage of blood from the intra-testicular veins into the extra-testicular veins [8, 11].
There are several hypotheses concerning the physiological role of the winding and straight segments of the intra-testicular arteries, as well as the arterial conglomerations inside the mediastinum of the bovine testis [9, 17, 18, 20]. However, the absence of these structures in the human testis prompts speculation. It is possible that the winding course of the arteries in bulls compensates for the shorter length of the terminal branches of the testicular artery compared with those of humans. If this is the case, its main role would be to decrease arterial pressure inside the gonad. It is possible that conglomeration of arteries, as in bulls, and a vascular network, as in humans, are two alternative ways for efficient thermoregulation and metabolite exchange with the venous vessels.