Читать книгу Bovine Reproduction - Группа авторов - Страница 72

In beef bulls, average temperatures at the top, middle, and bottom were 30.4, 29.8, and 28.8 °C (scrotal surface); 33.3, 33.0, and 32.9 °C (scrotal subcutaneous); and 34.3, 34.3, and 34.5 °C (intratesticular), respectively [12]. Therefore, top‐to‐bottom differences (gradients) in temperature were 1.6, 0.4, and –0.2 °C for scrotal surface, scrotal subcutaneous, and intratesticular temperatures, respectively. Moving dorsal to ventral, the scrotum gets cooler, whereas the testis (independent of the scrotum) gets warmer. These temperature gradients are consistent with their vasculature, as the scrotum is vascularized from top to bottom, whereas the testis is essentially vascularized from bottom to top. In that regard, the testicular artery exits the bottom of the testicular vascular cone, courses the length of the testis (under the corpus epididymis), and at the bottom of the testis ramifies into multiple branches that spread dorsally and laterally across the surface of the testis before entering the testicular parenchyma [13]. Blood within the testicular artery was a similar temperature at the top of the testis compared to the bottom of the testis, but was significantly cooler at the point of entry into the testicular parenchyma (intra‐arterial temperatures at these locations were 34.3, 33.4, and 31.7 °C, respectively [14]). Therefore both the scrotum and the testis are warmest at the origin of their blood supply (top of scrotum and bottom of testis), but they both get cooler distal to that point (i.e. bottom of scrotum, top of testis). Remarkably, these opposing temperature gradients collectively result in a nearly uniform intratesticular temperature in situ [15].

In beef bulls, internal temperatures of the caput, corpus, and cauda epididymis averaged 35.6, 34.6, and 33.1 °C (gradient, 2.5 °C), respectively [12]. That the caput was warmer than the testicular parenchyma at the top of the testis was attributed to the proximity of the caput to the testicular vascular cone. Furthermore, it was noteworthy that the cauda, critical in sperm storage and maturation, was cooler than testicular parenchyma.

Bulls fed moderate‐energy diets after weaning have better semen quality than those fed high‐energy diets. In one study, beef bulls fed a moderate‐ versus high‐energy diet for 168 days after weaning had a larger scrotal surface temperature gradient (3.9 vs 3.4 °C, P < 0.02), more morphologically normal sperm (68.8 vs 62.5%, P < 0.01), and a higher proportion of progressively motile sperm (53.4 vs 44.5%, P < 0.006) [16]. Perhaps increased dietary energy reduced heat loss, thereby increasing temperatures of the testes and scrotum.