Читать книгу Bovine Reproduction - Группа авторов - Страница 95

Anatomical changes of the testicular vascular cone as the bull ages and starts to produce sperm are indications that the efficiency of the counter‐current heat exchange mechanism between the testicular artery and veins in the pampiniform plexus needs to improve in order to cope with increasing testicular metabolism. In B. taurus beef bulls the testicular vascular cone diameter measured by ultrasonography increases until approximately 13.5 months of age, or until 1–8 weeks before SC reaches a plateau [65] (Figure 6.11). In crossbred beef bulls, the length and diameter of the testicular artery in the vascular cone increased from 6 to 12 months of age (1.8 m and 1.9 mm vs 3.1 m and 3.5 mm, respectively), but did not increase significantly thereafter [66]. The testicular artery length and volume in the vascular cone were 1.6 m and 6 ml in 15‐month‐old Angus bulls and 2.2 m and 11.4 ml in 28‐month‐old crossbred bulls, respectively [57]. Other studies have reported that the testicular vascular cone length is approximately 10–15 cm and that testicular artery length varies from 1.2 to 4.5 m in adult beef bulls of several breeds [67–69]. In addition to the lengthening of the testicular artery, the distance between the arterial and venous blood in the testicular vascular cone also decreases with age as a result of thinning of the artery wall (317 and 195 μm at 6 and 36 months of age, respectively) and reduction of the distance between the artery and the closest veins [66].

The epididymis continues to grow until at least 6 years of age in Holstein bulls and epididymal weight increases from 9 g at 8 months of age to 15, 23, 27, and 38 g at 12, 18, 25–48, and 73–96 months of age, respectively [51, 70]. The greatest dimension of the epididymis tail measured by ultrasonography from an oblique plane near the distal pole of the testis in Friesian bulls increased from 0.9 cm at 3 months of age to 1.8 and 2.7 cm at 12 and 24 months of age, respectively [71]. The tube‐like vesicular glands in newborn calves increase in length and become lobulated during development. The weight of the vesicular glands increased until approximately 4 years of age in Holstein bulls, from 13 g at 8 months of age to 26, 35, 54, and 78 g at 12, 18, 25–48, and 73–96 months of age, respectively [51, 70]. The maximum diameter of vesicular glands in Friesian bulls increased significantly only from age 3 months (1.1 cm) to 9 months (1.6 cm), suggesting that further increase in gland weight is a result of increase in length [71]. Prostate diameter also only increases significantly until 12 months of age (0.85 cm from 0.49 cm at 3 months), whereas maximum diameter of the bulbourethral glands increased from 1.2 cm at 3 months of age to 1.5 and 2.1 cm at 12 and 24 months of age, respectively [71].

In B. taurus bulls, the sigmoid flexure of the penis begins to develop at about 3 months of age, penis length increases by up to five times by the onset of puberty, and length continues to increase until sexual maturity [72]. The penis in Friesian bulls 13–19 months old measured 73–89 cm [73], whereas the penis in Holstein bulls aged 25 months or older measured 95–106 cm [70]. First protrusion of the penis during mounting was observed at approximately 8 months of age, whereas complete separation of penis and sheath was observed at approximately 8.5 months of age in Angus, Charolais, and Hereford bulls [47, 74]. Complete sheath–penile detachment evaluated during electroejaculation was observed around the same time of puberty, whereas first completed service evaluated during libido testing was only observed approximately one month after puberty [48].