Ultrasonography of Testicular Maturation and Correlation with Body Growth and Semen Evaluation in Beagle Dog Model

Abstract

This prospective study investigated the ultrasonographic appearance of the canine testis from birth to adulthood. Eight purpose-bred laboratory Beagle-breed dogs were monitored from 4 to 40 weeks of life. The following parameters were evaluated every two weeks: bodyweight and height, scrotal and testicular volume, ultrasonographically measured testicular volume, echogenicity, heterogeneity, blood-flow score, ratio of the grayscale intensity value of the testis to the capsule, ejaculate volume, motility, viability, and number of spermatozoa. A correlation analysis was carried out between the various measurements obtained. Fertility was achieved in the 36th week of life. The echogenicity of the testicular parenchyma increased with age, and subsequently to the 30th week of life remained constant. The heterogeneity of the testicular parenchyma, as was evaluated by the standard deviation of the values of grayscale intensity of the parenchyma, also increased with age and was >19 at the onset of fertility. The ratio of grayscale intensity of testicular parenchyma had values < 200 at maturity. A colour Doppler evaluation first detected blood flow in the testis in the 22nd week. After the 32nd week, distinct signals were visible. In the 36th week, >80% of the testes imaged had visible vessels. A significant correlation was found between all the evaluation methods. The findings of the study may help clinicians detect the onset of fertility in dogs, especially when semen evaluation is not feasible; however, their applicability in all breeds or individuals might possibly vary due to genetic, physiological, and developmental differences. In summary, the study ultrasonographically explores the testicular maturity in dogs, with the aim to improve clinical assessments and health management in these animals.

Keywords: colour Doppler; dog; echogenicity; grayscale intensity; heterogeneity; maturity; puberty; semen; testis; ultrasound.

Figure 1

Colour Doppler scores of the testicular parenchyma vessels. (a). Score 1: one vessel (colour pixels) is visible (yellow arrow). (b). Score 2: distinct signals (three vessels) are visible (yellow arrows). (c). Score 3: distinct signals (four vessels) are visible (yellow arrows). (d). Score 4: the course of the vessels is visible (yellow arrows).

Figure 2

The spot-meter technique; a total of nine square-shaped spots (2 mm²) of testicular parenchyma are selected for evaluating grayscale intensity values (coloured squares). The grayscale intensity values of the capsule of the testis and mediastinum testis are also evaluated ((a): white rectangles; (b): black rectangles).

Figure 3

The region-area technique of grayscale analysis: the entire image of testicular parenchyma is outlined (yellow line), excluding mediastinum testis (mt), and corresponding histograms are created for the calculation of mean pixel grayscale intensity (numerical pixel values) and pixel heterogeneity (pixel values standard deviations).

Figure 4

Median testicular volume (cm³) from 4th to 40th week of life (blue arrow: first ejaculate; orange arrow: spermatozoa first seen in ejaculate; green arrow: total sperm count > 200 × 10⁶ in ejaculate).

Figure 5

Median total number of spermatozoa (×10⁶) from 30th to 40th week of life (orange arrow: spermatozoa first seen in ejaculate; green arrow: total sperm count > 200 × 10⁶ in ejaculate).

Figure 6

Median ultrasonographically (US) evaluated testicular volume (cm³) from 4th to 40th week of life (blue arrow: first ejaculate; orange arrow: spermatozoa first seen in ejaculate; green arrow: total sperm count > 200 × 10⁶ in ejaculate).

Figure 7

Median grayscale intensity values (scale 0–255) of testicular parenchyma, from 4th to 40th week of life (blue arrow: first ejaculate; orange arrow: spermatozoa first seen in ejaculate; green arrow: total sperm count > 200 × 10⁶ in ejaculate).

Figure 8

Median of standard deviation (SD) of grayscale intensity values of testicular parenchyma, from 4th to 40th week of life (blue arrow: first ejaculate; orange arrow: spermatozoa first seen in ejaculate; green arrow: total sperm count > 200 × 10⁶ in ejaculate).

Figure 9

Median grayscale intensity values (scale 0–255) of mediastinum testis from 4th to 40th week of life (blue arrow: first ejaculate; orange arrow: spermatozoa first seen in ejaculate; green arrow: total sperm count > 200 × 10⁶ in ejaculate).

Figure 10

Median grayscale intensity values (scale 0–255) of tunica albuginea (testis capsule) from 4th week to 40th week of life (blue arrow: first ejaculate; orange arrow: spermatozoa first seen in ejaculate; green arrow: total sperm count > 200 × 10⁶ in ejaculate).

Figure 11

Box and whisker plot of grayscale intensity values (scale 0–255) during pre-pubertal (blue), pubertal (green), and post-pubertal (violet) period (solid blocks: testicular parenchyma; lined blocks: mediastinum testis; rhombus mesh: testis capsule) (p < 0.0001 for all comparisons).

Figure 12

A cross-plot of grayscale intensity values (scale 0–255) of all the area of testicular parenchyma (except the mediastinum testis) and the area of separate sampling nine different regions of interest.

Figure 13

Ratio of grayscale intensity values of mediastinum testis/testicular parenchyma (blue line) and ratio of grayscale intensity values of capsule/testicular parenchyma (green line) (blue arrow: first ejaculate; orange arrow: spermatozoa first seen in ejaculate; green arrow: total sperm count > 200 × 10⁶ in ejaculate).

Figure 14

A cross-plot of the grayscale intensity values of testicular parenchyma and the total number of spermatozoa in the ejaculate collected the same day with the US examination (r_sp = 0.726, p = 0.001).

Figure 15

A cross-plot of the grayscale intensity values of testicular parenchyma and the total number of spermatozoa in the ejaculate two weeks after the US examination (r_sp = 0.680, p = 0.003).

Figure 16

A cross-plot of the grayscale intensity values of testicular parenchyma and the total number of spermatozoa in the ejaculate four weeks after the US examination (r_sp = 0.617, p = 0.008).