The testes remain small until about 10 months, when they increase in size slowly till at 17-18mths, when they enlarge rapidly. About 80% of horses have a left testis larger than the right.
According to Johnson and Neaves (1981) reports that the testis is 117g at 2 years, 161g at 4-5 years and 213g at 13 to 20 years.
Figure 1. Normal testis and epididymis from a stallion.
Figure 2. Normal testis and epididymis from a stallion. Sectioned .
The efferent ductules and head of the epididymis are to the right, the body is dorsal and the tail is to the left. The spermatic cord is intact dorsally.
When sectioned sagitally, the testicular parenchyma is visible. The mediastinum testis is central and there are fibrous bands that run radially from the mediastinum. The normal colour is white pink. Spermatozoa should ooze from the tail of the epididymis.
Histologically, the testis is similar to other that of other species. The seminiferous tubules are coiled, but basically run from the rete testis in the mediastinum to the tunica albuginea and back, this in a vertical axis. A histological section taken from the tunic to the mediastinum will often capture several cross sections of the same tubule.
Figure 3: Histology of a normal testis.
Figure 4: Histology of a normal testis.
The completely normal testis should have normal spermatogenesis in each seminiferous tubule.
There are 8 stages of spermatogenesis recognised.
Swierstra EE, Pickett BW, Gebauer MR. Spermatogenesis and duration of transit of spermatozoa through the excurrent ducts of stallions. J Reprod Fertil Suppl. 1975; 23: 53-57.The interstitial compartment of the testis has the interstitial endocrine cells, blood vessels, and macrophages, dentritic cells, mast cells, testicular T cells (in some species) and interstitial fibroblastic cells.
Spermatozoa travel from the seminiferous tubules, or more correctly the convoluted seminiferous tubules to the straight seminiferous tubule into the rete testis. The rete testis is a network of ducts in the mediastinum testis that are lined by cuboidal epithelial cells. The intratesticular rete become the extratesticular rete tubules and these become the efferent ductules. The efferent ductules are lined by cuboidal and ciliated cells. The efferent ductules number between 13 and 15. These are derived from the mesonephric tubules.
Figure 6: Normal efferent ductules(upper left);these number 13-15; compared to the head of the epididymis (lower left, larger tube).
The efferent ductules join the single epididymal mesonephric duct. Some may be blind ended, but this is regarded as an 'anomaly' as it can lead to disease. The volume of fluid entering the epididymis (called rete testis fluid) is large and much is absorbed in the head of the epididymis.
Dym M. (1976) The mammalian rete testis--a morphological examination. Anat Rec 186(4):493-523.
McEntee K. Reproductive Pathology of Domestic Mammals. Academic Press San Diego, Ca 1990 Ch 13.
Johnson L, Neaves WB. Age-related changes in the Leydig cell population, seminiferous tubules, and sperm production in stallions. Biol Reprod. 1981; 24: 703-712.
