A study of some factors influencing fertility and sterility in the bull

by Herbert Lester Gilman

Testes: The testicles of the bull are relatively large. Varying with the size and age of the animal they measure from fourteen to seventeen centimeters in length, including the epididymis, and from six to eight centimeters in diameter. Each testicle is enclosed within a serous sac, the tunica vaginalis, whose visceral layer is very intimately fused with the underlying covering of the organ, the tunica albuginea. The tunica albuginea is quite thin and consists of connective tissue which is rich in elastic fibres. Muscular tissue is not present as it is in the case of many mammals. Inside the tunica, and closely attached to, though separated from, the parenchyma by a thin layer of connective tissue, is a layer of very loose connective tissue, which because of its rich supply of blood vessels is termed the tunica vasculosa. The parenchyma is of a yellowish gray color, and of a rather soft consistency. It is made up of the seminiferous tubules, rete, and the connective tissue stroma, the mediastinum testis. On section, the mediastinum appears as the center or axis of the entire organ. It is star-shaped, and radiates connective tissue septa out into the parenchyma to support and separate the tubules. Ellenberger states that the testis of the bull and all ruminants lacks a closed system of interlobular septa, because of the feeble development of the connective tissue.

The principal blood vessels and rete tubules are found in this structure, the function of the latter being to connect the seminiferous tubules and the efferent tubules of the epididymis. The epithelium of the rete is quite irregular,—consisting in places of a single layer; in others, of two layers. At some points there are formed groups of several cells lying over one another, with swollen homogeneous basal cells, which sometimes form projections into the lumen.

The interstitial tissue, besides conveying the blood vessels to the organ, contains many “interstitial cells.” These cells are relatively sparse in the adult bull, and are comparatively delicate, slightly granular, often shuttle-shaped, with a rather small nucleus. Embryologically they are derived from a syncitium arising from the mesothelium of the genital ridge, differentiating out by growth of the cytoplasm. They contain large quantities of fat, and elaborate the internal secretion of the testis. This secretion governs the development of the secondary sexual characters, and has a profound influence on the general body metabolism, and development of the skeleton. The interstitial cells appear early in embryonic life even before there is any differentiation of sex, and their greater relative development in the fetus is indicative of a future male development. In very young embryos, the growth is very rapid, followed, however, by a period of atrophy, during which the seminiferous tubules undergo marked development. Pende (21) states: “There seems to be an inverse relation between the growth of the tubular and interstitial tissues, as one is hypoplastic when the other is in full activity.” From birth to the onset of sexual maturity, which may be called a period of rest for the testicle, the cells are few in number. With the accentuation of the secondary sexual characters, and the beginning of sexual life, these cells again increase in number and activity.

The parenchyma of the testis consists for the most part of the seminiferous tubules, which, on account of the courses they take in the different regions, are divided into groups. The peripheral tubules are the much-contorted tubuli contorti. These anastomose to form the much shorter tubuli recti. These in turn anastomose frequently, uniting to form the rete testis. The rete proceeds through the mediastinum to form the efferent ductules which break through the tunica albuginea to form the greater part of the head of the epididymis. The tubuli contorti are the longer and more numerous of the tubules, for it is here that practically all the spermatozoa are produced. The straight tubules are relatively so short that they may be regarded more in the light of the beginning of the system of excretory ducts.

The seminiferous tubules consist of a thin peripheral membrana propria upon which rests the seminal epithelium, which is made up of the essential semen forming cells, and the cells of Sertoli. The spermatogenic cells may be divided into three groups, from within outward: the peripheral single layer of small cuboidal spermatogonia; one or two rows of large spermatocytes; and three to five rows of spheroidal spermatids. The cells of Sertoli are more or less of the syncitial type,—large in size and irregular in outline. They occur at various intervals between the layers of spermatogenic cells, with their bases resting upon the membrana propria. Centrally they send out protoplasmic processes for variable distances,—some even reaching the border of the innermost cell layers.

Spermatogenesis: In this process, the primary germinal cells, the spermatogonia, divide to form the primary spermatocytes. Maturation consists of two cell divisions of the primary spermatocytes, and these in turn form four spermatids. During the process, the number of chromosomes is reduced to half the number characteristic of the species. The spermatids then become converted into mature spermatozoa. This mode of transformation may be seen in Plate I. In the process, the nucleus of the spermatid forms a large part of the head: the centrosome divides, part passing to the extremities of the neck. One centrosome becomes the anterior, and remains attached to the head, while the other divides to form the posterior centrosome. The latter is divided into the anterior part, and the posterior nodule or annular ring. Besides this, the posterior centrosome becomes elongated to form the axial filament, and the cytoplasm forms the sheaths of the neck and tail. The spiral filament of the connecting piece is derived from the cytoplasmic mitochondria. At this time, a large part of the cellular cytoplasm is cast off. Meanwhile, the spermatozoa sink their heads into the long protoplasmic processes of the Sertoli or “nurse” cells which furnish nutritive material for their complete development. Finally the adult cells are cast off into the lumen. The structure of the spermatozoa, and a discussion of the semen will be taken up later.

Epididymis: The epididymis is divided into three parts: the head, body and tail. The head is made up principally of the lobules formed by the much-coiled efferent ductules proceeding from the rete. The ductules, about twelve in number in the bull, unite to form the body, which remains coiled and runs along the postero-medial part of the testicle to which it is more or less closely attached. To quote Ellenberger (23): “The transition from the rete into the ductules is gradual, as the characteristic epithelium of the latter (ductules) begins in cavities without walls, and at first, gradually form a wall which is well marked out as a thin ring of interstitial tissue.... The epithelium of the ductules is in sharp contrast to the rete in that it has a single-layered ciliated columnar epithelium, in which here and there one finds round basal cells. The dark and light columnar cells alternate; the cilia are often cemented together, and form cone-shaped, homogeneous appearing protuberances. The secretory activity is quite clearly observable. In the light cells one finds secretory globules, accumulating in rows, sometimes above, other times below, arches of cells. The secretory droplets pass from the cells into the lumen, and often lie in irregular layers on the epithelium; also the basal cells appear swollen and shoved out between the cylindrical cells.” At the lower extremity of the testicle, the tail is formed, which is globular in shape, and more or less loosely attached to the testicle. Here the ductules anastomose freely, gradually become less coiled, and end in the single excretory tube, the vas deferens. The epithelium at the tail part is more or less of the pseudo-stratified columnar, ciliated variety. Outside this is a membrana propria, a circular muscular layer, and a connective tissue coat. The secretory activity is very marked here and one finds much secretion in the lumen, Courrier (24), working on the bat, suggests that the glandular activity is conditioned by the secretion from the interstitial (endocrine) gland. The action of the secretion is to dilute the large mass of spermatozoa present, nourish them to some extent and also stimulate them to active motility. Stigler (25) states that the properties of the sperms are modified in the epididymis; the motility, the ability to resist heat, and other properties are augmented, at least in the case of the guinea pig, rat, and mouse. Some authors state that the sperms first become motile when in contact with the prostatic secretion, but I have repeatedly examined the contents of the tail of the epididymis of the bull, rabbit, and guinea pig, finding full motility in each case, though the duration is not nearly as long as when the sperms are ejaculated in the semen.

The Vas Deferens is quite narrow (2 mm.) and runs from the tail of the epididymis to the verumontanum, or colliculus seminalis, where it empties into the urethra in common with the duct of the vesicle. At first it is lined by epithelium similar to that of the vas epididymis, but this changes over into a peculiar low stratified type. Ellenberger describes it as follows: “The epithelium shows a very pronounced basal coat. The overlying cell zone shows more (at the most, three) rows lying over each other of elongated nuclei, while an outline of cell form is not ordinarily noticeable, so that it may be spoken of as a syncitium, and at the same time as a many layered epithelium.” The mucosa forms low, broad folds into the lumen. The tunica propria is a thin connective tissue layer. The submucosa consists of thin connective tissue. Three muscular coats are present: an inner thin longitudinal layer, middle circular layer, and an outer longitudinal layer. All are more or less intimately blended, and are surrounded by the adventitia, made up of connective tissue, elastic fibres and scattered longitudinal muscle cells of the internal cremaster muscle. Near the dorsal surface of the bladder, the ducts come in close apposition, and for ten to twelve centimeters dilate to form the ampullae. Here the mucous membrane becomes much plicated, forming long folds which anastomose freely. The function of the vas is to convey the spermatozoa and secretions from the epididymis to the urethra. Disselhorst (26) believes the ampulla acts as a seminal reservoir and states that he has found spermatozoa stored up in the little pockets in the walls of this structure in animals during the rutting time. He suggests, further, that there is a relation between the state of development of the ampulla and the time occupied by copulation. When the organ is small or absent, as in dogs, cats, and boars, the coition is a slow process, but when the ampulla is large and well developed, as in horses and sheep, the coitus requires a relatively short time. Inasmuch as coitus is so rapid in the bull, and the ampulla is so well developed, it seems as though this function is very probable.

The Seminal Vesicles are very compact glandular structures lying on either side of the median line, on the dorsal side of the bladder, and ventral to the rectum. In the mature bull they measure ten to twelve centimeters in length, four centimeters in width, and about two and one-half to three centimeters in thickness. The glands are distinctly lobulated, quite tortuous, and are often asymmetrical in size and shape. They converge posteriorly, to empty into the urethra at the colliculus seminalis with the ampulla, in a slightly oval slit in the mucosa. Microscopically, the gland is of the anastomosing tubular type, with very poorly developed excretory ducts to the glandular cavities. Posteriorly one finds centrally a few sinus-like narrow excretory passages, which open into the somewhat larger collecting and excretory duct. The epithelium is of the simple columnar type and produces a relatively large amount of secretion. The gland cavities are surrounded by a membrana propria, over which is a relatively thick layer of smooth muscle. Outside this is a connective tissue covering which sends trabeculae or septa in between the lobules. The secretion of the seminal vesicles is a tenacious albuminous fluid with a slightly yellowish tinge, all or part of which appears in the ejaculate in the form of swollen sago-like grains which are soon dissolved following ejaculation and the liquefaction of the semen. The proteid compounds belong to the group of histones. The secretion is liquid when warm and coagulates when cold. Some say that the filling of the vesicles serves to excite sexual feeling, but this is doubtful in view of the fact that in some animals the sexual desire exists before the vesicles are filled. Likewise, Steinach found that rats, whose seminal vesicles had been removed, still retained their desire for copulation. The function of the secretion is to furnish much of the volume to the semen, and in some way it has a distinct bearing on fertility, inasmuch as extirpation of the organs in rats leads to lowered fertility. The vesicles of the bull are in no sense a store-house for spermatozoa, as is usually understood. Repeated examinations in a large number of bulls have led to the finding of spermatozoa there only in very rare instances. That they serve as a resorption place for sperms that are not ejaculated is also very unlikely. Normally, one sees on smear of the vesicles, occasional cells, leucocytes, lecithin granules, sago bodies, and rarely a few degenerated spermatozoa.

The Colliculus Seminalis is a rounded or cone-shaped eminence in the posterior urethra, upon which the ducts of the seminal vesicles and vasa deferentia open. The ducts open separately at the bottom of two narrow slits, one on each side of the mound, there being no distinct ejaculatory duct as in man. The function of the colliculus or verumontanum is not definitely known. It is generally believed that the structure is made up of blood spaces which become engorged during erection, causing a blockage of the posterior urethra, which prevents regurgitation of the semen. Rytina (27), however, demonstrated that the structure is not composed of any unusual number of blood vessels or spaces, and that removal of the organ was not followed by regurgitation of the semen into the bladder during ejaculation. He believes, and quite logically, that its function is to afford a prominence upon which the ducts may empty. The mixture of the thick gelatinous semen with the thin prostatic secretion must occur at the moment of ejaculation and must be perfectly homogeneous, otherwise large numbers of the organisms remain in the thick gelatinous portion of the fluid. The eminence serves this purpose in that the prostatic ducts which converge toward it, may eject their secretion toward the eminence, producing an admixture more evenly and quickly.