The accessory glands

The accessory glands are a series of four glands (sometimes reduced to three with the omission of the ampulla) responsible for the production and secretion of the majority of the fluid component of semen, termed seminal plasma. Different glands are responsible for the secretion of different fractions or jets. The epididymis of the testis is the other (minor) contributor to seminal plasma. The accessory glands are situated between the end of the vas deferens and the root of the penis (Fig. 3.9).

Access to (and, therefore, evaluation of the function and products of) the accessory glands is particularly difficult, due to their position within the pelvic cavity of the stallion. Hence little information is available on these glands, their relative importance and the characteristics of their secretions. Some information can be gleaned from ultrasonic examination - for example, the order of gland secretions and gland activity during ejaculation (Weber and Woods, 1993) - and from 1H nuclear magnetic resonance (Magistrini et al., 1995). Such examination confirms the involvement of the accessory glands in seminal plasma production and indicates that sexual preparation and ejaculation are associated with an increase, and subsequent decrease, in the size of the bulbourethral glands, prostate gland and vesicular glands (Weber et al., 1990). Table 3.1 illustrates the major secretions of the various accessory glands in some farm livestock; however, it is evident that considerable variation exists between and within stallions in the amounts of the components produced and their exact site of secretion (Magistrini et al., 1995). With recent developments, information on these glands is increasingly available (Weber and Woods, 1992, 1993). The following section will discuss each gland in turn. A more detailed account of seminal plasma is given in Chapter 4.

Bulbourethral glands

The bulbourethral glands, also known as the cowpers glands or glandula bulbourethralis, are situated nearest the root of the penis. They are paired;

Ultrasound Bulbourethral Glands
Fig. 3.9. Ventral view of the stallion's reproductive system, illustrating the relative positions of accessory glands.
Table 3.1. Major secretions of accessory glands in selected farm livestock (Mann, 1964; Mann and Lutwak-Mann, 1981; Setchell, 1991).

Gland

Stallion

Bull

Ram

Boar

Seminal vesicles

Major secretions

Citric acid

Fructose

Fructose

Citric acid

Potassium

Citric acid

Citric acid, Prostaglandins

Ergothionine Inositol

Minor secretions

Protein

Inositol

-

Fructose

Virtually absent

-

Ergothionine

-

-

Prostate

Major secretions

Protein Citric acid Zinc

Minor secretions

-

-

-

Citric acid

Virtually absent

-

-

-

-

Bulbourethral

Major secretions

Sodium chloride

-

-

Sialoproteins

Minor secretions

-

-

-

-

Virtually absent

-

-

-

Citric acid Inositol

Ampulla

No gland

Major secretions

Ergothionine

-

-

Minor secretions

Inositol

Fructose Citric acid

-

Virtually absent

Fructose Citric acid

-

-

they measure, on average, 19 mm by 32 mm and weigh in the order of 11 g each (Gebauer et al., 1974b; Little and Woods, 1987). They lie near the ischial arch on either side of the urethra into which they exit. They are normally oval in shape, but become increasingly flattened and ovoid during ejaculation (Weber and Woods, 1993). The secretions produced by the bulbourethral glands are clear, watery and thin in appearance, and high in sodium chloride (Mann, 1975). These secretions are mainly produced during the main sperm-rich part of the ejaculate (Weber and Woods, 1993) but possibly also contribute to the pre-spermatozoan fraction (Mann, 1975). The volume of the bulbourethral gland after sexual preparation, assessed by ultrasonography, is reported to correlate with the number of false mounts attempted by the stallion during sexual preparation (Weber et al., 1990).

Prostate gland

The prostate gland is a bilobed, single gland, each lobe being approximately 70 mm by 40 mm by 20 mm, weighing in its entirety 40-50 g (Gebauer et al., 1974b). Both lobes of the gland are interconnected via an isthmus 30 mm in length with a diameter of 6 mm, and exit via a single opening into the urethra near the bulbourethral glands (Little and Woods, 1987). The secretions of the prostate gland are thin and watery, with an alkaline pH, and are high in protein, citric acid and zinc. The significance of these levels is unclear. The secretions of the prostate gland make a significant contribution to the pre-spermatozoan fraction, whose likely importance is the flushing out of the system prior to the passage of the spermatozoa. The lobular and isthmus thickness of the prostate gland increase significantly after sexual preparation and significantly decline post ejaculation (Weber et al., 1990).

Vesicular glands

The vesicular glands, or seminal vesicles, are paired, multi-lobed glands lying either side of the bladder and exiting again near the exit of the bulbourethral glands. They are, on average, 160-200 mm in length, 25 mm in width and 13 mm in height, and weigh 25-30 g (Gebauer et al., 1974b; Little and Woods, 1987). The main function of these glands seems not to be (as first suggested) spermatozoan storage, but rather the production of the gel-like last fraction of semen ejaculated. Work by Weber et al. (1990) indicated that the decrease in size of the vesicular glands post ejaculation is positively correlated with the volume of gel evident in that ejaculate. The vesicular glands' function, however, is apparently not exclusively gel production. Weber and Woods (1993) demonstrated that there was fluid in the vesicular gland's excurrent ducts in only six out of 17 ejaculates that resulted in gel. It is likely that the vesicular glands are also responsible for secretions within the main sperm-rich fraction of the ejaculate. Vesiculectomy results in a significant decline in seminal volume not entirely accounted for by a lack of gel fraction secreted (Klug et al., 1979; Webb et al., 1990). The secretions of the vesicular glands are reported to be high in potassium and citric acid (Rossdale and Ricketts, 1980; Mann and Lutwak-Mann, 1981). The vesicular glands are the most testosterone dependent of all the accessory glands, their secretions being significantly reduced as a result of declining testosterone levels - for example, during the non-breeding season (Thompson et al., 1980).

Ampullae

The ampullae are not always classified as accessory glands as they do not strictly conform to the definition 'discrete structures separated from their site of secretion, that is the vas deferens, by duct(s)'. Rather they are paired out-foldings of the vas deferens, at its junction with the urethra. They form a pouch approximately 12 mm in diameter, compared with the normal vas deferens diameter in this area of 4-5 mm. Their average length is 16 mm (Little and Woods, 1987), making them relatively large compared with those of other farm livestock studied (Leone, 1954; Mann et al., 1956; Mann, 1975). These two pouches consist of a series of crypts and folds, along with the general increase in the diameter of the vas deferens lumen. Little is known of the contribution, in terms of volume, made by the ampullae towards seminal plasma (Amann, 1993a,b), but it is evident that any contribution they make occurs primarily before penile contraction or ejaculation (Weber and Woods, 1993). The secretions of the ampullae are thought to include ergothionine with some inositol; concentrations of fructose and citric acid are negligible (Rossdale and Ricketts, 1980; Mann and Lutwak-Mann, 1981). It has also been suggested that their function may be one of sperm storage, but work done by Gebauer et al. (1974a) does not support this theory, at least in regularly used stallions. The secretions of the ampullae vary with season (as do those of some of the other glands): in general they tend to be greater at the beginning and end of the season rather than at the peak. This is indicated by similar seasonal changes in ergothioine concentrations (Mann, 1975).

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  • leyton
    What accessory gland makes the gel fraction?
    8 years ago

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