The Poll Glands in the Dromedary

History

One of the most noticeable anatomic characteristics related to reproductive phenomena in the dromedary is the presence of the poll glands. These glands were first described at the end of the 19^th century⁽⁶⁶⁾ and were called temporal glands. The term “poll glands”, which is now used by most authors, was introduced by Leese in 1927, because the glands are found in the skin 5 to 6 centimeters below the apex of the head on the back of the poll.^{(39, 40, 57, 60)} Other authors refer to these glands as nape or occipital glands.⁽¹⁴⁾ In the French literature these glands are also termed "glandes retro-occipitales" or "glandes de la nuque".

Early reports stated that these glands do not develop until late in life (Lambordini 1879, cited by Curasson, 1947).⁽¹⁴⁾ However, later studies showed that the glands are present even in the new-born dromedary but they do not become active until puberty.⁽⁴⁰⁾

Anatomical characteristics

The poll glands are oval bodies located in the poll region of the male dromedaries.⁽⁶⁰⁾ They are easily differentiated from the surrounding skin (being slightly elevated, darker and covered by sparse short hair) (Figure 1.30). Each gland is broader at the base (towards head) and narrower at the apex. In the adult, these are situated in the skin about 5 centimeters below the apex of the head, on the back of the poll, on each side of the midline and are easily identified as two dark spots, especially during the rutting season (Figure 1.31).^{(64, 68)} This color of the poll gland is due to the brownish secretions that are produced. The amount of secretion is increased during the breeding season so that it often runs down the neck (Figure 1.31 and 32). This area is the first to show patches of sweat during hot weather or when the animal is put to hard work.⁽⁵⁷⁾ Many excretory pores are visible on the surface of the glands and can be demonstrated by squeezing the adjacent skin fold.

The size of the glands varies according to authors. Leese⁽⁴⁰⁾ reported that the glands were 2.5 inches long and 1.5 inches wide on either side. According to others, the glands are 3.6 inches in length, 3 inches in width and 1.6 inches in thickness.^{(57, 60)} Taha and Abdalla⁽⁶⁸⁾ give dimensions of 10 x 6.5 x 2.5 cm.

The glands are richly vascularized with the major arterial blood supply coming from the common carotid and vertebral arteries.⁽⁶⁰⁾
 

Histological characteristics

Histologically, the poll glands are considered to be modified sweat glands because of the similarities with the sweat glands in the dromedaries.^{(39, 57)} However, this theory is refuted by other authors because the poll glands have different histochemical and histological proprieties, from those found in the sweat glands.⁽⁶⁸⁾ The poll glands are also classified as compound, tubulo-alveolar glands due to the encapsulated glandular masses.^{(39, 57, 60, 68)} The association with the hair follicle and opening of the excretory ducts of the poll glands observed in the dromedaries are suggestive of an apocrine mode of secretion.⁽⁶⁰⁾

The poll glands consist of lobules and alveoli separated by a dense connective tissue sheet, from which a number of septae penetrate into the glandular tissue, dividing the glands into lobules of various shapes and sizes.⁽⁶⁸⁾ During the inactive state (before puberty and outside of the breeding season) the poll glands consist mainly of connective tissue and scattered groups of lobules and alveoli. The interlobular connective tissue, which forms the bulk of the glands during this inactive stage, consists of densely arranged collagenous fibers and only a few elastic ones. During activity (breeding season), the amount of connective tissue decreases and is visible as thin septae which encloses large lobules. The alveoli are spherical, ovoid or irregular in shape and vary considerably in size. The tubules of the glands are lined by a simple, tall (columnar) epithelium with apical protrusions.^{(57, 68)} The lining of the gland duct varies from simple cuboidal or columnar to stratified columnar.⁽⁶⁸⁾ The tubules become lined with low cuboidal or flat epithelium during the non-rutting season and after castration.^{(39, 57, 60, 68)} The effect of age on the activity of the poll glands is unclear.

The epithelial lining of the poll glands tubules is cuboidal. All the cells of this lining present microvilli. These cells present numerous mitochondria and vacuoles within their cytoplasm and a well-developed granular endoplasmic reticulum and Golgi complex.^{(60, 68, 71, 75)}

The alveoli are lined either with tall cells considered to be secretory, or with flat or simple cuboidal epithelium indicative of inactivity.^{(68, 71, 75)} A mixture of all these types of linings can be found in the same gland because not all lobules are in the same stage of activity at one particular time.^{(68, 71, 75)}

Ultrastructural features suggest that poll gland cells are involved in intracellular protein synthesis and secretion. The variations in size and electron density of the alveolar cells in the active poll gland indicate accumulation and transformation of secretory products associated with biosynthetic activity.⁽⁷¹⁾

Secretion of the poll gland is described as apocrine. This is illustrated by the common ultrastructural appearance of apical cytoplasmic protrusions, devoid of microvilli and organelles apart from ribosomes. These protrusions correspond to luminal bleb-like projections observed by the light microscope.^{(57, 71, 75)}

Function and variations

Activity of the poll glands is highly seasonal and closely follows testicular activity.⁽⁶⁰⁾ The highest activity of the poll glands is observed during the winter months (breeding season) and the lowest activity during the summer months (non breeding season). This observation, as well as the gradual regression of glandular tissue and its replacement by connective tissue, show that the activity of the poll gland is highly dependent on androgen activity.⁽⁴³⁾ Also, the poll glands are not visible in the female dromedary.^{(39, 40, 57, 60)}

Steroidogenic activity of the poll glands has been suggeted based on histochemical studies^{(71, 75)} and androgen itself is present in the secretions of the rutting male.⁽⁸⁰⁾ Steroid secretion and storage is also confirmed by the ultrastructural characteristics of the cells. Steroidogenic activity in cells of the poll glands resides entirely in the mitochondria and not the agranular endoplasmic reticulum. However, it is possible that the poll gland is steroid-dependent and accumulates testosterone rather than synthesizes it.^{(71, 75)}

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