The Uterus

The non gravid uterus

Morphology and location

The uterus in camelidae is bicomuate (Uterus bicornis). In the mature non-pregnant dromedary and Bactrian female, this organ is located in the abdominal cavity at the level of the 5^th, 6^th and cranial half of the 7^th lumbar vertebrae. It is more caudal in young females, extending cranially from the 6^th or 7^th lumbar vertebra. In nulliparous females the uterus is very small and can be found entirely within the pelvic cavity. The position of the uterine horns gives the organ a "Y" or a "T" shape depending on the reproductive history of the animal (e.g. number of pregnancies, age) (Figure 5 39).^{(9, 25-27, 30, 59, 65, 66, 74, 78, 87, 89, 107)}

In fresh specimens, the uterus appears grayish-white or red. Although it has been described as flaccid on postmortem specimens, the uterine tone and consistency varies in live animals depending on the status of ovarian activity. The uterus is normally contracted and hard during the follicular phase and flaccid during the luteal phase. ^(7,86,109) The average weight of the fresh uterus in young and mature females is 79.2 g and 316.5 g respectively.⁽⁵⁹⁾
 

Externally, the uterus shows two distinct parts: a cranial transverse part formed by the free portions of the uterine horns and a caudal longitudinal part formed by the fused parts of the horns, the body and the cervix of the uterus. Upon dissection, a long intercomual septum is found dividing the longitudinal part of the uterus into two distinct horns which join briefly into a small uterine body briefly before reaching the cervix (Figure 5.40).

The dorsal and ventral intercomual ligaments found in the bovine and small ruminant species are absent in camelidae.⁽⁵⁹⁾

The size of the uterine horns varies greatly amongst animals according to their reproductive history (Table 1). Asa general rule, the left horn is always longer and thicker than the right one and this discrepancy increases with age and number of parturitions because nearly all pregnancies are carried in the left uterine hom. The length of the free portion of the right hom (from the external bifurcation to the tip of the hom) measures 1/3 to 4/5 that of the left uterine hom. The diameter of the left and right uterine horns are respectively 3.4 ± 1.1 cm and 2.4 ± 0.61 cm. The length and diameter ratios are respectively 0.6 and 0.74 in favor of the right hom. We have found that this discrepancy in size between the left and right uterine horns is already present in the fetus and prepuberal animals (Figure 5.41).

The uterine body (Corpus uteri) is relatively short and measures 2.5 to 4 cm in length and diameter (Table 2).
 

The average total uterine weight is 284 ± 9.21 g and 272 ±77.1 g during ovarian activity and anestrus respectively. The open uterine horn reveals an endometrium with variable degrees of endometrial-folds development. The inner surface of the uterine hom can appear smooth and flat or undulating (Figure 5.40).^{(10, 50, 51, 85, 87, 88, 105)}

The major means of fixation of the uterus is the broad ligament which is attached to the lateral and caudal mesometrial borders of the longitudinal and transverse parts of the uterine hom respectively (Figure 5.40). The broad ligament extends caudally and attaches to the sacrum at the level of the third sacral vertebra (the sacrum is formed by 4 fused vertebrae).⁽⁷³⁾ On its cranial aspect, the ligament crosses the insertion tendon of the psoas minor muscle at the level of its attachment into the shaft of the ilium.⁽⁵⁹⁾ The right broad ligament is attached to the last lumbar vertebrae by its thick (4 mm) medial part which joins the parietal layer of the peritoneum. The thin (1 to 2 mm) lateral part of the right broad ligament joins the parietal layer of the peritoneum at the level of the crest of the ilium. The lateral part of the broad ligament merges with the parietal layer of the peritoneum at the level of the last rib, 6-8 cm lateral to the ipsilateral kidney.⁽⁵⁹⁾ The left broad ligament is divided into a medial and a lateral part at the level of the last lumbar vertebra. The medial portion attaches to the medial border of the left kidney and continues with the rumino-renal ligament. The lateral portion continuous cranially with the suspensory ligament of the spleen.^{(59, 96, 107)} The cranial (free border) of the uterine broad ligament is thin and forms folds that contain the uterine tube and vascular elements.⁽⁵⁹⁾

The uterus is also fixed by its round ligament which rises from the ventro-lateral aspect of the broad ligament and is contained within a double peritoneal fold. It measures 2 to 3 mm in diameter, runs in front of the shaft of the ilium and ends below the parietal layer of the peritoneum.⁽⁵⁹⁾

In the Bactrian camel, the anatomy of the uterus is very similar to that of the dromedary. The uterine body measures 2.5 to 3 cm. The left and right uterine horns measure 8 to 12 cm and 6 to 8 cm in length, respectively. A difference of 0.4 to 1.5 cm in length in favor of the left horn is also present in the newborn. The left horn has a larger diameter which measures 3.5 to 4 cm at its base.⁽²⁶⁾

In the llama and alpaca, the uterus is also similar in morphology to that of the dromedary. It is suspended by the broad ligament)^{(38, 49, 98)} and can be located within the pelvic canal or at its brim, depending on the age and reproductive history of the female.⁽⁹⁵⁾ The uterine body is short and measures 2.5 to 3.5 cm in length and 2.5 to 3.5 cm in diameter.^{(38, 49)} The same size discrepancy between the left and the right uterine horn described for the dromedary exists in llamas and alpacas (Figure 5.42) and increases after the first parturition. However, some authors have stated that both horns are comparable in size in the nulliparous female.⁽⁴⁹⁾ The uterine horn in the llama measures 20 to 22.5 cm from the caudal border of the septum to the tip. The free portion of the uterine horn varies between 8.5 to 15 cm in length and 2.5 to 4 cm in diameter.⁽³⁸⁾

Histology

Uterine histological features in camelidae are similar to those of other domestic species with only a few peculiarities. The uterine wall is composed, from the external surface to the uterine cavity, of a serosa, a muscular layer and a glandular layer. The serosa, or perimetrium, is the outer-most layer. It is a very smooth but strong tissue. The muscular layer (musculosa) is formed by two distinct layers of muscular fibers - the external longitudinal layer and the internal circular layer. The glandular layer (endometrium) is the most important layer from the theriogenolist's point of view because it is the structure directly responsible for the implantation and development of the conceptus and therefore is very important in fertility. The endometrium can be subdivided into two main sections: a glandular section containing the endometrial glands, and a surface epithelium which is the inner-most layer of the uterine wall (Figure 5.43, 5.44, 5.45).

The surface and glandular epithelium of the endometrium consists of a single layer of tall columnar epithelial cells, 17-22 µm in length and 1.7 - 2.3 µm in width, resting on an undulating membrane. The nuclei of epithelial cells are large, oval and parabasal.^{(37, 52)} This epithelium shows significant variations during the "estrous cycle".⁽¹⁾ It is usually described as low columnar during the early follicular phase and tall columnar or pseudostratified during the late follicular phase.⁽⁷⁸⁾ There is a slight difference between the cells of the surface epithelium and those of the epithelium of the endometrial glands. The former are cylindrical in shape while the latter tend to be broader at their base.⁽³⁷⁾

The cells in the epithelial lining of the endometrium and endometrial glands can be differentiated into 3 types according to the ultrastructural appearance of their borders. These border types are cilia, microvilli or zeiotic blebs. ⁽³⁷⁾ Ciliated cells are the most frequent cell type. The cilia are long, with a maximal length of 5.4 µm. The microvilli vary in number from few to moderate and are irregularly arranged. Their size also varies with a maximum length of 1.4 µm.⁽³⁷⁾ Adjacent cells are connected by desmosomes and gap junctions.

The endometrial stroma consists of irregular stromal cells, blood vessels, a large amount of collagen fibers, a few mast cells, and lymphocytes. Stromal cells have an oval or ellipsoidal nucleus and a small cytoplasm. The stromal cells around the blood vessels are more elliptical or even spindle-like in shape, and are associated with an increase in the amount of the surrounding collagen fibers.⁽³⁷⁾

The uterine glands are simple, branched, and tubular. They open in the surface of the epithelium and are relatively few in number compared to other domestic species. Branching of the uterine glands increases during the luteal phase)^{(52, 78)} (Figure 5.44, 5.45). Sub-epithelial edema is increased during the follicular and early luteal phase.⁽⁵²⁾
 

In the llama and alpaca, the histological arrangement is similar to that of the dromedary. The endometrial epithelium is columnar. The connective tissue of the submucosal stroma is dense and contains a few small uterine glands⁽⁸¹⁾. No obvious cyclic changes of the endometrium histology are identified. Also, administration of estradiol cypionate does not affect the histology of the endometrium.

A more complete discussion of the histological features of the endometrium is presented elsewhere in the present publication (cf. Uterine biopsy).

The gravid uterus

The size, tone, and location of the uterine horns change throughout pregnancy. During the early luteal phase, the uterus becomes very relaxed. The left uterine horn becomes noticeably distended at 1.5 months of pregnancy and its width is increased by 30 to 407c (7.1 cm as compared to 5.4 cm). At 2 months the left uterine horn is almost doubled in size and the uterus hangs into the abdominal cavity. The right uterine horn becomes enlarged and reaches 3 times its normal diameter at 75-90 days. At 150 days the diameters of the gravid and non-gravid horns are nearly 4 times their original sizes.^{(28, 30, 31, 61, 63, 66)}

Blood supply increases tremendously with advancing stage of pregnancy. The endometrium increases in size and the uterine glands become more numerous. The epithelial surface develops maternal crypts during implantation, providing an attachment of the placenta.

The postpartum uterus

Morphology and location

The non-involuted postpartum uterus hangs over the brim of the pelvis. It feels thickened and has longitudinal grooves on its external aspect. The uterine serosa is reddish-pink in color. The uterine wall is thick, edematous, non-resilient, and shows well-developed arteries and veins. In the early postpartum period (4 to 5 days), the uterus contains a small amount of brown viscous material. The surface of the endometrium is dark brown, probably because of the presence of small quantities of old blood.⁽²⁹⁾ The uterus returns to its normal non-pregnant characteristics and position between 20 and 45 days after parturition (cf. Uterine involution, physiology).^{(28, 32, 68, 84, 87, 90)} In the llama and alpaca, uterine involution with resumption of ovarian activity is faster. This is probably due to the fact that there is little loss of tissue during parturition because of the microcotyledonary type of placentation.^{(18, 39, 49, 95, 99, 100, 104)}

Histology

The histology of the endometrium during the postpartum period is characterized by an increased edema and infiltration by inflammatory cells. The microcaroncules disappears after 15 days postpartum and the regeneration of the epithelium is completed after 15 to 21 days. The uterine glands are few during the first week postpartum and increase thereafter.⁽⁷⁹⁾

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