Arabian camels (Camelus dromedaries) are primarily found in dry regions of Africa and Asia, particularly the Middle East. These camels have significant importance in these areas due to their remarkable ability to withstand hot weather and scarcity of water while also providing valuable resources such as milk and high-quality meat and serving as transportation animals. In addition to their traditional roles, camel racing and beauty contests have gained popularity, especially in Gulf countries like Saudi Arabia [1,2]. Nevertheless, raising camels faces several biological challenges, including lower reproductive efficiency due to several factors. Among these factors are early embryonic losses, exclusive pregnancies in the left horn (98%), and pregnancy losses in the LH, even in the case of twins [2,3]. Asynchrony of conceptus signaling and uterine receptivity is believed to be the leading cause of implantation failure [4,5]. Therefore, the impaired interaction between the maternal endometrium of the right horn and the developing embryo requires further investigation to shed insights into improving the reproductive efficiency and fully utilizing the reproductive capacity of dromedary camels.

Dromedary camels, like pigs and horses, have a diffuse, epitheliochorial placenta [6]. Their ovulation is induced within 24–48 h after mating, with alternative ovarian activity occurring approximately 50% between both ovaries. The functional corpus luteum (CL) has a lifespan between Days 8–10 post-ovulation [2,7,8]. The early hatched blastocyst enters the uterus by Day 6–6.5 post-ovulation and rapidly grows, reaching the elongating stage by Day 10. During this period, the developing conceptus releases a signal to the endometrium to prevent CL luteolysis or exert luteotrophic mechanism. This embryonic signal must be released before Day 8 post-ovulation, and the MRP must occur before Day 10, coinciding with embryo elongation [3,4,9,10].

In all mammalian uteri, the endometrium is protected by a heavy glycoprotein called glycocalyx, which acts as a biological barrier against microbial and proteolytic attacks. One of the major components of this endometrial glycocalyx is mucin 1 (MUC1), which acts as an anti-adhesion molecule by sterically hindering interactions with other adhesion molecules [[11], [12], [13], [14]]. MUC1 has been extensively studied in the context of embryo attachment during the implantation window in mammals [5,15,16]. During the peri-implantation period, MUC1 plays a crucial role in preventing embryo adhesion by inhibiting the attachment of trophoblast cells to the uterine luminal epithelium cells. Therefore, the transition of the endometrium from the non-receptive to the receptive phase requires either the concentration of adhesion-promoting molecules or the depletion of anti-adhesive molecules. In most mammalian species, successful embryo attachment requires a mandatory remodeling in the endometrial MUC1 during the receptive phase [5,14,[17], [18], [19], [20], [21]].

Regulation of MUC1 in mediating endometrial receptivity and its anti-adhesion function has been studied in several mammalian species. The expression of MUC1 during this period is mediated by progesterone and/or estrogen. In several species (mice, rats, pigs, sheep, cows, and buffalos), MUC1 was highly expressed on the luminal epithelium during the non-receptive phase. However, it downregulated/removed during the receptivity phase, allowing adhesion-promoting molecules to interact with that of trophectoderm [20,[22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]]. Conversely, in humans and rabbits, MUC1 expression is increased across the luminal surface during the receptive phase. However, during embryo attachment, MUC1 is removed at the attachment site, likely regulated by the blastocyst through a local embryonic enzymatic-proteolysis [16,[34], [35], [36], [37]].

In camelids such as alpacas, it has been documented that there was a reduction of the MUC1 mRNA in the LH compared with the RH in the non-pregnant females at Day 15 post ovulation. However, no differences were seen between the left and right horns of the pregnant group. Additionally, when comparing the ipsilateral horns of pregnant and non-pregnant animals, there was less expression of MUC1 in the pregnant horns [38]. In our previous study conducted in advanced pregnant dromedary camels applying only the immunohistochemical technique, MUC1 was not detected in either LE or GE in both uterine horns. However, it was detected with strong intensity in the stroma of the RH compared with the LH [39].

During the pre-implantation period, glycoproteins like MUC1 expressed by the zona pellucida serve as a protective and antiadhesive coat for the conceptus [11,12]. The full extent of MUC1 expression in the conceptus remains unclear, but studies in equine embryonic capsules have identified mucin-like glycoproteins [40]. Transcriptomic analysis confirmed the presence of MUC1 and MUC20 in equine blastocysts [41]. In equine fetal membranes, MUC1 immunoreactive protein was detected on trophoblasts during the pre-attachment period, persisting on the surface of non-invasive trophoblast cells in allantochorion throughout attachment and placental development [42]. In addition, MUC1 was identified in trophoblasts isolated from early gestation macaques and found in endovascular trophoblasts within placental-decidual tissues during the same period [43].

The molecular dialogue between the dromedary conceptus and its mother endometrium during peri-implantation is still not sufficiently clear. Therefore, further investigation to understand how the molecular picture of MRP and implantation occurs in the left horn and what is the difference in the right horn is requested in this species. We hypothesized that MUC1 has a differential permissive role in adhesion, attachment and establishing implantation of camel conceptus in the left and right uterine horns. Therefore, we investigated for the first time the spatial and temporal expression of MUC1 and its regulation between the left and the right uterine horns of pregnant dromedary camel and its trophectoderm at Days 8, 10 and 12 of gestation in order to shed new insights on the molecular event at the peri-implantation period of the dromedary camels.