Birds are considered an important livestock resource and play an important nutritional role for humans in terms of producing meat and eggs. It is considered one of the laboratory animals on which many studies are conducted. Therefore, it is important to know the physiological, topographic, anatomical, and histological characteristics of the internal organs of birds in order to compare them with pathological tissues when birds are infected with bacterial, viral, and parasitic diseases. The difference occurs in the morphological and histological changes occurring in the digestive, respiratory, cardiac, and urinary systems according to the surrounding environmental conditions. In order to adapt and suit the environment, we notice here some changes that occur in the body systems of birds, and a difference can be noticed when comparing females and males as well. Birds that live in an aquatic environment differ in structure from birds that live in a desert or dry environment. Therefore, it is important to study the internal systems of birds and know their morphological and histological changes.

The physiological makeup of birds, or their anatomy, demonstrates a variety of unusual adaptations, most of which are beneficial to flying. Birds are able to fly because of their light skeleton, strong muscles, and respiratory, digestive, and urinary systems, which can all function at extremely high metabolic rates and deliver oxygen.

Digestive System

The development of the digestive system is a critical component of avian performance. During the last stage of embryonic development, the organs that comprise this system undergo significant development. Approximately 1% of the embryo's weight is made up of them during 18 days of incubation, and by the time they hatch, that weight has already increased to 3.5%. This increase in intestinal weight is attributed to the growth of intestinal villi [124]. The digestive tract of the bird is physically complete at hatch, but it lacks functional capability. Significant physiological and morphological changes occur in the gastrointestinal tract, improving the efficiency of the digesting and absorption processes [11;91]. The mucosa, submucosa, muscularis, and serosa are the four tunicas that make up the avian esophagus [34]. In all age groups, there were no gender differences observed in the histological characteristics of the esophagus in Ross broilers, which is consistent with findings published by Olsen et al. [2002] in Adelie penguins. Three layers make up the esophageal wall of kingfishers, according to histopathological observation: adventitia, muscularis, and mucosa. The submucosal layer wasn't there. The outcome was accepted [15]. Hoopoe's esophagus is divided into four separate functioning layers: the outermost serosa, muscular, submucosa, and mucosa. In addition, no crop was seen in the esophagus of the hoopoe species. This might be because food is stored along the length of the esophagus of bird species that do not have crops. The mucosal folds expand, leaving only a small channel in the esophageal centrum [1]. found with a different search. At D1, D14, and D28, the average esophageal lengths were 3.73±0.278 cm, 8.45±0.210 cm, and 13.75±0.478 cm, respectively. The mean weights of the esophagus at D1, D14, and D28 were 0.75±0.067 gm, 4.13±0.153 gm, and 7.45±0.341 gm, in that order [87]. The majority of research on birds [44, 134] concurred that the stomach appeared in both birds as comprised of two parts: the proventriculus and the gizzard. Like other birds, the native sparrow has four layers in its proventriculus: mucosa, submucosa, muscular, and serosa [101]. There have also been reports of four levels in the construction of the starling proventriculus [108]. Additionally, findings indicate that proventriculus has four layers in red junglefowl [65], seagulls [112], and domestic ducks and pigeons [56]. Nevertheless, reports indicate that the yellow-billed grosbeak's proventriculus only possesses three layers: mucosa, muscularis, and serosa [135]. The stomach of a quail weighed 4.06 ± 0.22 g and measured 2.38 ± 0.20 cm, with a relative weight of 31.6% and a relative length of 2.54%, respectively [2]. The pigeon's stomach measured 6.86 ± 0.69 g and 2.75 ± 0.25 cm in length, however its relative weight percentage was 3.2% and its stomach weight was 45.3%, according to reference [58]. The gizzard muscles of quails were characterized by Starck & Rahmaan [119] as consisting of smooth muscles that were divided by layers of connective tissue, giving the gizzard muscles their onion-like form. According to Mathias and Abdul Rahman [80], Japanese quails that were two weeks old had gizzard muscles that were around 180.52 mm thick. After being exposed to the experiment for two weeks, the average gizzard mass was 4.8±0.5 g. It's interesting to note that in Japanese quail, gizzard size increased during a 4-week period, peaking at 6.01±0.5 g [119]. Following the abdominal cavity's exposure, the intestines emerged as a tube-like structure that divided into the small and big intestines and extended from the gizzard to the cloaca. The big intestine was divided into the caecum and colorectum, while the small intestine consisted of the duodenum, jejunum, and ileum. The duodenum, which resembled a U-shaped loop, was the initial portion of the small intestine. There were ascending and descending limbs in this duodenal loop. The duodenum's ascending and descending limbs are where the pancreas is situated. The anterodorsal side of the gizzard, which terminates at the terminal portion of the ascending limb, is where the duodenum starts. The jejunum, caecum, and ileum covered the dorsal side of the duodenum, while the left side was connected to the gizzard. The liver and lateral body wall were observed to be in touch with the right side of the duodenum [129]. Numerous previous investigations, such as those conducted on houbara birds by Bailey et al. [21], quails by Zaher et al. [134], and barn owls by Oyelowo et al. [94], also identified the small (duodenum, jejunum, and ileum) and big (caecum and colorectum) intestines. Additionally, Dyce et al. [36] in poultry and Oyelowo et al. [94] in barn owls noted the U-shaped loop of the duodenum. The duodenum ceased at the terminal portion of the ascending limb and began at the anterodorsal face of the gizzard. The jejunum, caecum, and ileum covered the dorsal side of the duodenum, while the left side was connected to the gizzard. The liver and lateral body wall were observed to be in touch with the right side of the duodenum. These results are consistent with observations made by Nasrin et al. [87] and Dyce et al. [36], although Partha et al. [98] were unable to identify any notable differences in the topographic architecture of the duodenum between ducks, quails, and poultry. The jejunum is the longest segment of the gut and is the duodenum's continuation after the ascending limb. It is shaped like coils and is held in place by mesenteric folds. The jejunum loops were connected to the duodenum and gizzard ventrally and were visible in the dorsal part of the abdominal cavity. There was no discernible gross difference between the jejunum and the duodenum. The jejunum was described as the longest portion of the intestine by King and McLelland [70], Nickel et al. [88], Verma [125], and Hassouna [57], in agreement with the results of the current investigation. In contrast, the ileum was shown to be the longest segment of the intestine in quails and three native genotypes of chicken from Nigeria, respectively, by Zaher et al. [134] and Mahmud et al. [77]. Nonetheless, the ileum was shown to be the smallest portion of the barn owl's small intestine by Oyelowo et al. [94]. Different bird species may be the cause of the difference in gross architecture observed in different intestinal segments. The ileum was observed as a short, straight tube between two caecae near the jejunum's end. There was no discernible difference between the ileum and the jejunum. These findings are completely consistent with those of Fitzgerald [43] about Japanese quail, Hodges [62] regarding poultry, Nickel et al. [88], and Hassouna [57] regarding birds. There were two caecas—the right and left—that originated ventrolaterally, around the point where the ileum and colorectum meet. The peritoneal folds that connected both the caecal and the ileum were known as ileocaecal ligaments. Every caecum displayed three sections: the proximal, middle, and distal segments. The distal segment of the caecum displayed a blind-ended structure resembling a rounded cul-de-sac. The colorectum was a short, straight tube that extended from the caeca's origin to the cloaca. It was likewise the continuation of the ileum. The comparable origin and architecture of the caeca in quails and the broiler chickens' Nasrin et al., [87], as well as the results of Kumary et al., [73], Zaher et al., [134], Rajathi [100], and EI Deeb et al., [39], are in support of these findings. 

Digestive system accessories Pancreas

According to several theories, the pancreas in vertebrates always develops from the rear end of the foregut, with variations in the number of primordia [115]. However, in the bulk of the vertebrates, the number is believed to have never gone over the maximum of 3 [68, 105]. No matter how many primordia there were, a rat's pancreas could only function when all of the primordia fused together [49]. A review of the literature found that there hasn't been much research done on the histogenesis of the pancreas in Japanese quail. As a result, efforts have been undertaken to investigate this topic as much as possible [117]. The dorsal, ventral, and splenic lobes, which are situated between the duodenum's ascending and descending loops [62, 72], make up the avian pancreas. There is also another lobe that is referred to as the third lobe. In chickens [72], quails [118], and geese [52], the pancreas is split into four lobes; in ducks and many other bird species, however, only three lobes are recognized [103]. The glandular epithelial cells, known as acinus cells, with their small and large excretory ducts, make up the exocrine pancreas. The acinar cytoplasm contains a high concentration of rough endoplasmic reticulum, crystal-type mitochondria, free ribosomes, and secretory zymogene granules [85]. Centro-acinus cells, so termed because of their euchromatic nuclei and presence in the lumen of acini, are not observed in chickens but occasionally occur in starlings [131] and domestic geese [52]. Acinus cell secretions are expelled into the duodenum by excretory channels that are intercalated, intralobular, interlobular, and interlobar [38]. The dorsal and ventral pancreatic ducts, as well as a third duct in some other bird species, are the primary excretory ducts in birds that enter into the duodenum [24]. Alpha, beta, and mixed islets, which are tiny and big islets, make up the endocrine pancreas of avian species [30]. Silver-stained alpha islets are also referred to as black islets, while beta islets without a silver stain are referred to as light islets [62, 84]. Chickens' A, D, and B cells and quails' A, D, and Type IV cells make up the large population of alpha islets that are located in the caput area of the splenic lobe and third lobe [122]. A, B, and D cells make up beta islets, which are abundant in all lobes [122]. B and D cells are found in chickens [84], quails [118], and geese [52]. The embryonic development, immunohistochemistry, and electron microscopic architecture of the alpha and beta cells in the fowl pancreas (usually in chickens) have been described in earlier research [24,30,103,116;128].

Liver
Duck The liver of quails is made up of left and right lobes. The liver's caudal side contains the gallbladder and extrahepatic biliary system. The quail tract contains two primary ducts: the hepatic-enteric duct and the cystic-enteric duct, in contrast to the one duct seen in the human biliary system [95, 96]. The ducts begin in the center of the liver, between the lobes on the left and right, and go straight along a parallel path toward the duodenum. The little branches are not visible. Subsequently, both biliary ducts open at the same location to the duodenum's ascending portion. The extrahepatic region is not where the two ducts form anastomoses. The cystic-enteric duct is the sole conduit connecting the gallbladder. These two biliary ducts also have a duodenal entrance that gives birth to the dorsal and ventral pancreatic ducts. The ventral duct connects to the ascending portion of the duodenum, whereas the dorsal pancreatic duct continues to the pancreatic lobe, which joins to the descending portion. The celiac artery trunk serves as the origin of the major arteries in the hepatobiliary system. The common hepatic artery and the left gastric artery are the two main arteries that emerge from the celiac trunk. The biliary tract is supplied by the former after it travels straight to the right hepatic artery. In order to serve the pancreas and duodenum, the gastroduodenal artery splits off from the common hepatic artery. The hepatobiliary system is supplied by the portal vein. The pancreaticoduodenal vein is big and provides several minor branches in the duodenum, matching the length of the duodenum. Certain segments of the hepatic-enteric duct do not travel parallel to the portal vein, but the distal portion does [114;45]. There are several structural distinctions between the quail and pigeon hepatobiliary systems. The pancreatic and bile ducts of the quail had a common intestinal entry, but the pigeon's ducts opened to the duodenum at distinct levels. Thus, the entire morphology of the biliary system changed, in addition to gallbladder status. It is unclear, nevertheless, if biliary tract variations are connected to gallbladder loss. The anatomy of the biliary system is one of the vertebrate body's most malleable parts. For instance, rats possessing a gallbladder have several ductal variants. The duodenal entrance of the pancreatic duct and the biliary duct are completely distinct in many situations, while many rodents have the same opening for both ducts [50]. In other circumstances, the biliary tract branches as shown in chinchillas [92]. It is believed that dietary and life history variables have little impact on this difference in connection. Despite these difficulties, comparing our findings with those of other research should assist elucidate the evolutionary patterns of the avian biliary system and determine whether system is more ancestral—the quail or the pigeon system. According to the current study, the quail's hepatobiliary system's topology is quite similar to that of the chicken, which is a close relative [95, 96]. The branching architecture of the biliary system seen in modern quails is similarly retained throughout a number of duck species, despite the lack of knowledge regarding blood vessels [75]. The Japanese bush warbler, brambling, Japanese quail, duck, tiny egret, pigeon, tree sparrow, and chicken were all studied in comparison by Yamagishi [132]. There are two biliary ducts present in these animals. Nearly all of the duodenum's branches originate from the ascending portion, despite minor variations in the locations of the pancreatic ductal and bile trunks. The pancreatic duct opens to the ascending part of the duodenum, separate from the biliary duct [23]. Hence, although the blood vessel pattern remains unclear, the hepatobiliary system of the ostrich is largely the same as that of the pigeon, with the exception of the cystic-enteric duct. The above comparisons suggest that avian gallbladder loss may be related to changes in the connectivity of the biliary tract itself. However, given the plasticity of the biliary tract in rodents, this hypothesis requires additional evidence. To further evaluate gallbladder loss in birds, morphological data are needed from columbiform taxa that have not lost their gallbladder, such as Ptilinopus, Ducula, and Gymnophaps [45, 53]. Separate from the biliary duct, the pancreatic duct opens to the ascending portion of the duodenum [23]. Thus, the ostrich's hepatobiliary system is similar to the pigeon's, except for the cystic-enteric duct, even if the blood-vessel arrangement is yet unknown. The aforementioned parallels imply that alterations in the biliary tract's connection may be connected to the disappearance of the gallbladder in birds. However, more proof is needed for this theory because of how flexible the biliary system is in rodents. Morphological data from columbiform taxa—such as Ptilinopus, Ducula, and Gymnophaps—that have not lost their gallbladder is required in order to assess gallbladder loss in birds in greater detail [45, 53]. Similar to mammals, avian hepatobiliary system arteries are fed by the celiac trunk, while the portal vein supplies the venous system. The avian vasculature is mostly similar to that of mammals, despite the fact that the peripheral arteries in this area, at least in the major arterial trunks, tend to display notable diversity even within the same species [60;71; 82;83]. The existence of two bile channels is the most distinctive aspect of the avian hepatobiliary system. Given that other amniotes only have a single ductal trunk, this twin biliary trunk is a developed feature based on evolutionary theory. As members of the same Archosauria ancestry as birds, crocodilians "nearly" have two bile ducts [130]. The many anastomoses between the two bile trunks of A. mississippiensis's most prevalent pattern set it apart from birds. But even within the same species, morphology varies, and a small percentage of individuals have a split double-trunk condition, which is a pattern like that of birds [130]. Therefore, it seems that the twin biliary duct originated in the archosaurian lineage in a sequential manner. In addition to the twin biliary duct, birds also seem to have a highly derived developmental mechanism. The majority of the morphology of the hepatobiliary primordia is preserved over the pharyngula phase, despite significant variance in the early embryonic stages [40]. As a hepatic diverticulum, the hepatobiliary system initially emerges from the ventral side of the posterior foregut [114]. After that, the diverticulum gives birth to many hepatic buds as well as one biliary branch in non-avian vertebrates. A dorsal pancreatic bud emerges from the dorsal side of the foregut at the same time as a ventral pancreatic bud forms in the area caudal to the biliary bud [61, 114]. Although the ventral pancreatic bud is typically single, it can occasionally be split into left and right halves [41, 67]. Hepatic buds can vary in number, but throughout development, they are usually grouped inside a single duct and proceed straight to the biliary duct [109, 126]. In any event, it is believed that the lack of a biliary bud during the whole developing period causes gallbladder loss in mammals [61]. In the meantime, birds have two hepatobiliary buds. The hepato-enteric duct originates from the cranial one, and the cystic-enteric duct from the caudal one. These are divided craniocaudally. Many bird species, such as chickens [25], quail [74], and black-tailed gulls [66], have been seen to do this. How these two hepatobiliary buds relate to the original hepatic and biliary buds is unknown. These primordia have a similar topology and are present throughout development in pigeons as well [110]. Moreover, the posterior hepatic duct gives rise to an apparent biliary bud in pigeon embryos; the gallbladder subsequently "degenerates" throughout development [110]. Therefore, the loss of the gallbladder in birds is a reflection of secondary degeneration of the biliary bud during development; this is not the case in rats, whose biliary bud never develops during development [61], but rather is comparable to the loss of the eye in cavefish [64]. This is also different from the lamprey, which matures and loses its gallbladder [110;133]. Therefore, it's possible that developmental alterations in mammals and birds may not lead to gallbladder loss in the same way. The creation or maintenance of the gallbladder primordium may be mediated by a shared molecular process; however, this is unknown. If the mechanisms are comparable, there may be differences in time between the effects in these two animal species.

Respiratory System

Animals' respiratory systems are generally similar, but because of differences in function and complexity, different species exhibit different morphologic, morphometric, and physiological variances [104]. Accurate diagnosis and prognosis of illnesses, as well as a deeper comprehension of physiology, depend on an awareness of morphological standards and the factors that lead to their changes [107]. The organs that enable the union of blood and air to enable gas exchange are known as the respiratory system [63]. With mean body weights of 159.51±8.19g for the Japanese quail and 265.78±4.88g for the pigeon, respectively, all the birds employed were adults, and this was deemed to be highly significant. The quail's tracheal weight, length, and diameter were 0.55±0.06g, 7.61±0.37cm, and 0.47±0.04 cm, respectively, whereas the pigeon's comparable values were 0.79±0.11g, 8.13±1.10 cm, and 0.51±0.06 cm. The lower respiratory tract was seen to extend from the caudal area of the mouth cavity to the neck and thoracic region in both the quail and the pigeon. The trachea, syrinx, bronchus, lungs, and the structures that support them make up the lung and respiratory tract (LRT), although in this research the syrinx was regarded as a component of the trachea. The trachea, bronchus, and lungs were the LRT structures utilized in this investigation [59]. The quail's mean total weight of the LRT features was 2.38±0.18g, or around 1.49% of its entire body weight; the pigeon's mean total weight of the LRT features was 4.03g, or 1.52% of its total body weight. In contrast to the data given by Ajayi et al. [3], who reported 1% as the weight of the LRT relative to the body weight in grasscutter, the comparative proportion of the overall weight of the LRT in both bird kinds was roughly 1.5% of their entire body weights. Since these birds are fliers, their respiratory systems must be subjected to greater demands, which might account for the larger weight percentage of the respiratory apparatus in these animals. In both quail and pigeons, the trachea was found to be ventral to the esophagus along the neck's length. In the quail, the tracheal weight, length, and diameter were 0.55±0.06g, 7.61±0.37cm, and 0.47±0.04 cm, respectively [59]. In contrast, the pigeon had a tracheal weight, length, and diameter of 0.79±0.11g, 8.13±1.10 cm, and 0.51±0.06 cm, respectively. The mean tracheal diameter measured in this study for both bird species is different from Reznik's [104] findings, which indicated that adult rats had a diameter of 0.2 to 0.3 cm. It has been noted that in both species, the right and left bronchus enter the corresponding left and right lobes of the lungs after emerging from the trachea. The lengths of the right and left bronchus in the quail and pigeon, however, revealed very substantial variances. The weights of the bronchi in both bird kinds were not measured because they were too small. Both birds had their bronchial diameters measured, although the observed differences were not statistically significant. A linear relationship was observed by Choi et al. [29] between the number of mucous glands in mammals and the tracheal diameter. They deduced that an increase in the size of a conductive airway, indicated by an increase in tracheal diameter, causes an increase in the inspired air's inflow velocity, which in turn causes an increase in the rate of foreign body deposition. The thoracic vertebrae make contact with the dorsal surface of the lungs of both quail and pigeons, causing them to become brilliant red in color and leaving a distinct vertebral impression on the lung surfaces. The quail's right lung's weight, length, and diameter (thickness) were 0.92±0.15g, 2.26±0.13cm, and 0.66±0.04cm, respectively, whereas the pigeon's counterpart values were 1.53±0.28g, 2.78±0.37cm, and 0.52±0.07cm, respectively. Despite this, there was no statistically significant difference between the two. The quail's left lung measured 0.91±0.15g in weight, 2.29±0.14cm in length, and 0.73±0.03cm in diameter. In contrast, the pigeon's left lung measured 1.71±0.34g, 2.61±0.36cm, and 0.52±0.07cm in weight, length, and diameter, respectively. The thickness of the left lung was deemed statistically significant, although the weight and length of the lung were not [59]. The lung is a fairly tiny compartment that serves as a large surface area for gas exchange with the blood. It is a very hard structure that does not expand or retract with breathing. The degree to which each lung was entrenched into the ribs in the two birds under study is a highly significant observation that led to a separation of the lungs into portions. This observation is consistent with the results of research conducted on guinea fowl by Ibe et al. [63]. Similar observations on ostriches were also made by Maina and Nathaniel [79]. According to the research [106], the lungs in both birds were compactly placed against the dorsal wall of the thorax and did not expand or contract. The lungs' primary duties include storing blood from the right side of the heart until it enters the systemic circulation, filtering blood, and facilitating gas exchange. The digestion of proteins and fats, the activation or inactivation of hormones, and the metabolism of foreign substances that may enter the lungs by the airways or the vasculature are other metabolic tasks carried out by the lungs. Despite their diminutive size, birds' lungs are linked to eight or nine air sacs that circulate throughout their bodies and are subsequently connected to air gaps found inside their bones. Despite having 27

We found that studying the morphological and histological changes in birds’ organs helps us to discover how birds can adapt to their external surroundings, and this reveals to us the importance of studying the birds’ internal organs and the way their changes are observed in them according to the adopted way of living and their adaptation, and also comparing them to the changes that occur in infected birds when studying disease cases.

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