Open access peer-reviewed chapter
Abstract
Goat in Pakistan has been raised both for getting meat and milk with a total production of 748 tons of mutton and 965 tons of milk. In Pakistan, goat meat is the most preferred protein source among muttons. Goats possess a healthy alimentary canal and are mostly offered low-cost grazing land-grown roughage. A chain of tubular organs/tissues and allied glands are observed in the goat alimentary system. A healthy digestive system transforms the available nutrients after digestion and absorption into better growth performance. Only few reports available in literature focusing different local raising goat breeds, its feeding behavior, and the morphological structures of their guts. In this chapter, we tried to draw attention to the embryological development of the goat, the nutrition, and the gross/microscopic anatomy and physiology of gut, which is comprised of oral cavity, pharynx, esophagus, stomach, intestines, liver, and pancreas. The histology of the mucosal structures is focused for better understanding because this tunic is in contact with the ingested food particles and is playing a key role in the process of digestion and absorption. Furthermore, health and clinical issues that can occur with goat alimentary systems are also highlighted in this chapter.
Keywords
- microscopic anatomy
- physiology
- management
- gastrointestinal system
- small ruminants
1. Introduction
Pakistan has 35 goat breeds (mostly of medium size) with a total population of 78.2 million. Goats belong to Bovidae family and their scientific name is
It has been observed that flock and family size is contributing depressingly while training and education of the family putting in positively to implementation index of fruitful management practices. Due to low literacy, lack of technology, the pitiable/high-cost veterinary services and extension infrastructure, un-accessibility of grazing land, the obtaining of value animals are the vital constraint. The prim indicator of the farm is mortality rate and it is observed that the mortality animal is predominantly due to diseases affecting the alimentary system where goats with kids and adults are nearly likewise affected [11, 12]. It is suggested that a state-level knowledge and inspiration of the people toward management development may also be accorded to the people. Lack of awareness about better management may possibly be attributed to the reality that the majority are illiterates and they do not have access to the latest information in the print media and also scientific lectures impart less on them. It is anticipated that government should arrange awareness
The ruminants in Pakistan are alienated into browsers and grazers. It has been reported that the browsers have considerable foregut functions, whereas the grazers mainly depend on abomasum, caecum, and intestines. Due to this anatomical difference, grazers can digest high-quality feed, whereas browsers can digest the fibrous parts of the plant. The digestive system is composed of many tubes-like organs and the associated glands [14]. Its prime role is to cut down the ingested food into smaller parts which can be absorbed into the circulation and used for the maintenance of the organism [15]. Considerable unique morphological structures of the tubular system, including oral cavity, stomach, and intestine, play important roles in digestive physiology. For example, the teeth of goats are made to grind roughages, and the stomach particularly the fore-stomach makes possible the microbial digestion of rough fibrous food [16]. There are numerous accessory/supportive glands, such as salivary gland, liver, and pancreas, located aside from the GIT tube but are connected with the duct system. Their ducts go through the walls of tube-like organ and pour their secretary products into lumina. The morphological study is of utmost necessary to precisely understand the function, nutrition, and pathology of the alimentary system; therefore, the current report is aimed to enlighten the anatomy and histology of the GIT since it is intricately involved with the conversion of food by enzymatic digestion into useful products in form of milk, meat, and skin [17, 18].
2. Key goat breeds and their geographical location in Pakistan
The goat breeds of Pakistan are found in various geographical and climatic environments ranging from cold mountains to hot plains [1]. Beetal bread is the most popular goat breed in Punjab region of Pakistan as its meat, milk, and skin is liked by the community (Figure 1). Kamori breed is an inhabitant of the Sind region and is another loved breed of the area particularly for getting milk. Dera Din Panah and crossbreeds are reared for milk and meat in Punjab [19, 20, 21]. Barbari breed is raised for milk and meat in Sindh and Punjab [22]. Bugri, Chappar, Jattan and Kamuri, Pateri, Sindh Desi, Tapri, and Tharri breeds are raised in the Sindh province of Pakistan [23]. Gaddi, Damani, and Kaghani breed are present in KPK and mostly used for meat purpose [24, 25]. Gulabi, a giant goat breed is preferred for meat and milk. Teddy breed is raised for mutton in Punjab and Azad Kashmir [26]. The home tract of Nachi goat breed encompasses Bahawalpur and Multan districts living in hot climatic conditions [25]. Khurasani and Lehri breeds are present in Balochistan region of Pakistan [23, 27]. Jattal, Beiari, Buchi, Jarakheil, Labri, Pamiri, Shurri, and Baltistani goat breed are kept in cold environment of the mountainous areas of Kashmir, Mirpur, Kotli, and northern areas of Pakistan [21, 22, 23, 25, 28, 29].
Figure 1.
Local goat breeds kept in the research center, Ravi campus, UVAS, Pattoki.
3. Nutrition of goats
In juvenile ruminants, the initial three gastric compartments are underdeveloped [30] and the animal in this stage acts as monogastric where effective utilization of colostrum and milk occurs in the gut. The animal later consumes a high fibrous diet and there is an establishment of microbial population in the stomach compartment stimulating the rumen development. In some areas, the fattening protocol is adopted for the goats and fed excessive concentrates to attain earlier growth and higher quality of the carcass.
4. Embryological development and baseline morphometrics of the alimentary system
During embryogenesis, the gut portion of the animal is derived from the endodermal germ layer, which lies ventrally to the embryo and forms the top surface of the yolk sac. With the passage of time, a greater part of this germ layer is integrated inward into the embryo to develop a gut tube, which is comprised of the foregut, midgut, and hindgut. The midgut is shared with the yolk sac through a narrow and longer vitelline duct. The foregut at the cranial end is enclosed by the oropharyngeal membrane, which later developed a passageway between the oral cavity and the primitive gut [39, 40]. The lateral plate mesoderm of the embryo is participating in the body cavity formation. The hindgut terminates at the cloacal region and forms an anal opening [41]. The whole developmental cycle of the goat is mentioned in Figure 2.
Figure 2.
Embryological development of goat.
Goat lifecycle is completed according to the following stages:
Embryo stage: It is a developmental period between fertilization of ova to the endometrial attachment of the conceptus in the female reproductive tract. This process begins with fertilization leads to cleavage, followed by compaction, differentiation of cells, cavity formation, zona hatching, and then implantation with the endometrial lining [40, 41].
Fetus: It is a period of development between attachments of conceptus to the uterus to birth and is characterized by the development and growth of body system. This ranges from cell mass differentiation of hypoblast, epiblast, and trophoblast leads to bilaminar disc formation, development of ectoderm, mesoderm, and the innermost endoderm, which later produce the gut tissue, liver, and pancreas, and then the formation of embryo, chorion, amnion and development of Cotyledonary (epitheliochorial) placenta formation and histogenesis occurs [41, 42].
Neonatal stage: It is an early period of an individual (from birth to physiological independence).
Prepubertal stage: It is period up to which animal becomes sexually mature.
Puberty: It is the period of animal life when it attains sexual maturity to produce young ones [43].
Adult: It is the prime and transition stage of animal reproduction.
Senescence: It is the old age of animal when there is a cessation of reproduction and the overall decline of body system leading to death [44].
5. Digestive system of the goat
The organs present in this system are responsible to get the food, chemically and mechanically breaking it down into smaller pieces and then absorbing them so that it can be used to get energy and body growth and renewal of cells and tissues. Later, it has to remove the unabsorbed portion of the food. This system extends from mouth to anus, including other accessory gland and organs (Figure 3). The anal canal is a short tube and is the terminal portion of the elementary canal. This is controlled by internal consists of smooth muscle and external anal sphincters consists of striated muscles [45].
Figure 3.
The alimentary system of goat (flow diagram).
6. Oral cavity
The oral cavity plays an important role in procuring and mastication of food particles. The oral cavity includes tongue, lips, teeth gum, cheeks, palate (hard and soft), and the vestibular space. The tongue is the mobile muscular tissue covered dorso-ventrally by stratified squamous epithelium and has three parts: root, body, and apex. Within the oral cavity, the tongue is supported by the hyoid bone caudally, mandible rostrally, and ventrally, and is attached dorsally through frenulum linguae. A large number of papillae are present on its dorsal surface and the filiform (mechanical papillae) are most numerous. Other (gustatory papillae) including fungiform are scattered on the tips, circum vallate is located on the anterior root dorsally, and foliate on the sides of the tongue. The tongue executes functions like reception of dry leaves, mastication, and deglutition of food and the muscles of the intrinsic and extrinsic groups facilitate fodder movement inside the oral cavity [15, 46]. The group of muscles, including masseter muscle, internal pterygoid muscle, medial pterygoid muscle, and temporal muscles, are responsible for mastication.
7. Salivary glands
In mammals, the main function of the salivary glands is to lubricate the ingested food, which helps in mastication and deglutition to protect oral tissues and in some species to initiate enzymatic activity. Salivary glands are present outside the wall of the digestive system and linked with the oral cavity through the duct. The two types of salivary glands include the major salivary glands and the minor salivary glands. The secretions of the salivary glands are serous, mucous, or seromucous (mixed). Serous cells produce a watery secretion having enzymes, ions, and a small amount of mucin, whereas mucous cells produce a viscous, stringy secretion called mucus [15, 47].
8. Minor salivary glands
Minor salivary glands are present within the wall of the oral cavity and oropharynx, and have short ducts [48]. They are named on the basis of their location:
Labial glands present in the lips.
Buccal glands present in the cheeks.
Palatal glands present in the hard and soft palate.
Lingual glands situated on the tongue.
The minor salivary glands are present in all three forms, that is, serous, mucous, and seromucous, which are the contributors of serous and mucous secretions through secretary ducts to saliva. Secretary units exist in a variety of forms (i.e., acinar, tubuloacinar, or tubular). Mucous tubules surrounded by the serous acini frequently develop serous demilunes; however striated ducts (the small ducts) are not its characteristics. The epithelium present in it is the simply squamous to low-cuboidal in shape. Later in the oral cavity, it is changed into stratified squamous form. Glands in the cheeks lie in the middle, dorsal, and ventral rows. The glands of the labial regions are assemblage as superior (nasolabial glands), commissural labial (under the skin along the mouth angle), and inferior labial glands which are connected to the commissural labial glands [15].
The composition of the submandibular gland varies in terms of saliva secretion among different species. The mandibular salivary gland is responsible for the production of a major portion of saliva. These are situated in the ventral and caudal part of the angle of the mandible and are irregularly triangular in shape. The gland has three angles, two surfaces, and three borders. The lateral surface is covered partly by the ventral part of the parotid gland. The medial surface is related to retrophyrangeal lymph node, the pharynx, larynx, and the lingual artery. The mandibular salivary gland is cream-colored or pale yellow, weighing from 5 g to 11 g. It is tubule-alveolar in composition. A thin connective tissue capsule covers the gland and the gland is derived into lobes and lobules by connective tissue septa emerging from the capsule. These connective tissue septa contain ducts, blood vessels, lymphatic, and nerves. The mixed alveoli (seromucous) are more abundantly present in the parenchyma than the other two forms. The duct system of the gland consists of intercalated, striated, and excretory duct forms [49].
Like other ruminants, goats are known to process saliva that acts mainly as a bicarbonate phosphate buffer, which aids in rumination and maintaining electrolyte and water balance, thus the saliva has a role in producing alkaline activity and evenness of the food contents within the sac of rumen and reticulum [50]. After suckling, the milk is overstepped by the reticulo-rumen through the esophageal groove into the true stomach (abomasums) in goat kids. The rumen remains very small in kids due to consuming liquid contents only until and unless the animal receives the fibrous diet. The juvenile animal relies on milk to neutralize the acidic environment in the stomach.
9. Lymphoid tissue of oral cavity of goat
Lymphoid tissue comprises mainly of lymphocytes within the oral cavity and is responsible for maintaining immunity. Such tissues are arranged in diffused and nodular form also called as lymphoid patches. There are two types of lymphoid organs, the primary lymphoid organs comprise of bone marrow and thymus where the development and schooling of the lymphocytes occurs, and secondary lymphoid organs include spleen, lymph nodes, and associated tissues of skin, mucosa, gut, and bronchi,. where the mature naive T and B lymphocytes identify the antigen where they get activated, proliferated and differentiated into effectors and memory types. The tonsils comprise of lymphoid cells (diffuse and nodular), including lingual and palatine, lined by keratinized stratified squamous epithelium [48]. On the other hand, para-epiglottic, pharyngeal, and tubal mucosae are lined by pseudostratified columnar ciliated epithelium on major areas contributing largely to immunity [51].
10. Esophagus
After the pharynx, which is the common cavity for ingested material and the air to pass, a long tube called esophagus is present in the goats. This hollow muscular tube connects the stomach to pharynx. The esophagus in goats passes over the dorsal border of the liver and enters the stomach at the cardia. The cardiac opening in the stomach is opposite to the middle of the eighth intercostal space; it is just to the left of the median plane and about two to three inches below the vertebral column. It opens into stomach at the junction of rumen and reticulum. In kids, the esophagus travels to form an esophageal groove, which serves as a bypass to transfer milk directly into abomasum. In goats, the esophagus consists of majority of the striated muscles. The esophagus has a stratified squamous epithelium lining the hollow organ. It has mucosal folds present for distension. The lamina propria and submucosa have connective tissue (CT) fibers and at the area of the junction with the stomach smooth muscles are arranged in a circular pattern in the inner layer and outer longitudinal within the tunica muscularis [15, 52].
11. Stomach of goat
Like other ruminants, a goat’s stomach has four compartments, including rumen, reticulum, omasum, and abomasum (Figure 4). Feed undergoes microbial digestion in the fore stomach followed by acidic digestion in the abomasums (true stomach). The larger portion of the fore stomach, the rumen, is regarded as a fermentation vat having the boundless hovering of plant particles related with microbes that are affixed to the mucosal lining cells of the rumen. The inner surface of rumen is coated with minuscule ridge called papillae, which enhances the contact surface area and facilitates improved absorption of digested food ingredients. The inner of the reticulum is honeycomb like in appearance which further adds to the nutrient contact surface area extension [48]. Some of the bacterial digested feed material is absorbed
Figure 4.
Digestive system of goat.
The small intestine of most domestic species is quite similar morpho-physiologically. Some of the structural and functional differences in specific regions of the small intestine impart differential functional capabilities to these segments. The small intestine is divided into the following three distinct segments: duodenum, jejunum, and ileum. The major functions of the small intestine are digestion, secretion, and absorption [59, 60]. The small intestinal mucosa has several anatomic adaptations that serve to create an immense surface area to digest and absorb nutrients. These include the plicae circulares (intestinal folds), villi, and microvilli. The villi are the most important area for digestion and absorption of intra-luminal nutrients [61]. It is more elongated in the anterior portion of the gut getting broader toward the ileum. Number of mucous-producing goblets cells increases anterior to posteriorly. The columnar epithelium has a brush border on the top and the mucosa has crypts of Lieberkuhn [62, 63, 64]. Columnar cells, Paneth cells, and goblet cells are present in the crypts [24]. Fibrous connective tissue (FCT), blood and lymph vessels are present in the lamina propria and submucosa. Few of the aggregated lymphoid tissues called Payers patches are present and increase in number toward ileal region. The thickness of the smooth muscles in the muscular mucosa is variable and arranged in thin continuous inner circular and outer longitudinal arrangements. The tunica submucosa is formed by CT, mononuclear cells, having fine blood capillaries along with CT cells, elastic, collagen, and reticular fibers. In duodenum, there are Brunner’s glands present in the submucosa which secrete alkaline mucus secretions protecting the intestinal mucosa from the acid released out from the stomach and the viscous mucus lubricates the gut contents [61, 63, 65]. Tunica muscularis thick layer comprising smooth muscle layers similar to the layers present in the mucosa. Mesenteric plexus is present between these layers. Tunica serosa is the last layer containing CT cells, fibers, small capillaries, and flat mesothelial cells. This layer represents the peritoneum and continues with mesentery. The main function of duodenum is to accomplish the initial phase of digestion and mixing of the stomach contents with pancreatic and bile secretions.
Because of the larger surface area of the jejunum, it plays an important role in the absorption of amino acids, fatty acids, sugar, water, minerals, and vitamins. The word ileum means eiliein (Greek word) and is the last part/section of the small intestine in higher vertebrates followed by jejunum. This has fewer diameters than other parts. This portion has smaller circular folds or is even absent in the terminal part and the final absorption of vit. B12, bile salts, and all the remaining important products that are not absorbed in the anterior segments are absorbed in the ileum. These compounds are then absorbed into the bloodstream. The capacity of the small intestine of goats is approximately 2.5 gallons [66].
12. The large intestine
Caecum, colon, and rectum are the three portions of the large intestine. This portion also has all four tunics present in the small intestine that works together to achieve the organs’ function. The mucosa is lined by simple columnar epithelium with the handsome number of goblet cells. The lamina propria have rich glandular regions and lymphoid tissues. The muscularis layer is of smooth muscles. There are solitary lymphoid follicles present in the colon over which the dome epithelium is lining on the luminal surface consisting of columnar enterocytes and M cells. Nutrient absorption occurs in the gut
Rectum is the final part of the large intestine located dorsally to the urogenital tract and ends on anus. The recto-anal Junction marks the termination of the lamina muscularis and longitudinal layer of the tunica muscularis, which forms the internal muscular anal sphincter. The anal sphincter is mostly comprised of skeletal muscle. The transition of epithelium columnar to stratified squamous non-keratinized occurs at the junction. The leftover material is getting rid of this portion.
Some differences can be are as follows:
The lumen of large intestine is much broader.
There are no villi present in the large intestine.
Number of goblet cells increased in the intestine proximo-distally.
Payer’s patches are absent in the large intestine.
There are no plicae circulars in the large intestine.
13. The liver and pancreas
Liver is red-brown in color and is the largest gland of the goat’s body, positioned on the right side and just caudal to the diaphragm within the abdominal cavity. This organ has parietal and visceral surfaces. The liver is covered by an FCT Glisson’s capsule from which the septa emerge downward and divide the parenchyma into partially completed hexagonal lobules. The central vein is present in the center of the hepatic lobule. The polygonal-shaped hepatocytes are radiated from the central vein area in the form of cords. Sinusoids are present in between the cords where the endothelium and fenestrations are visible. Prominent CT is present in the hepatic triad area at the corner of classical hepatic lobules where hepatic artery, portal vein, and bile ducts are present [68, 69, 70].
The liver and pancreas play a pivotal role in digestion. The liver performs metabolic and immunologic functions. It produces bile, which is also stored and then secreted by gallbladder helping the emulsification of fats for digestion. Proteins, fats, and carbohydrates are digested with the help of enzymes secreted by the pancreas in the small intestine. A goat liver also performs different functions like the destruction of hemoglobin, storehouse of glycogen and further converting it into glucose, transformed area into uric acid, detoxification, metabolizing drugs, synthesizing phospholipids and cholesterol, storehouse of vitamins and iron, and the production of almost 10% of the erythropoietin in adult animals [71].
The gall bladder of goat is a pear-shaped sac, yellowish-white in color, which remains filled with green-colored bile juice located at the level of the ninth rib. This structure is lined internally by tall columnar epithelium with occasional goblet cells. Thick muscles at the neck region are thought to be the sphincter of the gall bladder [61].
Pancreas has both the exocrine and endocrine portions and is present in the abdominal cavity. Its head lies in the loop formed by the duodenum and its tail is headed toward the spleen. The major portion is the exocrine comprised of the acini lined internally by the pyramidal acinar cells forming lobules having zymogen granules at their apical portion. The granules are precursors of several digestive enzymes which are secreted into the duodenum
14. Health and clinical issues that can occur with goat alimentary system
In goats, a range of diseases affecting the gastro-intestinal tract is found, chiefly including bacterial, viral, parasitic, and fungal infections. Oral cavity affections caused by bacterial pathogens are stomatitis caused by a number of bacteria such as Staphylococci, Streptococci, Fusobacterium necrophorum, Sphaerophorus necrophorus, and Actinobacillus lignieresii (Wooden Tongue). The common viral infections include PPR (PPR Virus, mainly Lineage IV), Bluetongue (Orbi virus), ORF (Parapoxvirus), Goat Pox (Goat Pox virus), Foot and Mouth Disease (Aptho virus), etc. Fungal infections are frequently caused by Monilia spp., Candidia spp., Fusarium spp., etc. Pharyngitis and esophagitis are caused by bacteria such as Actinobacilli and Fusobacterium necrophorum besides the pathogens invading down from the oral cavity to pharynx and esophagus. Ovine Herpes virus-2 is the viral agent causing esophagitis in goats [73, 74].
Rumenitis in goats accounts for a significant subclinical disease in survivors of acute episodes, favoring it by serving as a portal for the entry of fungi in this pivotal organ of digestion in goats. Fusobacterium necrophorum may cause secondary infections of abomasum. In Abomasum, a number of parasites belonging to the genera Haemonchus and Mecistocirrus have been recorded. They are large abomasal blood-sucking Trichostrongyles, capable of causing severe anemia and hypoproteinemia. In addition, Ostertagia spp. and related genera, such as Camelostrongylus, Teladorsagia, Marshallagia, and Trichostrongylus axei, are also found in various ruminants, causing chronic abomasitis with mucous metaplasia, achlorhydria, diarrhea, and plasma protein loss [75, 76].
Intestinal diseases affecting goats include Johne’s disease generally associated with wasting of body condition, but often not diarrhea mostly targeting the small intestine. The large bowel may be involved in a minority of cases, but the ileum is consistently affected. Acute undifferentiated diarrhea in neonatal goats is frequently associated with enterotoxigenic E. coli, rotavirus, coronaviruses, and Cryptosporidium parvum. Coccidiosis (a protozoan disease) is a multifaceted ailment caused by Eimeria spp. in caprine kids and may occur in animals as young as 3 weeks to 5 months of age. Raised white plaques of coccidia-infected proliferative epithelial cells are found in the terminal ileum; there may be some degree of hemorrhage in severe cases. Some parasites use the peritoneal cavity as their final habitat. Setaria spp. are Onchocercid filarioid nematodes inhabiting the peritoneal cavity of many wild and domestic ungulates, including goats [26, 74, 77].
Gastroenteritis has been recorded in goats >3 weeks of age. The major causes of diarrhea and ill-thrift in goats at pasture are parasitic. The main helminth endoparasite species causing this syndrome are Ostertagia, Nematodirus, and Trichostrongylus. Diarrhea and enterocolitis may also be associated with Clostridium perfringens type D enterotoxemia in goats, although; animals may die quite suddenly without showing any premonitory signs such as diarrhea. C. perfringens type D Enterotoxemia (“pulpy kidney” disease also known as “overeating” disease) is an important disease of goats worldwide resulting in significant mortality. Mycoplasma mycoides may cause acute fibrinous peritonitis in goats, although acute death from septicemia, or arthritis and mastitis are more common. Paratuberculosis caused by Mycobacterium avium subspecies paratuberculosis frequently produces nodular granulomatous lymphangitis in the mesentery and sometimes caseous or mineralized lymphadenitis [10, 26, 74, 78, 79, 80].
15. Conclusion
Both anatomical and physiological study of the GIT and associated organs of domestic goats are essential to be reported. Therefore, the development, histological structures, and functions of the goat gastrointestinal tract, and immune system, nutrition, and clinical issues of the gut are highlighted in this chapter.
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