Lesson 19 - Digestive System (I) PDF

Cytology and Histology _____________ LESSON 19 _____________ DIGESTIVE SYSTEM (I) I. INTRODUCTION The main function of the digestive system is the preparation of food for absorption by the body. This process takes place in 5 phases: ingestion, fragmentation, digestion, absorption and elimination o...

Cytology and Histology _____________ LESSON 19 _____________ DIGESTIVE SYSTEM (I) I. INTRODUCTION The main function of the digestive system is the preparation of food for absorption by the body. This process takes place in 5 phases: ingestion, fragmentation, digestion, absorption and elimination of the waste products. From a histological point of view, the digestive system is basically a muscular tube lined by a mucous membrane (with regional variations derived from the function of each part of the tube) and connected to various exocrine glands. II. GENERAL STRUCTURE OF THE DIGESTIVE SYSTEM The digestive system is made up of: 1) Primary areas: They are the tubular or membranous organs such as the mouth, pharynx, oesophagus, stomach and intestine (small and large). 2) Secondary areas: They are the parenchymatous organs or attached glands such as the salivary glands, the liver and the pancreas. Histologically, the primary areas of the digestive system are made up of tubular organs (they have a lumen) that have four components arranged in concentric layers that form the wall of the organ. From the inner (the light) to the outmost, the layers of the wall are: mucosa, submucosa, muscularis or muscular layer and serosa or adventitia (Figure 1). The mucosa (in contact with the lumen) is made up of: - An epithelial sheet (in contact with the lumen), which is an epithelial lining tissue of a variable type depending on the location. A lamina propria, which is a loose connective tissue with blood and lymphatic vessels. A muscularis mucosae, made up of one to several layers of smooth muscle fibres. The muscularis mucosae is absent in the mucosa of the mouth, pharynx, some parts of the esophagus, and rumen. The submucosa is made up of loose connective tissue with blood and lymphatic vessels and, in some sections, glands. The muscularis or muscular layer is made up of several layers of smooth and/or striated muscle tissue depending on the location. The adventitia is a layer of loose connective tissue located around the muscularis externa layer. In the stomach and intestine (except the rectum), this layer of loose connective tissue is surrounded by a squamous, simple-type lining epithelial tissue called mesothelium, and this layer receive then the name of serosa. SEROSA S M Me MUSCULARIS EXTERNA SM SUBMUCOSA M MUCOSA Figure 1.- Diagram of the histological constitution of the different layers in the tubular organs of the digestive system. III. ORAL CAVITY Its function is the ingestion and the fragmentation of food to form the food bolus. Its general histological structure is as follows: 1. Mucosa: - Epithelial lamina: keratinized stratified squamous epithelium (variable degree). The degree of keratinization is different depending on the species. In carnivores it is scarce or practically non-existent, while in ruminants it is very abundant. - Lamina propria: It is made up of two layers: • • - Superficial layer: It is thin, with loose connective tissue with elastic and collagen fibres. Deep layer: Thicker and with more collagen fibres (dense irregular connective tissue). Muscularis mucosae: It does not exist at this level. 2. Submucosa: loose connective tissue with abundant adipocytes. 3. Muscularis: Striated muscle tissue. 4. Adventitia: loose connective tissue. There are variations to the general histological structure in the different parts of the mouth, which are as follows: Cytology and Histology The gingiva or gums are made up of a squamous stratified epithelium, keratinized or non-keratinized depending on the species, whose lamina propria continues with the periosteum of the maxilla or jaw at the level of the dental socket. The cheeks (Figure 2) have a middle or central portion of striated muscle tissue internally lined by oral mucosa, and externally by skin. The mucosa is usually very keratinized, and sometimes pigmented (dog and ruminants). In ruminants it forms conical papillae. The epithelial lamina is a keratinized squamous stratified epithelium with its lamina propria and scant submucosa. In the thickness of the muscle tissue there are molar glands (ventral, dorsal and medial), whose ducts cross the mucosa and lead to the inner portion of the cheeks. These glands are minor salivary glands of mixed secretion (serous and mucous) in horses and pigs, and of serous (ventral molar gland) or mucous (dorsal and medial) secretion in ruminants. In carnivores, the dorsal gland (zygomatic gland) is mixed, while the ventral is mucous. Finally, the layers corresponding to the skin are, from inside to outside, connective tissue or dermis and epidermis. Mucosa Muscle AdventitiaSubcutaneous Skin Figure 2.- Photomicrograph of the different layers of the cheek. HE, 4X. The palate separates the oral and nasal cavities and has a fixed part (the hard palate) and a mobile part (the soft palate). The hard palate is characterized by the mucosa forming a series of longitudinal and transverse folds, the palatal rugae. It is made up of a mucosa whose epithelium of keratinized stratified squamous epithelium forms the folds. In the submucosa there are abundant mucous minor salivary glands and blood vessels. The soft palate is made up of a portion of skeletal muscle tissue, a portion that faces the oral cavity, and a portion that faces the laryngo-pharynx. The portion that faces the oral cavity is a typical digestive mucosa with non-keratinized stratified squamous epithelium, and the portion that faces the larynx-pharynx has a respiratory mucosa with a ciliated pseudostratified columnar epithelium. The lips (Figure 3) owe their shape to the striated muscle tissue that forms the orbicularis oris muscle, and their shape varies in different species but they always have the same histological structure. Around the striated muscle tissue there are: 1. The external or cutaneous area: Keratinized stratified squamous epithelium with basement membrane and connective tissue (facial skin) that has hairs, sebaceous glands and serous glands except in carnivores. In some species there are sensitive hairs or tylotrich hairs. 2. The red transition area or free border: It is skin without hairs and without glands. It is richly vascularized what gives the name to this zone. 3. The internal area: It is oral mucosa of keratinized stratified squamous epithelium with minor salivary glands in the submucosa. External or cutaneous area Dorsal face Apex Ventral face Internal area Lip Tongue Figure 3.- Schemes of the histological structure of the lip (left) and tongue (right) with HE. Taken from the “Atlas of Histology Di Fiore, Víctor P. Eroschenko, Lippincott. Williams and Wilkins, 10th Edition, 2009 ”. The tongue (Figure 3) is a muscular and “tubular (non-cavitary)” organ located on the floor of the mouth that plays an important role in the apprehension, chewing and swallowing of food, as well as in the sense of taste. It has two faces, the ventral face and the dorsal face, and a central zone made up of the muscular layer with intersecting bundles of striated skeletal muscle tissue interspersed with fibrous connective tissue and adipose tissue. The tongue has numerous glands of serous secretion (located in the terminal sulci of the foliate and fungiform papillae), mucous secretion (at the base and edge of the tongue) and mixed secretion (in the submucosa of the ventral face and in the intermuscular connective tissue). These lingual glands are minor salivary glands. On the ventral face (or base of the tongue) the muscle tissue is covered by the submucosa, first, and the mucosa of non-keratinized stratified squamous epithelium. On the dorsal face, the muscular tissue is covered by the mucosa directly (there is no submucosa), which is of keratinized stratified squamous epithelium. This dorsal face of the tongue has elevations on the surface called papillae, which are morphologically divided into filiform, fungiform, circumvallate and foliate (Figure 4). All papillae, except the filiform ones, contain taste buds, structures that detect different flavours (bitter, sour, sweet, salty). The filiform papillae serve to detach food from the surfaces of the oral cavity and to prevent the backwards movement of food, since they are oriented towards the pharynx. They are very abundant, particularly in the cat. They are lined by a keratinized stratified squamous epithelium. They lack taste buds. The fungiform papillae are scattered among the filiform papillae and are shaped like a mushroom. They are lined by non-keratinized stratified squamous epithelium so they can be seen as red dots on the tongue. They have taste buds on the dorsal surface of the cap, which are rare in horses and cattle, abundant in sheep and pigs, and very functionally active in dogs and goats. Circumvallate papillae are very rare, calyx-shaped and submerged in the surface of the tongue so that they are surrounded by a groove, the annular groove. The groove is lined by a stratified squamous epithelium. At its base the exit orifices of some minor salivary serous glands located in the thickness of the tongue, the von Ebner glands, can be seen. The main function of these glands is to keep the papilla clean. The taste buds are on the lateral faces of these papillae in all species except the cat, in which there are hardly any taste buds. The foliate papillae are leaf-shaped. They are found next to the fungiform papillae and can be located not only on the lingual dorsum, but also on the lingual Cytology and Histology border. Ruminants lack these papillae. They are separated from the lingual mucosa by a groove that is called the gustatory groove. The taste buds are located in the lateral portions and in the bottom of the groove where, in addition, there are excretory ducts of serous glands. Filiform Fungiform Circumvallate Foliate Figure 4.- Microphotographs and diagrams of the histological constitution of the lingual papillae. Diagrams taken from the “Atlas of Histology Di Fiore, Víctor P. Eroschenko, Lippincott. Williams and Wilkins, 10th Edition, 2009 ”. HE. The taste buds, also called taste corpuscles, are intraepithelial sensory organs whose function is to perceive flavours. They are oval structures made up of elongated cells of pale cytoplasm located in the thickness of the stratified squamous epithelium of the papillae. The taste buds reach the surface of the epithelium and contact the taste pore, an orifice formed by the epithelial cells of the papilla. The taste bud cells have microvilli at the apical pole near the taste pore and form synaptic junctions with nerve fibres. The teeth are highly mineralized structures of the oral cavity that serve domestic mammals to grasp, cut and crush food and as offensive and defensive weapons. The tooth consists of a highly mineralized outer part (enamel, dentin and cementum) that surrounds a central cavity called the pulp, made up of loose connective tissue, blood vessels and nerves. The enamel is the hardest substance in the body that is made up of 99% mineral substances (hydroxyapatite) and 1% organic substances. The enamel is organized in parallel bundles. It is synthesized by ameloblasts. The dentin is the second hardest tissue in the body, highly mineralized and constitutes the largest component of the tooth. Dentin is made up of 70% mineral substance and 30% organic substance. The dentin is perforated by the dentinal tubules. Odontoblasts synthesize the organic matrix of dentin. The cementum is slightly modified bone tissue. It is made up of lamellae oriented parallel to the tooth surface. Cementocytes are the cells that synthesize cementum and occupy the gaps. Cementum forms the perforating cementum fibres and the periodontal membrane or ligament that fixes the tooth to the bone/dental socket. The dental pulp (central area of the tooth) is made up of loose connective tissue, numerous blood and lymphatic vessels, and nerves. The most peripheral part of the pulp has a layer of odontoblasts, from which the odontoblastic processes extend into the dentin tubules. The pulp cavity shrinks with age due to the production of dentin. Domestic animals have two types of teeth, which differ in their growth rate and in the arrangement of the layers of mineralized tissue: brachydonts (Figure 5) (carnivores, man, ruminant incisors and all pig teeth except the boar tusks or canines) and hypsodonts (horse, ruminant molars and boar tusk or canine). In brachydont teeth, the portion that protrudes from the gingiva is called the crown and is covered by enamel that extends below the neck region, which is the narrowest region located just below the gingiva. The root or roots are the portions inserted into the socket (bony cavity) and which is lined by cementum. Underneath the enamel and the cementum there is a thick layer of dentin and internally is the dental pulp. Hypsodont teeth lack a crown and neck but nevertheless have an elongated body that forms the root. Cementum covers the outside of the tooth both above and below the gingiva. The enamel layer, located under the cementum, extends throughout the entire body and almost to the tip of the root and underneath the enamel is a thick layer of dentin and pulp. A B Ameloblasts Enamel Dentin Predentin Odontoblasts Figure 5. (A) Diagram of the sagittal section of an incisor tooth located in the mandibular bone (brachydont). (B) Photomicrograph of an immature tooth section showing enamel and dentin. Ameloblasts (enamel-producing cells) and odontoblasts (dentin-producing cells) are both arranged in a palisade. Pararosaniline and toluidine blue staining. Images taken from the book of Basic Histology of Junqueira and Carnero, 2005. IV. MAJOR SALIVARY GLANDS The major salivary glands (Figure 6) are paired glands that are outside the oral cavity and discharge their secretion product through their excretory ducts. The secretion of these glands facilitates the fragmentation and swallowing of food. The major glands are the parotid, mandibular and sublingual, and the secretion product forms saliva, of mixed nature (serous and mucous), whose main function is to keep the mucous membranes moist, soften and lubricate food to facilitate swallowing. The major salivary glands are compound tubuloacinar exocrine glands with a capsule of dense irregular fibrous connective tissue that emit extensions into the organ, subdividing it into lobes and lobules. In addition, the acini are surrounded by reticular connective tissue. The secretory portions, tubules and acini, have three types of cells, serous, mucous and myoepithelial. The excretory ducts are highly branched and are called, from smaller to bigger, intercalated, striated, excretory intralobular, interlobular and interlobar, and terminal duct. V. PHARYNX The pharynx is a tubular organ that is responsible for the swallowing process by which the food bolus passes from the oral cavity to the esophagus. It has 3 parts, the Cytology and Histology naso-pharynx, the oro-pharynx and the larynx-pharynx. The mucosa of the naso- and laryngo-pharynx is made of ciliated pseudostratified columnar epithelium, whereas at the oro-pharynx it presents a keratinized stratified squamous epithelium. Below the epithelium there is a lamina propria of loose connective tissue with numerous elastic fibres as well as diffuse lymphoid tissue and abundant isolated lymphoid aggregates or pharyngeal tonsils (posterior part of the naso-pharynx). The submucosa is made up of connective tissue with elastic fibres and numerous tubular sero-mucous glands. The muscular layer is made up of longitudinal and circular skeletal striated fibres. The adventitia is of loose connective tissue. Myoepithelial cells Lobule Acinus Figure 6.- A) Diagram of the histological constitution of the major salivary glands. B) Microscopic image of the histological constitution of a mixed salivary gland. HE, 20X. Scaled duct Intralobular duct Interlobular duct Lobular duct Terminal duct A B VI. OESOPHAGUS The oesophagus carries the food bolus from the oropharynx to the stomach. The mucosa has a keratinized or non-keratinized stratified squamous epithelium with a lamina propria of loose connective tissue with lymphatic vessels (and glands in pigs) and its longitudinally arranged smooth muscle fibres in the muscularis mucosae. The muscularis mucosae is missing in the pig and is only present in the initial part of the esophagus in the dog. The submucosa is made up of connective tissue with mucous glands and their excretory ducts, blood vessels, nerves, and lymphatic vessels. In the dog there are oesophageal glands along their entire length. In pigs, there are oesophageal glands up to the middle of the oesophagus, from there and up to the stomach cardia they become more scarce and smaller. In horses, ruminants and cats, there are only glands at the beginning of the esophagus. The muscularis is made up of striated skeletal muscle tissue at the proximal segment of the oesophagus and smooth muscle tissue in the vicinity of the stomach. Skeletal muscle fibres are oblique and intersect at first. When the muscularis is made up of smooth muscle fibres they are arranged in two layers, an internal circular and an external longitudinal one. The circular layer increases in thickness in all animals towards the stomach and increases especially in thickness in the horse. The longitudinal layer thickens towards the stomach only in the horse. The outer layer is an adventitia in the cervical portion of the esophagus and is made up of loose connective tissue, while in the thoracic and abdominal portions it is a serosa (connective tissue and mesothelium). Bird oesophagus It has a mucosa of keratinized stratified squamous epithelium that forms longitudinal folds and papillae. The lamina propria has sac-like mucous glands and excretory ducts with simple columnar epithelium. There are lymphoid clumps around the glands near the stomach called the oesophageal tonsil. The muscularis mucosae is made up of smooth muscle fibres, the submucosa of connective tissue, the muscularis of smooth muscle fibres that are arranged in two layers, an internal circular and an external longitudinal one, and the adventitia by loose connective tissue. The crop (“buche” in Spanish) in birds is a dilation of the esophagus and has a structure similar to that of the esophagus with the difference that the folds of the mucosa are less high.