Digestive Tract

Introduction

The digestive system consists of the -oral cavity( and ) - digestive tract : (esophagus, stomach, small intestine, large intestine, rectum, appendex and anus) -accessory glands of digestive system: (salivary glands, pancreas, and liver).

Its function is to obtain from ingested food the molecules necessary for the maintenance, growth, and energy needs of the body. Macromolecules such as proteins, fats, complex carbohydrates, and nucleic acids are broken down into small molecules that are more easily absorbed through the lining of the digestive tract, mostly in the small intestine. Water, vitamins, and minerals from ingested food are also absorbed. In addition, the inner layer of the digestive tract is a protective barrier between the content of the tract's lumen and the internal milieu of the body.

The has a form of general histology with some differences that reflect the specialization in functional anatomy. It is a hollow tube with a lumen of variable diameter and a wall made up of four main layers:

1- Mucosa

2- Submucosa

3- Muscularis externa (the external muscular layer)

4- Adventitia or serosa

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Mucosa The mucosa is the innermost layer of the gastrointestinal tract that is surrounding the lumen, or open space within the tube. This layer comes in direct contact with digested food . The mucosa is made up of three layers:

A- Epithelium - innermost layer. Responsible for most digestive, absorptive and secretory processes.

B- Lamina propria - a thin layer of connective tissue .

C- Muscularis mucosae - is a thin layer of smooth muscle that supports the mucosa and provides it with the ability to move and fold. The mucosae are highly specialized in each organ of the gastrointestinal tract to deal with the different conditions. The most variation is seen in the epithelium. In the oesophagus, the epithelium is stratified, squamous and non-keratinising, for protective purposes. In the stomach it is simple columnar, and is organised into gastric pits and glands to deal with secretion.

The gastro-oesophageal junction is extremely abrupt.

The small intestine epithelium

(particularly the ileum) is specialized for

absorption; it is organized into plicae

circulares and villi, and the enterocytes

have microvilli. This creates a brush border which greatly increases the surface area for absoption. The epithelium is simple columnar with microvilli.

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In the ileum there are occasionally Peyer's patches in the lamina propria. The colon has simple columnar epithelium with no villi. There are goblet cells The appendix has a mucosa resembling the colon but is heavily infiltrated with lymphocytes.

The ano-rectal junction is again very abrupt; there is a transition from simple columnar to stratified squamous non-keratinising epithelium (as in the oesophagus) for protective purposes.

Submucosa consists of a dense irregular layer of connective tissue that contains arteries, veins, lymphatic and nerves

Muscularis externa

The muscularis externa consists of an inner circular layer and a longitudinal outer muscular layer. The circular muscle layer prevents food from traveling backward and the longitudinal layer shortens the tract. These two layers move perpendicularly to one another and form the basis of peristalsis.

Between the two muscle layers are the myenteric or Auerbach's plexus. This controls peristalsis. The gut has intrinsic peristaltic activity (basal electrical rhythm) due to its self-contained enteric nervous system. thickness of muscularis externa varies in each part of the tract. In the colon, for example, the muscularis externa is much thicker because the faeces are large and heavy, and require more force to push along. The outer longitudinal layer of the colon thins out into 3 discontinuous longitudinal bands, known as tiniae coli (bands of the colon). This is one of the 3 features helping to distinguish between the large and small intestine.

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The pylorus of the stomach has a thickened portion of the inner circular layer: the pyloric sphincter. Alone among the GI tract, the stomach has a third layer of muscularis externa. This is the inner oblique layer, and helps churn the chyme in the stomach

3 Adventitia/serosa

The outermost layer of the GI tract consists of layer of simple squamous epithelium, called the mesothelium and small amount of underlying connective tissue .The serosa containing blood vessels, nerves, and fat

Intraperitoneal parts of the GI tract are covered with serosa. These include most of the stomach, first part of the duodenum, all of the small intestine , caecum and appendix , transverse colon,sigmoid colon and rectum.

The adventitia consists of connective tissue. In the portions of the tract within the peritoneal cavity, it is lined by the adventitia , These include the oesophagus, pylorus of the stomach, distal duodenum, ascending colon, descending colon and anal canal. In addition, the oral cavity has adventitia.

Oral Cavity

Food enters the digestive tract in the oral cavity, where it is masticated into particles on which digestive enzymes can act more efficiently. In the mouth, food particles are mixed with saliva, which lubricates them and initiates their digestion.

The of the face is a keratinized stratified squamous epithelium with hair follicles, while red margin of the lacks hair follicles or glandular tissue.

The oral cavity is lined with stratified squamous epithelium, keratinized or nonkeratinized, depending on the region with the labial minor salivary glands present beneath the epithelium.. The layer protects the from damage and is best developed on the gingiva (gum) and hard . The lamina propria in these regions has many papillae and rests directly on bony tissue. Nonkeratinized squamous epithelium covers the , lips, , and the floor of the mouth. The lamina propria has papillae similar to those in the skin and is continuous with a submucosa containing diffuse small salivary glands. The soft palate also has a core of skeletal muscle and lymphoid nodules.

4 Lips

When we think of lips we usually only think of a small part, the vermilion border (or prolabium), of the "anatomical" lips, which comprise the entire fleshy fold surrounding the oral orifice. The outside and inside of the lips are lined by skin and oral mucosa respectively. Between the two, we find labial vessels, nerves, the orbicularis oris muscle (striated), which shapes the lips, and labial salivary glands.

The vermilion border is the area of transition from the oral nonkeratinized epithelium to the keratinized epithelium of the skin. The epithelium is somewhat thicker than in other parts of the facial skin. Connective tissue papilla extend deep into the epithelium and are heavily vascularized. It is the proximity of these vessels to the surface of the epithelium which gives the prolabium it's red appearance.

Tongue

The tongue is a mass of striated muscle covered by a mucous membrane whose structure varies according to the region. The muscle fibers cross one another in three planes and are grouped in bundles separated by connective tissue. Because the connective tissue of the lamina propria penetrates the spaces between the muscular bundles, the mucous membrane is strongly adherent to the muscle. The mucous membrane is smooth on the lower surface of the tongue. The tongue's dorsal surface is irregular, covered anteriorly by a great number of small eminences called papillae. The posterior third of the tongue's dorsal surface is separated from the anterior two thirds by a V-shaped groove, the terminal sulcus. is divided into an oral part, the anterior two-thirds, and a pharyngeal part, the posterior one-third Behind this boundary is the root of the tongue, whose surface shows the many bulges of the and smaller collections of lymphoid nodules

5 The dorsal surface of the oral part has a characteristic appearance due to the presence of a large number of small projections, the lingual papillae. The epithelium of the pharyngeal part forms a somewhat irregular surface which covers the lingual tonsils.

The lingual papillae consist of a connective tissue core covered with a stratified squamous epithelium. On the basis of their appearance four types of papillae can be distinguished

1- Filiform papillae

Have an elongated conical shape, and are heavily keratinized, which gives their surface a gray or whitish appearance. Their epithelium lacks taste buds and their role is mechanical in providing a rough surface that facilitates food movement during chewing.

2- Fungiform papillae Are less numerous, lightly keratinized, and mushroom-shaped with connective tissue cores and scattered taste buds on their upper surfaces. They are irregularly interspersed among the filiform papillae

3- Circumvallate papillae are the largest and less numerous, lightly keratinized, and mushroom-shaped with connective tissue cores and scattered taste buds on their upper surfaces. They are irregularly interspersed among the filiform papillae

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4- Foliate papilla are not well developed in humans and may be absent in aged individuals. If present, they form lamellae along the posterior and lateral border of the tongue.

The muscles of the tongue (skeletal muscle) are organized into strands oriented more or less perpendicular to each other. Their actions provide the tongue with the necessary motility to participate in the formation of speech and to aid in the initial processing of foods.

Taste Buds

Taste buds are most numerous in the

fungiform, circumvallate and foliate papillae. In histological sections they appear as ovoid lightly stained bodies, which extend

perpendicular from the basement membrane to

a little opening formed in the epithelium, the taste pore. The elongated cells that form the can functionally be divided into three groups: sensory cells, supporting (or sustentacular) cells, and basal cells. different

7 Sensory cells extend microvilli into the taste pore. These microvilli contain the receptors for the basic taste modalities (sweet, salty, bitter and acid). Basal cells regenerate the two other cell types.

Pharynx

The , a transitional space between the oral cavity and the respiratory and digestive systems, forms an area of communication between the nasal region and the larynx .The pharynx is lined by stratified nonkeratinized squamous epithelium in the region continuous with the esophagus and by ciliated pseudostratified columnar epithelium containing goblet cells in the regions close to the nasal cavity.The pharynx contains tonsils and the mucosa also has many small mucous salivary glands in its lamina propria. The constrictor and longitudinal muscles of the pharynx are located outside this layer.

Teeth

In the adult human there are normally 32 permanent teeth, arranged in two bilaterally symmetric arches in the maxillary and mandibular bones. Each quadrant has eight teeth: two incisors, one canine, two premolars, and three permanent molars. Twenty of the permanent teeth are preceded by deciduous (baby) teeth which are shed; the others are permanent molars with no deciduous precursors. Each tooth has a crown exposed above the gingiva, a constricted neck at the gum, and one or more roots below the gingiva that hold the teeth in bony sockets called alveoli, one for each tooth

Salivary Glands

Saliva is a mixed secretion, which is derived from numerous large and small salivary glands that all open into the oral cavity. Small salivary glands are situated in the connective tissue beneath the epithelia lining the oral cavity, and, in the case of the tongue, they may also be found between the muscular tissue. Depending on the localisation they are grouped into lingual, labial, buccal, molar and palatine glands.

The large salivary glands form three paired groups:

1. the sublingual glands, which are positioned beneath the tongue and embedded deeply in the connective tissue of the oral cavity, 2. the submandibular glands and 3. the parotid glands, which lie outside the oral cavity.

8 All of these glands are tubuloacinar glands, i.e. they have secretory acini but the first part of the duct system originating from the acini also participates in the secretory process. The salivary glands are divided by connective tissue septa into lobes, which are further subdivided into lobules.

Functionally the secretory acini can be divided into two groups: those that secrete a rather liquid product - serous acini, and those that secrete a very viscous product - mucous acini. This functional differentiation is reflected in the appearance of these acini in histological sections.

 The cells forming the serous acini contain a round or slightly ovoid nucleus which is placed basally in the cell.the apical cytoplasm may appear pinkish/red. The granules represent the vesicles which contain the secretory products of the cell. The digestive enzyme α-amylase is also secreted by the acinar cells.  The cells forming the mucous acini usually contain The cells forming the mucous acini usually contain flattened nuclei which appear to be "pressed" against the basal surface of the cell. Secretory vesicles fill much of the apical cytoplasm

Glands located close to the oral cavity have mainly mucous secretions, whereas glands located further away from the oral cavity have mainly serous secretions. Following this general rule, the parotid glands contain almost exclusively serous acini, the submandibular glands contain both serous and mucous acini, and the sublingual glands contain mainly mucous acini or mucous acini with serous demilunes.

9 Esophagus

The part of the gastrointestinal tract called the esophagus is a muscular tube whose function is to transport food from the mouth to the stomach. It is lined by nonkeratinized stratified squamous epithelium with stem cells scattered throughout the basal layer. In general, the esophagus has the same major layers as the rest of the digestive tract. In the submucosa are groups of small mucus-secreting glands, the esophageal glands, secretions of which facilitate the transport of foodstuffs and protect the mucosa. In the lamina propria of the region near the stomach are groups of glands, the esophageal cardiac glands, which also secrete mucus, which resemble the glands in the adjacent mucosa of the stomach.

Swallowing begins with controllable motion, but finishes with involuntary peristalsis. In the proximal third of the esophagus the muscularis is exclusively skeletal muscle like that of the tongue. The middle third contains a combination of skeletal and smooth muscle fibers (Figure 15–14) and in the distal third the muscularis contains only smooth muscle. Also, only the most distal portion of the esophagus, in the peritoneal cavity, is covered by serosa. The rest is enclosed by a layer of loose connective tissue, the adventitia, which blends into the surrounding tissue.

The adventitia consists only of a layer of loose connective tissue. Only the lowest part of the oesophagus (approx. the lowest 2 cm) enters the peritoneal cavity. A serosa forms the outermost part of the adventitia of this short intraperitoneal segment of the oesophagus.

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