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Digestive Tract

Introduction

The digestive system consists of the -oral cavity( lips and tongue) - 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 gastrointestinal tract 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

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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 skin of the face is a keratinized stratified squamous epithelium with hair follicles, while red margin

of the lip 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 keratin layer

protects the oral mucosa from damage and is best developed on the gingiva (gum) and hard palate. The

lamina propria in these regions has many papillae and rests directly on bony tissue. Nonkeratinized

squamous epithelium covers the soft palate, lips, cheeks, 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.

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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 lingual tonsils and

smaller collections of lymphoid nodules

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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 taste bud can functionally be divided into

three groups: sensory cells, supporting (or

sustentacular) cells, and basal cells. different

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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 pharynx, 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.

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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.

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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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