The Digestive System

• 1. Gastrointestinal Tract(oral cavity till anal orifice).

• 2. Assosciated Glands.

• ORAL CAVITY

• Within the oral cavity are the tongue,teeth and the salivary glands ,which empty their secretory products into the oral cavity.

• The epithelial lining is divided into two types:

• 1.Masticatory epithelium covers (tongue, gingivae and hard palate). The epithelium is keratinized stratified squamous epithelium.

• 2.Lining epithelium, i.e. non-keratinised stratified squamous epithelium, covers the remaining surfaces of the oral cavity.

• The tongue is a muscular organ covered by mucosa, which consists of epithelium and lamina propria containing blood vessels, nerves, lymphatic nodules, serous and mucus secreting glands. These glands are also found within the connective tissue separating the muscle bundles.

• The muscles of the tongue are skeletal muscle are organized into strands oriented in different directions and perpendicular to each other.

• The dorsal surface of the tongue is divided into anterior two-thirds containing small projections, the lingual papillae, and the posterior one-third containing the lingual tonsils.

• The lingual papillae consist of a connective tissue core covered with a stratified squamous epithelium. Four types of papillae can be distinguished - filiform, fungiform,circumvallate and foliate papillae.Filiform papillae are the smallest & most numerous papillae(conical). Fungiform papillae occur singly.

• Circumvallate papillae are the largest and least numerous papillae - in humans there are between 8 and 12 of them. Taste buds are particularly numerous on the lateral surfaces of these papillae. The excretory ducts of serous glands of von Ebner open into the papillae.

• Foliate papillae not well developed in humans .

• The epithelium of the dorsal surface of the tongue rests on a dense layer of connective tissue, which connects the epithelium firmly with the underlying muscular and connective tissues.

• Taste buds are most numerous in the fungiform, circumvallate and foliate papillae. In addition, taste buds are found in the palate, in the pharynx and larynx.

• 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. Sensory cells extend microvilli into the taste pore. These microvilli contain the receptors for the different basic taste modalities.Basal cells regenerate the two other cell types.

• Salivary Glands

• 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.

• The large salivary glands form three paired groups:

• 1.the sublingual glands 2.the submandibular glands and 3.the parotid glands, which lie outside the oral cavity.

• All of these glands are acinar glands, i.e. they have secretory acini

• The salivary glands are divided by connective tissue septa into lobes, which are further subdivided into lobules.

• The secretory acini can be divided into two groups: serous acini, and mucous acini.

• Serous cells and mucous cells may form compound or mixed acini.

• The serous cells form half-moon or crescent-shaped structures, which attach to mucus producing acini and empty their secretory product between the mucus-producing cell. From their appearance they are called serous demilunes.

• Both serous and mucous acini and parts of the secretory duct system are surrounded by myoepithelial cells which by their contraction participate in the secretory process.

• The parotid glands contain almost exclusively serous acini,

• the submandibular glands contain both serous and mucous acini with serous demilunes, and the sublingual glands contain mainly mucous acini or mucous acini with serous demilunes.

• Ducts of the Salivary Glands

• The ducts of the salivary glands can, according to their position in relation to the lobes and lobules of the glands, be divided into two parts. Interlobular and interlobar ducts are embedded in the connective tissue surrounding the lobes and lobules of the glands

• Intralobular ducts are located in between the secretory acini within the lobules. Intralobular ducts can on the basis of their function be divided into intercalated ducts and striated ducts. The secretory acini empty into intercalated ducts which merge into the striated ducts.Cells forming the intercalated ducts are formed by cuboidal epithelium.

• Striated ducts are formed by columnar cells.The striations of the striated duct are found in the basal part of the cytoplasm of the cells where numerous mitochondria are found between infoldings of the basal cell membrane.

• Interlobar and interlobular ducts are formed by a stratified cuboidal or stratified columnar epithelium. The epithelium is replaced by the stratified squamous epithelium as they approach the opening into the oral cavity.

• The Pancreas

• The capsule, is formed by a thin layer of loose connective tissue. It is both an exocrine and endocrine gland. The endocrine part, which accounts for ~1% of the pancreas, consists of the cells of the islands of Langerhans

• Components of the exocrine pancreas: The exocrine pancreas consists of acinar glands. A single layer of pyramidal shaped cells forms the secretory acini. The first portion of the duct system extends into the centre of the acini, which is lined by small centroacinar cells. These cells form the first part of intercalated ducts.

• Intercalated ducts are lined by low columnar or cuboidal epithelium. They empty into interlobular ducts, which are lined by a columnar epithelium. Interlobular ducts in turn empty into the main pancreatic duct (of Wirsung), which is lined by a tall columnar epithelium.

• Teeth:

• A tooth consists of a crown, covered by very hard translucent enamel and a root covered by yellowish bone-like cementum. These meet at the neck or cervical margin. The dentine surrounds a central pulp cavity.

• The pulp is a connective tissue, continuous with the peridontal ligament via the apical foramen. It contains vessels for the support of the dentine and sensory nerves

• The root is surrounded by alveolar bone, its cementum separated from the osseous socket (alveolus) by the connective tissue of the periodontal ligament . Near the cervical margin, the tooth, periodontal ligament and adjacent bone are covered by the gingiva.

• Enamel

• Enamel is an extremely hard and rigid material which covers the crowns of teeth. It is a heavily mineralized cell secretion, containing 95-96% by weight crystalline apatites and less than 1% organic matrix

• The organic matrix comprises mainly unique enamel proteins, amelogenins and non-amelogenins such as enamelins, tuftelins. As its formative cells (ameloblasts) are lost from the surface during tooth eruption, enamel is incapable of further growth.

• Dentine:

• Dentine is a yellowish avascular tissue which forms the bulk of a tooth is formed by odontoblast. It is a tough and compliant composite material, with a mineral content of 70% dry weight (largely crystalline hydroxyapatite with some calcium carbonate and 20% organic matrix (type I collagen, GAGs & phosphoproteins).

• Its clear feature is the regular pattern of microscopic dentinal tubules, which extend from the pulpal surface to the enamel-dentine junction. Each tubule encloses a single cytoplasmic process of an odontoblast whose cell body lies in the layer which lines the pulpal surface.

• Dentine is formed slowly throughout life, and so there is always an unmineralized zone of predentine at the surface of the mineralized dentine, adjacent to the odontoblast layer at the periphery of the pulp.

• Dental pulp

• Dental pulp provides the nutritive support for the synthetic activity of the odontoblast layer. It is a well-vascularized, loose connective tissue, enclosed by dentine and continuous with the periodontal ligament via apical and accessory foramina.

• Several thin-walled arterioles enter by the apical foramen and run longitudinally within the pulp.

• As well as typical c.t cells, pulp uniquely contains the cell bodies of odontoblasts whose long processes occupy the dentinal tubules and dendritic antigen-presenting cells. Approximately 60% of collagen is type I, and the bulk of the remainder is type III.

Cementum:

• Cementum which is formed by cementocytes, is a bone-like tissue which covers the dental roots, and is 50% by weight mineralized (mainly hydroxyapatite crystals). However,unlike bone,cementum is avascular and lacks nerves.

• Like bone, cementum is perforated by Sharpey's fibres, which represent the attachment bundles of collagen fibres in the periodontal ligament. Cementum is not remodelled but small areas of resorption with evidence of repair may be seen.

Periodontal ligament:

• The principal functions of the periodontal ligament are to support the teeth, generate the force of tooth eruption and provide sensory information about tooth position and forces to facilitate reflex jaw activity.

• The periodontal ligament is a dense fibrous connective tissue which contains cells associated with the development and maintenance of alveolar bone (osteoblasts and osteoclasts) and of cementum (cementoblasts and odontoclasts).

• The majority of collagen fibres of the periodontal ligament are arranged as variously oriented dense fibre bundles that connect alveolar bone and cementum and which may help to resist movement in specific directions.

• About 80% of the collagen in the periodontal ligament is type I, most of the remainder is type III. The rate of turnover of collagen is probably the highest of any site in the body.

• The periodontal ligament has a rich nerve and blood supply. The nerves are both autonomic and sensory (for pain and proprioception). The majority of proprioceptive nerve endings appear to be Ruffini-like endings

Alveolar bone:

• That part of the maxilla or mandible which supports and protects the teeth is known as alveolar bone. Like bone in other sites, alveolar bone functions as a mineralized supporting tissue, gives attachment to muscles,provides a framework for b. marrow and acts as a reservoir for ions, especially calcium.

• It is dependent on the presence of teeth for its development and maintenance, and requires functional stimuli to maintain bone mass. Where teeth are congenitally absent, as for example in anodontia, it is poorly developed, and it atrophies after tooth extraction.