Histology of Digestive System-I Handout PDF

HISTOLOGY OF DIGESTIVE SYSTEM-I  THE DIGESTIVE SYSTEM Histology of digestive system comprises I. Oral cavity and its contents (Oral histology) II.Alimentary tract/Digestive tract or Gastro- intestinal tract (GIT): a single tube which extends from oral cavity to anus...

HISTOLOGY OF DIGESTIVE SYSTEM-I  THE DIGESTIVE SYSTEM Histology of digestive system comprises I. Oral cavity and its contents (Oral histology) II.Alimentary tract/Digestive tract or Gastro- intestinal tract (GIT): a single tube which extends from oral cavity to anus including pharynx, esophagus, stomach, small intestine and large intestine. III.Accessory/extrinsic glands of digestive system: these include major salivary glands (parotid, submandibular & sublingual glands) Liver Gall bladder and pancreas  Functions of Digestive System 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 Oral/Mouth Cavity Extends: from the lips and cheeks (externally) to pillars of the fauces (internally), where it continues posteriorly into the oropharynx through oropharyngeal isthmus. It has anterior and side walls, roof, and floor. Subdivided into the oral cavity proper and the vestibule, which communicate posteriorly behind the last molar (wisdom) tooth when the mouth is closed. The oral vestibule: a slit like space between lips and cheeks externally and gums and teeth internally). receives the opening of parotid duct opposite to the second upper molar tooth. Mouth cavity Proper extends from inside of the teeth/ gingivae to oropharyngeal isthmus, which leads to Oropharynx.  The Oral/Mouth Cavity Functions: 1. Ingestion of food, mastication & swallowing 2. Speech and ventilation (breathing) Contents of oral cavity: 1.Tongue 2.Teeth 3. Minor/accessory salivary glands  Boundaries of the Oral Anteriorly: Lips Cavity Sidewalls: formed by Cheeks externally and by gums and teeth internally. Roof (superior boundary): formed by the hard palate anteriorly and soft palate posteriorly. Floor (inferior boundary): formed mainly by myelohyoid muscle covered by oral mucosa -occupied mainly by the tongue, lined with thin epithelium- str, sq, non-keratinized type NB. Oral cavity is lined with stratified squamous epithelium, keratinized or nonkeratinized, depending on the region. Boundaries of the Oral Cavity  LIPS Fleshy fold of tissues forming the anterior boundary of oral cavity. Normally (Competent lips), the upper and lower lips are lightly closed at rest forming the oral seal. Each lip is attached to its respective gingiva by a fold of mucosa known as frenulum on its internal surface: – The upper lip is attached to the gingiva of the upper jaw by frenulum of upper lip – The lower lip is attached to the gingiva of the lower jaw by frenulum of the lower lip Each lip has a central bulk of skeletal muscle (orbicularis oris) covered by mucous membrane on inner side, & by hairy skin on external/outer side.  Competency of Lips Competent lips - lips lightly closed at rest forming the oral seal Incompetent lips - a lip seal is not produced at rest, i.e. the lips are merely held apart habitually. Potentially competent lips - lips that could be closed at rest in the absence of interference caused by the protruding incisors LIPS-EXTERNAL surface Cutaneous portion Red Border Red Border Cutaneous portion lips-Internal surface (Mucous portion) Frenulum of Lower lip Mucous portion  Histology of the Lips 1. Cutaneous Portion (External surface of lip) Consists of thin hairy skin – keratinized stratified squamous epithelium (epidermis), typical of thin skin – the dermis consists of hair follicles, sebaceous and sweat glands. – passes through a transition zone to merge with oral mucosa of the inner surface. – irregular in outline due to hair follicles and ducts of sweat glands which open onto the surface. 2. Red (Vermilion) Border or Transition Zone Constitutes the free, red portion of the lip, vermilion refers to a brilliant red or scarlet color of this border Consists of thin skin where epidermis is very thin, lightly keratinized (with translucent prekeratin only) stratified squamous epithelium, and transparent to blood in the rich microvasculature of the underlying connective tissue dermis from which it derives its color.  Vermilion border is devoid of hair follicles, sebaceous and sweat glands, as a result it is prone to excessive dryness and chapping in cold, dry weather.  As a result requires continuous moistening from saliva to prevent its dehydration and cracking.  Its reddish color in the living state is from numerous blood vessels in the connective tissue (lamina propria).  it is highly sensitive especially to touch due to its rich sensory innervations. 3. Mucous Portion (Internal surface) This is oral mucous membrane portion Has smooth outline when compared to the outer cutaneous portion The mucosa consists of Thick epithelium: non-keratinized stratified squamous Compact Lamina propria: houses minor salivary glands of mucous type known as labial glands. LIP TRANSVERSE SECTION (Cutaneous Portion) (Mucous Portion) Histology of Lip cont’d… A: Skin (cutaneous portion) B: Vermilion zone C: Oral (labial) mucosa D: Minor salivary glands (labial glands) LIP SAGITTAL SECTION Histology of Lip vertical section G= Salivary glands OM= Oral mucosa M= Orbicularis oris muscle F= Hair follicle S= Skin (Cutaneous part) V= Vermilion border  General Histological Characteristics of The oral mucosa Oral Mucosa consists of two layers: Surface epithelium and Lamina propria The epithelia form ridges that protrude towards lamina propria known as epithelial ridges. The papillary layer of the lamina propria protrudes towards the oral epithelium (in between the ridges) and carries blood vessels and nerves. The epithelium is separated from the connective tissue by means of a basement membrane (*basal lamina + reticular lamina). The oral mucosa is attached to the underlying structures by a layer of tissue, the submucosa consisting of connective tissue of varying thickness, glands, blood vessels, nerves lymphatic vessels and adipose tissue.  THE CHEEKS Forms the external side wall of oral cavity Extend intraorally from the labial commissures anteriorly to the ridge of mucosa overlying the ascending ramus of the mandible posteriorly Bounded superiorly by the upper and lower vestibular sulci Consists of a central bulk of skeletal muscle (mainly buccinator) covered by mucosa on both surfaces. 1.Outer/External surface (cutaneous portion) – Covered by thin hairy skin (epidermis= keratinized stratified squamous epithelium, and dermis with sweat glands, sebaceous glands and hair follicles) – Under skin is buccal pad of fat 2.Inner/Oral surface (mucous portion) – Lined by oral mucosa  The Oral mucosa of Cheeks The Epithelium: thick non keratinized stratified squamous Lamina propria: – similar to the oral mucosa that lines the inner part of the lips, but with abundant elastic fibers, which permit over stretching of this tissue during blowing and full mouth chewing. – It is also smoking tissue – Houses accessory (minor) salivary gland of mucous, serous and mixed types called Buccal glands. Ectopic sebaceous glands may be evident in the mucosa as yellowish patches called Fordyce’s spots The parotid duct drains into the cheek/vestibule opposite the maxillary second molar tooth and its opening is covered by a small fold of mucosa termed- parotid papilla.  ROOF OF ORAL CAVITY (Hard and Soft Palates palates) form the roof of oral cavity (oral or lower part) and floor of nasal cavity (nasal or upper part). HARD PALATE Constitutes the anterior 2/3rd of palates. Its central bulk is bony and formed by palatine processes of maxillae and horizontal processes of palatine bones. Covered by thin mucosae on both surfaces. Lower surface (Oral part): Lined by thin oral mucosa: – Epithelium: non keratinized stratified squamous epi. – A dense fibrous lamina propria: devoid of glands, near the midline and adherent to periosteum of the bones.  The lamina propria reveals transverse ridges to assist the process of mastication. As a result the mucosa is thrown into several transverse folds known as palatine rugae or papillae. These are equivalent to the dermal papillae of thick skin which forms the finger and foot prints. Except in the median palatal raphae & near the gum, the lamina propria contains: – Fat: in its anterior third (i.e. in its anterolateral aspect) – Few accessory salivary glands of mucous type: in its posterolateral part continuous with soft palate only. Upper surface (Nasal part): – Lined by respiratory mucosa formed by pseudostratified columnar epithelium and very thin lamina propria.  SOFT PALATE Constitutes the posterior 1/3rd of palates Its central bulk consists of a thick bundle of striated muscles Covered by a thick mucosa on both surfaces Lower surface (Oral part): Lined by an oral mucosa formed by: 1.Epithelium: thick non keratinized stratified squamous 2.Lamina propria: Infiltrated with lymphocytes Consists of accessory minor salivary glands of mucous type known as palatal glands and a few taste buds. Upper surface (Nasal part): PALATES Surface features - palatine rugae - papillae - free & attached gingivae - stippling only on the attached gingivae - NB: finger prints vs palatine rugae  TONGUE is the major content of oral cavity used for speech, manipulation of food and for sensory perception. is a pyramidal muscular organ with a wide base (root) & a narrow apex (tip),upper (dorsal) and lower (ventral) surfaces placed horizontally in the oral cavity in anterior- posterior axis, its apex directed anteriorly. is a mass of striated muscle (forming its central bulk) covered by a mucous membrane (oral mucosa) whose structure varies according to the region. its mucous membrane is specialized for manipulation of food, general sensory reception and special visceral sensory function of taste. The mucous membrane is smooth on the lower surface, while the dorsal surface is irregular & rough.  THE TONGUE  Dorsum of Tongue The tongue's dorsal surface is divided into anterior two third & posterior third by a V-shaped groove, the terminal sulcus (Sulcus terminalis). The vertex (apex) of this sulcus is directed posteriorly and ends in a shallow pit, the foramen cecum. Foramen cecum marks the embryonic origin of thyroid gland from the floor of mouth. Irregular and roughened by a great number of small surface projections or eminences. Anterior two-third of dorsum of tongue: – lies in front of sulcus terminalis – constitutes the body and apex or tip of the tongue – roughened by mucosal projections called lingual papillae of four types. Tongue (Dorsum) NORMAL TONGUE, TRANSVERSE SECTION Posterior one-third of dorsum of tongue: – lies behind the sulcus terminalis – constitutes the root or base of the tongue – roughened by masses of lymphoid nodules separated by shallow grooves , tonsilar crypts, all of which comprise the lingual tonsils. – have deferent embryonic origin Lingual papillae are elevations of the mucous membrane of the dorsum of the anterior two-third of the tongue they assume various forms and functions all contain cores of connective tissue (lamina propria) covered by stratified squamous epithelium (keratinized in filiform, in the others all, non- keratinized)  Four types are recognized: filiform, fungiform,foliate & circumvallate  1. Filiform papillae quite numerous and makes up majority of the papillae present over the entire anterior surface of the tongue short (2.5-3mm high), narrow, conical, bristle-like highly keratinized projections thus resistant to abrasion they facilitate mastication they erect & directed towards the throat and assist in movement of food towards that direction whitish appearance devoid of taste buds  2. Fungiform papillae wider (1 mm in diameter) than filiform, but only about 1.9 mm high & interspersed among them resemble mushrooms in that they have a narrow stalk & a smooth surfaced, dome- shaped upper dilated part the lamina propria rich in blood vessels & imparts translucent, reddish color covered by a thin non- keratinized stratified squamous epithelium. more numerous near the tip of the tongue contain one or more transversely placed taste buds FUNGIFORM PAPILLA Non-keratinized, stratified squamous epithelium Core of lamina propria  3. Foliate papillae leaf-like, appear as closely Foliate Papillae packed folds bilaterally located along the posterior border of the body of tongue rudimentary in human adult present on the postero- lateral margins of the tongue consist of 4 to 11 parallel ridges that alternate with deep grooves or furrows in the mucosa, which receive the openings of Von Ebner’s glands contain several taste buds FOLIATE PAPILLAE WITH TASTE BUDS Filiform papillae FOLIATE PAPILLAE WITH TASTE BUDS Taste buds  4. Circumvallate papillae The largest (3 mm in diameter) & least common papillae Not more than 12 (7- 12) in number Arranged as a single row in front of sulcus terminalis Each is surrounded by a moat-like deep groove, called trench Core of lamina The trench receives propria the openings of the ducts of Von Ebner's glands (serous salivary glands)  Circumvallate papillae Contain numerous taste buds The taste buds are strategically placed along the trenches so that the substances to be tasted can easily reach & dissolved in the watery secretion of Von Ebner’s glands. This secretion also washes away chemicals that are being tasted & prepares the receptors for new stimuli. Ebner’s glands also secrete a lingual lipase, that probably prevents the formation of hydrophobic layer over the taste pores that could hinder the function of receptor cells. Lingual lipase is also active in the stomach and can digest up to 30% of dietary triglycerides.  Taste Buds Are special visceral sense organs located in epithelium of dorsum of tongue along the trenches and furrows. Few taste buds are also found in soft palate, epiglottis, larynx, and pharynx Onion or barrel-shaped intraepithelial structures whose opening (taste pores) face trenches Also referred to as Neuroepithelial structures  A Taste Bud is about 80 microns in height and 40 microns in width. composed of about 30 to 50 modified epithelial cells, some of which are supporting cells called sustentacular cells and others of which are taste (Gustatory) cells. The taste cells are continually being replaced by mitotic division of surrounding epithelial cells, so that some taste cells are young cells. Others are mature cells that lie toward the center of the bud; these soon break up and dissolve. The life span of each taste cell is about 10 days in lower mammals but is unknown for humans. The outer tips of the taste cells are arranged  Taste Buds Each taste bud has three types of cells: taste cells, supporting & basal cells. 1.Taste (Gustatory) cells receptor cells of taste, and are most numerous cells are light cells whose apex poke numerous microvilli or taste hairs outward into the taste pore to approach the cavity of the mouth these microvilli provide the receptor surface for taste. Receive contact tastants dissolved in saliva Afferents nerve fibers are associated to the base of these cells 2. Supportive cells (appear darker in routine stains) secrete amorphous substance that surrounds microvilli in a taste pore  Taste Buds 3. Basal cells Undifferentiated stem cells Replace all cells of the buds by division Taste Bud  A Taste Bud Interwoven around the bodies of the taste cells is a branching terminal network of taste nerve fibers (unmyelinated) that are stimulated by the taste receptor cells. Some of these fibers invaginate into folds of the taste cell membranes. Many vesicles form beneath the cell membrane near the fibers. These vesicles are believed to contain a neurotransmitter substance that is released through the cell membrane to excite the nerve fiber endings in response to taste stimulation. Location of the Taste Buds. The taste buds are found on three types of papillae of the tongue. Additional taste buds are located on the palate, and a few are found on the tonsillar pillars, on the epiglottis, TASTE BUD  A person can perceive hundreds of different tastes. They are all supposed to be combinations of five primary taste sensations, just as all the colors we can see are combinations of three primary colors (red, green & blue) Taste buds detect at least five broad categories of tastants: metal ions (salty); hydrogen ions from acids (sour); sugars and related organic compounds (sweet); alkaloids and certain toxins (bitter); and certain amino acids such as glutamate (umami; Jap. umami, savory). Umami Taste: is a separate, fifth category of primary taste stimuli. Umami is a Japanese word (meaning "delicious") designating a pleasant taste sensation that is qualitatively different from sour, salty, sweet, or bitter. – Umami is the dominant taste of food containing L- glutamate, such as meat extracts and aging cheese.  Salty and sour tastes are produced by ion channels; the other taste categories are mediated by G-protein- coupled receptors. Receptor binding produces depolarization of the gustatory cells, stimulating the sensory nerve fibers which send information to the brain for processing. Conscious perception of tastes in food requires olfactory and other sensations in addition to taste bud activity. A taste receptor for l-glutamate may be related to one of the glutamate receptors expressed in neuronal synapses of the brain. However, the precise molecular mechanisms responsible for umami taste are still unclear. Specificity of Taste Buds for a Primary Taste Stimulus. Microelectrode studies from single taste buds show that each taste bud usually responds mostly to one of the five primary taste stimuli when the taste substance is in  Muscles of the form the bulkTongue of the tongue all are skeletal (striated) muscles The tongue 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 mucous and serous minor salivary glands (lingual glands) are interspersed between the bundles of muscles Are of two groups: INTINSIC & EXTRINSIC Intrinsic muscles: - they change the shape of the tongue – fibers oriented in three planes: Longitudinal ,Transverse and Vertical Extrinsic muscles: change the position of the tongue  Tongue skeletal muscle in three planes: A, longitudinal fibers in x plane; B, vertical fibers in y plane; C, transverse fibers in z plane direction NORMAL TONGUE, TRANSVERSE SECTION TONGUE, STRATIFIED SQUAMOUS EPITHELIUM AT HIGH MAGNIFICATION NORMAL TONGUE, TRANSVERSE SECTION Von Ebner’s glands  TEETH (DENTES) Introduction In human, there are two sets of teeth (dentition): 1.Primary dentition (temporary; deciduous; milk teeth; baby teeth)- 20 in number 2. Secondary dentition (permanent; adult teeth)-32 in ♯ In adult humans, 32 permanent teeth - Disposed in two bilaterally symmetric arches in the maxillary and mandibular bones, with eight teeth in each quadrant: two incisors, one canine, two premolars, and three permanent molars. Twenty of the permanent teeth are preceded by deciduous predecessors (baby teeth); the remainder (the premolars and the last molar or wisdom teeth) have no deciduous Left lateral view of dentition. Arrows, unerupted permanent teeth.  Types of Teeth and their eruption time NB. The years under brackets indicate time of eruption of permanent teeth  Parts of a Tooth Each tooth consists of the crown, neck and root(s) 1.Crown is exposed portion that projects above the gingiva is covered by extremely hard substance – enamel  In young, anatomical crown is covered by enamel, but in older an individuals most enamel is lost by erosion and the exposed crown is called clinical or functional crown. 2. Root(s)= Radix – is the portion in side the gum below the gingiva that hold the teeth in bony sockets called alveoli, – can be one or more for each tooth. – covered by another bone-like mineralized tissue, the cementum. 3. Neck (cervix of the tooth) - The constricted region between crown & roots - The two coverings (enamel & cementum) meet at neck VS through a tooth  Composition of a Tooth 1.Dental Tissue Proper Dentin Enamel Cementum and Dental pulp- in pulp cavity (pulp chamber and root canal) 2. Supportive Tissues Periodontal ligament Alveolar bone and Gingivae  The main bulk of a tooth is composed of a calcified material, dentin, that surrounds the pulp cavity. The pulp cavity has a coronary portion (the pulp chamber) and a root portion (the root canal), extending to the apex of the root, where an orifice (apical foramen) permits the entrance and exit of blood vessels, lymphatics, and nerves of the pulp cavity. Denti n that is harder than bone A calcified tissue because of its higher content (70% of dry weight) of calcium salts in the form of calcium hydroxyapatite. Its organic matrix contains type I collagen fibers, glycos-aminoglycans, phosphoproteins, phospho-lipids. Its inorganic portion is highly mineralized & consists of  The organic matrix of dentin is secreted by odontoblasts, Odontoblasts are slender, tall polarized cells that line the pulp cavity are bound together by junctional complexes as a layer, with no basal lamina, and produce organic matrix only at the dentinal surface. neural crest origin (head mesenchyme) are polarized, protein secreting cells with oval-shaped basal nucleus (cell body) & apical cytoplasmic secretory granules their cell body face the pulp cavity and hence they are called pulp cells have slender, branched apical extensions that penetrate perpendicularly through the width of the dentin—the odontoblast processes (Tomes' fibers), which run in small canals known as dentinal tubules.  Dentinal Tubules are living canals which carry blood vessels & unmyelinated afferent nerve fibers to the odontoblast processes limited by uncalcified dental sheath of Newman. also houses sensory receptors for pain, cold, warm, PH, trauma, etc all of which are interpreted as pain in CNS. extensively branch near the dentino-enamel junction. As the dentin-predentin layer thickens, Tome’s fibers lengthen and send fine branches into smaller lateral branches of these tubules. The Dentine Matrix The organic matrix produced by odontoblasts is initially unmineralized/uncalcified and is called predentin or dentinoid.  The mineralization of developing dentin begins one day of secretion, when membrane-limited vesicles—matrix vesicles—appear, produced by odontoblasts. The process of mineralization is similar to that occurring in osteoid of developing bones & involves binding of calcium & phosphate ions to collagen fibrils (in the matrix vesicles), resulting in formation of fine crystals of hydroxyapatite that grow and serve as nucleation sites for further mineral deposition on the surrounding collagen fibrils. Odontoblasts remain active in predentin secretion into adult life, gradually reducing the size of the pulp cavity. Odontoblast processes can be silver-stained and shown to branch near the junction of dentin with enamel (E) and along their length closer to their source (c), with the lateral branches occupying smaller canaliculi within dentin. Histology of Tooth  Ultrastructure of dentinal tubule & odontoblast processes Odontoblast process (OP) running in small canals called dentinal tubules. Note its close association with an unmyelinated nerve fiber (N) extending there from fibers in the pulp cavity.  These odontoblast process (OP) connections to the odontoblasts (O) are important for the maintenance of dentin in adult teeth. Cross-section of an odontoblast process (OP) near predentin (P) shows its close association with an unmyelinated nerve fiber (N) extending there from fibers in the pulp cavity (the above slide b). These nerves respond to various stimuli reaching the nerve fibers through the dentinal tubules, such as heat, cold, trauma, and acidic pH, and all are perceived as pain. Therefore, pain is the only sensory modality perceived in teeth. According to the hydrodynamic theory, the different stimuli cause the movement of fluids inside dentinal tubules, which stimulate the nerve fibers located near odontoblast processes.  Dentine is similar in mineral & organic composition to bone, but differs from it in the following ways:  No Haversian system  No odontocytes equivalents of osteocytes  More harder than bone (due to its higher mineralization than bone), that is why dentine persists as a mineralized tissue long after destruction of the odontoblasts and thus tooth remains undecomposable It is therefore possible to maintain teeth whose pulp and odontoblasts have been destroyed by infection (canal treatment). In adult teeth, destruction of the covering enamel by erosion from use or dental caries (tooth decay) usually triggers a reaction in odontoblasts that causes them to resume the synthesis of dentin components  Enamel the hardest component of the human body has highly calcified matrix composed of about 96% mineral (98% in dry state), 1% organic material, and 3% water as the remainder in living state. the inorganic component of enamel is mostly calcium hydroxyapatite crystals. Other ions, such as strontium, magnesium, lead, and fluoride, if present during enamel synthesis, can be incorporated or adsorbed by the crystals. enamel containing fluorapatite is more resistant to acidic dissolution caused by microorganisms, hence the addition of fluoride to toothpaste and water supplies.  Enamel The organic enamel matrix is composed not of collagen fibrils but of at least two heterogeneous classes of glycoproteins called amelogenins and enamelins. The organic enamel matrix is secreted by cells of ectodermal origin called ameloblasts Ameloblasts ectodermal in origin, whereas most of the other structures of teeth derive from mesodermal or neural crest cells Their cell body found at periphery  Amelobla sts polarized cells with numerous are tall columnar, mitochondria, well-developed RER & Golgi apparatus Each ameloblast has an apical extension, known as a Tomes' process. Tomes' process contains numerous secretory granules that contain the proteins that make up the enamel matrix. After finishing the synthesis of enamel, ameloblasts form a protective epithelium that covers the crown until the eruption of the tooth. This protective function is very important in preventing several enamel defects. After tooth eruption, they degenerate.  Enamel consists of elongated rods or columns— enamel rods (prisms)—that are bound together by interrod enamel. Enamel rods are externally covered by enamel sheath Both interrod enamel and enamel rods are formed of hydroxyapatite crystals; they differ only in the orientation of the crystals. Each rod extends through the entire thickness of the enamel layer and has a sinuous track. The arrangement of rods in groups is very important for enamel's mechanical properties. MEDICAL APPLICATION The susceptibility of enamel crystals to dissolution in acidic pH is the basis for dental caries, and some of these crystals (e.g.  Ameloblasts and enamel Ameloblasts (A) are tall polarized cells whose apical ends initially contact dentin (D). Ameloblasts are joined to form a cell layer surrounded basally by connective tissue (CT). As odontoblasts secrete predentin, ameloblasts secrete a matrix lacking collagens, but rich in a few glycoproteins which quickly initiate calcium hydroxyapatite formation to make enamel (E), the hardest material in the body. Enamel forms a layer, but consists of enamel rods or prisms, solidly fused together by more enamel. No cellular processes occur in enamel and the layer of ameloblasts surrounding the developing crown is completely lost during tooth eruption. Teeth that have been decalcified for histological sectioning typically lose their enamel layer completely.  Dental Pulp and Pulp Cavity Pulp Cavity a soft connective tissue-filled space surrounded by dentin. richly vascularized & contains afferent nerve fibers. All sensations from the pulp are interpreted as pain in CNS. its periphery contains the organized odontoblasts has two portions: 1. Pulp chamber- the upper, expanded coronary portion 2. Root canal (root portion)- a narrow portion which lies in the root(s), extending to the apex of the root, which ends at apical foramen.  Dental (Tooth) Pulp is loose gelatinous mucoid, embryonic CT, contained within the pulp chamber and root canals. its main components are: - odontoblasts - fibroblasts - mesenchymal cells, all of neural crest origin - reticulin & thin collagen fibrils, and a ground substance that contains glycosaminoglycans - blood vessels, lymphatics & nerve fibers Pulp is a highly innervated and vascularized tissue. Blood vessels and myelinated nerve fibers enter the apical foramen and divide into numerous branches. Some nerve fibers lose their myelin sheaths and extend for a short distance into the dentinal tubules. These pulp fibers are sensitive to pain only. Micrograph of Dental Pulp O-Odontoblasts D-Dentine V-Vessels The periphery of the dental pulp contains the organized odontoblasts (O) contacting the surrounding dentin (D). Centrally the pulp consists of delicate, highly cellular connective tissue resembling undifferentiated mesenchyme but with many thin-walled venules (V) and capillaries. Pulp has reticulin fibers and other fine collagen fibers, with much ground substance. Nerves fibers are also present. The blood and nerve supplies enter the pulp cavity via the apical foramen at the apex of the roots.  Periodontium (around tooth)responsible for maintaining comprises the structures the teeth in the maxillary and mandibular bones. consists of the cementum, periodontal ligament, alveolar bone, and gingiva. Cementum is a calcified tissue covering dentine of the root forms a thin layer of bone-like material secreted by large, elongated cells called cementoblasts. is similar in composition and calcification to bone, although Haversian systems (osteons) and blood vessels are absent. It is thicker in the apical region of the root, where there are cementocytes, cells with the appearance of osteocytes.  Like osteocytes, they are encased in lacunae; unlike those cells, however, cementocytes do not communicate through canaliculi, and their nourishment comes from the periodontal ligament. Like bone tissue, cementum is labile and reacts to the stresses to which it is subjected by resorbing old tissue or producing new tissue. Continuous production of cementum in the apex compensates for the physiological wear of the teeth and maintains close contact between the roots of the teeth and their sockets. Compared with bone, the cementum has lower metabolic activity because it is not irrigated by blood vessels. This feature allows the movement of teeth by orthodontic appliances without significant root resorption.  micrographs of Periodontium cementum (C), periodontal ligament (PL), alveolar bone (B), and gingiva FG = free gingiva, v = vessels, D = Dentine, P = periosteum covering the bone, LP = lamina propria of the gingiva, SF = Sharpey’s fibers SF PL SF  Periodontal Ligament (membrane) a dense fibrous connective tissue (about 150 to 350 µm) with bundles of collagen fibers inserted into the cementum and alveolar bone, fixing the tooth firmly in its bony socket (alveolus). permits limited movement of the tooth within the socket and the fibers are organized to support the pressures exerted during mastication. this avoids transmission of pressure directly to the bone which would cause localized bone resorption. unlike typical ligaments, it is highly cellular and has a rich supply of blood vessels and nerves, giving the periodontal ligament supportive, protective, sensory, and nutritive functions.  Collagen of the periodontal ligament has an unusually high turnover rate (as demonstrated by autoradiography) and a high content of soluble collagens, with the space between its fibers filled with glycosaminoglycans (GAGs). MEDICAL APPLICATION The high rate of collagen renewal in the periodontal ligament allows processes affecting protein or collagen synthesis, e.g., protein or vitamin C deficiency (scurvy), to cause atrophy of this ligament. As a consequence, teeth become loose in their sockets; in extreme cases they fall out. This relative plasticity of the periodontal ligament is important because it allows orthodontic intervention, which can  Alveolar Bone (alveolar processes of the maxillae and mandible) The alveolar bone is in immediate contact with the periodontal ligament, which serves as its periosteum. It is an immature type of bone (primary bone) in which the collagen fibers are not arranged in the typical lamellar pattern of adult bone. Many of the collagen fiber bundles of the periodontal ligament penetrate this bone and bind it to the cementum. The bone closest to the roots of the teeth forms the tooth sockets. Vessels (perforating vessels) run through the alveolar bone and penetrate the periodontal ligament along the root. Some vessels and nerves from alveolar bone run to the apical foramen of the root to enter the pulp  Alveolar ManyBone (B) fibers of the periodontal of the collagen ligament (L) are arranged in bundles that penetrate this bone and the cementum (C), forming a connecting bridge between the two structures known as Sharpey's fibers (SF). SF SF  THE GINGIVA a mucous membrane firmly bound to the periosteum of the maxillary and mandibular bones. composed of Keratinized stratified squamous epithelium and lamina propria containing numerous connective tissue papillae. a very specialized part of this epithelium, the junctional epithelium, is bound to the tooth enamel by means of a cuticle that resembles a thick basal lamina and forms the epithelial attachment of Gottlieb. the epithelial cells are attached to this cuticle by hemi- desmosomes. between the enamel and the epithelium is the gingival sulcus or gingival crevice, a small deepening or groove up to 3 mm surrounding the crown.  Gingiva may be divided into two: 1. Attached Gingiva: Part of the gingiva which provides protective covering over the alveolar bone 2. Free Gingiva: Part of the gingiva which forms a protective cuff around the enamel at the neck of the tooth. Between this gingiva and enamel is a potential space, the gingival crevice or sulcus, which extends from the tip of the free gingiva to the cemento-enamel junction. At the tip of free gingiva, the epithelium undergoes abrupt transition to a thin layer of epithelial cells tapering to only two or three cells thick at the base of the gingival crevice. This crevical epithelium is easily breached by pathogens and the underlying lamina propria is thus.  THE GINGIVAE Attached gingiva Attached gingiva Free gingiva Medical Application The depth of the gingival sulcus, measured during clinical examination, is very important and may indicate periodontal  Histology of gingiva A.Epithelum stratified squamous type smooth in free gingva, stippled in attached gingiva. according to the behavior of the surface layer, different types of gingival epithelium can be noticed. 1.Fully Keratinized epithelium- the surface layer consist of flat tightly packed horny scales, without nuclei (15%) 2.Parakeratinized epithelium- the surface cells retain the pyknotic and condensed nuclei. Keratinization is not complete (50%). Incomplete parakeratosis may be observed 3.Non Keratinized epithelium- the surface cells are flay with nuclei (10%)  Histology of gingiva B. Lamina propria: consists of dense CT not highly vascular. Macrophages are present. Fibroblasts are abundant. Plasma cells and lymphocytes are also present normally under the crevice. There are also blood vessels, rich network of lymphatic vessels and different nerve endings such as Meissner or krause corpuscles, end bulbs, etc.  Tooth Development involves the following major stages: bud stage, cap stage, bell stage and stage of root development. each tooth is derived from two embryological sources: the enamel is of epithelial (ectodermal) whilst the dentine, cementum, pulp, periodontal ligament and alveolar bone are of mesenchymal (mesodermal) origin tooth development in human begins during the 6th week of intrauterine development. appear as a thickening of a horseshoe-shaped (C-shaped) epithelial ridge, known as dental  Maxillary Process Stomodeum Dental lamina Developing Tongue Mandibular process  In each quadrant of the mouth, the dental lamina develops four globular swellings- the enamel organs (this marks the bud stage). The enamel organs are the future deciduous central & lateral incisors, canine & first molar teeth. Subsequently, the dental lamina proliferates backwards in each arch successively giving rise to the enamel organs of the future second deciduous molar & the three permanent molars. The permanent successors of the deciduous teeth will later develop from  1. Bud Stage Intramembr anous Meckel’s ossification cartilage Bud stage is characterized by rounded, localized growth of epithelium surrounded by proliferating mesenchymal cells, which are packed closely beneath and around the  1. Bud Stage In the bud stage, the enamel organ consists of peripherally located low columnar cells and centrally located polygonal cells  2. Cap Stage The primitive mesenchyme subjacent to the developing enamel organ proliferates to form a cellular mass known as dental papilla, whilst at the same time, the enamel organ becomes progressively cap-shaped (marks cap stage), as seen in micrograph (a), enveloping the dental papilla. During the cap stage, the cells lining the concave face of the enamel organ in contact with the dental papilla begin to differentiate into tall columnar cells , the future ameloblasts, which will form the enamel. This, in turn, induces the differentiation of a layer of columnar odontoblasts (dentine-  Stages of Tooth formation and layers of developing OEE= Outer enamel/tooth dental epithelium, DP= Dental papilla, O= Odontoblasts, A= Ameloblasts, PD= Predentin, D= Dentin, B= alveolar Bone,  2. Cap Stage Enamel Organ Dental Papilla Condensation of the ectomesenchyme immediately subjacent to the tooth bud caused by lack of extracellular matrix secretion by the cells thus preventing separation. Histodifferentiation begins at the end of cap stage. Epithelial outgrowth called Enamel Organ because it will eventually form the enamel  2. Cap Stage Enamel organ Enamel knot Dental papilla Dental follicle or sac Dental follicle or dental sac is the condensed ectomesenchymal tissue surrounding the enamel organ and dental papilla. This gives rise to cementum and the periodontal ligament (support structures for tooth)  The outer surface of the enamel organ consists of simple cuboidal epithelium- the outer/external enamel epithelium By the cap stage of development, the dental lamina connecting the enamel organ to the oral mucosa fragmented and, around the whole developing bud, a condensation of mesenchyme forms the dental follicle (sac) which will eventually become the cementum, periodontal ligament & alveolar bone. Although enamel production is confined to the crown, the rim of the bell of the enamel organ nevertheless continues to proliferate, inducing dentine formation & thereby determining the shape of the tooth root.  3. Bell stage As the enamel organ develops further, it assumes a characteristic bell shape, its free edge proliferating so as to determine the eventual shape of the tooth crown. As the dentine of the crown and root are progressively laid down the dental papilla shrinks and eventually becomes the dental pulp contained within the pulp chamber and root canals. The dentine and dental pulp directly develop from the dental papilla, while the cementum, periodontal ligament as well as the alveolar bone develop from the dental follicle of the surrounding mesenchyme  3. Bell Stage (Early) Outer dental epithelium Stellate reticulum Stratum intermedium Inner dental epithelium Dental papilla Inner dental epithelium: Short columnar cells bordering the dental papilla. These will eventually become ameloblasts that will form the enamel of the tooth crown by differentiating into tall columnar cells. The cells of inner dental epithelium exert an organizing influence on the underlying mesenchymal cells in the dental papilla, which later differentiate into odontoblasts. Outer dental epithelium: Cuboidal cells that cover the enamel organ. Their function is to organize a network of capillaries that will bring nutrition to the ameloblasts. In preparation to formation of enamel, at the end of bell stage, the formerly  3. Bell Stage (Early) Outer dental epithelium Stellate reticulum Stratum intermedium Inner dental epithelium Dental papilla Stellate reticulum: Star-shaped cells with processes, present between the outer and the inner dental epithelium. These cells secrete glycosaminoglycans, which attract water, thereby swelling the cells and pushing them apart. However, they still maintain contact with each other, thus becoming star-shaped. They have a cushion-like consistency that may support and  Outer dental epithelium Stellate reticulum Stratum intermedium Inner dental epithelium Dental papilla Dental Papilla: Before the inner dental epithelium begins to produce enamel, the peripheral cells of the mesenchymal dental papilla differentiate into odontoblasts under the organizing influence of the epithelium. First, they assume a cuboidal shape and then a columnar form and acquire the specific potential to produce dentin. The basement membrane that separates the enamel organ and the dental papilla just prior to dentin formation is called the “membrana preformativa”  3. Bell Stage Dental lamina (and the lateral lamina) will disintegrate and loose contact with oral epithelium. Sometimes, these epithelial cells will persist when they are called “epithelial pearls” or “cell rests of Serre” Clinical significance: Cysts will develop in these (eruption cysts) and prevent eruption, or they Eruption Cyst Future crown patterning also occurs in the bell stage, by folding of the inner dental epithelium. Cessation of mitotic activity within the inner dental epithelium determines the shape of a tooth.