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ALIMANTARY SYSTEM 1
MOUTH, PHARYNX ANDRELATED STRUCTURES
TOPICS• HIGHLIGHTS
• MOUTH
• TEETH
• TONGUE
• SALIVARY GLANDS
• TONSILS
• PHARYNX
• TIMETABLE OF SOME EVENTS DESCRIBED
HIGHLIGHTS
• The oral cavity is derived from ectoderm (stomatodaeum) and endoderm (foregut).
• These two are separated by the buccopharyngeal membrane.• Teeth are formed in relation to the dental lamina.• Enamel organ is an enlargement of the lamina for each tooth.• Ameloblasts (ectoderm) form the enamel.• Odontoblasts (mesoderm) form dentine.• The pulp is formed by mesenchyme that invaginates into the
enamel organ.• Three swellings are formed in the floor of the pharynx, in relation to
the 1st pharyngeal arch.– The right and left lingual swellings.– Tuberculum impar (the median swelling).– Hypobranchial eminence = Another median swelling is formed in relation
to the 3rd and 4th arches.
• The anterior 2/3rd of the tongue is formed from the lingual swellings and the tuberculum impar.
• The posterior 1/3rd of the tongue is formed by the cranial part of the hypobranchial eminence.
• Salivary glands develop as outgrowth of the buccal
epithelium.
• The pharynx is derived from the foregut.
HIGHLIGHTS (continue)
MOUTH
• It is derived partly from stomatodaeum and partly from foregut.
• Its epithelial lining is partly ectoderm and partly endoderm.
• The epithelium of the tongue is derived from endoderm.
• The epithelium of the of the lining inside of the lips and cheeks, and the palate, is most probably ectodermal.
• The teeth and the gums are ectodermal origin.
• In the region of the floor of the mouth, the mandibular processes take part in the formation of three structures:
1. The lower lip (and lower part of cheeks).
2. The lower jaw.
3. The tongue.
• At first these structures are not demarcated from each other.
• Soon the tongue forms a recognizable swelling.
• Linguo-gingival sulcus separates laterally the swelling from the rest of the mandibular process.
• Labio-gingival sulcus appears more laterally.
• The lower lip (or cheek) is formed.
• The alveolar process are formed, with deepening of these two sulci,
• This process forms the jaw, and the teeth develop in relation to it.
• The roof of the mouth is formed by the palate.• Just as in the lower jaw, the alveolar process of the upper jaw
is separated from the upper lip and cheek by the appearance of labio-gingival furrow.
MOUTH (continue)
1. Lateral lingual swelling
2. Tuberculum impar
3. Foramen cecum
4. Copula
5. Epiglottal swelling
6. Laryngeal orifice
7. Arytenoid swellings
8. Pharyngeal arches
TEETH
• The teeth are formed in relation to the alveolar process.
• Dental lamina projects into the underlying mesoderm.
• Dental lamina is curved as the alveolar process.
• Dental lamina is formed before the alveolar process is defined.
• Dental lamina now shows a series of local thickenings
(dental organs), each of which is destined to form one milk tooth.
• There are ten enamel organs (5 on each side) in each alveolar process.
The Stages in the Formation of an Enamel Organ and the development of a Tooth are as follows:
1. Enamel organ is formed from dental lamina.
2. Cap stage; the enamel organ grows into the alveolar process, its lower edge assumes a cup-shaped appearance, this cup is occupied by the dental papilla (mesenchymal mass). The enamel organ and the dental papilla together constitute the tooth germ. The developing tooth looks like a cap.
3. Ameloblasts (are cells of the enamel organ that line the papilla become columnar).
4. Bell stage; odontoblasts (mesodermal cells adjacent to the ameloblasts and separated from ameloblasts by a basement membrane), arrange themselves as a continuous epithelium-like layer. The remaining cells of the papilla form the pulp of the tooth.The developing tooth looks like a bell.
5. Ameloblasts lay down enamel on the superficial (outer) surface of the basement membrane. The odontoblasts lay down dentine on the deeper surface. This process is similar to the formation of bone by osteoblasts. Ameloblasts and odontoblasts become away from each other.
6. Ameloblasts disappear leaving a thin membrane, the dental cuticle, over the enamel. The odontoblasts continue to separate the dentine from the pulp throughout the life of the tooth.
7. The root of the teeth become surrounded by bone.
The Stages in the Formation of an Enamel Organ and the development of a Tooth are as follows:
The Root of the Tooth
• The root of the tooth is established by continued growth into underlying mesenchyme.
• Odontoblasts in this region lay down dentine.
• As layers of dentine are deposited, the pulp space become narrower and is gradually converted into a canal, (through which nerves and blood vessels pass into the tooth).
• There are no ameloblasts in the region of the root.
• The dentine is covered by mesenchymal cells that differentiate into cementoblasts.
• Cementoblasts lay down a layer of dense bone called cementum.
• Mesenchymal cells form the periodontal ligament which connect the root to the socket in the jaw bone.
Formation of Permanent Teeth
The dental lamina gives off a series of buds.
• One bud lies on the medial (mesial) side of each developing milk tooth.
• The buds form enamel organs (as mentioned above).
• These medial buds give rise to the permanent incisors, canines and premolars.
Buds that arise from the dental lamina posterior to the region of the last milk tooth, give rise to the permanent molars.
• The dental lamina is established in the 6th week of intrauterine life.
• At birth,
– the germs of all the temporary teeth and of the permanent incisors, canines and 1st molars show considerable development.
– The germs of the permanent premolars and of the 2nd molars are rudimentary .
• After birth, the germ of 3rd molars is formed.
Formation of Teeth (germs)
• The developing tooth germs undergo calcification.
• All the temporary teeth and the permanent lower 1st molar begin to calcify before birth.
• The other permanent teeth begin to calcify at a varying ages after birth.
Formation of Teeth (calcification)
Eruption of Teeth
• The eruption of a tooth is preceded by a major development of its root.
• The ages at which teeth erupt vary considerably.
• The average age of eruption is as follows;A. Temporary or milk teeth
i. Lower central incisor 6-9 months
ii. Upper incisors 8-10 months
iii. Lower lateral incisor 12-20 months
iv. First molar 12-20 months
v. Canines 16-20 months
vi. Second molars 20-39 months
B. Permanent teeth
1) First molar 6-7 years
2) Central incisors 6-8 years
3) Lateral incisors 7-9 years
4) Premolars 10-12 years
5) Canines 10-12years
6) Second molars 11-13 years
7) Third molars 17-21 years
Eruption of Teeth
Ectoderm Ameloblasts ---- Enamel
Mesoderm (of neural crest) Odontoblasts ----- dentine
Mesenchyme around tooth CementumPeriodontal ligament
Summary of Derivation of Parts of Tooth
Anomalies of Teeth1. Anodentia (complete absence)/ one or more.2. Supernumerary.3. Too large X too small. Supernumerary cups or roots.
Cups and roots may be less than normal.4. Gemination (fusion of two or more).5. Malocclusion (incorrect occlusion).6. Precocious eruption.7. Delayed eruption.8. Formation in Abnormal situations; ovary/ hypophysis
cerebri.9. Improper formation of enamel or dentine.
Tongue • The tongue develops in relation to the pharyngeal arches in
the floor of the developing mouth.
• The medial-most parts of the mandibular arches proliferate to form two lingual swellings.
• Tuberculum impar is a midline swelling partially separates the two lingual swellings.
• The epithelium immediately behind the tuberculum impar proliferates to form thyroglossal duct.
• The site of this downgrowth is called the foramen caecum.
Caption = Pattern of the branchial arches. I-IV branchial arches, 1-4 branchial pouches (inside) and/or pharyngeal grooves (outside)a Tuberculum lateraleb Tuberculum imparc Foramen cecumd Ductus thyreoglossuse Sinus cervicalis
Tongue (continue)
• Hypo-branchial eminence is another midline swelling is seen in relation to the 2nd, 3rd, and 4th arches.
• Hypo-branchial eminence shows subdivision into a cranial and caudal parts.
• Cranial part (copula) is related to the 2nd and 3rd arches.
• Caudal part (epiglottis) is related to the 4th arch.
• The anterior two-third of the tongue is formed by fusion of:
1) The tuberculum impar.
2) The two lingual swellings.
• The anterior 2/3rd of the tongue is derived from the mandibular arch.
Tongue (continue)
The posterior one-third of the tongue;
• Is derived from copula (the caudal part of the hypobranchial eminence).
• The 2nd arch mesoderm gets buried below the surface.
• The 3rd arch mesoderm grows over it to fuse with the mesoderm of the 1st arch.
• The posterior 1/3rd of the tongue is formed by 3rd arch mesoderm.• The posterior-most part of the tongue is derived from the 4th arch.
• The anterior 2/3rd of the tongue is supplied by lingual nerve branch of the mandibular nerve, the post-trematic nerve of the 1st arch.
• Chorda tympani is the pre-trematic nerve of this arch.
• The posterior 1/3rd of the tongue is supplied by the glossopharyngeal nerve (nerve of the 3rd arch).
• The most posterior part of the tongue is supplied by the superior laryngeal nerve ,(branch of vagus nerve), which is the nerve of the 4th arch.
• The musculature of the tongue is derived from the occipital myotomes.
• The hypoglossal nerve is the nerve of the occipital myotomes.
• The epithelium of the tongue is at first made up of a single layer of cells.
• Later the epithelium becomes stratified and papilla become evident.
• Taste buds are formed in relation to the terminal branches of the innervating nerve fibers.
• Development of the tongue starts in the 4th week of intrauterine life.
Tongue (continue)
Anomalies of the Tongue
1. Macrogloosia X Microglossia.
2. Bifid tongue.
3. Ankyloglossia = tongue tie. // Ankyloglossia superior.
4. Persistent tuberculum impar.
5. Thyroid tissue may be present in the tongue, either under mucosa or within the muscles.
6. Thyroglossal cyst at the base of the tongue.
7. Fissure tongue.
Part of the tongue
Embryonic part from which derived
General sensation
Taste sensation Motor sensation
Epithelium over anterior 2/3rd
1st arch Mandibular (lingual nerve)
Chorda tympani ?(facial nerve)
Epithelium over posterior 1/3rd
3rd arch Glossopharyngeal nerve
Glossopharyngealnerve
Epithelium over posterior-most part
4th arch Superior laryngeal nerve (vagus)
Superior laryngeal nerve (vagus)
Muscle Occipital myotome
Hypoglossal nerve
Summary of Derivation of Components of the Tongue
Salivary Glands• Develop as outgrowths of the buccal epithelium.
• The out growths are at first solid and later canalized.
• They branch repeatedly to form the duct system.
• The terminal part of the duct system develop into secretory acini.
• It is difficult to determine whether they are ectodermal or endodermal.• The outgrowth for the parotid gland arises in relation to the line along which
the maxillary and mandibular processes fuse to form the cheek.
• Parotid gland is generally considered to be ectodermal.• The outgrowths of the submandibular and sublingual glands arise in relation
to the linguo-gingival sulcus.
• Submandibular and sublingual glands are considered to be endodermal.
• One or more of the salivary glands may sometimes be absent.
Tonsils
• The palatine tonsil develops in relation to the lateral part of the 2nd pharyngeal pouch.
• The endodermal lining of the pouch undergoes considerable proliferation and invades the underlining mesoderm of the 2nd arch, which forms the tonsillar stroma.
• Most of the pouch is obliterated.• Lymphocytes collect in relation to the tonsillar stroma beneath the epithelium.
• It is not certain whether these lymphocytes differentiate in situ or derived from blood.
• (Possibly, they come to the tonsil from the liver as lymphoblasts).• The intratonsillar cleft or tonsillar fossa is believed to represent a persisting
part of the 2nd pouch.• Similar epithelial proliferations and aggregations of lymphoid tissue give rise to
the tubal tonsil, the lingual tonsil and the pharyngeal tonsils.
Pharynx • The pharynx is derived from the cranial-most part of the tongue.• Most of the endodermal pouches lose contact with the
pharyngeal wall.• The endodermal pouches are formed in relation to the lateral wall
of the pharynx.
• The floor of the foregut gives rise to a midline diverticulum from which the entire respiratory system is developed.
• The opening of the pharyngo-tympanic tube represents the site of origin of the tubotympanic recess.
• The site of the midline respiratory diverticulum is represented by the inlet of the larynx.
• The pharynx shows a subdivision into nasopharynx, oropharynxand laryngopharynx, with the establishment of the palate of the mouth.
• The muscles forming the wall of the pharynx are derived from the third and subsequent pharyngeal arches.
Age Developmental events
4 weeks Tongue starts forming ,i.e. two lateral lingual swelling and tuberculum impar become visible
5 weeks Hypobranchial eminence becomes visible.
6 weeks Dental lamina of upper and lower jaws are established .
7 weeks Salivary glands starts developing .
8 weeks Enamel organs are formed .
10 weeks Enamel organ becomes cup- shaped .
3 months Formation of tonsil begins .
5 months The tonsil is infiltrated by lymphatic tissue .
6 months Enamel and dentine have formed considerably.Formation of tongue is almost complete .
Just before birth Cementum is formed .
After birth Periodontal ligament are formed before eruption of teeth .
