ALIMANTARY SYSTEM 1
MOUTH, PHARYNX AND
• SALIVARY GLANDS
• TIMETABLE OF SOME EVENTS DESCRIBED
• The oral cavity is derived from ectoderm (stomatodaeum) and
• 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
• 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
• The pharynx is derived from the foregut.
• 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
• This process forms the jaw, and the teeth develop in relation
• 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.
• 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
• Dental lamina now shows a series of local thickenings
(dental organs), each of which is destined to form one milk
• There are ten enamel organs (5 on each side) in each
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
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.
The Stages in the Formation of an Enamel Organ and
the development of a Tooth are as follows:
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
7. The root of the teeth become surrounded by bone.
The Root of the Tooth
• The root of the tooth is established by continued growth into
• 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 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
Formation of Teeth (germs)
• The dental lamina is established in the 6th week of
• At birth,
– the germs of all the temporary teeth and of the
permanent incisors, canines and 1st molars show
– 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 (calcification)
• 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.
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
Eruption of Teeth
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
Summary of Derivation of Parts of Tooth
Ectoderm Ameloblasts ---- Enamel
Mesoderm (of neural crest) Odontoblasts ----- dentine
Mesenchyme around tooth Cementum
Anomalies of Teeth
1. Anodentia (complete absence)/ one or more.
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
9. Improper formation of enamel or dentine.
• 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 laterale
b Tuberculum impar
c Foramen cecum
d Ductus thyreoglossus
e Sinus cervicalis
• 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
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
• 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
• 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
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.
Summary of Derivation of Components of the Tongue
Part of the Embryonic General Taste sensation Motor
tongue part from sensation sensation
Epithelium over 1st arch Mandibular Chorda tympani
anterior 2/3rd (lingual nerve) ?(facial nerve)
Epithelium over 3rd arch Glossopharyn Glossopharyngeal
posterior 1/3rd geal nerve nerve
Epithelium over 4th arch Superior Superior laryngeal
posterior-most laryngeal nerve (vagus)
part nerve (vagus)
Muscle Occipital Hypogloss
myotome al nerve
• 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
• One or more of the salivary glands may sometimes be absent.
• The palatine tonsil develops in relation to the lateral part of the 2nd pharyngeal
• The endodermal lining of the pouch undergoes considerable proliferation
and invades the underlining mesoderm of the 2nd arch, which forms the
• 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.
• The pharynx is derived from the cranial-most part of the tongue.
• Most of the endodermal pouches lose contact with the
• 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, oropharynx
and laryngopharynx, with the establishment of the palate of the
• 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
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 .