Embryology

1. Embryologic derivation Of
Oral and Dental Structures
Lubna Mohammad Abu Al-Rub

2. consider the following :
neural tube situated dorsally.
below is a notochord.
further ventrally hollow gut tube.
mesoderm surrounds these
structures.
the investing layer of ectoderm .

3. Neural tube

4. formation of the somites
 by the middle of the third week the
embryo consists of three layers :
1. ectoderm which is continuous with
the wall of the amnion above.
2. the endoderm - which is continuous
with the yolk sac below
3. the mesoderm - continuous with the
extraembryonic mesoderm
peripherally.

5. 3 weeks Embryo

6.  the mesoderm separates the endoderm and
the ectoderm except centrally where it is
interrupted by the notochord.
 the buccopharengeal membrane laterally and
the cloaclal membrane coadally.
Mesoderm
 initially forms a thin sheet of tissues on either
side of the midbrain.
 around day 17 , mesoderm on either side
thickens to form a mass called PARAXIAL
MESODERM

7. PARAXIAL MESODERM – the
formation of somites
 by the end of the third week of embryo life , mesoderm
condensations are seen laterally from the notochord
 starting from the cephalic end adjacent to the tip of the
notochord they start segmental condensation to form
paired
cubic masses called the somites
 this process is referred to as segmentation or
metamerism.
 somites are formed of whorls of cells called
somitomeres.
 somites mould the surface ectoderm to form humps
which we use to predict the age of the embryo.
 by the end of the 5th week we have 42-45 pairs of
somites which are classified into ( according to their
fate) 4 occipital 8 cervical 12 thoracic 5 lumbar 5 sacral
and 8-10 coocgeal.

8. The formation Of Somites

9. Further differentiation of the
somites
 by the end of the fourth week each somite undergoes more
diffrentiaiton and has a
 myoceol - slit like cavity contains condensation of cells
which divides and by this way somites grow
 dermatome - beneath the myoceal- will eventually spread
beneath the ectoderm to form the dermis of the skin.
medial to the myoceol we have 2 divisions :
 1.medially sclerotome : differentiates into connective tissues
MESENCHYME which migrates and condenses around the
neural tube to form vertebral column the ribs and the sternum
the sclerotomes of the metotic somites are believed to
contribute to the scull formation.
 2. laterally myotome : forms the flexor and the extensor
muscles of the vertebral column the intercostal muscles and
part of the limb muscles .

10. Somites
 paraxial mesoderm undergoes metameric segmentation .
 the resulting somites become divided into dermotome ,
sclerotome, and myotome .
 in the head region the formation of myotomes can be
described as " atypical" , a number of somitomeres can
reach to seven ( but this number varies according to species
studied) are found within the mesoderm alongside the
forebrain , midbrain and the cranial half of the hindbrain . and
these do not progress to form somites.
 those somitomeres give rise to the striated muscles of the
face jaw and throat.
 the most cephalic fully formed somites are four in number and
are called the occipital or metotic behind the ottic capsule )
somites.

12.  an older and more classic interpretation of
head somites describes three pairs of prootic
somites ( anterior to the ottic capsule ).
prootic somites do not form seperate and
identifiable structures in modern species.
 their myotomes give rise to extraoccular
muscles.
 the nerves supplying their myotomes (
occlumotor trochlear and abducent) are
equivilant to the ventral roots of the spinal
nerves through which the muscles of the
trunk are supplied .

13.  dermatomes of prootic somites are supplied
by cranial nerves equivalent to dorsal roots of
the spinal nerves.
 these cranial nerves are the ophthalmic of the
trigiminal and the combined maxillary and
mandibular of the trigeminal.
 the dermatomes of the first and second
prootic somites are enlarged thus the
corresponding trigiminal divisions are
enlarged and carry many sensations
 the dermatome of the third prootis somite is
small and the cutanous content of the facial
nerve supplying it thus is small

15. occipital somites
 there are controversy regarding their number.
 according to the CLASICAL view they are 6 on each side but
the first has disappeared and the second is a transient
structure .
 so usually we describe four instead of 6 ( the original
number)
 the mesoderm of the occipital myotome migrates to the floor
of the mouth where it forms the musculature of the tongue
.___> the nerves supplying theose myotomes are ventral root
of spinal nerves.
 in higher vertebrates they are fused to form the hypoglossal
nerve
 corresponding dorsal nerves are fused to form the vagues
nerve.
 the glossopharengeal nerve is the nerve of the first missing
occipital somite according to the classical view.
 the sclerotomes of the occipital somites become
incorporated in the posterior part of the scull base -
parachordal

16.  unlike the spinal dorsal nerves , the
dorsal nerves of the head region fail to
fuse with the corresponding ventral
nerves .
the contain :
1. sensory fibers
2. visceral motor fibers
3. motor fibers which supply the striated
muscles associated with the pharengeal
(brachial )arches , thats why they are
sometimes called the brachial nerves .