Embryology1 /certified fixed orthodontic courses by Indian dental academy

Embryology

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EMBRYOLOGY

Definition: It is the study of the formation
and development of the embryo or fetus
from the moment of its inception up to the
time when it is born as an infant.


Embryogenesis is divided
into three phases
► Pre implantation period (first 7 days)
► Embryonic period (next 7 weeks)
► Fetal period (3 – 9 months)


First week of development
► Ovulation
► Fertilization
► Cleavage
► Blastocyst formation


Fertilization
► It is the process by which male and female

gametes fuse .

► It occurs in ampullary region of the uterine

tube.


Cleavage
► Development of zygote from two cell stage

to the morula


Blastocyst formation


Second week of development
(bilaminar germ disc)


Day -8
► Trophoblast proliferates to form

-- Inner cytotrophoblast
--Outer syncytiotrophoblast


Bilaminar germ disc
(day -8)
► Embryoblast differentiates into two layers

--Hypoblast
-- Epiblast


Lacunar stage (day-9)


Day-11 and 12


Two types of extraembryonic
mesoderm
► Extraembryonic somatopleuric

mesoderm
► Extraembryonic splanchopleuric
mesoderm


Secondary yolk sac formation


► The second week of development is

known as week of two

► The trophoblast differentiates into -2

--the cytotrophoblast
--the synctiotrophoblast
► The embryoblast forms two layers

--epiblast
-- hypoblast

Third week of development
(trilaminar germ disc)
► Gastrulation
► Formation of the notochord
► Neurulation


Gastrulation
►

Definition: it is process that establishes
all three germ layers i.e.
-- ectoderm
-- mesoderm
--endoderm


Migration of cells b/w hypoblast
and epiblast


Formation of notochord
► Primitive streak
► Primitive node
► Canalization of cord cells


Primitive streak and node


Neurulation
► End of 3 week
► Formation of neural tube and neural

plate


Formation of neural tube


Neural crest cells
► Ectomesenchymal tissue termed neural

crest cells appears in the intermediate zone
between the surface ectoderm and the
neural tube during neurulation.


Characteristics of neural crest cells.
Pluripotent cells
Migration
Cyto-differentiation


Derivatives of neural crest cells.
Connective tissue- Ectomesenchyme of facial prominences and
branchial arches
- Bones and cartilages of facial visceral skeleton
- Dermis of face and neck
- Stroma of salivary ,thymus ,thyroid,
parathyroid and pituitary gland
- Corneal mesenchyme
- Aortic arch arteries

-Dental papilla

-Portions of periodontal ligament ,Cementum
Muscle tissue-Ciliary muscles
-Covering connective tissue of
branchial arch muscles

Nervous tissue-Leptomeninges.
-Schwann sheath cells

Sensory ganglia-Autonomic ganglia.
-Spinal dorsal root ganglia.
-Sensory ganglia.
ANS-Sympathetic ganglia.
-Parasympathetic ganglia.

Migration of neural crest cells.
► Active migration
► Passive migration


Factors affecting migration
► Extra cellular molecules: like fibronectin
► Vitamin A
► 13 cis-retinoic acid
► Mutant genes

Factors effecting the normal growth
of neural crest derivatives
► Number of cells leaving the neural tube
► Rate of migration
► Proportion of dividing
► Minimal period between successive

divisions
► Number of cells that die


Characteristics of neural crest cells
► Regulation


Significance of neural crest cells
► In craniofacial region

Differentiate to form most of the connective
tissue of head - ectomesenchyme


Source and pattern of migration to the
developing face and branchial arch
system


Significance in development of
dentition
► Neural crest is primary material for

odontogenesis.

► Interaction between neural crest,

pharyngeal endoderm and oral ectoderm
produces the dental lamina.

► Neural crest provides material for dentin,

pulp and cementum.


Branchial arches or
pharyngeal arches


Each pair of branchial arches contain a
basic set of structure
► Cartilage component
► Muscular component
► Vascular component
► Nervous component


First branchial arch
(mandibular arch)


COMPONENTS OF 1st ARCH
►

Cartilage : MECKEL’S CARTILAGE
--Arises 41st – 45th Day I.U
--It provides a template for subsequent
development of the mandible.


Structures formed by cartilaginous
component


Derivatives of Meckel’s Cartilage :
Mental Ossicle (Endochondral oss.)
► Head and neck of Malleus..
► Ant. Ligament of the Malleus.
► Sphenomandibular Ligament.
►


Muscles of first pharyngeal arch
►
►
►

Muscles of mastication
Ant. Belly of digastric
Tensor tympani and
palatini


Nerve supply
► Mand. Branch of trigeminal nerve
►

Sensory component supplies
: Mandible and covering mucosa.
: Mandibular teeth including gingiva
: Mucosa of ant. 2/3 of Tongue.
: Floor of the mouth.
: Skin of the lower third of Face.

Nerve supply


Second branchial arch
Components:
►

(Hyoid Arch )

Cartilage : Reichert’s Cartilage
(45th – 48th I.U)
-- Stapes
-- Styloid process of the temporal bone.
-- Stylohyoid ligament.
-- Lessor horn of sphenoid.
-- Cranial part - Body of Hyoid.

►

Muscles :
-- Stapedius
-- Stylohyoid
-- Post. Belly of Digastric
-- auricular
-- muscles of facial expression

.

► Nerve

:
-- Facial / VII Cranial nerve.
-- Chorda tympani nerve
(Ant 2/3rd of Tongue)

►

Artery :
-- Stapedial artery
-- Transient i.e. disappears during fetal
life


Development of tongue


Clinical correlation
► Tongue tie
► Macroglossia
► Microglossia
► Bifid tongue
► Fissured tongue
► Rhomboid tongue


Position of pharyngeal arches


Development of the face


► Dev. Of the face starts at the end of 4

week

► Facial prominences

--maxillary prominence
--mandibular prominence
- lat. Nasal prominence
--med. Nasal prominence
--frontonasal prominence


Facial prominences


► Upper lip

-- 2 med. Nasal prominence
-- 2 max. prominence
►

Lower lip and jaw
-- Mand. Prominences

► Nose

-- Five facial prominence


Development of palate


► Primary palate
► Secondary palate
► Primary palate

--intermaxillary segment
-- labial component
-- upper jaw
-- palatal component

Intermaxillary segment
and maxillary process


6.5 week old embryo


7.5 week old embryo


10 – week embryo


Clinical correlates
Facial clefts
1. Failure of fusion of maxillary
prominence with medial nasal process
►

Anterior to incisive foramen
--lat. Cleft lip
--cleft upper jaw
--cleft b/w pri. And sec. palate

► Post. to incisive foramen

-- cleft palate
--cleft uvula
2 . Failure of fusion of maxillary process
-- oblique facial cleft
3. Incomplete merging of 2 medial nasal
prominences
--Median cleft lip

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Embryology1 /certified fixed orthodontic courses by Indian dental academy

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