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.
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Embryogenesis is divided into three phases ► Pre
implantation period (first 7 days) ► Embryonic period (next 7 weeks) ► Fetal period (3 – 9 months)
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First week of development ► Ovulation ► Fertilization ► Cleavage ► Blastocyst
formation
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Fertilization â–ş It
is the process by which male and female gametes fuse .
â–ş It
occurs in ampullary region of the uterine tube.
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Cleavage â–ş Development
of zygote from two cell stage to the morula
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Blastocyst formation
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Second week of development (bilaminar germ disc)
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Day -8 â–ş Trophoblast
proliferates to form
-- Inner cytotrophoblast --Outer syncytiotrophoblast
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Bilaminar germ disc (day -8) â–ş Embryoblast
differentiates into two layers
--Hypoblast -- Epiblast
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Lacunar stage (day-9)
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Day-11 and 12
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Two types of extraembryonic mesoderm â–ş Extraembryonic
somatopleuric
mesoderm â–ş Extraembryonic splanchopleuric mesoderm
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Secondary yolk sac formation
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► 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 www.indiandentalacdemy.com
Third week of development (trilaminar germ disc) ► Gastrulation ► Formation
of the notochord ► Neurulation
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Gastrulation â–ş
Definition: it is process that establishes all three germ layers i.e. -- ectoderm -- mesoderm --endoderm
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Migration of cells b/w hypoblast and epiblast
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Formation of notochord ► Primitive
streak ► Primitive node ► Canalization of cord cells
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Primitive streak and node
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Neurulation ► End
of 3 week ► Formation of neural tube and neural plate
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Formation of neural tube
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Neural crest cells â–ş Ectomesenchymal
tissue termed neural crest cells appears in the intermediate zone between the surface ectoderm and the neural tube during neurulation.
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Characteristics of neural crest cells. Pluripotent cells Migration Cyto-differentiation
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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 www.indiandentalacdemy.com
-Dental papilla -Portions of periodontal ligament ,Cementum Muscle tissue-Ciliary muscles -Covering connective tissue of branchial arch muscles
Nervous tissue-Leptomeninges. -Schwann sheath cells www.indiandentalacdemy.com
Sensory ganglia-Autonomic ganglia. -Spinal dorsal root ganglia. -Sensory ganglia. ANS-Sympathetic ganglia. -Parasympathetic ganglia. www.indiandentalacdemy.com
Migration of neural crest cells. â–ş Active
migration
â–ş Passive
migration
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Factors affecting migration ► Extra
cellular molecules: like fibronectin
► Vitamin ► 13
A
cis-retinoic acid
► Mutant
genes www.indiandentalacdemy.com
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
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Characteristics of neural crest cells â–ş Regulation
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Significance of neural crest cells â–ş In
craniofacial region
Differentiate to form most of the connective tissue of head - ectomesenchyme
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Source and pattern of migration to the developing face and branchial arch system
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Significance in development of dentition â–ş Neural
crest is primary material for odontogenesis.
â–ş Interaction
between neural crest, pharyngeal endoderm and oral ectoderm produces the dental lamina.
â–ş Neural
crest provides material for dentin, pulp and cementum. www.indiandentalacdemy.com
Branchial arches or pharyngeal arches
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Each pair of branchial arches contain a basic set of structure ► Cartilage
component ► Muscular component ► Vascular component ► Nervous component
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First branchial arch (mandibular arch)
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COMPONENTS OF 1st ARCH ►
Cartilage : MECKEL’S CARTILAGE --Arises 41st – 45th Day I.U --It provides a template for subsequent development of the mandible.
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Structures formed by cartilaginous component
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Derivatives of Meckel’s Cartilage: Mental Ossicle (Endochondral oss.) ► Head and neck of Malleus.. ► Ant. Ligament of the Malleus. ► Sphenomandibular Ligament. ►
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Muscles of first pharyngeal arch ► ► ►
Muscles of mastication Ant. Belly of digastric Tensor tympani and palatini
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Nerve supply â–ş Mand. â–ş
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. www.indiandentalacdemy.com
Nerve supply
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Second branchial arch
(Hyoid Arch)
Components: ►
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. www.indiandentalacdemy.com
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â–ş
Muscles : -- Stapedius -- Stylohyoid -- Post. Belly of Digastric -- auricular -- muscles of facial expression
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â–ş Nerve
: -- Facial / VII Cranial nerve. -- Chorda tympani nerve (Ant 2/3rd of Tongue)
â–ş
Artery : -- Stapedial artery -- Transient i.e. disappears during fetal life
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Nerve supply
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Development of tongue
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Development of tongue
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Nerve supply
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Clinical correlation ► Tongue
tie ► Macroglossia ► Microglossia ► Bifid tongue ► Fissured tongue ► Rhomboid tongue www.indiandentalacdemy.com
Position of pharyngeal arches
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Development of the face
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â–ş Dev.
Of the face starts at the end of 4 week
â–ş Facial
prominences --maxillary prominence --mandibular prominence - lat. Nasal prominence --med. Nasal prominence --frontonasal prominence
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Facial prominences
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► Upper
lip -- 2 med. Nasal prominence -- 2 max. prominence
►
Lower lip and jaw -- Mand. Prominences
► Nose
-- Five facial prominence
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Development of palate
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► Primary
palate ► Secondary palate ► Primary
palate --intermaxillary segment -- labial component -- upper jaw -- palatal component www.indiandentalacdemy.com
Intermaxillary segment and maxillary process
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6.5 week old embryo
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7.5 week old embryo
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10 – week embryo
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Clinical correlates Facial clefts 1. Failure of fusion of maxillary prominence with medial nasal process â–ş
Anterior to incisive foramen --lat. Cleft lip --cleft upper jaw --cleft b/w pri. And sec. palate www.indiandentalacdemy.com
â–ş 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 www.indiandentalacdemy.com
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