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H_N Embryology (Week1/2/4) Early Development / Embryonic Period / Foetal…
H_N Embryology (Week1/2/4)
Early Development / Embryonic Period / Foetal period
Pharyngeal Arches
Lateral aspects of pharynx
Arches (Meso)
Skeletal Structure
Cartilageous Precursor
1: Mandibular Arch -
Meckle's Cartilage
(
CN5
)
Anterior
Posterior
Malleus
Incus
Adjacent Tissue
Mandible
Muscles of mastication
2: Hyoid Arch -
Reichert's Cartilage
(
CN7
)
Stapes
Styloid Process (Temporal Bone)
Hyoid Bone (Lesser horns & Upper body)
Platysma
Muscles of Facial Expression
Arch II overlaps other arches Caudally
3: Thyrohyoid Arch -
CN9
Mesoderm: Carotid
4:
CN10
Mesoderm: Aortic Arch
6:
CN10
Mesoderm: Pulmonary Arteries
Cleft/Grooves (Ecto)
Diverticulm
Glands / Duct / Canals
2: Disappear
Arch 2 overlaps
Forms a space called
cervical sinus
3: Disappear
Arch 2 overlaps
Forms a space called
cervical sinus
4: Disappear
Arch 2 overlaps
Forms a space called
cervical sinus
Epicardiac Ridge
1
Epithelium of External Acoustic Meatus
Tympanic Membrane
Cervical Sinus
Squamous Epithelium
Cyst
: Fluid-filled Closed Sac
Fistula
: Connection between 2 Sacs
Branchial Cyst
Only with inside
Pharyngocutaneous Fistula
Connects with both inside and outside
Allows saliva to run out during mastication
Pharyngeal Fistula
Only with outside
Pouches (Endo)
Diverticulm
Glands / Duct / Canals
Migration
: Generally
Cadual
Ventral
4 + 5
Ventral
C-cells Parafollicular (calcitonin)
Dorsal (Migrates)
Superior Parathyroid
3
Ventral (Migrates)
Thymus Gland
(Primordia)
Dorsal (Migrates)
Inferior Parathyroid
Both migrate caudally
2
Ventral (Stays)
Palatine Tonsils
Dorsal (Nil)
1
Ventral (Nil)
Dorsal (Stays)
Eustachian Tube
Mastoid Antrum
Middle Ear
General Principles
Primordia
Becomes
Organs
Size
Shape
Site
Structure
Diverticula
Becomes
glands
Remains a tube
Clinical Implications
Commonly during 4th - 8th Wk
Genetic
Nutritional
Chemical
Viral
Irradiation
Development
Tongue & Thyroid
Ventral Wall / Floor of pharynx
References to arches
Tongue
Mesoderm
Endoderm
Bulge on pharnyx
Congenital Malformations
Ankyloglossia
Program cell death on ventral (insufficient)
Persistent median lingual swelling
Cleft Tongue
: Incomplete fusion of LLS posteriorly
Bifid Tongue
: Complete fusion failure
Thyroid
Endoderm
Primordium
Endodermal diverticulum
Between 1st & 2nd Arch
Congenital malformations
Thyroglossal Cyst
/
Sinus
/
Fistula
Remanent of thyroglossal duct
Fail to degenerate
Midline of neck / Hyoid bone
Lingual / Cervical Thyroid
Very rare
Fails to descend
Fully functional
Lingual
: On foramen caecum
Palate
Secondary
: Fusion of maxillary process
Horizontal projections from both sides
Primary
: Fusion of medial nasal process
Triangle anterior
General Timeline
(Embryonic Age)
Foetal Period (21st Wk - Delivery)
Embryonic Period (Fertilization - Wk 8)
Early Development (Wk 1 - 3)
Fertilization
Unicellular Zygote
Multicellular Zygote
Morula
to
Blastocyst
Tri Laminar Germ Disc (Wk 3)
Folding
(4 directions)
Endodermal tube
(Primitive Gut)
Endodermal tube
(Primitive Gut)
Germ Layers
Ectoderm
Mesoderm
Bones
Muscles
Connective Tissue
Blood Vessels
Endoderm
Diverticula
Glands
Endocrine
Exocrine
Remains a tube
Blastocyst
Embryoblast
/Inner Cell Mass:
Primitive Endoderm
(Amniotic Sac; Protection)
Epi-blast
(Endoderm/Mesoderm/Ectoderm)
Trophoblast
:
Combines with endometrium to become placenta (Organ)
Morula
: Solid ball of cells
4th - 8th Week
:
Early Embryo
Pharyngeal Arches
First seen
Gestational age: wk 5
Embryonic age: wk 4
Collection of Primodia
Positioning
:
Dorsal
Ventral
Cranial
Caudal
Primitive Pharynx
Tongue
Tonsil
Eustachian Tube
Thyroid
Thymus
1) Lateral lingual swelling: Fuses together
2) Median lingual swelling: Overgrown by LLS
3) 3rd
arch component
hypobranchial eminence overgrows 2nd
4) 3rd arch component joins with LLS; (Sulcus Terminalis)
Anterior 2/3
: Lateral lingual swelling
Poster 1/3
: Hypobranchial Eminence (Cranial Part)
Muscle
: Caudal part of brain CN12
5) Programmed cell death frees tongue
A) Thyroid diverticulum connected to tongue by thyroglossal duct
B) Thyroglossal duct becomes foramen caecum
https://discovery.lifemapsc.com/library/review-of-medical-embryology/chapter-54-the-face
https://discovery.lifemapsc.com/library/review-of-medical-embryology/chapter-55-development-of-the-palate
https://discovery.lifemapsc.com/library/review-of-medical-embryology/chapter-56-congenital-malformations-of-the-lip-and-palate
