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A ‘Teach to Reach’ Initiative
CLINICAL EMBRYOLOGY OF THE EYE An Illustrated Manual 1st Edition Dr Pranesh Medical Officer Aravind Eye Hospital and PG Institute of Ophthalmology Coimbatore
PREFACE Embryology of eye is a difficult topic to master due to the obscure understanding of general principles and lack of application of the learned concepts in our routine practice. This handwritten manual aims to bridge the knowledge gaps & missing links - thereby presenting itself as a Complete Course on Ocular Embryology from Genes to Clinics. Highlights : ① Profusely illustrated with colourful diagrams and clinical photographs ② Flowcharts to aid understanding of complex derivatives ③ Every detail on individual structure is accompanied by relevant applied aspects ④ Must read for residents, practicing ophthalmologists and paediatricians
CONTENTS 1. General Embryology - Overview Basics 2. Ocular Embryology - Overview 3. Brief summary of Derivatives 4. Development of specific Ocular structures a) Optic Nerve b) Retina c) Retinal Vasculature d) Hyaloid Vasculature e) Vitreous f) Sclera & Choroid g) Iris & Ciliary Body b) Lens i) Cornea j) Angle of Anterior chamber k) Lids & Adnexa
with relevant
clinical aspects
5. Genetics - in Brief + Discussion on Coloboma 6. Big Picture + Summary repeated 7. References
GENERAL EMBRYOLOGY 3
Germinal Layers
I
b
Ectoderm t Surface
Endoderm
Mesoderm
Neuro
4 Neural
deum
Ectoderm
t
g
I No contributionto
oculardevelop
Mesenchyme
Note Contributes to Eye Development
Mesenchyme
Nut Mesoderm
NCC's contribution
Mesodermal Mesenchyme contribution
So for all practical purposes here
Ncc
Mesenchym
3 Layers formed
finishesKydd wk
0
Gastrulation T
Neurulation
formationof3germindlayes
Formationof neural
tube
cranial a pole cross
section
caifdalpole
Neuralcrest
T
Neuro
ectoderm
from neuro ectoderm
Neurulation
03
c
Neural Tube 3 Vesiclesformation
is
OCULAR Day22
EMBRYOLOGY
lsteventin eye develop
Neuro ectoderm
Tencephalon
Thickening of surface ectoderm
dotes
by 7thwk
OpticCup
if not closed
Inner layer Outer Layer
Coloboma
NeurosensoryRetina RetinalPigment Epithelium
migrating
Ncc x
cells Ncc
SUMMARY
must know
Optic
Nerve
Neuro ectoderm
Hyaloid
vessels
Central
Retinal Hall
Congenital OpticDisc Anomalies Congenital oD Pit
focal excavation
on
0N Head
Examine in
Discserous central
petinopMM
I
Serious detachment
If
1
pit
OD Dusen
focal collections of axonal metabolic products
Buried
dmsen
Pseudo papilledema
calcified deposits within ONH
Anomaly variant of Glory Morning oboma OD lol
Axial Coloboma
Geneinvolved PAX6 M
c systemic
Association
CNS disorders associated
FrontoNasal Dysplasia Neuroimaging required
Large disc
tunnel shaped excavation Ring of
Chorioretinaldisturbance
spokes of wheel pattern emerging vessels
of radially
discussed in the coloboma OD Details of
Last section of the manual
Myelinated Nervefibres Medullated Nerve fibres
progression
Lamina Cribwsa of Sdera CLA
of
starts from optic tract ends Lc myelination
t
if Oligodendrocytes migrate
into intraocular area
Abnormal myelination of Ganglion cell Axons b'yond LC
I
Optic N Hypoplasia
fed Nerve
f
snes
Uk
Squint
Bk
Nystagmus
I
de Morsier syndrome septo optic dysplasia7
7
Absent Corpus Callosum Septum Pellucidum
Bk on Hypoplasia Hypopituitarism
Reti na
RPE
n Neuro
sensory retina
inn
Edmond
d L
Marginalzone
TransientLayer of Chievitz
Macularitoveal Development Foreal maturation completes
4thmonth
Fixation
Reflex
I
if impaired develops
Nystagmus
Albinism eg.in
Retinal Vasculature
t
I
2nd
Temporal Retina
fully
Of
vascularised
by
I month after birth
1st
Nasal retina
fully Vascularised
by 8thmonth of GA
H i Incomplete Vascularisation of Retina in Preterm infants Vascularisation occurs in Low O2 tension in utero
RETINOPATHY of PREMATURITY
n't
In Preterm infants
02supplementation given for
lung
immaturity
902Tension
1
Growth of Peripheral Retinal Vesselsout off Retina thinks no need for vessels dueb ROD Oz removed
Ischemic response
t Leahy 4
new vessels
abnormal vessel production
fibrovascular
Retinopathy of Prematurity prop
Tissues
Retinal Vascular Anomalies
familial Exudative VitteoRetinopathy
I
1119 to
Rop
Tt failure of Xascularisation
of Temporal Retina
Screen the relatives of PE E FEUR
Hyaloid Vasculature
portionof
pinkaneural Hyaloid vessels
t ya
central
Retinal A'eV
aeagyeof
Tunica Vasculosa Lenh's
yfgawdue.se
TUL
3 components AnteriorVascular Capsule
Ant pupillary membrane Capsulopupillary portion
PosteriorVascularCapsule
Post Pupillarymembrane
refresses in Thrimestv Regressed PrimaryVitreous
t
Cloquet's Canal x
Details below
Persisting pupillary Membran
Mittendorf
Dot BergMeister papilla
t
ft
Bergmeister papilla
Mittendorf Persistent
fetalVasculature Retrolentalmass
persistent
HyaloidArtery
Vitreous
Vitreous Development t.de Pnimary
PrimiliveSeconday Definite.ve
Tertiary
ViFeousPwpev From Mesenchyme
of Ncc
From NeuroEctoderm
Surface Ectoderm
1
month
Vascular whenzolineous
fills cavity it
becomes Cloquet's Canal
2ndmonth
AVascular 1
fills the cavity fwmjtfyafoyt.es
7mm Neuro
Ectoderm
374
month
Becomes
Vitreous Basee
Ciliary2onules
Persistent Hypoplastic PrimaryVitreous
Persistent Fetal Vasculature
t
t
t
Persistent Posterior
Persistent Anterior Fetal Vasculature t Reholental fibrovascular Mass
Fetal Vasculature
t
Retinal Detachment
LEUCOCORIA 11
DifferentialDiagnosis Retinoblastoma
Persistent 7daL
Rop Retinal Dysplasia
congenital Cataract
Toxocara Endophthalmitis
Vasculature
Goat's disease Coloboma Choroid
ODisc
Persistent
fetal
Vasculature
Persistent Anterior Fetal Vasculature
Scleras choroid
Ncc
continues Anteninly
sclera choroid
Substantia
cornea
propria of
Ciliary Body
Posteriorly
Dura
Pia Arachnoid
SCLERA
T Tempyalsdera t
from Mesoderm mesenchyme
hotherparkofsdera
I from NCC Mesenchyme
Iris ciliary Body
Outerlayer of optic
up
Inner
layer of opticCup
In's D Mff
fromNeuwectoderm
Muscleof CB Ciliarymuscle
from NCC
Vascular mesenchyme Pupillary membrane
optic Cupmargin
Neuroectoderm
cm a ciliary muscle
CB Ciliary Body
Tm Trabecular Mesh SS
Iris
J Retina
scleralspur
schlemm'scanal 0
Cp Ciliary Processes Posterior Non Pigment
U
Epithelium
of developing in's
Attains melanin pigment Melanogenesis
Ut Becoming Posterior Pigmented Iris Epithelium of
Melanocytes
Ncc derived
FT
t
Neuwedndermal derived
t
choroid skinehair
RPE
f Race related
Epithelium't Irisa ciliary body
to
pigmented in all races
If
affected melanin production
d ALBINISM
Oaelocutaneous
Ocular
Oculocutaneous Albinism Tyrosinase negative complete til Not capable of producing melanin
Ilusinase Incomplete
Hariable Melanin production
c
Let us review
Iris developmen
again
PAX 6 gene mutation
Aniridia Autosomal Dominant
BIL condition
not complete absence of
misnomer
Abnormal Iris seen
as a
stump
of
hypercellular stroma
Leads to malformed outflow system
syndrome associated
L WAGR
syndrome
WT mutation
iris
Glaucoma 75 t
WIM'sTumor Aniridia abnormalities
Genitourinary Retarded mental status
Do 456 Abdomen for all cases of sporadic Aniridia to detect Wim's Tumor
Partial Aniridia
Total Aniridia
Associated features
of Aniridia
calaraff
701gal subluxation
Assoc Limbalstem
cell deficiency
Hypoplasia
Poor'Exation to Nystagmus
Lens
f
formed in 4th7kWh
Surface Ectoderm
Affected in
Rubella in 1sttrimester y
congenital
primam lens fibres
cataract
f
Equatorial LensBow
Embryonic
Nucleus
Lens Vesicle
d
v
Anterior epithelium
posterior
1
epithelium
t
equatortorms
secondary
1
Lensfibres
SLF
elongates
loses
formprimaylensmoki fibres
fetal Nucleus Infantile Nucleus Adult Nucleus Coney
1 Embryonic Nucleus H
central core
i oldestfibres
SLF
grows anteriorly posteriorly
Feet t
y sutures
anteriorly straight'Y posteriorly
suture dotted area
inverted1
iii
i i
CONGENITAL CATARACT
t
I
I
43rd
43rd
H
y
Assoc I
Notassoci systemicillness
t
Nonsyndromic
Metabolic disorder
Hereditary
Intrauterine Infections
1
Syndromes
Human
gamma Erystallins intens
tens proteins Genes encoding crystalline mutations 1
Cataract duets
I
protein misfolding
Assoc E l
t Metabolic disorders
Galactosemia
Intrauterine Infections
Syndromes
GPUT enzyme deficiency
oil droplet cataract
Lowe syndrome
t
Galactose 1 Phosphate
Luridyltransferase
Oculocerebro renal syndrome
XLR
Fabry disease Mannosidosis
deficiency a mannosidase
Assoc E l
EitEfinne syndromes
Moffat
b
TORCH Toxoplasma
es
syphilis Herpes
RIbeyatcytomg.gg
Congenital
Rubella Syndrome
Gregg's syndrome
H Classic Triad
Dense Nu ear cataract
Rubell
Retinopathy
Congenital syphilis saddle
shaped Note
Clutton's Joints
SabreTibia
notched smallteeth Hutchison's
Teeth
Interstitial
keratitis 1
Sensorineural
Hearing
Loss
HUTCHISON'sTRIAD
Congenital Toxoplasmosis SABIN's TETRAD
Cartwheel
Madison
Intracranial
calcification
Mental Retardation
Hydrocephalus
microcephaly
Syndromes t Downs
Trisomy21
t Edwards
Trisomy18
t
Patan
Trisomy 13
Down's syndrome congenital cataract
flat nasal bridge Brushfield spots
1
Y
focal areas of stromal
Tissue Hyperplasia Upslanting palpebral fissure epicanthal folds
Got Hypermetropia
error
Mc
Refractory
M
Typeof Astigmatism
c
M c Typeof strabismus
NLD obstruction
Oblique
Esotropia
common
Defect information of Lenscapsule
H
Lenticonus
Anterior
1 Assoc
c
ALPORT's SYNDROME
Posterior
t Assoc E
Lowe's
SYNDROME
t microspherophahia He
smaller diameter
t microphatia
shatter diameter f in Lowe's
spherical
A
Risk of Pupillary Block Angle
closure
Weil Marchasani Marfan's Hyperlysinemia Peter's Anomaly
syndrome
Ectopia Lentis t congenital t
EL
J
t
Acquired t
Post Traumatic
Marfan's
Weil Marchasani Homocystinuria
Ehler Danlos syndrome Sticklersynd
Most common
oath
2
manifest
Retinal
Detach
Inferonasal EL micyspherophahia
Superoptemporeaf
Homotustinuria
Inmate.IT
pYanfmidsi'sIIIan
ui
eg
Homocystinuria
Inferonasal EL
Marfan's
Weitmarchasani Syndrome
BIL
Inferior EL
Superolemporal
EL t
Zonulav Disintegration
Intactanules
t
Microspherophab
40 Accomodation IntactAccomod 2 Matanoid features t
Thrombo
embolism
MentalRetard
Tall.thin Arachnodactyly
GothiclHighanhed palate
Tnt'Laxity
t pupillaryblouk glaucoma
Opposite K marfans
I
short fingers
shortstature
Stickler syndrome Hereditary Arthro
3main features High myopia Vitreous Retinal degeneration 4
RD
Cataract
Ophthalmopathy
3
3
waves L
1st
wave
waves
of NCC migration bln Lens
1
3rd
2nd wave
Endothelium of cornea Descend's memb
surface ectoderm
1
v
t
Ref
of NCC
t
t
Shoma of cornea
stoma of Iris pupillary
AAO 2019 woo
wave
membrane
some books say 2 l 3rd
wave
YnaeaYs mmas
in's
cornea
t
t
1
other layers
Epithelium t
t
from NCC
from surface Ectoderm
corneal Transparency 1
due to maturation of corneal Lamellae but failure of collagen
heratocytes
fibres
in
to
produce
Lamellar
t
Scleralisation of
cornea
Sotero cornea
Array
moderate Sdewcome a
severe Sdew
cornea
Differentials for Congenital Corneal Clouding
clew cornea Tears
in Des cemets
post Trauma
Iker M ucopolysaccharidosis Metabolic disorders
Peder's Anomaly
Posterior axialstromal defect
Edema CongenitalHereditary Dermoid
EndothelialDystrophy
Microcornea
us
Megalocornea
t
t Horizontal corneal diameter Elomm
in adults
Horizontal corneal
diameter 213mm in adults
f
Cor
9mm in newborn
Nole
corneal diameter 10mm 1
Neonates
12mm
Assoc Pigment
2 yrs till Adult
pension
Syndrome
I
s
a
Anterior Chamber
Posterior Chamber
pig
see
Primo
men
E ftp.eo.i.mesenchsylimteihbln developing in's Hens posterior
chamber
Pc
me
Trabecular Meshwork corneal endothelium
elopingcornea
Dgeveloping
Ncc Mesenchyme
e iieiine
t.fi Ac
CollectionFest
of
undifferentiated Ncc mesenchyme
t
Trabecular Anlage b
mu mean eY
Cinema
t
it
a
Trabecular 13Wh
Anlage
0
capillaries
y Trabecular
Ant chamber
to Pupillary
membrane
cells
Schlemm's canal Sc 22Wh
giant Vacuoles in SC
Enlarged Connective
Tissue
Inter Trabecular spaces
LTrabewfaveys
Schemm's Canal
me.sentiemme
Fm
from Ncc mesenchyme
Deep ATM collection of small capillaries fuse to form
single
elongated
slit likevessel
CANAL of Sc LE nm
from Mesodermaf Mesenchyme
vessel vacuoles
w
Endothelial cells
Trabecular Anlage
t
Trabeculen Meshwork Develops into x
3pqHs I
Innermost H Unreal Meshwork
l
Middle
H
Comeosderal Meshwork
I cord like
Lamellar
1
Outermost
Hi
Juxta canalicular
f Cribriform Zone f Loosely
arranged
Schlemm
Tuxtacanaticular Meshwork corneosderal Meshwork
fffcandoc
j 99
saved meshwork
Tn
Er Microscopic Anatomy of Tnt in Adults
GONIO DYSGENESIS Maldevelopment
of
Iridocorneal Angle
t
t
Trabeculedysgenesis
Fido
only
Isolated TM
corner
dysgenesis maldevelop
t
a
Impaired Aqueous outflow W o other major ocular abnormalities
I PRIMARY CONGENITAL GLAUCOMA
Mostimportantgene CYPIBI
1
Axenield Riegersyndrome
2
Peter'sAnomaly
3 Aniridia SECONDARY CONGENITAL
GLAUCOMA
Primary
Congenital Glaucoma
Triad
1 Epiphora
2 Photophobia
D Blephawspasm
I HAAB's STRIAE cut'rvilinear
healed breaks in Descend's membrane
Axenfeld Riegersyndrome to Axenfeld Anomaly
Riteger
Anomaly
f
4 Rieger syndrome
Reiger Anomaly Extraocular
1
prominenteanteriorly
manifestations
displaced
Schwalbe's line _posterior
f
Embryotoxon
peripheral
iris strands extending into cornea
Rieger's Anomaly mimics In do cornealEndothelial
ICE syndrome
Rieger's syndrome
Telecanthus
BroadNasalBridge MaxillaryHypoplasia
Dental Anomalies microdontia small teeth
Hypodontia
fewteeth
Peter's Anomaly
2
1
Corneal involvement
only Type
I
Corneatlens Type 2
Hallmark Central defect in Descemets's Endothelin Thinning opacified corresponding Shona
Central corneal opacity
Ii docorneal Adhesions
Lenticulo corneal Adhesions
Eye Lids
fusion of Lids complete
GA
10wks of
mesoderm
yureof 1
Tarsal
plate
lqwksremainfusedtill28thwhlSacendosedbyfusedlidsft.y
Conjunct sac
Corneal Epithelium
lined
I
by ectoderm
r
cornea
Note
failure of I
Lidformation
t
t c
Lidfusion
Lidseparation
CotoBomA
CRYPTOPHTHALMOS
Assoc
T
Fraser
Syndrome
Away BLEPHARON
I
LID
COLOBOMAS
2 syndromes Treacher Collins H
Lower Lid coloboma details given later
Goldenhar H Upper lid coloboma
1
Triad
Peribulbar dermoid Pre auricularappendages Pre tragal fistulas
Goldenhav syndrome
limbaldermoid
Preauriculattags
Epicanthus in versus
foldof skin extending from lower lid to medial 1
canthus
1
Assoc I
BPES
Blepharophimosis
Ptosis Epicanthus inversus syndrome
Telecanthus f
Bendohyperklorism
Abnormally ton
medial
canthallIndons
I fed distance bln medialcanthi
f
TInnercanthal distance
OCD
IPD
us
Hypertelorism 4
wide bonyseparation of orbits
f
orbital Hyperklorism I
manifestation of
a
craniofacial deformity TICD OCD IPD OCD Outer Canthal Distance
IPD Inter
pupil Dist
ICD Inner
canthd pistons
edetainthus Hyterlelorism
Orbit
contents
Striated ExtraocularMuscles
Bones Tendons Intermusadarseplae ConnectiveTissue AdiposeTissue
Neural CrestCells
Note All bones
are membranous except
cartilaginous
sphenoid
3rdmonth
Ossification
2
of GA
f 7thmonth
7
fusion
Mesoderm
Congenital Craniofacial Abnormalities
f
t Non synostosis
craniosynostosis
t
t
Branchial ArchSyndrome 4
Premature fusion of
Cranialsutures
failure
of Development of 151 2 Branchial Arches
f
I
3 syndromes L
2 syndromes
I
Aperisterouson Pfeiffer
Treacher
Goldenhar
coins
t Mandibulofacial Dysostosis
i
nYdD
Apeitsyndrome most
severe
craniosynostosis
mid facial Hypoplasia
Parrotbeaknose
syndactyl
oxycephaly
CConicalskull Lowsetears
Gougon Syndrome Midfacial Hypoplasia shallow orbits
Proptosis
Hypertelorism
Shot anlocoposterior Diameter of Shul
Pfeiffer syndrome a
cloverleaf shaped
skull
1
Exorbilism spontaneous profusion of eyes
due to shallow orbit
Treacher Collins Synd Downward slantingeyes
Bk
Hypoplasia
of mandible
Zygoma
Lower
Lid
colobomas
Punctal Atresia
Lacrimal Gland NasoLacrimal Drainage
surface Ectoderm
LacrimalGland go.sepittdialbudsfwmfoyyngngn.nl
Nonfunctioning till
bwnsglik
Newborn don't tearup when
they cry
Congenital ALacrimia ltypoplastiahbsI.tljyeeye.inyoung
Lacrimal Drainage
Vaso lacrimal Groove
blnfwntonasal Maxillary process
Isolated cavities
solidepithelial cord
5 swhsot GA
complete
canalisation
4h7 9thmon
NasoLacrimal Duct patency
1
completes
monthof 7th by life
H Incomplete canalisation of NLD
1 Congenital
NLD obstruction
th MC
c
Imperforate Hasner's Membrane
Value of Hasner lowerendof Last part to canalised
NLD
Epiphora in Newborn
t Congenital Is
L
l
Acquired
EffITgenimeDaoyogstowe NLD
Punto
IngenitalNLDObslmoti
H
Agenesis
Atresia
Fistula
t
Eanalicular Agenesis
Atresia
Most common cause of Epiphora in
Newborn H
Non Patent NLD
Competent Valve
of Hasner
Collection of Amnioticfluid Mucus in Lacrimal sac
Amniontocede Congenital Dacryocele
perinatal bluish cystic swelling
or
below medial canthus
DD
Encephalocele
k
above medial canthat
tendon
GENETICS Master Gene in
developmentof Eye
PAX 6 gene mutation
PAX G
Paised Box 6gene
t
Coloboma Microphthalmia Peters
t
Anomaly
Generesponsible for Division of Eyefield into 2 eye Globes SHH Soni'T Hedgehog
PAX6 expression
PAX
G
switch
for eye development
D
D
Holo Prose N CEPHALY
Defect in SHH
Gene
Proboscis
Cyclopean
fSyn
Eye
ophthalmia7
ANOPHTHALMIA failure offormation of opticVesicle to
attain e
any
visible globe structure
MICROPHTHALMIA to Complex
simple_Pure
f
Nanophtralmos
t
Structurally
Associated features
eyeball
of Dysgenesis
short Axial length
oboma eg.co orbital cyst Bllolobomas
Orbital
cyst
COLO BOMA
Mutilation
PAX 6 gene mutation
Chtoidal
1
fissure hot closed Normally closes by 7th week
Coloboma
Isis ciliary
Body 2onules
H
t
Retinae choroid Recoloboma
optic Disc x
µ
ODpit
Axial Coloboma of OD
MORNING Glory
SYNDROME
M.c.sie Inkronasalheyhole defect of iris Typical
Coloboma
Calamitous lens
Hq
keyholedefectinin's
I
ymissingzonules
2ondaddoboma
Types I
Typical
It
Inferonasal quadrant closure fissure choroidal site
of
Atypical other areas macular'Toloboma
is also atypical Atrial
Complete
fulllhickness
defect
E Incomplete
Partial thickness defect of epithelium stroma
Retinochoroidal Globo ma
r Retinalvessels down
dip
Excavated area Exposing
yo RPE
bare sclera
Edges dean cut
Choroid
Histopathology Inner Nyagkey ppenfierarm
SplitinNeurosensory Retina
Layer
a
ignerlayer
Intercalary membrane
7 t.TT
sFsRPElchaoid
outer layer fusey.si RPE
Termination
Disorganised mass
of choroid
H Junction Locus Minoris Resistentiae
Pathophysiology fetal fissure closure
Defective
t
Inner Layer of opticCup growsfaster than outerLayer future RP E
f
Eversion 11
Gradual displacement of RPE t
Double Layer of photoreceptors
facing
each other
Absent RPE
1
Absent choroid
Ida Mann
Classification
AboveDisc
superior bogda
Below disc
Nimue area of
bn
Retina OD Globo ma
Isolationism
J so
pigmendfoftwbana
extreme
Classification of RC Globoma Gopal Lingam 19967
NO Disc
I
OD is outside fundus coloboma
Abnormal Disco
I
I I
Colobomalous Disc
isolated
Disc
OD is within Fundus coloboma
I
ColobomatousDisc 4
I
Disc shape not identified Blood vessels emanating from superior aspectof large fundalwlobom
Classification Gopal Lingam
I
I
o
E
x f
I
r
f
I
abnormal
TI
I
I
r
TI
III
I
I
Visual Acuity Good myopia High
Microphthalmos
common
OD Colo boma focalglistening while bowel shaped excavation
Inferior NRR
1
Absent
Thin
Assoc O D pit
Association
of
Hoboma
Rhegmatogenous
RD
CHARGE syndrome
I
401
CHD7 geneinvolved
Edoboma HeartDefects
Atresia choanae Retarded growth
Genital hypoplasia
Ear malformation
AICARDI syndrome WARBURG syndrome
Note
Lens Coloboma
Misnomer CB
coloboma
I Absence
ofwhales
1
Indented Lens
BIG PICTURE
orbit
Efm
NYids
migrating
Sark
Ectoderm
sypienophab
1
i
II
NTeuralabe
ftp 7Iweloph.g
mean Lens Hyaloidvessels
Neuroectoderm
Future Retina
Neuroectoderm
Neural crest Mesenchyme surface Ectoderm mesodermal Mesenchyme
SUMMARY
Go throughagain
the derivatives been of Summary Repeated to have the
the of understanding
final
Big
Picture
References
For relevant video based discussions on embryology and more topics Visit YouTube.com/kingsclasses