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EARLY EMBRYOLOGICALDEVELOPMENT
Oral Histology
Dent 206Dr Ashraf Shaweesh
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Intra-uterine stages
Pre-embryonic (proliferative period) 0 2 weeks
Fertilization
Implantation
Bilaminar embryonic disc
Embryonic 2 8 weeks
Different types of tissues develop
Formation of organ systems Fetal
8 weeks until birth
Increase in body wt & size
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Pre-embryonic period (1st week)
Fertilization
zygote, 0.1-0.2 mm
Cleavage (Mitotic) divisions
2 cell (blastomere) stage
4 cell stage
12-16 cell stage (morula, 0.1-
0.2mm) Blastocyst
100-150 cells, 0.1-0.2 mm
Blastocoel
Zona pellucida
Inner cell mass embryo proper
Outer cell mass
Future trophoblast
Attachment
Implantation
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Pre-embryonic period (2nd week)
Bilaminar Embryo
Inner cell mass becomes 2 layereddisc (7-12 days) Embryonic Epiblast
Columnar cells
Face the cytotrophoblast
Future ectoderm
Embryonic hypoblast Flattened cells
Face the blastocoel
Future endoderm
Amniotic cavity
Exocoelomic membrane Continuous with endoderm
With endoderm enclose:
Primitive yolk sac (exocoelomic cavity)
Still 0.1-0.2 mm
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Bilaminar embryo
Extra-embryonic
mesoderm
From
cytotrophoblasts
Secondary yolk sac
Connecting stalks
Placental circulationestablished
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Embryonic period (3rd week) Prochordal (cephalic) plate
Slight thickening of endoderm
Indicates the future head end
Buccopharyngeal membranelater
Caudal end (cloacal
membrane) Intra-embryonic mesoderm
Appears from ectoderm at 17days
Rounding up toward caudal
midline Spreading between ectoderm
and endoderm
Formation leaves
Primitive streak
Primitive (Hensens) node
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Trilaminar embryo (Gastrula)
Three germ layers Ectoderm
Mesoderm
Endoderm Primitive streak & node
Notochordal process Blind-ended tube
From primitive node
Up to the prochordal plate
Progenitor of the backbone andthe vertebral column
Mesoderm separates
ectoderm & endoderm except in Prochordal plate
Notochord
Cloacal membrane
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Germ layers
These cells are considered pluripotent: each is capable ofproducing descendants representing all of the hundreds ofdifferentiated cell typesEctoderm Mesoderm Endoderm
Skin and appendagesBone & Muscle
Lingual tonsils
Oral and anal mucosa Connective tissue Linings of lungs
Linings of nose and
sinuses
All dental tissues
except enamelDigestive system
EnamelLymphatic tissue &
spleen
Linings of excretory
system
Nervous systemBlood cells, heart &
lungs
Pituitary & mammary
glandsReproductive system
Lens of the eye Excretory system
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Notochordal process
From the primitive knot,mesoblastic cells migratetoward the prochordal plate
Acts as a template for thenotochord
Cannot go through theprochordal plate
Some cells migrate aroundthe prochordal plate(cardiogenic area)
In prochordal plate, theembryonic endoderm andectoderm layers are fused
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Further development
Notochord Fusion with endoderm
Rounding up and separationfrom endoderm
Mesoderm
Paraxial mesoderm (Somites) Pairs on each side of
notochord
Cuboidal masses, mould theectodermal surface
42 45 pairs by the end of 5thweek
Intermediate mesoderm Urinary system
Adrenal cortex
Much of reproductive system
Lateral plate mesoderm
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Further development
Lateral plate mesoderm More widespread than
somites
Spreads cephalic (ahead) toprochordal plate
Intraembryonic coelom Forms by coalescence of
vacuoles within lateral platemesoderm
U shaped
Anterior part Primitive pericardial cavity Cephalic (ahead) to primitive
pericardial cavity lies what willbecome septum transversumin which liver later develops
Lateral part is primitiveleural & eritoneal cavities
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Somites
Differentiation of somites
Dermatome
Dermis of the skin
Lamina propria of oral mucosa
Myotome
Vertebral musculature
Intercostal musculature
Some limb musculature
Sclerotome
Vertebrae, ribs and sternum
Head somites Prootic somites (3 pairs)
Myotome - eye muscles
Metotic (occipital) somites
Myotome of 3rd -6th metotic somites tonguemuscles
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Neurulation
Formation of CNS
Neural plate: thickened mass forms inthe overlying ectoderm. and becomesknown as the
Neural groove
A crease or fold soon appears in thisplate
Rapidly deepens - precursor of theembryos CNS, the first organs todevelop
Neural folds: arch over and fuse witheach other at several points along thelength of the neural tube
Neural tube: zippered closed as bythe neural folds, concomitant with thebudding somites
Anterior & posterior neuropores
The entire embryo is lengthening asthis happens
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Neural crest
Junction of neural platewith ectoderm
Unite then pinched off as
neural tube separatesfrom ectoderm
Neural crest cellsmigrate within
mesoderm
So far embryo is 1.5-3
mm
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Neural crest
Ectomesenchyme tissue in head region Dermis of head region
All dental tissue except enamel
Branchial arches Skeleton
Part of musculature Pigment cells
Melanocytes
Meninges
Spinal & cranial nerve ganglia
Sympathetic & parasympathetic systems Adrenal medulla
Schwann cells
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Folding of embryonic plate
Embryonic plate bulges upwards into amniotic cavity Folding turns the plate into a portion of a spheroid
Causes Most of growth happens in the upper surface
Neural tube growth exceeds that of the rest of the embryonicplate
Consequences Buccopharyngeal & cloacal membranes
Folded under cephalic & caudal ends, respectively
Their ventral surfaces become dorsal
Part of the yolk sac becomes incorporated in the embryo asforegut, midgut and hindgut
Primitive pericardial cavity lies beneath the foregut
The most cephalic lateral plate mesoderm (septum transversum)lies caudal to the pericardial cavity in which liver will form
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Folding
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Development of epithelial
structures
Epidermis surface ectodermal cells
Dermis underlying mesoderm of somites
Ectodermal cells thicken into 4 layers by 11-12
weeks Basal layer superficial layer of epithelium
Melanocytes invade epidermis
Structures developing from a combination of
dermal and epidermal tissues Mammary, sebaceous, salivary glands
Teeth, nails, hair
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Development of connective tissue
CN develop from somites as migrating fromeither side of neural tube
Somite Sclerotome
Medial portion mesenchymal cells Osteoblasts, chondroblasts, fibroblasts
Myotome Skeletal muscles
Smooth muscles & mesenteries
Dermatome Dermis
Visceral mesoderm lamina propria of GIT
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Development of cartilage & bone
Cartilage is the initial skeletal component Functions in supporting the soft embryo
Maintains its 3-D configuration
Cartilage migrate to surround notochord froming spinalcolumn
Cartilage growth Appositional new layers on the surface
Interstitial proliferation & expansion of cells
Hyaline elastic fibrous cartilage
Bone formation Intramembranous
Endochondral E.g. long bone
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Endo-chondral bone Formation
Deposition of bone matrix on a pre-existing cartilage matrix
Mesenchymal tissue Cartilage Bone
The primary transitional
cartilage is a hyalinecartilage whose shaperesembles a smallversion of the bone to beformed
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Intra-membranous Bone FormationDirect mineralisation of matrix secreted by osteoblasts
Mesenchymal tissue
(Condensed)
Bone
E i h l th
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Epiphyseal growth
Endo-chondral oss i f icat ion in a long b one Intra-membranous bone collar forms
within the perichondrium of the cartilagemodel
Cartilage degeneration (by hypertrophy)and calcification starting at the centralportion of diaphysis
Blood vessels penetration bringingosteoblasts
Continuous primary bone deposited overcalcified cartilage
Calicified cartilage resorbed by giantmutinucleated cells
Primary ossification center
Secondary ossification centers at the
epiphyses in a similar pattern In secondary ossification centers
cartilage remains in 2 regions The art icular cart i lage
Protection and mobility
The epiph yseal plate Growth until closure at 20 ys
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Development of muscle
By 10th week, myoblasts migrate from
myotomes
Muscle
Skeletal
Smooth
Cardiac
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Development of CVS
Originate from angioblasts
Angioblasts are from angiogenic clusters in the walls ofthe yolk sac
Angiogenic clusters Outer cells elongating tubes Inner cells blood cells
Nutrition of embryo At first - vatelline vascular system
Then umbilical vascular system
Hearts beats by 4th week Starts as a tube internal partitioning
An opening between atria remain until birth