1.1early Embryological Development Shortened 150212

7/28/2019 1.1early Embryological Development Shortened 150212

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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

Documents

1.1early Embryological Development Shortened 150212