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EmbryologyPRESENTED BYPRESENTED BY
HANAA ALY
PROFESSOR OF ORAL BIOLOGYPROFESSOR OF ORAL BIOLOGY
ALEXANDRIA UNIVERSITYALEXANDRIA UNIVERSITYProf. Hanaa Aly
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It is the study of growth and differentiationwhich an organism undergoes during its
life from a sin le cell to a com lex
independent living being.
Development of individual is divided into :
A)Prenatal Development B) Postnatal Development
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A- Prenatal DevelopmentIt can divided into 3 periods:
The proliferation period
The embryonic periodThe fetal period
B- Postnatal Develo ment
It is divided into six periods :
i) Neonatal period
ii)Infancy
iii) Childhood
iv) Puberty
v) Adolescence
vi) Early adulthood Prof. Hanaa Aly
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First week: In the 1st week fertilization takes
place, the morula forms and is converted into a
blastocyst
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The blastocyst consists of embryoblast,
and the trophoplast function?
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Abnormality in number of chromosomes iseither increase or decrease in number
decrease in one chromosome is called
monosomy which is usually lethal .
Increase in number is called teratogenic
and an example for trisomy is Downsyndrome ,Mongolism
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Genetic Abnormalities
Examples of Autosomal Dominant Genes are
1)Achondroplasia
2)cleidocranial dysostosis
3) Mandibulofacial dysostosis
4)Dentinogenesis imperfecta
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Second week:
morphologic changes occur in the inner cell mass.It differentiates into a plate
called the bilaminar embryonic disk or germ
formed of two layers:An endoderm layer consisting of cuboidal cells facing the blastocele
and an ectoderm layer of columnar cells
adjacent to the endoderm and facing the trophoblast
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prochordal platethickened area ofendoderm
oropharyngeal membrane The prochordal plate fuseswith the overlying embryonic ectoderm
Two embryonic cavities are
The u er cavit or the amniotic cavit soon
becomes lined with amnioblast derived fromthe ectodermal layer which forms its floor.
The lower cavity or (the secondary yolk sac)becomes lined with endoderm developed from
the migration of cells at the periphery ofendodermal layer which forms its roof
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Third week: IntraembryonicMesoderm Formation:
groove appears in the midline of the surface
of the ectoderm caudally, theprimitive groove.The end of this groove cranially deepens to formtheprimitive pitand the elevated margin around the pit iscalled theprimitive node.
The entire structure is called theprimitive streak
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Cells from the primitive streak migrate between the ectoderm and
endoderm layers to form intraembryonic mesoderm. The mesodermal cells migrate anteriorly on each side of the
midline until they meet at the cephalic end of the embryonic diskin front of the prochordal plate.
m gra e a era y un ey ecome con nuous w e
extraembryonic mesoderm The mesoderm, therefore, exists between the ectoderm and the
endoderm everywhere Expect
at the prochordal plate (the oropharyngeal membrane) craniallyand at caudally, the cloacal membrane (tail end).
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Induction ?The first inductive event occurs on day 18
neural plate: a thickening of
ectoderm dorsal to thenotochord. It is oval in shapewhen viewed from theectodermal surface.
neural fold neural tube
anterior or rostral neuropore
posterior neuropore. These
pores close during the 4thweek to establish the centralnervous system (brain andspinal cord)
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neural crest cells
multipotent ectomesenchyme
mesenchyme produced from ectoderm which formsp nen v n e . e e e e en
ganglia and the sympathetic neurons, meninges, Schwanncells and pigment cells, most of the mesenchyme of thebranchial arches, face and teeth.
if they fail to migrate they result in a syndrome calledmandibulofacial dysostosis which is a congenital
malformation ?
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Fate of the Three Germ Layers
ectoderm will give rise to the nervous system and the sensory epithelium of the eye, ear, and nose, the
epidermis of the skin and its appendages (hair, nails and glands), the surfaceepithelium lining the oral cavity, the nasal cavities and the sinuses, the parotidsalivary glands and the anterior lobe of the pituitary gland, the surface epithelium
endoderm gives rise to the surface epithelium lining the gastrointestinal tract and all associated glands,
including the submandibular and sublingual salivary glands, the surfaceepithelium lining the pharyngeal mucosa, the soft palate and the posterior 1/3 ofthe tongue.
mesoderm gives rise to
the muscles including the muscles of the tongue, all the structures derived fromconnective tissue e.g., dermis of the skin, lamina propria of the oral mucosa, bone,cartilage, blood, dentin, pulp, cement and periodontal ligament.
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Suggested Reading
Avery, J. K. (1997) OralDevelopment and Histology, 3rd edition St Louis: Mosby Company.
Bhaskar, S. N. (ed) (1991) Orban's Oral Histology and Embryology , 11th
Edition St, Louis; Mosby Year Book, Inc
Lersen, W. J. (1998)Essential of Human EmbryologyEdinburgh: Churchill Livingstone.
Prof. Hanaa Aly