EMBRYOLOGY
Dr Marwa Magdy Saad Abbass
PhD Oral Biology
Phases of Intra-Uterine Life
The intra-uterine is divided into three phases
Phase I
The zygotic phase
From fertilization
till
2-3rd week
Phase II :
The embryonic
phase
From the 3-4th week
till
the 8th week
Recently the first 2 phases collectively
are called embryonic phase
Phase III :
The fetal phase
From the 9th week
till
delivery
Phase I: Zygotic (proliferation) phase:
(Fertilization-----end of 3rd week)
Fertilization is the
fusion of male and
female germ cells
(ovum and sperm) to
form the zygot
Then a series of mitotic cell divisions occur to
form a ball of cells that is called morula (32 cells)
Morula
Morula
The fluid accumulate inside the Morula and the cells
realign themselves to form a Blastocyst
5 days IUL
The blastocyst implants itself into the uterine wall
Embryoblasts
(inner cell mass)
Trophoblast
1ry yolk sac
Blastocyst
Bi-laminar embryonic disc (2nd WIU)
The inner cell mass realign themselves at the center and
differentiate into bi-layered disc
Lined by ectoderm
Amniotic cavity
L.S coronal section
Ectoderm
2ry Yolk sac
Endoderm
Lined by endoderm
Trilaminar embryonic disc
(Gastrulation) (2nd - 3rd WIU)
T.S
Top view
The embryonic disc has a pear
shaped appearance
Broad cephalic end
Primitive node
A groove called
Primitive
streak
Narrow caudal end
Trilaminar embryonic disc
(Gastrulation) (2nd - 3rd WIU)
Top view
T.S
L.S coronal section
The ectodermal cells migrate from the streak in a
lateral direction to form the mesoderm between
ectoderm and endoderm
Formation of the notochord (2nd - 3rd WIU)
L.S sagittal section
The ectodermal cells migrate from the streak in a
cephalic direction to form the notochordal process
between ectodem and endoderm
The notochordal process canalizes to form the Notochord
that support the primitive embryo
(Notochord is vertebral column of primitive animals).
By the end of the zygotic phase the embryonic disc
has a pear shaped appearance
T.S
L.S coronal section
Broad cephalic end
Amniotic cavity
Dorsal
aspect
Ectoderm
Mesoderm
Ventral
aspect
Endoderm
Yolk sac
Narrow caudal end
Notochord
Phase II: The embryonic period:
(3-4th week-8th week)
1- Folding of the embryonic disc in
an anteropotserior (cephalocaudal)
and in lateral direction
2- All organ and systems are developed
•Development of major external and internal
structures of the embryo
SO
Any maternal illness specially of viral origin and
drug therapy could cause congenital deformities
3-4th WIU
Development of the
Nervous System
Folding of the Embryo
The ectoderm at (the midline of the dorsal surface of
the cranial portion of the embryonic disc)
differentiate into
neuroectoderm
T.S
Amniotic cavity
Yolk sac
L.S coronal section
Amniotic cavity
Yolk sac
The neuroectoderm thicken to form
neural plate.
L.S coronal section
Non neural ectoderm
Neural plate
Notochord
The cells at the margins of the neural plate
proliferate at faster rate, forming raised margins
called neural folds encompasses a midline
depression called neural groove.
L.S coronal section
Neural folds
Non neural ectoderm
Notochord
Neural groove
The neural groove deepen Until the
neural folds fuse to form
neural tube
(that separate from the ectoderm and sink in the
underlying mesoderm).
A group of cells separate from the lateral aspect
of the neural plate
called neural crest cells
sink in the underlying mesoderm
Form the connective tissue in the head region
which is called ectomesenchyme
Neural groove
Neural tube
Folding of the Embryo
24 days -till the end of week 4
Folding in lateral direction
Changes the embryonic disc into tubular embryo covered
by ectoderm and lined by endoderm
Forms a head fold with the primitive stomodeum or oral
cavity beneath ;The stomodeum is lined by ectoderm
Head fold
Tail fold
Stomodeum
27
By the end of the embryonic period
Development of major external and internal structures
of the embryo occur
Phase III: The fetal period
(9th week----------birth)
There is a rapid increase in the overall size of the fetus.
9th week
Birth
Neural Crest Cells
Neural Crest Cells
As the nueral tube forms
The neural crest cells separate
from the lateral aspect of the
neural plate (neuroectoderm)
Migrate
Differentiate extensively within
the embryo
Migration of cranial neural crest cells
Anterior midbrain
FNM
E
Anterior midbrain
TG
Posterior midbrain
Posterior midbrain
E
Anterior hindbrain
TG
Md
Anterior hindbrain
E
TG
Md
Imai et al., 1996
Derivatives of Neural
Crest Cells
• Meninges
•Pigment cells
(Melanin cell)
• Spinal sensory ganglion
• Sympathetic neurons
• Schwann cells
All tooth structures
and surrounding
tissues except
enamel
Connective tissue of the
head
(Ectomesenchyme)
Avian neural crest cells
In the mammalian embryo
these stem cells separate
from the lateral aspect of the
neural plate rather than the
crest ; even though the term
neural crest remains
Clinical Correlation
Treacher Collins Syndrome is due to failure of
neural crest cells to migrate properly to the
facial region
defects of structures that are
derived form them
Stem Cells
Stem Cell
To be a stem cell, 2 requirements are needed
1- Self-renewal: ability to go
through multiple cell
division cycles while
remaining undifferentiated.
2- Potency: ability to
differentiate into
specialized cell types.
39
According to their Potency
Stem cells are
Totipotent stem cells Can differentiate into an entire organism.
Cells from early embryos (1-3 days)
Pluripotent stem cells .Can differentiate into any tissue except
placenta. eg. Embryoblasts
Multipotent stem cells Can differentiate into multiple cells of
related family.
Oligopotent stem cells Can differentiate into a few cell types
eg. lymphoid stem cells.
Unipotent stem cells
Can differentiate into only one cell type
eg. muscle stem cells.
40
Stem Cell Differentiation
6/15/2019
Princeton University
Dr. Hariom Yadav
According to the source
Stem cells are
Embryonic stem cells Derived from the inner cell mass of
blastocyst
-They are Pluripotent cell population
Adult stem cells
Found in developed organs and can divide
to give more differentiated cells
-Act as a repair system for the body
-Adult stem cells are multipotent
e.g Mesenchymal SCs, Blood SCs, Pulp SCs
Induced pluripotent Somatic cells reprogrammed through
genetic engineering to become stem cells
stem cells
42
Embryonic stem cells
Adult stem cells
Sources of oral
stem cells
PDLSCs
G-MSCs
DFSCs
Periodontal
ligament cells
Inner layer
Neural crest
ectomesenchyme
Dental
follicle
Outer layer
Cementoblasts
Gingiva
Alveolar bone
Dental
papilla
BM-MSCs
DPSCs and SHED
SCAP
Sources of oral stem cells
2
4, 5
1
6
3
7
1- DFSCs: Dental follicle stem cells,
2- G-MSCs: Gingival mesenchymal stem cells,
3- PDLSCs: periodontal ligament stem cells,
4- SHEDs: stem cells from the human exfoliated deciduous teeth,
5- DPSCs: Dental pulp stem cells,
6 -BM-MSCs: Bone marrow mesenchymal stem cells,
7- SCAP: Stem cells from the apical papilla.
THANK YOU