Download the neural crest cells

Embryology
What is embryology?
• Embryo is a Greek word “Embryon”= unborn
• The study of developmental events that
occur during the prenatal stage .
• The branch of biology and medicine
concerned with the study of embryos and
their development.
INTRODUCTION
• The human somatic cell contains 46
chromosomes, called as the diploid number.
Out of which 44 are autosomes and the
remaining 2 are sex chromosomes,
designated as X and Y.
• The sex chromosomes in females are XX
and in males are XY.
• There are two series of division of somatic
cells- MITOSIS and MEIOSIS.
• MITOSIS produces the same number of
chromosomes in the resulting daughter cell
while MEIOSIS produces half the number i.e.
23 designated as haploid, with resultant
formation of gametes .
• Development begins with FERTILIZATION,
the process in which the male gamete- the
sperm, and the female gamete- the oocyte,
unite to form a ZYGOTE.
Embryonic period vs. Fetal
period
– Embryonic – first 8 weeks. Development of the
three primary germ layers give rise to all structures
and Basic body plan takes shape
– Fetal period – remaining 30 weeks. Structures
and organs continue to grow and develop.
Stages of Development
1. Fertilization
2. Cleavage
3. Gastrulation
4.Organogenesis
5.Maturation
Embryogenesis
1- Fertilization
• The process of fusion or union of the
spermatozoon with the mature ovum is known
as conception / fertilizaiton/impregnantation.
• Which produced the fertilized single mononucleated cell called the zygote.
• The sperm and ovum contribute half the
complement of chromosomes to make a total
of 46.
• The sperm and ovum is known as the male and
female gametes and the fertilized ovum as the
zygote
THE PROCESS OF FERTILIZATION
THE MALE GAMETE (SPERM) FUSES WITH THE FEMALE
GAMETE (OVUM)
2- Cleavage
• Day 2 and 3: Cleavage
• Cleavage is a series of rapid mitotic
divisions (without cell growth)
• The original zygote divides about 30
hours after conception into two
daughter cells called blastomeres.
• Continued subdivisions of the
original cell result in increasing
numbers of blastomeres.
• During cell division the dividing
cells decrease in size. This type of
cell division is called cleavage.
Morula
By the time the zygote is ready to enter the
uterus, it contains a solid ball of 16
blastomeres called the morula (from the latin
word for mulberry).
The blastomeres continue to divide by
binary division through 4, 8, and 16 cell
stage until a cluster of cells is formed–
Morula, resemblibg a mulberry.
Morula
Morula
Day 4: Formation of the blastocyst
• Fluid within the intercellular spaces
of the morula gradually increases,
and spaces on one side of the inner
cell mass come together, forming a
single cavity, the blastocele.
• The cavity of the blastocele fills with
fluid, and the conceptus is now called
the blastocyst.
Blastocele
cavity
Blastocyst
• The cell of the outer cell
mass forms the wall of
the blastocyst and is
known as trophoblast.
• The inner cell mass is
concerned with the
development of the
embryo.
Two Distinct Cell Types
1. An outer cell layer, the trophoblast.
2. An inner cell mass, the embryoblast.
Embryoblast
Trophoblast
Blastocyst
Trophoblast
Placenta
Chorion
Inner cell mass (embryoblast)
Fetus
Amnion
umbilical cord
IMPLANTATION : 6TH DAY
• The trophoblast attaches to the sticky
endometrial surface on the posterior wall of
the body of the uterus.
2nd week
• Embryoblast forms two
layers:
• epiblast and hypoblast.
• Together they make up
the bilaminar embryonic
disc.
• Formation of 2 cavities:
– amniotic and yolk sac
cavities.
21
Week 2
• 2 fluid filled sacs
1. Amniotic sac from epiblast
within which the embryo and
later the fetus develop until
birth.
2. Yolk sac from hypoblast
which is one of the
structures through which the
mother supplies nutrients to
the early embryo.
3. Gastrulation
• The process by which the bilaminar disc is converted
to a trilaminar disc (known as the ectoderm,
mesoderm, and endoderm)
• Consist of the formation of primitive streak, three
germ layer and the notochord.
The three germ layers*
1. Endoderm—formed from migrating cells that replace
the hypoblast
2. Mesoderm
3. Ectoderm—formed from epiblast cells that stay on
dorsal surface
*All layers derive from epiblast cells
Week 3
• Bilaminar to trilaminar disc
• Three primary “germ” layers: all body tissues
develop from these
• Ectoderm
• Endoderm
• Mesoderm
Each of the three germ layers gives rise to
specific tissues and organs:
• Ectoderm forms the outer layer. Ectoderm forms
skin, hair, sweat glands, epithelium, brain and
nervous system.
• Endoderm forms the inner layer. The endoderm
forms digestive, respiratory systems, liver,
pancreas, all bladder, and endocrine glands such
as thyroid and parathyroid glands.
• Mesoderm forms the middle layer. The mesoderm
forms body muscles, cartilage, bone, blood,
reproductive system organs and kidneys
Neurulation
• The process of formation of the neural tube is
known as neurulation
Neurulation
• The nervous system
develops as a thickening
within the ectodermal layer
at the rostral end of the
embryo. This thickening
constitutes the neural plate
called neurectoderm, which
rapidly forms raised margins
(the neural folds).
• These folds in turn
encompass and delineate a
deepening midline
depression, the neural
groove.
• The neural folds eventually
fuse so that a neural tube
separates from the
ectoderm.
• As the neural tube
forms, changes occur in
the mesoderm adjacent
to the tube and the
notochord. The
mesoderm first thickens
on each side of the
midline to form:
1- Paraxial mesoderm:
Along the trunk of
the embryo, this
paraxial mesoderm
breaks into segmented
blocks called somites.
Each somite has three components:
(a) the sclerotome, which eventually contributes
to two adjacent vertebrae and their disks;
(b) the myotome, which gives origin to a
segmented mass of muscle;
(c) the dermatome, which gives rise to the
connective tissue of the skin overlying the
somite. In the head region, the mesoderm only
partially segments to form a series of numbered
somatomeres, which contribute in part to the
head musculature.
2- Lateral plate mesoderm
• Further laterally the
mesoderm thickens again
to form the lateral plate
mesoderm, which gives rise
to (1) the connective tissue
associated with muscle
and viscera; (2) the
serous membranes of the
pleura, pericardium, and
peritoneum; (3) the blood
and lymphatic cells; (4)
the cardiovascular and
lymphatic systems; and (5)
the spleen and adrenal
cortex.
3- Intermediate mesoderm:
• At the periphery of the
paraxial mesoderm, the
mesoderm remains as a
thin layer, the
intermediate mesoderm,
which becomes the
urogenital system.
THE NEURAL CREST CELLS
• As the neural tube
forms, a group of cells
separate from the
neuroectoderm called
neural crest cell. These
cells have the capacity
to migrate and
differentiate extensively
within the developing
embryo.
• Neural crest cells in the
head region have an
important role. In
addition to assisting in
the formation of the
cranial sensory ganglia,
they also differentiate
to form most of the
connective tissue of
the head.
• Embryonic connective tissue elsewhere is
derived from mesoderm and is known
as mesenchyme, whereas in the head it is
known as ectomesenchyme, reflecting its
origin from neuroectoderm.
• In a dental context the proper migration of neural
crest cells is essential for the development of the
face and the teeth. In Treacher Collins syndrome ,
for example, full facial development does not
occur because the neural crest cells fail to
migrate properly to the facial region. All the
tissues of the tooth (except enamel and
perhaps some cementum) and its supporting
apparatus are derived directly from neural crest
cells, and their depletion prevents proper dental
development.