Transcript Week 2 of development

Lecture Outline
• Terminology review
• Fertilization
• First week of
development
• Week 2 of development
• Week 3-4 of
development
• Endochondral and
intramembranous bone
formation
The second week of
development is
significant for:
1. The formation of
the bilaminar disc
(two-layers) –
this will give rise
to all the tissues
and organs of the
body
2. The completion
of implantation
Syncytiotrophoblast
cells displace the
endometrium
around the
implantation site
The endometrium
has undergone
changes
Cells have become
filled with
glycogen and
lipids
The nutrients spill
into the
connective tissue
This is called the
decidua reaction
The syncytiotrophoblast is responsible for hormone production.
hCG maintains the corpus luteum in the ovary, allowing it to
continue to produce P+E.
Ovary
Uterus
hCG
Produces P+ E to
maintain the pregnancy
The cells of the
embryoblast will also
differentiate into 2
layers:
1. The epiblast- a layer
of high, columnar
cells adjacent to the
amniotic cavity.
2. The hypoblast- A
layer of small
cuboidal cells adjacent
to the blastocyst
cavity.
Together these layers
form a flat disc.
Amnioblasts (derived
from the epiblast)
separate and form
the lining of the
amniotic cavity.
Cells from the
hypoblast form a
membrane that lines
the inner surface of
the cytotrophoblast.
This forms the
exocoelomic cavity
or primitive yolk sac
The cavities allow
movement of the disc
The primordial
uteroplacental
circulation is
established
The yolk sac contains no
yolk- the embryo is
nourished from the
lacunar networks- but
may have a role in
selective transfer of
nutrients
New cells appear
between the yolk
sac and the
cytotrophoblast
They form a layer of
loose connective
tissue:
extraembryonic
mesoderm.
Cavities or spaces
appear in the
extra-embryonic
mesoderm
The cavities form a new
space- the chorionic
cavity
The primitive yolk sac is
pinched off- a
secondary or
definitive yolk sac is
formed
The cavity divides the
extraembryonic
mesoderm into the
1. Extraembryonic
somatic mesodermlining trophoblast and
amnion
2. Extraembryonic
splanchnic
mesoderm- lines the
yolk sac
The chorion is formed
by
1. Extraembryonic
somatic mesoderm
2. Cytotrophoblast
3. Syncytiotrophoblast
The chorion forms the
wall of the
chorionic cavitythe amniotic cavity
and yolk sac are
suspended in the
chorionic cavity by
the connecting
stalk.
3
2
1
By day 14 the
embryo has the
form of a flat,
bilaminar disc,
ovoid in shape.
In a localized area
of the hypoblast,
the cells become
more columnar
and form a
thickened circle
area, the
prechordal plate.
At the end of the
second week:
Trophoblast has
had a period of
growth- greater
than the
embryoblast.
The 2 layer
bilaminar disc is
formed and will
give rise to
other tissues
and structures.
Lecture Outline
• Terminology review
• Fertilization
• First week of
development
• Week 2 of
development
• Week 3-4 of
development
• Endochondral and
intramembranous
bone formation
The third week is
Floor of the amniotic cavity or epiblast
significant for the
conversion of the
bilaminar disc to
the trilaminar disc
Gastrulation: The
formation of all 3
germ layers.
• Begins with the
formation of the
primitive streak.
• Cells of the epiblast
proliferate and
migrate to the median
plane of the
embryonic disc.
• Primitive node
surrounds the
primitive pit.
The primitive groove and pit form from migration of
epiblast cells towards the primitive streak. Upon
arrival to the region of the streak, they detach from
the epiblast and slip beneath it.
Once the cells have invaginated:
• Some displace the hypoblast = embryonic endoderm.
• Cells between the epiblast and newly created
endoderm form mesoderm.
• Cells remaining in the epiblast then form ectoderm.
Section
Is Here
The endoderm and ectoderm are closely
adhered at the prechordal plate and the
cloacal membrane.
The prechordal plate is the primordium of
the oropharyngeal membrane, the cloacal
membrane is the primordium of the anus.
Prechordal
plate
Cloacal
membrane
View looking down
on the dorsal surface
Longitudinal
Section
staff.um.edu
Mesenchymal cells invaginating in the primitive pit
move cranially forming a median process -the
notochordal process. The notochordal process
grows cranially between the ectoderm and the
endoderm until it reaches the prechordal plate.
The notochord is a cellular rod that develops by
transformation of the notochordal process. The
notochord serves as a basis of development of the
axial skeleton.
The notochordal process
eventually becomes
the notochordal plate.
The cells proliferate
and the ends of the
plates fold to form the
notochord.
The notochord is important because:
• The vertebral column and base of skull develop around it
• It will degenerate, only adult remnant is the nucleus pulposus
• It will induce the ectoderm to form the neural plate (when one
population of cells influences the development of another population of
cells -INDUCTION)
Lateral
mesoderm
Neurulation
The appearance of the
notochord induces
the overlying
ectoderm form the
neural plate.
Cells of the neural
plate make up the
neuroectoderm.
The induction of these
cells and the
formation of the
neural tube is called
neurulation.
• The nervous
system
develops as a
thickening of
the ectoderm
• The thickening
is called the
neuroectoderm
and constitutes
the neural
plate
• From this, the
brain and
spinal cord
will develop
Oropharyngeal membrane
Neural plate
Newly added
cells
Cloacal
membrane
Approximately
18 days
The neural plate
invaginates along
the central axis to
form a median
neural groove
surrounded by
neural folds. By the
end of the 3rd week
the neural folds
approach each
other in the
midline where they
fuse. Fusion begins
in the cervical
region and proceeds
cranially and
caudally.
As a result the
primordium of the
CNS, the neural
tube, is formed.
Approximately
20 days
The neural tube
separates from the
surface ectoderm
(which differentiates
into the epidermis).
Until fusion is
complete, the cephalic
and caudal ends of the
neural tube
communicate with the
amniotic cavity by
way of the cranial and
caudal neuropores.
Closure of the anterior
neuropore occurs day
25, posterior day 27.
As the neural folds elevate and fuse, cells at the lateral border
of crest of the neuroectoderm begin to disassociate from
their neighbors.
These neural crest
cells will
undergo
epithelial to
mesenchymal
transformation
as they leave the
neuroectoderm
by active
migration and
displacement to
enter the
underlying
mesoderm.
Summary
Neuroectoderm consists of :
1. Neural plate> neural tube>>brain and spinal cord.
2. Neural crest>>consists of pluripotent cells which
migrate to all areas and give rise to formation of
many organs and tissues.
Intraembryonic mesoderm
As the neural tube forms, the embryonic mesoderm
on each side of it proliferates forming 3 distinct
longitudinal columns of cells.
The paraxial mesoderm (most medial) gives rise to the somites
The intermediate mesoderm gives rise to the urogenital system
The lateral mesoderm is continuous with the extraembryonic
mesoderm covering the yolk sac and the amnion.
Segmented blocks,
called somites,
first appear in
the cephalic
region- their
formation
proceeds
cephalocaudally.
The somites are
located on each
side of the
neural tube.
www.neoucom.edu
During this stage of development the age of the embryo
(approximate number of days) is correlated to the number of
somites
The somites eventually
shift their position
around the notochord
and give rise to:
Sclerotome- form
vertebral column and
base of the skull
Myotome- segmental
muscle component
Dermatome- gives rise to
the dermis and
subcutaneous
connective tissue
www.neuro.wustl.edu
Paraxial mesoderm cells arrange in
concentric whorls around a small cavity
Cells of the ventral/medial wall become less compact and
migrate to the direction of the somite- these cells form
the sclerotome
Cells of the dorsal
lateral aspect
migrate as the
dermomyotome
The myotome cells
are precursors to
limb and body
wall musculature
The dermotome
cells spread out
under the
ectoderm/epider
mis to form the
dermis
epidermis
Folding of the Embryo
Folding occurs in 2
planes- longitudinal
(cephalocaudal) and
transverse planes
(lateral folding).
Longitudinal folding:
due to brain
development.
Lateral folds: due to
growth of the somite.
Folding begins day 24usually complete by
day 28
Longitudinal folding
or
cephalocaudal fold
because
it is most
pronounced in the
head and tail regions
Head folding :
• septum transversum,
• the primordial heart,
• pericardial cavity
• oropharyngeal
membrane
• The endoderm (which
is ventral and lines the
yolk sac) is
incorporated into the
embryo and forms the
foregut
Head region
The head fold
forms the
stomatodeum –
the primitive
oral cavity
Thus ectoderm
lines the oral
cavity and is
separated from
the foregut by
the
oropharyngeal
membrane
Oropharyngeal membrane
stomatodeum
Amniotic cavity
Tail folding:
• allantois
• primitive streak
• cloacal
membrane
• connecting stalk
The endoderm
layer and a
portion of the
yolk sac is
incorporated as
the hindgut
Tail fold
Lateral Folding
In the transverse
plane
As a result of the
rapid growth of
somites
Embryo forms a
round appearance
Forms the ventral
body wall and
incorporates the
midgut , which
remains in
communication
with the yolk sac
Initiation of folding- approximately 24 days
Transverse
section through
the midgut
showing the
connection
between the
midgut and the
yolksack
Amniotic
cavity
ectoderm
Connection b/w
Gut and yolksac
Intra
embryonic
cavity
Section below the
midgut to show
the closed ventral
abdominal wall
ectoderm
Intraembryonic
cavity
Amniotic
cavity
Lecture Outline
• Terminology review
• Fertilization
• First week of
development
• Week 2 of
development
• Week 3-4 of
development
• Endochondral and
intramembranous
bone formation
Development of Bone and
Cartilage
Mesenchyme is
embryonic
connective tissue
Mesenchymal cells
migrate to various
areas and
differentiate into
several different cells,
including fibroblasts,
chondroblasts,
osteoblasts
The paraxial mesoderm
forms segmented blocks
on each side of the neural
tube:
Somitomeres in the head
region
Somites from the occipital
region down
Bone formation is NOT
restricted to the
sclerotome
It also occurs in somatic
mesoderm of the body
wall and to form the
pelvic and shoulder
girdles and long bones of
the limbs
• NCC’s also differentiate into mesenchyme and participate in
the formation of the bones of face
• In some bones, primarily flat bones, mesenchyme
differentiates directly into bone (intramembranous
ossification)
• In most bones, mesenchymal cells first give rise to hyaline
cartilage models which ossify by endochondral ossification
Membranous neurocranium
Cartilaginous neurocranium
Mesenchyme derived
from neural crest cells
(viscerocranium)
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Development of Cartilage
First appears during 5th week of development
Mesenchyme condenses to form chondrification
centers
The cells loose their processes, become rounded and
cluster together to form chondrification centers
Chondroblasts (cartilage forming cells) secrete
collagen and ground substance
There are 3 types of cartilage- hyaline, fibrocartilage,
and elastic cartilage
Mesenchymal
cells
Chondrification
Center
Secreting the
Fibers and
Ground subs.
Chondrocytes
In lacunae
Three types of cartilage are
distinguished based on
the make-up of the
extracellular matrix:
• Hyaline cartilage – most
abundant in body, lies
the template for
endochondral bone
formation. Articular
cartilage is a specialized
hyaline cartilage.
• Elastic cartilage – elastic
fibers added to matrix
increase flexibility
• Fibrocartilageincreased tensile
strength
Development of Bone
Initial bone formation in two ways:
Intramembranous bone formation – bone develops in
well vascularized mesenchyme. Most flat bones
develop this way (scapula, flat bones of the head)
Intracartilaginous (endochondral) bone formation- bone
forms in a cartilage model (limb bones)
These names refer only to how the bone is initially
formed
The initial bone is replaced by remodeling that occurs
later
Intramembranous
Ossification
Bone is formed by
differentiation of
mesenchymal cells into
(osteoprogenitor)
osteoblasts
Occurs in the
mesenchyme where
mesenchymal cells
aggregate in the specific
area that bone is to be
formed
A. The newly organized
tissue becomes more
vascularized- the cells
more rounded
B. The now differentiated
osteoblasts secrete
collagen and bone
matrix
C. The osteoblasts become
separated as more bone
matrix is produced
D. When the matrix
calcifies the cells are
termed osteocytes, still
interconnected by
channels- osteoclasts are
apparent- participate in
bone modeling
Endochondral
Ossification
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(intracartilaginous)
Occurs in preexisting
cartilage
In long bones this occurs
in the diaphysis- the long
part between the ends
(shaft)
This is the primary
center of ossification
Initially, a hyaline
cartilage model is formed
in the shape of bone
The first sign of
ossification is the
formation of a bone
collar around the
cartilage model
The cartilage in this area
no longer gives rise to
chondrocytes- instead
osteoblasts (this part
ONLY is
intramembranous
development)
After the bone collar forms,
the cells in the midregion
of the cartilage become
hypertrophic
The cartilage matrix is
resorbed and the cells are
on a scaffolding of thin
matrix
The hypertrophic cells secrete
a substance that causes the
remaining cartilage matrix
to calcify- this impairs
diffusion and the
chondrocytes die
Blood vessels
penetrate the
bony collar and
vascularize the
cavity
The vessels carry
osteoprogenitor
cells that
differentiate into
osteoblasts and
lay down bone
Ossification of limb bones
begins at the end of the
embryonic period- thus
there is more demand on
the maternal supply of
calcium
At birth the shafts are
ossified, but the ends or
the epiphyses are still
cartilage
The secondary ossification
centers are in the
epiphyses and are a postnatal event