Primitive streak formation

LCF
Embryogenesis
Dr Ismail Memon
BBS Department
AKU
For
Class of 2016
March 1, 2012

Learning Objectives
• To have three dimensional understanding of
blastocyst, yolk sac, amniotic cavity, epiblast, and
hypoblast
• Identify various areas in the epiblast e.g. primitive
streak, buccopharyngeal membrane, cloacal
membrane, prechordal plate
• To have concept of body axes
• To describe the ingression of epiblast cells in
primitive streak and formation of three germ layers
• To describe the formation of notochord
• To describe the fate map of epiblast/ gastrulation
• To describe the developmental changes in
trophoblast and initiation of placenta a formation

Third week of development
Gastrulation
• Formation of three germ layers and
body axes
Formation of primitive groove
Formation of primitive streak
Invagination
Formation three germ layers
Formation of notochord

Primitive streak formation
• Primitive groove
• Primitive pit
• Primitive node
• Primitive streak
• Prechordal plate
• Oropharyngeal
membrane
• Cloacal
membrane

Establishment of body axes
• Formation of primitive streak determines the
body axes
• The primitive streak appears in midline, in
caudal portion of embryonic disc facing to
amniotic cavity
• Part of embryonic disc in front of primitive
streak is cranial, hence cranial-caudal axis is
formed
• Looking down the primitive streak from
inside the amniotic cavity, what lies to the
right side of the primitive streak represent
the right side of the embryo and what lie son
the left , represent the left side, hence left to
right axis is formed
• At the time of primitive streak formation,
ectodermal view forms dorsal surface and
endodermal view forms ventral surface
hence, dorso-ventral axis is formed

Epiblast cells move
towards primitive streak,
undergo epithelial-tomesenchymal
transformation, detach
and sneak in the space
between epiblast and
hypoblast to form three
primary germ layers
Gastrulation

Endoderm formation
The first ingressing
epiblast cells cells
replace hypoblast cells
and form DEFINITIVE
ENDODERM
The definitive endoderm
gives rise to lining of
future gut and its
derivatives

Mesoderm formation
• Next population of epiblast
cells coming through
primitive streak settles
between epiblast and newly
formed endoderm to form
the INTRAEMBRYONIC
MESODDRM
• Shortly thereafter the
intraembryonic mesoderm is
recognized to form its four
subdivisions:
cardiogenic mesoderm
paraxial mesoderm
intermediate mesoderm
Lateral plate mesoderm

Ectoderm formation
• Once the formation of
endoderm and mesoderm is
complete, the remaining
epiblast cells do not move to
primitive streak and constitute
the ectoderm
• The ectoderm quickly
differentiate into neural plate
and surface ectoderm
• With the formation of ectoderm
the process of gastrulation is
complete.

Buccopharyngeal membrane and
• During 3 rd weak, two faint
depressions appear in the
ectoderm
• One lies at cranial end, in front
of prechordal plate and other at
caudal end behind the primitive
streak
• The ectoderm in both the areas
fuse tightly with the underlying
endoderm, excluding mesoderm
• The cranial membrane is
buccopharyngeal membrane and
caudal is the cloacal membrane
Cloacal membrane

Prechordal plate
• The prechordal plate forms by
ingression of epiblast cells from
primitive node between tip of
notochord and buccopharyngeal
membrane
• It contributes in the formation of
oropharyngeal membrane
(endodermal part) and head
mesenchyme hence called
MESENDODERMAL

Formation of Notochord
Notochord formation
• Prenotochordal cells move through primitive
node to reach the prechordal plate forming a
hollow tube the notochordal process

Notochord formation
Notochord
• Ventral floor of the tube fuse with
underlying endoderm/hypoblast
• Then separate from endoderm and
form notochordal plate
• At the level of pit, the amniotic
cavity transiently communicates
with yolk sac at neurenteric canal

Notochord
Notochord formation…….
• Notochordal plate completely
detach from endoderm and form
definitive notochord between
ectoderm and endoderm
• As it is present in mesodermal
layer hence it is considered as
mesodermal
• It induce forebrain and vertebrae
formation

Notochord formation…….
During development rudiments
of vertebral bodies coalesce
around the notochord
Notochord serves as basis for
axial skeleton and it form the
nucleus pulposus in the center
of the intervertebral discs

Paraxial mesoderm in Head region
• Paraxial mesoderm flanks notochord
• The paraxial mesoderm in the head region forms bands of unsegmented
cells
• It loosely fills the developing head as head mesenchyme
• It gets supplemented by neural crest cells
• The head mesenchyme gives rise to striated muscles of face, jaw,
and throat

Paraxial mesoderm in trunk region
• The mesoderm in the trunk region forms bands of
segmented cells the somites
• 1 st pair of somites appear on day 20, then somites appear
cranio-caudally 3-4 pair per day till day 30, with final
count of 37 pairs
• Somites give rise to most of axial skeleton, voluntary
muscles of neck, body wall, and limbs and the dermis of
the neck

Intermediate Mesoderm
• It forms in trunk region
only
• It develops lateral to
somites
• It give rise to urinary
system and parts of
genital system

Lateral plate Mesoderm
• It also forms in trunk region only
• It develops lateral to intermediate mesoderm
• It splits into two layers
ventral associated with endoderm called as splanchnic
mesoderm. It forms mesothelial covering of viscera and
parts of wall of viscera
Dorsal associated with ectoderm is called somatic
mesoderm. It forms inner lining of the body wall and
parts of limbs

Fate map of gastrulation
A. Early stage of primitive streak showing
prospective (GE) gut endoderm, (PP)
prechordal plate
B. Early stage of primitive streak showing
prospective (CM) cardiogenic
mesoderm, (PEEM) prospective
extraembryonic mesoderm
C. Mid stage of indicating prospective
mesoderm (N) notochord, (HM) head
mesoderm, (S) somites, (IM)
intermediate mesoderm, (LPM) lateral
plate mesoderm
D. Elongated stage of primitive streak
showing prospective (NP) neural plate
(SE) surface ectoderm, (NC) neural
crest cells, (PE) placodal ectoderm

Development of Trophoblast
• During 3 rd week, the trophoblast forms primary villi
• The primary villus consists of core of cytotrophoblast
covered by syncytiotrophoblast
• Mesoderm penetrate the core of primary villi and grow towards
the decidua and form the Secondary villi
• By the end of 3 rd week, the mesenchyme in the core
differentiate into blood cells and blood vessels and form the
tertiary villi

Feto-placental circulation
• The capillaries formed in the tertiary villi establish
contact with capillaries developing in the chorionic
plate and connecting stalk
• These vessels in turn, establish contact with
intraembryonic circulatory system in placent

Feto-placental circulation
• Meanwhile the overlying cytotrophoblast erode the
maternal uterine endometrium
• When the heart start beating, the villi are ready to supply
the embryo with nutrients and oxygen

To be continued………