Primitive streak formation

Primitive streak formation



Dr Ismail Memon

BBS Department



Class of 2016

March 1, 2012

Learning Objectives

• To have three dimensional understanding of

blastocyst, yolk sac, amniotic cavity, epiblast, and


• 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


• Formation of three germ layers and

body axes

Formation of primitive groove

Formation of primitive streak


Formation three germ layers

Formation of notochord

Primitive streak formation

Primitive groove

Primitive pit

Primitive node

Primitive streak

• Prechordal plate

• Oropharyngeal


• Cloacal


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


• 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


Endoderm formation

The first ingressing

epiblast cells cells

replace hypoblast cells



The definitive endoderm

gives rise to lining of

future gut and its


Mesoderm formation

• Next population of epiblast

cells coming through

primitive streak settles

between epiblast and newly

formed endoderm to form



• Shortly thereafter the

intraembryonic mesoderm is

recognized to form its four


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


Buccopharyngeal membrane and

• During 3 rd weak, two faint

depressions appear in the


• One lies at cranial end, in front

of prechordal plate and other at

caudal end behind the primitive


• 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


• It contributes in the formation of

oropharyngeal membrane

(endodermal part) and head

mesenchyme hence called


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


• 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 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


• It induce forebrain and vertebrae


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


• 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


• It develops lateral to


• 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………

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