Making it easy for you the 2nd week of embryo development.

1. EMBRYOLOGY Second week of the IUL
BY: Ankita Nanda

2. INTRODUCTION
Refers to 8th to the 14th day
Known as the week of 2’s
Related to the important events named
i. Bilaminar germ disc
ii. Implantation
iii. Formation the UTEROPLACENTAL CIRCULATION

3. RECAPITULATION OF THE 1ST WEEK
By the 3th day, morula (16 cell stage embryo) formed by cleavage comprises of :
the inner cell mass the outer cell mass
(future embryo proper) (future trophoblast)
At 5th day, morula travels through uterine cavity, fluid penetrates in the
intercellular spaces forming a cavity called the Blastocoel. At this time , embryo
is given the name as a Blastocyst.

4. DAY 8
Trophoblast at the embryonic pole differentiates into :
a) An inner layer of mononucleated cells, the cytotrophoblast
b) An outer multinucleated zone devoid of cell boundaries, the
syncytiotrophoblast
erosive syncytiotrophoblast invades endometrial tissue
blastocyst partial embedment
(endometrial cells undergo apoptosis)
Concurrently, the embryoblast differentiates into:
a) Epiblast layer, columnar cell layer related to amniotic cavity
b) Hypoblast layer, cuboidal cell layer related to blastocyst cavity
These two layers now constitute the BILAMINAR GERM DISC.

5. CONT.
Significance :
Formation the bilaminar germ disc , primitive dorso- ventral axis maybe
determined as
( epiblast is dorsal and hypoblast is ventral)
The syncytiotrophoblast produces the glycoprotein hormone, human
chorionic gonadotrophin (hGC). It
i. Maintains the activity of corpus luteum
ii. Forms basis of pregnancy tests

6. DAY 9
A fibrin coagulum: closes the penetration effect in epithelium
Heuser membrane : originating from hypoblast,
lines inside of cytotrophoblast
Heuser membrane along with hypoblast: lining of the PRIMITIVE YOLK SAC.
Vacuoles in the syncytium (trophoblast)
Fuse to form Lacunae
Hence this phase of development is also called the LACUNAR STAGE.

7. DAY 11 AND 12
Implantation is complete, fibrin coagulum entirely covers the
epithelial defect.
(maternal blood rapidly anastomose with the lacunae)
Sinusoids – dilated and congested maternal capillaries
THE UTEROPLACENTAL CIRCULATION : maternal blood
entering the lacunar system due to eroding of the trophoblast

8. EXTRAEMBRYONIC MESODERM
AND CAVITY
Derived from the yolk sac cells, b/w the cytotrophoblast and the
exocoelomic cavity forms the loose, extraembryonic mesoderm.
Development of large cavities within becoming confluent
Forming a new space called the extraembryonic cavity or CHORIONIC
CAVITY.
Are of two kinds :
1. Extraembryonic somatic
2. Extraembryonic splanchnic

9. DAY 13
Primary villi: cellular columns of the cytotrophoblast with the
syncytial cover
Secondary yolk sac : also called the definitive yolk sac
: inside of the exocoelomic membrane by
hypoblast
The extraembryonic coelom, surrounds the amnion and umbilical
vesicle, except where they are attached to the chorion by the
connecting stalk by the development of blood vessel forms the
UMBLICAL CORD.

10. WHY WEEK OF 2’S ?
BECAUSE
1. Trophoblast differentiates into 2 : cytotrophoblast and
syncytiotrophoblast
2. Embryoblast into 2 : the epiblast and the hypoblast
3. Extreaembryonic mesoderm splits into 2: the somatic and
splanchnic layers
4. Two cavities : the amniotic and yolk sac cavities are formed

11. CLINICAL CORRELATIONS
Decidua reaction :. As the conceptus implants, the endometrial cells
swell because of the accumulation of glycogen and lipid in their
cytoplasm, The primary function is to provide nutrition for the early
embryo.
Ectopic pregnancy: implantation other than uterus, eg. Rectouterine
pouch, mesentery etc
Bleeding due to increased blood flow in lacunae can interfere with the
judgement of the delivery date.

12. REFERENCES
1. Langman’s Medical Embryology 13th edi
2. The Developing Human by Keith L Moore 9th edi
3. Larsen’s HUMAN EMBRYOLOGY 5th edi

13. THANKYOU