The Second week of Life:

Implantation and formation of the bilaminar embryo



Professor Alfred Cuschieri

Department of Anatomy

University of Malta



During the first week the zygote undergoes cleavage divisions.  It becomes a morula and then a blastocyst, consisting of embryoblast and trophoblast.  The whole structure, irrespective of the stage of development is referred to as the conceptus. 


On day 4 the conceptus reaches the uterine cavity, and is at the blastocyst stage.


A blastocyst is formed by accumulation of fluid between the blastomeres.  The cells form two distinct groups:

  1. The trophoblast – a single layer of cells surrounding the blastocyst cavity.  It will eventually form the placenta and membranes.
  2. The embryoblast – a mass of cells situated within the trophoblast.  It will give rise to the embryo.

As a result of the accumulation of fluid the blastocyst increases rapidly in size.  The zona pellucida ruptures and disintegrates.


The blastocyst has a distinct polarity:

The embryonic pole is the one where the embryoblast is situated. The opposite pole is the abembryonic pole.

The trophoblast is now divided into two parts:

The polar trophoblast  is situated at the embryonic pole and caps the embryoblast.

The mural trophoblast lines the blastocyst cavity.


On approximately day 5 the trophoblast differentiates into two distinct layers:

1.       Text Box:  Cytotrophoblast :

–        The inner layer

–        Consists of a single layer of cuboidal cells

–        Is the source of dividing cells

2.     Syncitiotrophoblast

–        The outer layer

–        Consists of a mass of multinucleated cytoplasm with irregular finger-like processes

–        Formed by coalescence of cells derived from the cytotrophoblast

–        Does not contain mitotic figures

Differentiation of the syncitiotrophoblast begins at the embryonic pole and spreads over the blastocyst.


The syncitiotrophoblast has two important secretory functions:

1.       Secretion of hydrolytic enzymes

1.       essential for erosion and penetration of the endometrium

2.     Secretion of Human Chorionic Gonadotrophin (HCG)

–        Has properties of LH

–        Is essential for maintenance of the corpus luteum, which enlarges to form a corpus luteum of pregnancy

–        Is essential for maintenance of pregnancy

–        The embryo’s signal saying “Hi Mum!  I’m here!”


Day 7

The embryoblast differentiates into two germ layers to fom a bilaminar embryo (or germ disc):

1.       Epiblast – a columnar epithelium adjacent to the trophoblast

2.     Hypoblast – a cubical epithelium adjacent to the blastocyst cavity


Two cavities are also formed:

1.       Amniotic cavity lined by the amniotic membrane, a thin layer of cells derived and growing out from epiblast


2.      Primary yolk sac lined by Heuser’s membrane, a thin layer of cells derived and growing out from the hypoblast




Text Box:  Separation of Heuser’s membrane from the cytotrophoblast gives rise to two new cavities:

1.       The secondary (definitive) yolk sac shrinks away from the cytotrophoblast and becomes re-lined by a new layer of cells derived from the hypoblast


2.     The chorionic cavity or extra-embryonic coelom forms between the lining of the yolk sac and the cytotrophoblast. It contains cells – the extra-embryonic mesoderm- of uncertain origin.







The chorionic cavity expands greatly by accumulation of fluid within it. As a result : 

–        The chorionic cavity becomes the dominant cavity  

–        the amniotic cavity and yolk sac become progressively smaller.

–        The chorionic cavity is lined by extra-embryonic mesoderm

–        The cytotrophoblast is lined by a layer of extra-embryonic mesoderm.

–        The chorion is the three-layered membrane surrounding the chorionic cavity.  It consists of  syncytiotrophoblast, cytotrophoblast and extra-embryonic mesoderm.

–        Both the amnion and the  yolk sac are covered externally by a layer of extra-embryonic mesoderm.






The embryo is now a bilaminar disc consisting of the:

1.       Epiblast that forms the floor of the amniotic cavity

2.     Hypoblast that forms the roof of the yolk sac






Day 14

The embryonic disc with its amnion and yolk sac become suspended in the chorionic cavity by a thick layer of mesoderm, which elongates to form the connecting stalk








Implantation begins on approximately day 5and is completed by the end of the second week (approximately day 13).

Outgrowths from the syncitiotrophoblast invade the deciduas


They erode the maternal blood vessels.


They become surrounded by trophoblastic lacunae containing maternal blood


A utero-placental circulation is established by day 13







Three stages of development of chorionic villi:

  1. Primary villi

–        consist of syncitiotrophoblast and cytotrophoblast

–        day 13



  1. Secondary villi

3.     have a core of extra-embryonic mesoderm

4.     day 16




  1. Tertiary villi

–        have blood vessels in the mesoderm

–        day 21




Anomalies of development

  1. Anomalies of the embryoblast

–        “Blighted ovum” is a failure of development of the embryoblast, while the trophoblast develops.  This results in spontaneous abortion of an empty gestational sac

  1. Anomaly of the trophoblast

–        Hydatidiform mole is the result of hyper-development of the trophoblast, forming swollen and vesicular chorionic villi.  These secrete an excess of HCG.   The embryo fails to develop normally.  It results in spontaneous abortion.  Some hypertrophied villi may remain embedded in the deciduas; the pregnancy test remains positive with a high titre of HCG.  It may also undergo malignant change causing choriocarcinoma.  


Uniparental disomy is the condition in which both nuclei of a zygote are derived from the same parent.  Although the chromosome complement is diploid, subsequent expression of some genes depends on whether the gene was maternally derived or paternally derived.   Thus maternal or paternal disomy give rise to different clinical syndromes. This differential expression of genes is termed maternal imprinting.


Paternal disomy occurs when both sets of homologous chromosomes are derived from the father.  This may result from dispermy or double fertilization of the oocyte. Subsequent elimination of the maternal nucleus as a polar body results in the zygote having two sets of paternally derived chromosomes. 

This results in hydatidiform mole.



Maternal disomy occurs when both sets of homologous chromosomes are derived from the mother.  This may result from failure of extrusion of the male pronucleus as the second polar body after fertilization. This results in a zygote with two sets of maternal chromosomes.

This results in a small, underdeveloped placenta and embryo.


Paternally imprinted chromosomes are necessary for the development of the placenta and membranes.  Maternally imprinted chromosomes are necessary for the development of the embryoblast.



This is the situation in which the zygote has a triploid rather than a diploid set of chromosomes.   


If the extra set of chromosomes is of paternal origin, resulting from  dispermy, it forms a partial hydatidiform mole, which ends in spontaneous abortion.



If the extra set of chromosomes is of maternal origin, resulting from  failure  of extrusion of the polar body, it forms the triploidy syndrome.  The foetus may survive beyond the embryonic period but ends in foetal or neonatal death.