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6.2 Implantation stages   Normal implantation zone 

  Stages 

o  Adplantation of the blastocyst on the uterine mucosa o  Adhesion of the blastocyst to the endometrium 

o  Invasion of the trophoblast and embedding 

Invasion of the trophoblast and embedding 

The trophoblast differentiates into two different cell masses, shortly

 before it comes into contact with the endometrium:

  the outer syncytiotrophoblast (ST)  the inner cytotrophoblast (CT)

The cytotrophoblast, deep inside, consists in an inner irregular layer of 

ovoid, single- nucleus cells. This is also where intensive mitotic

activity takes place.

In the periphery the syncytiotrophoblast forms a syncytium, i.e., amulti-nucleic layer without cell boundaries that arises from the fusion of cytotrophoblast cells. The syncytiotrophoblast produces lytic enzymes 

Quiz

Quiz

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and secretes factors that cause apoptosis of the endometrial epithelial

cells. The syncytiotrophoblast also crosses the basal lamina and

 penetrates into the stroma that lies below, eroding the wall of capillaries. With the implantation of the blastocyst in the endometrium

the syncytiotrophoblast develops quickly and will entirely surround the

embryo as soon as it has completely embedded itself in theendometrium.

The uterine mucosa reacts to the implantation by the decidual reaction.

The syncytiotrophoblast cells phagocytize the apoptotic decidual cellsof the endometrium and resorb the proteins, sugars and lipids that have

 been formed there. They also erode the canals of the endometrial

glands and the capillaries of the stroma.

 Mor

The d

reactio  

chara

by ..  

Fig. 18 - Implantation:

6th-7th day 

Fig. 19 - Implantation:

7th-8th day 

Legend 

1 

2

3

4 

Epithelium of the

uterine mucosa

Hypoblast

Syncytiotrophobla

Cytotrophoblast

5

6 Epiblast

Blastocyst cavity 

Fig. 18 

Free blastocyst

(following the

dissolution of

the pellucid

zone) in

adplantation

phase on the

uterine wall

(6th to 7th

day). Thetrophoblast

cells of the

embryonic pole

differentiate

themselves,

multiply, and

form the

invasive

syncytiotrophobl

ast. The

abembryonic poleconsists of

cytotrophoblast

cells.

Fig. 19 

Didermic

embryonic disk

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(hypoblast and

epiblast) after

8 days. The ST

continues its

invasive, lytic

activity into

the maternal

tissue. 

Fig. 20 - Implantation: 8th day  Fig. 21 - Implantation: 9th day 

1

2

3

4

56

7 

Syncytiotrophoblast

(ST)

Cytotrophoblast (CT)

Epiblast

HypoblastBlastocyst cavity

Maternal blood

capillary

Amniotic cavity 

8

9

10

11 

Amnioblasts

Fibrin plug

Trophoblast lacunae

Multiplying hypoblast 

Fig.

21

Comp

impl

of t

into

endo

and

of t

impl

loca

fibr

The

cavi

expa

cell

laye

(amnnow

it f

CT.

hypo

cell

begi

mult

.

Extr

vacu

appeST a

to f

lacu  

In the middle of the 2nd week extracellular vacuoles appear in the ST.They join together forming lacunae. Initially these lacunae are filled

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with tissue fluids and uterine secretions. Following the erosion of the

maternal capillaries, their blood fills the lacunae that later develop

further into intervillous spaces. The invasive growth of the ST ceasesin the zona compacta of the endometrium. At around the 13th day the

primitive utero-placental circulatory system arises.

Fig. 22 - Implantation: 9th-10th

day 

Fig. 23 - Implantation: 10th-11th

day 

1 

2 Hypoblast growing

ventrally

Eroded maternal

capillaries 

3

4

5

6

7

8 

Extraembryonic

reticulum

Heuser´s membrane

Amniotic cavity

Cytotrophoblast

Syncytiotrophoblast

Lacunae, filled with

blood 

Fig.

Fig.

The

dest

lyti

acti

the

reac

capi

of t

endo

The

bloo

into

lacu

ST e

the

capi

expa

lacu

netw

form

arte

infl

veno

outf

syst  

At the end of the 2nd week, when implantation has ended, the

embryonic bud consists schematically of two hemispheric cavities that

lie on one another: the amniotic cavity (dorsal) and the umbilical vesicle(ventral).

The floor of the amniotic cavity is formed by the epiblast, and the roof of the umbilical vesicle by the hypoblast. These two layers, which lie on

one another, form the embryo or the double-layered embryonic disc.

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