PLACENTA DEVELOPMENT:
The first thing is to understand how placentation occurs in a normal pregnancy,
to comprehend the pathological alternatives that exist.
Placentation will develop in the place where the implantation of the blastocyst takes place,
around day 8 post-fertilization.
Factors that can cause the implantation not to be in a suitable place are also risk factors for an abnormal placentation,
not only by alterations of the location (previous,
ectopic) but by alterations in the penetration (accretism).
These risk factors are: previous surgeries (caesarean section as major and most important risk factor,
curettage,
myomectomies),
advanced maternal age,
uterine anomalies. Fig. 1
STAGES:
1.
PRELACUNAR STAGE: The 8th day of development,
when the blastocyst contacts the endometrial mucosa,
begins a series of changes that include the trophoblast,
the embryoblast and the endometrium (the uterine mucosa starts to be called decidua after the process of endometrial change).
The placenta is the consequence of the development of decidua and chorion.
The trophoblast begins to specialize and it is divided,
at the level of the union with the decidua,
in cytotrophoblast and syncytiotrophoblast (this is specifically the one who breaks through the blood vessels into the decidua.)
The embryoblast is divided into 2 germinative layers,
one endodermal and the other ectodermal.
The embryoblast will detach completely from one of the two sides of the trophoblast.
The other end will be taking off but maintaining a cell layer end,
which will be the umbilical cord.
The amniotic cavity will be formed by an amniotic membrane (which comes from the fetal ectoderm) and a chorionic membrane (coming from the trophoblast).
At the end of the third trimester,
the amnion and the chorion merge.
The chorion is divided into:
- Chorion frondosum: in contact with the decidua.
It will form the placenta.
- Chorion laeve: very thin layer.
It will form the second membrane of the amniotic cavity.
2.
LACUNAR STAGE: The spiral arteries (which irrigate the endometrium) undergo trophoblast invasion,
with loss of the middle muscular layer,
turning them into vessels of little resistance,
different from the rest of the spiral arteries of the uterus that are not invaded by the trophoblast and maintain their high resistance .
3.
PLACENTAL VILLI STAGE: The trophoblast when invading the decidua is forming structures similar to "fern leaves",
they have blood vessels inside.
These structures are called villi.
- Primary villi: formed only by trophoblast.
- Secondary villi: trophoblast cells begin to develop into connective tissue-like cells (mesenchymal axis).
- Tertiary villi: when that mesenchyme becomes blood vessels.
CLASSIFICATION: Fig. 2
There are 3 varieties of abnormal adherence of the placenta:
- Placenta accreta: the villi are inserted directly into the myometrium without penetrating it.
- Placenta increta: the villi penetrate the thickness of the myometrium.
- Placenta percreta: the villi reach the peritoneal serosa or even invade neighboring organs,
most often,
the bladder.
By its extension three types are recognized: a) Focal: only involves small areas of the placenta; b) Partial: one or more cotyledons are involved in the process; c) Total: the entire surface of the placenta is abnormally attached.