الصقر الجارØ
المدير العام عمرو محمد
عدد المساهمات : 3025
سمعة المساهمه : 52437
تاريخ الميلاد : 30/11/1996
تاريخ التسجيل : 25/04/2009
العمر : 27
 |  موضوع: Implantation. By the end of first week of development  الأحد أغسطس 28, 2011 2:02 pm | |
| Implantation. By the end of first week of
development, the human zygote has passed through the morula and blastocyst
stages and has begun its implantation into the uterine mucosa. Implantation is a process of blastocyst introducing
into the endometrium, usually it occurs along the posterior or anterior wall of
the uterine body.There are
two phases of implantation: adhesion and invasion. Till that moment tunica of
fertilization dissappears and at first blastocyst attaches to the uterine
mucosa with embryoblast pole.
The trophoblastic cells begin to penetrate between the epithelial cells of
endometrium. The penetration and erosion of the mucosa cells results
from proteolytic enzymes produced by trophoblast.Hence, by
the end of the first week of development, the human zygote has passed through
the morula and blastocyst stages and has begun its implantation in the uterine
mucosa.Implantation, or nidation, involves penetration through the uterine epithelium, with
little signs of necrosis of the connective tissue stroma and blood vessels of
the endometrium.This type
of human embryo implantation is called interstitial
implantation, in which the blasocyst comes to lie entirely within
the endometrium.Process of
implantation continues (lasts) at about 40 hours. Till the end of second week
the embryo is completely embedded in the endometrial stroma and covered with
epithelium. The
adhesive mechanisms are those which attach the blastocyst to endometrium.
Uterine mucosa contains numerous simple nonbranched uterine glands (crypts),
usually embryo attaches between their openings by the embryonic pole,
containing embryoblast. At this period blastocyst actively absorbs fluid from
uterine secretion and expands to 0,25 mm in diameter. Contacting with endometrium trophoblast
rapidly proliferates and begins to invade inside. Its outer layer
syncytiotrophoblast is multinucleated cytoplasmic mass with irregular villi
over the surface. It contains a lot of lysosomes, whose lytic activity
erodes the maternal tissues. Inner layer of cytotrophoblast consists of
mitotically active mononucleated ovoid cells.The
fingerlike processes of syncytiotrophoblast extend through the endometrial
epithelium and invade the connective tissue and blood vessels. The blastocyst
sinks into the endometrial stroma through the implantation window. Blastocyst
penetration causes firstly defect of endometrium, thus forming spaces around
villi (lacunae) and secondly the rupture of the endometrial blood vessels, with
overflow of blood into these lacunae. Later they become filled by a mixture of
maternal blood and secretions from eroded uterine glands. At this stage of
embryogenesis the earlier histiotrophic nutrition
takes place. Embryo is nourished by diffusion. Endometrial
surface over implanting embryo is gradually closed by fibrin and by
prolipheration of adjacent epithelium.In 12-day
embryo adjacent lacunae fuse to form network and maternal blood flows into
lacunae oxygen and nutrients become available to the embryo. So hematotrophic nutrition of embryo begins.Stromal
cells of endometrium are transformed into decidual cells that accumulate
glycogen and lipids.Process of
embryo implantation is under endocrine control: progesterone of ovarial corpus
luteus and estrogens. The syncytiotrophoblast produces chorionic gonadotrophin,
which enters the maternal blood and maintains the hormonal activity of corpus
luteum. By the way, determination of this hormone in urine allows identifying
pregnancy at the early stages.It
has to be noticed that successful implantation occurs “fifty-fifty”. | |
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