د.منى زهير Lec.1
Bilaminar germ disc
Second week of development
Eighth day of development (Day 8):
The blastocyst is partially embedded in the endometrial stroma .
The endometrial stroma near the implantation site is edematous and highly vascular and the glands secrete a lot of glycogen and mucous.The trophoblast differentiate into two layers :
Cytotrophoblast :an inner layer of mono nucleated cells (with distinct cell boundaries).
Syncytiotrophoblast or syncytium : an outer layer of multinucleated cells without distinct cell boundaries.
4.The embryoblast differentiate into two layers :
Epiblast or ectodermal germ layer : a layer of tall columnar cells.
Hypoblast or endodermal germ layer : a layer of small cuboidal cells. Each germ layer will form a disc, and together they are called bilaminar germ disc .
5. A small cavity appears in the epiblast, this cavity enlarges to become the amniotic cavity. The epiblastic cells will form the lining of the cavity, and the cells that form the roof are called amnioblast .
Ninth day of development (Day 9) Lacunar stage :
(Day 9) :
The blastocyst is deeply embedded in the endometrial stroma and the site of penetration is closed by a fibrin coagulum.
Vacuoles appear in the syncytium, when these vacuoles fuse they form large lacunae. So this is called lacunar stage .
Flattened cells arise from the hypoblast, these cells will form a membrane called exocoelomic membrane or heuser's membrane along the inside of the cytotrophoblast. This membrane with the hypoblast will form the lining of the exocoelomic cavity (primitive yolk sac)
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Eleventh to twelfth days of development (Day 11&12):
(Day 11&12):
Blastocyst is completely embedded in the endometrial stroma and the site of implantation is almost entirely covered by the surface epithelium.
The endometrial cells become polyhedral and filled with lipid and glycogen .the intercellular spaces are filled with fluid, and the tissue is edematous… . these changes known as the decidual reaction .
Due to continuous invasion of the syncytial cells into the endometrial stroma, the lacunae become continuous with the maternal sinusoids (sinusoids is a congested and dilated capillaries), so blood will flow from the sinusoids to the lacunae, thus establishing the uteroplacental circulation.
A new group of cells appear (originating from the cytotrophoblast) between the cytotrophoblast and the exocoelomic membrane, forming a thin, loose connective tissue layer called the extra embryonic mesoderm . many cavities develop in the extraembryonic mesoderm and when they fuse , a large single cavity will form known as the extra embryonic coelom . the extraembryonic mesoderm lining the cytotrophoblast is called the extra embryonic somatopleuric mesoderm , and that covering the primitive yolk sac and amniotic cavity is called the extra embryonic splanchnopleuric .
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Thirteenth day of development (Day 13) :
(Day 13) :
The site of penetration is healed (although bleeding may occur at this time, which may be confused with the normal menstrual bleeding, and may cause an inaccuracy in determining the date of delivery).The cytotrophoblastic cells grow and penetrate into the syncytium forming cellular columns inside the syncytium is known as the primary stem villi .
Another group of cells arise from the hypoblast forming a membrane bounding a new cavity inside the original exocoelomic cavity. This cavity is known as the secondary or definitive yolk sac. The original exocoelomic cavity degenerates and represented by small cyst called exocoelomic cyst.
The extra embryonic coelom enlarges forming the chorionic cavity. The extra embryonic mesoderm lining the cytotrophoblast is then known as the chorionic plate. The only place where the extra embryonic mesoderm traverses the chorionic cavity is in the connecting stalk (with the development of blood vessels, the stalk will become the umbilical cord).
The hypoblastic cells (in the cephalic region of germ disc) become columnar and firmly attached to the epiblast. This region is known as the prochordal plate or buccopharyngeal membrane.
Note: the second week of development is referred to as the period of two since during this period, the trophoblast differentiate into two layers, the embryoblast differentiate into two layers, and two cavities are formed, amniotic and yolk sac cavities.