9TH DAY OF DEVELOPMENT
● The blastocyst is more deeply embedded in the endometrium, and the penetration defect in the surface epithelium is closed by a fibrin coagulum.
● The trophoblast shows considerable progress in development, particularly at the embryonic pole, where vacuoles appear in the syncytium.
● When these vacuoles fuse, they form large lacunae, and this phase of trophoblast development is known as the LACUNAR STAGE (very significant during day 9).
● At the abembryonic pole, meanwhile, flattened cells probably originating from the hypoblast forms a thin membrane that lines the inner surface of the cytotrophoblast.
● This membrane, together with the hypoblast, forms the lining of the exocoelomic cavity, or primitive yolk sac.
● The hypoblast with the continuation of cells lining the
cytotrophoblast is your exocoelomic membrane.
● Cytotrophoblast (light green in color shown in Fig. 15), invades some villi and syncytiotrophoblast.
● During the 8th day, syncytiotrophoblast is the outer layer and is multinucleated, but during the 9th day, it forms vacuoles.
● Epiblast near the amniotic cavity together with hypoblast is called bilaminar germ disc.
● Blood vessels and enlarged and engorged.
11th and 12th DAY OF DEVELOPMENT
● Mid of the second week: formation of your lacunar and maternal site.
● The blood vessels from the mother side enter the lacunar space - formation of fetal and maternal circulation.
● By the 11 th and 12 th day of development, the blastocyst is completely embedded in the endometrial stroma.
● And the surface epithelium almost entirely covers the original defect in the uterine wall.
● The blastocyst now produces a slight protrusion into the lumen of the uterus. The trophoblast is characterized by lacunar spaces in the syncytium that form an intercommunicating network.
● This network is particularly evident at the embryonic pole, the trophoblast still consists mainly of cytotrophoblastic cells.
● Concurrently, cells of the syncytiotrophoblast penetrate deeper into the stroma and erode the endothelial lining of the maternal capillaries. These capillaries, which are congested and dilated, are known as SINUSOIDS.
● As the trophoblast continues to erode more and more sinusoids, maternal blood begins to flow through the trophoblastic system, establishing the UTEROPLACENTAL CIRCULATION.
● Extraembryonic mesoderm- derived from yolk sac cells, form a fine, loose connective tissue, splits into 2:
○ Somatic mesoderm - lining the cytotrophoblast (and
amnion)
○ Splanchnic mesoderm - lining covering the primitive yolk
sac (exocoelomic cavity).
13th DAY OF DEVELOPMENT
● Surface defect in endometrium healed.
● Bleeding occurs at the implantation site as a result of increased
blood flow into the lacunar spaces, and occurs near the 28th day of
menstrual cycle.
● Because this bleeding occurs near the 28th day of the menstrual cycle, it may be confused with normal menstrual bleeding, therefore causing inaccuracy in determining the expected date of delivery.
● Bleeding occurring near or at the 27-28th day of fertilization is an implantation bleeding due to an extra position of capillaries of blood coming from the maternal side going to lacunar spaces.
● The trophoblast is characterized by a villous structure.
● Cells of the cytotrophoblast proliferate locally and penetrate into the syncytiotrophoblast forming cellular columns surrounded by syncytium.
● Cellular columns with the syncytial covering are known as primary villi.
● Secondary yolk sac (definitive yolk sac) - a new cavity within the exocoelomic cavity as a result of proliferation and migration of additional cells produced by hypoblast to the inside of the exocoelomic membrane; much smaller than the original exocoelomic cavity (primitive yolk sac).
● Exocoelomic cysts - large portions of the exocoelomic cavity that are pinched off and are often found in the extraembryonic coelom.
● Chorionic cavity (extraembryonic coelom) - the result of
extraembryonic coelom expansion, forming a large cavity. The extraembryonic mesoderm lining the inside of the cytotrophoblast is then known as the chorionic plate.
● Connecting stalk - the place where extraembryonic mesoderm traverses the chorionic cavity. With development of blood vessels, the stalk becomes the umbilical cord.
1th and 12th DAY OF DEVELOPMENT
● The second week of development is known as the week of 2's:
○ The trophoblast differentiates into two layers:
cytotrophoblast and syncytiotrophoblast.
Syncytiotrophoblast is responsible for hormones especially
the human chorionic gonadotropin (Perfect time to take
pregnancy test) - as early as second week and that can be
detected.
■ By day 9, lacunae develop in the syncytiotrophoblast. Subsequently, maternal sinusoids are eroded by the syncytiotrophoblast, maternal blood enters the lacunar network, and by the end of the second week, a primitive
uteroplacental circulation begins.
■ The cytotrophoblast, meanwhile, forms cellular
columns penetrating into and surrounded by the
syncytium. These columns are primary vili. By the
end of the second week, the blastocyst is
completely embedded, and the surface defect in
the mucosa has healed.
○ The embryoblast forms two layers: the epiblast and
hypoblast. This will be your bilaminar germ disk.
○ The extraembryonic mesoderm (cells extending from
hypoblast) splits into two layers: the somatic and splanchnic layers. Splanchnic lines trophoblast going to primitive yolk sac.
Somatic lining trophoblast.
■ Extraembryonic mesoderm lining the
cytotrophoblast and amnion is extraembryonic
somatic mesoderm; the lining surrounding the yolk
sac is extraembryonic splanchnic meso- derm.
○ The cavities form: the amniotic adjacent to epiblast and yolk sac cavities adjacent to trophoblast.
■ Extra-embryonic mesoderm cells migrate between
the cytotrophoblast and yolk sac and amnion.
■ At the end of the second week, it is possible to
distinguish the dorsal (amniotic cavity) from the
ventral (yolk sac) side of the embryo.
MENSTRUAL CYCLE
● The average menstrual cycle is 28 days. Day 28 of the ending cycle is the same as day zero of the cycle that is about to start.
● The normal range for a menstrual cycle is 25 to 35 days.
● The typical amount of blood lost during menstruation is 30 mL.
● Relation of body temperature whether having an ovulation.
● Anterior pituitary hormone - produces Follicle-stimulating hormone (FSH) and Luteinizing hormone (LH)
● Ovarian hormones - estrogen and progesterone
● A woman's menstrual cycle is regulated by several hormones: estrogen, progesterone, luteinizing hormone (LH), and follicle stimulating hormone (FSH). The menstrual cycle can be divided into 2 phases: the follicular or proliferative cycle and the luteal or secretory
phase.
● Day 1 of the menstrual cycle is marked by the first day of bleeding.
This also marks the beginning of the follicular phase which persists until ovulation. During this time, body temperatures are lower and the development of ovarian follicles begins.
● After the peak in LH stimulating ovulation, there is a drop in estrogen levels.
The next phase- the luteal phase, is dominated by progesterone. The luteal phase is typically 14 days long. Following ovulation, the remaining cells that are not released at ovulation become enlarged. This eventually forms the corpus luteum.
The corpus luteum is a transient (short lived) endocrine organ that primarily secretes progesterone.
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