Second Week of Development Flashcards
What are extra-embryonic structures
- Structures not part of the embryo but essential for growth
- Amniotic cavity
- Amnion. (coming from the epiblast)
- Yolk Sac or umbilical vesicle (coming from the blastocystic cavity)
- Chorionic sac and connecting stalk
What happens in the second week of development
- Implantation of blastocyst into the uterine wall
- Bilamar embryonic disc forms
- 2 trophoblastic layers contiune developing
- Extra-embryonic structures form
- Blastocystic cavity becomes exocoelomic cavity (primitive yolk sac)
- End is marked by the appearance of the primary villi
How is the bi-laminar disc formed?
- A small space is created in the between the cytotrophoblast and the inner cell mass called the amniotic cavity
- A bilaminar plate of cells consisting of the:
-epiblast (high columnar cells related to the amniotic cavity) and the
-hypoblast (small cuboidal cells related to the exocoelomic cavity) which differentiates to mesodermal cells
Explain the development of the chorionic sac
- The primary chorionic villi from the cytotrophoblast protrude into the syncytiotrophoblast.
- The extraembryonic mesoderm forms between the cytotrophoblast, yolk sac and the amniotic cavity
- The extraembryonic mesoderm is split into two layers by a coelom:
- Extraembryonic somatic mesoderm lining the trophoblast and amnion
- Extraembryonic splanchnic mesoderm, surrounding the yolk sac
- The extraembryonic somatic mesoderm and whole trophoblast form the chorion which is the wall of the chorionic sac
- The embryo, amniotic cavity and yolk sac are suspending by the connecting stalk (extraembryonic mesoderm)
- The coelom = chorionic cavity
What development of the hypoblast happens in the second week of development?
The hypoblast thickens to form the prechordal plate which indicates the future site of the mouth and the future cranial region of the embryo
What is done to prevent the shedding of the endometrium?
- Human Chorionic Gonadotropin (HCG) is released by the blastocyst causing the corpus luteum to produce estradiol and progesterone
- In week 20 the placenta takes the hormone production control from the corpus luteum
How can the age and development of the embryo be evaluated?
By measuring the chorionic sac diameter with a transvaginal ultrasonography
How and when can we detect the implantation of the blastocyst to the uterine wall?
- With ultrasonography
- With highly sensitive radio-immuno-assays of HCG
- The end of the second week
What is the condition where implantation occurs in the lower segment of the uterus?
Placenta Previa
What is an ectopic pregnancy?
- This is when the blastocyst implants somewhere other than the uterus
- The most common ectopic site is the uterine tube
What are extra things occurs in the second week?
- Adjacent lacunae fuse to form lacunar networks
- Maternal blood seeps in and out of lacunar networks when the syncytiotrophoblast destroys endometrial blood vessels
