embryology 2 - fertilization to trilaminar Flashcards
fertilization - 4 steps
1) Sperm binds in a human specific interaction with the zona pellucida glycoprotein (ZP3)
2) Acrosomal enzymes released from sperm head; the sperm digests its way into egg.
3) Egg and sperm plasma membrane fuse and sperm contents enter the egg
4) Sperm entry triggers: - completion of meiosis 2 (fully matures) - release of cortical granules by oocyte (no other sperm can enter)
* typically occurs in the ampulla of the uterine tube
cleavage - first cell divisions
- The zygote now undergoes a series of mitotic divisions which subdivide the fertilised egg into many smaller daughter cells called blastomeres. Up to the 8 cell stage blastomeres are thought to be totipotent
what does totipotent mean
can form any cell including embryonic or placental
mosaicism
- Individuals with two or more cell line with different chromosome complements
when does mitotic nondisjunction occur
can occur in early division of the zygote
nondisjunction
when cell splits into 2 daughter cells it splits unevenly
what is a morula
16+ blastomeres
what are the blastomere cells confined in
within the zona pellucida, outer protective lining around cell
compaction of the morula
- Maximise available space by coming into closer contact with each other and begin to form cell junctions = start to adhere to each other
- Outer cells (in contact with ZP) form extensive gap junctions
- Fluid enters through ZP
blastocyst
consists of trophoblast, blastocyst cavity, inner cell mass and the zona pellucida
trophoblast
forms placenta
inner cell mass
forms embryo
blastocyst cavity
collection of fluid between cell masses
what is the embryonic pole
the eventual site of implantation into the uterus
why is it important for the blastocyst to hatch out the ZP
allows the embryo to directly interact with the endometrial lining of uterus for implantation
when does implantation occur
- Typically, by day 6 on posterior or anterior uterine wall. Cell pole attaches
corpus luteum
in ovary, forms every time an egg is released, it is a temporary endocrine gland. Secrets progesterone which tells the uterus to keep the thick endometrium lining for implantation. Eventually it gives up which is how periods are started
bilaminar embryo - day 7.5
- Trophoblast divides into Cytotrophoblast and Syncytiotrophoblast
- Embryo organizes into 2 layers – epiblast and hypoblast. Amniotic cavity begins to form
- Epiblast forms dorsal surface of embryo, hypoblast is ventral
- Syncytiotrophoblast produces hCG, bases of pregnancy test and tells Corpus luteum to keep producing Progesterone
day 9
- implantation complete (closed by fibrin coagulum)
- Vacuoles appear in the syncytiotrophoblast and unite to form lacunae (eventually sets up blood supply from mother to baby)
- Formation of 2 cavities – amniotic cavity (made by epiblast) primitive yolk sac (hypoblast)
day 12
- Establishment of the uteroplacental circulation
- Maternal blood flows into lacunae to diffuse to the embryoblast
- Extraembryonic mesoderm develops (outside embryo) and eventually degenerates forming the chorionic cavity
day 13
- Further development of the chorionic cavity and presence of connecting stalk (eventually umbilical cord).
- Second wave of hypoblast cells migrate to form the definitive (secondary) yolk sac
features of the implanted embryo
- Uteroplacental circulation forms as syncytiotrophoblast extends and contacts maternal blood supply
- The amniotic cavity is present above the bilaminar embryonic disc and the definitive yolk sac below
- The chorionic cavity has developed and encircles the entire embryo
ectopic pregnancy
- Implantation in a location outside the uterus.
- Dangerous and potentially life-threatening.
- Abnormal sites = ovary, uterine tube, peritoneum
formation of primitive streak
- Week 3
- Primitive streak appears in midline at caudal end of epiblast
- Cranial end of Primitive streak = Primitive Pit and Node
- Sets up body rotation
