2nd and 3rd weeks Flashcards
what day does the embryo implant in the uterus? what day is this usually finished
- what hormones influence this and how?
- where does the embryo implant? what layers does it penetrate
- what mediates this implantation?
- day 5-12 or week 2 the embryo implants. usually finished by day 9.
- progesterone and estrogen levels influence the initiation of implantation to prep the uterine wall
- embryo implants on the uterine wall on the EMBRYONIC POLE and this is receptor mediated by proteoglycans and glycoproteins
- this triggers a DECIDUAL REACTION in the uterine endometrium causing a thickening in the endometrium cells and an increase in cells for implantation
- compact layer forms closest to the embryo
-embryo penetrates the compact layer of the endometrium and forms SYNCYTIOTROPHOBLAST: formed by fusion of the cytotrophoblast cells and its invasion into the uterine endometrium. this synctiotrophoblast contains microvilli. proteases and other enzymes help with invasion
what are lacunae
they form within the syncytiotrophoblast. these are spaces that allow the MATERNAL vessels and glands to empty into them so that the embryo can receive nutrients BEFORE the formation of the true placenta.
-later develops into blood vessels of the placenta
the actual embryo only develops from the ___
epiblast
choriocarcinoma
tumor of the chorion which can be formed from the hydatiform mole due to two paternal pronuclei present in the embryo
where is the site of implantation? what are clinical correlations/ abnormal attachments?
You want the attachment to be intrauterine: middle of uterus or high on the walls.
abnormal:
-placenta previa: placenta attaches near or over the cervix. This can lead to premature detachment
- extrauterine: ectopic pregnancy outside the uterus. commonly in the outer half of the oviduct for a tubal pregnancy or abdominal/pelvic cavity. leads to nonviable pregnancy and mother gets bleeding 2-3 months into pregnancy.
- failure to attach intrauterinely can lead to spontaneous abortion (50%)…which actually is also a way for body to abort abnormal pregnancies caused by chromosomal abnormalities
formation of germ layers:
describe the formation of the amnion and its function
- it is formed from the epiblast
- this is the source of amniotic fluid. this fluid is clinically important for protection of embryo, proper lung formation, fluid homeostasis, and regulation of movement and body temperature
formation of germ layers :describe the formation of the yolk sac and its function
aka___
the yolk sack is also known as the primary umbilical vessel.
-formed from the hypoblast and coats the blastocyst cavity
- primary yolk sac forms around day 11 (remember bc 11 letters in the word) on the VENTRAL side of the embryo
- around day 14, bc trophoblast grows faster than the embryo, the SECONDARY yolk sac structure forms from an outgrowth of hypoblast cells
- thus the primary yolk safe degenerates
- yolk sac also has outgrowth called the ALLANTOIS which extends into the body stalk and is site of blood vessel formation.
fx: metabolism and transfer of nutrients from trophoblasts to embryo
- site of initial vascular formation and primordial germ cell production
- some portions become part of primitive gut
formation of the germ layers: describe the formation of extra embryonic mesoderm
what is the function
Extra embryonic mesoderm is MESCHYMAL layer rather than epithelium.
- undergoes epithelial meschymal transformation where the epithelial cells lose polarity and shape. they also down regulate their adhesion molecules (CAMs) (they can also change back)
- once detached, it coats the outer surface of amnion and yolk sac.
- function:
- source of vascular precursor cells
- will associate with the trophoblast to form the CHORION
- chorion: EEM, cytotrophoblast, syncytiotrophobast—> form chorionic villi (precursors to placenta)
-EEM will condense to form the BODY STALK –this suspends the embryo and connects it to the chorion. eventually this becomes the umbilical cord.
gastrulation: describe the formation of primitive streak
what is it?
and the function
what is gastrulation?
where does the embryo form from?
gastrulation is the formation of the 3 layered embryo (ectoderm, mesoderm, and endoderm)
- embryo forms from epiblast
- primitive streak is the thickening of the epiblast where cells undergo epithelial, mesenchymal transformation.
this leads to polarity of the embryo!differential growth leads to elongation to a more oval shape.
fx:
- primitive streak defines the caudal end of the embryo.
- allows formation of additional layers
- cell change allows migration and shift
what is the fate of the mesenchymal cells?
primitive streak cells of gastrulation migrate.
- first layer forms the definitive ENDODERM
- this eventually becomes the gut structures
- then the intraembryonic mesoderm will form
describe regression of the primitive streak
-when does it occur
occurs after formation, the primitive streak begins to regress around day 17 to 26
-it just isn’t keeping up with the cell
describe the formation of the primitive node and the function
thi is another site of the epithelial-mesenchymal transformation
-this occurs at the cranial end of the primitive streak which is the caudal end of the embryo!
function -important signaling center for the growth of future MESODERM structures
describe the formation of the intraembryonic mesoderm
what tissues arise?
- layer to form after the endoderm is the intraembryonic MESODERM
- this layer fans out laterally and cranially from the primitive streak and primitive node.
- becomes sandwiched btwn epiblast and the endoderm.
tissues:
-muscle, connective tissue, heart and blood vessels, NOTOCHORD
-kidneys, lungs, certain glands (from interaction with ectoderm and endoderm)
what are the 2 regions without mesoderm migration?
1) cloacal plate -anus
2) oral plate- becomes mouth
these 2 regions the ectoderm and the endoderm will touch without the mesoderm in between.
describe the formation of the notochord
- where does it form ?
- steps?
formed as part of mesoderm
- forms from central axis of the embryo
- begins as rod that grows cranially
1) invagination of primitive node
2) this forms notochord PROCESS
3) incorporated into the endoderm as notochord PLATE
