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what happens in the week of 2's?

By 2nd week!>> Day 8


2 distinct cellular layers emerge from:

  • outer cell mass

– syncytiotrophoblast

– cytotrophoblast

  • inner cell mass becomes the bilaminar disk

– epiblast

– hypoblast 



Define implantation & what day does it occur? how long does it take? when does it end?

early stage of preganancy in which conceptus adheres to the wall of the uterus (endometreium)

the uterine epithelium is breached and the conceptus implants w/ in the stroma

begins 6-7 days after fertilization!>> when endometrium is propper and ready


takes about 5 days and usually compeltes 12th day after ovulation (normally when the endomterium walls sloughs off)


why does the blastocyst have to wait an extra 1-3 days before it implants on the endometrium? what is keeping it alive until then?

after reaching the uterus, the developing blastocyst just chills there for extra 1-3 days before it implants...until is nourished from the uterine endometrial rich glycoprotein secretion "uterine milk"

bc weyre waiting for the endometrium to be properly ready!


Describe the processes of implantation of the conceptus into the endometrium 


describe the blastocyst structure!


what occurs by the end of the 2nd week?

the conceptus has implanted (not yet done with the process tho)


what does implantation acheive?

(explain the 3 different villi)

during implantation, trophoectoderm cells begin to differentiate to cellular subtypes that will characterize the placenta.


its gunna establish the major unit of exchange!


–  primary villi: early finger-like projections of trophoblast

–  secondary villi: chornionic extensions grows into primary villi

–  tertiary villi: fetal BV form in the vill!


Implantation defects r characterized into 2 catagories, name them...

  1. Implantation in the wrong place 
  2. Incomplete invasion (invasive element of implantation r incomplete!)


Implantation in the wrong place (2 conditions)

–  Ectopic pregnancy

–  Placenta praevia 


cells of the endometrium r critically important for modulating the invasive reaction during implantation.

many of the enzymes released by the trophoblasts allows the the process of implantation to occur and allow access of conceptus into the endometrium!


these enzymes can be extremely how does endomterium cope with it?

what does that tell u if u had ectopic pregnancy at a site where u dont have endometrium?...

The decidual reaction provides the balancing force for the invasive force of the trophoblast!!


in ectopic pregancy>> no decidua layer>> no control>> invasion can go right into the wall of the uterine tube and out into the pelvic cavity where uve got a larger room.


thats why its a life-threatening emergency!


Ectopic pregancy

where does it most commonly occur?

most common in fallopian tube

can be peritoneal as well

life threatening


Placenta previa

what can it cause?

implanatatin at lower uterine segment

if it crosses the internal os of cervix>>  can cause hemmorhage in preganacy

birth canal is blocked>> requires C- section



what happens to the embryonic spaces during placentation? (yolk sac, amniontic sac, chorionic sac)

  • yolk sac dissappears (pinches off and later become gut tube)
  • amniotic sac enlarges
  • chorionic sac is occupied by expanding amniontic sac! (amniotic sac keeps growing until it closed it up)


what is placentation?

formation of the placenta "flat cake"

a temporary organ that originated both from embryonic & maternal (endometrial) tissues.


the placenta is specialisation of what?

the placenta is specialization of the chorionic membrane


endometrium undergoes "decidual change"

describe it

when the conceptus implants the endometrium, via the continued release of progesterone, the endometrium cells SWELL further & store more nutrients.

these cells r now called Decidual cells and the total mass of cells r called decidua!

decidual reaction is "transformed epithelium"


what is a chorionic villi? chornionic frondosum?


explain the different villi made 

 major unit of exchange!


–  primary villi: early finger-like projections of trophoblast

–  secondary villi: chorionic extensions grows into primary villi

–  tertiary villi: fetal BV form in the vill!


Describe the structure of the placenta and its explain its adaptation for the exchange of materials between fetal and maternal blood 


-has a mass of tree-like placental cotyledons called "villi" which r bathed in maternal blood!

blood enters space btw villi (intervillus space) through high-flow maternal spiral (endometrial) vessels.

fetal vessels r located w/ in the cotyledons (villi)

cytotrophoblast surrounding tip of each villi will make a "outer-trophoblastic shell"

The outer layer of the placental villae is coated by a syncytiotrophoblast.

The syncytiotrophoblast is directly bathed in maternal blood in the mature placenta




The______________is directly bathed in maternal blood in the mature placenta

The syncytiotrophoblast is directly bathed in maternal blood in the mature placenta


Structure of the chorionic villus and thinning of placenta over time (in 1st trimester & 3rd trimester )

1st trimester villus >> thick barrier

3rd trimester villus >> thin barrier


Describe the arrangement of fetal blood vessels within the placenta 

• 2 umbilical arteries>> Deoxygenated blood from fetus to placenta

• 1 umbilical vein >> Oxygenated blood from placenta to fetus 


umbilical A. winds around the vein


what is the function of the cytotrophoblast surrounding the villia?

it acts as a stem cell layer for the syncitotrophoblast.

allows it to keep growing > so we can build room for growth

(cytyotrophoblast is a specilaised version of the syncitiotrophoblast)




What tissues constitute the placental barrier at the 3rd trimester?

Only syncytiotrophoblast & fetal capillary endothelium 

(its gotten thinner cuz baby metabolic needs r high and we need good transfer)



what gives the placenta its glossy appearnece?

the amniotic transparent memebrane


Placental functions


list hormones secreted by placenta?

-secretes progesterone & estrogen

-HcG & HcS


Human Chorionic Gonadotrophin (hCG) 

produced by? how many days after fertilization is it secreted for? how long is it secreted for? function?

• produced during the first 2 months of pregnancy

• supports the secretory function of CL

• produced by syncytiotrophoblast therefore is pregnancy specific

excreted in maternal urine therefore used as the basis for pregnancy testing

• used to diagnose trophoblast disease


when does placenta take over the function of the CL

by the 11th week (1st trimester)