(2) Placenta, Fetal Membranes & Hormones

zygote

Fusion of the two nuclei and intermingling of the maternal and paternal chromosomes will create a ___________

fertilization

Process involved from the time of the release of the oocyte during ovulation to the fertilization to the development of the blastocyst

Fallopian tube (oviduct)

Where does fertilization normally occur?

Fimbriae

What structure catches the ovum after ovulation and transports it into the fallopian tube?

Spermatozoa

What must be present in the fallopian tube when the oocyte arrives for fertilization to occur?

Within a few hours, not exceeding 24 hours

How long after ovulation must fertilization normally occur?

Slow cleavage begins while the zygote is still in the fallopian tube

What happens to the zygote after fertilization?

zygote

Diploid cell with 46 chromosomes

Fallopian tube

Where does cleavage of the zygote occur?

2-cell → 4-cell → 8-cell

What are the stages of zygote cleavage?

Blastomere

What is each individual cell produced during cleavage called?

morula

This is a 16-cell stage; solid, mulberry-like ball of cells

3 days after fertilization → enters the uterine cavity

When is the morula formed, and where does it go?

blastocyst

Formed from the gradual fluid accumulation between morula cells

4–5 days after fertilization

When is the blastocyst formed?

Inner cell mass + outer cell mass

What are the two major cell populations of the blastocyst?

Embryo

What does the inner cell mass develop into?

Trophectoderm → trophoblasts → placenta

What is the outer cell mass also called, and what does it develop into?

6–7 days after fertilization

When does the blastocyst usually implant?

upper ⅓ of the posterior uterine wall

Where does the blastocyst usually implant?

Receptive endometrium primed with estrogen and progesterone from the corpus luteum

What is required for successful blastocyst implantation?

Decidualization

What is the process of preparing the endometrium for implantation called?

Apposition

What is the first phase of implantation, where the blastocyst is attracted to and attaches to the uterine wall?

Embryonic pole

During apposition, which part of the blastocyst attaches first?

Adhesion

What is the phase in implantatiom where the blastocyst firmly adheres to the endometrial surface with increased physical contact?

Invasion

What is the phase in implantation where trophoblasts proliferate and penetrate into the receptive endometrium?

Syncytiotrophoblast + cytotrophoblast

Which trophoblast layers penetrate the decidua during implantation?

Inner third of the myometrium + uterine vasculature

During invasion, how far can the trophoblast penetrate into the uterine wall?

Hemochorial placentation

What type of placentation occurs when maternal vascular structures come into contact with the trophoblast?

8 days after fertilization

When does the trophoblast differentiate into two layers?

Syncytiotrophoblast

What is the outer, multinucleated layer of the trophoblast?

Penetrates the decidua and myometrium → firmly attaches the placenta

What is the main function of the syncytiotrophoblast during implantation?

Cytotrophoblast

What is the inner layer of primitive, mononuclear cells in the trophoblast?

Germinal cells for the syncytiotrophoblast + covers the embryo

What is the role of cytotrophoblast cells?

Day 10 after fertilization

When does Blastocyst becomes completely encased within the endometrium?

Trophoblast + embryo-forming inner cell mass

What are the two zones of the thicker blastocyst wall opposite the uterine lumen?

Continues invading the myometrium and forms lacunae

What happens to the syncytiotrophoblast on Day 12 after fertilization?

Maternal blood

What fills the lacunae formed by the invading syncytiotrophoblast?

Hemochorial placentation

What type of placentation begins around Day 12 after fertilization when maternal blood contacts the syncytiotrophoblast?

Villous trophoblast + extravillous trophoblast

What are the two main trophoblast pathways formed after implantation?

Chorionic villi

What does the villous trophoblast generate?

chorionic villi

What transport oxygen, nutrients, and other substances between fetus and mother

Decidua and myometrium

Where does the extravillous trophoblast penetrate?

Maternal vessels

What can some extravillous trophoblasts penetrate into?

Interstitial + endovascular trophoblast

What are the two types of extravillous trophoblasts?

Decidua → myometrium → placental-bed giant cells

What does the interstitial trophoblast invade and eventually form?

Spiral arteries

What does the interstitial trophoblast surround?

Spiral artery lumens → cellular plugs

What does the endovascular trophoblast penetrate and form?

Destroys vascular endothelium via apoptosis → modifies vascular media → low-resistance blood flow

How does the endovascular trophoblast create the low-resistance blood flow characteristic of the placenta?

Transform them into large, low-resistance vessels

What do extravillous trophoblasts do to the spiral arteries during early pregnancy?

Coiling is removed → vessels become large and straightened

What happens to the spiral arteries after trophoblast invasion?

Ensures adequate blood flow to the placenta

Why is spiral artery transformation important?

Preeclampsia

What condition may result from failure of trophoblast invasion and spiral artery transformation?

Fetal growth restriction

What fetal complication may result from inadequate spiral artery transformation?

Continue to branch

What happens to the chorionic villi during placental development?

Macrophages (Hofbauer cells)

Which cells infiltrate the villous stroma during placental development?

Thins

What happens to the syncytiotrophoblast as the placenta matures?

↓ Cytotrophoblasts + ↓ stroma

What happens to the number of cytotrophoblasts and the villous stroma?

↑ Capillaries → closer to the syncytiotrophoblast surface

What happens to placental capillaries as the villi mature?

decidualization

Transformation of proliferating endometrial stromal cells into specialized secretory cells that support blastocyst implantation

Estrogen + progesterone + androgens + factors from the implanting blastocyst

What hormones and factors are essential for decidualization?

decidua

Specialized, highly modified endometrium of pregnancy

Allows maternal blood to contact the trophoblast during trophoblastic invasion

Why is the decidua essential for hemochorial placentation?

Factors regulating endometrial receptivity and immune and vascular cell functions

What does the decidua produce to support the maternal-fetal environment?

Decidual natural killer (NK) cells

Which cells mediate the immunomodulatory relationship between the decidua and invading trophoblasts?

Prevent maternal immune rejection of the fetal semiallograft

Why are decidual NK cells important during pregnancy?

Contains both maternal and paternal chromosomes

Why is the fetus considered a semiallograft?

decidua basalis/placentalis

Decidua directly beneath the implanted blastocyst and placenta → contributes to the placental basal plate

decidua capsularis

Decidua that covers the developing embryo and separates it from the rest of the uterine cavity

decidua parietalis

Decidua that lines the remainder of the uterus

Decidua vera

What forms when the decidua capsularis and parietalis come together as pregnancy progresses?

Uterine cavity is functionally obliterated → becomes a closed system

What happens to the uterine cavity when the decidua capsularis and parietalis fuse?

15–16 weeks → they fuse

Until approximately when does a virtual space exist between the decidua capsularis and parietalis?

After ~15 weeks, the uterine cavity becomes functionally sealed

What happens to the risk of intrauterine infection after the capsularis and parietalis fuse?

Trophectoderm

What embryonic structure gives rise to the trophoblast layer that surrounds the blastocyst?

naur

Is the placenta already fully formed during the early part of pregnancy?

hCG and LH

During early pregnancy, what maintains progesterone production by the corpus luteum?

Corpus luteum regresses → progesterone decreases

What happens to the corpus luteum if pregnancy does not occur and LH decreases?

Progesterone is thermogenic → increases body temperature

Why can basal body temperature be used to monitor ovulation and pregnancy?

Continued progesterone production by the corpus luteum

What does a sustained elevated basal body temperature after ovulation indicate?

No pregnancy → corpus luteum regression → ↓ progesterone

What does a drop in basal body temperature after ovulation suggest?

Spiral → radial → arcuate → ovarian/uterine arteries

What is the arterial pathway supplying the uterus and placenta?

Ovarian artery → abdominal aorta; uterine artery → internal iliac artery

Where do the ovarian and uterine arteries originate?

Trophoblast + mesenchyme

What is the chorion composed of?

Body stalk

What structure develops from condensed mesenchymal cells and eventually forms the umbilical cord?

hemochorial placenta

Direct contact between trophoblast and maternal blood

anchoring villi

What is a chorion frondosum villi extending from the chorionic plate to the decidua basalis

branching villi

What is a arborizing villi extending toward the intervillous space

Protect the baby, Nutrition, Respiration, Excretion, Hormone production

What are teh functions of placenta?

Placenta grows faster than the fetus

During the 1st trimester, how does placental growth compare with fetal growth?

Placenta and fetus are approximately equal in size

At approximately 17 weeks (mid-trimester), how do the sizes of the placenta and fetus compare?

Fetus becomes larger than the placenta

During the 3rd trimester, how does fetal size compare with placental size?

About ⅙ of fetal weight

At term, what proportion of the total fetal weight does the placenta weigh?

~470 g

What is the average weight of the placenta at term?

Round to oval; ~22 cm diameter and ~2.5 cm thick

What is the typical shape and size of the placenta at term?

Glistening, covered by transparent amnion

What are the characteristics of the fetal surface of the placenta?

Chorionic vessels

What structures course beneath the amnion on the fetal surface?