Implantation, placentation and week 2 development

what is the definition of infertility?

the failure to achieve a pregnancy after 12 months or more of regular unprotected sexual intercourse

how is IVF performed?

eggs are fertilised in a culture medium, then its placed in the uterus once it reaches the 8-cell stage

how is intracytoplasmic sperm injection performed?

a single sperm is injected into an egg cytoplasm

what is embryo cryopreservation?

its the freezing of a blastocyst

in vitro techniques come with an increased risk of what?

birth defects, prematurity, low birth weight and multiple births

when does implantation begin?

6-8 days post fertilisation

what happens on day 7 with regards to implantation?

trophoblast cells of the embryo contact and penetrate the endometrial epithelial cells

trophoblasts also divide and differentiate into cytotrophoblasts and syncytiotrophoblasts

the embryoblast of the embryo differentiates into epiblasts and hypoblasts

what happens on day 8 with regards to implantation?

cell mass begins to flatten and fluid collects between the epiblast and cytotrophoblast which forms the amniotic cavity.

syncytiotrophoblasts grow and begin to expand around the embryo

the bilaminar embryonic disc is formed

the decidual reaction occurs

what is a syncytiotrophoblasts?

a cell which loses their membrane and becomes a syncytium which then projects into the endometrium

what happens when the decidual reaction occurs?

the endometrium thickens and cells begin to enlarge, the endometrium becomes highly vascular allowing nutrients to be provided to the embryo

what happens on day 9 in regards to implantation?

the embryo is completely implanted in the endometrium

hypoblast cells migrate to form Heuser’s membrane

syncytiotrophoblasts completely surround the embryo and the trophoblastic lacuna forms

the amniotic cavity is expanding and the coagulation pug is marking the point of implantation

what happens on days 10-12 in regards to implantation?

extraembryonic mesoderm appears between Heuser’s membrane and the cytotrophoblast

trophoblast lacunae anastomose with maternal sinusoids, the lacunae fill with blood forming the uteroplacental circulation

what happens on days 12-13 with regards to implantation?

the chorionic cavity begins to form within the extraembryonic mesoderm, the extraembryonic mesoderm splits into the extraembryonic splanchnic mesoderm and the extraembryonic somatic mesoderm.

the chorionic cavity also begins to expand and leave a wedge of attachment where the 2 parts of the extraembryonic mesoderm meet

definitive or secondary yolk sac begins to form, the hypoblast has a wave of cell divisions and migration out over the inside of the extraembryonic splanchnic mesoderm pushing the primary yolk sac away causing it to pinch and degenerate

what happens on days 14 and 15 with regards to implantation?

thick stalk of extraembryonic mesoderm known as the connecting stalk begins to form, this stalk suspends the bilaminar embryonic disc containing the amnion and yolk sac. this allows the umbilical cord to form.

when does the placenta start to provide nutrients to the embryo?

Weeks 4-12

what are the functions of the placenta?

gas, nutrient and electrolyte exchange

barrier to teratogens

anchorage

hormone secretion

transmit maternal antibodies

when does placentation begin?

day 9

how is the primary chorionic stem villi formed?

its formed when the cytotrophoblast proliferates to form small projections into the syncytiotrophoblast

what is the 1^o villus composed of?

syncytiotrophoblasts and cytotrophoblasts

how is the 2^o villus formed and what is it composed of?

formed when extraembryonic mesoderm proliferates and invades the centre of the 1^o villus

composed of syncytiotrophoblast, cytotrophoblast and mesoderm

when is the 2^o villus formed?

early week 3

how is the villous capillary system formed?

formed when mesoderm cells begin to differentiate into blood cells and small blood vessels

what is the 3^o villus composed of and when is it formed?

syncytiotrophoblasts, cytotrophoblasts, mesoderm and villous capillaries

it is formed late into the 3rd week

when is the uteroplacental circulation fully established?

at the end of week 3

what happens during week 4 in regards to placentation?

cytotrophoblastic cells in the villi penetrate the overlying synctium until they reach the maternal endometrium

contact between villus systems is established

the outer cytotrophoblast shell gradually extends around the embryo surrounding the trophoblast entirely, attaching the chorionic sac to the maternal endometrium

what are the 2 parts of the foetal portion of the placenta?

chorion frondosum and chorion laeve

what are the 3 parts of the maternal portion of the placenta?

decidua basalis, decidua parietalis and decidua capularis

what is the placenta made up of?

chorion frondosum and decidua basalis

what happens during the second half of pregnancy in regards to placentation?

villi become very complex branched structures and are organised into cotyledons

chorionic frondosum has larger blood vessels that supply the villi

the placenta covers 15-30% of the internal surface of theuterus

what are cotyledons?

compartments within the placenta, clusters occur due to formation of decidual septa