Bilaminar Embryo Flashcards Preview

Histology / Embryology Unit 2 > Bilaminar Embryo > Flashcards

Flashcards in Bilaminar Embryo Deck (39):

What are the two stages of implantation?

Attachment and invasion


What are the two stages of attachment?

Apposition - Pinopodes (protrusions of uterine epithelium) interdigitate with microvilli on surface of trophoblast

Adhesion - Integrin receptors on embryonic pole of blastocyst bind extracellular matrix created by trophoblast and uterine epithelium


What are the two layers that the trophoblast differentiates into during invasion?

Cytotrophoblast - inner layer, mononuclear mitotically active which gives rise to outer layer
Syncytiotrophoblast - outer layer, multinucleated mass with no plasma membrane between cells. Penetrates through uterine epithelium first


What two compounds does the syncytiotrophoblast secrete?

1. Matrix metalloproteases - degrade components of basement membrane and extracellular matrix of uterine epithelium
2. Human chorionic gonadotropin - responsible for maintaining corpus luteum in ovary, preventing it from becoming corpus albicans


What are the two distinct cell layers that emerge once the blastocyst invades the uterine epithelium?

Embryoblast cells differentiate into epiblast (facing trophoblast) and hypoblast (facing blastocyst cavity)


What are amnioblasts and when do they form?

Differentiated from epiblast cells to form the amniotic cavity, they are adjacent to the trophoblast and will form the amniotic membrane.


When is the bilaminar embryo considered formed?

Around day 8-9, two adjacent layers of hypoblast and epiblast


What forms the Heuser's membrane?

Also called the exocoelomic membrane, hypoblast cells proliferate and migrate along inner surface of trophoblast / blastocyst cavity. The new layer of hypoblast cells around the cytotrophoblast is called the Heuser's membrane.


What is the primary (primitive) yolk sac?

Former blastocyst cavity which is now surrounded by Heuser's membrane. It forms around the same time as the amniotic cavity.


What forms the extraembryonic mesoderm?

Some of the hypoblast cells of Heuser's membrane differentiate into mesenchymal cells and fill the space between Heuser's membrane and the cytotrophoblast cells with mesenchymal cells


What is epithelial to mesenchymal transformation?

EMT - epithelial cells get off basal lamina and begin expressing mesenchymal proteins like vimentin and demonstrate invasive motility and shape change. First occurs to form extraembryonic mesoderm


What lies between the amnion and cytotrophoblast around days 11-12?

Extraembryonic mesoderm expands around this area (originally proliferates between Heuser's membrane and cytotrophoblast)


How is the extraembryonic coelom formed?

Small lacunae appear in the extraembryonic mesoderm and coalesce. Within 24 hours, this chorionic cavity is formed.


What are the two layers of the extraembryonic mesoderm after the chorionic cavity is formed?

1. Splanchnopleuric mesoderm - lines the primary yolk sac
2. Somatopleuric mesoderm - lines the cytotrophoblast and continues around amniotic cavity


What forms the extraembryonic endoderm?

While the chorionic cavity is forming, a second wave of hypoblast cells proliferates and displaces the Heuser's membrane lining the primary yolk sac. These hypoblast cells which line the splanchnopleuric mesoderm become the extraembryonic endoderm.


What forms the secondary / definitive yolk sac?

Cavity lined by migrating extraembryonic endoderm, it is much smaller than primary yolk sac.


How are the exocoelomic cysts formed?

Chorionic cavity expands and pinches off the primary yolk sac during formation of secondary (definitive) yolk sac. These are pushed to the abembyronic pole of the chorionic cavity and gradually disappear into the extraembryonic mesoderm.


What is the function of the connecting stalk?

It is a mass of extraembryonic mesoderm which connects the embryo to the chorionic plate, since the chorionic cavity is getting much larger.


What is the chorionic plate?

Also called chorion, it is the wall of the chorionic cavity. It is somatopleuric mesoderm + cytotrophoblast + syncytiotrophoblast


What is the role of the secondary (definitive) yolk sac in humans?

Transfer nutrients in weeks 2 and 3 before the uteroplacental circulation is established. Hemopoesis first occurs in the vascularized extraembryonic splanchnopleuric mesoderm until week 6 it starts in liver. Will also give rise to primordial germ cells in endodermal layer.


When does the primitive placenta start to form?

Around the same time as the formation of the primary yolk sac (Heuser's membrane bounds it)


What are trophoblastic lacunae?

Structures of the syncytiotrophoblast which anastomose to create a lacunar network. They surround capillaries and get filled with maternal blood.


What forms the primary stem villi / primary chorionic stem villi?

Proliferating cytotrophoblast cells. These chorionic stem villi undergo many transformations to come in contact with the lacunar network inside syncytiotrophoblast cells


What are the three stages of transformation of the chorionic stem villi?

1. primary - columns of cytotrophoblast surrounded by syncytiotrophobolast
2. secondary - extraembyronic somatopleuric mesoderm invades to form core
3. tertiary - mesenchymal cells in the extraembryonic somatopleuric mesoderm differentiate into capillaries and blood cells


How does blood get into the embryo?

Tertiary chorionic stem villi capillaries form arteriocapillary networks. They connect with embryonic blood vessels through the connecting stalk and attach to embryonic heart. Blood is flowing by end of week 3. (primitive placenta)


What is early spontaneous abortion? (ESA)

Pregnancy loss during first 3 weeks, might be high as 50% rate. random event that often goes undetected


What is recurrent spontaneous abortion? RSA

Three or more consecutive pregnancy losses prior to the 20th week of gestation. Causes may include genetic / anatomical anamolies, endocrine or immunological factors, infections, or teratogens. Happens more with increasing maternal age.


What is the primary cause of most RSA?

90% is caused by genetic anomalies, especially poly or monoploidy of X chromosome. 54% have normal karyotype, just some wrong genes


What is ectopic pregnancy and its main causes?

Pregnancy resulting from embryo implantation outside normal uterine sites. Blastocyst implants in abdominal cavity or uterine tube. Most are tubal pregnancies.

Causes: infections, inflammation, scar tissue from previous surgery in the area, or abnormal uterine tube development


What is the most common cause of ectopic pregnancy?

Pelvic inflammatory disease -> generally as a result of certain STDs such as gonorrhea or chlamydia


What are the diagnostic signs and treatment of ectopic pregnancy?

Severe abdominal pain, often associated with severe internal bleeding. Can be diagnosed by ultrasound and treated by surgical removal of ectopic embryo


What is a complete hydatidiform mole (CHM)?

Human conceptus that develops into a disorganized mass of placental derivatives, generally associated with two sets of paternal chromosomes. They are mostly cytotrophoblast with some syncytiotrophoblast


How can two paternal pronuclei occur?

1. Two sperm invade egg and maternal pronuclei is expelled
2. Materal pronuclei is expelled and paternal pronuclei divides


How is genetic imprinting involved in CHM formation?

When two male pronuclei are in the blastocyst, maternally expressed imprinted genes are not expressed. For trophoblast, maternally imprinted genes are not required, so this will proliferate. Maternally and paternally expressed is required for embryonic development.


What is recurrent biparental mole?

Familial cases of CHM can happen with both maternal and paternal chromosomes as a result of autosomal mutations which suppression maternally imprinted genes


How is CHM diagnosed and treated?

Vaginal bleeding towards end of first trimester. Ultrasound shows no fetus. hCG levels will become very high early on
Surgical removal and chemotherapy with methotrexate if cannot be fully removed (cancerous)


What is choriocarcinoma?

Aggressive cancer from malignant transformation of a trophoblast. ~50% are preceded by CHM, rest are normal preg, ectopic, or abortion.


How does choriocarcinoma metastasize and what are the signs?

Generally early spread to the lungs. Other regions may be included.
Excessive hCG.
Vaginal bleeding
Chest pain and chest X-ray infiltrates (lung metastasis)
Coughing up blood, SOB


What is the treatment for choriocarcinoma?

1. Surgical removal
2. Methotrexate chemotherapy.
Remission can be monitored by hCG levels