GENERAL EMBRYOLOGY

What are the main activities that take place in week 1?

1. Fertilisation

2. Cleavage

3. Implantation

Approximately 24 hours post fertilisation, what happens to the zygote?

It undergoes cleavage

What is cleavage?

Cleavage is the process whereby the zygote undergoes rapid mitotic division to produce any smaller cells in each division

What are the “many smaller cells “ called?

Blastomeres

At which stage is the cell termed as Morula

16-32 cell stage on day 3

What notable activity happens concurrent to cleavage

As cleavage occurs, the blastocyst moves along the uterine tube heading towards the uterus

As the blastocyst enters the uterine cavity, fluid moves from the walls into it creating a cavity called…

Blastocystic cavity

The blastocystic cavity or blastocoele divides the blastocyst into two portions. And outer portion and an inner portion called

Trophoblast and Embryoblast respectively

What happens to the Zona Pellucida after the two layers are formed?

It disintegrates

What is the time frame for implantation?

Day 7-11

What is another name given to week two due to the activities that occur there?

“The week of twos"

What happens to the trophoblast in week 2?

It divides into two;

Cytotrophoblast(inner)

Syncitiotrophoblast(outer)

On the first day of implantation, a layer of cuboidal cells form on the embryoblast facing blastocoele called

The hypoblast

The remaining embryoblast becomes a layer of columnar cells called

Epiblast

After the epiblast forms a layer of cells line the blastocoele. At this stage, the blastocoele is called

Primary/primitive yolk sac

What is the name of the layer of cells lining the primary yolk sac?

Heuser's membrane

Later on in development, a portion of the primitive yolk sac pinches of giving rise to

Definitive yolk sac and Exocoelomic cyst

At some point the hypoblast swells to become columnar and fuse with the epiblast forminga structure called

Prochordal plate

What structure develops btwn the exoceolomic lining and the cytotrophoblast?

Extraembryonic mesoderm

What are the two layers of the extraembryonic mesoderm

Somatic and Splanchnic embryonic mesoderm

What does the somatic extraembryonic do?

It covers the exoceolomic cavity, forms the connecting stalk and covers the amnion

How about the splanchnic extraembryonic mesoderm?

It covers yolk sac

After the formation of the exocoelomic cyst and definitive yolk sac, the cavity formed is termed as

Chorionic cavity

What are the components of the Chorion?

The two layers of the trophoblast and the somatic extraembryonic mesoderm

The syncitiotrophoblast produces the hormone hCG to cause what to continue producing what?

The Corpus leteum to keep producing Progesterone

Until what time does the syncitiotrophoblast produce hCG?

Until week 8 that the placenta takes over progesterone production

By what day should the bilamimar embryonic disc be fully implanted in the endometrium?

By day 13

Projections of cytotrophoblast called…….penetrate into the syncitiotrophoblast

Primary villi

What are the components of umbilical cord?

Umbilical veins

And Umbilical arteries

How many umbilical veins are there?

1

How many umbilical arteries are there?

2

What type of blood does the umbilical artery carry and from where to where?

The umbilical arteries carry DEOXYGENATED blood from the the fetus to the mother via the placenta

What type of blood does the umbilical vein carry and from where to where?

It carries OXYGENATED blood from the mother to the fetus

What is the embryonic period?

Week 3 -8

During the formation of the placenta what causes the primary villi to become secondary villi?

Mesoderm invades the cytotrophoblast core of the primary villi

What causes the secondary villi to become the tertiary villi

Mesenchymal cells in the mesoderm differentiate into hemangioblast

The part around the embryo with yhe most villi is

Basal residua

Which part of the syncitiotrophoblast eventually becomes the placenta?

Chorion frondosum

The part with lesser villi which layer atrophies is called

Chorion laeve

When is placental circulation established?

Day 17

When does gastrulation, appearance of primitive streak and development of notochord start?

Week 3

What is the primitive streak?

A longitudinal groove of mesenchymal cells that form in the media plane of the epiblast