Fertilization to Trilaminar Embryo

Question 1

What is the process of fertilization?

Answer 1

1. Sperm binds I a human specific interaction with the zona pellucida glycoprotein (ZP3).
2. Acrosomal enzymes released from sperm head; the sperm digests its way into egg.
3. Egg and sperm plasma membrane fuse and sperm contents enter the egg.
4. Sperm entry triggers: - completion of meiosis 2
   - release of cortical granules by oocyte.

Question 2

What is the only self propulsive cell in the body?

Answer 2

Sperm, so requires a large proportion of ATP.

Question 3

Where does fertilization typically occur?

Answer 3

Fertilization typically occurs in the ampulla of the uterine tube.

Question 4

How long does it take for the fertilized egg to get from the ampulla to the uterus?

Answer 4

It takes about 6 days.

Question 5

Zygotes undergo a series of what type of divisions?

Answer 5

mitotic divisions.

Question 6

What do the mitotic divisions do to the zygote?

Answer 6

They subdivide the fertilised egg into many smaller daughter cells called blastomeres.

Question 7

Up to what cell stage are blastomeres thought to be totipotent?

Answer 7

Up to the 8 cell stage blastomeres are thought to be totipotent.
After 8 cells they are said to be pluripotent.

Question 8

What is the definition of mosaicism?

Answer 8

It’s individuals with two or more cell lines with different chromosomal complements.

Question 9

What is mitotic nondisjunction?

Answer 9

failure either of two homologous chromosomes to pass to separate cells during the first meiotic division, or of the two chromatids of a chromosome to pass to separate cells during mitosis or during the second meiotic division. As a result, one daughter cell has two chromosomes or two chromatids, and the other has none.
This can occur in the early division of the zygote.
This can result in a zygote with trisomy 21 (Which is the most common form of down syndrome). Some cells will have a normal complement of cells and some with trisomy. Therefore they will show less prominent features.

Question 10

What is a morula?

Answer 10

A morula is what the zygote is called once it has 16+ blastomeres.

Question 11

What is the compaction of the morula?

Answer 11

Cells are confined within the zona pellucida (ZP).
They maximise the space by coming into closer contact with each other and begin to form cell junctions. Outer cells (in contact with ZP) form extensive gap junctions. Fluid enters through the ZP.

Question 12

What are 4 parts that make up a blastocyst?

Answer 12

Trophoblast
Blastocyst cavity
Inner cell mass
Zona pellucida

Question 13

What does the inner cell mass eventually give rise to?

Answer 13

The embryo proper/embryoblast, amnion and yolk sac.

Question 14

What will the trophoblast eventually form?

Answer 14

The placenta

Question 15

Embryos can interact with what for implantation?

Answer 15

Embryos can interact directly with the endometrial lining of the uterus for implantation.

Question 16

Eventually the blastocyst hatches from the zona pellucida why?

Answer 16

In order to get rid of the restrictive zona pellucida as it prohibits it from increasing in size.

Question 17

When and where does implantation usually occur?

Answer 17

It typically occurs by day 6, on the posterior or anterior uterine wall.

Question 18

In a bilaminar embryo day 7.5 Syncytiotrophoblast produces what and why is it important?

Answer 18

It produces human chorionic gonadotropin (hCG). This can be produced as early as the second week.
This is the basis for pregnancy tests and stimulates corpus luteum.

Question 19

In a bilaminar embryo day 7.5 what does a trophoblast divide into and when?

Answer 19

When the bilaminar embryo touches the endometrial wall, the trophoblast divides into: Cytotrophoblast (Cyto = cell - made of individual cells)
Syncytiotrophoblast (Syncytium = single multinucleated cell).

Question 20

In a Bilaminar embryo day 7.5 the embryo begins to organise into 2 cell layers, what are these?

Answer 20

Epiblast (The dorsal surface of the embryo where the cells are tall and columnar) and Hypoblast (The ventral surface of the embryo where the cells are more cuboidal).

Question 21

What feature begins to form in a bilaminar embryo day 7.5?

Answer 21

The abdominal cavity.

Question 22

What has happened by day 9 in a bilaminar embryo?

Answer 22

Implantation is complete and there is closure the fibrin coagulum.
Vacuoles appear in the syncytiotrophoblast and unite to form lacunae.
There is also the formation of two cavities:
Amniotic cavity (epiblast) (this gets a bit bigger)
Primitive yolk sac (hypoblast).
There is some apoptosis that creates holes/vacuoles.

Question 23

What happens in a day 12 Bilaminar embryo?

Answer 23

There is establishment of the uteroplacental circulation. The embryo gets it’s nutrients through diffusion.
Maternal blood flows into lacunae to diffuse to the embryoblast.
Extraembryonic mesoderm develops and eventually degenerates.
Forming the chorionic cavity. increase in cell structure.

Question 24

What happens in a bilaminar embryo at day 13?

Answer 24

There is further development of the chorionic cavity and presence of connecting stalk (eventually umbilical cord).
Second wave of hypoblast cells migrate to form the definitive (secondary) yolk sac.
All the holes that were surrounding the embryo have merged to form the extraembryonic cavity.

Question 25

What happens to the embryo’s circulation once its implants to the uterine wall?

Answer 25

Uteroplacental circulation forms as syncytiotrophoblast extends and contacts maternal blood supply.

Question 26

Where is the amniotic cavity present in an implanted embryo?

Answer 26

The amniotic cavity is present above the bilaminar embryonic disc and the definitive yolk sac below.

Question 27

A chorionic cavity develops in an implanted embryo. Where is it located?

Answer 27

It encircles the entire embryo.

Question 28

What is the chorionic cavity?

Answer 28

It is the space surrounding the primary yolk sac and the amniotic sac.

Question 29

What is the definition of ectopic pregnancy?

Answer 29

It is where implantation of the embryo occurs in a location outside the uterus. It is very dangerous and potentially life-threatening.

Question 30

What are some of the abnormal sites that ectopic embryo’s implant in?

Answer 30

Ovary
Uterine tube (Various locations within this)
Peritoneum
(90% of ectopic pregnancies occur in the fallopian tubes. It starts to create an area around it that is filled with blood. If it implants in an area not designed to expand to the size the embryo requires, then it will rupture. If it implants in the right fallopian tube, this can mimic the pain of appendicitis).

Question 31

What happens in week 3 of embryo development?

Answer 31

The primitive streak appears in midline at caudal end of epiblast.
The cranial end of the primitive streak = Primitive pit and Node.

Question 32

What is the definition of gastrulation?

Answer 32

It is the movement of epiblast cells through the primitive streak to form the three primary germ layers.
(This is where we go from 2 layers to 3. The 3 layers are the springboard from where systems are formed, the 3rd layer formed is the ectoderm).

Question 33

How are body axes established?

Answer 33

They are established by the presence of primitive streak.

Question 34

During gastrulation, Hypoblast cells are replaced by?

Answer 34

Hypoblast cells are replaced by the definitive endoderm.

Question 35

During gastrulation, cells migrate through what?

Answer 35

Cells migrate through the primitive streak between the epiblast and the definitive endoderm.
The epiblast becomes ectoderm.
2 germ layers become 3 (trilaminar embryo).
These 3 layers are the ectoderm, mesoderm and endoderm.

Question 36

What derives from the ectoderm?

Answer 36

The epidermis of skin, hair, nails, associated glands.

The entire nervous system also derives from the ectoderm e.g. brain, spinal cord, peripheral nerves.

Question 37

What happens to the mesoderm?

Answer 37

The mesoderm becomes divided into 3 regions either side of the notochord. These are called:
Paraxial mesoderm
Intermediate mesoderm
Lateral plate mesoderm:
- Somatic layer
- Visceral layer

Question 38

What is the fate of the paraxial mesoderm?

Answer 38

Axial skeleton, voluntary (skeletal) muscle, parts of the dermis (back).
(It forms all skeletal muscles from the neck down and forms a dermis).

Question 39

What is the fate of the intermediate mesoderm?

Answer 39

Urogenital systems

• Kidney and ureter
• Gonads and associated structures

Question 40

What is the fate of the lateral plate mesoderm, somatic part?

Answer 40

It creates most of the dermis, lining of body wall, parts of limbs.

Question 41

What is the fate of the lateral plate mesoderm, visceral part?

Answer 41

Cardiovascular system, mesothelial covering of organs, smooth muscle.
(The cardiovascular system and the heart form first to transport nutrients around the body).

Question 42

What is the fate of the endoderm?

Answer 42

Lining of the gut tube
Lining of respiratory tract
Lining of the bladder and urethra.
(Endoderm forms the gut tube)