Embryology

Question 1

Define the term morula.

Answer 1

It is a solid ball of cells formed from a series of motorists divisions of the fertilised ovum, each of the cell is identical to the fertilised ovum.

The blastocyst is formed from the morula

Question 2

Define the pre-embryonic period?

Answer 2

The pre-embryonic period is the first two weeks of gestation.

During the pre-embryonic period we see cleavage of the fertilised ovum to form the morula. We also see compaction to form the blastocyst from the morula, and then implantation of the blastocyst into the wall of the uterus.

Question 3

Define the term zygote

Answer 3

A zygote is a fertilised oocyte

Question 4

Where does fertilisation of the oocyte normally occur?

Answer 4

Fertilisation usually occurs in the ampulla of the Fallopian tube

Question 5

Where is the ideal site for implantation?

Answer 5

High up on the posterior Uterine wall

Question 6

Why is there a difference between the time of clinical gestation and biological gestation?

Answer 6

Because clinical gestation is calculated from the beginning of the mother’s last menstrual cycle, which is actually 2 weeks before fertilisation occurs. Oocyte is release from the ovaries on the 14th day of the menstrual cycle.

Question 7

What is the first developmental process seen in the zygote?

Answer 7

First process seen is cleavage. The zygote divides into two identical daughter cells of equal size, these are called blastomeres. It begins 30 hours after fertilisation. Cleavage continues, mitotic divisions continue and each daughter cell progressively gets smaller and smaller, leads to the formation of a morula.

Question 8

What is the zona pellucida?

Answer 8

It is a glycoproteins shell which was created during maturation of the oocyte inside the oocyte. It acts as a protective outer coat and ensures that once one sperm has fertilised the oocyte no further sperm are capable of entering the zygote.

Blastocyst hatches from the zona pelucida in order to implant into the endometrium.

Question 9

Describe briefly the process of assisted reproductive techniques?

Answer 9

Oocytes are fertilised in vitro by male gametes and allowed to divide to the 4 or 8 cell stage

The morula is then transferred into the uterus

Question 10

Describe the process of compaction?

Answer 10

It is the developmental process which results in the formation of the blastocyst from the morula.

Is where we see the first differentiation, generates two different cell populations, the trophoblast and the embryoblast.

Also see the first cavity appearing, the blastocyst cavity.

Before compaction the cells are totipotent, could become any cell type.

After compaction the cells are pluripotent, can become many cell types but not any.

Question 11

Why is it essential for the blastocyst to hatch from the zona pelucida?

Answer 11

Blastocyst is unable to interact with the endometrium when the zona pelucida is present. Needs to hatch in order to continue development, interaction with the maternal tissue is essential for the maintenance of the pregnancy.

Zona pelucida also restricts the growth of the blastocyst, in order for the blastocyst to grow it needs to hatch from the zona pelucida.

Question 12

What is the embryoblast and the trophoblast?

Answer 12

Embryoblast is the inner cell mass of the blastocyst that will give rise to the embryo.

Trophoblast is the outer cell mass of the blastocyst that will give rise to the the supporting tissues of the embryo

Question 13

Define the syncytiotrophoblast.

Answer 13

It is the barrier across which the transport between the embryo and the mother will occur.

It is a multinucleated sheet that sends finger like projections out into the endometrium to access the glands and sinusoids

Question 14

Define the cytotrophoblast

Answer 14

It is a epithelial stem cell layer for the syncytiotrophoblast.

Question 15

What occurs during implantation of the blastocyst in the uterine wall?

Answer 15

The conceptus becomes embedded in the tissues, fibrin plug closes the opening by which the conceptus entered the uterine wall.

Maternal blood flow is established within the placenta

The chorionic villus is established, the structure by which the embryo receives oxygen from the mother and removes CO2

Question 16

Name for conditions linked to implantation defects?

Answer 16

Intrauterine growth restriction pre-eclampsia
ectopic pregnancy
placenta praevia

Question 17

Why can an ectopic pregnancy progress to a life threatening emergency?

Answer 17

In an ectopic pregnancy implantation occurs at an inappropriate site within the female reproductive tract, most commonly the Fallopian tube as a result of the fertilised ovum not being transported into the uterus.

This can become a life threatening emergency because implantation is very invasive and only the endometrium of the reproductive tract is capable of tolerating that invasive power.

Would have unchecked invasion into nearby blood vessels of the pelvis leading to catastrophic intraperitoneal haemorrhage

Question 18

What is placenta praevia? Why is it dangerous in pregnancy?

Answer 18

Placenta praevia is a condition whereby the implantation of the conceptus has occurred in the lower uterine segment.

It is dangerous because it can cause haemorrhage during pregnancy as the placenta grows across the exit of the reproductive tract

Question 19

What percentage of zygotes are lost in the first 2-3 weeks of gestation?

Answer 19

50% are lost in the first 2-3 weeks

Question 20

What percentages of diagnosed pregnancies will miscarry?

Answer 20

15%

Question 21

At which stage of gestation is the conceptus most sensitive to teratogenic agents and why?

Answer 21

Embryonic stage (3-9 weeks post fertilisation), this is because this is this stage where the systems of the body are initially generated from a bilaminar disc, embryonic cells begin to specialise.

Question 22

At which stage of development would exposure to teratogenic agents most likely lead to lethal effects?

Answer 22

Pre-embryonic (0-2 weeks)

Question 23

What is the primitive streak?

Answer 23

It is a morphological feature which appears on the dorsal surface of the epiblast at the beginning of the third week of development. It’s appearance signals the start of gastrulation.

Question 24

Why can human embryos not be manipulated after the appearance of the primitive streak?

Answer 24

It is because the appearance of the primitive streak indicates the presence of a single individual

Question 25

What occurs during gastrulation?

Answer 25

Gastrulation occurs in the third week of development and marks the start of the embryonic period

Initiated by the appearance of the primitive streak, forms in the caudal epiblast

Epiblast cells proliferate and migrate to and through the primitive streak

As they pass through the primitive streak they undergo a degree of differentiation

Migrating epiblast cells displace the hypoblast, form a trilaminar disc consisting of the three germ layers, ectoderm, mesoderm and endoderm

Convert the one epiblast layer into 3 embryonic germ layers

Question 26

Name a two tissue types derived from each of the germ layers?

Answer 26

Ectoderm - epidermis of the skin
Nervous system

Mesoderm - heart, skeletal muscle

Endoderm - epithelial lining of the GI tract and Respiratory Tract, parenchyma of glands

Question 27

How is left-right asymmetry established?

Answer 27

Cells in the node of the primitive streak express cilia

These cilia beat in a co-ordinated manner which pushes signalling molecule responsible for the development of left sided features to the left

The presence of these signalling molecules on the left side of the trilaminar disc elicits a series of side specific signalling cascades which ensure left-right asymmetry

The absence of these signalling molecules on the right side leads to the development of right sided features

Question 28

Why does gastrulation occur?

Answer 28

To ensure the correct placement of precursor tissues to allow subsequent morphogenesis to take place