embryology

Question 1

What is fertilisation?

Answer 1

Fertilisation is the first step.

This is when a sperm and an oocyte unite to create a diploid cell called a zygote (46 chromosomes)

Only 1 sperm can fertilise the oocyte due to changes in the cell membrane.

This occurs in the uterine tube.

Chromosomal sex of the embryo is determined at fertilisation.

Initiation of cleavage then commences

Question 2

Cleavage

Answer 2

This is a series of mitotic divisions increasing the number of cells – blastomeres.

After 3 days there are 16 cells (called a morula)

Day 5 – the morula reached the uterine cavity and is now called a blastocyst

Question 3

What is Blastocyst?

Answer 3

The blastocyst implants in the endometrium of the body of the uterus 6-7 days after ovulation

The endometrial glands enlarge and there is an increase in blood supply

The blastocyst burrows into the endometrium and becomes surrounded

There are 2 parts of the blastocyst
- Embryoblast – develops into the embryo
- Trophoblast – outer later that becomes part of the placenta.

Implantation is complete is complete by day 14

Question 4

What us the bilaminar embryonic disc?

Answer 4

The trophoblast erodes the lining of the maternal capillaries of the endometrium

Maternal blood begins to flow through the trophoblastic system – forming a uterus / placenta circulation

The embryoblast forms a flat disc called the bilaminar embryonic disc.

A small cavity forms within the blastocyst to form the amniotic cavity

Question 5

Development of the yolk sac

Answer 5

Day 10:

Cells of the embryoblast form a thin membrane which is the wall of the yolk sac.

The yolk sac:
Supplies nutrients to the embryo during weeks 2-3
Is a source of blood cells during weeks 3-6
Contains the first germ cells (eventual oocytes)
Is a shock absorber.
Stops the embryo drying out.

Question 6

Development of the chorion

Answer 6

The chorion is formed from the cells of the embryoblast

The chorion surrounds the embryo and later the fetus

It becomes the embryonic part of the placenta

It protects from an immune response from the mother
Secretes proteins blocking antibody production by the mother

Promotes the production of lymphocytes that suppress immune response in the uterus

Produces HSG

Question 7

Day 15 – gastrulation

Answer 7

A third layer of cells form in the embryoblast

These layers form the body systems
Ectoderm – nervous system, skin, skeletal

Mesoderm – major organs

Endoderm – lining of the digestive, urinary and respiratory tract

The embryonic disc forms the body of the embryo

The body and organs of the embryo begin to form - embryogenesis

Question 8

embryology: Week 4

Answer 8

Embryonic disc continues to gro

A bulge develops and projects into the amniotic cavity

There is a head and tail fold

Question 9

Ectoderm

Answer 9

By day 22 ectoderm thickens and forms neural folds. This becomes a tube

The anterior end becomes the brain, the rest is the spinal cord

By the end of week 8 the brain hemispheres are in place

Develops the internal linings of the GI tract and the respiratory tract

Also forms the linings of the endocrine, auditory and urinary systems

Question 10

Mesoderm

Answer 10

Major organs

Forms the gonads
In the early stage the tissue formed may be male of female

But the germ cells form gonads in week 7 for males and week 8 for females

Question 11

By the end of the embryonic period (8 weeks)

Answer 11

All adult body systems are present in some form

The major brain regions are present

The liver is beginning to form blood cells

Limbs and digits are present

Cardiovascular system is fully functioning and circulating blood