Embryonic development

Question 1

Zygote

Answer 1

The single cell that results from fertilisation of an ovum by a sperm, has the potential to grow into a new individual human.

Question 2

Embryonic development

Answer 2

is the process by which a fertilised egg (zygote) grows and develops into an embryo, undergoing various stages of cell division, differentiation, and organisation to form the basic structures and systems of a new organism.

This process typically occurs in the early weeks of pregnancy.

Question 3

Cleavage divisions

Answer 3

The zygote remains safely enclosed in the zona pellucida and continues to be slowly transported down the fallopian tubes towards the uterus.

Within 24 hours of fertilisation, the first mitotic division occurs. This is known as a cleavage division.

The nuclear division is a normal mitotic division, but there is no cell growth occurring. The large egg cell, following nuclear division, simply divides in two, producing 2 smaller cells.

The embryo will undergo more cleavage divisions over the next few days. After 2 days, the embryo is made of 4 cells, 3 days – 8 cells

Question 4

Late Cleavage and Morula

Answer 4

At the 8 cell stage, all cells still have contact with the zona pellucida. Beyond this stage, new cells start to form centrally in a ball of cells that becomes known as a morula.

This stage is important as no longer are all of the cells totipotent. In particular the cells that are around the outside.

They start to form tight connections between their cells
The morula is still inside the zona pellucida.

Question 5

Blastocyst formation

Answer 5

At day 5-6 fluid begins to accumulate within the structure, trapped by the tight junctions of the outer cell cells, pushing all of the inner cell cells into a collection on one side of its structure - entire structure known as a blastocyst.

Question 6

Blastocyst definition

Answer 6

A hollow ball of cells formed during early embryonic development.

The outer cells are known to be trophoblasts

The inner cells are known as embryo-blasts (inner cell mass), which is composed of stem cells that will differentiate into the different body cells to form the embryo

Question 7

Hatching

Answer 7

At about this time, the blastocyst has reached the uterus and is producing enzymes that will allow it to bury into the endometrium to access nutrient there.

However, it first needs to ‘hatch’ from the zona pellucida

Question 8

Steps

Answer 8

1. fertilisation
2. Early cleavage
3. Late cleavage
4. Blastocyst formation

Question 9

Implantation

Answer 9

The blastocyst remains free within the cavity of the uterus for two to three days, and then sinks into the soft endometrium (uterine lining) to become firmly attached to the wall of the uterus.

This process is called implantation, and enables the blastocyst to gain nourishment for growth and development by absorbing nutrients from the glands and blood vessels of the uterine lining.

Question 10

Hormone production

Answer 10

The continued development of the blastocyst depends on the endometrium being maintained.

High levels of ostrogen and progesterone in the blood stop the endometrium breaking down, and so the menstrual cycle ceases.

During the early stages of pregnancy, it is the corpus luteum that produces these hormones until the developing placenta can take over the role after approximately 8-12 weeks.

The first two months of pregnancy are referred to as the embryonic period; after the second month, the developing individual is called a foetus.

Question 11

stem cells

Answer 11

Cells that are not specialised and are completed/divided by mitosis repeatedly, a process called proliferation.

Given the right conditions stem cells can differentiate into specialised cells.

They are also the cells that make up the inner cell mass of the blastocyst

Question 12

Differentiation

Answer 12

The process by which unspecialised cells like stem cells develop special characteristics to suit particular functions.

During differentiation specific genes are activated causing the cells to produce proteins that shape its role in the body.

Question 13

Stem cells are different from normal cells

Answer 13

• They are not specialised for any particular role.
• They are capable of repeated cell division by mitosis (proliferation)
• Given the right conditions, they can differentiate into specialised cells.

Question 14

Signals for cell differentiation

Answer 14

• Internal signals are controlled by the cell’s genes
• External signals include chemicals secreted by other cells.

Question 15

Totipotent

Answer 15

A type of stem cell that has the potential to become any type of cell that is necessary for embryonic development (embryo and embryonic membranes).

Ex - Zygote

Question 16

Pluripotent

Answer 16

Stem cells that can develop into any cell type of the body but cannot form tissues such as the placenta.

They are formed from Totipotent Stem cells.

Ex: Inner cell mass of the blastocyst

Ex: Embryonic stem cells

Question 17

Multipotent

Answer 17

Stem cells that can develop into a limited range of cell types within a specific tissue or organ system.

Arise from pluripotent stem cells after germ layers form.

Examples:
Hematopoietic stem cells → RBCs, WBCs, platelets

Mesenchymal stem cells → bone, cartilage, fat

Adult stem cells in various tissues

• Cannot become certain cells

Question 18

PROCESSS

Answer 18

• After fertilisation, the fertilised oocyte is a totipotent stem cell.
• Inner - cell mass of a blastocyst is a pluripotent stem cell.
• Pluripotent goes through further specialisation into another type of stem cell multipotent.

Question 19

Primary germ layers

Answer 19

Formation of three germ layers due to the inner cell mass undergoing changes as the cells change to multipotent.

Question 20

Ectoderm

Answer 20

The ectoderm is the outermost germ layer. This will form the outer layers of the body, such as the skin, hair and mammary glands, as well as the nervous system.

Question 21

Mesoderm

Answer 21

The mesoderm is the middle germ layer.
The skeleton, muscles, connective tissue, heart, blood and urogenital tract form from the mesoderm. The mesoderm also allows the formation of the stomach and intestines.

Question 22

Endoderm

Answer 22

The endoderm is the innermost germ layer. It forms the lining of the digestive system as well as the lungs and thyroid.

Question 23

4 embryonic membranes

Answer 23

• Yolk sac
• Amnion
• Allantois
• Chorion

Question 24

Amnion

Answer 24

The amnion is the first membrane to develop. By the eighth day after fertilisation, it surrounds the embryo, enclosing a cavity into which it secretes amniotic fluid.

This fluid serves to protect the embryo against physical injury by acting as a shock absorber.

It also helps to maintain a constant temperature and allows the developing embryo, and later the foetus, to move freely.

The amnion expands as growth takes place. It usually ruptures just before childbirth, releasing the amniotic fluid, an event commonly referred to as ‘breaking of the waters’.

Question 25

Chorion

Answer 25

Another embryonic membrane is the chorion. It is formed from the outer cells of the blastocyst together with a layer of mesodermal cells. The chorion surrounds the embryo and the other three embryonic membranes. As the amnion enlarges, it fuses with the inner layer of the chorion. Eventually, the chorion becomes the main part of the foetal portion of the placenta.

Question 26

Yolk sac and allantois

Answer 26

In addition to the chorion and the amnion, there are two other membranes - the yolk sac and the allantois. These are not as important in humans as they are in the development of many other animals; however, they do form the outer structure of the umbilical cord.

Question 27

Trophoblast

Answer 27

Trophoblasts are cells forming the outer layer of a blastocyst, which provides nutrients to the embryo, and develops into a large part of the placenta.

Question 28

After implantation... Cytotrophoblast (1 distinct part of the trophoblast)

Answer 28

remains as a thin layer surrounding the embryo and cavity of the blastocyst

Question 29

Syncytiotrophoblast (2nd distinct part of the trophoblast)

Answer 29

Forms cells that invade the endometrial space. These cells largely coalesce to form very large, multinucleated cells and spaces called lacunae.

Question 30

Embryoblast

Answer 30

An embryoblast, also known as the inner cell mass, is a cluster of cells inside a blastocyst that will develop into the embryo itself and ultimately the fetus. (pluripotent)

Question 31

Bilaminar disc

Answer 31

Where the epiblast and hypoblast are next to each other will become the embryo. It involves lots of cells moving around and folding to occur before it really looks anything like a small person