Embryology

Question 1

What is a Morula?

Answer 1

A morula is a cluster of 16 or more cells (held together by tight junctions) formed when cells have divided to maximise contact with each other.

Question 2

What stage is a morula formed?

Answer 2

Day 4

Question 3

What is a blastocyst?

Answer 3

A blastocyst is the first sign of cellular differentiation. It is made of:
- An inner cell mass (epiblast and hypoblast) - this will form the embryo
- An outer cell mass (trophoblast, tropho = ‘eat/nourish’ which is what the placenta does). - this will form the placenta

The blastocyst also has a fluid-filled blastocyst cavity.

Question 4

What happens when the blastocyst runs out of nutrients?

Answer 4

When the blastocyst starts to run out of nutrients, the trophoblast secretes enzymes which break down the zona pellucida to allow for implantation.

Question 5

When does implantation occur?

Answer 5

7-12 days after fertilisation

Question 6

What is implantation?

Answer 6

The interaction between the implanting embryo and the endometrium of the uterus

Question 7

Which cells implant first? What happens when these implant?

Answer 7

Trophoblast cells (as these are on the outside and so will encounter the endometrium first). These trophoblasts will then form a cytotrophoblast and a syncytiotrophoblast which will invade the stroma of the endometrium - allowing attachment.

Question 8

Following implantation, a bilaminar disc forms. What are the layers of the bilaminar disc and what does it do?

Answer 8

The layers are the Epiblast and the Hypoblast. These structures allow formation of extra-embryonic membranes.

Question 9

What are the 3 extra-embryonic membranes at the point of week 2/implantation? What is each of these membranes function?

Answer 9

• The Amnion = has a cavity filled with amniotic fluid and gives protection for the embryo
• The Yolk Sac = Important for nutrient transfer in weeks 2-3 and disappears after week 20.
• The Chorion = Forms the foetal component of the placenta

Question 10

What are the 3 3’s of gastrulation

Answer 10

Occurs in week 3
Forms 3 layers (ectoderm, mesoderm, endoderm)
Has 3 important structures (primitive streak, notochord, neural tube)

Question 11

What is the first event of gastrulation?

Answer 11

Formation of an invagination into the epiblast. This invagination is called the Primitive Streak.

Question 12

What occurs at the primitive streak?

Answer 12

Cells move into the invagination/primitive streak and begin to differentiate, forming the 3 layers (ectoderm, mesoderm and endoderm).
There is also formation of the notochord and the neural tube

Question 13

Where does most muscle formation occur?

Answer 13

In the mesoderm layer.

Question 14

What are the regions of the mesoderm?

Answer 14

Notochord, paraxial mesoderm, intermediate mesoderm, lateral plate mesoderm, extraembryonic mesoderm

Question 15

When does the mesoderm differentiate?

Answer 15

Days 17-21

Question 16

What are the 3 main regions of the mesoderm? What is the function of each region?

Answer 16

• Paraxial Mesoderm - Lies adjacent to the notochord and neural tube. It forms the somites in the embryo.
• Intermediate Mesoderm - Forms the Genitourinary System
• Lateral Plate Mesoderm - Split by a cavity into 2 layers (visceral layer and parietal layer)

Question 17

Where does skeletal muscle arise from?

Answer 17

Paraxial mesoderm

Question 18

Where does smooth muscle (of the gut) arise from?

Answer 18

Visceral Lateral Plate Mesoderm

Question 19

Where does smooth muscle (of the pupil, mammary and sweat glands) arise from?

Answer 19

Ectoderm

Question 20

Where does Cardiac Muscle arise from?

Answer 20

Visceral Lateral Plate Mesoderm

Question 21

What is a Somite?

Answer 21

A block of Paraxial Mesoderm which gives rise to skeletal muscles

Question 22

What is the process of somite formation?

Answer 22

Somites form alongside the neural tube in a craniocaudal sequence from day 20 at roughly 3 pairs a day until the end of week 5 (this rate is used to accurately determine embryo age).

Question 23

What controls the patterning of somites?

Answer 23

The Notochord influences somite formation.
- Genes (FGF family, Wnt and Notch) also control somites via the clock and wave mechanism.

Question 24

What is the clock and wave mechanism?

Answer 24

Where genes (FGF, Wnt and Notch) tell cells to switch between permissive and non-permissive states in a timed fashion (clock).
Then, a wave of factors sweeps along the length of the embryo and interacts with the cells that are permissive at the right time and the right area

Question 25

What happens in somite differentiation?

Answer 25

Once all somites have formed, they start to differentiate.
- Cells in the ventral and medial area undergo epithelial-mesenchymal transition - forming the Scleratome (which forms the vertebrae and the ribs, sclera = white, like bone)
- Cells in the dorsal half differentiate to form the dermomyotome. This splits into the Dermatome and the Myotome (which form the Dermis and the Muscles respectively)

Question 26

What are MYOD and MYF5?

Answer 26

These are Mylogenic transcription factors which activate muscle-specific genes to enable the differentiation of myogenic precursor cells in the dermomyotome into myoblasts

Question 27

What are myoblasts?

Answer 27

Myoblasts are the precursors of muscle cells. They undergo cell division under the influence of growth factors t