Embryology 3

Question 1

What signals the start of gastrulation?

Answer 1

The formation of the primitive streak. The primitive streak is made up of the primitive groove, primitive pit and primitive node. The primitive streak is formed by the migration of epiblast cells - it is an indentation formed in the dorsal aspect of the embryo.

Question 2

How is a caudal/cranial axis established?

Answer 2

Towards the cranial end, there is the future development of the oropharyngeal membrane. At the caudal end, there is future development of the cloacal membrane (important in the urinary and reproductive systems).

Question 3

What is situs invertus?

Answer 3

It is when the body systems are inverted in the body along the left-right axis. As a result the organs are in the wrong place. This occurs due to problems with gastrulation. It is usually asymptomatic in individuals with complete situs invertus, it only becomes a problem in partial inversions where the connections have not changed e.g. in dextrocardia.

Question 4

What is the trilaminar embryonic disc?

Answer 4

The bilaminar disc becomes trilaminar during gastrulation where the three germ layers are formed.

Question 5

How is the trilaminer embryonic disc formed?

Answer 5

1. Epiblasts migrate through the primitive streak. They then detach from the epiblast and replace the hypoblasts. Once they have replaced the hypoblasts the first germ layer is formed - definitive endoderm.
2. The epiblasts continue to migrate through the primitive streak to form a layer between the definitive endoderm and the epiblasts. This forms the second germ layer = mesoderm (The intraembryonic mesoderm ins continuous with the extra embryonic mesoderm).
3. The remaining epiblasts make up the final germ layer known as the ectoderm.

Question 6

What is derived form the endoderm?

Answer 6

• Epithelial lining of the respiratory tract, GI tract, urinary bladder and urethra.
• Liver
• Gall bladder
• Pancreas
• Thymus
• Thyroid gland

Question 7

What is derived from the mesoderm?

Answer 7

• Endothelium
• Epithelial lining of body cavities and joint cavities
• Muscle
• Connective tissue
• Adrenal Cortex
• Heart
• Kidneys and ureters
• Internal reproductive organs
• Spleen

Question 8

What is derived from the ectoderm?

Answer 8

• Epidermis of the skin and epidermal derivatives (hair, sweat glands, nails and mammary glands)
• Nervous tissue and sense organs
• Pituitary glands
• Adrenal medulla
• Enamel of lymph
• Lens of eye

Question 9

How does the intraembryonic mesoderm organise?

Answer 9

It organises into paraxial mesoderm, intermediate mesoderm and lateral plate mesoderm. Each will then form different structures. Paraxial mesoderm organises further into somites to form the vertebrae. The somites form on day 20. They form in pairs at a rate of 3/4 pairs a day until 42-44 pairs are formed starting at the cranial aspect towards the caudal aspect. The formation of somites is a very predictable process and so can be used to date an a pregnancy.

Question 10

What is Sirenomelia? What is its cause?

Answer 10

Sirenomelia is ‘mermaid syndrome’. The legs are not separated and the urinary system is not fully developed. This is due to insufficient mesoderm. This may be due to premature regression of the primitive streak or reduced migration of epiblasts.

Question 11

What is the cause of a sacrococcygeal teratoma?

Answer 11

Too much mesoderm was formed leading to a large tumour in the lower region ion the spine. The primitive streak stays around for too long and so there is excessive migration of the epiblasts through the streak. There is a predisposition to females. It is a quite common tumour and relatively easy to treat.

Question 12

Give an account of the formation of the notochord

Answer 12

The primitive streak is a transient structure. There are cells that migrate cranially and as they do this they form the notochord process. As the primitive streak regresses, it pulls the notochord with it, causing it to extend caudally.

The notochord process then fuses transiently with the underlying endoderm to form a notochordal plate. It then detaches to form a solid structure called the notochord.

Question 13

Describe the role of the notochord in inducing neurulation and formation of the vertebrae

Answer 13

The notochord signals the ectoderm to divide to form a neural plate. This will eventually fold over to form the neural tube.
The notochord also initiates the formation of the vertebrae from somites. The notochord induces the differentiation of the somites into 3 parts:
- Sclerotome: Goes onto form bone and cartilage (ands so goes onto form vertebrae)
- Myotome: Goes onto form skeletal muscle
- Dermatome: Goes onto form the dermis

Question 14

How is the vertebrae formed from sclerotome?

Answer 14

The notochord signals the somites to differentiate. The sclerotome goes onto form the vertebrae. The vertebrae first covers the notochord and then the neural tube. This is known as the neural arch.

Question 15

What happens to the notochord?

Answer 15

The notochord is also a transient structure. It is thought it becomes apart fo the intervertebral discs specifically the nucleus pulpous.

Question 16

Explain the consequences of abnormal formation of the vertebrae.

Answer 16

Induction of the sclerotome can go wrong and lead to congenital malformations.

Spina bifida - The neural arch does not form along the whole of the neural tube. As a result there are regions where the neural tube is protected by only skin and subcutaneous fate.

Meningocele: The neural arch does not form around the neural tube causing the meninges to produce.

Myelomeningocye: In addition to the protrusion of the meninges, some aspect of the neural tube protrude through the gap in the neural arch.

Question 17

What is the clinical presentation of spina bifida?

Answer 17

Dimpled skin and a tuft of hair on the back.