Embryology Lecture 3 - Early Embryonic Development 3

Question 1

How is the neural tube formed?

Answer 1

Notochord in the mesoderm directs the conversion of overlying ectoderm to neuroectoderm

Notochord signals the overlying ectoderm to thicken and form the key shaped neural plate

The edges of the neural plate elevate out of the plate of the ectoderm and curl towards each other, creating the neural tube

Question 2

What are somites and how do they develop?

How many somites are there at the end of their development and what is the significance of this?

Answer 2

Somites are segments of paraxial (either side of axis) mesoderm.

The first pair forms on day 20 in the occipital region. MOre then appear in craniocaudal sequence at a rate of 3 pairs a day

There are 42-44 pairs by the end of week 5

Some then dissapear, leaving 31 pairs in total that correspond to the 31 pairs of spinal nerves/levels

Question 3

What is the significance of the organisation of the somites?

Answer 3

The organisation of the mesoderm into somites gives rise to repeating structures

E.g. Vertebae, ribs, intercostal muscles

This guides innervation

Question 4

Label this diagram

Answer 4

Top figure:

Mesoderm

Bottom figure:

Labels from top right clockwise:

Paraxial mesoderm

Intermediate mesoderm

Somatic mesoderm

Interaembryonic coelom

Splanchnic mesoderm

Question 5

Describe the formation of the Coelom

Answer 5

When mesoderm differentiates the intraembryonic coelom is formed

When the embryo undergoes lateral folding the lateral branches of the mesoderm connect to form the coelom within (the intraembryonic coelom join)

Question 6

What is derived from the paraxial mesoderm?

Answer 6

Axial skeleton (vertebral column and ribs)

Dermis

Muscles of the axial body wall

Some limb muscles

Question 7

What is derived from the somatic mesoderm?

Answer 7

Connective tissues of the limbs

Connective tissues of the axial body wall

Question 8

What is derived from the splanchnic mesoderm?

Answer 8

Smooth musculature

Connective tissues and vasculature of the gut

Question 9

What is derived from the intermediate mesoderm?

Answer 9

Urogenital system

E.g. Kidneys, ureters, gonads

Question 10

Describe the differentiation of somites into the dermomyotome and sclerotome

Answer 10

1. Somites appear as regular blocks of mesoderm arranged around a small cavity.
2. This is followed by organised degeneration of the ventral wall of the somite, which breaks down to form the sclerotome.
3. Further organisation of the dorsal portion forms the dermomyotome.
4. Myotome proliferates and migrates. Dermatome disperses.

Question 11

What 3 structures arise from the somites and what is derived from these structures?

Answer 11

Dermatome - Skin

Myotome - Muscle

Sclerotome - Bones

These are the functional precursors of the musculo-skeletal system

Question 12

Label this diagram

What process it showing?

Answer 12

Top left figure, from top clockwise:

Neural tube

Notochord

Somite

Top right figure:

Sclerosome

Bottom left figure, left to right:

Dermomyotome

Sclerotome

Bottom right figure, top to bottom:

Dermatome

Myotome

Showing the differentiation of somites into dermatomes, myotomes and sclerotomes

Question 13

What is the differnece between the develpomental and clinical definitions of ‘dermatome’?

Answer 13

Developmentally:

For a given segment, a dermatome is a part of a somite that gives rise to the dermis

Clinically:

A dermatome is a strip of skin supplied by a single spinal nerve. Can be examined clinically

Question 14

What’s the difference between the developmental and clinical definitions of ‘Myotome’?

Answer 14

Developmentally:

The mytome is part of a somite that gives rise to muscles

Clinically:

The muyotome is a muscle/muscle group supplied by a single spinal nerve. Can be examined clinically

Question 15

What nerves are derived from the neural tube?

Answer 15

Neural tube produces the nerves that innervate the dermomyotome

Question 16

Describe the formation of the head/tail fold

Describe the movement of key structures in the embryo that occurs during this folding

Along what othe axes does folding occur?

Answer 16

Cephalocaudal folding occurs

Heart lies at the cranial end and moves inward until it lies in the future chest

The yolk sac extends out of the embryo to form a connection between the yolk sac and the gut

There is also lateral folding

Question 17

What is the result of embryo folding?

Answer 17

Achieves 3D structure

Creates the ventral body wall

Pulls the amniotic membrane around the disk so the embryo becomes suspended in the amniotic sac

Pulls the connecting stalk ventrally

Lateral folding creates a new cavity within the embryo, the coelom

Question 18

What three major processes occur during week 4 of development?

Answer 18

Neurulation

Organisation of the mesoderm E.g. Somite formation/segmentation

Embryonic folding