18. Early Embryonic Development - The Fate of the Mesoderm

Question 1

What has happened by the end of week 3 in early embryonic development?

Answer 1

The embryo has gastrulated and the bilaminar disk is converted to a trilaminar disk. The axes have been set and the three germ layers have developed: ectoderm, mesoderm and endoderm.

Question 2

What is the objective of the mesoderm?

Answer 2

Notochord formation for neurulation, organisation of the mesoderm into somites, intermediate and lateral plate mesoderm.

Question 3

What is the objective of segmentation?

Answer 3

To form dermatomes and myotomes.

Question 4

What is the notochord responsible for?

Answer 4

Releasing signals to surrounding ectoderm.

Question 5

What are the two parts of neurulation?

Answer 5

Notochord - driven induction of ectoderm to form the nervous system.
Neural plate - folds to develop the brain and spinal cord from the neural tube.

Question 6

Where is the notochord found?

Answer 6

Between the ectoderm and endoderm.

Question 7

How does neurectoderm form?

Answer 7

The notochord directs conversion of overlying ectoderm to neurectoderm.

Question 8

How is the neural tube formed?

Answer 8

Cells at the edges of the neural plate so they rise up and curl round to meet each other and form the tube.

Question 9

What is the paraxial mesoderm?

Answer 9

The mesoderm found either side of the axis.

Question 10

What is the intermediate mesoderm?

Answer 10

The mesoderm found inbetween the axis and the edge.

Question 11

What is the somatic mesoderm?

Answer 11

The mesoderm that goes on to give the skeletal muscle and is to do with body and body structures.

Question 12

What is the splanchnic mesoderm?

Answer 12

The mesoderm to do with viscera/ organs, e.g. intestines.

Question 13

What is the intraembryonic coelom?

Answer 13

The space inside the embryo (new cavity in gastrulation).

Question 14

What are somites?

Answer 14

The organisation of paraxial mesoderm into segments.

Question 15

When does the first somite appear in embryonic development?

Answer 15

At day 20.

Question 16

Why can somites be used to age the foetus?

Answer 16

Because they have very predictable presentation. The first appears at day 20 and then 3 pairs for every day after that until there are 42-44 pairs by the end of week 5. Some then disappear, leaving 31 pairs total.

Question 17

What is organised degeneration in embryonic development?

Answer 17

The ventral wall of somites breaks down leading to the formation of the sclerotome (gives rise to hard tissue).

Question 18

What are the following derivatives of somites?

a. Dermatome
b. Myotome
c. Sclerotome

Answer 18

a. Skin section/ dermis
b. Muscle section/ muscle
c. Hard tissue section/ bones.

Question 19

Why is having 31 pairs of somite important for later development?

Answer 19

There are 31 pairs of spinal nerves.

Question 20

What are the two meanings of dermatome and myotome (developmental and clinical)?

Answer 20

Dermatome = part of the somite that gives rise to the dermis (dev.) or the strip of skin supplied by a single spinal nerve (clin.)
Myotome = gives rise to the muscles (dev.) or the muscle/groups of muscles supplied by a single spinal nerve (clin.).

Question 21

What is the cardiogenic area in embryonic development?

Answer 21

The future heart.

Question 22

What is the buccopharyngeal membrane in embryonic development?

Answer 22

The future mouth, the opening to the oral cavity.

Question 23

How does embryonic folding happen?

Answer 23

Cephalocaudal folding driven by the size of the neurotube - the head (cranial end) folds under, then the tail folds under.
Lateral folding driven by the size of the developing somite - the sides fold under.

Question 24

What happens to the cardiogenic field and buccopharyngeal membrane in embryonic folding?

Answer 24

They are both folded into the inside in a new cavity, with the ectoderm facing outside still.

Question 25

What does folding achieve in embryonic development?

Answer 25

It draws together the margins of the disk to create a ventral body wall, pull amniotic membrane around the disk, pull connecting stalk ventrally.

Question 26

What has happened by the end of the fourth week in embryonic development?

Answer 26

The nervous system has started to form, segments have appeared and the embryo has folded to put everything in the right place.