18. Early Embryonic Development - The Fate of the Mesoderm Flashcards Preview

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1
Q

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

A

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.

2
Q

What is the objective of the mesoderm?

A

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

3
Q

What is the objective of segmentation?

A

To form dermatomes and myotomes.

4
Q

What is the notochord responsible for?

A

Releasing signals to surrounding ectoderm.

5
Q

What are the two parts of neurulation?

A

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

6
Q

Where is the notochord found?

A

Between the ectoderm and endoderm.

7
Q

How does neurectoderm form?

A

The notochord directs conversion of overlying ectoderm to neurectoderm.

8
Q

How is the neural tube formed?

A

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

9
Q

What is the paraxial mesoderm?

A

The mesoderm found either side of the axis.

10
Q

What is the intermediate mesoderm?

A

The mesoderm found inbetween the axis and the edge.

11
Q

What is the somatic mesoderm?

A

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

12
Q

What is the splanchnic mesoderm?

A

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

13
Q

What is the intraembryonic coelom?

A

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

14
Q

What are somites?

A

The organisation of paraxial mesoderm into segments.

15
Q

When does the first somite appear in embryonic development?

A

At day 20.

16
Q

Why can somites be used to age the foetus?

A

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.

17
Q

What is organised degeneration in embryonic development?

A

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

18
Q

What are the following derivatives of somites?

a. Dermatome
b. Myotome
c. Sclerotome

A

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

19
Q

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

A

There are 31 pairs of spinal nerves.

20
Q

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

A
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.).
21
Q

What is the cardiogenic area in embryonic development?

A

The future heart.

22
Q

What is the buccopharyngeal membrane in embryonic development?

A

The future mouth, the opening to the oral cavity.

23
Q

How does embryonic folding happen?

A

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.

24
Q

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

A

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

25
Q

What does folding achieve in embryonic development?

A

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

26
Q

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

A

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