Embryology - mesoderm, segmentation and folding (week 4)

Question 1

Which component of the embryo directs the conversion of overlying ectoderm to neuroectoderm?

Answer 1

notochord

Question 2

Notochord signals cause overlying ectoderm to thicken into what slipper-shaped component of the embryo?

Answer 2

Neural plate

Question 3

How does the neural tube form?

Answer 3

The neuroectoderm in the edges of the neural plate multiplies so much that the edges of the neural plate elevate out of the plane of the disk and curl towards each other, creating the neural tube. The ectoderm then closes over the top of this tube.

Question 4

What are the four new differentiated mesoderm types that form?

Answer 4

1. Somatic mesoderm - to do with body structures
2. Splanchnic mesoderm - to do with the organs
3. Paraxial mesoderm - para to the neural tube
4. Intermediate mesoderm

Question 5

What are somites?

Answer 5

Somites are division of the body of an animal or embryo. They are bilaterally paired blocks of paraxial mesoderm that form along the head to tail axis of the developing embryo in segmented animals.

Question 6

Why are somites useful in dating the age of embryos?

Answer 6

They have a regular, predictable appearence e.g. first pair appear at day 20 in the occipital region.

Question 7

Somites appear in a craniocaudal sequence. How many appear per day?

Answer 7

3 pairs/day

Question 8

How many somites are present by the end of week 5? How many are present in the adult?

Answer 8

43-44 pairs

some disappear, leaving 31 pairs

Question 9

What do somites look like when they first appear?

Answer 9

A regular block of mesoderm cells arranged around a small cavity.

Question 10

What happens during “organised degeneration” of somites?

Answer 10

The ventral wall breaks down, leading to the formation of the schlerotome (hard section of tissue).

Question 11

What happens to the dorsal portion of the degenerated somite?

Answer 11

It forms a combined dermomyotome (consisting of myotome and dermotome).

Question 12

What happens to the dermamyotome?

Answer 12

The myotome proliferates and migrates and the dermatome disperses - but both still stay affiliated to their original somite.

Question 13

Name the three somite derivatives

Answer 13

Dermatome
Myotome
Sclerotome
These are the beginnings of the muscular-skeletal system.

Question 14

What does dermatome differentiate into?

Answer 14

Dermis (skin)

Question 15

What does myotome differentiate into?

Answer 15

Muscle

Question 16

What does scleratome differentiate into?

Answer 16

Bones (sclero- means hard)

Question 17

What is the position of the dermomyotome relative to the sclerotome?

Answer 17

Dermomyotome is dorsal relative to the sclerotome.

Question 18

In what relation to each other are, the derivatives of the dermomyotome, the back muscles to the ventral body wall muscles & limb muscles?

Answer 18

The back muscle are medial to the ventral body wall muscles & limb muscles.

Question 19

In what relation to each other are the vertebral body and the vertebral arch (both derivatives of the sclerotome)?

Answer 19

The vertebral body is medial to the the vertebral arch.

Question 20

In what relation to each other are the proximal and distal ribs?

Answer 20

The proximal ribs are medial to the distal ribs.

Question 21

In what relation to each other are the vertebral body and vertebral arch to the proximal and distal ribs?

Answer 21

The ribs are ventral to the vertebrae.

Question 22

What are the implications of segmentation?

Answer 22

1. It gives rise to repeating structures (vertebrae, ribs, intercostal muscles and spinal chord segments (functionally not anatomically segmented)).
2. Guides innervation.

Question 23

What is an epimere?

Answer 23

The dorsal portion of a somite, from which are formed muscles innervated by the dorsal branch of a spinal nerve for that segment. These muscles are the skeletal muscle of the back.

Question 24

What is an hypomere?

Answer 24

The ventral portion of a somite, from which are formed muscle innervated by the ventral branch of a spinal nerve for that segment. These muscle include the lateral and anterior muscle of the abdomen and thorax.

Question 25

How many pairs of spinal nerves are there?

Answer 25

31 segments in the body (originating from 31 pairs of somites) and therefore 31 pairs of spinal nerves.

Question 26

Clinically what does a dermatome refer to?

Answer 26

A strip of skin supplied by a single spinal nerve.

Question 27

Clinically what does a myotome refer to?

Answer 27

A muscle/ group of muscles supplied by a single spinal nerve

Question 28

What does the paraxial mesoderm develop into?

Answer 28

Axial skeleton - vertebral column and ribs
Dermis
Muscles of anterior/lateral body wall
Some limb muscles

Question 29

What two types of mesoderm make up the lateral plate?

Answer 29

Somatic

Splanchnic

Question 30

What does the somatic mesoderm develop into?

Answer 30

Somatic means body- it develop into body structures:

the connective tissue of limbs, contributes to the A/L body wall.

Question 31

What does splanchnic mesoderm develop into?

Answer 31

Smooth musculature, connective tissue and vasculature of the gut.

Question 32

What does intermediate mesoderm develop into?

Answer 32

Urogenital system e.g. kidneys, ureter, gonads.

Question 33

In the 4th week what types of folding occur?

Answer 33

1. Cephalocaudal folding (head and tail fold in)

2. Lateral folding (sides fold in)

Question 34

These two types of folding leave one opening. Why?

Answer 34

It’s the opening to the umbilicus.

Question 35

What affect does craniocaudal folding have?

Answer 35

1. The amniotic sac now surrounds the entire embryo.
2. The cardiogenic area is now located in its normal location (and primordium of diaphragm).
3. The connecting stalk is pulled ventrally -> true umbilical chord.

Question 36

What affect does the lateral folding have?

Answer 36

The space between ‘the somatic and splanchnic leaves of mesoderm creates a:

1. cavity inside the embryo: intraembryonic coelom
2. Ventral body wall
3. Part of yolk sac pinched off -> secondary yolk sac formed (creates the primordium of the gut)

Question 37

What does folding acheive?

Answer 37

Draws together the margins of the disk:

1. Creating a ventral body wall
2. Pulling amniotic membrane around the disk and suspending the embryo within the amniotic sac
3. Pulling connective stalk ventrally -> true umbilical chord

Question 38

What has happened by the end of this fourth week?

Answer 38

1. Neuralation :the NS has started to form.
2. Segmentation: segments have appeared, assigning specific tasks to specific cells.
   Organisation of the mesoderm: the embryo has folded, putting everything in its right place.