Lecture 13 - Differentiation of the Endoderm + Mesoderm

Question 1

What tubes does the endoderm form? (2)

Answer 1

Digestive tube - extends entire length of body

Respiratory tube - forms as an outgrowth of the digestive tube

Question 2

What are the digestive and respiratory tubes initially closed by?

Answer 2

The presence of the oropharyngeal and cloacal membranes

Question 3

What will the oropharyngeal and cloacal membranes be replaced by?

Answer 3

The openings of the mouth and anus, respectively.

Question 4

What associated glands bud off from the digestive tube? (3)

Answer 4

Liver

Gallbladder

Pancreas

Question 5

List two structures of endodermal origin

Answer 5

Digestive tube

Respiratory tube

Question 6

How is the mesoderm formed?

Answer 6

Formed by cells migrating through the primitive streak that place themselves between the ectoderm and endodermis

Question 7

Describe the initial structure of the mesoderm

Answer 7

Disorganised and loosely packed (mesenchymal)

Question 8

What does mesenchymal mean?

Answer 8

Loosely packed mesodermal cells

Question 9

List the four types of mesoderm

Answer 9

Chordamesoderm

Paraxial (somatic) mesoderm

Intermediate mesoderm

Lateral plate mesoderm

Question 10

What does the chordamesoderm form?

Answer 10

A rod like structure known as the notochord along the midline of the embryo up to the hindbrain.

Question 11

What is the function of the chordamesoderm?

Answer 11

Forms the notochord which plays an important role in the development of the nervous system; eventually disappears

Question 12

Describe the paraxial mesoderm and where it can be found

Answer 12

Paired segmented structures known as somites on both sides of the embryo near the midline.

Question 13

Describe the process of somite creation

Answer 13

Somites start of as loosely packed cells called somitomeres and become compacted to become somites

Question 14

What structures will somites form? (5)

Answer 14

Cartilage of vertebrae and ribs

Muscles of trunk and limbs

Tendons that connect muscles and bones

Dermis of dorsal skin

Structures of the circulatory system

Question 15

How many regions can be identified once somites have formed?

Answer 15

Four regions

Question 16

True or false: each region of the somite gives rise to a different mesodermal structure

Answer 16

True

Question 17

List the four regions of a somite and where they are found in relation to one another

Answer 17

Sclerotome - closest to the neural tube and notochord (innermost part of somite)

Syndetome - between the sclerotome and the myotome

Myotome - found between the syndetome and the dermatome

Dermatome - region lying beneath the ectoderm (outermost part of somite)

Question 18

What do the four somite regions give rise to? How does this happen?

Answer 18

Sclerotome - cells become mesenchymal and migrate towards midline - cartilage of vertebrae (becomes bone) and large part of each of the ribs

Syndetome - gives rise to tendons that attach muscles to bones

Myotome - cells migrate out - gives rise to all skeletal muscles of the body (except head muscles)

Dermatome - cells migrate underneath the ectoderm - contributes to the connective tissue of the dermis of the dorsal skin

Question 19

Do somitomeres become compacted in the cranial (head) region?

Answer 19

No, they remain unpacked and combine with chordamesoderm to form the head mesoderm.

Question 20

What is the function of the head mesoderm?

Answer 20

Gives rise to the connective tissues and muscles of the head and neck that aren’t derived from the neural crest

Question 21

Do all somites give rise to the same structures?

Answer 21

No, each pair of somites give rise to specific structures in the embryo

Question 22

What does the intermediate mesoderm give rise to?

Answer 22

The urogenital system

• kidneys
• gonads
• associated ducts

Question 23

What does the lateral plate mesoderm give rise to?

Answer 23

Heart

Blood vessels

Blood cells

Lining of the body cavities

Bones of limbs

Question 24

What happens to the lateral plate mesoderm around 3 weeks post fertilisation?

Answer 24

It splits into two sheets

Question 25

Name the two sheets that the lateral plate mesoderm gives rise to and where it is found

Answer 25

Visceral (splachnic) lateral plate mesoderm - nearest to endoderm

Parietal (somatic) lateral plate mesoderm - next to endoderm

Question 26

What does the visceral lateral plate mesoderm give rise to?

Answer 26

Heart

Smooth muscle

Connective tissue of the gut

Question 27

What does the parietal lateral plate mesoderm give rise to?

Answer 27

Forms smooth layer of tissue that lines the inside of the body wall (parietal peritoneum)

At level of limbs, cells proliferate and migrate to form skeletal structures and limbs

Question 28

Which sheet of lateral plate mesoderm forms limbs?

Answer 28

Parietal lateral plate mesoderm

Question 29

Which lateral plate mesoderm gives rise to the connective tissue of the wall of the gut?

Answer 29

The visceral plate mesoderm

Question 30

Which lateral plate mesoderm gives rise to the smooth layer of tissue that lines the inside of the body wall?

Answer 30

The parietal lateral plate mesoderm gives rise to the parietal peritoneum

Question 31

What is the parietal peritoneum?

Answer 31

A smooth layer of tissue that lines the inside of the body wall

Question 32

What does the space between the visceral and parietal lateral plate mesoderm become?

Answer 32

The body cavity or coelomic cavity (coelom)

Question 33

What does the coelomic cavity eventually convert to?

Answer 33

A series of separate cavities:

• pericardial coelomic cavity
• 2 pleural coelomic cavities
• peritoneal cavity