Differentiation of the endoderm and mesoderm

Question 1

What does the endoderm form?

Answer 1

The digest tube that extends the entire length of the body

Question 2

What membrane have only two tissue types? What tissue types are they?

Answer 2

Buccopharyngeal membrane and cloacal membrane They only have the ectoderm and endoderm

Question 3

Label the diagram

What developmental stage is it in at the moment?

Answer 3

Day 20 just before longitudinal bending and transverse folding

Question 4

Label the diagram

Where on the embryo is this found?

Answer 4

The Buccopharyngeal membrane

Question 5

Label the diagram

Where is this found on the embryo?

Answer 5

The neural plate

Question 6

Label the diagram

Where does this cross section pass through?

The intermediate mesoderm is connected onto two other mesoderm tissue types, what are they called and where are they found?

Answer 6

Throught the somites

The parietal lateral plate mesoderm, adjactent to the ectoderm (top)

The visceral lateral plate mesoderm, adjacent to the endoderm (bottom)

Question 7

Label the diagram

Where is this cut from?

Has this section undergone full gastulation? What indicates this?

Answer 7

Cut through the primitive streak at the tail end

No, because there is still mesenchyme (mesoderm that is still loosely packed)

Question 8

Label the diagram

What is the developmental stage of the embryo at this point?

What is happening at this point?

Answer 8

21 days

The embryo is beginning to undergo longitudinal bending and transverse folding

Question 9

Label this diagram

What is the embryos development stage?

What are the 3 vesicles that form the brain?

Answer 9

23 days

Forebrain vesicle, midbrain vesicle and hind brain vesicle

Question 10

What happens to the buccopharyngeal membrane and cloacal membrane during longitudinal bending? What happens to these as the embryo develops? What do they form?

Answer 10

They block the ends of the tube These membranes disappear Forms the mouth and the anus

Question 11

What forms on the ‘embryonic tube’ once folding and bending is complete? What does this develop into?

Answer 11

Budding begins, this leads to the development of all of the digestive glands and the lungs

Question 12

How does the somitomere tissue vary during development based on body location?

Answer 12

In the head it remains as somitomeres (FYI a pseudo segmeneted tissue) while from the head lower (trunk) it develops into somites

Question 13

What is the process of forming somites from somitomere?

Answer 13

Somitogenesis

Question 14

How does the formation of somites progress during embryonic development (eg regular/irregular)? What does this indicate?

Answer 14

Somites are formed at very regular intervals

Inidcates that there is some kind of clock device within the embryo

Question 15

How does the clock within the embryo work? What is a developmental sequence that is linked closely with this clock?

Answer 15

The levels of mRNA production fluctuate with a regular time interval creating a genetic clock

The formation of somites is syncronised with the peaks of mRNA production of this gene

Question 16

What is the clock and wave form model? What does this control and how do they work together?

Answer 16

The clock is the genetic clock determined by the mRNA production

The wave is a chemical signal that determines the competency of a cell (if it has passed a cell it become competent, if not then it is incompetent) that travels from the rostral to caudal end (head to tail)

Only when the wave is competent (chemical signal has gone by the somitomere) and the gene expression is at its maximum will a somite form

Question 17

Once the clock and wave form model has passed through the entire embryo, what is formed?

Answer 17

Somites that are lined along the length of the notohord

Question 18

What segments are the somites divided into?

Answer 18

The medial and lateral axis (left and right side) and the dorsal and ventral surface (top and bottom)

Question 19

How are the dorsal and ventral surface distinguished? Where does it come from?

Answer 19

Through the concentration differences from the chemical sonic hedgehog produced by the notochord

Question 20

How does the concentration of sonic hedgehog influence what surface phenotypes the somites develop?

Answer 20

The side which gets a higher concentration will develop ventral phenotypes while the side which gets a lower concentraiton will develop dorsal phentypes

Question 21

What kind of chemical is sonic hedgehog? What does this make the notochord?

Answer 21

It is a morphogen however the notochord is not regarded as an orgnaiser, it only has orngaiser properties

Question 22

What does sonic hedgehog do to the somites?

Answer 22

It causes the dorsal side to express the Pax3 gene and the ventral side to express the Pax1 gene

Question 23

Label the diagram

What have the dorsal and ventral somites developed into?

Answer 23

Dorsal = dermatome adn myotome, Ventral = syndetome and sclerotome

Question 24

What is the differentiation of the dorsal and ventral somites determied by? Explain the conditions for each of the four types formed

Answer 24

It is determied by what gene is expressed and the position of the cells

Dermatome: Pax3 expressed and furtherest away from notochord/closest to epithelium

Myotome: Pax3 expressed adjacent to somite cavity

Syndetome: Pax1 expressed adjacent to somite cavity

Sclerotome: Pax1 expressed and closest to notochord/furthest from epithelium

Question 25

What part of the embryo will form the the epidermis and dermis?

Answer 25

The outside layer forms the epidermis and the dermatome forms the dermis layer (as it is just below the epithelial tissue of the embryo which will form the epidermis FYI)

Question 26

What do each of the layers from the somites form in a fully develped organism?

Answer 26

Dermatome: dermis

Myotome: skeletal muscle

Syndetome: tendons

Sclerotome: skeletal structures

Question 27

What does myotome do before forming skeletal muscle?

Answer 27

It migrates out of the somites first

Question 28

Do sclerotome form bones? Explain how

Answer 28

Eventually it does but first is becomes mesenchymal and migreates towards the midline where it then normally first forms cartilage which then transforms into bones

Question 29

What kind of skeletal structures do the sclerotome form?

Answer 29

Forms the ribs and verterbrate

Question 30

How are limbs formed?

Answer 30

Cells from the lateral plate mesoderm migrate and form limb buds, these then develop into skeletal structures (yes, limb bones are formed from lateral plate mesoderm NOT sclerotome)

Question 31

What two main embryonic tissue types are limbs formed from? Why not all three types?

Answer 31

Ectoderm and mesoderm, not endoderm becasue that is used for forming organs which are not presnet in the limbs

Question 32

What are the differences between the embryonic formation of the trunk vs the limbs?

Answer 32

The trunk has derivatives of the endoderm, the limbs don’t

Skeletal structure of the limbs is from parietal lateral plate mesoderm (lateral plate mesoderm closest to the ectoderm) while the trunk’s is derived from sclerotome (somatic mesoderm)

Question 33

When and how is mesoderm first formed? What does it initiatlly form? What does it develop into?

Answer 33

First formed during gastrulation as the cells migrate through the primitive streak and placing themeselves between teh ectoderm adn endoderm

First forms mesenchyme (loosely packed mesenchmal cells)

Forms chordamesoderm (notochord), Paraxial mesoderm (somites), intermediate mesoderm and lateral plate mesoderm

Question 34

Where is the notochord formed? Is it a permanent structure? What does it do?

Answer 34

Along the midline up to the level of the hindbrain (about the level where the ear will form)

No it is not, it is important in the development of the nervous system but eventually disappears

Question 35

Where is paraxial mesoderm found? What does it form?

Answer 35

It is formed at both sides of the embryo near the mid line

Form somitomeres (loosely packed mesodermal cells) that soon become compacted and form somites

Question 36

Are all somites the same? How do we know this?

Answer 36

No

Each somite will form a different structure

Question 37

How does the somitomeres develop differently in the cranial region?

Answer 37

Somitomeres do not become compacted to fomr well defined somites, instead it combines with chordamesoderm (aka notochord) which then gives rise to connective tissues and muscles of the head and neck

Question 38

What does the intermediate mesoderm form?

Answer 38

Forms the urogenital system (aka the organs associated with the reproductive and urinary system, eg kidneys, gonads etc.)

Question 39

What does the lateral plate mesoderm split into? At what developmental stage does it do this?

Answer 39

The visceral lateral plate mesoderm and the parietal lateral plate mesoderm

At week three

Question 40

What does each layer of the lateral plate mesoderm develop into?

Answer 40

Visceral: heart, smooth muscle and connective tissue of the gut
Parietal: forms inner lining of the body wall and the skeletal structues of the limb