Mesoderm (3 germ layers)

Question 1

Gastrulation

Answer 1

Begins with the formation of the P.S.
Ends with the formation of the 3 embryonic germ layers (ectoderm, endoderm and mesoderm)

Question 2

Delamination

Answer 2

Detachment of blastomeres from inner cell mass into blastocoel forming hypoblasts
Remaining cells proliferate and form epiblast (source of germ layers)

Question 3

Hypoblast

Answer 3

Detached cells that line the inner side of blastocoel
Presumptive endoderm cells
Line the yolk sac

Question 4

Epiblast

Answer 4

Remaining cells of the inner cell mass
Gives rise to the ectoderm and mesoderm

Question 5

Formation of the mesoderm

Answer 5

Proliferation of epiblast cells
Cells move through the P.S. occupying the space ebtween epiblast and hypoblast
Formed first caudally then expands laterally and cranially

Question 6

What does the mesoderm expand to form?

Answer 6

Paraxial
Intermediate
Lateral (splanchnic and somatic)
Extraembryonic

Question 7

How is the cr. mesoderm formed?

Answer 7

Accumulation of multipotential cells at the cr. end of the P.S.
Forms Hansen’s/ Primitive Node, head mesoderm and plays a role in forming the notochord

Question 8

Extraembryonic mesoderm

Answer 8

Expands between the trophoblast and extraembryonic endoderm
Continuous with embryonic germ layers

Question 9

Where is the rate of mesoderm expansion the slowest?

Answer 9

In horses

Question 10

Early embryo

Answer 10

Mesoderm is a loose aggregation of cells (mesenchyme)

Question 11

Later embryo

Answer 11

Differentiation begins
Mesodermal “fate map” is possible

Question 12

Lateral mesoderm

Answer 12

Lateral plate splits to form somatic and splanchnic mesoderm
Cavity between the two is coelom

Question 13

Somatic mesoderm

Answer 13

Somatopleure (mesoderm + ectoderm)
Parietal
Forms pelvic and shoulder girdles, long bones of the limbs, sternum

Question 14

Splanchnic mesoderm

Answer 14

Splanchnopleure (mesoderm + endoderm)
Visceral
Must be present for BVs to form
GI tract of thorax and abdomen

Question 15

Intermediate mesoderm

Answer 15

Runs lengthwise along the entire “trunk” of the embryo
Later in development cell clusters (nephrotomes) and nephrogenic cord form

Question 16

What does the intermediate mesoderm give rise to?

Answer 16

Most but not all of the urinary system and parts of the reproductive system

Question 17

Paraxial mesoderm

Answer 17

Aggregation of mesoderm to form 2 parallel columns
Beginning cranially, transverse fissures appear and form somites

Question 18

What does the somite number indicate?

Answer 18

the age of embryo and is constant for a species

Question 19

Myotome

Answer 19

Forms skeletal muscles of body

Question 20

Dorsal (epaxial) part of myotome

Answer 20

Muscles of the back
Muscles of vertebral column dorsal to transverse process

Question 21

Ventral (hypaxial) part of myotome

Answer 21

Ventro-lateral muscles (intercostal and abdominal)
Limb muscles

Question 22

Formation of the multinucleate cell

Answer 22

Mesenchyme –> myogenic cells –> myoblast –> cells fusion to form multinucleate cell

Question 23

Sclerotome

Answer 23

Verterbrae and ribs

Question 24

Dermatome

Answer 24

Dermis (not epidermis)

Question 25

Mesoderm of the head

Answer 25

No lateral mesoderm or coelom Mesoderm migrates to the "brachial arches" Neural crest cells contribute (ectomesenchyme)

Question 26

Occipital somites

Answer 26

Somites that extend only into the cd. regions of the head

Question 27

Somitomeres

Answer 27

Rostral to occipital somites, Formed by paraxial mesoderm Extend only as far as the rostral extent of the notochord (cd. forebrain/ infundibulum)

Question 28

Skeletal system

Answer 28

Somatic mesoderm Sclerotome Occipital somites Neural crest cells

Question 29

What do neural crest cells form?

Answer 29

Ectomesenchyme Face and skull bones

Question 30

How are bones formed?

Answer 30

Via endochondral and intramembranous ossification

Question 31

Skeletal dysplasia

Answer 31

Cranio-facial defects- abnormal neural crest migration

Question 32

T/F: After the formation of the mesoderm, P.S. regresses and has no derivative structures in the adult

Answer 32

TRUE

Question 33

How is the notochord formed?

Answer 33

Mesodermal cells from Hensen's node migrate toward the pre-cordal plate forming the notochordal plate Plate folds to form notochord (solid mass of cells) Formed cr. to cd. as the primitive streak regresses

Question 34

Function of the notochord

Answer 34

Inductor for ectoderm to form neuroectoderm (neural plate, CNS, somite) Organizing center for vertebral bodies

Question 35

Formation of the neural tube

Answer 35

Thickening of the ectoderm overlying notochord neural plate Folding of neural plate Formation of neural tube