Germ Layers II

Question 1

From what does skeletal muscle arise?

Answer 1

from the myotome component of the somites

Question 2

What are myoblasts?

Answer 2

muscle precursor cells

Question 3

Expression of myogenic determination genes (transcription factors) in myoblasts regulates what?

Answer 3

the expression of structural muscle proteins

Question 4

Myoblasts migrate to the site of muscle formation. Individual myoblasts fuse into _____ (micronucleate cells) and eventually assemble into _____ expressing contractile proteins.

Answer 4

Myoblasts migrate to the site of muscle formation. Individual myoblasts fuse into MYOTUBES (micronucleate cells) and eventually assemble into MYOFIBERS expressing contractile proteins.

Question 5

The great majority of the muscles in the face, including those for mastication and facial expression, originate from what?

Answer 5

somitomeres in the head region

• these are the 7 pairs of cranial-most somitomeres that did not develop into somites
• note that neural crest cells do not contribute to facial muscles

Question 6

Limb muscles develop in place from ______ surrounding bones.

Answer 6

mesenchyme

*this mesenchyme migrated into the limb bud from the myotome region of the somites

Question 7

The mesenchyme surrounding bones that develop into limb muscles migrated into the limb bud from where?

Answer 7

from the myotome region of the somites

Question 8

Where is smooth muscle surrounding visceral organs derived from?

Answer 8

splanchnic mesoderm layer surrounding the endoderm of the gut

Question 9

For most blood vessels and lymphatic vessels, smooth muscle comes from where?

Answer 9

somatic mesoderm

*it seems that endothelial cells secrete signals that recruit smooth muscle cells to the developing blood vessels

Question 10

The smooth muscle cells in the iris, sweat glands, and mammary glands arise from where?

Answer 10

ectodermal tissue

Question 11

For the ascending aorta and carotid blood vessels, the surrounding smooth muscle is derived from what?

Answer 11

neural crest cells that migrated into the pharyngeal arches

Question 12

Smooth muscle development also involves a myoblast precursor cell. Smooth muscle cells do not fuse, but remain mononucleate. Unlike skeletal and heart muscle, smooth muscle is not what?

Answer 12

striated

Question 13

Myocardial cells that form heart muscle arise form what?

Answer 13

the splanchnic mesoderm in the cardiogenic area

• Genes involved in heart development begin to be expressed in the cardiogenic region very soon after gastrulation
• cardiogenic region is at the very top, above, kind of around the oral membrane (opposite end of the streak)

Question 14

One gene known to regulate development of heart muscle is called what?

Answer 14

Nkx2-5

*it is a homeobox gene related to the ‘tinman’ gene in Drosophila
Tinman mutant flies have no heart
Similar genes are involved in normal heart development in all organisms

Question 15

Nkx2-5, what is it?

Answer 15

• gene known to regulate development of heart muscle
• it is a homeobox gene related to the ‘tinman’ gene in Drosophila
  Tinman mutant flies have no heart
• Similar genes are involved in normal heart development in all organisms

Question 16

Cardiac precursors remain close to the endodermal layer and a signal from the _____ is essential for heart development.

Answer 16

endoderm

Question 17

The heart primordia essentially foom a ‘blood vessel’ on each side of the embryo. What is the eventual fate of these ‘blood vessels’?

Answer 17

these will eventually fuse into one heart tube at the ventral midline.

Question 18

Does fusion of cardiomyocytes occur?

Answer 18

No.
Like smooth muscle, no fusion of cardiomyocytes occurs. Normal myogenic determination genes are not involved in heart muscle differentiation.

Question 19

Expression of myogenic determination genes (transcription factors) in what cells regulates the expression of structural muscle proteins?

Answer 19

myoblasts (specifically for skeletal muscle)

Question 20

Most of the bone in the body forms from what?

Answer 20

mesoderm

*it is important to remember however, that bones of the face are formed from neural crest cells

Question 21

Bones of the face are formed from what?

Answer 21

neural crest cells

*not the mesoderm, unlike most of the other bones of the body

Question 22

The axial skeleton is formed from what?

Answer 22

the sclerotome component of the somite

*in the case of the vertebral column, sclerotome cells migrate to surround the notochord and the neural tube; these cells then differentiate to form bone

Question 23

Bones of the limb form from what?

Answer 23

• mesodermal mesenchyme that migrates into the limb buds and then condenses in place to form cartilage
• the cartilage is later ossified to generate the bones

Question 24

The endothelial cells that are the inner lining of blood vessels, and also the smooth muscle cells that surround the vessels, are both of what origin?

Answer 24

mesodermal

Question 25

In the extra-embryonic mesoderm, the first endothelial cells and blood cells are found together in structures called what?

Answer 25

blood islands

Question 26

What two things are found in blood islands?

Answer 26

- the first endothelial cells - blood cells *blood islands are found in the extra-embryonic mesoderm

Question 27

Within the embryo itself, endothelial precursor cells (angioblasts) arise throughout all mesodermal tissues, except which?

Answer 27

the notochord

Question 28

The angioblasts migrate and proliferate and then assemble into loose structures called what?

Answer 28

vascular cords *these cells will only form normal blood vessel tubes in response to a signal from the endoderm. This process of blood vessel formation from individual angioblasts is called vasculogenesis

Question 29

What is vasculogenesis?

Answer 29

the process of blood vessel formation from individual angioblasts mesodermal cell --> angioblast --> angioblasts aggregate --> ECs Aggregate (cord) --> Nascent Vessel (lumenized)

Question 30

Blood vessels in the extra-embryonic region fuse with vessels within the embryo to make a _____ vascular network.

Answer 30

continuous

Question 31

Vasculogenesis is only responsible for making the major blood vessels in the embryo. All subsequent formation of blood vessels, including the provision of a blood supply to previously avascular tissues like the brain, is achieved by branching and extension from the primary vessels. This growth from pre-existing vessels is called what?

Answer 31

angiogenesis

Question 32

What is angiogenesis?

Answer 32

the growth of new vessels from pre-existing vessels *this forms all blood vessels besides the primary vessels, it is achieved by branching and extension from the primary vessels

Question 33

At the end of gastrulation, the embryo consists of 3 germ layers, but is essentially flat. As neural tissues start to develop the embryo elongates and forms ridges on its dorsal surface. The are of most rapid growth is the brain. As it enlarges, it pushes forward and expands inside what? What does this expansion cause?

Answer 33

the amnion | *this expansion causes the head fold

Question 34

The same movements that pushes forward and expands the inside of the amnion, causing the head fold, pushes the developing heart around from where to where?

Answer 34

from the cranial to the oral plate to a position on the chest wall

Question 35

Similar growth (to that of the brain) of the tail produces what? And causes what?

Answer 35

- produces a tail fold | - causes the embryo to arch into the amniotic space

Question 36

What causes the embryo to arch into the amniotic space?

Answer 36

the growth of the tail and subsequent production of a tail fold * I think this all happens by the end of the first month after fertilization * this folding along the body of the embryo becomes more pronounced as development proceeds

Question 37

Together these folding movements (head fold, tail fold, arching of embryo into amniotic space) establish what?

Answer 37

- foregut (ends at the oral membrane, now also called the buccopharyngeal or oropharyngeal membrane) - midgut - hindgut (ends at the cloacal (anus) membrane)

Question 38

At the same time of the folding movements (head fold, tail fold, arching of embryo into amniotic space, gut formation), lateral growth of the somites causes the embryo to do what? What does this result in?

Answer 38

fold ventrally *these movements enclose most of the endodermal tissue, leaving only a small continuity with the yolk sac, the vitelline duct *the endoderm becomes surrounded by the splanchnic mesoderm layer to form the gut tube. Mesoderm continuous with the gut, and extending between the dorsal body wall and the gut forms the dorsal mesentery.

Question 39

After folding ventrally and enclosing most of the endodermal tissue leaving just the vitelline duct (a small continuity with the yolk sac), the endoderm becomes surrounded by the splanchnic mesoderm layer to form what?

Answer 39

the gut tube

Question 40

Mesoderm continuous with the gut, and extending between the dorsal body wall and the gut forms what?

Answer 40

the dorsal mesentery

Question 41

What are three medical problems associated with lateral folding?

Answer 41

- Meckel's diverticulum - Gastroschisis - Ectopia Cordis

Question 42

Medical problems associated with lateral folding: Meckel's Diverticulum

Answer 42

failure of complete regression of the yolk sac results in a very common birth defect (about 2% of the population) called Meckel's diverticulum, which is an out-pocketing of the ileum at the end of the small intestine

Question 43

Medical problems associated with lateral folding: Gastroschisis

Answer 43

- a fairly common birth defect (1 in 2000 births) | - visceral organs, usually intestine, lie outside of the body wall

Question 44

Medical problems associated with lateral folding: Ectopia Cordis

Answer 44

- in very rare cases, folding of the body wall leaves the heart outside of the chest - this is usually lethal around birth, bot some individuals have survived beyond 30 years of age

Question 45

In response to signals from the adjacent tissues, the gut tube begins to differentiate into different visceral organs. In the case of the liver, signals from what are required for initiation of the liver development pathway?

Answer 45

signals from the heart *note that the developing heart and the endodermal tissue that will develop into liver are adjacent in the embryo