Germ Layers

Question 1

What induces the formation of the neural plate?

Answer 1

the notochord induces overlying ectodermal tissue to form the neural plate
*this is the result of specific signaling molecules from the notochord altering the gene expression patterns in the adjacent ectoderm

Question 2

How does the neural plate increase in surface area?

Answer 2

through a combination of cell proliferation and cell movements

Question 3

The neural plate increases in surface area through a combination of cell proliferation and cell movements. The plate sinks down into the underlying mesoderm while rolling into a tube. The edges or crests of the neural plate are called what?

Answer 3

the neural folds

Question 4

Fusion of the neural folds into a tube first occurs in the ____ of the embryo (about the region of the 4th somite) and then proceeds both ____ and _____.

Answer 4

Fusion of the neural folds into a tube first occurs in the MIDDLE of the embryo (about the region of the 4th somite) and then proceeds both CRANIALLY and CAUDALLY.

Question 5

Failure of the neural tube to fuse correctly results in birth defects. What are two resulting birth defects?

Answer 5

• Anencephaly (failure of fusion at the cranial end)

- Spina Bifida (failure to fuse at the caudal end)

Question 6

What might neural tube defects be due to?

Answer 6

folic acid deficiency (a B group vitamin), although the molecular basis for the effect is unknown

• supplementing folic acid levels in diet of pregnant women have led to a decrease in neural tube birth defects
• folic acid is only found in animals, for vegans we have put this in food items

Question 7

What is Anencephaly?

Answer 7

neural tube defect: failure of fusion at the cranial end

Question 8

What is Spina Bifida?

Answer 8

neural tube defect: failure of fusion at the caudal end

Question 9

At about the same time that the neural tube closes, during the third week, the first signs of the developing eyes and ears become visible.

What is the optic placode?

Answer 9

The optic placode is a thickening of ectodermal tissue that will later invaginate and develop into the lens of the eye.

Question 10

At about the same time that the neural tube closes, during the third week, the first signs of the developing eyes and ears become visible.

What is the otic vesicle?

Answer 10

the otic vesicle will invaginate to form the inner ear and the balance organs

Question 11

What are neural crest cells primarily responsible for?

Answer 11

the face

Question 12

Where are neural crest cells located?

Answer 12

neural crest cells are initially located at the crests of the neural plate that form the neural folds. this is also the lateral extreme of the developing neural tissues

Question 13

At approximately the time of neural tube closure, neural crest cells become mobile and migrate away from the developing neural tube. Depending on their final destination, neural crest cells develop into a remarkable array of different cell types. Neural crest derivatives include, but are not limited to:

Answer 13

• the dorsal root ganglia
• the peripheral nervous system
• most of the bones and cartilage of the face
• pigment cells
• tooth papillae
• CT of aortic arch arteries and especially the septum separating the aorta and pulmonary vessel

Question 14

What is believes to determine the fate of neural crest cells?

Answer 14

it is believed to be primarily determined by signals from adjacent tissues at their final location, not by an intrinsic property of the cells prior to migration

Question 15

What are three clinical consequences of neural crest defects?

Answer 15

• Treacher-Collins syndrome (insufficient neural crest contribution to face)
• Waardenberg’s syndrome (facial and pigmentation defects and digestive system problems)
• Neurofibromatosis (autosomal dominant disease; mostly benign tumors of neural crest cells including melanocytes and Schwann cells; due to mutation in a tumor suppressor gene)

Question 16

What is Treacher-Collins syndrome?

Answer 16

insufficient neural crest contribution to face

Question 17

What is Waardenberg’s syndrome?

Answer 17

facial and pigmentation defects and digestive system problems

Question 18

What is neurofibromatosis?

Answer 18

• autosomal dominant disease affecting about 1 in 4000
• mostly benign tumors of neural crest cells including melanocytes and Schwann cells
• due to mutation in a tumor suppressor gene

Question 19

The embryonic ectoderm develops into what specialized tissue types?

Answer 19

(primarily those that come in contact with the outside environment)

• epidermis, including hair and nails
• sensory epithelium of the ear and nose and the lens of the eye
• sweat glands, mammary glands, pituitary gland
• enamel of the teeth

Question 20

Mesoderm lateral to the notochord is divided into what three subsections?

Answer 20

• paraxial mesoderm
• intermediate mesoderm
• lateral mesoderm
• respectively at increasing distances from the midline

Question 21

Which subsection of the mesoderm forms the somites? What are somites?

Answer 21

• paraxial mesoderm (closest to midline)

- somites are repeating structures that serve as the primitive musculature of the embryo

Question 22

What does the intermediate mesoderm form?

Answer 22

it will form mesodermal structures of the urinary system and the reproductive system

Question 23

The lateral mesoderm will split into what two layers separated by the embryonic coelem?

Answer 23

• SOMATIC mesoderm (parietal mesoderm; the outer layer in contact with the ectoderm)
• SPLANCHNIC mesoderm (visceral mesoderm; inner layer in contact with the endoderm)

Question 24

What is the somatic mesoderm?

Answer 24

• aka parietal mesoderm
• the outer (initially dorsal (upper, back)) layer in contact with the ectoderm
• goes on to form tissues that line the outer body wall, including CT, bone, fat and tendons

Question 25

What is the splanchnic mesoderm?

Answer 25

• aka visceral mesoderm
• inner (initially ventral (underside, abdominal side)) layer in contact with the endoderm
• contributes the smooth muscle, mesothelia (cells lining the body cavities), and the supportive tissue of organs derived from the gut (e.g. liver, lungs, etc.)

Question 26

Mesodermal cells adjacent to the notochord cluster into epithelialized balls - what are these called?

Answer 26

somites

• commences at about day 20
• formation of somite involves a transient structure, less well organized than the somite, called the somitomere

Question 27

What is the somitomere?

Answer 27

the transient structure, less well organized than the somite, that becomes the somite
*except for 7 pairs of somitomeres located at the cranial end, all the other somitomeres proceed to develop into somites

Question 28

Where does somite formation start?

Answer 28

at the cranial end of the embryo, and then progresses in a developmental wave towards the caudal end
*ultimately, about 44 pairs of somites form in the human embryo

Question 29

Based on their location within the somite, regions of tissue go on to form what three things?
(The somite contains the precursor cells of 3 different tissues, name them)

Answer 29

• myotome
• dermatome
• sclerotome
• the lower medial portion breaks down and becomes sclerotome, which will go on to form the vertebrae and the ribs

Question 30

Which of the three (myotome, dermatome, sclerotome) goes on to form the vertebrae and the ribs?

Answer 30

sclerotome (the lower medial portion of the somite thing breaks down and becomes sclerotome)

Question 31

After the sclerotome has dissociated, the remaining epithelialized cells become known as what?

Answer 31

dermamyotome

Question 32

The dermamyotome then separates (after sclerotome dissociates) into the outer dermatome, which will from the ______, and the more medial myotome, which will from the _____.

Answer 32

outer dermatome forms the CT (dermis) of the skin

medial myotome forms the body muscle

Question 33

All skeletal muscle below the level of the neck is derived from what?

Answer 33

somitic myotome

Question 34

The repeating structure of the somites is an example of segmentation of the original body plan. This is an evolutionary relic of the segmental structure of the worms that were our distant ancestors. In the embryo, segmentation is also visible in development of the rhombomeres of the brain. In the adult, segmentation remains visible in the pattern of the vertebrae and ribs.

The body plan of animals can be viewed as a set of repeating segments. This is particularly evident in the early embryo, in structures such as ______, but also in adult structures such as the ribs and vertebrae. Later development alters these basic segments so that they adopt individual morphologies.

Answer 34

somites

• just keep reading this to understand segmentation
• A segmental identity can be specified by patterns of gene expression. Indeed certain transcription factors of the homeodomain family are known to play a role in defining segmental identity in the vertebrate embryo.

Question 35

Some of the problems caused by embryo exposure to alcohol result from what?
(Fetal Alcohol Syndrome)

Answer 35

neural crest defects
e.g. Facial abnormalities are due to deficiency of
neural crest cells. Children also commonly exhibit heart
defects.