third to eighth weeks

Question 1

what is the difference between mesoderm and mesenchyme

Answer 1

Mesoderm refers to cells derived from the epiblast and extraembryonic tissues.
Mesenchyme refers to loosely organized embryonic connective tissue regardless of origin

Question 2

how does the neural crest form

Answer 2

As the neural folds elevate and fuse, cells at the lateral border or crest of the neuroectoderm begin to dissociate from their neighbors, this population is the neural crest

Question 3

what happens to the neural crest as it leaves the neuroectoderm

Answer 3

will undergo an epithelial-to-mesenchymal transition by active migration and displacement to enter the underlying mesoderm

Question 4

Crest cells from the trunk region leave the neuroectoderm after closure of the neural tube and migrate along one of two pathways:

Answer 4

1) a dorsal pathway through the dermis, where they will enter the ectoderm through holes in the basal lamina to form melanocytes in the skin and hair follicles, and
(2) a ventral pathway through the anterior half of each somite to become sensory ganglia, sympathetic and enteric neurons, Schwann’s cells, and cells of the adrenal medulla.

Question 5

what cells contribute to the craniofacial skeleton

Answer 5

neural crest cells migrating from the cranial neural folds, leaving the neural tube before closure

Question 6

other than craniofacial skeleton what do neural crest cells contribute to

Answer 6

neurons for cranial ganglia, glial cells, melanocytes

Question 7

what are the neural crest cells sometimes referred to as

Answer 7

fourth germ layer

Question 8

what do high and low levels of BMP cause

Answer 8

High BMP induce epidermis formation, intermediate levels, at the border of the neural plate and surface ectoderm, induce the neural crest; and very low concentrations cause formation of neural ectoderm

Question 9

intermediate concentrations of BMPs, together with _________________, induce _____ and other transcription factors that “specify” the___________

Answer 9

FGF and WNT proteins
PAX3
neural plate border

Question 10

what do the transcription factors from intermediate BMP levels induce

Answer 10

a second wave of transcription factors, including SNAIL and FOXD3, and SLUG

Question 11

what is the function of

a) SNAIL and FOXD3
b) SLUG

Answer 11

a) specify cells as neural crest

b) promotes crest cell migration from the neuroectoderm

Question 12

what organs and structures does ectodermal layer give rise to

Answer 12

• CNS
• PNS
• The epithelium of external auditory canal
• The sensory epithelium of the ear, nose, and eye
• The epidermis, including the hair and nails

Question 13

what else does the ectodermal layer give rise to

Answer 13

 Subcutaneous glands,
 The mammary glands,
 The pituitary gland,
 And enamel of the teeth

Question 14

what causes anencephaly

Answer 14

failing of the neural tube to close in the cranial region

Question 15

what happens if the closure fails in the neural tube anywhere from the cervical region caudally

Answer 15

a defect called spina bfida

Question 16

what is the most common site for spina bfida to occur

Answer 16

lumbosacral region

Question 17

what reduces the incidence of spina bfida

Answer 17

folic acid (folate)

Question 18

what happens to the cells of the mesodermal layer by approximately the 17th day

Answer 18

cells close to the midline proliferate and form a thickened plate of tissue known as paraxial mesoderm

Question 19

what happens to the mesodermal layer more laterally

Answer 19

remains thin and is known as the lateral plate

Question 20

what connects the paraxial and the lateral plate mesoderm

Answer 20

Intermediate mesoderm

Question 21

what happens with the apperance and coalescence of intercellular cavities in the lateral plate and what forms as a result

Answer 21

this tissue is divided into two layers
• A layer continuous with mesoderm covering the amnion, known as the somatic or parietal mesoderm layer
• A layer continuous with mesoderm covering the yolk sac, known as the splanchnic or visceral mesoderm layer

these layers line a newly formed cavity, the intraembryonic cavity

Question 22

what segments does the paraxial mesoderm form

Answer 22

somitomers which first appear in the cephalic region (cephalocaudally)

Question 23

where do somites first appear

Answer 23

in the occipital region of the embryo

Question 24

new somites appear in__________ sequence at a rate of approximately ________________, at the end of the fifth week

Answer 24

craniocaudally

three pairs per day

Question 25

At the end, embryo will have 34-36 pairs 3 ______, 8 ______, 12 _______, 5 ______, 5 ______ and 1 to 5 _________ pairs

Answer 25

occipital, 
cervical, 
thoracic,
lumbar, 
sacral 
coccygeal pairs

Question 26

what does Formation of segmented somites from unsegmented presomitic (paraxial) mesoderm depend on

Answer 26

NOTCH and Wnt

Question 27

what does increase in Notch protein do

Answer 27

activates other segment-patterning genes that establish the somite

Question 28

Boundaries for each somite are regulated by what

Answer 28

retinoic acid (RA) and a combination of FGF8 and WNT3a

Question 29

RA is expressed at____________ cranially and __________________ caudally, whereas the combination of FGF8 and WNT3a proteins is expressed at _____________ caudally and ______ ones cranially

Answer 29

high concentrations
decreases in concentration
higher concentrations
lower

Question 30

what do the somites first form

Answer 30

presomitic mesoderm, as a ball of mesoderm

Question 31

what happens to the somites by the beginning of the fourth week

Answer 31

cells in the ventromedial walls of the somite lose their epithelial characteristics become mesenchymal again and shift their position to surround the neural tube and notochord. these cells form the sclerotome

Question 32

what does the sclerotome differentiate into

Answer 32

vertebrae and ribs

Question 33

what do Cells at the dorsomedial and ventrolateral edges of the upper region of the somite form and what do the cells between them form

Answer 33

precursors for muscle cells, while cells between these

two groups form the dermatome

Question 34

what happens to cells from muscle precursor groups

Answer 34

become mesenchymal again and migrate beneath the dermatome to create the dermomyotome

Question 35

what do cells from the ventrolateral edge of the somite form

Answer 35

migrate into the parietal (somatic) layer of lateral plate mesoderm to form most of the musculature for the body
wall and most of the limb muscles

Question 36

what do cells in the dermomyotome ultimately form

Answer 36

dermis for the skin of the back and muscles for the back, body wall (intercostal muscles) ,and some limb muscles