early embryonic development

Question 1

When do somites appear and what is the rate they appear at? how many somites do you end up with?

Answer 1

1st pair at 20 days, then grows 3pairs/day at craniocaudal sequence until 42-44 pairs (end of week 5)
some then disappear, leaving 31 pairs - spinal nerves

Question 2

What does organisation of mesoderm to somites give rise to?

Answer 2

ribs, vertebrae, intercostal muscle

all guides the distribution of organs & nerves

Question 3

What structures are there in the differentiation of mesoderm?

Answer 3

notochord, somites, intermediate mesoderm & lateral plate mesoderm (somatic, splanchnic)

Question 4

What does lateral plate mesoderm give rise to?

Answer 4

somatic (top): skeletal muscles (contributes to body structures)
splanchnic (bottom): organs

Question 5

What does intermediate mesoderm give rise to?

Answer 5

gonads & kidneys

Question 6

What does paraxial mesoderm give rise to?

Answer 6

somite –> sclerotome, myotome, dermatome

Question 7

What is the space between splanchnic & somatic mesoderm called?

Answer 7

intraembryonic coelom

Question 8

How do somites appear?

Answer 8

as a block of mesoderm cells arranged around small cavity

Question 9

How do somites differentiate into sclerotome?

Answer 9

through ‘organised degeneration’

ventral (front) wall of somite breaks down

Question 10

How do somites differentiate into dermomyotomes?

Answer 10

further organisation of dorsal portion

forms combined dermomyotome - skin, muscle, tissue

Question 11

How does dermomyotome separate?

Answer 11

myotome proliferates & migrates (spreads out around body)
dermatome disperses

Question 12

What are somites?

Answer 12

segments in pairs, either side of axis, surrounding neural tube & notochord

Question 13

What is segmentation and what does it achieve?

Answer 13

organise mesoderm into somites to give:
vertebrae (spinal cord segment), ribs, intercostal muscles
& guide innervation: distribution of nerves

Question 14

What is epimere & hypomere?

Answer 14

epimere: supplied by dorsal branch of spinal nerves
hypomere: supplied y ventral branch of spinal nerves

Question 15

Explain the use of the term ‘dermatome’ in describing the innervation of skeletal muscle in adults

Answer 15

Developmentally: part of somite which gives rise to dermis
Clinically: a strip of skin supplied by a single spinal nerve

Question 16

Explain the use of the term ‘myotome’ in describing the innervation of skeletal muscle in adults

Answer 16

Developmentally: part of smite which gives rise to muscles
Clinically: muscle / group of muscles supplied by a single spinal nerve

Question 17

List the different regions of mesoderm

Answer 17

1. notochord
2. intermediate mesoderm
3. lateral plate mesoderm
4. paraxial mesoderm

Question 18

What is the derivative of paraxial mesoderm?

Answer 18

sclerotome, myotome, dermatome

axial skeleton (vertebral column & ribs)
dermis, muscles of A/L wall
some limb muscles

Question 19

What is the derivative of intermediate mesoderm?

Answer 19

urinary system e.g. kidneys, ureters, gonads

Question 20

What is the derivative of the lateral plate mesoderm?

Answer 20

Somatic (top): C.T. of limbs, contributes to anterior/lateral body walls (skeletal muscles)

Splanchnic (bottom): (organs) smooth musculature, C.T. & vasculature of gut

Question 21

Describe the formation of the head, tail & body folds

Answer 21

cephalocaudal folding (head --> tail)
so endoderm on inside: gives rise to lining of GI

Question 22

Describe the formation of the lateral body folds

Answer 22

the sides (determined by size of developing somites)
so amniotic cavity (top) folds over yolk sac

Question 23

What is the purpose of folding? What doe it achieve?

Answer 23

draws together margins of disk

1. create ventral body wall (active closure)
2. pull amniotic membrane around everything (embryo suspended in sac)
3. lining of GI
4. heart move from head –> chest
5. yolk sac forms connection with gut (umbilical out of belly)

Question 24

Describe the formation of the coelom

Answer 24

when the embryo undergoes lateral folding & the lateral branches of mesoderm connect to form the coelom (inside embryo where the organs grow within) (intraembryonic coelom join)

Question 25

What is the purpose of notochord in neurulation?

Answer 25

notochord: solid rod of cells running in midline, important signalling role directs conversion of overlying ectoderm to neuroectoderm (lead to development of nervous tissue)

Question 26

Describe the formation & fate of the neural plate

Answer 26

notochord signals: ‘overlying ectoderm’ to thicken (neural plate - key hole shape)
the edges elevate out of the plane of disc (up & above) curls towards each other, creating the neural tube

Question 27

Describe what has happened by the end of week 4

Answer 27

neurulation: formation of primordium of nervous system
organisation of the mesoderm:
1. derivatives of mesoderm regions (somites) - lateral plate, intermediate, paraxial, notochord (signal to form neuroectoderm) - overlying ectoderm
2. segmentation - distribution of nerves & formation of ribs etc. (somites break down)

Question 28

describe what has happened by the end of week 3

Answer 28

embryo gastrulated: bilaminar (trophoblast & embryoblast) –> trilaminar disk (endo, meso, ectoderm)
axes set - through notochord signalling