7.5 Muscle Formation and Innervation

Question 1

What are the three phases of human development within the mother? Describe the timeline of these three phases.

Answer 1

• Embyrogenic (weeks 0-2)
• Embyronic (weeks 3-8)
• Foetal (months 3-9)

Question 2

How many days after fertilisation is the blastocyst formed and ready for implantation?

Answer 2

7 days (1 week)

Question 3

What are the respective roles of the cyto vs syncitiotrophoblast cells?

Answer 3

Cyto: anchor to endometrium
syncytiotropho: invade endometrium

Question 4

What do the epiblast and hypoblast become? What are the functions of these?

Answer 4

• Hypoblast becomes yolk sac; provides nutrients, gas exchange, blood cell production
• Epiblast becomes amnion; will eventually surround the whole foetus. Epiblast also gives rise to the three germ layers

Question 5

Describe gastrulation

Answer 5

• Primitive streak forms caudally, epiblast cells begin to invaginate
• Epiblast cells that displace hypoblast become endoderm
• Epiblast cells that are invaginated after this are mesoderm
• Remaining epiblast becomes ectoderm

Question 6

Describe neurulation

Answer 6

• Notochord releases factors that causes the overlying ectoderm to thicken, forming the neural plate
• Neural plate begins to fold and invaginate anteriorly, forming the neural crests
• Once closed, the neural tube is formed (which will be CNS), and neural crest cells are still present (this will be PNS).

Question 7

Does the neural tube fold all along all at once? Describe the pattern.

Answer 7

• No
• Folds in center, then spreads cranially and caudally

Question 8

After gastrulation, what three segments does the mesoderm divide into? What does each give rise to?

Answer 8

• Paraxial: axial skeleton and skeletal muscle
• Intermediate: urinary and reproductive tract
• Lateral: (somatic forms appendicular skeleton; splachnic forms cardio system + smooth muscle of resp and GI)

Question 9

Describe somite differentiation.

Answer 9

• Somites become sclerotomes and dermomyotomes
• Sclerotome (sclero = hard) becomes axial skeleton
• Dermomyotome become dermatome, and myotome, and myotome becomes epimere (epiaxial = postural musclces) and hypomere (hypaxial = locomotion muscles)

Question 10

Describe the two directions of embryonic folding. In what planes do they occur?

Answer 10

• Lateral folding (in horizontal plane)
• Cranial caudal folding in the sagittal plane

Question 11

Describe cranial caudal folding

Answer 11

Embryo folds inward along the sagittal plane, causing the endoderm, which was previously on the ventral aspect, to be incorporated on the inside. Some of the yolk sac remains outside the embryo.

Question 12

What three sections of the endoderm are formed during embryonic folding?

Answer 12

• Foregut
• Midgut
• Hindgut

Question 13

Describe lateral folding of embryo.

Answer 13

Embryo folds ventrally and medially, closing off the gut tube and bringing the two lateral aspects of the ectoderm closer together on the ventral side, like closing it up.