Embryology

Question 1

What is gastrulation?

Answer 1

Bilaminar disk (2 layers) becomes trilaminar disk (3 layers)

• forms from upper layer of bilaminar germ disk (epiblast cells)
• cells invaginate + differentiate (pluripotent = can differentiate into a number of tissue/organ/cell types)
• body axes are established

Question 2

What are the 3 germ layers?

Answer 2

Endoderm, mesoderm and ectoderm

Question 3

What do each of the 3 germ layers go on to form?

Answer 3

Ectoderm = nervous system, epidermis, hair + nails, teeth enamel, sensory epithelium
Mesoderm = MSK system, notochord, muscular layer of stomach/intestine, circulatory system
Endoderm = epithelial lining of digestive + respiratory tracts, lining of urethra/bladder/reproductive system, liver, pancreas

Question 4

How do the 3 germ layers form?

Answer 4

invagination of some epiblast mesenchyme cells
days 14-15: replace hypoblast becoming endoderm
day 16: mesoderm (new 3rd layer) formed in between
epiblast cells remaining on surface = ectoderm

Question 5

What is organogenesis?

Answer 5

trilaminar germ disk differentiates into specific tissues + organs
(involves neurulation, development of somites + lateral plate mesoderm + development of all major organ systems)

Question 6

Which trilaminar disc layers are involved in formation of the MSK system?

Answer 6

mesoderm = forms forms nearly all connective tissue of musculoskeletal system
some ectodermal form neural crest cells

Question 7

What are the 3 parts of the mesoderm?

Answer 7

• paraxial (axial skeleton)
• intermediate
• lateral plate (apendicular skeleton)

Question 8

Describe somites

Answer 8

• paraxial mesoderm separates into segments called somites (day 20 onwards)
• somites form craniocaudally
• have ventral + dorsal portions
• ventral = sclerotome
• dorsal = dermomyotome

Question 9

Sclerotome

Answer 9

• form bone/connective tissue
• migrate more medially (ventromedial) in somite (closer to neural tube)
• form bones of vertebral column + ribs

Question 10

Dermomyotome

Answer 10

• migrate more laterally (dorsolateral) in somite
• further divide into myotomal + dermatomal cells
• dermatomal = develop skin
• myotomal = develop muscles

Question 11

What are the 2 main types of bone formation?

Answer 11

Intramembranous (most flat bones)

Endochondral (most limb bones)