Embryology

Question 1

What 4 main things does the endoderm (internal layer) develop into?

Answer 1

• Epithelial lining of GI, respiratory tracts & bladder
• Epithelia of tympanic cavity and auditory tube
• Parenchyma of thyroid, parathyroid, liver, pancreas
• Notochord

Question 2

What 7 main things does the mesoderm (middle layer) develop into?

Answer 2

• Circulatory system
• Skeletal muscle cells, myotomes
• Kidneys and their ducts
• Red blood cells
• Cartilage and bone
• Dermis of skin
• Muscle, connective tissue, peritoneal compartments of gut wall

Question 3

What 6 main things does the ectoderm (external layer) develop into?

Answer 3

• Epidermis of skin, hair, nails
• CNS & PNS
• Sensory epithelia - nose, eye, ear
• Pituitary, mammary and sweat glands
• Enamel of teeth
• Lens of eye and parts of middle ear

Question 4

At how many cells is a morula?

Answer 4

16

Question 5

When is a blastocyst formed?

Answer 5

Further cell division results in cavity at one end of morula

Question 6

What are the 2 different cell masses the blastocyst develops into?

Answer 6

Embryoblast, inner

Trophoblast, outer

Question 7

What develops into the placenta?

Answer 7

Trophoblast

Question 8

What 2 layers does the embryoblast differentiate into & what cells are these made up of?

Answer 8

Epiblast (columnar cells)

Hypoblast (cuboidal cells)

Question 9

What do these 2 layers of the embryoblast form?

Answer 9

Bilaminar disc

Question 10

What do the epiblasts migrate to?

Answer 10

Line the newly formed cavity (between epiblast and trophoblast)
Becomes amniotic cavity

Question 11

What do the hypoblasts migrate to?

Answer 11

Line inner surface of trophoblast

Enclose blastocyst cavity, which becomes primitive yolk sac

Question 12

When does the trophoblast develop extensions to invade the endometrium?

Answer 12

When the blastocyst has formed

Question 13

What are the initial 2 layers of the trophoblast?

Answer 13

Syncytiotrophoblast (lies against endometrium)

Cytotrophoblast (surrounds embryoblast)

Question 14

How does the embryo develop freedom from he trophoblast?

Answer 14

New layer of cells form, the extra-embryonic mesoderm
Between the cytotrophoblast and the hypoblast cells lining the primative yolk sac
Eventually a thick layer separating the embryoblast and the trophoblast

Question 15

How does the embryo receive its primitive blood supply?

Answer 15

Trophoblastic lacunae

Question 16

What is the hypoblast replaced by?

Answer 16

Cells from ectoderm to form endoderm

Migrate to line primitive yolk sac, making it the secondary yolk sac

Question 17

What occurs during the 1st week of development?

Answer 17

Fertilization, formation of the morula and blastocyst

Question 18

What occurs during the 2nd week of development?

Answer 18

Implantation of the blastocyst and formation of bilaminar embryonic discand early placenta

Question 19

What occurs during the 3rd week of development?

Answer 19

Further differentiation of cell layers to form the tri-laminar embryonic disc

Question 20

What occurs during the 4th week of development?

Answer 20

Folding of embryo

Question 21

What occurs from the 5th-8th weeks of development?

Answer 21

Development of all organs

Question 22

What is the primitive streak?

Answer 22

Folding occurs along caudal midline as ectoderm grows

Question 23

What forms the mesoderm?

Answer 23

Cells from the base of the primitive streak break off and migrate to lie between the layers of the ectoderm and endoderm

Question 24

What are the 3 germ layers?

Answer 24

Ectoderm
Mesoderm
Endoderm (lines yolk sac)

Question 25

What does the notochord develop from?

Answer 25

Tube from pit at end of primitive streak

Endoderm