Embryology

Question 1

What 4 main things does the endoderm (internal/’lower’ 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
(see from page 57 in syllabus)

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

8

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

Fertilisation and the formation of the blastocyst

Question 18

What occurs during the 2nd week of development?

Answer 18

Implantation and formation of the bilaminar embryonic disc

Question 19

What occurs during the 3rd week of development?

Answer 19

Further development of the embryo and formation of trilaminar embryonic disc

Question 20

What occurs during the 4th week of development?

Answer 20

Folding of the embryo

Question 21

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

Answer 21

Development of all organs
By end of 8th week, embryo looks like person and is called a foetus - the kidneys, liver, brain and lungs are all beginning the function; the fingers and toes are separate and external genitalia are formed.

Question 22

What forms the mesoderm?

Answer 22

Invaginating cells

cells that have detached from the epiblast that migrate towards the primitive streak

Question 23

What does the notochord develop from?

Answer 23

Prenotochordal cells invaginating in the node of the primitive streak.

Question 24

What is the period of time called from fertilisation to end of 8th week?

Answer 24

Embryonic period

Question 25

What is period of time called from beginning of 9th week to birth?

Answer 25

Foetal period

Question 26

What does the notochord form?

Answer 26

A midline axis which serves as the basis of the axial skeleton