Embryology 3

Question 1

What does endoderm form?

Answer 1

• Important for inducing the formation of
  mesodermal organs
• (e.g. heart, blood vessels)
• Forms lining of the digestive tract
• Digestive tract: whole length of the body
• Buds off liver, gall bladder, pancreas, lungs
• Cranial part (anterior of lungs): pharynx
• Epithelial pockets of pharynx: thyroid, thymus, and
  parathyroid

Question 2

Describe formation of gut tube

Answer 2

• Lateral folding of the embryo completes the gut
  tube
• With lateral folding, mesoderm is recruited to the
  gut wall
• The mesodermal layer of the gut tube from
  splanchnic mesoderm
• somatic mesoderm lines the body cavity

Question 3

Describe the allantois

Answer 3

• Endodermal, surrounded by blood vessels
  (becomes umbilical arteries and veins)
• Sac-like structure
• Helps with gas exchange and excretion
• Becomes urachus (connects fetal bladder to yolk
  sac)

Question 4

Explain development of gut and its blood supply

Answer 4

• Primitive gut develops at beginning of fourth week
• Closed at cranial end: oropharyngeal membrane
• Close at caudal end: cloacal membrane
• 3 parts: Foregut, Midgut and Hindgut defined by
  their blood supply

o Foregut from celiac artery
o Midgut from superior mesenteric artery
o Hindgut from inferior mesenteric artery

Question 5

Explain regional patterning of gut

Answer 5

 Regional patterning of the gut tube (Development
of other endodermal organs)
o Endodermal thickening
o Cells proliferate to form bud
o Continuous lengthening and bifurcation/branching

Question 6

Explain development of stomach

Answer 6

• Distal part of foregut, around middle of fourth
  week, slight dilation
• Enlarges and broadens ventro-dorsally
• Dorsal part grows faster than ventral part: greater
  curvature of stomach
• While growing: rotation 90 degrees clockwise
• Ventral border (smaller curvature) moves to right,
  dorsal border (greater curvature) to left
• Rotation superiorly bends duodenum into a C-
  shape

Question 7

Explain development of lungs

Answer 7

• Ventral out-pocketing of endoderm = respiratory
  diverticulum > will form trachea
• Growths ventro-caudally
• Bifurcation > right and left primary tracheal buds >
  will form bronchi
• Next bifurcation: secondary bronchial buds (3 on
  right, 2 on left) > lung lobes
• Third bifurcation: tertiary bronchial buds >
  bronchopulmonary segments
• 14 more branchings > terminal bronchioles

Question 8

Memorise the “gut and its derivatives” table

Answer 8

Yes

Question 9

Describe pharyngeal arches

Answer 9

• Many skeletal structures unusual: formed from neural crest cells rather than mesoderm
• Human embryo has 4 pairs of well-defined pharyngeal arches
   Pharyngeal arches (also called branchial arches)
  o Outer covering of ectoderm
  o Mesenchymal core derived from mesoderm as well as neural crest cells
  o Lined inside with endoderm
  o Each pharyngeal arch contains:
   Central cartilaginous skeletal element (derived from neural crest)
   Striated muscle rudiment (derived from head mesoderm)
   Arch-specific cranial nerve
   Aortic arch artery (endothelial cells derived from mesoderm)

Question 10

Describe formation of germ cells

Answer 10

 Primordial germ cells
- Precursors of sperm and egg
- Specified early in development, the embryonic disc with two layers
- Specified by inductive signalling from surrounding cells
 PGC migration
- PGCs are not specified within the gonads
- PGCs at base of allantois
- Gonads develop as thickening of ventro-medial surface of mesonephros (intermediate mesoderm)
 Migration:
o Along hindgut
o Through dorsal mesentery
o Split up to coalesce with left and right genital ridges
o Stop migrating