Lecture 8: Embryology II

Question 1

How does embryonic lengthening occur during gastrulation?

Answer 1

1. Oriented cell division/migration: cells migrate/divide along long axis
2. Convergent extension: cells merge from wider into longer and extend along long axis

Question 2

When does body folding start?

Answer 2

Week 4, right after the trilaminar germ disc stage

Question 3

How are the TGD layers initially oriented?

Answer 3

Endoderm is ventral, ectoderm dorsal, mesoderm in between

Question 4

Cephalocaudal folding process (head to toe)

Answer 4

Cephalic end -> head fold (rapid brain development, creates foregut, places heart ventral)
Caudal end -> tail fold (convergent extension, spinal cord lengthening, creates hindgut, places connecting stalk ventral)

Midgut also forms continuous with the yolk sac

Question 5

Lateral folding process

Answer 5

Lateral aspects of the embryo fold ventrally. Amniotic cavity wraps around, coelomic cavity created, gut tube sealed off (except yolk sac)

Question 6

Neuroectoderm

Answer 6

Surface ectoderm dorsal to the notochord which is induced to differentiate. Begins as neural plate and becomes nervous system + others

Question 7

Neurulation

Answer 7

Formation of the neural tube:
1. Induction of neural plate
2. Lateral folding + convergence of folds
3. Fusion of folds into tube
4. Separation of neural crest cells

Question 8

Neural crest cells

Answer 8

Neuroectoderm cells that pinch off and undergo EMT to become ectomesenchyme. “4th germ layer”; form many essential structures

Question 9

Neural crest cell migration pathways

Answer 9

Ventral pathway: through anterior half of somites -> peripheral ganglia
Dorsal pathway: re-enter ectoderm through basal lamina holes -> melanocytes
Cranial NCCs -> facial structures

Question 10

Neuropores

Answer 10

Open ends of neural tube at cranial/caudal ends; become brain/end of spinal cord

Question 11

Direction of embryonic development

Answer 11

Embryo develops from cranial/rostral end to caudal.

Question 12

Surface ectoderm

Answer 12

Completely covers embryo after folding. Contains invaginations for future mouth/anal canal; future skin epidermis

Question 13

Stomodeum

Answer 13

Primitive mouth

Question 14

Proctodeum

Answer 14

Primitive anal canal

Question 15

Mesoderm sections

Answer 15

Midline/axial: notochord
Paraxial: segments into somites
Intermediate: excretory units
Lateral plate: splits into parietal/visceral (somatic/splanchnic) layers; cavity/body wall lining

Question 16

Somite

Answer 16

Condensed mesoderm segments; differentiates into dermatome, myotome, sclerotome

Question 17

Dermatome

Answer 17

Migrates with myotome to form skin dermis. Retains innervation from spinal segment of origin. Segmental nerves follow.

Question 18

Myotome

Answer 18

Migrates with dermatome to form muscle. Retains innervation from spinal segment of origin. Segmental nerves follow.

Question 19

Sclerotome

Answer 19

Becomes vertebrae/ribs.

Question 20

Intermediate mesoderm

Answer 20

Becomes urogenital structures; gonadal ridge, mesonephric ducts, etc.

Question 21

Lateral plate mesoderm, somatic layer

Answer 21

AKA parietal layer. Lines coelomic cavity along body wall. Along with visceral layer, comprises the serous membranes AKA the mesothelium, undergoing MET.

Question 22

Lateral plate mesoderm, splanchnic layer

Answer 22

AKA visceral layer Lines internal organs facing cavities. Along with parietal layer, comprises serous membranes (mesothelium), undergoing MET.

Question 23

Coelomic cavity

Answer 23

Intraembryonic cavity. Becomes the 3 main adult cavities (peritoneal, pericardial, pleural)

Question 24

What structure does the endoderm become after folding?

Answer 24

Gut tube (“tube within a tube”) epithelial lining (ONLY the epithelium)`

Question 25

Umbilical ring

Answer 25

Region of ventral body wall open to yolk sac, connecting stalk

Question 26

Gut herniation

Answer 26

Rapid intestinal elongation leads to the gut herniating through the umbilical ring. Later returns and rotates back into the body.

Question 27

Gut rotation

Answer 27

During herniation, the gut rotates 90° counterclockwise around the superior mesenteric artery. Later, it rotates another 180° within the abdominal cavity; essential for proper gut positioning

Question 28

Umbilical hernia

Answer 28

Pathological developmental herniation; small bowel protrusion through umbilical ring, covered by skin

Question 29

Omphalocele

Answer 29

Pathological developmental herniation; protrusion through the umbilical ring entirely; can be a failure of intestinal return.

Question 30

Gastroschisis

Answer 30

Pathological developmental herniation; failure of anterior body wall to close properly, not just at the umbilical ring

Question 31

Origins of gut tube sections

Answer 31

Gut epithelium = endoderm
Lamina propria to serosa (all other layers) = splanchnic mesoderm

Question 32

Heart development

Answer 32

Primitive blood island and initial vasculature develop in extraembryonic mesenchyme, initially rostral to rest of embryo. Cranio-caudal folding brings these into the ventral body wall.

Question 33

Branchial arches

Answer 33

AKA pharyngeal arches. Bilateral ecto/mesoderm evagination; pharyngeal arch fusion is critical for normal facial development.

Question 34

Prechordal plate

Answer 34

Mesoderm cells between the cranial notochord end and the oropharyngeal membrane. Important for forebrain induction, head/neck CT.