Gastrulation And Onwards

Question 1

Gastrulation: from blastula to gastrula

Answer 1

• a coordinated series of cell and tissue movements during which the single layered blastula is reorganised into a multilayered structure known as the gastrula
• results in the formation of the three germ layers (ectoderm, mesoderm, endoderm)

Question 2

Three types of gastrulation movements

Answer 2

Ingression

Invagination

Involution

Question 3

Ingression

Answer 3

• migration of individual cells from surface epithelium to interior of the embryo

Question 4

Invagination

Answer 4

• in folding of the surface epithelium of the blastula usually at the vegetal pole, to form a central tube called the archenteron
• point of invagination is known as the blastopore

Question 5

Involution

Answer 5

In turning of and expanding outer layer of cells

Question 6

Ingression in sea urchins (Lytechinus variegatus)

Answer 6

• vegetal pole thickens to form the vegetal plate
• primary mesenchyme cells ingress into the blastocoel
• look at slides for diagrams

Question 7

Invagination in sea urchins

Answer 7

• vegetal plate invaginates, buckling inwards at the blastopore to form the archenteron
• a group of secondary mesenchyme cells form at the tip, producing pseudopodia that pull the archenteron towards the animal pole
• the archenteron dudes with the animal pole to form continuous tube - the gut. The blastopore forms the anus

Question 8

Invagination: apical constriction

Answer 8

• buckling of the vegetal plate is caused by localised contraction of actin myosin micro filaments near the apical surface of the blastula epithelium
• apical contraction narrows the apical surface relative to the Basal surface

Check diagram on slides for better visual explanation

Question 10

Elongation: requires convergent-extension

Answer 10

• elongation of the archenteron is mediated by convergent extension
• cells acquire lamellipodia perpendicular to the invagination and intercalate with each other
• this narrows the archenteron (convergence) and at the same time elongates it (extension)

Question 11

There are 3 germ layers

Answer 11

Ectoderm, endoderm and mesoderm

Question 12

Example of ectoderm germ layer

Answer 12

Epidermis

Nervous system
(Cartilage/bone in the head)

Question 13

Example of mesoderm germ layers

Answer 13

Cartilage/ bone
Connective tissue
Muscle
Kidney
Blood
Vasculature
Heart
Gonads

Question 14

Examples of endoderm germ layers

Answer 14

Digestive tract (epithelium)
Lungs
Liver
Gall bladder
Pancreas

Question 16

The mammalian blastocyst

Answer 16

• undergo Holoblastic cleavage
• at the morula stage (~16 cells) cells undergo compaction whereby cells maximise their contacts
• cells at the centre of the compacted embryo are known as the inner cell mass, while peripheral cells are known as the trophoblast
• a cavity, the blastocoel, forms in the centre of the embryo, which is now called a blastocyst
• the inner cell mass cells are pluripotent

Question 17

Implantation

Answer 17

• after attaching to uterine wall the trophoblast proliferates to form the syncytiotrophoblast, a highly invasive tissue that penetrates the uterine wall
• the ICM divides into the epiblast and hypoblast layers
• the epiblast forms the amniotic membrane
• the hypoblast forms the Heuser’s membrane

Question 18

Ingression: mammalian gastrulation

Answer 18

• most similar to birds (although mammals have lost the large yolk mass)
• epiblast cells migrate through the primitive streak as individual cells via Ingression
• the first cell to ingress displace the hypoblast and form the embryonic endoderm
• subsequently Ingressing cells form the mesoderm
• cells remaining in the epiblast form the ectoderm

Question 19

Ingression - mesoderm formation

Answer 19

• cells migrating through the node from the axial mesoderm, the prechordal mesoderm and the notochord
• cells ingressing through the primitive streak condense either side of the notochord, forming the paraxial, intermediate and lateral plate mesoderm

Question 20

Somite development

Answer 20

• Paraxial mesoderm forms pairs of somites, sequentially from rostral to caudal
• in response to signals they subsequently divide into sclerotome, myotome and dermatome layers
• the myotome forms the skeletal muscle, the sclerotome forms the axial skeleton and the dermatome form the connective tissue of the dermal layer of the dorsal skin

Somites form every 3 minutes in zebrafish, every 90 in chicks and every 5 hours in humans

Question 21

Neurulation: the neural plate

Answer 21

• the ectoderm anterior to the node forms the neural plate, the precursor of the nervous system
• cells of the neural plate elongate to form a pseudostratified columnar epithelium
• the primitive streak regresses

Question 22

Neurolation: neural tube closure

Answer 22

Thought to function via apical constriction (just like Involution)

Neural tube closure defects are one of the most common severe congenital malformations

Question 23

Neural crest

Answer 23

• dorsal roof of neural tube forms the neural crest
• neural crest are migratory multi potent stem cells
• unique to vertebrates and may have played a key role in vertebrate evolution

Question 24

The vertebrate body plan: phylotypic stage

Answer 24

• at the end of neurolation, vertebrate embryos display the basic vertebrate body plan
• they are the most similar to each other at this stage - which is known as the phylotypic stage

They also share a common ‘molecular anatomy’ at this stage