sea urchin development

Question 1

deuterostome

Answer 1

blastopore becomes anus

Question 2

sea urchin symmetry

Answer 2

echinoderms (sea urchins, sand dollars, star fish) have radial symmetry as adults but are part of Bilateral because embryos and larvae are bilaterally symmetric

Question 3

tunicates

Answer 3

closest invertebrates to vertebrate animals
- tunicate larvae have notochord and dorsal nerve cord that degenerate at metamorphosis

Question 4

chordates

Answer 4

include phyla that all exhibit a notochord, a structure that induces formation of the spinal cord in vertebrates

Question 5

sea urchin cleavage

Answer 5

urchins exhibit radial holoblastic cleavage

Question 6

sea urchin cleavage expanded

Answer 6

• first seven cleavages are stereotypic (always occur the same way)
• at division four, animal cells divide meridionally (dividing left to right) to form 8 similar mesomeres
  -vegetal cells divide equatorially (up and down) and unequally to form 4 large macromeres and 4 small micromeres
  -at division five, division planes swap with animal mesomeres producing two cell tiers by equatorial division; vegetal macromeres divide meridionally; micromeres divide unequally yielding 4 larger and 4 smaller micromeres
• cleavage continues, creating two animal layers (an1 and an2) and two vegetal layers (veg1 and veg2) and a vegetal cluster of micromeres
  -at the 120 cell stage the embryo is considered a blastula

Question 7

fate map of the sea urchin blastula

Answer 7

• by the 60 cell stage, before onset of gastrulation, cell fates are specified across the urchin embryo
  -animal pole cells (an1 and an2) = ectoderm
  -veg1 = ectoderm and endoderm
  -veg2= endoderm and mesodermal derivatives (coelom + non-skeletogenic mesenchyme)
• large micromeres = mesodermal cells (skeletogenic mesenchyme)
  -small micromeres= germ line

Question 8

coelom

Answer 8

produces an internal body wall

Question 9

skeletogenic mesenchyme

Answer 9

produces a larval skeleton

Question 10

non skeletogenic mesenchyme

Answer 10

produces muscle, pigment, and immune cells

Question 11

endoderm

Answer 11

internal layer –> digestive tube, pharynx, respiratory tube (stomach cells, thyroid cells, lung cells)

Question 12

mesoderm

Answer 12

middle layer –> notochord, bone tissue, tubule cells of the kidney, red blood cells, facial muscle

Question 13

ectoderm

Answer 13

outer layer–> epidermal cells of skin, neuron of the brain, and pigment cells

Question 14

120 cell stage – sea urchin

Answer 14

by the 120 cell blastula stage, all cells are similar in size, are in contact with the blastocoel, are polarized w/ apical cilia facing outward, and make tight junctions with each other

• ciliary movements cause the blastula to begin rotating within the fertilization envelope, while vegetal cells thicken into the vegetal plate and animal cells secrete enzymes that degrade the fertilization envelope and allow the blastula to hatch

Question 15

blastula cell fates are specified in a two step process

Answer 15

1. by the 16 cell embryo stage, the large micromeres are autonomously specified by inheriting maternal determinants from the vegetal pole
2. large micromeres send signals to conditionally specify veg2 cells to become endomesoderm (both endo and mesodermal fates). these signals are strong enough to re-specify ectoderm into endodermal like fates

Question 16

large micromeres autonomously specified

Answer 16

-can be observed by isolating them, they divide and differentiate normally by themselves
-also transplantation of large micromeres to animal pole mesomeres induces vegetal duplication. these transplanted micromeres from a second skeletogenic mesenchyme, while also inducing animal mesomeres to become vegetal plate endoderm and begin invagination

Question 17

micromere fates are specified by nuclear B-catenin activity

Answer 17

maternal disheveled activity in the vegetal egg is distributed by cleavage and prevent b-catenin degradation
- this accumulated b-cetenin enters the nuclei at high level, specifying micromere fate
-promotion of b-catenin signaling = endoderm and mesoderm formation
-blocking b-catenin activity causes a loss of endoderm and mesoderm

Question 18

b-catenin function in micromere vs. macromere cell fate

Answer 18

• veg2 macromeres have b-catenin but not at a high enough level so HesC promotes end-mesodermal fate by repressing micromere genes
  -in micromeres, high levels of maternal b-catenin and Otx together activate Pmar1, which inhibits HesC expression–> blocking this allows for activation of micormere genes such as Delta and Notch to promote Veg2 endomesodermal fate
  -pmar1 inhibition of HesC inhibition to achieve activation is referred to as “double-negative gate” which is a new name for disinhibition

Question 19

gastrulation in sea urchin

Answer 19

sea urchin gastrulation creates mesoderm and endoderm in two separate events
1. mesenchymal cells (skeletogenic, mesoderm) form by ingression from the vegetal plate into the blastocoel
2. gut tube (archenteron) endoderm is made secondarily by invagination of the vegetal plate

Question 20

skeletogenic mesenchyme are produced by EMT

Answer 20

large micromere descendants undergo 5 major changes during EMT
1. vegetal cells elongate
2. apical constriction
3. remodeling of basal lamina
4. de-adhesion
5. increased motility

note: (idk ??)
- apical surface faces outward toward the hyaline layer
-the “pink” basal lamina is degraded to allow ingression
-the blastocoel is not just fluid, it contains ECM that interacts with mesenchyme

Question 21

EMT

Answer 21

epithelial to mesenchymal transition; during gastrulation, mediates the formation of mesoderm

Question 22

signaling events promote skeletal positioning and formation

Answer 22

observation 1. skeletogenic mesenchyme position themselves near ectodermal cells with high level
b-catenin –> how and why?

observation 2. mesenchyme sense the environment in part by sampling with long filopodia (but cannot sense for b-catenin activity)

observation 3. ectodermal regions expressing the RTK signals FGF and VEGF direct mesenchyme positioning (these cells express the receptors!!)

leads to ==> B-catenin likely controls the RTK ligand expression and blocking RTK signaling disrupts skeleton formation

Question 23

skeletogenic mesenchyme –> structural support

Answer 23

-signals direct skeletogenic mesenchyme to form a ring of cells around the blastocoel along the animal/vegetal axis
- once aligned, these cells fuse to form syncytial “cables” that begin to secrete calcium carbonate, forming spicules
-the calcium carbonate skeleton forms in stereotypic fashion, providing structural support for the larva

Question 24

archenteron formation and gastralation

Answer 24

-in early gastrulation vegetal plate cells change shape via apical constriction and forms the blastopore as the plate invaginates
-first cells to enter are mesenchymal then endoderm
-invagination stalls when apical constriction reaches its limit
-later in gastrulation, invagination resumes as cell divide, move, intercalate
-casues convergent extension; elongates and narrows the archenteron within the blastocoel
-last part of archenteron extension is provided by non skeletogenic mesenchyme, which extend and attach filopodia to the upper blastocoel ectoderm –> filopodia shorten –> pulls archenteron into final position –> non mesenchymal cells disperse into the blastocoel to proliferate and form mesodermal organs
-a mouth opening forms in the ectoderm that contacts archenteron
- fusion of mouth cells with the archenteron forms a gut tube that supports the larva