Sea Urchin and Tunicate Development

the outside of the embryo is ____ side and the inside is ___ side

apical, basal

This pole of the egg is the future ectoderm

animal pole

This pole of the egg is the future endoderm

vegetal pole

The cleavage sea urchins exhibit

radial holoblastic

These divisions are meridional and perpendicular to each other, cleavage furrow passes through both poles and yields identical cells

1st and 2nd

This division is equatorial and separates the animal pole from the vegetal pole—the 8 new cells are not equal in size; the cells containing the vegetal pole are slightly larger and morphogens are now only present in these bottom 4 cells

3rd

8 same volume blastomeres the animal cells divide into during the 4th division

mesomeres

Unequal division of the vegetal cells where macromeres and micromeres are produced

5th

what drives cellular development in the sea urchin

beta-catenin

formed after the 7th division where the cells in the 120 cell blastula arrange into a hollow sphere surrounding a central cavity

blastocoel

thickened cells belonging to the vegetal pole of the blastula before gastrulation

vegetal plate

Larval stages

blastula, prism, pluteus

in the 64 cell blastula these cells are autonomously determined

• responsible for making skeletal rods and when they migrate they pull in endoderm from the bottom and help start gastrulation

large micromere cells

If micromeres are isolated and placed in culture they will…

create skeletal rods/spicules

1st cells whose fates are determined autonomously

skeletogenic micromeres

This germ layer is autonomously specified in the animal hemisphere of the 64 cell blastula

ectoderm

The micromeres conditionally specify ectoderm cells to make…

neural components

what happens when micromeres are transplanted to the animal hemisphere of the blastula

a 2nd gut formation will be induced

maternal determinants micromeres use to conditionally specify tissues

disheveled and beta-catenin

what micromeres use to specify the endoderm and mesoderm, and to pattern the animal-vegetal axis

• GSK3 binds to APC and regulates beta-catenin degradation normally, disheveled is only present in the micromeres and allows beta-catenin to accumulate

canonical wnt pathway

wnt is only ______ in the micromeres

active

Otx is a maternally derived TF that interacts with the beta-catenin/TCF complex at the enhancer of the Pmar1 gene to activate Pmar1 transcription in the micromeres shortly after their formation

• Pmar1 represses the HesC gene, HesC is active in every embryonic cell except the micromeres

• When the genes repressed by HesC are activated, skeleton-forming genes are activated, which stabilize the regulatory state of the skeleton mesenchyme cells

Pmar HesC double negative gating

a mechanism where a repressor locks the genes of specification and these genes can be unlocked by the repressor of that repressor

• allows for tight regulation of fate specification

double negative gate

Another way micromeres retain their specification—sets up a positive feedback loop between blimp1 and wnt that establishes a source of beta-catenin for the micromere nuclei, serves to lock both genes “on”

• can amplify expression levels and make the resulting cell type (skeletal) irreversible

beta-catenin autocrine loop

the process by which endoderm (gut) and mesoderm (skeleton) destined cells on the outside of the hollow blastula are brought inside the embryo

gastrulation

Movement of primary mesenchyme cells—occurs simultaneously to gastrulation

• happens after the blastula hatches from the fertilization envelope

• epithelial cells change shape, lose their adhesions to neighboring cells via dissolving blastomere cadherins, losing affinity to the hyaline layer and gaining affinity to basal lamina, and break away from the epithelium to enter the blastocoel as skeletogenic mesenchyme cells

epithelial to mesenchymal transition

After EMT these cells gain mobility and begin extending and contracting filopodia

• cells become localized within the prospective ventrolateral region of the blastocoel by following chemotactic signals of VegF and FGF from ectoderm

• then they fuse into syncytial cables that will form the axis of the CaCO3 spicules of the larval skeletal rods

PMC

Sequential invaginations and flattening and thickening of cells along with apical constriction, the blastopore is formed during this process

• apical actomyosin complex undergoes contraction to buckle epithelium, shape helps pull cells up during invagination and initiate gastrulation

archenteron formation (early gut)

The first group of cells to invaginate during early gut formation—forms that tip of the archenteron and leads the way into the blastocoel

• these cells eventually form pigment cells and musculature around the gut

non-skeletogenic mesenchyme

These cells migrate the farthest into the blastocoel during gastrulation

• eventually becoming foregut, midgut, hindgut and anus

endoderm

process where actin microfilaments collect in the apical ends of vegetal cells, causing these ends to constrict, forming bottle-shaped vegetal cells that pucker inward

• happens during archenteron formation

apical constriction

phenomenon where cells intercalate to narrow the tissue and at the same time lengthen it

• Along with pulling from filipodia, how the archenteron lengthens dramatically to form intestines

convergent extension

Gene that drives archenteron formation, is activated by beta-catenin

Endo16

deuterostomes, the most basic chordate animal

• when going from larva to adult, the larva cements its head to a structure, digests its tail, and the nerve chord turns into cerebral ganglion

tunicates

The 1st cell division creates a right and left side in the embryo and the 2nd division is slightly unequal meridian—asymmetrical cell division leads to unequal distribution of cytoplasmic determinants

tunicate cleavage

yellow lipid inclusions in the tunicate egg that migrate along with the male pronucleus after fertilization

• extends from the vegetal pole to the equator, is responsible for mesoderm cells (mainly tail muscle)

yellow crescent

Located in the yellow crescent is mRNA that codes for a TF that is required for the activation of several mesodermal genes such as snail

• activates a TF cascade that promotes muscle differentiation while at the same time inhibiting notochord specification

macho-1

a protein that prevents brachyury expression in presumptive muscle cells, prevents muscle precursors from becoming notochord cells

snail

these pathways control germ layer differentiation in tunicates

macho-1 and FGF