Cell and Embryology
Development of Nematodes, Sea Urchins, Ascidians, and Slime Molds
Wolpert L, Beddington R, Jessell T, Lawrence P, Meyerowitz E, Smith J. (2001) Principles of Development. 2th ed. London: Oxford university press.
Gilbert SF. (2003) Development Biology. 7th ed. Sunderland: Sinaure Associates Inc.
Fig 6.1 Phylogentic tree showing relationship between the organisms considered in this book
Mosaic versus regulative development
Mosaic development depends upon localized cytoplasmic factors while regulative development depends upon cell-cell (group) interactions .
In some invertebrates, cell fate is often specified at the single cell level, not in groups of cells, and does not rely upon positional information.
This process results from asymmetric cell division to distribute the cytoplasm unequally and allow determination of different cell f t
fates.
Most organisms, both mosaic and regulative development.
Cell lineage of Caenorhabditis elegans (nematodes)
The complete cell lineage of Caenorhabditis elegans is invariant . The larva of 558 cells grows to an adult of 959 cells (plus germ cells). 113
undergo programmed cell death.
No polarity exists in the unfertilized egg, but sperm entry controls the first p y gg p y cleavage. (very different )
A cap of actin microfilaments form at the anterior end and P granules become localized to the posterior. Microfilaments participate the location of P granules.
After fertilization, egg enter unequal cleavage. Fig 6.3
The AB cell becomes the anterior and P cell (contain P granules) is posterior. P granules are ribonucleoprotein that specify the germ cells, it
p g p p y g ,
regulate translation.
P granule movement is the result (not a cause) of posterior determination, by cytoskeleton. Fig 6.3
P granule move depend on sperm entry.
Fig 6.3 Localization of P granules after fertilization
Sperm entry region is posterior Egg nucleus is anterior
Fusion sperm and egg nucleus P granules move
to posterior
Two cell stage
P granules localized in posterior. Small cell contain P
Stain DNA
g cell contain P
granules.
26 cell stage
All the P granules are in the P4 cell, it only germline only
Cell lineage of Caenorhabditis elegans Axes depend on asymmetric division & cell interactions.
Before fertilization, there is no evidence of any asymmetry in the nematode egg.
The first division gives a large anterior AB cell and a small P1 cell. (P1, a stem cell, produces a P* cell and another.) The first cleavage, defines the , p ) g , future A/P axis, with the large AB cell marking the anterior and smaller P1 cell the posterior.
During first three divisions, P cells give rise to a number of lineages but the P4 cell then gives rise to only germ cells.
The AB cell divides into anterior ABa (neurons, epidermis plus pharynx mesoderm) and the posterior ABp (neurons, epidermis and specialized cells).
P1 divides into P2 and EMS. EMS divides to make MS (mesodermal pharynx) and E (gut).
P2 becomes P3 and C (epidermis & muscle).
P3 becomes P4 and D (muscle).
All the cells undergo further invariant divisions, and by about 100 minutes after fertilization, gastrulation begins.
Fig 6.2 Early cell lineage in the nematode
.
Large small 1
2 Fertilized egg
↓ First cleavage
3
4 First cleavage
↓ Microfilament A/P axis- P granules move
AB cell
↓ Cleavage
↓ D/V axis
a four-cell-stage Caenorhabditis elegans embryo, Wnt signaling polarizes an endoderm precursor called EMS
Maternal gene in nematodes : A/P axis
Maternal gene par-1 encode PAR-1 protein, which localized in the future posterior region after fertilization
PAR-1 and P granules are contribute to the development of A/P axis ( l it )
(polarity).
The distribution of PAR-1 and P granules are regulated by microtubule- organizing center at the posterior of the fertilized egg.
The center arise from the aster of microtubules contributed by sperm nucleus.
A t l (fi t) PAR 1 l di t ib ti
Asymmetry cleavage (first) → PAR-1 unequal distribution
Egg (PAR-1 and P granules) → After fertilization → microtubles move → posterior → egg division → asymmertic distribution
elegans: cell-cell interactions (not groups) for D/V axis develop From second cleavage, Cell-cell
interactions specify cell fate in the early nematode embryo.
Cell fate is invariant but experiments reveal that cell-cell interactions are crucial. Fig g 6.4
D/V change → R/L change
ABp must contact the P2 cell (or it becomes an ABa cell).
D/V determined in cleavage 2 R/L determined in cleavage 3 (Fig 6.5) ABp and ABa contain the same substance ABp and ABa contain the same substance,
it may form the equal division.
Different contact, may induced different differentiation.
EMS → ventral part development (gut) AB → Bilateral development
Fig 6.5 Reversal of handedness in nematodes
AB al and AB ar from the equal division.
Specification of left and right occurs that at the third cleavage, and handedness can be reversed by experimental manipulation at this stage (6 cells).
Asymmertic division and cell-cell interaction specify cell fate in the early nematode embryo
AB and P1 cell are unequal division, but ABa and ABp are equal division.
(positional signal different → different fate)
ABa and ABp must be by interactions with adjacent cell (P1 cell). For removing P1, a normal product of ABa, but no ABp cell.
P2 cell responsible for specifying ABp cell.
Maternal gene glp-1 and apx-1 involve of the P2 induced ABp cell.
glp-1encodes a transmembrane receptor. Its mRNA is uniformly distributed but translation is repressed in the P cell and is restricted to the AB cell.
At two cell, glp-1 protein only expression in AB cell. After 3’ cleavage produced ABa and ABp also has expressed.
p p p
After 2nd cleavage, P2 expresses apx-1 protein on its surface which activates Glp-1 receptor. This causes ABa and ABp descendants to respond to signals from the P2 cell differently. Fig 6.6
Other maternal gene skin in EMC cell (higher than AB cell). Mutant skin gene, EMC only produce muscle.
Induction → posterior development
Fig 6.6 An early inductive specifics ABp
ABa and ABp are symmertic division form AB cell. But ABp received signal from P2 cell.
1. Maternal gene glp-1 and apx-1 involve of the P2 induced ABp cell.
2. glp-1 encodes a transmembrane receptor. Its mRNA is uniformly distributed but translation is repressed in the P cell and is restricted to the AB cell.
3 At two cell glp 1 protein only 3. At two cell, glp-1 protein only expression in AB cell. After 3’
cleavage produced ABa and ABp also has expressed.
4. After 2nd cleavage, P2 expresses apx- 1 protein on its surface which activates Glp-1 receptor. This causes ABa and ABp descendants to respond
P2 cell → signal → gut development
p p
to signals from the P2 cell differently.
Fig 6.6
5. Other maternal gene skin in EMC cell (higher than AB cell). Mutant skin gene, EMC only produce muscle.
Gut development Depend on induction
Only 4-cell stage EMS did not formed the gut.
Remove P2 cell, did not formed the gut. But P2 + EMS can produced gut Cell-cell interaction play an important role of the development of gut.
Cell differentiation (Fig 6.7 Cell fate is ( g linked to the pattern of cell division) It is closely linked to cell division MS cell: from the sequence p-a-a-p-p →
apoptosis.
For asymetry cell division. Unequal cleavage produced different concentration of some protein → diff t ff t
different effect .
Each division produces an anterior (a) and a posterior (p) cell.
At 80-cell (gastrula stage), a fate map can be made for the nematode embryo (Fig.6.8)
At this stage, cell from different lineages that contribute to the same tissue or organ
v A
a: anterior l:left r:right p:posterior
Experiments Show that Cell-Cell Interactions Are Required for the EMS Cell to Form Intestinal Lineage Determinants
(A) Isolated shortly after its formation, the EMS blastomere can produce gut-specific
l If l f i granules. If left in place for longer periods, it can not (B) If the EMS cell is recombined with either or both derivatives of the AB blastomere it will not blastomere, it will not form gut-specific granules.
(C) If recombined with the P2 blastomere, the EMS cell give rise to gut specific structures
Nematodes: Homeobox genes
No segmentation, but has homeobox gene
A small cluster of homeobox genes specify cell fate along the A/P axis.
Four homeobox genes, similar to the HOM/HOX genes, plus an additional less related homeobox gene make up the C elegans Hox additional less related homeobox gene, make up the C. elegans Hox cluster. Fig 6.9
They are expressed in different positions along the A/P axis.
Expressed during embryogenesis, their primary role is in larval development.
Less effect
Adult nematode: 550 cells, and 4 larval stages.
Critical development: cell lineage and position, gene control at right time and right place.
Temporal information is important in the nematode.
Heterochronic mutants alter the timing of developmental events. Fig 6.10 Gene control graded temporal information in nematode development
(Mutation that alter the timing of developmental events are called heterochronic mutant)
lin-14(from lateral hypodermal T-cell lineage) mutants affect the T.ap lineage(epidermis, neurons and support cells). Fig 6.10
Lin-14 mutant → disturbance in the timing of cell division→ change in the
tt f ll li patterns of cell lineage Gain-of-function (gf) mutations result in retarded development (happens laterthan normal).
Loss-of-function (lf) mutations result in precocious development (happens earlier than normal).
Normal, T cell generates (T.ap; T blast cell) → epidermis, neurons and support cells in first (L1) and second (L2) larval stages;
Later larval stages (L3 and L4), some of T cell descendants divide to other structure.
The concentration of lin-14 protein is post-transcriptionally regulated in an interesting and unusual way. The translation of lin-14 mRNA can be repressed by lin-4 RNA, which complexes with lin-14 mRNA.
Lin-4 mRNA ↑(during the later larval stages) → temporal gradient in lin-14 protein; so mutation of lin-4 → gain of function mutations in lin-14
p g
Gene control graded temporal information in nematode development
Gene control timing of development: by control the concentration of some substance, causing it to decrease with time Fig. 6.11g
Temporal gradient control development Lin-14 mRNA → translate to protein, when
protein to high, inhibited by lin-4 mRNA (formation of complex)
gf: gain-of-function mutation (high level lin-14) go on induction
if: loss-of-function mutation (low level lin- 4) loss inhibition
Ascidians (tunicates)
Cell lineage of Ascidians (tunicates)
It marine animals, tunicates, urochordates.
Ascidian larvae appear to be similar to vertebrate neurulas 神經軸胚(notochord, neural tube and muscles).
Ascidian larva.
Notochord cell
The egg and early embryo are regulative (part) and mosaic, but after a few cleavages, the cells are mostly determined. Fig 6.20
Ascidians embryogenesis is cleavage pattern (mosaic development), cytoplasmic factors specifying cell fate, cell interaction are plays relative part.
Notochord cell are labeled green
Muscle may be specified by localized cytoplasmic factors
Before fertilization, the yellow granules are uniformly distributed through the egg.
The cells that acquire yellow cytoplasm-the myoplasm-during cleavage are those that will give rise to the muscle cells of the larval tial.
Egg → fertilization →yellow pigment granules (YPG) → rearrangement → myoplasm → move to vegetal and lateral by microtuble move (two stage) → form of crescent → posterior end ,gastrulation start The crescent marks the future posterior end.
During cleavage, the myoplasm becomes confined to particular cells for posterior end.
Cytoplasmic Rearrangement in the Fertilized Egg of Styela partita
(A) Before fertilized, the yellow cortical cytoplasm surrounds the gray yolk inner cytoplasm (B) After sperm entry (in vegetal
oocyte)), the yellow cortical
t l d th l
yellow pigment granules (YPG)
cytoplasm and the clear cytoplasm derived from the breakdown of the oocyte nucleus contract vegetally toward the sperm
(C) As the sperm pronucleus migrates animally toward the newly formed egg pronucleus, the yellow and clear cytoplasms move with it
(D) The final positions of the yellow cytoplasm marks the location where cells give rise to tail muscles.
Bilateral Symmetry in the Egg of the Tunicate Styela partita
(A) Uncleavaged egg.
The regions of cytoplasm destined to form particular organs are labeled here and coded
Blastomeres
by color throughout the diagrams. (mosaic development)
(B) 8-cell embryo, showing the blastomeres and the fates of various cells. The embryo can be viewed as two 4-cell halves;
each division on the right side each division on the right side of the embryo has a mirror image division on the left (C,D) views of later embryos from the vegetal pole.
The dashed line show the plane of bilateral symmetry.
Two B4.1 cell contain myoplasm →primary muscle, endoderm The blastomeres adjacent to B4.1, developed secondary muscle cells.
The myoplasm (yellow pigmented in Styela embryos) gives rise to the muscles of the larval tail by apparent mosaic development. It may be regulated or induced by autonomous specification by a morphogentic factor. Morphogentic factor → affect cell → cell cleavage → affect differentiate
This experiments provided the involvement of cytoplasmic determinants in muscle differentiation.
Good correlation between muscle development and yellow cytoplasm, these observations alone do not establish that something in the yellow cytoplasm causes differentiation of the cells into muscles. Only one factor did not induced muscle, must be combination of many signal. It is regulative development
Autonomous specification by a morphogenetic factor. The macho-1 mRNA message is localized to the muscle-forming tunicate cytoplasm. In situ hybridization shows the macho-1 message found first in the vegetal pole cytoplasm, then migrating up the presumptive posterior surface of the egg and becoming localized in the B 4.1 blastomere
Cell-autonomous: If only the mutant cell exhibit the mutant phenotype and are not rescued by the normal cells, the gene is acting cell-autonomously; a gene product not influencing other cells. (only one mutant → affect phenotype;
directly)
Notochord development require induction
A-lineage cells give notochrod, from 32 cell; B-lineage cells give notochord from 110 cells
FGF can induce notochord formation
Notochord induced by vegetal region, between 32 to 110 cell stage.
However, the notochord is induced by vegetal cells and involves the homologue of the Brachyury (T) gene (Regulative development).
It not only i
The lineage of the ascidain notochord
mosaic developm ent for notochord developm ent
Gastrulation in Tunicates
A-C: cross sections and D-F:
scanning electron
micrographs from the vegetal pole.
A,D: the invagination of the ectoderm
B,E: involution of the mesoderm
C,F: epiboly of the ectoderm.
Types of cell movement during gastrulation
Invagination Involution Involution Ingression Delamination
Eiboly: ectoderm covers embryo
Comparison of Normal Tunicate Embryos and Embryos from which Posterior Vegetal Cytoplasm has been Removed
Wild type
Posterior vegetal cytoplasm removed
Vegetal view 76 cell embryo Posterior vegetal cytoplasm removed Can not formed tail and muscle
SLIME MOLDS AND WATER MOLDS
Slime molds(see photograph) and water molds are protistans that superficially resemble fungi. Like fungi, the slime molds and water molds are not photosynthetic.
In addition, many of the slime molds and water molds have bodies formed from thread-like structures called hyphae , which many fungi possess as well. However, several characteristics differentiate slime molds and water molds from fungi, including the fact that fungi have cell walls composed of chitin, while slime molds and water molds do not. Slime molds and water molds play an important role in the recycling of nutrients by digesting decaying organic material. The slime molds and water molds, like the other protistans, have complex life cycles.
Cell lineage of the Cellular Slime Mold ( Dictyostelium discoideum)
Cellular slime molds are eukaryotes that diverged earlier than plant and animals and share properties with each (animals: cell movements during morphogenesis;
plants: cellulose cell walls).
Slime mold alternate between a unicellular and a multicellular phase of life cycle. Fig 6.23
“Di t ” i i ll l b t t t f lti ll l “ l ” ( 100 000 ll ) d i
“Dicty” is unicellular but aggregates to form a multicellular “slug” (~100,000 cells) during famine .The slug forms a fruiting body with a mass of spore cells on top of a stalk.
Aggregation:
chemotactic mechanism Gene (smlA) control
Unicell, ameba-like aggregation size.
Prestalk A cell: green Prestakl O cell: red
dicty
Patterning of the slime mold slug involves cell sorting and positional signaling
In slug, anterior end: prestalk; posterior end: prespore cell
Prestalk cell : migration down through the prespore region through the prespore region
→ push the prespore region upward
Possible mechanism:
1. Some information → positional information →form morphogen gradient→ the slug relative position is specified
2. Differentiation of cells at random → sort out to give the normal pattern
Patterning of the slime mold slug involves cell sorting and positional signaling
Patterning from a diversity of cell types. Which expressed different gene (ecmA and ecmB) and code for extracellular matrix protein
Prestalk AB expressed ecmA and ecmB and arranged in a funnel-shaped region near the tip of slug.
Pst B only expressed ecmB, it near prestalk and prespore, and move downward at the culmination stage.
Pst O/ALC is anterior-like cell (expressed ecm A). Pst O/ACL become pst O cell then pst A cell
The arrangement of cell types seems to be due to some form of cell sorting (, may not positional signaling.
Chemical signals direct cell differentiation in the slime mold
Extracellular cyclic AMP can induced prespore cell differentiation.
Chemotaxis
Developing cell release → differentiation inducing factor (DIF) → increase cytoplasmic enzyme↑ negative feedback destroy DIF
cytoplasmic enzyme↑ → negative feedback → destroy DIF DIF is graded, posterior region is highest concentration.
Nucleosome and Chromatin Structure
Nucleosome and Chromatin Structure Where gene regulation takes place
• Opening of chromatin
T i ti
• Transcription
• Translation
• Protein stability
• Protein modifications
Transcriptional Regulation
Strongest regulation happens during transcription
Best place to regulate:
No energy wasted making intermediate products
However, slowest response time After a receptor notices a change:
1. Cascade message to nucleus
2. Open chromatin & bind transcription factors
3 R it RNA l d t ib
3. Recruit RNA polymerase and transcribe 4. Splice mRNA and send to cytoplasm 5. Translate into protein
Transcription Factors Binding to DNA
Transcription regulation:
Certain transcription factors bind DNA
Binding recognizes DNA substrings:
Regulatory motifs Regulatory motifs
Promoter and Enhancers
• Promoter necessary to start transcription
• Enhancers can affect transcription from afar
Regulation of Genes
Transcription Factor (Protein)
RNA polymerase (Protein)
Regulatory Element Gene DNA
Regulation of Genes
RNA polymerase Transcription Factor
(Protein)
Regulatory Element Gene DNA
Regulation of Genes
New protein
RNA polymerase Transcription Factor
p
Gene Regulatory Element
DNA
Example: A Human heat shock protein
TATA SP1
CCAAT AP2
AP2 HSE CCAAT SP1
--158 0
GENE
• TATA box: positioning transcription start
• TATA, CCAAT: constitutive transcription
promoter of heat shock hsp70
• GRE: glucocorticoid response
• MRE: metal response
• HSE: heat shock element
The Cell as a Regulatory Network
A B C Make D
If C then D
gene D
A B C Make D
If B then NOT D If A and B then D
D
• Genes = wires
• Motifs = gates
D Make B
If D then B
C
gene B
The Cell as a Regulatory Network (2)
Cell lineage of Molluscs (snails , oysters)
A spiral pattern of cleavage is seen in some molluscs.
The animal cells are displaced in a spiral arrangement. (clockwise
= dextral; counterclockwise = sinistral) Two common features of mollusc development are:
first and second unequal cleavages give 3 small cells and 1 big cell (the D blastomere becomes posterior-dorsal embryo).
By the 64 to 128 cell stage, cell fates can be mapped.
Handedness is maternally specified and dextral spiral is genetically dominant to sinistral .
The body axes are defined by early cleavages and cytoplasmic The body axes are defined by early cleavages and cytoplasmic
determinants are clearly important.
Spiral cleavage of the mollusc
Cell lineage of Annelids (earthworms, leeches)
• Early development of the Annelids is similar to the Molluscs (spiral cleavage).
• The teloplasm is segregated into the D blastomere which divides into the teloblasts, the source of mesodermal and ectodermal segmented structures.
• Centrifugation experiments alter the distribution of the teloplasm.
• Two sets of 5 teloblasts, one on each side of the embryo, behave as stem cells and undergo repeated asymmetric cell divisions with one cell maintaining the parental identity.
• This generates a string of daughter blast cells, the first-formed giving rise to the most anterior segments.
Sea urchins and starfish
Good model for developmental study: transparency and easy handling Sea urchin is simple regulative development model.
Sea urchin 1-3 cleavage is symmetric, after 4 cleavage is assymmetric, it produced four small micromeres at one pole of the egg (the vegetal).
1 2 l di id l i l l i 3 l i i l d
Echinoderm
1-2 cleavages divide along animal-vegetal axis. 3 cleavage is equatorial and divides the embryo into animal and vegetal halves.
During the 4th cleavage, 4 small animal cells divide equally but the 4 large vegetal cells undergo asymmetric division to generate 4 vegetal micromeres and 4 macromeres which multiply into 1000 ciliated cells enclosing the blastocoel.
Gastrulation starts with entry of the primary mesenchyme about 40 cells
( d ) hi h l d k l t l d f ll d b th d d d
(mesoderm) which lays down skeletal rods, followed by the endoderm and secondary mesenchyme which together stretch across the blastocoel to form the digestive tract.
Maternal factors specify the animal/vegetal axis and likely specify the vegetal organizer in the micromeres.
Sea urchins have a large capacity to regulate along the A/V axis with signals similar to that of the frog D/V axis.
The development of sea urchin embryo.
Gastrulation begins at the vegtal pole, with the entry of about 40 primary mesenchymy cells into the interior of the blastula.
Invertebrate: Sea Urchin
Radial holoblastic cleavage (isolecithal)
The 4thcleavage, very different from the first three. In animal pole, four cell divide to 8 blastomeres and with the same volume (the 8 cells also called mesomeres). In vegetal pole, undergoes an unequal cleavage to four large cells (macromeres) and four small cells (micromeres)
The animal mesomeres divide equatorially to produced two tiers: an1 and an2.
The vegetal macromeres divide a small cluster beneath the large tier.
cells (macromeres) and four small cells (micromeres).
128 cells blastula.
Meridionally
Sea Urchin: blastula formation
The blastula stage of sea urchin development begins at the 128 cells.
Blastulation: The cells form a hollow sphere surrounding a central cavity (blastocoel). Every cell contact with proteinaceous fluid of the bastoceol (inside) and with the hyaline layer on the outside.
About 9thor 10thcleavage, cells become specified and they end develop cilia.
Ciliated blastula → rotate within fertilization envelop (E→F) → vegetal pole of Ciliated blastula → rotate within fertilization envelop (E→F) → vegetal pole of Bastula become thicken
(forming vegetal plate) → then animal pole synthesis and secret hatching enzyme → digest fertilization envelope → embryo is a free swimming embryo is a free swimming hatched blastula.
rotate
Fate maps and the determination of sea urchin blastomeres
Fate map and cell lineage of the sea urchin.
Fate map of the zygote
Late blastula with ciliary tuft and flattened vegetal plate
blastula
Pluteus larva Prism-stage larva
Formation of syncytial cables by primary mesenchyme cells of sea urchin
SEM of spicules formed by the fusing of primary mesenchyme cells into syncytial y y cables
C: SEM of primary mesenchyme cells enmeshed in the extracellular matrix of early gastrula
gastrula.
D: Gastrula-stage mesenchyme cell migration The extracellular matrix fibrils of the bastocoel lie parallel to the animal- vegetal axix
Ingression of primary mesenchyme cells
Invagination of the vegetal plate
SEM of external surface
f th l t l
CSPG release → into inner lamina → osmotic gradient ↑→ absorb water → swell inner lamina ,but outer lamina attached does not swell → inward
of the early gastrula inward
Entire sequence of gastrulation in sea urchin
The sea urchin egg is polarized along the animal-vegetal axis Defined the animal-vegetal polarity in the ovary.
Pigment in vegetal region,
Animal-vegetal axis is stable, it develop to different region.
Cytoplasmic difference along the animal-vegetal axis
Important development signals are produced by micromeres in the vegetal i
region
Fig 6.13 Development of isolated sea urchin blastomers
The D/V axis is related to the plane of the first cleavage
D/V axis, mouth is ventral side Not identified in the egg It is labile up to Not identified in the egg. It is labile up to
as late as the 16 cell stage. Not at the first cleavage.
The D/V axis lies 45oclockwise from the first cleavage plane as viewed from the animal pole (Fig 6.14)
Cleavage clockwise is related with cytoskeleton.
The fate map is finely specificed by regulation
At 60cell, 4 region (different color) along animal-vegetal axis
Large and small micromeres: to mesoderm (formed the primary mesenchyme → skeleton) mesenchyme → skeleton) Micromeres is maternally determined.
Vegetal plate (Veg 1 and 2) : to endoderm, mesoderm (secondary mesenchyme → muscle, connective tissue) and some ectoderm.
Mesomeres (ectoderm): to anterior and
t i i
posterior region
Sea urchin embryo is regulative development, not cleavage pattern.
Half embryo still developed a adult.
Fig 6.16
Fig 6.16 Regulation in sea urchin development
The vegetal region acts as an organizer
Experiment proof: Vegetal region play an important role of organization
New micromeres Micromeres
↓ Signal
↓ Veg 2
micromeres can induced a new gastrulation Micromeres can induced
animal half to form a gut, and ectoderm correctly
Maternal gene gradient may regulated the D/V axis development
↓ Skeleton-forming
↓ organizer
Ability of the Micromeres to Induce a Secondary Axis in Sea Urchin Embryos
Ability of the Micromeres to Induce Presumptive Ectodermal Cells to Acquire Other Fates
Different regulatory region control sea urchin develop
Drosophila: even-skipped contains many binding sites for transcription factor, these site are encode regulatory modules
In sec urchin: endo-16 as modular; In blastula, vegetal region (presumptive endoderm) →secreted glycoprotein endo-16 (unknow function)…
Endo-16 control by 2200 base pair (at least 30 target sites, 13 different
t i ti bi d)
transcription can bind).
Endo-16 regulatory region likely pair-rule gene in Drosophlia.
Modular organization of the endo-16 gene regulatory region. A-G are modular sburegions.
