Bio 2 Slide Set 5

Front 1

Mechanisms Controlling Morphogenic Development:

Back 1

1) Cell Movement/Rearrangement
2) Cytoplasmic Determinants
3) Cell Communication
4) Pattern Formation

Cells are guided through development via molecular signals that induce changes

Front 2

Cell Movement/Rearrangement

Back 2

Development requires cells to divide (mitosis), move, change shape, and differentiate in a coordinated fashion

Cells move, rearrange, and change shape during gastrulation and neurulation

1) Gastrulation
2) Changes in Cell Shape (Neurulation)

Front 3

Mechanisms of Cell Movement/Rearrangement in Gastrulation:

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1) Convergent Extension
2) Other Movements (Invagination, ingression, involution)

Front 4

Convergent Extension

Back 4

a process in which the cells of a tissue layer rearrange themselves in such a way that the sheet of cells becomes narrower (converges) and longer (extends)

Cells crawl in-between each other (converge) then the mass elongates (extends)

Ex: Elongation of the archenteron

Front 5

Invagination

Back 5

an indentation of cells (creating a "dimple")

Front 6

Ingression

Back 6

Cells detach and migrate freeely inward

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Involution

Back 7

layer of cells rolls inward

Front 8

Changes in Cell Shape

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Ex: Neurulation

Neural Tube Formation:

Extension of microtubules elongates cells
Contraction of microfilaments causes cells to become wedge shaped

Front 9

Cytoplasmic Determinants (CDs)

Back 9

Molecular "signals" like maternal CDs in the egg later regulate gene expression and influence early development in descendent cells

If CDs are distributed asymmetrically, cells receiving different CDs will have different "instructions" (signals) and different embryonic fates

signals --> induce changes --> cell development

Front 10

Three Body Axes of Symmetry (of frog):

-- determined by signals

Back 10

Established relative to poles of the zygote and first cleavage

determined by signals

1) Anterior-Posterior Axis
2) Ventral-Dorsal Axis
3) Left-Right Axis

Front 11

Anterior-Posterior Axis

Back 11

Animal Pole = "Anterior" end of from embryo

Vegetal Pole = "Posterior"

Front 12

Ventral-Dorsal Axis

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Ventral = Front
Dorsal = Back

determined by the point of sperm entry

Front 13

Gray Crescent

Back 13

marks the future dorsal side

exposed non-pigmented cytoplasm

Where invagination will occur to begin gastrulation, forming the dorsal lip of the blastopore
-- This determines the dorsal-ventral axis of the embryo

Front 14

Cortex

Back 14

the cytoplasm just under the CM (?)

this shifts (= cortical rotation) after fertilization and leaves behind a "gray crescent" region of cytoplasm

Front 15

How do Axes of Symmetry in the Frog Arise via Cytoplasmic Determinants?

Back 15

1) CDs in gray-crescent cells convey positional (dorsal) information for the embryo and induce differentiation

2) Cortical Rotation (at fertilization) --> gray crescent --> site of invagination --> gastrulation starts --> dorsal lip of blastopore forms --> dorsal-ventral axis

3) Maternal CDs in the unfertilized egg affect the "Fate" of future cells: the distribution of a CD called Bicoid protein determines the anterior-posterior axis in the fruit fly

Front 16

Cortical Rotation

Back 16

At fertilization, the egg surface (the plasma membrane and associated cortex) rotates with respect to the inner cytoplasm

This rotation is always TOWARD the point of sperm entry

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Morphogen

Back 17

a substance that provides positional information in the form of a concentration gradient

Ex: Bicoid protein

Front 18

Explain how Bicoid protein provides positional information in the form of a concentration gradient.

Back 18

Maternal CDs in the unfertilized egg affect the "fate"of future cells: the distribution of bicoid protein (a CD) determines the anterior-posterior axis in the fruit fly, based on the concentration gradient

Bicoid gene --> bicoid mRNA is transcribed --> concentrated at one end of unfertilized egg --> translated into bicoid protein (AFTER fertilization)

High bicoid protein Concentration signals "Anterior Development" via selective gene expression

A morphogen that acts as a transcription factor
-- Where bicoid level is high, it activates genes to transcribe other TFs and induces a cascade of gene expression at the anterior end of the embryo, guiding the development of anterior structures

Front 19

Cell Communication

Back 19

Cell-to-cell communication (via cell signaling) is necessary to coordinate cell movement and cell differentiation during development

Front 20

Evidence for Cell Communication:

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Holtfretter's Study of Frog Late Gastrula Embryo

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Holtfretter:

Back 21

An experimental embryologist

1955 Study of Frog Late Gastrula Embryo

a) Dissociation of cells (from the 3 gastrula germ layers)
b) Reaggregation of Cells
c) Resegregation of distinct germ layers

Front 22

Holtfretter's Interpretation:

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Embryonic Cells "Communicate"

-Cells somehow recognize other cells
-Cells selectively adhere to some cells and not others
-This Coordinates cell movement and placement that eventually lays out a body plan

Front 23

Cell-Cell Recognition and Adhesion:

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Achieved by the binding of cell membrane proteins
-this can coordinate cell movement during development

Ex: Cell Adhesion Molecules (CAMs)

Front 24

Cell Adhesion Molecules (CAMs)

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proteins located on the cell surface and involved with binding to other cells through cell adhesion

Promote interaction between pairs of cells

Ex: Cadherins

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Functions of CAMs:

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allow cells to adhere or bind to each other, or not

Ex: CAMs hold skin cells together

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Cadherins:

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A class of CAMs that require calcium

-They are Glycoproteins
-Display homophilic bindin ("like binds to like"

Ex: the role of Cadherins in Neural Tube Formation

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The Role of Cadherins in Neural Tube Formation:

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E-Cadherin is expressed (so other cells "bind" or adhere to each other)

E-Cadherin expression decreases (cell adhesion weakens allowing neural folds to uplift)

N-cadherin expression increases (green neural tube cells lose adhesion for overlying surface ectodermal cells)

Some neural crest cells Stop Expressing any cadherin: these lose adhesion and can freely migrate (completely detach from neighboring cells)

End Result: Neural tube is completely separate from overlying ectoderm (epidermis)

Front 28

Induction:

Back 28

* A very important mechanism in development
- A mechanism of cell communication

The ability of one GROUP of embryonic cells (an "organizer" region) to influence the development of other cells via SIGNAL molecules ("inducers") that alter gene expression

Ex: The Amphibian Dorsal Lip "Organizer" of Spemann and Mangold

Front 29

Embryonic Organizer

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a group of embryonic cells to influence the development of other cells via signal molecules (inducers) that alter gene expression

Front 30

Spemann-Mangold's Amphibian Dorsal Lip "Organizer"

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Experiment:
-Some cells from dorsal lip (fated to become neural ectoderm) of 1 gastrula are transplanted to the ventral side of a 2nd gastrula

The transplanted dorsal lip cells induced nearby ventral cells on the 2nd embryo to develop uncharacteristically into dorsal features (Ex: notochord and neural tube where they don't belong)

Dorsal Lip Organizer cells INHIBIT the effect of BMP-4, which induces ectoderm to develop into epidermis (skin)

Once inhibited, a 2nd neural plate can develop

Front 31

BMP-4

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a signal molecule that induces ectoderm to develop into epidermis (skin)

Front 32

Pattern Formation

Back 32

the development of spatial organization in an embryo

The development of spatial organization in an embryo, the arrangement of organs and tissues in their characteristic places in 3D space

Can be achieved by induction using morphogens and organizers

Ex: bicoid factor in flies
Ex: vertebrate limb formation

Front 33

Vertebrate Limb Formation

(an example of pattern formation)

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AER secretes FGF protein signals --> promotes limb-bud outgrowth

ZPA cells emit signals that INDUCE "posterior" limb development in adjacent cells

Front 34

2 Limb Organizer Regions:

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ZPA and AER

each sets up a morphogen gradient to provide positional information to developing cells in the limb

Front 35

Zone of Polarizing Activity (ZPA)

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A block of mesodermal tissue located underneath the ectoderm where the posterior side of the bud is attached to the body

sets up anterior-posterior axis of limb

creates a morphogen called "sonic hedgehod" (SHH) that induces limb level

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Apical Epidermal Ridge (AER)

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A thickened area of ectoderm at the tip of the bud

sets up proximal-distal axis of limb

secretes protein signals (FGF)

Front 37

Sonic Hedgehog (Shh)

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a morphogen secreted by ZPA cells

induces limb development

Ex: Low Shh induces thumb development
HIgh Shh induces little finger development