Developmental Biology

Development

formation of a complex multicellular organism from a single cell

Model Organisms

animals that are used to study many biological mechanisms; observations can be used to understand other species

Key Model Organism Traits

easy to obtain, raise, and maintain in the lab; small with short generation time; easy to work with in experiments; relation to other species

What are some model organims?

flies, mice, zebra fish, C. elegans, sea urchins, arabidopsis

What area from the evolutionary tree are model organisms usually from?

bilateria (vertebrates)

Are model organisms usually evolutionary close to humans?

yes

Where does development start?

2 gametes (N) come together to form a single fertilized egg (2N) and grow via mitosis to become an embryo

Animal Embryo

may be protected in egg or mother or exist freely in environment

Land Plants Embryo

sporophyte is always protected; no gametophyte

When is the embryo stage complete?

when most or all organ systems are formed

What do heterotrophs prioritize in embryonic development?

organization- interior (gut) vs. exterior (skin) & body axes (back/belly, head/tail, left/right)

What is the land plant embryo protected in?

seed with energy source & building materials

What do land plants prioritize in embryonic development?

high surface area with cotyledons

What are cotyledons?

first leaves on a plant

What are the key similarities in early development?

polarity, cleavage, migration

Polarity

establish axes of symmetry

When does polarity occur in plants?

two-cell stage

When does polarity occur in animals?

before or after fertilization

Cleavage

growth via cell division throughout embryo development

Migration

movement of cells

Does migration occur in both animals and plants?

no- plant cells are held in place by cell wall

Germ Cells

give rise to gametes in animal cells; set-aside very early

Somatic Cells

all cells of the body except those for reproduction; genetically identical

Zygote

single diploid cell formed immediately after fertilization

Syngamy

fusion step of broader term, fertilization

embryo

early developmental stage after fertilization but before it fully develops into juvenile or fetus

What do somatic cells differentiate into?

3 germ layers

Ectoderm

outermost germ layer

Ectoderm gives rise to

skin and nervous system

Mesoderm

middle germ layer

Mesoderm gives rise to

muscles, circulatory systems, excretory systems, bones

Endoderm

innermost germ layer

Endoderm gives rise to

lining of the gut and derivatives

Gastrulation

radical cell movements reposition cells in the three germ layers to generate a multi-layered organism

Organogenesis

rearrangement of tissues to form distinct organs and organ systems

In vertebrates, organogenesis begins with

neurulation and somitogenesis

Neurulation

formation of the neural tube (spinal cord and brain)

Somitogenesis

formation of protective vertebrae and muscle blocks

What are the mechanisms of embryonic development?

differential gene expression, cellular signaling, cell fate and differentiation, patterning

Differential gene expression

activation of different sub-sets of genes in different cells that is necessary for cell specialization

Cellular signaliing

communication between cells that is also necessary for cell specialization

Cell fate and differentiation

generation of cell specialization and phenotype

patterning

establish specialized body regions and structures

What mechanisms of embryonic development are cellular?

differential gene expression & cellular signaling

What mechanisms of embryonic development are tissue/full organism?

cell fate and differentiation & patterning

What lineages all have blastula?

animal

When do germ cells develop?

before 3 germ layers differentiate

When a concussion damages neurons, what are the signals?

inflammation & repair

Tissue specialization

same differentiation of cells that work together

Gastrulation is in

animals

Differential Gene Expression

expression of different sub-sets of genes that result in specialized cells that are genetically identical but functionally and morphologically different

Cellular Signaling

signals sent between cells and those receiving cells respond by activation or inhibition of different sub-sets of genes

Cell Fate and Differentiation

differential gene expression and cell communication provide positional information to cells across the embryo

Patterning

differential expression of homeotic genes establishes specialized body regions and location of structures

Central Dogma

DNA → RNA → Protein (aka genotype → phenotype)

Transcription Factors

external signals trigger changes inside cell to release these factors that regulate the expression of genes

How do transcription factors regulate gene expression?

activate or inhibit

Translation

converts RNA information into Amino Acid chain (protein)

Transcription

copies information from DNA to RNA

Expression of specialized proteins can lead to

polarity, cell determination, additional transcription factors and/or signals to other cells

Morphogens

chemical signals

Induction

secretion of morphogens by a group of cells to nearby target cells to trigger a cascade of changes in cell

What can induction cause?

differential gene expression

What determines how an inductive signal will be interpreted by the responder cell?

concentration of morphogens & competent to respond

Concentration of inducer cell

graded target cell response depends on how much morphogens reaches

Competent to Respond

number and type of inducer receptors have to match morphogens

Animal Polarity/Axis Determination

all of the above

Land Plant Polarity/Axis Determination

apical-based

Cytoplasmic Segregation

regional localization of maternally-derived materials in the egg as it is maturing leads to specific orientation of the cell division planes → uneven division of these materials between daughter cells

When does cytoplasmic segregation occurs?

before fertilization

Mechanisms to generate positional information

cytoplasmic segregation & cell communication

Cell Communication with Polarity/Axis determination

morphogens can form concentration gradient across cells that will have different repsonses based on concentration

Cell differentiation

when cells achieve different identities

Germ Layer differentiation

after polarity is established, differential gene expression is used to establish germ layers

Determination

process that sets a cell’s overall fate

Differentiation

the process of becoming a specialized cell

Determination Example

gene is activated → gene transcription factor

Differentiation Example

transcription factor interacts with many genes → signal cascade

Animal Repair

stem cells migrate to damaged area then differentiate into appropriate cell types

Plant Repair

cells de-differentiate into meristem cells and then re-differentiate to produce needed structures

Homeotic Genes

master control genes that initiate a cascade of gene expression and affect the timing and amount of synthesis of proteins encoded by large number of genes

What happens if homeotic genes malfunction?

all of the above

What do homeotic genes include?

hox genes & organ identity genes

Hox Genes have

collinearity & highly conserved

Collinearity

order of hox genes on a chromosome is the same as the position in the body when (and where) they will be activated

Highly conserved

last common ancestor (insect/chordate 670 mya) had hox proteins; distantly related species can substitute hox proteins for each other

Evolutionary Developmental Biology

understanding how changes in developmental mechanisms can create alterations in body plans over evolutionary time

Eukaryotic evolution involves building different ______ to create novel body plans during embryo development from the same _____ and using the same ____ and similar ______.

structures; materials; genes; regulators

Can changes that regulate the expression patterns of the SAME genes generate different phenotypes?

yes

Heterotopy

“where”- differences in the location of gene expression; hox gene limits/divides expression (tells what region/position the genes are in and their function)

Heterotopy example

bird neck contain more vertebrae than mammals necks because Hox gene is expressed in different sections

Heterochrony

“when”- differences in the timing of expression

Heterochrony example

giraffes, maturity (timing of signals to cease bone growth) is delayed so bones grow longer compared to other mammals since 7 cervical vertebrae in mammals

Heterometry

“how much”- differences in the level of gene expression; amount of expression

Heterometry example

beak depth and length are controlled by the relative expression of two genes; more beak depth/width expression, more beak depth/width expression

Whorls

circular arrangement of plant organs that emerge from same level

Organ Identity Gene

controlled by transcription factors (2 polypeptides that come together to form dimers) and start a cascade of differential expression to cause different phenotypes