Chapter 21- Development and Gene Expression

Central questions:

1) How are cells in various locations of the body different -is the variation in their genome or in the proteins they express?

2) Can differentiated cells be coaxed to retrace their steps and become de-differentiated?

3) How do cells become different from one another to form different body parts when they all develop from ONE cell - the zygote?

4) Are there difference or similarities in the way genes behave during development between various species?

Figure 21.1 From early embryo to tadpole: what a difference a week makes

Types of stem cells:

Embryonic totipotent

Embryonic pluripotent

Adult stem cell (misnomer)

Embryonic development—single-celled zygotes

2 cell stage

4 cell stage

Blastula (hollow ball with cells on the outside). Humans - this is called blastocyst

A cell upto the 8 cell stage is an embryonic stem cell - it is able to differentiate into a full organism - totipotent!

A cell from the blastula/blastocyst is also an embryonic stem cell - it is able to differentiate into many types of cells but not a full organism - pluripotent!

Embryonic blastocyst stem cells in animals are pluripotent because epigenetic modifications are minimal- that is = DNA methylation/histone acetylation (turning off its genes) is minimal

Embryonic development—Blastula

Gastrula(Ectoderm, Mesoderm, Endoderm)

Tissues

Organs

Organ Systems

Adults/babies (humans ) have adult stem cells - in special places like the bone marrow which are also pluripotent but to a lesser degree than embryonic stem cells!

Once the cell is committed to its fate during gastrula formation, they become differentiated - they lose their pluripotency (genes turned off)!

Stem Cells In The Human Adult

Bone Marrow Cells – make blood cells all through lifeBrain Stem Cells – can make neurons and glial cellsSkin stem cells – keratinocytes, hair follicles, epidermisAre human stem cells PLURIPOTENT? (-can differentiate into multiple cell types)Yes, but to a limited extent

Stem Cells - another property

Stem Cells have telomerase (immortal) - capable of self-renewal

Cell division (mitosis) of the zygote increases the number of cells in an organism.

Differentiation is when each cell becomes specialized in both structure and function (genes turned off by epigenetic processes).

Morphogenesis is when the eventual shape (body plan) of the organism forms - head-tail axes; top-down axes….

Plants Animals

N/A Movement of cells and tissue needed to transform embryo

Continuous differentiation, and morphogenesis throughout life

Differentiation only during embryonic development and in some adult stem cells like bone marrow cells

Any cell in a plant can be a stem cell at any stage - ‘totipotent’. Meristems - regions of growth and differentiation.

Totipotent - only upto 8 cell embryonic cell stage; Pluripotent - blastocyst, bone marrow adult stem cells (can only make certain type of cells like blood cells)

If pluripotent - this means some genes are still off (epigenetics)!

Carrot cells are totipotent.

In plants, cells remain totipotent—all genes can be activated, and any

cellcan form any part of the organism

Review Questions: Are there differences in gene number or type between undifferentiated (stem) cells and differentiated (mature/adult) cells?

Are there stem cells in an adult human body? Where?

Can adult differentiated cells be induced to make any (and all) types of body cells - retrace and become ‘embryo like’?

NO! And Yes! And Yes!

Are there differences in genes between different cells in the body?

No = Genomic EquivalenceAre there differences in genes expression

between undifferentiated (stem) cells and differentiated (mature/adult) cells?

Yes. Genes get inactivated/activated through processes like methylation (epigenetics)

When cells differentiate, are genesinactivated irreversibly? Can they retrace to become de-differentiated?

Well, it depends on the organism! Dec 2007 - scientists de-differentiated the human skin cells by using transcription factors!

Can adult skin cells retrace steps to become de-differentiated?

Somatic Cell Nuclear transfer (SCNT)

1) Take egg cell and remove nucleus (haploid) - throw it away

2) Take skin cell and remove nucleus (diploid) - save this

3) Insert skin cell nucleus (somatic cell) into egg cell cytoplasm. No need for sperm! Why?

4) Allow egg cell to divide and become blastocyst

5) Now you can extract stem cells (THERAPEUTIC CLONING) OR carry out REPRODUCTIVE CLONING - implant blastocyst in a surrogate mom and grow a clone!

Dolly and Bonny!

Reproductive Cloning is an offshoot of stem cell research

SCNT made Dolly the sheep!-Mammary glandcells from donor arrested in G0

phase apparently“dedifferentiated.”

Review: Dolly’s mitochondrialDNA is from theegg donor sheep.

Remember in cloning - an egg nucleus is replaced with a nucleus of a differentiated cell. Ability of differentiated nucleus to support normal development is related to its age - Dolly may have died prematurely and developed arthritis at a young age! (epigenetics controls this)

Therefore… in animals - review

Nuclei change as cells differentiate

The DNA sequence usually doesn’t change, but chromatin structure may be altered

Nuclear “potency” is restricted as cells develop and become more differentiated.

How does a stem cell make a differentiated cell?Determination & differentiation of muscle cells

Master control gene => codes for transcription factors => turned on (determination) => transcription factors => turs on other genes=> more transcription factors => muscle protein genes turned on => muscle protein (myosin) made => cell has differentiated (These are INTERNAL SIGNALS)

Ques. 3) How do cells become different from one another to form different body parts when they all develop from ONE cell - the zygote?

What turns on the Master control gene?

Figure 21.9 Determination and differentiation of muscle cells (Layer 1)

Figure 21.9 Determination and differentiation of muscle cells (Layer 2)

Figure 21.9 Determination and differentiation of muscle cells (Layer 3)

Model organisms for development studies—

observable embryos

short generation times

relatively small genomes

knowledge about the organism and its genes

* Drosophila, C. elegans, mouse, zebrafish, Arabidopsis

What tells a cell (and triggers the master gene) what its fate will be?

Cytoplasmic determinants (from the mom - internal signals in egg)

Induction—signal molecules from cells nearby (neighbors)

1) Cytoplasmic determinantsinclude mRNA, proteins,chemicals, and organelles and how they are distributed in the egg. They are distributed unevenly - and this can set up gradients that says ‘head’ side, ‘tail’ side , etc.

Cytoplasmic determinants are coded for by maternal effect genes (or egg-polarity genes)

Example: --Bicoid mRNA is present at the anterior end of the egg

-Bicoid protein is essential for head formation.

Background on Drosophila:Cytokinesis does not occur inthe early Drosophila embryo.

Nuclei migrate to the peripheryin the blastula.

Bicoid is a morphogen—a substance that establishes an organism’s axis or other 3Dfeatures.

Bicoid helps create the anterior/posterioraxis.

It’s a transcription factor that activates expression of segmentation genes

3 types of segmentation genes:

Gap genes map out basic subdivisions along anterior/posterior axis

Pair-rule genes define smaller regions

Segment-polarity genes determine the anterior/posterior axis of each specific segment.

2) Induction = signals impinging on an embryonic cell from other nearby embryonic cells. These can be transcription factors - remember cell communication?

Our friend Drosophila !

Has 3 parts—head, thorax, and abdomenhas an anterior/posterior axis and a dorsal/ventral axis

Cytoplasmic determinants and induction together lead to PATTERN FORMATION

Anterior Posterior

Dorsal

Ventral

Pattern formation is the development of thespatial organization of an organism.

Molecular clues (positional information) tell cells

where they’ll be located in the body

who their neighbors will be

how to respond to other molecular signals

Cell Lineage of all 959 C. elegans cells!

Homeotic genes determine the segment on which appendages or other structures will formThe expression of these genes is activated byTranscription factors coded by segmentation genes.

All homeotic genes contain a homeobox domain.

Homeobox domains have been found in many other animals besides flies, and most geneswith a homeobox are related to development.

The homeobox domain is actually a DNA-binding domain!So proteins containing it are likely to betranscription factors!!

Flies and micehave homologousgenes coding forproteins involvedin development.

Induction is when cells signal other cells tochange in a specific way—mostly activatingor inactivating transcription.

Induction has been studied most in the nematode, C. elegans.

Vulva precursor cellscan develop into 3different types of cells.

Signals from theanchor cell induce thedetermination of eachcell.

Effects of inducerscan vary dependingon concentration.

Apoptosis is programmed cell death—occurs at various stages of development. Suicide proteins are activated - cell blebs (becomes multilobed), nucleus condenses, and then slowly degrades due to nucleases and proteases….how painful! It is then eaten by neighboring cells.

Apoptosis is programmed cell death—occursat various stages of development ex: ‘web retraction’ between digits/fingers (textbook activity)

MUTANT MICE GALLERY!

In the name of Science…..Are you ready for the gore?

Figure 21.x2a Laboratory mice: brachyury mutant

Figure 21.x2b Laboratory mice: eye-bleb mutant

Figure 21.x2c Laboratory mice: Hfh11 mutant

Figure 21.x2d Laboratory mice: Lama2 mutant

Figure 21.x2e Laboratory mice: Lepr mutant

Figure 21.x2f1 Laboratory mice: Mgf mutant

Figure 21.x2f2 Laboratory mice: Pax3 mutant

Figure 21.x2g Laboratory mice: Otc mutant

Figure 21.x2h Laboratory mice: Pax6 mutant

Figure 21.x2i Laboratory mice: Pit1 mutant

Figure 21.x2j Laboratory mice: pudgy mutant

Figure 21.x2k Laboratory mice: ruby-eye mutant

Figure 21.x2l Laboratory mice: stargazer mutant

Figure 21.x2m1 Laboratory mice: ulnaless mutant

Figure 21.x3 Nude mouse

Figure 21.x4 Normal and double winged Drosophila