1. Chapter 21- Development and Gene Expression
Central questions:
How are cells in various locations of the body different -is the variation in their genome or in the proteins they express?
Can differentiated cells be coaxed to retrace their steps and become de-differentiated?
How do cells become different from one another to form different body parts when they all develop from ONE cell - the zygote?
Are there difference or similarities in the way genes behave during development between various species?
An organism arises from a fertilized egg cell as the result of three interrelated processes: cell division, cell differentiation, and morphogenesis.
From zygote to hatching tadpole takes just one week.

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

3. Types of stem cells:
Embryonic totipotent
Embryonic pluripotent
Adult stem cell (misnomer)

4. 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 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!
A cell upto the 8 cell stage is an embryonic stem cell - it is able to differentiate into a full organism - totipotent!
The red receptors on the sperm come into contact with the egg jelly, yellow. This induces the acrosome reaction causing the acrosome in green to fuse with the plasma membrane of the sperm. The actin in pink goes from a globular state to a filamentous state pushing the front of the sperm outward exposing the binding receptors, blue. The binding receptors can now bind with the egg
Sperm undergoes the acrosome reaction and binds with the egg. Fusion occurs, actin polymerizes in the egg, the cortical granules fuse with plasma membrane and release their contents. This causes the fertilization membrane to rise. The sperm is pulled into the egg with help from the fertilization cone. Microtubules started at the male centrosome help push the sperm to the center of the egg to fuse with the female pro-nucleus.
In normal development the sperm fertilizes the egg. The cortical reaction occurs raising the fertilization membrane and cell divisions occur until the blastula stage. When the embryo reaches the blastula stage the embryo releases an enzyme that dissolves the fertilization membrane and the young embryo swims free to continue development.
In insects the egg is too large to allow for the division of the cytoplasm at first. This will occur later as the nuclei get closer to the surface of the egg. For clarity, most of the mitotic cycle has been left out. (microtubules, etc.)
Insects are protostomes as opposed to deuterostomes (sea urchins and humans) and exhibit other characteristics not seen here.
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

6. Embryonic development— Blastula Gastrula
(Ectoderm, Mesoderm, Endoderm)
Tissues
Organs
Organ Systems
Once the cell is committed to its fate during gastrula formation, they become differentiated - they lose their pluripotency (genes turned off)!
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!
At the forth division the micromeres form The swimming blastula "hatches" Gastrulation forms the mouth from the anus Prism stage. Embryo starts to feed Early Pluteus, about 3 days from start
Late Pluteus, about one week Metamorphosis happens at several months Producing the baby sea urchin

8. Stem Cells In The Human Adult
Bone Marrow Cells – make blood cells all through life
Brain Stem Cells – can make neurons and glial cells
Skin stem cells – keratinocytes, hair follicles, epidermis
Are human stem cells PLURIPOTENT? (-can differentiate into multiple cell types)
Yes, but to a limited extent
Hematopoietic stem cells give rise to all the types of blood cells: red blood cells, B lymphocytes, T lymphocytes, natural killer cells, neutrophils, basophils, eosinophils, monocytes, macrophages, and platelets.
Bone marrow stromal cells (mesenchymal stem cells) give rise to a variety of cell types: bone cells (osteocytes), cartilage cells (chondrocytes), fat cells (adipocytes), and other kinds of connective tissue cells such as those in tendons.
neural stem cells in the brain give rise to its three major cell types: nerve cells (neurons) and two categories of non-neuronal cells—astrocytes and oligodendrocytes.
Epithelial stem cells in the lining of the digestive tract occur in deep crypts and give rise to several cell types: absorptive cells, goblet cells, Paneth cells, and enteroendocrine cells.
Skin stem cells occur in the basal layer of the epidermis and at the base of hair follicles. The epidermal stem cells give rise to keratinocytes, which migrate to the surface of the skin and form a protective layer. The follicular stem cells can give rise to both the hair follicle and to the epidermis.
Adult stem cell plasticity and transdifferentiation. A number of experiments have suggested that certain adult stem cell types are pluripotent. This ability to differentiate into multiple cell types is called plasticity or transdifferentiation. The following list offers examples of adult stem cell plasticity that have been reported during the past few years.
Hematopoietic stem cells may differentiate into: three major types of brain cells (neurons, oligodendrocytes, and astrocytes); skeletal muscle cells; cardiac muscle cells; and liver cells.
Bone marrow stromal cells may differentiate into: cardiac muscle cells and skeletal muscle cells.
Brain stem cells may differentiate into: blood cells and skeletal muscle cells.
Current research is aimed at determining the mechanisms that underlie adult stem cell plasticity. If such mechanisms can be identified and controlled, existing stem cells from a healthy tissue might be induced to repopulate and repair a diseased tissue

9. Stem Cells - another property
Stem Cells have telomerase (immortal) - capable of self-renewal
Another hot research areas involves stem cells.
As relatively unspecialized cells, they continually reproduce themselves and under appropriate conditions, they differentiate into specialized cell types.
The adult body has various kinds of stem cells, which replace nonreproducing specialized cells.
For example, stem cells in the bone marrow give rise to all the different kinds of blood cells.
A recent surprising discovery is the presence of stem cells in the brain that continues to produce certain kinds of nerve cells.
Stem cells that can differentiate into multiple cell types are multipotent or, more often, pluripotent.
Scientists are learning to identify and isolate these cells from various tissues, and in some cases, to culture them.
Stem cells from early embryos are somewhat easier to culture than those from adults and can produce differentiated cells of any type.
These embryonic stem cells are “immortal” because of the presence of telomerase that allows these cells to divide indefinitely.
Under the right conditions, cultured stem cells derived from either source can differentiate into specialized cells.
Surprisingly, adults stem cells can sometimes be made to differentiate into a wider range of cell types than they normally do in the animal.
Beyond the study of differentiation, stem cell research has enormous potential in medicine.
The ultimate aim is to supply cells for the repair of damaged or diseased organs.
For example, providing insulin-producing pancreatic cells to diabetics or certain brain cells to individuals with Parkinson’s disease could cure these diseases.
At present, embryonic cells are more promising than adult cells for these applications.
However, because embryonic cells are derived from human embryos, their use raises ethical and political issues.

10. 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….
Early events of morphogenesis lay out the basic body plan very early in embryonic development.
These include establishing the head of the animal embryo or the roots of a plant embryo.
The overall schemes of morphogenesis in animals and plants are very different.
In animals, but not in plants, movements of cells and tissues are necessary to transform the embryo.
In plants, morphogenesis and growth in overall size are not limited to embryonic and juvenile periods. Apical meristems, perpetually embryonic regions in the tips of shoots and roots, are responsible for the plant’s continual growth and formation of new organs, such as leaves and roots.
In animals, ongoing development in adults is restricted to the differentiation of cells, such as blood cells, that must be continually replenished.
The importance of precise regulation of morphogenesis is evident in human disorders that result from morphogenesis gone awry.
For example, cleft palate, in which the upper wall of the mouth cavity fails to close completely, is a defect of morphogenesis.

11. Movement of cells and tissue needed to transform embryo
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)!
Early events of morphogenesis lay out the basic body plan very early in embryonic development.
These include establishing the head of the animal embryo or the roots of a plant embryo.
The overall schemes of morphogenesis in animals and plants are very different.
In animals, but not in plants, movements of cells and tissues are necessary to transform the embryo.
In plants, morphogenesis and growth in overall size are not limited to embryonic and juvenile periods. Apical meristems, perpetually embryonic regions in the tips of shoots and roots, are responsible for the plant’s continual growth and formation of new organs, such as leaves and roots.
In animals, ongoing development in adults is restricted to the differentiation of cells, such as blood cells, that must be continually replenished.
The importance of precise regulation of morphogenesis is evident in human disorders that result from morphogenesis gone awry.
For example, cleft palate, in which the upper wall of the mouth cavity fails to close completely, is a defect of morphogenesis.

13. Carrot cells are totipotent.
One experimental approach to the question of genomic equivalence is to try to generate a whole organism from differentiated cells of a single type.
In many plants, whole new organisms can develop from differentiated somatic cells.
During the 1950s, F.C. Steward and his students found that differentiated root cells removed from the root could grow into normal adult plants when placed in a medium culture.
These cloning experiments produced genetically identical individuals, popularly called clones.
The fact that a mature plant cell can dedifferentiate (reverse its function) and then give rise to all the different kinds of specialized cells of a new plant shows that differentiation does not necessarily involve irreversible changes in the DNA.
In plants, at least, cell can remain totipotent.
They retain the zygote’s potential to form all parts of the mature organism.
Plant cloning is now used extensively in agriculture.
In plants, cells remain totipotent—
all genes can be activated, and any cell
can form any part of the organism

14. 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!

15. Are there differences in genes between different cells in the body?
No = Genomic Equivalence
Are 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 genes
inactivated irreversibly? Can they retrace to become de-differentiated?
Well, it depends on the organism! Dec scientists de-differentiated the human skin cells by using transcription factors!
Much evidence supports the conclusion that nearly all the cells of an organism have genomic equivalence - that is, they all have the same genes.
An important question that emerges is whether genes are irreversibly inactivated during differentiation

16. Can adult skin cells retrace steps to become de-differentiated?
Somatic Cell Nuclear transfer (SCNT)
Take egg cell and remove nucleus (haploid) - throw it away
Take skin cell and remove nucleus (diploid) - save this
Insert skin cell nucleus (somatic cell) into egg cell cytoplasm. No need for sperm! Why?
Allow egg cell to divide and become blastocyst
Now you can extract stem cells (THERAPEUTIC CLONING) OR carry out REPRODUCTIVE CLONING - implant blastocyst in a surrogate mom and grow a clone!

17. Dolly and Bonny!
Reproductive Cloning is an offshoot of stem cell research
In July 1998, researchers in Hawaii reported cloning mice using nuclei from mouse ovary cells.
Since then cloning has been demonstrated in numerous mammals, including farm mammals.
The possibility of cloning humans raises unprecedented ethical issues.
In most cases, only a small percentage of the cloned embryos develop normally.
Improper methylation in many cloned embryos interferes with normal development.
AFTER THE CIRCUS PROcession of cloned sheep, cows, mice, and goats in the past couple years, humans seemed likely to join the list soon. Now this sobering news: A cloned calf in France dropped dead seven weeks after its birth.
The calf appeared healthy until days before her death; then she developed severe anemia and collapsed. An autopsy revealed a withered thymus gland, where white blood cells mature, suggesting that her immune system never started working. Jean-Paul Renard of the National Institute for Agronomic Research in Jouy-enJosas, who cloned her, thinks a defective donor cell might be at fault. He points out that the cloning process can work fine--other clones produced with his technique are thriving--but concedes that 30 to 50 percent of cloned calves die shortly before and immediately after birth. "If we want to apply this technique outside of research," he says, "such a high rate of abortion and mortality will not be acceptable."
Failure is actually the norm in the cloning business. Ryuzo Yanagimachi at the University of Hawaii has had perhaps the greatest success, producing five generations of cloned mice. Recently he created the first male clone, also a mouse. Yet Yanagimachi and his team had to transplant 274 embryos just to produce three live male mice, two of which died almost immediately That's not much of an improvement over the 276 failures that preceded Dolly the sheep, the first mammal clone.
There are other signs of trouble. The cloned mice were born with mild breathing problems. More disturbing is that Dolly's chromosomes are worn down at the edges, possibly showing signs of premature aging. It is not clear if she will die early For now, she appears healthy and has given birth to four lambs, also doing fine. But Margaret Mellon of the Washington, D.C.-based Union of Concerned Scientists warns: "There are many things that could be seriously wrong with her that would be very difficult to detect."

18. SCNT made Dolly the sheep! -Mammary gland
cells from donor arrested in G0
phase apparently
“dedifferentiated.”
The ability to clone mammals using nuclei or cells from early embryos has long been possible
However, it was not until 1997 when Ian Wilmut and his colleagues demonstrated the ability to clone an adult sheep by transplanting the nucleus from an udder cell into an unfertilized egg cell from another sheep.
They dedifferentiated the nucleus of the udder cell by culturing them in a nutrient-poor medium, arresting the cell cycle at the G1 checkpoint and sending the cell into the G0 “resting” phase.
These arrested cells were fused with sheep egg cells whose nuclei had been removed.
The resulting cells divided to form early embryos which were implanted into surrogate mothers.
One, “Dolly,” of several hundred implanted embryos completed normal development.
Review: Dolly’s mitochondrial
DNA is from the
egg donor sheep.

19. 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)
Differentiated cells from animals often fail to divide in culture, much less develop into a new organism.
Animal researchers have approached the genomic equivalence question by replacing the nucleus of an unfertilized egg or zygote with the nucleus of a differentiated cell.
The pioneering experiments in nuclear transplantation were carried out by Robert Briggs and Thomas King in the 1950s and extended by John Gordon.
The ability of the transplanted nucleus to support normal development is inversely related to the donor’s age.
Transplanted nuclei from relatively undifferentiated cells from an early embryo lead to the development of most eggs into tadpoles.
Transplanted nuclei from differentiated intestinal cells lead to fewer than 2% of the cells developing into normal tadpoles.
Most of the embryos failed to make it through even the earliest stages of development.

20. 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.
Developmental biologists agree on several conclusions about these results.
First, nuclei do change in some ways as cells differentiate.
While the DNA sequences do not change, chromatin structure and methylation may.
In frogs and most other animals, nuclear “potency” tends to be restricted more and more as embryonic development and cell differentiation progress.
However, chromatin changes are sometimes reversible and the nuclei of most differentiated animals cells probably have all the genes required for making an entire organism.

22. How does a stem cell make a differentiated cell?
Ques. 3) How do cells become different from one another to form different body parts when they all develop from ONE cell - the zygote?
How does a stem cell make a differentiated cell?
Determination & differentiation of muscle cells
During embryonic development, cells become obviously different in structure and function as they differentiate.
The earliest changes that set a cell on a path to specialization show up only at the molecular level.
Molecular changes in the embryo drive the process, termed determination, that leads up to observable differentiation of a cell.
The outcome of determination - differentiation - is caused by the expression of genes that encode tissue-specific proteins.
These give a cell its characteristic structure and function.
Differentiation begins with the appearance of mRNA and is finally observable in the microscope as changes in cellular structure.
In most cases, the pattern of gene expression in a differentiated cell is controlled at the level of transcription.
These cells produce the proteins that allow them to carry out their specialized roles in the organism.
For example, lens cells, and only lens cells, devote 80% of their capacity for protein synthesis to making just one type of proteins, crystallins.
These form transparent fibers that allow the lens to transmit and focus light.
Similarly, skeletal muscles cells have high concentrations of proteins specific to muscle tissues, such as muscle-specific version of the contractile protein myosin and the structural protein actin.
They also have membrane receptor proteins that detect signals from nerve cells.
Muscle cells develop from embryonic precursors that have the potential to develop into a number of alternative cell types, including cartilage cells, fat cells or multinucleate muscle cells.
As the muscles cells differentiate, they become myoblasts and begin to synthesize muscle-specific proteins.
Researchers developed the hypothesis that certain muscle-specific regulatory genes are active in myoblasts, leading to muscle cell determination.
To test this, researchers isolated mRNA from cultured myoblasts and used reverse transcriptase to prepare a cDNA library.
Transplanting these cloned genes into embryonic precursor cells led to the identification of several “master regulatory genes” that, when transcribed and translated, commit the cells to become skeletal muscle.
One of these master regulatory genes is called myoD, a transcription factor.
It binds to specific control elements and stimulates the transcription of various genes, including some that encode for other muscle-specific transcription factors.
These secondary transcription factors activate the muscle protein genes.
The MyoD protein is capable of changing fully differentiated non-muscle cells into muscle cells.
However, not all cells will transform.
Non-transforming cells may lack a combination of regulatory proteins, in addition to MyoD.
What turns on the Master control gene?
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)

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

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

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

26. 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
When the primary research goal is to understand broad biological principles - of animal or plant development in this case - the organism chosen for study is called a model organism.
Researchers select model organisms that lend themselves to the study of a particular question.
For example, frogs were early models for elucidating the role of cell movement during animal morphogenesis because their large eggs are easy to observe and manipulate, and fertilization and development occurs outside the mother’s body.
For developmental genetics, the criteria for choosing a model organism include, readily observable embryos, short generation times, relatively small genomes, and preexisting knowledge about the organism and its genes.
These include Drosophila, the nematode C. elegans, the mouse, the zebrafish, and the plant Arabidopsis.
The fruit fly Drosophila melanogaster was first chosen as a model organism by geneticist T.H. Morgan and intensively studied by generations of geneticists after him.
The fruit fly is small and easily grown in the laboratory.
It has a generation time of only two weeks and produces many offspring.
Embryos develop outside the mother’s body.
In addition, there are vast amounts of information on its genes and other aspects of its biology.
However, because first rounds of mitosis occurs without cytokinesis, parts of its development are superficially quite different from what is seen in other organisms.
The nematode Caenorhabditis elegans normally lives in the soil but is easily grown in petri dishes.
Only a millimeter long, it has a simple, transparent body with only a few cell types and grows from zygote to mature adult in only three and a half days.
Its genome has been sequenced.
Because individuals are hermaphrodites, it is easy to detect recessive mutations.
Self-fertilization of heterozygotes will produce some homozygous recessive offspring with mutant phenotypes.
For studying the molecular genetics of plant development, researchers are focusing on a small weed Arabidopsis thaliana (a member of the mustard family).
One plant can grow and produce thousands of progeny after eight to ten weeks.
A hermaphrodite, each flower makes ova and sperm.
For gene manipulation research, scientists can induce cultured cells to take up foreign DNA (genetic transformation).
Its relatively small genome, about 100 million nucleotide pairs, has already been sequenced.

27. Cytoplasmic determinants (from the mom - internal signals in egg)
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)
Two sources of information “tell” a cell, like a myoblast or even the zygote, which genes to express at any given time.
Once source of information is both the RNA and protein molecules, encoded by the mother’s DNA, in the cytoplasm of the unfertilized egg cell.
Messenger RNA, proteins, other substances, and organelles are distributed unevenly in the unfertilized egg.
This impacts embryonic development in many species.
These maternal substances, cytoplasmic determinants, regulate the expression of genes that affect the developmental fate of the cell.
After fertilization, the cell nuclei resulting from mitotic division of the zygote are exposed to different cytoplasmic environments.

28. 1) Cytoplasmic determinants
include 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.

29. Example: --Bicoid mRNA is present at the anterior end of the egg
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.
Cytoplasmic determinants establish the axes of the Drosophila body.
These maternal effect genes, deposited in the unfertilized egg, lead to an abnormal offspring phenotype if mutated.
In fruit fly development, maternal effect genes encode proteins or mRNA that are placed in the egg while in the ovary.
When the mother has a mutated gene, she makes a defective gene product (or none at all), and her eggs will not develop properly when fertilized.
Using DNA technology and biochemical methods, researchers were able to clone the bicoid gene and use it as a probe for bicoid mRNA in the egg.
As predicted, the bicoid mRNA is concentrated at the extreme anterior end of the egg cell.
The results of detailed anatomical observations of development in several species and experimental manipulations of embryonic tissues laid the groundwork for understanding the mechanisms of development.
In the 1940s, Edward B. Lewis demonstrated that the study of mutants could be used to investigate Drosophila development.
He studied bizarre developmental mutations and located the mutations on the fly’s genetic map.
This research provided the first concrete evidence that genes somehow direct the developmental process.
In the late 1970s, Christiane Nüsslein-Volhard and Eric Weischaus pushed the understanding of early pattern formation to the molecular level.
Their goal was to identify all the genes that affect segmentation in Drosophila, but they faced three problems.
Because Drosophila has about 13,000 genes, there could be only a few genes or so many that there is no pattern.
Mutations that affect segmentation are likely to be embryonic lethals, leading to death at the embryonic or larval stage.
Because of maternal effects on axis formation in the egg, they needed to study maternalNüsslein-Volhard and Wieschaus focused on recessive mutations that could be propagated in heterozygous flies.
After mutating flies, they looked for dead embryos and larvae with abnormal segmentation among the fly’s descendents.
Through appropriate crosses, they could identify living heterozygotes carrying embryonic lethal mutations.
They used a saturation screen in which they made enough mutations to “saturate” the fly genome with mutations.
They hoped that the segmental abnormalities would suggest how the affected genes normally functioned.
After a year of hard work, they identified 1,200 genes essential for embryonic development
About 120 of these were essential for pattern formation leading to normal segmentation.
After several years, they were able to group the genes by general function, map them, and clone many of them.
Their results, combined with Lewis’ early work, created a coherent picture of Drosophila development.
In 1995, Nüsslein-Volhard, Wieschaus, and Lewis were awarded the Nobel Prize.

30. Background on Drosophila: Cytokinesis does not occur in
the early Drosophila embryo.
Nuclei migrate to the periphery
in the blastula.
(1) Mitosis follows fertilization and laying the egg.
Early mitosis occurs without growth of the cytoplasm and without cytokinesis, producing one big multinucleate cell.
(2) At the tenth nuclear division, the nuclei begin to migrate to the periphery of the embryo.
(3) At division 13, the cytoplasm partitions the 6,000 or so nuclei into separate cells.
The basic body plan has already been determined by this time.
A central yolk nourishes the embryo, and the egg shell continues to protect it.

31. (4) Subsequent events in the embryo create clearly visible segments, that at first look very much alike.
(5) Some cells move to new positions, organs form, and a wormlike larva hatches from the shell.
During three larval stages, the larva eats, grows, and molts.
(6) The third larval stage transforms into the pupa enclosed in a case.
(7) Metamorphosis, the change from larva to adult fly, occurs in the pupal case, and the fly emerges.
Each segment is anatomically distinct, with characteristic appendages.
The results of detailed anatomical observations of development in several species and experimental manipulations of embryonic tissues laid the groundwork for understanding the mechanisms of development.
In the 1940s, Edward B. Lewis demonstrated that the study of mutants could be used to investigate Drosophila development.
He studied bizarre developmental mutations and located the mutations on the fly’s genetic map.
This research provided the first concrete evidence that genes somehow direct the developmental process.

32. Bicoid is a morphogen—a substance that
establishes an organism’s axis or other 3D
features.
Bicoid helps create the anterior/posterior
axis.
It’s a transcription factor that activates expression of segmentation genes
After the egg is fertilized, the mRNA is transcribed into proteins, which diffuse from the anterior end toward the posterior, resulting in a gradient of proteins in the early embryo.
Injections of pure bicoid mRNA into various regions of early embryos results in the formation of anterior structures at the injection sites as the mRNA is translated into protein These maternal effect genes are also called egg-polarity genes, because they control the orientation of the egg and consequently the fly.
One group of genes sets up the anterior-posterior axis, while a second group establishes the dorsal-ventral axis.
One of these, the bicoid gene, affects the front half of the body with mutations that produce an embryo with duplicate posterior structures at both ends.
This suggests that the product of the mother’s bicoid gene is essential for setting up the anterior end of the fly.
It also suggests that the gene’s products are concentrated at the future anterior end.
This is a specific version of a general gradient hypothesis, in which gradients of morphogens establish an embryo’s axes and other features.

33. 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.
The bicoid protein and other morphogens are transcription factors that regulate the activity of some of the embryo’s own genes.
Gradients of these morphogens bring about regional differences in the expression of segmentation genes, the genes that direct the actual formation of segments after the embryo’s major axes are defined.

34. 2) Induction = signals impinging on an embryonic cell from other nearby embryonic cells. These can be transcription factors - remember cell communication?
The synthesis of these signals is controlled by the embryo’s own genes.
These signal molecules cause induction, triggering observable cellular changes by causing a change in gene expression in the target cell.

35. Has 3 parts—head, thorax, and abdomen
Our friend Drosophila !
Has 3 parts—head, thorax, and abdomen
has an anterior/posterior axis and a dorsal/ventral axis
Cytoplasmic determinants and induction together lead to PATTERN FORMATION
Dorsal
Cytoplasmic determinants, inductive signals, and their effects contribute to pattern formation, the development of a spatial organization in which the tissues and organs of an organism are all in their characteristic places.
Pattern formation continues throughout life of a plant in the apical meristems.
In animals, pattern formation is mostly limited to embryos and juveniles.
Pattern formation in animals begins in the early embryo, when the animal’s basic body plan - its overall three-dimensional arrangement - is established.
The major axes of an animal are established very early as the molecular cues that control pattern formation, positional information, tell a cell its location relative to the body axes and to neighboring cells.
They also determine how the cells and its progeny will respond to future molecule signals.
Pattern formation has been most extensively studies in Drosophila melanogaster, where genetic approaches have had spectacular success.
These studies have established that genes control development and the key roles that specific molecules play in defining position and directing differentiation.
Combining anatomical, genetic, and biochemical approaches to the study of Drosophila development, researchers have discovered developmental principles common to many other species, including humansFruit flies and other arthropods have a modular construction, an ordered series of segments.
These segments make up the three major body parts: the head, thorax (with wings and legs), and abdomen.
Like other bilaterally symmetrical animals, Drosophila has an anterior-posterior axis and a dorsal-ventral axis.
Cytoplasmic determinants in the unfertilized egg provide positional information for the two developmental axes before fertilization.
After fertilization, positional information establishes a specific number of correctly oriented segments and finally triggers the formation of each segment’s characteristic structures.
Anterior
Posterior
Ventral

36. Pattern formation is the development of the
spatial 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
Development of the fruit fly from egg cell to adult fly occurs in a series of discrete stages.

37. Cell Lineage of all 959 C. elegans cells!
A further important feature is that every adult C. elegans have exactly 959 somatic cells.
These arise from the zygote in virtually the same way for every individual.
By following all cell divisions with a microscope, biologists have constructed the organism’s complete cell lineage, a type of fate map.
A fate map traces the development of an embryo.
The mouse Mus musculus has a long history as a mammalian model of development.
Much is known about its biology, including its genes.
Researchers are adepts at manipulating mouse genes to make transgenic mice and mice in which particular genes are “knocked out” by mutation.
But mice are complex animals with a genome as large as ours, and their embryos develop in the mother’s uterus, hidden from view.
A second vertebrate model, the zebrafish Danio rerio, has some unique advantages.
These small fish (2 - 4 cm long) are easy to breed in the laboratory in large numbers.
The transparent embryos develop outside the mother’s body.
Although generation time is two to four months, the early stages of development proceed quickly.
By 24 hours after fertilization, most tissues and early versions of the organs have formed.
After two days, the fish hatches out of the egg case.
The study of the zebrafish genome is an active area.

38. Homeotic genes determine the segment on
which appendages or other structures will form
The expression of these genes is activated by
Transcription factors coded by segmentation genes.

40. All homeotic genes contain a homeobox domain.
Homeobox domains have been found in many
other animals besides flies, and most genes
with a homeobox are related to development.
The homeobox domain is actually a DNA-
binding domain!
So proteins containing it are likely to be
transcription factors!!

41. Flies and mice
have homologous
genes coding for
proteins involved
in development.

43. Induction is when cells signal other cells to
change in a specific way—mostly activating
or inactivating transcription.
Induction has been studied most in the
nematode, C. elegans.

44. Vulva precursor cells
can develop into 3
different types of cells.
Signals from the
anchor cell induce the
determination of each
cell.
Effects of inducers
can vary depending
on concentration.

45. 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.

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

47. MUTANT MICE GALLERY. In the name of Science…
MUTANT MICE GALLERY! In the name of Science….. Are you ready for the gore?

48. Figure 21.x2a Laboratory mice: brachyury mutant

49. Figure 21.x2b Laboratory mice: eye-bleb mutant

50. Figure 21.x2c Laboratory mice: Hfh11 mutant

51. Figure 21.x2d Laboratory mice: Lama2 mutant

52. Figure 21.x2e Laboratory mice: Lepr mutant

53. Figure 21.x2f1 Laboratory mice: Mgf mutant

54. Figure 21.x2f2 Laboratory mice: Pax3 mutant

55. Figure 21.x2g Laboratory mice: Otc mutant

56. Figure 21.x2h Laboratory mice: Pax6 mutant

57. Figure 21.x2i Laboratory mice: Pit1 mutant

58. Figure 21.x2j Laboratory mice: pudgy mutant

59. Figure 21.x2k Laboratory mice: ruby-eye mutant

60. Figure 21.x2l Laboratory mice: stargazer mutant

61. Figure 21.x2m1 Laboratory mice: ulnaless mutant

62. Figure 21.x3 Nude mouse

63. Figure 21.x4 Normal and double winged Drosophila