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U5. Embryonic Development. Opening Question. What do you know about stem cells?. Concept 14.1 Development Involves Distinct but Overlapping Processes. Development —the process any multicellular organism undergoes to take on forms that characterize its species/life cycle.
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U5 Embryonic Development
Opening Question What do you know about stem cells?
Concept 14.1 Development Involves Distinct but Overlapping Processes • Development—the process any multicellular organism undergoes to take on forms that characterize its species/life cycle. • After an egg is fertilized, it is now called a zygote. • In its earliest stages (before the organ systems are in place), a plant or animal is called an embryo. • This embryo must be protected as it develops in a seed, an egg shell, or a uterus.
Developmental Processes • As an embryo develops and adds more cells, it undergoes four processes of development: • Determination sets the fate of each new cell (e.g. location determines cells on the outside will be part of the skin) • Differentiation changes the cell into its determined form (e.g. the outer cells actually change form to become a skin cell) • Morphogenesis is the formation of the body structures (e.g. the skin cells group together to form skin tissue) • Growth is anincrease in body size by cell division and cell expansion (e.g. the skin tissue continues to add new skin cells to enable to organism to get larger)
Determined, but not Differentiated • Careful! Determination and Differentiation sound the same, but they are very different. • Experiments in which specific cells of an early embryo are grafted to new positions on another embryo show when cell fate (determination) has been decided, even if differentiation has not occurred.
How is a cell determined? • Determination is influenced by gene expression AND the external environment, usually the location in the developing organism • Reminder: Determination is a commitment (i.e. being accepted to college); the final realization of that commitment is differentiation (i.e. packing up your bags and going to college). • Differentiation is the actual changes in the cell itself to accept its cell fate. Once they know what they will be, they express the appropriate DNA to change and match their type.
Stem Cells • So what are Stem Cells anyway? • Stem Cells are cells that are not yet determined or differentiated. They can literally become any cell in the body they need to be. • How do they work? • Stem cells work because of genomic equivalence (all cells have the same and full copy of the DNA). As the cells pass through developmental the DNA is unchanged so at any point a cell *theoretically* should be able to become any other cell since they all have the same instruction manual.
Where do we find stem cells? • Embryos of any kind are essentially all stem cells • In mature mammals, stem cells occur in some tissues that require frequent replacement—skin, blood, etc. However, they are limited in what they can become once mature because there are over 300 different types of cells in a mammal. • Mature plants keep their stem cells throughout their life in meristems—areas of rapid division. Plant stem cells maintain the ability to become any part of the plant (even after maturity) since plants only have about 15-20 cell types
Stem Cells • Stem Cell Types • Totipotent stem cells—able to become any type of cell at any point of development forever; these cells may even be determined AND differentiated, but can be reversed (Plants only) • Ex - Because plant cells can be induced to dedifferentiate, roots will grow from a branch of a tree cutting if planted (asexual reproduction).
Stem Cells • Stem Cell Types • Multipotent stem cells—these cells can only differentiate into a few cell types. These are found in mature mammals in tissues that need frequent replacement. • Ex: Hematopoietic stem cells can produce red OR white blood cells. • Ex: Mesenchymal stem cells can produce bone OR connective tissue cells.
Stem Cells • Multipotent stem cells differentiate “on demand” when they receive a local or long distance signal; this enables the body to increase the type of cell they need most • For example, the body increases the amount of white blood cells when experiencing an infection. Other times it needs more red blood cells, such as when living at a higher elevation with less atmospheric oxygen. • All of these stem cells are found in the bone marrow. In addition, the signal to differentiate can come from adjacent cells or from the long distance signaling (glands or brain) through the blood stream. • This is the basis of a cancer therapy called hematopoietic stem cell transplantation (HSCP) or a bone marrow transplant.
Stem Cell Use • Bone Marrow Transplants to treat cancer • Therapies that kill cancer cells target cells that are rapidly dividing. This can also kill cells that are supposed to be rapidly dividing such as the bone marrow stem cells. Losing bone marrow stem cells weakens the immune system and lowers the blood count, resulting in risk of infection or anemia. • BEFORE cancer therapy begins, a sample of the stem cells are often removed and stored. Healthy tissue can then be returned to the bone marrow once the therapy is complete to speed healing. Bone marrow can also be taken from a donor if needed.
Stem Cells • Stem Cell Types • Pluripotent stem cells are found in the embryo, and like totipotent cells they can form any cell in the body. However, once they have differentiated, they cannot be reversed. Also called embryonic stem cells or ESCs • Ex: In mice, these cells have been removed from embryos and grown in laboratory culture almost indefinitely. In addition, we have learned that we can differentiate them using specific signals, such as using Vitamin A to form neurons or growth factors to form blood cells. This is very promising for stem cell therapy.
The controversy of ESC • ESC cultures may be sources of fresh, undifferentiated cells to repair damaged tissues with no cure, such as diabetes, Parkinson’s disease, traumatic brain injury, nerve damage, etc. • So why aren’t we using them more? Because they have to come from human embryos. Two issues… • Some people object to the destruction of human embryos for this purpose • Abortion vs In Vitro vs Cord Blood… • Just like any other organ transplant, the transplanted stem cells could provoke an immune response in the recipient
So how do all these stem cells know what part of the body to become? Developing the Tissues
Development of the Germ Layers • As the zygote undergoes mitosis and creates new cells, the cells create layers to organize their placement. In animals, this follows a very specific pattern and results in three layers of cells called germ layers. • Zygote (single cell) Blastula (ball of cells) Gastrula (folded-in ball of cells with three layers) Larvae and/or Adult (functioning organism) • Ectoderm (outer) layer becomes skin and nerves [blue] • Mesoderm (middle) layer becomes muscle and bones [red] • Endoderm (inner) layer becomes the digestive tract [yellow]
Cell Differentiation • Once the germ layers are established, the cells will begin to communicate with one another to determine what type of cell they should become. These communication signals notify each cell what DNA to express and what DNA to ignore. • Differentiated Gene Expression Cues • Location Cues • Ex: Asymmetrical factors that are unequally distributed in the embryo • Communication Cues • Ex: Differential exposure to an inducer released by a cell nearby
Example of a Location Cue • Polarity—establishing a “top” and a “bottom” in the embryo leads to cell determination and differentiation; the initial body axis is determined by the blastula’s orientation in the egg or womb • Example - Polarity was demonstrated using sea urchin embryos. • If an eight-cell embryo is cut vertically, it develops into two normal but small embryos. (top and bottom both represented) • If the eight-cell embryo is cut horizontally, the bottom develops into a small embryo, but the top does not develop at all. (only top or only bottom)
Example of a Location Cue • Cytoplasmic determinants (signals) are distributed unequally in the original egg. As the egg divides, some cells end up containing more of the determinants than others. The differing amounts of determinants will induce different cell fates.
Example of a Communication Cue • Induction refers to the signaling events in a developing embryo. Cells influence one another’s developmental fate via sending and receiving chemical signals. • Ex: Exposure to different amounts of inductive signals will lead to differences in gene expression. If your neighbor tells you to become a skin cell, you activate the genes to become a skin cell.
Morphogenesis • Pattern formation—the process that results in specific pattern forms in the body (i.e. hands) results from a combination of cell communication and location • To form a symmetrical structure… • Cells in body must “know” where they are • Cells must activate appropriate genes to embrace their appropriate fate
Location Detection • How does a cell know its location? • Positional information comes in the form of an inducer called a morphogen which will diffuse from one group of cells to another, setting up a concentration gradient. • Receiving different concentrations of the morphogen will result in different effects
Positional Information Example • The “French flag model” explains morphogens and can be applied to differentiation and development of vertebrate limbs. • Vertebrate limbs develop from paddle-shaped limb buds—cells must receive positional information from neighbors. • Cells on one side (ZPA = zone of polarizing activity) secrete a morphogen called Sonic hedgehog (Shh). As it floats from one side to the other, it forms a gradient that determines the posterior–anterior axis of the hand. More Shh = pinky, less Shh = thumb
Sculpting Structures • Pattern formation often includes the removal of cells that are in the wrong location. This is done through programmed cell death called apoptosis. Removing these cells “sculpts” structures (i.e. removing the webbing between fingers) Their expression in the human embryo guides development of fingers and toes. • Many cells and structures form and then disappear during development. (i.e. gills in humans, back legs in dolphins, etc. This is evidence of evolution! We have the DNA, but we don’t use it anymore.)
Morphogenesis: Setting up the Body Plan • Symmetrical animals must have defined body sections.A head (anterior), tail (posterior), back (dorsal), and belly (ventral) region must be defined. In addition, the body needs to know where to grow legs vs. where to grow arms. Several types of genes are expressed sequentially to define body segments: • FIRST: Maternal effect genes set up the anterior–posterior and dorsal–ventral axes based on the orientation of the blastula in the egg or womb. • NEXT: Segmentation genes determine rough boundaries of each segment using induction signals like the French Flag model. • FINALLY: Hox genes determine what organ will be made in each segment (i.e. antennae or legs here?).
The Homeobox: Your Cell Making Tool Kit. • The Hox genes are an interesting group… • Hox genes are a type of gene called a homeotic gene that are shared by all animals. As a group, the hox genes are called the homeobox and work like a construction team. The hox genes are BUILDER genes. They build legs or eyes or antennae, etc. The REST of the DNA tells them what kind of leg or eye or antennae to build. You can actually transfer hox genes from one species to another and they still function perfectly. (Evidence for Evolution)
EvoDevo • In development, there is a LOT of evidence for evolution. We have found that evolution does not make NEW structures; it simply modifies the development of EXISTING structures. • EvoDevo – The study of developmental mechanisms (like the homeobox) that have been conserved during evolution. This genetic toolkit has simply been modified over the course of evolution to produce the diversity in the world today. • Example: The giraffe’s neck has the same number of vertebrae as other mammals, but the bones grow for a longer period and therefore are larger. Ducks and Chicken foot formation is almost identical, but the duck keeps the webbing to assist with swimming.
Figure 14.17 Heterochrony in the Development of a Longer Neck
Figure 14.18 Changes in Gremlin Expression Correlate with Changes in Hindlimb Structure
Evidence of EvoDevo • Nearly all evolutionary innovations are modifications of existing structures. • In vertebrates, wings are modified limbs. • Genes that control development are highly conserved. • The homeobox is the same in all vertebrates • Organisms can lose structures through modification of development. • Ancestors of snakes lost their forelimbs as a result of changes in expression of Hox genes. Then they lost their hindlimbs by the loss of expression of the Sonic hedgehog gene in limb bud tissue.
