Bio 205 exam 3

What was the new name that Thomas Hunt Morgan gave Mendel's "hereditary factors?" Does the phenotype reveal the genotype? (Refresh your familiarity with these two terms) What is one variation on the central dogma (DNA → RNA → protein)?

"genetic loci" genotype-—the genetic information it inherits from its parents phenotype- The organism's visible appearance, internal structure, and biochemistry While the genotype certainly controls development, environmental fac- tors interacting with the genotype influence the phenotype- so not exactly some genes encode proteins that control the activity of other genes.

Body axes

(anterior-posterior, dorsal-ventral, left-right)

Blastomere

- cells derived from cleavage of the early embryo. (The almost identical cells produced by cleavage divisions are called)

Blastula

- the stage in animal development that results from cleavage. A hollow ball of cells, composed of an epithelial layer of small cells enclosing a fluid-filled cavity.

List, define and distinguish the four main developmental processes- 1. PATTERN FORMATION

1) pattern formation- the process by which cellular activity is organized in space and time so that a well-ordered structure develops within the embryo. -it is achieved by various cellular and molecular mechanisms in different organisms and at different stages of development. - initially involves laying down the overall body plan—defining the main body axes of the embryo that run from head (anterior) to tail (posterior), and from back (dorsal) to underside (ventral). -Even before the body axes become clear, eggs and embryos often show a distinct polarity, which in this context means that they have an intrinsic orientation, with one end different from the other. Gradient of a protein or other molecule, as the slope of the gradient provides direction. At the same time that the axes of the embryo are being formed, cells are being allo- cated to the different germ layers—ectoderm, mesoderm, and endoderm DDuring further pattern formation, cells of these germ layers acquire different identities so that organized spatial patterns of differentiated cells finally emerge, such as the arrangement of skin, muscle, and cartilage in developing limbs, and the arrangement of neurons in the nervous system.

List the four sequential locations (approximately) of sea urchin primary mesenchyme cells during gastrulation.

1) within the blastocoel to form a characteristic pattern on the inner surface of the blastocoel wall -first arranged in a ring around the gut in the vegetal region at the ectoderm-endoderm border 2) migrate to form 2 extensions toward the animal pole on the ventral (oral side 3) Move over inner surface of blastocoel wall by fine filopodia which can be 40 uM long and extend in several directions (avg 6)- moving toward portion of wall where filopodia make m out stable contact - random search for the most stable attachment - as migrate filopodia fuse with eachotehr - cable like extensions Lay down the skeletal rods of the sea urchin endoskeleton by secretion of matrix proteins

Epiblast -

A group of cells of the mouse or chick embryo, within the blastocyst or blastoderm respectively , that gives rise to the embryo proper. In the mouse, the epiblast develops from the cells of the inner cell mass

Epiblast (repeated from last time)

A group of cells of the mousee or chick embryo within the blastocyst or blastoderm respectively, that gives rise to the embryo proper. In the mouse, the epiblast develops from cells of the inner cell mass

Spemann-mangold experiment

A piece of tissue (yellow) from the dorsal lip of the blastopore of a newt (Triton cristatus) gastrula is grafted to the opposite side of a gastrula of another, pigmented, newt species (Triton taeniatus, pink). The grafted tissue induces a new body axis containing neural tube and somites. The unpigmented graft tissue forms a notochord at its new site (see section in lower panel) but most of the neural tube and the other structures of the new axis have been induced from the pigmented host tissue. The organizer region discovered by Spemann and Mangold is known as the Spemann organizer.

Adherens junction -

A type of adhesive junction that binds epithelial cells very tightly together to form an epithelium that seals of the environment on one side of the epithelium from the other

Involution

A type of cell movement that occurs at the beginning of amphibian gastrulation when a sheet of cells enters the embryo interior by rolling in under itself

What is one potential outcome of understanding stem cells?

Another area of medical research related to developmental biology is regenerative medicine—finding out how to use cells to repair damaged tissues and organs. The focus of regenerative medicine is currently on stem cells. Stem cells that can proliferate and give rise to all the different tissues of the body are present in embryos. These, and the stem cells with more limited developmental potential that are found in adult tissues, are discussed in Chapter 8.

What advantages have motivated the use of model organisms for the study of developmental biology?

As it is, developmental biologists have tended to focus their efforts on a relatively small number of animals, chosen because they were *convenient to study and amenable to experimental manipulation or genetic analysis* This is why some creatures, such as the frog Xenopus laevis (Fig. 1.2) and the fruit fly Drosophila melano- gaster, have such a dominant place in developmental biology. Similarly, work with the thale-cress, Arabidopsis thaliana, has uncovered many features of plant development. Amphibians have long been favorite organisms for studying early development because their eggs are large and their embryos are easy to grow in a simple culture medium and relatively easy to experiment on.

What are two types of cells known to do asymmetric cell division?

Asymmetric divisions are so called because they result in daughter cells having properties different from each other, independently of any environmental influence. Although some asymmetric cell divisions are also unequal divisions, in that they produce cells of different sizes, this is not usually the most important feature in animals; it is the unequal distribution of cytoplasmic factors that makes the division asymmetric. EGG CELL- there are well-known cases where eggs or cells divide so that some cytoplasmic determinant becomes unequally distributed between the two daughter cells This happens at the first cleavage of the nematode egg, for example, and defines the antero-posterior axis of the embryo. -The germ cells of Drosophila are also specified by cytoplasmic determinants, in this case contained in the cytoplasm located at the posterior end of the egg. A very particular type of asymmetric division is seen in stem cells. When they divide, these self-renewing cells can produce one new stem cell and another daugh- ter cell that will differentiate into one or more cell types (Fig. 1.32). Stem cells that are capable of giving rise to all the cell types in the body are present in very early embryos, and are known as embryonic stem cells (ES cells), . The difference in the behavior of the daughter cells produced by division of a stem cell can be due either to asymmetric distribution of cytoplasmic determinants or to the effects of external signals. The generation of neurons in Drosophila, for example, depends on the asymmetric distribution of cyto- plasmic determinants in the neuronal stem cells, whereas the production of the differ- ent types of blood cells seems to be regulated to a large extent by extracellular signals. Embryonic stem cells, which can give rise to all the types of cells in the body, are called pluripotent, while stem cells that give rise to a more limited number of cell types are known as multipotent. The hematopoietic stem cells in bone marrow, which give rise to all the different types of blood cells, are an example of multipotent stem cells. For most animals, the only cell that can give rise to a complete new organism is the fertilized egg, and this is described as totipotent

Polarization of cells during cleavage of the mouse embryo.

At the eight-cell stage, compaction takes place, with the cells forming extensive contacts with each other (top panel). The cells also become polarized. For example, microvilli, which were initially uniformly distributed over the cell surface, become confined to the outward- facing cell surface. Future cleavages can thus divide the cell either into two polarized cells (by a radial cleavage, bottom left panel) or into a polarized and a nonpolarized cell (by a tangential cleavage, bottom right panel). The tangential cleavages give rise to the inner cell mass.

What are three factors that have been tested as the determinant for MBT timing (two didn't turn out to be the essential)?

Cell division- Suppression of cleavage but not DNA synthesis does not alter the timing of transcriptional activation, and so the timing is not linked directly to cell division. Cell-cell interactions- dis- sociated blastomeres undergo the transition at the same time as intact embryos. *The ratio of DNA to cytoplasm—the quantity of DNA present per unit mass of cytoplasm.* Direct evidence for this comes from increasing the amount of DNA artificially by allowing more than one sperm to enter the egg or by injecting extra DNA into the egg. In both cases, transcriptional activation occurs prematurely, suggesting that there may be some fixed amount of a general repressor of transcription present initially in the egg cytoplasm.As the egg cleaves, the amount of cytoplasm does not increase, but the amount of DNA does. The amount of repressor in relation to DNA gets smaller and smaller until there is insufficient to bind to all the available sites on the DNA and the repression is lifted. Timing of the mid-blastula transition thus seems to fit with an hourglass egg-timer model (Fig. 4.16). In such a model something has to accumulate, in this case DNA, until a threshold is reached. The threshold is determined by the initial concentration of the cytoplasmic factor, which does not increase. WHAT IS THIS ? Mid-blastula transition (even tho actually occurs in late blastula)- the beginning of transcription from the embryo's own genes, often referred to as zygotic genes. The egg contains large amounts of maternal mRNAs, which are laid down during oogenesis. After fertilization, the rate of protein synthesis increases 1.5- fold, and a large number of new proteins are synthesized by translation of the maternal mRNA. There is, in fact, very little new mRNA synthesis until 12 cleavages have taken place and the embryo contains 4096 cells. At this stage, transcription from zygotic genes begins, cell cycles become asynchronous, and various other changes occur.

EMT- epithelial to mesenchymal transition

Cells lose adhesiveness and detach from the epithelium as single cells

What brings about change in embryo shape?

Changes in form in animal embryos are brought about by cellular forces that are generated in various ways: -cell division -change in cell shape -rearrangement of cells within tissues -migration of individual cells and groups of cells from one part of the embryo to another These cellular forces are most frequently generated within epithelial sheets by contraction of the cells' internal cytoskeleton Resisting these forces are the cells' own structure and their adhesive interactions with other cells and with the extracellular material, which hold them together in tissues. * isolated cells on their own cannot drive morphogenesis. It is their organization into coherent tissues that enables cells to coordinate their behavior and generate forces that model the embryo into a different shape.* 3 properties- morphogenesis: cell adhesiveness, shape, migration

Haploid

Describing a cell derived from diploid cells by meiosis and containing only one set of chromosomes, half the diploid number of chromosomes, and thus only one copy of each gene. In most animals, the only haploid cells are the gametes, the sperm and egg.

What does the cell adhesion molecule cadherin bind to on the extracellular surface? What does it bind to in the cytoplasm?

EXTRACELLULAR SURFACE The cadherins are transmembrane proteins that, in the presence of calcium ions (Ca2+), adhere to cadherins on the surface of another cell. CYTOPLASM They interact with the intracellular cyto- skeleton through the connection of their cytoplasmic tails with catenins and other proteins, and thus can be involved in transmit- ting signals to the cytoskeleton.

Filopodia

Finger-shaped cellular projections with an F-actin core, involved in pathfinding

As we have found out by studying the internalization of the mesoderm in Drosophila gastrulation, what is the most important protein involved in apical constriction? PAGE 381- this answer is not right!!!!!!!!

Gas- trulation begins with the invagination of a longitudinal strip of future mesodermal cells (9-10 cells wide) on the ventral side of the embryo, to form a ventral furrow and then a transient tube inside the body. The tube breaks up into individual cells, which spread out to form a single layer of mesoderm on the interior face of the ectoderm invagination Occurs in 2 phases (no cell div during) 1)apical constriction- central strip of cells develops flattened and smaller apical surfaces and nuclei move away from apical surfaces --> furrow 2)tube dissociate into individual cells which proliferate and spread out laterally In Drosophila, the transcrip- tion factors Twist and Snail are expressed in the prospective mesoderm before gastru- lation as part of the patterning of the dorso-ventral axis (see Section 2.19). It was then shown that mesoderm invagination is affected by mutations in these patterning genes and that they are associated with epithelial-to-mesenchymal transitions.

Re-draw Figure 1.19 (simply copy for the most part), taking into account what you know from Cell Biology that glycosylation usually occurs in the ER, and add steps to illustrate secretion of the protein of interest.

Gene expression and protein synthesis. A protein-coding gene comprises a coding region—a stretch of DNA that contains the instructions for making the protein—and adjacent DNA sequences that act as a control region. The control region includes: - the promoter region, to which general transcription factors and the enzyme RNA polymerase bind to start transcription -cis-regulatory region, consisting of one or more modules, at which other transcription factors bind to switch the gene on or off. This latter control region may be thousands of base pairs away from the promoter. When the gene is switched on, the DNA sequence is transcribed to produce RNA (1). The RNA formed by transcription is spliced to remove introns and the non- coding region at the start of the gene (yellow) and processed within the nucleus (2) to produce mRNA. This is exported from the nucleus to the cytoplasm (3) for translation into protein at the ribosomes (4). Control of gene expression and protein synthesis occurs mainly at the level of transcription but can also occur at later stages. For example, mRNA may be *degraded* before it can be translated, or if it is not translated immediately it may be *stored in inactive form in the cytoplasm* for translation at a later stage. mRNAs may also be *targeted by specific microRNAs so that translation is blocked*. Some proteins require a further step of post-translational modification (5) to become biologically active. A very common post-translational modification is the addition of carbohydrate side-chains (glycosylation) as shown here.*

Homologous gene

Genes that share significant similarities in their nucleotide sequence and are derived from the common ancestral gene In general, when an important developmental gene has been identified in one ani- mal, it has proved very rewarding to see whether a corresponding gene is present and is acting in a developmental capacity in other animals. Such genes are often identified by a sufficient degree of nucleotide sequence similarity to indicate descent from a com- mon ancestral gene. Genes that meet this criterion are known as homologous genes.

What methodology/ies were used by early developmental biologists, centuries ago?

Hippocrates- tried to explain develop- ment in terms of the principles of heat, wetness, and solidification. Aristotle- addressed the problem of how the different parts of the embryo were formed. He considered two possibilities: 1) one was that everything in the embryo was preformed from the very beginning and simply got bigger during development 2) that new structures arose progressively, a process he termed epigenesis (which means 'upon formation') and that he likened metaphorically to the 'knitting of a net'. Aristotle favored epigenesis and his conjecture was correct. 17th century- preformation (religious) Marcello Malpighi- preformation, . He argued that at very early stages the parts were so small that they could not be seen, even with his best microscope.

Spemann and Mangold grafted a specific part of the developing frog embryo onto another one in a different place. What was that grafted part sufficient to do? Again, what were the advantages of using frog embryos for this experiment?

Induction. This is where one cell, or tissue, directs the development of another, neighboring, cell or tissue. partial second embryo could be induced by grafting one small region of an early newt embryo onto another at the same stage The grafted tissue was taken from the dorsal lip of the blastopore—the slit-like invagination that forms where gastrulation begins on the dorsal surface of the amphibian embryo (see Box 1A). This small region they called the organizer, as it seemed to be ultimately responsible for controlling the organization of a complete embryonic body; it is now known as the Spemann-Mangold organizer, or just the Spemann organizer.

Intercellular adhesion

Junctions formed between 2 cells via the transmembrane proteins in the extracellular space. Such junctions serve various functions, including: - mechanical coupling -cytoplasmic communication -and membrane compartmentalization

Convergent extension

Literally coming together and getting longer; the process by which a sheet of cells changes shape by extending in one direction and narrowing in a direction at right angles to the extension, caused by intercalating between one another

Invagination

Local inward deformation of a sheet of embryonic epithelial cells

Mesenchyme

Loose connective tissue, usually of mesodermal origin, whose cells are capable of migration

What cytoskeletal polymer is most strongly implicated in positioning the cleavage plane? And which one in carrying out cleavage?

MICROTUBULES form the spindles that segregate chromosomes at mitosis and meiosis. ACTIN (MICROFILAMENTS) contraction of the contractile ring, bundles of actomyosin arranged in a ring in the cell cortex, pinches animal cells in two at cell division

What are the cell activities that drive gastrulation?

Movements of individual cells and cell sheets at gastrulation bring most of the tissues of the blastula (or its equivalent stage) into their appropriate position in relation to the body plan. Gastrulation thus involves dramatic changes in the overall structure of the embryo, converting it into a complex three-dimensional structure a pro- gram of cell activity involving changes in cell shape and adhesiveness remodels the embryo, so that the endoderm and mesoderm move inside and only ectoderm remains on the outside. The primary force for gastrulation is provided by changes in cell shape, as well as the ability of cells to migrate (see Box 9B). In some embryos, all this remod- eling occurs with little or no accompanying increase in cell number or total cell mass. Gastrulation involves various cell movements and cell behaviors that make use of the adhesive and motile properties of embryonic cells discussed in previous sections. Gastrulation begins with an epithelial-to-mesenchymal transition, in which the most vegetal mesodermal cells leave the blastula epithelium and become motile and mesenchymal in form. They become detached from each other and from the hyaline layer, and migrate into the blastocoel as single cells that have lost both their epithelial apico- basal polarity and their cuboidal shape. The transition to primary mesenchyme cells and entry into the blasto- coel is foreshadowed by intense pulsatory activity on the inner face of these cells, while still part of the epithelium, and on occasion a small transitory infolding or invagination is seen in the surface of the blastula before migration begins properly. Cell internalization requires loss of cell-cell adhesion and is associated with repression of cadherin expres- sion, the removal of cadherin from the cell surface by endocytosis, and the disappearance of the α- and β-catenins that link cadherins to the cytoskeleton (see Box 9A).

apical and basal

One type of polarity is called apico-basal polarity, and is seen in simple epithelia, single-layered sheets of cells in which the two sides of the sheet (the apical and basal sides) are different, and which are building blocks of tissues and organs

Focal adhesions

Points of contact between the cell and the ECM; - from inside out, they contain -F-actin -signaling and scaffold protein -and integrin transmembrane ECM receptors (and extracellularly, the ECM ligand of the integrins)

Cell adhesion molecules -

Proteins that bind cells to eachotehr and to the ECM -main classes important in developmental bio: -cadherins -immunoglobulibn superfamily -integrins

Devise your own every-day (macroscopic) example of apparent redundancy and propose a situation to which this redundancy is robust.

Redundancy is when there are two or more ways of carrying out a particular process; if one fails for any reason another will still function. . It is like having two batteries in your car instead of just one. Apparent redundancy, on the other hand, where a given process can be specified by several different mechanisms, is probably one of the ways that embryos can achieve such precise and robust results. the mechanisms underlying the developmental processes withstand large changes ( Internal fluctuations include small changes in the concentrations of molecules, and also changes due, for example, to mutations in genes not directly linked to the development of the organ in question. External factors that could perturb development include temperature and environ- mental chemicals) ---> THEY ARE ROBUST MY EXAMPLE- can clean up dirt using a broom or a vacuum- if the power goes out you can still clean up using a broom

What microscopic detail gave definitive insight Gregor Mendel's proposed mechanisms of heredity?

Sea urchin eggs! sperm and egg fuse to form zygote--> The climax of this line of research was the demonstration, towards the end of the nineteenth century, that the chromosomes within the nucleus of the zygote are derived in equal numbers from the two parental nuclei,

Lamellipodia

Sheet-shaped cellular projections packed with and extended by F-actin, involved in translocation

Blastopore

Slit-like or circular invagination on the surface of amphibian and sea urchin embryos where mesoderm and endoderm move inside the embryo at gastrulation

How can different collections of cells move past each other as if they were liquid?

Surface tension is also what drives cells to cohere, again as a way to reduce their overall surface tension by reducing the amount of surface exposed. Adhesive interactions affect the surface tension at the cell membrane, reflected by flattening of the cell at point of contact with another cell or with the substratum. Different surface tensions are what keep two immiscible liquids such as water and oil separate, and embryologists had long noted the 'liquid-like' behavior of the tissues in gastrulating frog embryos, observ- ing how the separate germ layers formed and remained distinct from each other, slid over each other, spread out, and underwent internal rearrangement to shape the embryo. The differential adhesion hypothesis explains these liquid-like tissue dynamics as the consequences of surface tensions and interfacial tensions gener- ated by cells that tend to stick to each other but are also able to change shape and to move.

Draw for yourself a row of 5 adjacent rectangles, representing cells, and then 2 or more stages, with the top surface of the middle 3 cells getting shorter, while the other 3 sides of those rectangles stay the same or extend. You have just explained the mechanics of endoderm invagination!

The entry of the primary mesenchyme is followed by the invagination and extension of the endoderm to form the embryonic gut (the archenteron). The endoderm invagi- nates as a continuous sheet of cells (see Fig. 9.17). Formation of the gut occurs in two phases. During the initial phase, the endoderm invaginates to form a short, squat cyl- inder extending up to halfway across the blastocoel. There is then a short pause, after which extension continues. In this second phase, the cells at the tip of the invaginating gut, which will later detach as the secondary mesenchyme, form long filopodia, which make contact with the blastocoel wall. Filopodial extension and contraction pull the elongating gut across the blastocoel until it comes in contact with and fuses with the mouth region, which forms a small invagination on the ventral side of the embryo (see Fig. 9.17). During this process the number of invaginating cells doubles, and this is in part due to cells around the site of invagination contributing to the hindgut

Primitive streak

The feature on the chick and mouse embryo that tis the site of gastrulation and the forerunner of the anteroposterior axis; a strip of ingressing cells that extends into the epiblast front eh posterior margin. Epiblast cells move through the primitive streak into the the embryo interior to become the mesoderm and the endoderm

Mesoderm

The germ layer that gives rise to the skeleton-muscular system, connective tissues, blood, and internal organs such as the kidneys and heart

Mitosis

The nuclear division that occurs during the proliferation of somatic diploid cells and results in both daughter cells having the same diploid complement of chromosomes as the parent cell

Epiboly

The process during gastrulation in which the ectoderm extends to cover the whole of the embryo

Neurulation

The process in vertebrates in which the ectoderm of the future brain and spinal cord (the neural plate) develops folds (the neural folds) that come together to form the neural tube

Gastrulation

The process in which animal embryos' prospective endodermal and mesodermal cells move from the outer surface of the embryo to the inside, where they give rise to internal organs

What two cell behaviors or properties combine to sort out cells in vivo and in experimental settings?

The sorting out is the combined result of differences in adhesiveness and surface tension between cells of the different types,so that the overall energy of interfacial interaction within the cell mass is eventually minimized. Cells of the same type tend to adhere preferentially to each other. Initially, cells in the mixed aggregate start to exchange weaker for stronger adhesions. Over time, the adhesive interactions between cells produce different degrees of surface tension that are sufficient to generate the sorting-out behavior, just as two immiscible liquids, such as oil and water, separate out when mixed. As the constituent cells form stable contacts with each other, the tissues become reorganized and the strength of intercellular binding in the system as a whole is maximized. In general, if the adhesion between unlike cells is weaker than the average of the adhesions between like cells, the cells will segregate according to type, with the more adhesive cell type on the inside of the aggregate. those cells with the higher surface tension become surrounded by those with lower surface tension.

What two processes work in concert to expand the volume of the blastocoel?

Tight junction formation and ion transport At the eight-cell stage, tight junctions start to be formed between the polarized cells of the outer layer of the mammalian morula. By the 32-cell stage this barrier is fully formed. At the same time, the Na+/K+-ATPase sodium pump and other membrane transport proteins become active in the basolateral membranes of the outer cells, transporting sodium and other electrolytes into the extracellular space, which is con- tinuous with the blastocoel. As the ion concentration in the blastocoel fluid increases, water is drawn into the blastocoel by osmosis through aquaporin water channels in the cell membranes, and the increase in hydrostatic pressure caused by the accumu- lating fluid stretches the surrounding epithelial layer.

Mid-blastula transition -

Time during embryogenesis when the embryos own genes (zygotic genome) becomes transcribed, and cleavages become asynchronous and slower

Gene is to allele, as Toyota is to ________. Compare and contrast forward and reverse genetics.

Toyota odyssey Reverse genetics- technique for identifying developmental genes; know the existence and sequence of a gene (for example, from genomic sequences) and work backwards to determine its function in the animal, usually by removing the gene or blocking its function. - Examples of reverse genetics include gene knock-out, in which the gene is effectively deleted from the animal's genome by transgenic techniques (discussed in Section 3.10), and gene knockdown or gene silencing, in which gene expression is pre- vented by techniques such as RNA interference or antisense RNA Forward genetics-technique for identifying developmental genes, approach of determining the genetic basis responsible for a phenotype. This was initially done by using naturally occurring mutations or inducing mutants with radiation, chemicals, or insertional mutagenesis (e.g. transposable elements). Subsequent breeding takes place, mutant individuals are isolated, and then the gene is mapped. ****Forward Genetic Screen make a mutation, see what happens. Mutants are available for a large number of genes and new mutations can be induced very easily by exposing flies to radiation or mutagenic chemicals*****

Tight junction -

Type of adhesive junction that binds epithelial cells tightly to form an epithelium and that seals off the environment on one side of the epithelium from the other along the apico-basal axis

Cadherins -

a family of cell-cell adhesion proteins (that are important for development) that bind homotypically (pair)

Morphogenesis

a process that brings about shape changed in the embryo

Compaction -

a process the early mouse embryo undergoes during early cleavage, in which the blastomeres flatten against each other and become polarized (i.e. microvilli only on the external surfaces)

Septate junction -

a type of adhesive cell junction in invertebrates with similar function to tight junctions

What is the function of septate junctions? What is the analogous structure in chordates?

a type of cell junction found in inverte- brates but not in chordates. The septate junction is thought to provide the same type of permeability barrier as the vertebrate tight junction—sealing the epithelium and so preventing water, small molecules, and ions from permeating freely between the cells. The epithelial cells can then control, through their cell membranes, what enters and exits the blastula.

Tissue -

an aggregate of cells that have a similar structure and function

Epithelium -

animal tissue, made of one or more layers of cells, that covers an internal or external surface and often serves a barrier function.

Somatic cell

any cells other than the germ cells. In most animals, somatic cells are diploid.

List, define and distinguish the four main developmental processes- 3. Cell differentiation

cells become structurally and functionally different from each other, ending up as distinct cell types, such as blood, muscle, or skin cells. gradual process, cells often going through several divisions between the time at which they start differentiating and the time they are fully differentiated (when many cell types stop dividing altogether) In humans, the fertilized egg gives rise to hundreds of clearly distinguishable types of cell. Pattern formation and cell differentiation are very closely interrelated,

List, define and distinguish the four main developmental processes- 1. morphogenesis

change in form, At certain stages in development, there are characteristic and dramatic changes in form, of which gastrulation is the most striking. Almost all animal embryos undergo gastrulation, during which endoderm and mesoderm move inside, the gut is formed, and the main body plan emerges. . During gastrulation, cells on the outside of the embryo move inwards and, in animals such as the sea urchin, gastrulation transforms a hollow spherical blastula into a gastrula with a tube through the middle—the gut (Fig. 1.16). Morphogenesis in animal embryos can also involve extensive cell migration. Most of the cells of the human face, for example, are derived from cells that migrated from a tissue called the neural crest, which originates on the dorsal side of the embryo. Morphogenesis can also involve developmentally pro- grammed cell death, called apoptosis, which is responsible, for example, for the sepa- ration of our fingers and toes from an initially solid plate of tissue.

How do gastrulation-related cell behaviors relate to adult pathogenesis?

changes in cell shape, changes in cell adhesiveness, and cell migration all work together to cause a major change in embryonic form. The epithelial-to-mesenchymal transition is regulated in part by the sea-urchin version of the gene snail, which encodes a transcription factor and, like many other developmental genes, was first identified in Drosophila (see Section 2.16). experi- ments using antisense morpholinos (see Box 6B) to knock down snail expression in early sea-urchin embryos show that it is required for the repression of cadherin gene expression and for cadherin endocytosis in the epithelial-to-mesenchymal transition. The role of the Snail family of transcription factors in this type of cellular change is highly conserved across the animal kingdom, as we shall see in this chapter. The actions of Snail and a related transcription factor, Slug, are also of medical interest. A similar epithelial-to-mesenchymal transition takes place in cancer cells, enabling them to undergo metastasis and migrate from the original site of the tumor to set up secondary cancers elsewhere in the body. In these metastatic cells, the normal low expression of snail and slug has increased.

Diploid

describing a cell that contains two sets of homologous chromosomes, one from each parent, and thus two copies of each gene

Compare and contrast generative and descriptive programs.

descriptive program - such as a blueprint or a plan, describes an object in some detail e.g. DNA contains a full description of the organism to which it will give rise generative program- describes how to make an object. -instructions for making the organism that determines where and when different proteins are synthesized and thus controls how cells behave. E.g. origami descriptive=To describe in any detail the final form of the paper with the complex relationships between its parts is really very difficult, and not of much help in explaining how to achieve it. generative- instructions on how to fold the paper. much more useful and easier to formulate because simple instructions about folding have complex spatial consequences. Genetic info in fertilized egg= folding instructions in origami

Spemann organizer

dorsal lip of the blastopore

Compare and contrast lamellipodia and filopodia.

embryonic cells migrate over a solid substratum such as the ECM - they move by: Lamellipodia- extending a thin sheet-like layer of cytoplasm known as a lamellipodium or Filopodia- extend long fine cytoplasmic processes Both structures (temporary) are pushed outward by actin assembly - contraction of the actomyosin network at the front of the cell then draws the cell forward To do this, the contractile system must be able to exert force on the substratum- this occurs at focal contacts- points at which the lamellipodium or filopodia are anchored to the surface over which the cell is moving

What tenet of evolutionary theory is substantiated by the observation that developmental mechanisms are held in common by many different animals?

he similarity in developmental mechanisms and genes in such very different ani- mals as flies and vertebrates is the result of the evolutionary process. All animals have evolved from a single multicellular ancestor, and it is therefore inevitable that some developmental mechanisms will be held in common by many different species, whereas new ones have arisen in different animal groups as evolution proceeded. The basic Darwinian theory of evolution by *natural selection* is that changes in genes alter how an organism develops, and that such changes in development in turn determine how the adult will interact with their environment. If a developmental change gives rise to adults better adapted to survive and reproduce in the prevailing environment, the underlying genetic change will be selected to be retained in the population. Thus changes in development due to changes in the genes are fundamental to evolution.

Determinant

historical term for heritable factors that dictate structure and function Weismann put forward a model of development in which the nucleus of the zygote contained a number of special factors, or determinants -these nuclear determinants would be distributed unequally to the daughter cells and so would control the cells' future development. The fate of each cell was therefore predetermined in the egg by the factors it would receive during cleavage. This type of model was termed 'mosaic,' as the egg could be considered to be a mosaic of discrete localized determinants. early cell divisions must make the daughter cells quite different from each other as a result of unequal distribution of nuclear components.

Do cells always respond to a given signal in the same way?

how a cell responds to a particular signal depends on its internal state. This state reflects the cell's developmental history—cells have good memo- ries—and so different cells can respond to the same signal in very different way We shall see many examples of the same signals being used over and over again by different cells at different stages of embryonic development, with different biologi- cal outcomes.

induction

induction. This is where one cell, or tissue, directs the development of another, neighboring, cell or tissue.

Compare and contrast epithelial and mesenchymal cell characteristics.

mesenchymal- motile, lost both their epithelial apico- basal polarity and their cuboidal shape, single cells

Apoptosis -

programmed cell death; a type of cell death that occurs widely during development during which a cell is induced to undergo "suicide," which involves fragmentation of the DNA and shrinkage of the cell. Apoptotic cells are removed by the body's scavenger cells and, unlike necrosis, apoptotic cell death does not do damage to surrounding cells.

8.3 (really 9.3) What is the general rule of cleavages, whether they be radial, spiral, unequal, or even syncytial?

radial cleavage, the divisions occur at right angles to the egg surface and the first few cleavages produce tiers of blastomeres that sit directly over each other.- This type of cleavage is characteristic of the deuterostomes, such as sea urchins (see Chapter 6) and vertebrates spiral cleavage- successive divisions are at planes at slight angles to each other, producing a spiral arrangement of cells. -eggs of molluscs and annelids, which are protostomes unequal cleavage- results in one daughter cell being larger than the other. *General rule: Cleavage always occurs occurs at right angles to this axis of the mitotic spindle* ------The plane of cleavage in animal cells is determined by the orientation of the mitotic spindle, which is in turn determined by the final positioning of the asters.

Name at least two factors about cadherins that drive the specificity of intercellular adhesion.

studies in which cells with different cadherins on their surface are mixed together. Fibroblasts of the mouse L cell line do not express cadherins on their surface and do not adhere strongly to each other. But if the gene encoding E-cadherin is transfected into L cells and expressed, the cells produce that cadherin on their surface and stick together, forming a structure resembling a compact epithelium. The adherence is both calcium-dependent, indicat- ing that it is due to the cadherin, and specific, as the transfected cells do not adhere to untreated L cells lacking surface cadherins. When populations of L cells are transfected with different types of cadherin and mixed together in suspension, only those cells expressing the same cadherin adhere strongly to each other: cells expressing E-cadherin adhere strongly to other cells expressing E-cadherin, but only weakly to cells expressing P- or N-cadherin. The amount of cadherin on the cell surface can also have an effect on adhesion. When cells expressing different amounts of the same cadherin are mixed together, those cells with more cadherin on their surface end up on the inside of the reaggregate, surrounded by the cells with less surface cadherin (Fig. 9.3). Thus, quantitative dif- ferences in cell-adhesion molecules could maintain differential cell adhesion. The cytoplasmic domain of a cadherin associates with the actin filaments of the cytoskeleton by means of a protein complex containing α- and β-catenins (see Box 9A), and failure to make this association results in weak adhesion. That is, just as there is information flowing from outside the cell to the inside via the cadherin-catenin complex, there is also information flowing from the inside of the cell outwards. In early Xenopus blastulas, for example, E-cadherin is expressed in the ectoderm just before gastrulation and N-cadherin appears in the prospective neural plate. If E-cadherin lacking an extracellular domain is produced in the blastulas from injected mutated mRNA, the mutant cadherin will compete with the embryo's own intact cad- herin molecules for the cytoskeletal association sites. The defective cadherin cannot influence cell-cell adhesion because it has no extracellular domain, but it blocks the intact cadherin's access to the cytoskeleton and affects the adhesion between cells. The result is disruption of the ectoderm during gastrulation, indicating that a cadherin molecule must bind both to its partner on an adjacent cell and to the cytoskeleton of its own cell to create a stable adhesion. The initial binding of the extracellular cadherin domains transmits a signal to the cytoskeleton, which then stabilizes the interaction.

Germ cell -

the cells that give rise to the eggs and sperm

Extracellular matrix -

the collective term for proteins, proteoglycans and other secreted materials that often forms a fibrous meshwork

What is the difference in cadherin localization between the 2 and 8-cell stages of the mouse embryo?

the earliest sign of structural differentiation is at the eight- cell stage, when the morula undergoes a process known as compaction (Fig. 9.11). Until then, cells divide equally without a preferred orientation and the blastomeres form a loosely packed ball, with each cell surface uniformly covered by microvilli. At compaction, blastomeres flatten against each other, maximizing cell-cell contact, and cells become polarized, with an apico-basal polarity, defined by the presence of E-cad- herin-based adherens junctions on the lateral faces and by microvilli confined to the apical surface (Fig. 9.12- At the eight-cell stage the cells have relatively smooth surfaces and microvilli are distributed uniformly over the surface. At compaction, microvilli are confined to the outer surface and cells increase their area of contact with one another.)

Zygote

the fertilized egg. It is diploid and contains chromosomes from both the male and female parents

Blastocoel

the fluid-filled cavity that develops in the interior of a blastula.

Ectoderm

the germ layer that gives rise to the epidermis and nervous system

Endoderm

the germ layer that gives rise to the gut and associated organs such as lungs and liver in vertebrates

Mesoderm

the germ layer that gives rise to the skeleto-muscular system, connective tissues, blood, and internal organs including the kidney and heart

List, define and distinguish the four main developmental processes- 4)Growth

the increase in size. little growth during early embryonic development and the basic pattern and form of the embryo is laid down on a small scale, always less than a millimeter in extent. Subsequent growth can be brought about in various ways: cell multiplication, increase in cell size, and deposition of extracellular materials, such as, for example, in bone. Growth can also be morphogenetic, in that differences in growth rates between organs, or between parts of the body, can generate changes in the overall shape of the embryo

Embryogenesis

the process of development of the embryo from the fertilized egg

Meiosis

the reductive division of a diploid cell that gives rise to haploid cells.

Cleavage plane

the region of the cell designated to ingress into a furrow

What happens if you kill one cell of a two-cell stage embryo?

the remaining cell developed into a well-formed half-larva The outcome of Driesch's experiment on sea urchin embryos, which first demonstrated the phenomenon of regulation. After separation of cells at the two-cell stage, the remaining cell developed into a small, but whole, normal larva. This is the opposite of Roux's earlier finding that when one of the cells of a two-cell frog embryo is damaged, the remaining cell develops into a half-embryo only was the first clear dem- onstration of the developmental process known as regulation. The experiment of Roux on frogs was later repeated by the American T. H. Morgan, who separated the two blas- tomeres instead of killing one of them and leaving it attached, and he obtained the same result as Driesch with sea urchins. showed the general ability of vertebrate embryos to regulate, that is, to restore normal development, even if some portions are removed or rearranged very early in development.

Cleavage

the series of rapid cell division without growth that occurs after fertilization and divides the embryo into numerous small cells. Can refer to the specific stage of cell division when cell pinch in half (cytokinesis).

Name at least three advantages of "model organisms."

their developing embryos are easy to obtain easy to manipulate experimentally, even at quite late stages. great deal is known about their developmen- tal genetics and they can be easily genetically modified. once a certain amount of research has been done on one animal it is more efficient to continue to study it rather than start at the beginning again with another species— chick- fertile eggs are easily available and the embryo withstands experimental microsurgical manipulation very well. but not much known about devel genetics mouse- lots known about devel genetics must be in mom to devel however, possible to fertilize mouse eggs in culture, let them develop for a short time outside the uterus, when they can be observed, and then re-implant the early embryos for them to complete their development. -it is also amenable to genetic modification by transgenic techniques, by which genes can be introduced, deleted, or modified in living organisms. zebrafish- easy to breed in large numbers, the embryos are transparent and so cell divisions and tissue move- ments can be followed visually Having the complete DNA sequences of the genomes of our model organisms helps enormously in identifying developmental genes and other developmentally important DNA sequences.

What is the function of tight junctions?

tight junction, which forms a seal that prevents water and other molecules passing between the epithelial cells.

Morula -

very early stage mammalian embryo when cleavage divisions have resulted in a solid ball of cells

Polarized (as in a cell) -

when one region of a cell is different in some way from the other

How did cadherins get their name?

which depend on calcium for their function and are components of a type of adhesive junction called the adherens junction, which is the junction that holds cells together in all epithelial tissues.