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| Extended Lecture Outline |
Chapter 20: Developmental Biology I: Morphogenesis And The Control Of Growth |
A. EARLY EMBRYONIC DEVELOPMENT
20.1 Fertilization activates an egg and initiates development.
a. When a sperm contacts an egg, a series of chemical and cellular reactions begins the process of embryonic development.
b. Outside its plasma membrane, an egg has a vitelline envelope, which contains structures that control a species-specific binding of the proper sperm.
c. The egg is further surrounded by a thick layer of glycoprotein jelly that may attract or activate sperm.
d. Section 11.6 covers chemotaxis, a process that guides sperm toward eggs of the same species.
e. Figure 20.1 shows how the sperm and egg are changed upon contact with each other:
1. The sperm has an influx of calcium ions, causing a release of digestive enzymes from its tip.
2. These enzymes digest the egg jelly.
3. A protein, bindin, is released and matched with egg receptors that ensure species specific fertilization.
4. Upon recognition, the vitelline envelope lyses as the sperm tip is attached.
5. Sperm and egg plasma membranes fuse, forming a fertilization cone that engulfs the sperm nucleus.
6. Ionic changes in the egg are triggered (Section 11.8 covers signal-transduction pathways), and sperm and egg nuclei migrate toward each other.
7. The two nuclei are fused to form a diploid zygote nucleus, and protein synthesis and DNA replication begin right away, followed by mitosis and the first cleavage of the zygote.
f. Though several sperm migrate to each egg (Figure 20.2), there are mechanisms, including membrane potential changes (Section 41.9 covers nerve cell action potentials), to prevent polyspermy in most species.
g. The cortical granule reaction (Figure 20.3), is a visible change that advances across the egg's surface from the point of sperm penetration, causing the vitelline membrane to form a fertilization membrane, and preventing other sperm from penetrating the egg.
h. Some species' eggs (e.g. amphibians) can be activated artificially without a sperm, and some can develop parthenogenetically, but mammalian embryos generally cannot develop this way.
i. Concepts 20.1 covers general terms in the area of body development (e.g. tissues) and anatomy (e.g. posterior, dorsal, ventral).
20.2 Animal embryos are shaped by large-scale movements of cells.
a. Each embryo begins as a single cell, which quickly divides repeatedly to form a generally round cluster of cells.
b. These clusters are not perfectly round in plants and animals, where each has an axis of polarity, along which the embryo develops.
c. Figure 20.4 illustrates some of these changes and the establishment of the axis of polarity.
d. Frog embryos (Figure 20.5) have been extensively studied, and are used in this section of text as a model for vertebrate development.
e. The darkly pigmented animal hemisphere of the egg contains pigmented cortical cytoplasm.
f. The vegetal hemisphere contains light-colored yolky cytoplasm.
g. Sperm penetration causes the cortical cytoplasm to rotate about 30 degrees relative to the inner cytoplasm, and toward the point of sperm entry.
h. A gray crescent area is thus created opposite the site of sperm entry; this generates a dorsal-ventral axis on the embryo, where the gray crescent represents the posterior end.
i. Rapid cleavage of the zygote next forms a morula (ball shape) of blastomeres (smaller cells resulting from division without growth).
j. The morula then forms a hollow blastula of cells, enclosing a fluid-filled blastocoel.
k. The blastula undergoes gastrulation, a complex movement of cells in both animal and vegetal regions into the blastocoel, which results in an indentation (blastopore) in the blastula, and forming a gastrula.
l. The blastocoel is obliterated during gastrulation, and a new cavity, the archenteron, is formed by the indentation of the blastula wall, and opens to the outside of the gastrula through the blastopore.
m. The archenteron represents the beginnings of the animal's digestive tract (gut).
n. During gastrulation, cells begin to be organized generally into one of three tissue germ layers: ectoderm, mesoderm, and endoderm.
o. Each of these layers develops into distinct sets of differentiated tissues (Figure 20.6).
1. Ectoderm, on the outside of the gastrula, generally forms skin and nervous system tissue.
2. Mesoderm, in between ectoderm and endoderm, generally forms bones, muscles, and parts of major organs such as kidneys and the heart.
3. Endoderm, on the inside of the gastrula (lining the archenteron), generally forms the gut lining, and parts of the liver, lungs, and pancreas.
p. The embryo is now characterized by two sets of very different cell types (Figure 20.7).
1. Epithelial cells form sheets or tubes, and adhere closely to one another.
2. Mesenchymal cells are loosely organized and act somewhat independently.
20.3 Reptile, bird, and mammal embryos develop extraembryonic membrane systems.
a. Embryos require nutrients, water, and a waste disposal system.
b. Aquatic embryos can easily exchange materials with their aqueous environment.
c. Terrestrial embryos evolved the amniote egg (Figure 20.9) as an adaptation to development on land.
d. Several extraembryonic membranes, made of various cell tissue layers, evolved:
1. The amnion surrounds the fluid-filled amniotic cavity in which the embryo grows.
2. The chorion allows for oxygen and CO2 gas exchange.
3. The allantoic membrane encloses the allantois, which forms a sac for nitrogenous waste disposal.
e. Aquatic species (e.g. sea urchin and frog) are holoblastic, with the entire fertilized egg dividing into blastomeres.
f. Adaptation to terrestrial growth required considerable modification of the embryo.
1. Reptile and bird embryos are meroblastic, with only a small part of the egg dividing into blastomeres, and forming a blastodisc, which sits on top of and is nourished by the much larger egg yolk.
2. The blastodisc splits into two layers: the epiblast above and hypoblast below (Figure 20.10).
3. The primitive streak, a linear invagination, forms down the middle of the epiblast before gastrulation occurs by epiblast cells migrating into this streak from both sides.
4. Cells forming endoderm will displace the hypoblast cells, and the remaining epiblast cells will form ectoderm; other cells between these two layers form the mesoderm.
5. The extraembryonic membranes occupy much of the egg (Figure 20.9).
6. The chorioallantoic membrane is rich in blood vessels and is the gas exchange membrane.
g. Mammalian development (Figure 20.11) is a modification of the bird-reptile system.
1. The zygote forms a ball-shaped blastocyst, with an inner cell mass forming the embryo and an outer trophoblast forming the chorion.
2. Division of the inner cell mass is followed by formation of a primitive streak and gastrulation, as it does in reptiles and birds.
3. Mammalian eggs have little yolk; the chorion and allantois combine with uterine wall tissues to form the placenta, through which the embryo is nourished.
4. Section 51.13 covers human development in further detail.
B. FACTORS THAT REGULATE MORPHOGENESIS AND GROWTH
20.4 Patterns of cell division determine the form of many tissues.
a. A major question in development concerns the forces that mold the embryo properly.
b. Two factors: cell division in different planes (this section) and cell adhesion with different strengths (Section 20.5) are involved in shaping the embryo.
c. Controlling the axis of cell division, as tissues grow, can result in filaments (with axes oriented in the same direction, Figure 20.12), sheets of cells (with cells dividing in one plane), or solid shapes (with cells constantly changing their axis orientation).
d. Figure 20.13 shows typical shapes attained by plants, algae, and bacteria, which are all constrained by morphologies that control the axis of cell division during development.
e. Animal cells are fairly free to move about and arrange themselves into different shapes.
20.5 Differential adhesion can determine the arrangement of some tissues.
a. Cells move and form structures due to properties on their surfaces and the way surface molecules (Figure 20.14) adhere to one another.
b. Henry Wilson did sponge reaggregation experiments in 1907 and showed that cells have recognition of each other and can reform a functioning sponge after dissociation.
c. Aron Moscona performed comparable experiments with chick embryos and got similar results.
d. Malcom Steinberg proposed that some organs form properly simply due to cells adhering to one another with different strengths.
e. Steinberg's proposal is born out by models of embryonic tissues (Figure 20.15) composed of three types of cells with different adhesion forces.
f. Cell adhesion molecules (CAMs) are transmembrane glycoproteins that are classified as either cadherins (which require calcium ions) or immunoglobulin CAMs (which do not require calcium ions).
g. These CAMs (Figure 20.16) have extracellular domains that bind specifically to other CAMs, and they are anchored in the cytoplasm to actin filaments (Chapter 11), all resulting in a matrix that can transmit forces across cells.
h. The binding strength of CAMs can be modified (Figure 20.17) chemically using sialic acid.
i. Each cell type may have a distinct set of recognition proteins.
20.6 Cells bind to an extracellular matrix and migrate on it.
a. Animal and plant tissues are composed of more than just cells (Section 32.8) and their junctional proteins (Section 6.11).
b. An extracellular matrix of proteins and polysaccharides is secreted by animal cells.
c. This matrix may constitute the majority of the tissue (e.g. in cartilage), or it may only be a thin basement membrane (e.g. in epithelia).
d. The animal cell matrix is complex:
1. it is largely a network of collagen fibrils,
2. it is supplemented by proteoglycans (Figure 20.18),
3. adhesive proteins are on top of these fibers,
4. integrins attach tissue cells to the adhesive proteins.
e. Integrins are a family of plasma membrane proteins that attach specifically to the cell matrix proteins.
f. They are named for their integrating of intracellular and extracellular connections (Figure 20.18).
g. Cells lose their connections during mitosis and assume a more spherical shape, but are otherwise loosely anchored into their tissue.
h. Cellular migrations determine embryogenesis, and the migrations must be controlled:
1. chemotaxis is one control method,
2. some cells migrate along protein gradients,
3. some cells move along a base of fibronectin, to which their integrins remain bound.
20.7 Contact inhibition normally restricts the division and movement of cells.
a. Contact inhibition represses cell division and movement when cells are in contact.
b. Animal cells in tissue culture extend a lamellipodium at their forward edge and two cells stop moving when their lamellipodia touch one another.
c. Contact inhibition can be regulated.
d. Cell contact may inhibit mitosis, and contact inhibition keeps the reproduction of normal cells to the proper limits in tissues and organs, rather than allowing unchecked growth.
e. Embryonic cells display no contact inhibition on growth, as cells may increase in size without dividing.
f. Tumor cells display abnormal contact inhibition mechanisms.
g. A wart is a type of tumor caused by a virus that alters a cell's reactions to inhibitory signals from other cells and allows abnormal growth.
20.8 Specific proteins regulate the growth of tissues.
a. The cell cycle (Chapter 13) is regulated by growth factors.
b. Chapter 39 addresses plant growth factors.
c. Animals have several types of factors:
1. nerve growth factor (NGF) stimulates some nerve cells,
2. a brain-derived growth factor stimulates other nerve cells,
3. an epidermal growth factor (EGF) stimulates epidermal cells and some others.
d. Some factors stimulate specialized cells:
1. cytokines are proteins that control immune system cells,
2. EGF is an example of a mitogen, which controls growth and mitosis.
e. Proto-oncogenes (Section 20.15) are those which, when mutated, promote the unregulated cell growth of cancer.
20.9 Sheets of tissue may be shaped by several factors.
a. Besides adhesion, cell elongation and contraction, differential cell division, and deposition of fibrous material are all involved in morphogenesis.
b. Cell elongation is involved in shaping the neural tube (Figure 20.19), as cells in the center elongate via their lengthwise microtubules and contract on one end via their actin filaments, forming a wedge shape.
c. Section 6.11 covers cytoskeleton linkages.
d. Sheets of epithelium are shaped into tubes in pancreas, liver, and other endoderm-derived organs, by cell-linkage mechanisms.
e. Figure 20.20 shows how these tubes become folded and branched as they mature.
f. Other factors affect branching and folding: mitosis rates, and clefts formed between branches of tubules, where extracellular matrix is laid down.
g. The largest remaining question about morphogenesis is not the mechanics of the known forces affecting cell change, but why these forces are set in motion at various times during development.
C. CANCER AND THE REGULATION OF GROWTH
20.10 Cancer cells escape restraints on reproduction and invade areas of normal tissue.
a. A tumor or neoplasm results from uncontrolled cell reproduction.
b. Benign tumors are composed of undifferentiated cells, grow in place, and cause relatively little damage except for when they obstruct the body mechanically.
c. Malignant tumors (cancers, Figure 20.21) can invade other tissues, become dedifferentiated, change their surface molecules, and lose their contact inhibition.
d. Cells can break away from their parent cells and migrate through the body in a process called metastasis, which accounts for the difficulty of fighting most cancers.
e. Carcinomas are cancers that grow from epithelial cells (e.g. skin, mouth, stomach, and colon) and account for about 80% of all human cancers.
f. Sarcomas are cancers of the muscle or connective tissue.
g. Leukemias are cancers of the blood cells.
20.11 Cancer, like evolution, involves natural selection.
a. Cancer can be better understood by applying the principles of natural selection at the cellular level, and seeing that cancer cells compete with and are eventually selected over normal cells as a cancer progresses.
b. The same factors applying to organisms in a population can be applied to cells in an organism: the number of cells involved, the rate at which the cells reproduce, and the selective advantage that mutants have over normal cells.
20.12 Cancers are derived from single cells through DNA modifications.
a. Cancers are monoclonal: derived from a single altered cell that proliferates uncontrollably.
b. Evidence for this conclusion includes the fact that:
1. all the cancer cells in a heterozygous woman have the same X chromosome inactivated (Section 19.7), thus they probably all came from the same cell,
2. all the myelogenous leukemia cells in a patient have the same gene translocation breakage site, and one which differs from the translocation sites of other patients.
c. The initial, or "founder," cell of a cancer arises from alteration of its DNA.
d. Most cancers arise from mutation of somatic cells.
e. Carcinogens are cancer-causing agents that interact directly with DNA or are converted to forms that interact with DNA.
f. Various chemicals, many substances (such as tar and soot), and radiations such as X-rays and UV light are either mutagens, carcinogens, or both.
g. Methods 20.1 covers the Ames test for screening compounds for their mutagenicity.
20.13 Several independent events are needed to change a normal cell into a fully cancerous cell.
a. Carcinogenesis is a progression of several different events occurring over time.
b. Older people are more likely to have cancer than are young people (Figure 20.22), further evidence that several events, which are more likely to have occurred in older people, are necessary for cancer to develop.
c. Intermediate stages between a cancer-free state and a cancerous state can often be identified; the Pap smear test for cervical cancer (Figure 20.23) is done to search for intermediate stages.
20.14 Both tumor initiators and tumor promoters contribute to the transition from normal cells to malignant ones.
a. Tumor initiators (e.g. tobacco smoke) are agents that alter DNA and start cells down the path toward malignancy.
b. Tumor promoters enhance tumor formation, generally by increasing proliferation of cancerous cells.
c. Increased levels of promoters result in faster appearance of cancer.
20.15 Some viruses can transform normal cells into tumor-producing cells.
a. In the early 1900s, Peyton Rous developed techniques for identifying cancer viruses such as the Rous sarcoma virus (RSV) in chickens, and he received the Nobel Prize for Medicine in 1966 for his work.
b. RSV is a retrovirus (Figure 18.34) that is copied into DNA and incorporated into a host's chromosome.
c. Cells infected with a retrovirus are transformed, as they change shape, lose contact inhibition, and grow beyond a single layer.
d. Viruses that are able to transform cells into tumors are tumor viruses, which incorporate tumor-promoting genes called oncogenes.
e. A proto-oncogene is a DNA region in a normal cell that closely resembles its modified counterpart, the oncogene, in the virus; all known oncogenes have proto-oncogene counterparts in vertebrate cells.
f. A virus that incorporates a proto-oncogene, which then later mutates into an oncogene, becomes a tumor virus that can introduce the oncogene into a normal cell.
20.16 Oncogenes are derived from cellular proto-oncogenes whose products are components of signal-transduction pathways.
a. Most proto-oncogenes encode cell communication proteins (Figure 20.24).
b. A corresponding oncogene interferes with a step in the cell-signaling pathway and creates an "outlaw" cell and uncontrolled cell growth.
c. Section 11.8 covers signal-transduction pathways.
d. Either growth factor genes or growth factor receptor genes can become oncogenes.
e. Figure 20.25 illustrates the erbB oncogene, which encodes an abnormal receptor, and is associated with breast, ovarian, and stomach cancers.
f. Tumor suppressor genes encode proteins that stop unregulated cell growth.
g. One suppressor gene is p53, which binds to a complex needed to pass the cell cycle S phase, thus preventing unchecked cell division.
h. Absence of p53 is linked to many cancers.
i. Though only one copy of a suppressor gene is needed to prevent cancer development, heterozygous individuals present a higher cancer risk to their offspring, as only one mutation is needed to lose the suppressor.
j. Thus, the tendency to develop cancer can be inherited.
20.17 Environmental factors cause most cancers.
a. Table 20.1 shows various cancer rates in several countries.
b. Though some countries have lower rates of some cancers, they have higher rates of others, and the total rates are about equal for all countries.
c. Genetic predisposition is considered to be a relatively minor factor leading to cancer development, compared with environmental factors such as smoke, air pollutants, industrial pollutants, and dietary factors.
d. The frequency of cancer in the U.S. is rising, due mainly to increasing pollution and an aging population.
20.18 More than one mutation is necessary to cause cancer in an animal, yet one oncogene can transform a normal cell.
a. In an individual organism, several mutations are necessary for cancer to develop, due to the complex interactions of cells and regulatory systems.
b. Oncogenes have been shown to arise through just one mutation in a proto-oncogene, mainly because they are studied in tissue culture, where the complexities of the organism's systems are not present.
c. Maintaining good immunity (Chapter 41) is just one of many healthy practices linked to cancer prevention.
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