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Chapter 42 Animal Development Sections 1-6

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1 Chapter 42 Animal Development Sections 1-6

2 42.1 Mind-Boggling Births In vitro fertilization (IVF) is an assisted reproductive technology which combines egg and sperm outside the body Prior to IVF a woman is given hormones to encourage maturation of multiple eggs and to prevent natural ovulation Each zygote undergoes mitotic divisions, forming a blastocyst that is placed in a woman’s womb to develop to term Louise Brown, born in 1978, was the first “test tube baby” conceived by IVF

3 Louise Brown with Husband and Son
Figure 42.1 Results of natural and assisted fertilization. A. Louise Brown, the first child conceived by IVF, with her husband and her son, who was conceived naturally.

4 Nadya Suleman with IVF Octuplets
Figure 42.1 Results of natural and assisted fertilization. B. Nadya Suleman with her octuplets conceived by IVF.

5 42.2 Stages of Reproduction and Development
Animals as different as sea stars and sea otters pass through the same stages in their developmental journey from a single, fertilized egg to a multicelled adult In all sexually reproducing animals, a new individual begins life as a zygote, the diploid cell that forms at fertilization

6 an egg, the egg and sperm nuclei fuse, and a zygote forms.
Sperm penetrates an egg, the egg and sperm nuclei fuse, and a zygote forms. Fertilization Mitotic cell divisions yield a ball of cells, a blastula. Each cell gets a different bit of the egg cytoplasm. Cleavage Cell rearrangements and migrations form a gastrula, an early embryo that has primary tissue layers. Gastrulation Organs form as the result of tissue interactions that cause cells to move, change shape, and commit suicide. Figure 42.2 Stages of development in vertebrates. Fertilization was described in Section 41.8. Organ Formation Organs grow in size, take on mature form, and gradually assume specialized functions. Growth, Tissue Specialization Figure 42-2 p752 6

7 Processes of Development
Fertilization Egg and sperm join to form a zygote Cleavage (blastula formation) Repeated mitotic divisions increase the number of cells (blastomeres), not the volume Gastrulation Gastrula (early embryo) forms with two or three germ layers (forerunners of tissues and organs)

8 Processes of Development
Organ formation The neural tube and notochord characteristic of all chordate embryos form early Growth and tissue specialization Many organs incorporate tissues derived from more than one germ layer In some animals, a larva undergoes metamorphosis – a drastic remodeling of tissues into the adult form

9 Three Primary Germ Layers
Outermost layer (ectoderm) gives rise to nervous tissue and to the outer layer of skin Middle layer (mesoderm) gives rise to muscles, connective tissues, and the circulatory system. Inner layer (endoderm) gives rise to the respiratory tract and gut linings

10 Table 42-1 p752

11 Life Cycle: Leopard Frog
Figure 42.3 Animated Above, overview of reproduction and development in the leopard frog. Opposite page, a closer look at what occurs during each stage.

12 gray crescent Figure 42.3 Animated Above, overview of reproduction and development in the leopard frog. Opposite page, a closer look at what occurs during each stage. 1 Here we show the first three divisions of cleavage, a process that carves up a zygote’s cytoplasm. In this species, cleavage results in a blastula, a ball of cells with a fluid-filled cavity. Figure 42-3b p753 12

13 blastocoel Figure 42.3 Animated Above, overview of reproduction and development in the leopard frog. Opposite page, a closer look at what occurs during each stage. 2 Cleavage is over when the blastula forms. blastula Figure 42-3b p753 13

14 ectoderm ectoderm yolk plug neural plate dorsal lip mesoderm
future gut cavity endoderm Figure 42.3 Animated Above, overview of reproduction and development in the leopard frog. Opposite page, a closer look at what occurs during each stage. 3 The blastula becomes a three-layered gastrula—a process called gastrulation. At the dorsal lip (a fold of ectoderm above the first opening that appears in the blastula), cells migrate inward and start rearranging themselves. Figure 42-3b p753 14

15 neural tube notochord gut cavity
Figure 42.3 Animated Above, overview of reproduction and development in the leopard frog. Opposite page, a closer look at what occurs during each stage. 4 Organs begin to form as a primitive gut cavity opens up. A neural tube, then a notochord and other organs, form from the primary tissue layers. gut cavity Figure 42-3b p753 15

16 Figure 42.3 Animated Above, overview of reproduction and development in the leopard frog. Opposite page, a closer look at what occurs during each stage. 5 The frog’s body form changes as it grows and its tissues specialize. The embryo becomes a tadpole, which metamorphoses into an adult. Tadpole, a swimming larva with segmented muscles and a notochord extending into a tail. Figure 42-3b p753 16

17 Figure 42.3 Animated Above, overview of reproduction and development in the leopard frog. Opposite page, a closer look at what occurs during each stage. 5 The frog’s body form changes as it grows and its tissues specialize. The embryo becomes a tadpole, which metamorphoses into an adult. Limbs grow and the tail is absorbed during metamorphosis to the adult form. Figure 42-3b p753 17

18 Figure 42.3 Animated Above, overview of reproduction and development in the leopard frog. Opposite page, a closer look at what occurs during each stage. 5 The frog’s body form changes as it grows and its tissues specialize. The embryo becomes a tadpole, which metamorphoses into an adult. Sexually mature, four-legged adult leopard frog. Figure 42-3b p753 18

19 ANIMATED FIGURE: Leopard frog life cycle
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20 Take-Home Message: How does an adult vertebrate develop from a zygote?
A zygote undergoes cleavage, which increases the number of cells. Cleavage ends with formation of a blastula. Rearrangement of blastula cells forms a three-layered gastrula. After gastrulation, organs such as the nerve cord begin forming. Continued growth and tissue specialization produce the adult body.

21 ANIMATION: Early frog development
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22 42.3 From Zygote to Gastrula
Localization of yolk and other material in the egg cytoplasm and specific cleavage patterns affect early development Cytoplasmic localization Many cytoplasmic components in an unfertilized egg, are localized in specific parts of the cytoplasm Hans Spemann showed that substances essential to amphibian development are localized in the gray crescent

23 animal pole pigmented cortex yolk-rich cytoplasm vegetal pole
sperm penetrating egg Figure 42.4 Animated Experimental evidence of cytoplasmic localization in an amphibian oocyte. A. Many amphibian eggs have a dark pigment concentrated in cytoplasm near the animal pole. At fertilization, the cytoplasm shifts, and exposes a gray crescent-shaped region just opposite the sperm’s entry point. The first cleavage normally distributes half of the gray crescent to each descendant cell. gray crescent fertilized egg Figure 42-4a p754 23

24 gray crescent of salamander zygote
First cleavage plane; gray crescent split equally. The blastomeres are separated experimentally. Figure 42.4 Animated Experimental evidence of cytoplasmic localization in an amphibian oocyte. B. In one experiment, the first two cells formed by normal cleavage were physically separated from each other. Each cell developed into a normal larva. Two normal larvae develop from the two blastomeres. Figure 42-4b p754 24

25 gray crescent of salamander zygote
First cleavage plane; gray crescent missed entirely. The blastomeres are separated experimentally. Figure 42.4 Animated Experimental evidence of cytoplasmic localization in an amphibian oocyte. C. In another experiment, a zygote was manipulated so one descendant cell received all the gray crescent. This cell developed normally. The other gave rise to an undifferentiated ball of cells. A ball of undifferentiated cells forms. Only one normal larva develops. Figure 42-4c p754 25

26 ANIMATED FIGURE: Cytoplasmic localization
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27 Cleavage –The Start of Multicellularity
Cleavage divides a fertilized egg into a number of small cells but does not increase its original volume Cleavage puts different parts of the egg cytoplasm into different cells (blastomeres) which will make them behave differently later in development Each species has a characteristic cleavage pattern

28 Main Cleavage Patterns
Protostomes (bilateral invertebrates) undergo spiral cleavage Dorsal lip from gastrulation forms the mouth Most deuterostomes (echinoderms and vertebrates) undergo radial cleavage Dorsal lip from gastrulation forms the anus Mammals undergo rotational cleavage

29 Gastrulation Starting with gastrulation, cells migrate about and rearrange themselves Experiments by Hilde Mangold showed how gastrulation is regulated in vertebrates Transplanted cells of the dorsal lip of the blastula (descended from the zygote’s gray crescent) induced gastrulation in salamanders

30 Gastrulation in a Fruit Fly
Figure 42.5 Gastrulation in a fruit fly (Drosophila). In fruit flies, cleavage is restricted to the outermost region of cytoplasm; the interior is filled with yolk. The series of photographs, all cross-sections, shows sixteen cells (stained gold) migrating inward. The opening the cells move in through will become the fly’s mouth. Descendants of the stained cells will form mesoderm. Movements shown in these photos occur during a period of less than 20 minutes.

31 Figure 42.6 Animated Experimental evidence that signals from dorsal lip cells initiate amphibian gastrulation. A dorsal lip region of a salamander embryo was transplanted to a different site in another embryo. A second set of body parts started to form. A. Dorsal lip excised from donor embryo,grafted to novel site in another embryo A Dorsal lip excised from donor embryo, grafted to novel site in another embryo. Figure 42-6a p755 31

32 B Graft induces a second site of inward migration.
Figure 42.6 Animated Experimental evidence that signals from dorsal lip cells initiate amphibian gastrulation. A dorsal lip region of a salamander embryo was transplanted to a different site in another embryo. A second set of body parts started to form. B. Graft induces a second site of inward migration. B Graft induces a second site of inward migration. Figure 42-6b p755 32

33 C The embryo develops into a “double” larva, with two heads, two tails, and two bodies joined at the belly. Figure 42.6 Animated Experimental evidence that signals from dorsal lip cells initiate amphibian gastrulation. A dorsal lip region of a salamander embryo was transplanted to a different site in another embryo. A second set of body parts started to form. C. The embryo develops into a“double” larva,with two heads,two tails, and two bodies joined at the belly. Figure 42-6c p755 33

34 ANIMATED FIGURE: Embryonic induction
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35 Take-Home Message: What are the effects of cytoplasmic localization and cleavage?
Enzymes, mRNAs, yolk, and other materials are localized in specific parts of the cytoplasm of unfertilized eggs. This cytoplasmic localization establishes polarity (an orientation of the fertilized cell) in the egg and thus influences early development. Cleavage divides a fertilized egg into a number of small cells but does not increase its original volume. The cells—blastomeres—inherit different parcels of cytoplasm that make them behave differently, starting at gastrulation.

36 42.4 How Specialized Tissues and Organs Form
After gastrulation, cells become specialized as their movement and interaction begin to shape tissues and organs Cell differentiation Process by which cell lineages become specialized Lays the groundwork for formation of specialized tissues and organs Based on selective gene expression Signaling molecules contribute to differentiation (cell signaling)

37 Responses to Morphogens
Signaling molecules encoded by master genes Diffuse from a source and form a concentration gradient throughout the embryo Have different effects depending on their concentration in each region

38 A Morphogen Bicoid protein of fruit flies is an example of a morphogen
The bicoid gene is a maternal effect gene expressed during egg production – its product influences development Bicoid mRNA accumulates at one end of the egg—an example of cytoplasmic localization A gradient of bicoid protein (a transcription factor) determines the front-to-back axis of the zygote

39 Embryonic Induction Gastrulation occurs when certain cells of the blastula make and release short-range signals that cause nearby cells to move about, either singly or as a cohesive group By the process of embryonic induction, cells of one embryonic tissue alter the behavior of cells in an adjacent tissue Example: Cells of a salamander gastrula’s dorsal lip induce adjacent cells to migrate inward and become mesoderm

40 Organ Formation After gastrulation, vertebrate organ formation begins with the neural tube Neural tube development is induced by signals from the notochord, which formed earlier from mesoderm Development begins when ectodermal cells overlying the notochord elongate, forming a thick neural plate

41 neural plate (ectoderm)
notochord (mesoderm) neural groove Figure 42.7 Animated Neural tube formation in a vertebrate embryo. 1.Chemical signals produced by notochord mesoderm induce the ectoderm above it to thicken and form a neural plate. 2.Changes in cell shape cause edges of the neural plate to fold in toward the plate center, forming a neural groove. 3.Further folding causes the edges of the neural plate to meet, forming the neural tube. neural tube Figure 42-7 p756 41

42 ANIMATED FIGURE: Neural tube formation
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43 Cell Migrations Cell migrations are an essential part of development
Cells travel by inching along in an amoeba-like fashion Actin microfilaments cause a portion of the cell to protrude forward, and adhesion proteins anchor it Cells may move in response to a concentration gradient of some chemical signal or it may follow a “trail” of molecules that its adhesion proteins recognize

44 How Cells Migrate Figure 42.8 How cells migrate. A cell extends its advancing edge and attaches to the underlying substrate (a cell or the extracellular matrix) by way of adhesion proteins. It then breaks the connection between its trailing edge and the substrate, and contracts, pulling its posterior portion forward.

45 Apoptosis Programmed cell death (apoptosis) helps shape body parts
An internal or external signal sets reactions in motion that result in the activation of self-destructive enzymes Example: Apoptosis eliminates the webbing between digits of a developing human hand

46 Apoptosis Figure 42.9 Apoptosis in the formation of a human hand. Left, forty-eight days after fertilization, tissue webs connect embryonic digits. Right, three days later, after apoptosis by cells in those webs, the digits have become fully separated.

47 Take-Home Message: What processes differentiate cells, tissues, and organs?
All cells in an embryo have the same genes, but they express different subsets of the genome. Selective gene expression is the basis of cell differentiation. It results in cell lineages with characteristic structures and functions. Cytoplasmic localization results in concentration gradients of signaling proteins called morphogens. Morphogens activate sets of master genes, the products of which cause embryonic cells to form tissues and organs in specific places. Migration, shape changes, and death of cells shape developing organs.

48 42.5 An Evolutionary View of Development
Similarities in developmental pathways among animals are evidence of common ancestry Cytoplasmic localization in the egg induces expression of localized master genes Concentration gradients of master gene products cause embryonic cells to form tissues and organs at certain locations

49 A General Model for Animal Development
Where and when particular genes are expressed determines how an animal body develops Positional information established by concentration gradients of master gene products affects expression of homeotic genes, which regulate development of specific body parts

50 Developmental Constraints and Modifications
Physical constraints Surface-to-volume ratio Architectural constraints Existing body frameworks, such as four limbs Phylogenetic constraints Master genes determine basic body form

51 Lethal Murtations Mutations that alter effects of master genes are often lethal Example: Development of somites Mesoderm on either side of the neural tube divides into blocks of cells that will develop into bones and muscles A complex pathway involving many genes governs somite formation – any mutation that disrupts this pathway so that somites do not form is lethal during development

52 Lethal Mutation Affecting Somites
Figure Lethal effect of a mutation in a zebrafish gene (fused somites) that functions in early development.

53 Take-Home Message: Why are developmental processes similar among animal groups?
In all animals, cytoplasmic localization affects expression of sets of master genes shared by most animal groups. The products of these genes cause embryonic cells to form tissues and organs at certain locations. Once a developmental pathway evolves, drastic changes to genes that govern this pathway are generally lethal.

54 42.6 Overview of Human Development
Humans begin life as a single cell and go through a series of prenatal developmental stages Second week: Blastocyst is embedded in the mother’s uterus, where it develops Embryonic period (first 8 weeks): All organs form Fetal period (9 weeks to birth): Organs grow and specialize Postnatal growth (after birth): Organ growth and maturation continues through adolescence to adulthood

55 Table 42-2 p759

56 Prenatal and Postnatal Changes
Figure Observable, proportional changes in prenatal and postnatal periods of human development. Changes in overall physical appearance are slow but noticeable until the teens. 8-week embryo 12-week embryo newborn 2 years 5 years 13 years (puberty) 22 years

57 Take-Home Message: How does human development proceed?
Humans are placental mammals, so offspring develop in the mother’s uterus. By the end of the second week, the blastocyst is embedded in the uterus. By the end of the eighth week, the embryo has all typical human organs. Most of a pregnancy is taken up with the fetal period, during which organs grow and begin to function.


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