Chapter 38 Reproduction and Development (Sections 38.9 - 38.11)

38.9 Overview of Animal Development All sexually reproducing animals begin life as a zygote, the diploid cell that forms at fertilization The same development steps and processes occur in all vertebrates – evidence of their common ancestry

5 Stages of Vertebrate Development Fertilization Sperm penetrates an egg, the egg and sperm nuclei fuse, and a zygote forms Cleavage Mitotic cell divisions yield a ball of cells (blastula); each cell gets a different bit of the egg cytoplasm Gastrulation Cell rearrangements and migrations form a gastrula, an early embryo that has primary tissue layers

5 Stages of Vertebrate Development Organ formation Organs form as the result of tissue interactions that cause cells to move, change shape, and commit suicide Growth and tissue specialization Organs grow in size, take on mature form, and gradually assume specialized functions

Overview: Frog Development

external fertilization) transformation to adult nearly complete adult, three years old Sexual reproduction (gamete formation, external fertilization) Figure 38.13 Above, overview of reproduction and development in the leopard frog. Opposite, a closer look at some stages. organ formation tadpole cleavage eggs and sperm larva (tadpole) zygote Fig. 38.13, p. 642

Overview: Frog Development adult, three years old transformation to adult nearly complete Sexual reproduction (gamete formation, external fertilization) eggs and sperm tadpole larva (tadpole) Figure 38.13 Above, overview of reproduction and development in the leopard frog. Opposite, a closer look at some stages. organ formation cleavage zygote Stepped Art Fig. 38.13, p. 642

Details of Frog Development (1) Cleavage divides a zygote’s cytoplasm into smaller blastomeres Number of cells increases, but the zygote’s original volume remains unchanged cleavage Mitotic division of an animal cell

Details of Frog Development (1)

Details of Frog Development (1) gray crescent Figure 38.13 Above, overview of reproduction and development in the leopard frog. Opposite, a closer look at some stages. 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. 1 Fig. 38.13.1, p. 643

Details of Frog Development (2) In this species, cleavage results in a blastula, a ball of cells with a fluid-filled cavity (blastocoel) Tight junctions hold cells of the blastula together blastula Hollow ball of cells that forms as a result of cleavage

Details of Frog Development (2)

Details of Frog Development (2) blastocoel Figure 38.13 Above, overview of reproduction and development in the leopard frog. Opposite, a closer look at some stages. blastula Cleavage is over when the blastula forms. 2 Fig. 38.13.2, p. 643

Details of Frog Development (3) The blastula becomes a three-layered gastrula by the process of gastrulation: Cells at the dorsal lip migrate inward and start rearranging themselves gastrula Three-layered developmental stage formed by gastrulation gastrulation Cell movements that produce a three-layered gastrula

Germ Layers A gastrula consists of three primary tissue layers (germ layers) Three germ layers give rise to the same types of tissues and organs in all vertebrates – evidence of a shared ancestry germ layer One of three primary layers in an early embryo

Three Embryonic Germ Layers ectoderm Outermost tissue layer of an animal embryo endoderm Innermost tissue layer of an animal embryo mesoderm Middle tissue layer of a three-layered animal embryo

Details of Frog Development (3)

Details of Frog Development (3) ectoderm ectoderm yolk plug neural plate dorsal lip mesoderm future gut cavity endoderm Figure 38.13 Above, overview of reproduction and development in the leopard frog. Opposite, a closer look at some stages. 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. 3 Fig. 38.13.3, p. 643

Details of Frog Development (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 Many organs incorporate tissues derived from more than one germ layer

Details of Frog Development (4)

Details of Frog Development (4) neural tube notochord Figure 38.13 Above, overview of reproduction and development in the leopard frog. Opposite, a closer look at some stages. gut cavity 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. 4 Fig. 38.13.4, p. 643

Details of Frog Development (5) In frogs, and some other animals, a larva undergoes metamorphosis: a remodeling of tissues into an adult form The tadpole is a swimming larva with segmented muscles and notochord extending into a tail During metamorphosis, the frog grows limbs, and the tadpole tail is absorbed

Details of Frog Development (5) Tadpole Metamorphosis Sexually mature, four-legged adult frog Figure 38.13 Above, overview of reproduction and development in the leopard frog. Opposite, a closer look at some stages.

ANIMATION: Leopard frog life cycle To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

38.10 Early Marching Orders Egg cytoplasm includes yolk proteins, mRNA transcripts, tRNAs and ribosomes, and other proteins Some cytoplasmic components are not distributed evenly, but localized in one particular region or another cytoplasmic localization Accumulation of different materials in different regions of the egg cytoplasm

Cytoplasmic Localization In a yolk-rich egg, the vegetal pole has most of the yolk and the animal pole has little In some amphibian eggs, pigment molecules accumulate in the cell cortex, close to the animal pole After fertilization, a gray crescent forms, where substances essential to development are localized

Experiment: Cytoplasmic Localization At fertilization, cytoplasm shifts, and exposes a gray crescent opposite the sperm’s entry point First cleavage normally distributes half of the gray crescent to each descendant cell

Experiment: Cytoplasmic Localization animal pole pigmented cortex yolk-rich cytoplasm vegetal pole sperm penetrating egg gray crescent Figure 38.14 Experimental evidence of cytoplasmic localization in an amphibian oocyte. fertilized egg 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. Fig. 38.14a, p. 644

ANIMATION: Cytoplasmic localization To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

Experiment: Cytoplasmic Localization In one experiment, the first two cells formed by normal cleavage were physically separated from each other Each cell developed into a normal larva

Experiment: Cytoplasmic Localization gray crescent of salamander zygote First cleavage plane; gray crescent split equally. The blastomeres are separated experimentally. Figure 38.14 Experimental evidence of cytoplasmic localization in an amphibian oocyte. Two normal larvae develop from the two blastomeres. B In one experiment, the first two cells formed by normal cleavage were physi-cally separated from each other. Each cell developed into a normal larva. Fig. 38.14b, p. 644

Experiment: Cytoplasmic Localization In another experiment, one descendant cell received all the gray crescent, and developed normally The other gave rise to an undifferentiated ball of cells

Experiment: Cytoplasmic Localization gray crescent of salamander zygote First cleavage plane; gray crescent missed entirely. The blastomeres are separated experimentally. Figure 38.14 Experimental evidence of cytoplasmic localization in an amphibian oocyte. A ball of undifferentiated cells forms. Only one normal larva develops. 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. Fig. 38.14c, p. 644

Cleavage: The Start of Multicellularity During cleavage, a furrow appears on the cell surface and defines the plane of the cut The plane of division is not random – it dictates what types and proportions of materials a blastomere will get Each species has a characteristic cleavage pattern

From Blastula to Gastrula At gastrulation, certain cells at the embryo’s surface move inward through an opening on the surface Cells in the dorsal (upper) lip of the opening are descended from a zygote’s gray crescent Gastrulation is caused by signals from dorsal lip cells

Gastrulation in a Fruit Fly The opening cells move in through will become the fly’s mouth; descendants of stained cells will form mesoderm

Gastrulation in a Fruit Fly Figure 38.15 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. Fig. 38.15a, p. 645

Gastrulation in a Fruit Fly Figure 38.15 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. Fig. 38.15b, p. 645

Gastrulation in a Fruit Fly Figure 38.15 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. Fig. 38.15c, p. 645

Gastrulation in a Fruit Fly Figure 38.15 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. Fig. 38.15d, p. 645

Experiment: Dorsal Lip Transplant Dorsal lip of a salamander embryo was transplanted to a different site in another embryo – a second set of body parts started to form

Experiment: Dorsal Lip Transplant Figure 38.16 Experimental evidence that signals from dorsal lip cells start 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. Fig. 38.16a, p. 645

Experiment: Dorsal Lip Transplant Figure 38.16 Experimental evidence that signals from dorsal lip cells start 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. Fig. 38.16b, p. 645

Dorsal Lip Transplant (cont.) The embryo develops into a “double” larva, with two heads, two tails, and two bodies joined at the belly

Dorsal Lip Transplant (cont.) C The embryo develops into a “double” larva, with two heads, two tails, and two bodies joined at the belly. Figure 38.16 Experimental evidence that signals from dorsal lip cells start 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. Fig. 38.16c, p. 645

ANIMATION: Embryonic induction To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

Specialized Cells, Tissues, and Organs All cells in an embryo have the same genes Selective gene expression causes different cell lineages in the embryo to express different subsets of genes Selective gene expression is the key to cell differentiation – the process by which cell lineages become specialized in composition, structure, and function

Cell Differentiation An adult human has about 200 differentiated cell types Example: Cells of one lineage turn on genes for crystallin, a transparent protein that forms the lens of the eye – no other cells in the body make crystallin A differentiated cell still retains the entire genome It is possible to clone an adult animal (a genetic copy) from one of its differentiated cells

Cell Communication in Development Long-range intercellular signals (morphogens) diffuse out from certain embryonic cells and form a concentration gradient in the embryo that controls differentiation morphogen Chemical encoded by a master gene; diffuses out from its source and affects development Effects on target cells depend on its concentration

Cell Communication in Development Other chemical signals only operate at close range Example: Cells of a salamander gastrula’s dorsal lip cause adjacent cells to migrate inward and become mesoderm embryonic induction Embryonic cells produce signals that alter the behavior of neighboring cells

Cell Movements and Apoptosis Long-range and short-range signals regulate development of tissues and organs Organs begin to form as cells migrate, entire sheets of tissue fold and bend, and specific cells die on cue

Cell Movements in the Brain Neurons form in the center of the brain, then creep along extensions of glial cells or axons of other neurons until they reach their final position Once in place, they send out axons

Cell Movements in the Brain Figure 38.17 How body takes shape. A Cells migrate. This graphic shows one embryonic neuron (orange) at successive times as it migrates along a glial cell (yellow). Its adhesion proteins stick to glial cell proteins. Fig. 38.17a, p. 646

Neural Tube Formation Sheets of cells expand and fold to form the neural tube Gastrulation produces a sheet of ectodermal cells Cells at the embryo’s midline elongate and neighboring cells become wedge-shaped, forming a neural groove Edges of the groove move inward, and flaps of tissue fold and meet at the midline, forming the neural tube The neural tube later develops into the brain and spinal cord

Neural Tube Formation

Neural Tube Formation Gastrulation produces a sheet of ectodermal cells. 1 A neural groove forms as microtubules constrict or lengthen in different cells, making the cells change shape. neural groove 2 Edges of the groove meet and detach from the main sheet, forming the neural tube. 3 Figure 38.17 How body takes shape. neural tube B Cells change shape. Here, shape changes in ectodermal cells form a neural tube. Fig. 38.17b, p. 646

Apoptosis Signals from certain cells activate self-destruction in target cells (apoptosis), which helps sculpt body parts Apoptosis causes a tadpole to lose its tail and it separates the digits of the developing human hand apoptosis Mechanism of cell suicide

Apoptosis As a human hand develops, cells in the webs of skin between digits die

ANIMATION: Formation of human fingers To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

Pattern Formation Pattern formation is the process by which certain body parts form in a specific place Example: Signals from AER (apical ectodermal ridge) at the tips of limb buds induce mesoderm beneath it to form a limb pattern formation Formation of body parts in specific locations

Experiment: AER and Limb Formation AER of a limb bud tells mesoderm under it to form a limb If AER from a chick’s wing bud is removed, wing development stops Earlier positional cues determined what the mesoderm will become – mesoderm from a chick’s hindlimb implanted under wing AER forms a leg

Experiment: AER and Limb Formation

Experiment: AER and Limb Formation mesoderm of chick embryo forelimb A Experiment 1: Remove wing bud’s AER AER removed no limb forms B Experiment 2: Graft a bit of leg mesoderm under the AER of a wing mesoderm from leg wing AER (region of signal-sending ectoderm) leg forms Figure 38.18 Experiments demonstrating interactions between AER (ectoderm) and mesoderm in chick wing development. AER at the tip of a limb bud tells mesoderm under it to form a limb. Whether that limb becomes a wing or a leg depends on positional signals that the mesoderm received earlier. Fig. 38.18, p. 647

Experiment: AER and Limb Formation mesoderm of chick embryo forelimb AER (region of signal-sending ectoderm) A Experiment 1: Remove wing bud’s AER AER removed no limb forms B Experiment 2: Graft a bit of leg mesoderm under the AER of a wing mesoderm from leg leg forms wing Figure 38.18 Experiments demonstrating interactions between AER (ectoderm) and mesoderm in chick wing development. AER at the tip of a limb bud tells mesoderm under it to form a limb. Whether that limb becomes a wing or a leg depends on positional signals that the mesoderm received earlier. Stepped Art Fig. 38.18, p. 647

Evolution and Development Where and when particular genes are expressed determines how an animal body develops: Localized molecules in an unfertilized egg induce expression of master genes in the zygote Products of master genes form gradients in the embryo Depending on where they fall within these gradients, cells activate or suppress other genes

Evolution and Development (cont.) Positional information set up by concentration gradients of products of master genes affects expression of homeotic genes, which regulate development of specific body parts All animals have similar homeotic genes homeotic gene Type of master gene; its expression controls formation of specific body parts during development

Evolution and Development (cont.) Evolution of body plans are influenced by physical constraints (such as surface-to-volume ratio) and existing body framework (such as four limbs) Interactions among master genes also restrain evolution, since a major change in any one probably would be lethal Mutations led to a variety of forms among animal lineages by modifying existing developmental pathways, rather than entirely new genetic innovations

Key Concepts Principles of Development The same processes regulate development of all animals Division of the single-celled zygote distributes different materials to different cells These cells go on to express different master genes that regulate formation of body parts in particular places

ANIMATION: Early frog development To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE