Campbell Biology · Chapter 47
Animal Development
pp. 1021–1044 · 3 sections
This chapter follows an animal from a single fertilized egg to an embryo with layers, a gut, a nervous system and the first versions of its organs, and then asks how each cell knows what to become. Most of the anatomy is beyond the current AP course, but the big idea is tested: cells with identical DNA become different by switching on different genes in response to stored molecules and signals from their neighbors.
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47.1 Fertilization and the first cell divisions
pp. 1022–1027
The steps of fertilization and the blocks to extra sperm aren't in the current course. What is tested: two haploid gametes join to make a diploid zygote (Topic 5.1), the cell cycle behind cleavage (Topic 4.5), signals relayed inside a cell (Topics 4.2 and 4.3) and vesicles releasing their contents by exocytosis (Topic 2.5).
In the course: Topic 5.1 Meiosis, Topic 4.5 Cell Cycle, Topic 4.2 Introduction to Signal Transduction, Topic 4.3 Signal Transduction Pathways, Topic 2.5 Membrane Transport (notes, videos and more questions)
Key points
- Animal development runs through a set order of stages: fertilization makes a zygote, cleavage turns it into a ball of many cells (the blastula), gastrulation sorts those cells into layers, and organogenesis builds the first versions of organs.
- Before a sperm can fuse with an egg, it has to get through the egg's outer coats. Enzymes released from a cap-like vesicle on the sperm's head (the acrosome) clear a path, and proteins on the sperm must match receptors on the egg, so sperm from other species usually can't get in.
- An egg must let in only one sperm, because extra sperm would give the zygote too many chromosome sets. In many animals, a quick change in the egg membrane's voltage blocks extra sperm within seconds, and then granules just under the membrane empty their contents outside the cell by exocytosis, building a lasting barrier.
- A rise in calcium ions (Ca²⁺) inside the egg is the trigger for that lasting barrier and also wakes the egg up: respiration and protein synthesis speed up. Early on, the egg runs on mRNAs and proteins the mother stored in it.
- Mammals fertilize internally. Sperm only become able to fertilize after time in the female tract (capacitation), and the barrier against extra sperm forms in the zona pellucida, the egg's outer coat.
- Cleavage divisions are fast because the cells skip the gap phases (G₁ and G₂) and don't grow. Each division splits the same amount of cytoplasm into smaller cells, called blastomeres.
- Yolk gets in the way of division. Eggs with little or moderate yolk divide all the way through (holoblastic cleavage), while very yolky eggs, as in birds, reptiles and many fish, divide only in a small yolk-free patch (meroblastic cleavage).
Key terms (15)
- fertilization
- The joining of a haploid sperm and a haploid egg to make one diploid cell. It also kicks off development.
- zygote
- The single diploid cell formed at fertilization. Every cell of the future animal descends from it.
- acrosome
- A cap-shaped sac covering the front of a sperm's head, filled with enzymes that digest a path through the egg's outer coats.
- polyspermy
- When more than one sperm enters an egg. The embryo ends up with extra chromosome sets and usually can't survive.
- fast block to polyspermy
- A quick shift in the egg membrane's voltage after the first sperm fuses, which stops other sperm from fusing for a short time.
- cortical reaction
- Granules just under the egg's membrane fuse with it and release their contents outside the cell, building a lasting barrier to extra sperm. A rise in Ca²⁺ sets it off.
- egg activation
- The burst of activity in an egg after fertilization, such as faster respiration and protein synthesis, that starts development.
- capacitation
- Changes a mammal's sperm go through inside the female reproductive tract that make them able to fertilize an egg.
- zona pellucida
- The thick, jelly-like coat around a mammal's egg. Sperm bind to it, and it changes after fertilization to keep more sperm out.
- cleavage
- The rapid run of cell divisions right after fertilization. The cells skip growth, so they get smaller with each division.
- blastomere
- Any one of the cells made by cleavage.
- blastula
- The ball of cells that cleavage produces, usually hollow with a fluid-filled center called the blastocoel.
- yolk
- Nutrients stored in an egg to feed the embryo. Lots of yolk changes how the egg divides.
- animal and vegetal poles
- The two ends of many eggs. The vegetal pole holds most of the yolk; the animal pole has much less.
- holoblastic vs. meroblastic cleavage
- Holoblastic: the whole egg divides, as in eggs with modest yolk. Meroblastic: only a small yolk-free part divides, as in very yolky eggs.
Check yourself: 47.1 Fertilization and the first cell divisions
4 questions on 47.1 Fertilization and the first cell divisions. Pick an answer to see if you got it, and why.
A human egg is released from the ovary paused partway through meiosis II. A sperm then fuses with it. Which event happens next in the egg?
Researchers studying a marine worm used electrodes to hold each egg's membrane at a fixed voltage, then added sperm. Unfertilized eggs of this species rest at about −70 mV, and the voltage jumps to about +20 mV within seconds of the first sperm fusing (invented data). Membrane voltage held at | Eggs with more than one sperm nucleus (%) Not held (normal) | 4 +20 mV | 2 −20 mV | 45 −70 mV | 88 Which conclusion do these data best support?
At a fertility clinic, a sperm is injected straight into an egg, yet the egg never begins dividing. Tests show the sperm lacks an enzyme that normally starts the release of Ca²⁺ inside the egg. Adding a chemical that raises Ca²⁺ in the egg lets many such eggs develop. What does this suggest?
In mice, granules just under the egg membrane release an enzyme after fertilization that cuts a sperm-binding protein in the zona pellucida. Which outcome is most likely in a mutant strain whose eggs can't make this enzyme?
0 of 4 answered
47.2 Germ layers, gastrulation and building organs
pp. 1027–1035
Germ layers, gastrulation, the neural tube and the membranes around an embryo aren't in the current course, and the cytoskeleton isn't its own topic. What carries over: apoptosis as a normal, controlled process (Topic 4.6), tissues signaling to each other (Topic 4.1) and surface area for exchange in the placenta (Topic 2.2).
In the course: Topic 4.6 Regulation of Cell Cycle, Topic 4.1 Cell Communication, Topic 2.2 Cell Size, Topic 2.1 Cell Structure and Function, Topic 6.6 Gene Expression and Cell Specialization (notes, videos and more questions)
Key points
- Morphogenesis means building the body's shape, mainly by moving cells and changing their shapes. Gastrulation turns the hollow blastula into a gastrula with layers: ectoderm on the outside, endoderm lining a new gut cavity (the archenteron), and in animals with bilateral symmetry a middle layer, mesoderm.
- Each germ layer makes its own set of tissues. Ectoderm gives skin's outer layer and the nervous system; mesoderm gives muscle, bone, blood, the heart and kidneys; endoderm lines the gut and airways and forms the liver and pancreas.
- The first opening of the gut, the blastopore, becomes the anus in deuterostomes such as sea stars and vertebrates; in protostomes it usually becomes the mouth.
- In vertebrates, dorsal mesoderm forms a stiff rod, the notochord, and signals the ectoderm above it to fold into the neural tube, the future brain and spinal cord. Neural crest cells peel off the tube's edges and travel widely, and blocks of mesoderm called somites become vertebrae and back muscles.
- Reptiles, birds and mammals (amniotes) grow four membranes outside the embryo: the amnion holds the embryo in fluid, the chorion handles gas exchange, the yolk sac holds food or makes early blood cells, and the allantois stores waste or forms umbilical vessels. In mammals the outer layer of the blastocyst (the trophoblast) invades the uterus and helps build the placenta, while the inner cell mass becomes the embryo.
- Cells change shape and move using their cytoskeleton. Contracting actin filaments at one end make cells wedge-shaped so a sheet bends; cells squeezing between each other make a sheet narrower and longer (convergent extension); crawling cells grip each other and the extracellular matrix with adhesion molecules.
- Planned cell death (apoptosis) is part of normal development. It removes extra cells, carves out spaces and gets rid of structures an earlier stage needed.
Key terms (15)
- morphogenesis
- The process that gives an embryo its shape and form, mainly by changing cells' shapes and moving them.
- gastrulation
- The stage where cells of the blastula move inward and rearrange into germ layers, forming a primitive gut.
- germ layers
- The cell layers made during gastrulation. Every tissue in the body comes from one or more of them.
- ectoderm
- The outer germ layer. It becomes the outer skin and the nervous system.
- mesoderm
- The middle germ layer. It becomes muscle, bone, blood, the heart, the kidneys and more.
- endoderm
- The inner germ layer. It lines the digestive tract and airways and forms organs like the liver and pancreas.
- archenteron
- The primitive gut cavity that forms during gastrulation, lined with endoderm.
- blastopore
- The first opening into the archenteron. In deuterostomes, including you, it becomes the anus.
- notochord
- A flexible rod of mesoderm along an embryo's back. It supports the embryo and signals the tissue above it to form the neural tube.
- neural tube
- The hollow tube that forms when a strip of ectoderm folds and closes. It becomes the brain and spinal cord.
- neural crest
- Cells from the edges of the folding neural tube that travel through the embryo and become nerves outside the brain, pigment cells, and some skull and tooth tissue.
- somites
- Paired blocks of mesoderm along the notochord. They become the vertebrae, ribs and back muscles.
- blastocyst
- The mammal version of a blastula: an outer cell layer (trophoblast) around a cavity, with a cluster of cells inside (inner cell mass) that becomes the embryo.
- extraembryonic membranes
- The amnion, chorion, allantois and yolk sac. An embryo makes them, but they lie outside its body and protect, feed and support it.
- apoptosis
- Planned, orderly cell death. Development relies on it to remove cells that aren't needed.
Check yourself: 47.2 Germ layers, gastrulation and building organs
4 questions on 47.2 Germ layers, gastrulation and building organs. Pick an answer to see if you got it, and why.
A mutation blocks mesoderm from forming in a frog embryo, while ectoderm and endoderm form normally. Which pair of structures would most likely be missing?
A flat sheet of cells lining an embryo's gut tube bends to form a pouch that will become a gland. Researchers measured the two ends of each cell in normal embryos and in mutants whose band of actin and myosin builds up at the far end of each cell instead of the end facing the cavity (invented data). Embryo | Width at the cavity end (µm) | Width at the far end (µm) | Shape after 4 h Normal, at the start | 8 | 8 | Flat sheet Normal, after 4 h | 2 | 9 | Pouch bulges out, away from the cavity Mutant, after 4 h | 9 | 2 | Sheet bulges into the cavity Which explanation best fits these data?
In a fruit fly embryo, a band of tissue starts 24 cells wide and 6 cells long. An hour later it is 6 cells wide and 24 cells long, with the same number of cells. No cells divided or died. What best explains the change?
As a human placenta forms, tissue from the embryo grows into thousands of branching, tree-like outgrowths (villi) bathed in the mother's blood. The two bloodstreams don't mix. What is the main advantage of this branching shape?
0 of 4 answered
47.3 How cells learn their fates
pp. 1035–1042
Topic 6.6 tests how cells with the same DNA become different by switching on different genes, and Topics 4.1 and 4.3 use signals like morphogens as examples of cell communication. You won't need the classic experiments, the names of limb signaling centers or specific signal proteins.
In the course: Topic 6.6 Gene Expression and Cell Specialization, Topic 6.5 Regulation of Gene Expression, Topic 4.1 Cell Communication, Topic 4.3 Signal Transduction Pathways (notes, videos and more questions)
Key points
- Every cell in an embryo carries the same genome. Cells become different because they express different genes, which is called differential gene expression.
- Determination is the moment a cell is locked into one fate, often before it looks any different. Differentiation is when it actually becomes specialized, making the proteins of a muscle, nerve or skin cell.
- Fate maps show which part of an early embryo gives rise to which later structures. Researchers make them by marking cells with dyes or genetic labels and following their descendants.
- Two main things set fates. Cytoplasmic determinants are molecules spread unevenly in the egg or a dividing cell, so daughter cells inherit different ones. Induction is one group of cells sending signals that change what nearby cells become.
- Body axes (head to tail, back to belly, left to right) are set up early, sometimes by molecules the mother puts in the egg, sometimes by signals after fertilization. In mammals, beating cilia in one region push fluid to the left, and that flow helps decide which side is which.
- Cells start out able to form many cell types, and their options narrow as development goes on. In some animals, including mammals, the very early cells can still adjust to their position.
- Pattern formation puts tissues in the right places. Cells read positional information, often the concentration of a morphogen that fades with distance from its source, and switch on different genes above or below certain thresholds. Hox genes give each region along the body its identity, so cells in different regions respond differently to the same signal.
Key terms (15)
- cell fate
- What a cell will become: its location, appearance and job in the finished body.
- determination
- The point at which a cell is committed to a fate, even if it doesn't look specialized yet.
- differentiation
- When a cell takes on its specialized structure and job, like a muscle cell building contracting proteins.
- fate map
- A map of an early embryo showing which later tissues come from each region.
- cell lineage
- The family tree of a cell: which cell it came from and which cells come from it.
- cytoplasmic determinants
- Molecules such as mRNAs and proteins placed unevenly in an egg or dividing cell, so daughters inherit different instructions.
- induction
- When signals from one group of cells change the fate of nearby cells.
- totipotent
- Able to give rise to every cell type, including the support tissues like the placenta. A zygote is totipotent.
- axis formation
- Setting up the main directions of the body plan, such as head–tail and back–belly, early in development.
- pattern formation
- Arranging tissues and organs in their proper places in three dimensions.
- positional information
- Signals that let a cell know its location in the embryo, so it can switch on the right genes.
- morphogen
- A signal molecule that spreads from a source and forms a gradient. Cells respond differently depending on how much they receive.
- organizer
- A small region of an embryo whose signals direct how surrounding tissue develops.
- Hox genes
- A family of genes that code for transcription factors and give each region along the head-to-tail axis its identity. They're found in nearly all animals.
- primary cilium
- A single, non-moving cilium on most cells that acts like an antenna, picking up developmental signals.
Check yourself: 47.3 How cells learn their fates
4 questions on 47.3 How cells learn their fates. Pick an answer to see if you got it, and why.
Researchers studied three groups of cells in a vertebrate embryo. They checked whether each group made a transcription factor that commits cells to cartilage and whether it made cartilage matrix proteins. Then they moved each group into a region that normally forms skin (invented data). Group | Commitment transcription factor | Cartilage matrix proteins | Fate after the move 1 | No | No | Skin 2 | Yes | No | Cartilage 3 | Yes | Yes | Cartilage Which group is determined but not yet differentiated?
Before a neural stem cell divides, a protein that switches off its self-renewal genes gathers on one side of it. Only the daughter cell that inherits this protein stops dividing and becomes a neuron; the other stays a stem cell. What would most likely happen if the protein were spread evenly through the dividing cell?
A signal protein spreads from a source in a developing tissue. Its concentration falls with distance (invented data). Cells switch on gene set X above 30 nM, gene set Y from 12 to 30 nM and gene set Z below 12 nM. Distance from source (cell widths) | Signal (nM) 0 | 80 5 | 40 10 | 20 15 | 10 20 | 5 Which gene set will cells 10 cell widths from the source switch on?
In a vertebrate embryo, an outgrowth of the brain (the optic vesicle) touches the head ectoderm above it, and that ectoderm forms the lens of the eye. When a thin, solid sheet is slipped between the two tissues, no lens forms. When an optic vesicle is moved under a patch of head ectoderm that would normally become skin, a small lens forms there, made of that patch's own cells. Which conclusion is best supported?
0 of 4 answered