I. Beginnings of the Embryo
1. Cells, sperm, and egg are called gametes.
a. sperm- male gametes
b. gametes- either an ovum (egg cell) or a sperm cell. [in plants: ova & pollen]
B. Animal sperm [and fern/moss sperm cells] has a flagellum they use to swim toward egg.
1. yolk- mass of nutrients stored in ovum
C. Each gamete has one-half of the chromosome set: haploid (1n).
D. Fertilization starts when sperm cell touches surface of egg cell and fuses with it.
E. Sperm nucleus enters egg cytoplasm, meets egg nucleus and the two nuclei fuse. Fertilization is complete.
F. Joining of nuclei connects full chromosome set and pairs of DNA gene sequence in every cell. Newly formed zygote is said to be diploid (2n).
1. zygote- fertilized egg
2. embryo- an organism in its earliest stages of development
G. Fertilization turns on egg’s metabolism and activation usually occurs within seconds of egg-sperm fusion.
1. activation- the turning on of egg’s metabolism that occurs in a newly formed zygote after fertilization.
H. New proteins are made when cell respiration increases which use messenger RNA molecules already in cytoplasm.
I. Activation has two major effects
1. rapid change in plasma membrane, which blocks fertilization by second sperm
2. rearrangement of zygote cytoplasm by movements in the cytoskeleton.
J. Events of activation start the
changing zygote into a complex multicellular organism.
A. Animal development includes growth, cell specialization, and formations of tissues and organs.
1. differentiation- the process where embryonic cells divide and some become different from others.
B. Cell is completely differentiated when it contains all features of a specific cell type.
1. ex. Muscle cell or a skin cell
C. Cells organize to form the tissues and organs of complete animal as they differentiate.
1. morphogenesis- embryonic development of structure of an organism
D. Each type of cell that differentiates during development has a unique structure and function.
1. skin cells are tough, thin, flat, and protect body
2. skeletal muscle cells contain protein fibers which enable them to contract (flex the muscle)
3. nerve cells have long, thin branches that are made to transmit information (both electrically and chemically)
E. The human red blood cell doesn't have a nucleus (it looses it soon after it is produced in the bone marrow)
1. They are made to transport oxygen.
F. Each cell type (tissue) and organ has a specific location and role (structure and function).
G. Proteins (coded for by the DNA) are the keys to differentiation in animal cells
H. Specific groups of genes are expressed in each cell type which leads to production of specific proteins (the other unneeded genes are turned off).
1. outer skin layer cells make an extracellular matrix of keratin (immune system functions)
2. muscle cells contain banded fibers that have actin and myosin proteins that form contractile units.
3. nerve cells contain neurotransmitter molecules that carry nerve impulses from one cell to another
J. Major protein in red blood cells is hemoglobin which transports oxygen to and from tissues
K. Differences between cells in
synthesis lead to differences in cell form and function
(different genes are turned off or on in different tissues).
III. From One Cell to Many: Making The Multicellular Organism
A. During cleavage, cells usually divide and double in number each cycle
1. cleavage- period where zygote divides into two cells after fertilization
B. Embryo consists of a mass of many cells called blastula at end of cleavage
1. blastula- an animal embryo after cleavage stage (a small ball of cells)
C. Shape of blastula depends on structure of original egg and how its yolk is arranged
D. Differentiation and morphogenesis becomes obvious when some cells move from surface to interior of blastula
E. Embryo becomes three-layered gastrula
1. gastrula- two or three-layered, cup shaped embryonic stage
F. Three layers are called primary germ layers
1. primary germ layers- form all of body’s tissues
a. ectoderm- outer layer; will form skin, nervous system and related structures
b. endoderm- inner layer; usually a tube that will become lining of the digestive system.
c. mesoderm- in-between-layer; will make skeleton, muscles, heart, blood, and other internal organs
G. Gastrulation: changing blastula into a gastrula, involves major changes
H. Morphogenesis includes:
1. coordinated movements of individual cells and tissues
2. changing cell shapes
3. folding or splitting of cell layers
4. tissue masses formation by local cell division
5. shaping of organs by genetically timed death of some cells
I. Cell movement during morphogenesis involves controlled breaking and remaking of chemical bonds.
J. In vertebrates the general shape, or body plan, of organism appears during gastrulation
body plan- general form of an organism’s boy structure
K. First mesoderm becomes the notochord
1. notochord- stiff rod that develops into part of the backbone/spinal chord.
L. Notochord runs down middle of embryo beneath dorsal ectoderm
M. This development establishes the anterior-posterior axis running from head to tail
N. At this time, dorsal-ventral direction, from back to belly, and right and left become obvious
O. Later, a large head, segmented backbone, and limbs complete vertebrate body plan
P. Above notochord, dorsal ectoderm folds up to become neural tube
1. neural tube- foundation of nervous system that will form brain, spinal chord, and nerves.
Q. Unlike mammals and birds who develop directly into young that are like the adult, others like frogs, sea stars, and insects first form a larva and them the larva forms into an adult. [example: egg, tadpole, frog]
1. larva- feeding individual that looks nothing like the adult
R. Larva goes through metamorphosis
IV. Developmental Patterns and Evolutionary Relationships
A. Basic developmental pattern varies among animals
B. In some animals like snails and worms, first two cells are unequal
1. small cell becomes ectoderm, large cell leads to mesoderm and endoderm
C. If two cells are separated experimentally, each develops only into its limited types of tissues
D. During cleavage, each cell receives molecules that control its fate
1. In all these embryos, opening of gastrula becomes the mouth
E. When sea star or vertebrate zygote cleaves, first two cells are identical
1. cell separation experiments show both cells can make all three kinds of tissue and develop into a complete embryo
2. accidental separation of these two cells makes identical twins (clones)
a. in these animals, first opening of gut forms the anus, while mouth forms later at other end of gut.
F. Developmental patterns are a clue to relationships among living groups of animals [field of embryology]
1. differences suggest a more distant relationship or adaptation to different environments
G. Charles Darwin was among the first biologists to compare developmental patterns to help determine relationships among animal species
H. Related species posses many of the same genes
1. similar genes in many animals are responsible for segmentation
a. segmentation- division of body into a number of similar sections
I. Segments of vertebrates are clearest in the skeleton (back bone is segmented)
J. Body-pattern genes were first discovered in fruit flies that carried errors in these genes
1. errors in these homeotic genes can transform one organ into another
a. homeotic genes- genes that determine which body parts are made at which locations on developing organisms
K. To study these genes, biologists compared DNA between abnormal flies and normal flies
1. found 11 homeotic genes close together on one chromosome<> 2. every gene contains one or more copies of a 180-base-pair sequence that codes for a protein segment ( called a homeodomain) which is 60 amino acids long.>
a. homeobox– DNA sequence
virtually identical in certain homeotic genes
3. each gene encodes a protein that includes the 60 amino-acid homeodomain
a. this part of protein bonds to DNA, regulating transcription of important genes
L. Homeotic genes are located close together, in same order as the body segments, whose development they control
M. Nearly identical gene sequences were found in mice
1. Mouse genes were names Hox genes for homeoboxes they contain
a. Hox genes- group of homeotic genes found in all animals
N. Changes in homeotic genes often lead to embryonic death or severe abnormalities
1. mutation in homeotic gene could produce a sudden evolutionary changehttp://www.zoo.utoronto.ca/dgodt/ZOO328/Zoo328%20Thea%20notes%2013+14.html
V. Human Development
A. Human egg (ova) is about 0.1 mm in diameter and contains no yolk
B. Fertilized zygote cleaves as it moves down the oviduct from ovary into uterus
C. About 5 days after fertilization, embryo, called a blastocyst, is a hollow blastula similar to that of other animals
1. blastocyst- mammalian embryonic stage that corresponds to blastula of other animals
2. blastocyst sinks into wall of mother's uterus to develop and grow [called implantation]
D. Thick mass of cells inside of blastocyst forms disk that becomes embryo
1. gastrulation occurs here and rest of blastocyst develops into membranes which surround, nourish, and protects embryo
E. Amnion immediately surrounds embryo
1. amnion- sac or membrane filled with amniotic fluid which encloses the embryo
F. Chorion encloses all other membranes, and forms blastocyst’s thin outer wall
1. chorion- embryonic membrane that surrounds all other embryonic membranes
G. Chorion extends finger like projections called villi into lining of uterus as gastrulation begins
1. villi-finger like projections
H. Chorionic villi and uterine lining form placenta
1. placenta- structure in uterus for exchange of materials between a fetus and the mother's blood supply, partly formed by each.
J. Mother’s blood flows through cavities in placenta
1. chorionic villi extend into these cavities
2. nutrients and wastes pass (diffuse) through villi and blood vessels, but the two blood supplies remain completely separate
K. Human takes about 40 weeks to develop in uterus
1. after beginning of eighth week, embryo is called a fetus
a. fetus- an older human embryo, first bone cells laid down
L. After 3 months, most organs begin to form and skeleton is visible in ultrasound images
rapid growth and maturation of organ systems occurs in last 3
<>VI. Birth Defects>
A. Some birth defects may be caused by defective genes or environmental factors (genetic or environmental diseases)
B. Polydactyly, condition of having extra fingers or toes is caused by an altered gene (genetic disease)
C. Biologists do not yet know how gene alters human limb development but did an experiment on chickens and got an idea
1. as first bud of chick’s limb begins to grow, particular gene is active in only posterior part of bud
a. it produces protein that seems to control pattern of digits in limb by regulating transcription of genes
2. experimenters exposed cells in anterior part of limb bud to protein
a. as embryo developed, treated limb developed extra digits in a mirror-image pattern
D. Neural-tube defects happen when part of neural tube does not close completely. Ex: spine bifida.
1. in spine bifida, posterior end of neural tube does not close and body wall remains open. Surgeons can sometimes partially fix it but, problems persist through out life. The lower on the spine this happens, the less the disability.
2. when anterior part does not close, large part of brain does not develop
a. this is called anencephaly, exposed brain degenerates and top of skull fails to form. These individuals usually don't survive after birth .
E. Both genes and environmental factors affect neural-tube development
F. Experiments suggest half of
defects could be prevented if pregnant women took folic acid and
VII. Mechanisms of Cell Differentiation
A. Just describing development of an embryo cannot tell us what cellular and molecular processes control this series of events
1. scientists came up with experiments to test their hypothesis on this subject
a. early experiments with embryo’s involved surgery
i. scientists removed certain cells or moved tissues to new locations
b. later method involved replacing nucleus of unfertilized egg with nucleus of differentiated cell
B. Molecular methods help determine which genes are active in particular cell
1. scientists can make large quantities of particular gene’s DNA and use chemicals to separate the DNA’s two strands
2. they match dye molecules to DNA to make visible
a. tagged molecules are used as probes to detect RNA with matching nucleotide sequence
i. this method is called DNA-RNA hybridization
a. DNA-RNA hybridization- paring of DNA molecules with RNA molecules by hydrogen bonds between complementary base pairs
b. cells that have transcribed the
mRNA that matches probe’s sequence
VIII. The Genetic Equivalence of Differentiating Cells
A. What happens to genes not used when cell differentiates?
1. selective gene-loss hypothesis says differentiating cells lose some genes
2. genetic-equivalence hypothesis states all cells contain same genes, but some become inactive during differentiation
B. In 1952, Robert Briggs and Thomas King injected nuclei of differentiated cells from leopard frogs into unfertilized frog eggs. (they cloned a frog!)
1. replaced egg’s nucleus with one of differentiated cell
a. egg’s never meet sperm; injection activated egg
b. nucleus from blastula supported development of egg all the way to becoming a tadpole
c. when researches used skin cell, development stopped after gastrulation
i. more differentiated cell still had all genes needed for development, but nucleus could not support production of all kinds of cells<> C. John Gurdon’s experiments with South African clawed frogs showed egg with nucleus of differentiated tadpole could develop into reproducing adult frog.>
1. nuclei from adult skin cells did not support development past the tadpole stage, no genes were lost as cell differentiated<> D. Results supported hypothesis that all cells in an individual are genetically equivalent, but differentiation does restrict expression of some genes.
A. Determination- process by which cell commits to particular course of development
B. Experiments with two-cell embryo’s of snail and frog demonstrate two extremes of determination
1. each frog cell produced complete tadpole, but snails did not produce normal larvae
a. in a snail, both cell continued to cleave, but smaller cell produced only ectoderm, larger cell made mesoderm and endoderm
2. snail embryo’s cells were already irreversibly determined, frog embryo’s two cells were not determined yet
Some proteins and other molecules are distributed unevenly in
which determines fates of snail embryonic cell
X. Cytoplasmic Determination
A. What makes snail’s first two cells different?
1. scientists used microneedles to remove lobe from zygote
a. first cleavage produced two cells of unequal size that developed into abnormal larva with no heart or intestine
i. evidence supported idea that large cell becomes mesoderm and endoderm from something received in lobe
B. Factors in snail lobes are still unknown, but embryo’s of tunicates (sea squirts) include RNA and proteins<> 1. differentiation in tunicates seems to begin with movements of zygote’s cytoplasm that carry pigment granules and regulating molecules into different regions of cell.>
a. cleavage distributes molecules to different cells
C. Yellow cells grown in isolation produce molecules found in muscle
1. RNA-DNA hybridization has shown RNA molecules attached to cytoskeleton, moving with pigment.
a. some RNA’s encode regulatory proteins that turn on genes that lead to muscle differentiation<> 2. RNA’s encode proteins that regulate differentiation by controlling either transcription or translation of genes that are active during development of types of cells>
D. Cells form anterior, middle, or posterior tissues
1. one kind of abnormal fly produces headless larvae, have two posterior ends and don't survive.
E. They develop normally when anterior end of zygote is injected with anterior cytoplasm taken from normal embryo.
F. Experimental results supported hypothesis that RNA and proteins in egg cytoplasm help control differentiation by regulating gene expression.http://www.fasebj.org/cgi/content/full/15/13/2390
1. hypothesized that signal from notochord shifts neighboring dorsal ectoderm cells from skin to neural-tube differentiation
2. transferred pieces of ectoderm between two gastrulas
a. cells that normally form skin moved to dorsal region, produced neural tube instead
b. dorsal cells that should form neural tube were placed in ventral area, produced skin
B. Experiment showed cells respond to other cells nearby as they become determined.
C. Signals to become neural tissue have received, and tissue does not turn back to become skin, called embryonic induction
embryonic induction- influence of one embryonic tissue on
another, causing second
tissue to specialize
D. Spemann and Mangold’s next experiment showed notochord is source of inducing individual
1. took piece of tissue that would later become notochord from blastula or early gastrula
2. when transplanted to another embryo, tissue formed extra notochord
a. extra notochord induced neighboring ectoderm to forms second neural tube
i. second notochord started series of events, led to a nearly complete second larva attached to individual
E. When piece of notochord is
ventral ectoderm, ectoderm cells soon induce to
become neural cells
F. To determine whether induction requires cells to be touching, scientists cultured tissues on opposite sides of plaster filter
1. pores of filter were so small, no cell contact could occur, but molecules could pass through.
2. induction occurred therefore contact is not required
a. notochord communicates with ectoderm by releasing substance that can pass through filter
G. Biologists started searching for inducing substance because of these experiments
H. More than 20 years later, molecular techniques finally revealed induction involves number of genes and proteins
1. two inducing proteins that notochord produces are called chordin and noggin
a. DNA-RNA hybridization shows chordin and noggin genes are active in cells that will form notochord
2. scientists injected chordin protein into ventral side of frog embryo
a. chordin induced ventral cells to form second anterior-posterior axis
i. showed chordin is important in notochord formation and establishing anterior-posterior axis of frog embryo
3. noggin protein causes ventral ectoderm in laboratory tissue cultures to become nervous tissue instead of skin
I. Chordin and noggin proteins do not regulate gene transcription directly
1. each interferes with action of another protein that controls production of family of other proteins
a. these latter proteins regulate transcription of many specific genes involved in development of nerve cells
J. Scientists have found more than 10 developmental control proteins in notochord cell alone.1. some help determine which part of neural tube forms brain and various types of nerves.