A. Development is the series of changes an organism undergoes from fertilization to death.

B. Prior to fertilization

1. meiosis + cytokenesis
2. spermatogenesis
3. oogenesis

C. Fertilization restores the diploid number

1. The haploid nucleus of the sperm fuses with the haploid nucleus of the egg.
2. zygote (2n) - a fertilized egg

D. A zygote is a tiny and relatively simple structure. Consider what it becomes. All the information needed for producing the adult animal is contained within that zygote.

1. differential gene expression - different genes are expressed at different times

E. The fertilized egg (a single cell) then begins to divide by mitosis + cytokenesis. Why mitosis?

F. Cleavage - initial period of cell division; embryo foes not increase in size

1. blastomeres - the cells of this early embryo
2. 2-cell stage
3. 4-cell stage
4. 8-cell stage
5. 16-cell stage
6. solid ball of cells

G. blastula (hollow ball of cells) forms

1. blastocoel - space inside blastula
2. forms when cells near middle of cell pump Na+ out, causing water to difuse into this area

H. gastrulation - formation of gastrula

1. blastopore - opening into archenteron
2. archenteron - "primative gut"
3. gastrula is bilaterally symmetrical
4. 3 tissue layers
a. ectoderm - the cells around exterior of gastrula; derivatives include - skin, nervous system
b. endoderm - the cells lining the archenteron; derivatives include most internal organs (stomach, lungs, liver, etc.)
c. mesoderm - cells that invade the space between the endoerm and ectoderm; derivatives include - notochord, bones, blood vessels, conenctive tissues, muscles

I. neurulation - formation of neural tube (from ectoderm)

1. neural tube separates from rest of ectoderm
2. neural tube eventually becomes brain and spinal cord

J. cell migration - some cells move to different parts of the embryo (e.g., neural crest cells pinch off from neural tube; some of these neural crest cells will give rise to various sense organs)

K. organogenesis and growth - tissues develop into organs; the embryo gets larger

L. variation among vertebrate groups

1. jawless fish
a. little to no yolk in egg
b. holoblastic cleavage
c. blastomeres of approximately equal size
2. bony fish, amphibians
a. relatively more yolk than jawless fish
b. vegetal pole, animal pole
c. boloblastic cleavage
d. blastomeres of vegetal pole larger
3. reptiles, birds, some fish
a. egg is almost all yolk with a tiny amount of cytoplasm at one end (the blastodisc)
b. meroblastic cleavage
4. mammals (placental)
a. egg is very similar to reptile egg but with very little yolk
b. holoblastic cleavage
c. inner cell mass at one end of blastula
- analogous to blastodisc
- goes on to become embryo
d. trophoblast
- the other cells of the blastula
- part contributes to placenta
5. gastrulation in amniotes (reptiles, birds, and mammals)
a. blastodisc (or inner cell mass) is flattened disc of cells; not a hollow ball as in blastula of other groups
b. lower cells of blastodisc become endoderm
c. upper cells of blastodisc become ectoderm
d. ectoderm invaginates along midline giving rise to primative streak
e. some of the invaginating cells give rise to mesoderm, in between ectoderm and endoderm.

M. induction - a cell switching from one development path to another due to interactions with other cells

1. molecules involved in unduction (morphogens) turn certain genes on and off
2. concentration of morphogen may be important; example in animal cells of clawed frog:
a. low concentration - epidermis
b. med. concentration - muscle
c. high concentration - notochord
3. the concentration of morphogen a cell receives is relative to its postition in the embryo

N. determination - commitment of a cell to a particular developmental path

1. a cell is totipotent if it is still capible of expressing any of its genes
2. up to the 8-cell stage in mammals, each blastomere is totipotent; implications:
a. can divide up to 8-cell state embryo and produce 8 genetically identical individuals (this type of technology has been used in breeding certain valuable lines of cattle)
b. can combine two 8-cell stage embryos and produce one individual that has four parents
3. as development proceeds, the fates of the cells become determined; example:
a. take cells from prospective brain region in early gastrula
b. place these cells elsewhere in gastrula
c. transplanted cells will develop like their new neighbors (i.e., not into brain tissue)
d. take cells from prospective brain region in late gastrula
e. place these cells elsewhere in gastrula
f. transplanted cells will develop into neural tissue regardless of where they are placed in the gastrula
4. Cells may become determined to become a certain type of tissue prior to differentiating into that type of tissue. (see imaginal discs below)
5. Cells may become partially committed to a particular fate before they are completely committed; example: wing bud, leg bud on chicken
6. determination cen be reversed; example:
a. producing the cloned sheep Dolly
b. used nucleus from a fully differentiated cell (mammary cell in udder)
7. procedure for cloning a sheep
a. remove cell from udder of sheep to be cloned; keep alive in culure
b. remove and egg cell from another
c. remove nucleus cell with micropipette
d. insert udder cell into egg cell
e. shock cell with electricity to release nucleus of udder cell and to trigger cell division
f. grow egg ni culture to blastula stage
g. insert blastula into surrogate sheep
h. ca. 5 months later the clone is born
i. clone is genetically identical to the sheep that "donated" the udder

O. pattern formation - forming the basic body arrangement; example from fruit fly:

1. the source of bicoid protein (a type of morphogen) is mRNA from mother
2. this mRNA stays near one end of egg
3. bicoid protein diffuses though embryo forming a morphogen gradient
4. the end of the embroy with the highest concentration of bicod protein becomes the head; posterior to his the thorax develops
5. embryos that can't make bicoid protein develop neither a head nor a thorax
6. if bicod protein is injected into either end of embryo that can't make it, it becomes the head
7. bicoid protein activates particular genes

P. homeotic genes - act as "master switches" that determine what form different body segments will take

1. example - in fruit flies, mutation in these genes can result in an extra set of wings or in legs forming where the antennae should be
2. the order of the genes matches the order of the body parts they control
3. homeobox
a. found in homeotic genes
b. 180 nucleotides long
c. highly conserved (found in fruit flies, mice, humans; evidence of common ancestry)

Q. apotosis - programmed cell death is a normal part of development

a. webbing between fingers in humans
b. death of some neurons in humans

R. metamorphosis - a major developmental change (reorganization of the body) after hatching or birth (e.g., many arthropods, mollusks, fish, amphibians)

1. fruit fly
a. zygote develops into a free-living larva (maggot - common term for fly larva)
b. larva eats, grows, passes through several instars (exoskeleton is shed to allow growth)
c. larva develops into pupa
d. imaginal discs - groups of cells "set aside" during embryonic development; in the pupa various discs give rise to legs, wings, eyes, etc.
e. adult emerges from pupal shell
f. this general pattern holds form many insects (e.g., caterpillars become butterflies; grubs become beetles)
2. leopard frog
a. zygote develops into a free-living larva (tadpole)
b. larva eats, grows
c. larva undergoes metamorphosis
d. apotosis is involved in breaking down a tadpole's tail
e. not all amphibians have a larval stage
- some caecillians, salamanders, and frogs hatch or are born as miniature adults
f. in the paradox frog, tadpoles are larger than adults
3. metamorphosis in brought on by signals from the endocrine system (hormones)
4. in some species/populations, the larva never metamorphoses (neoteny)

S. external vs. internal fertilization

1. internal fertilization is often necessary in dry environments
2. internal fertilization is necessary in species that lay shelled eggs

T. numbers of offspring and parental care

1. general trend - large numbers of offspring, less parental care; few offspring, more parental care

U. Aging - why do animals age? What causes the changes we see in individuals as they get older? Variety of hypotheses:

1. accumulated mutation hypothesis
a. cells accumulate mutations as they age
b. eventually these mutations are lethal
c. problem - no direct evidence that these mutations actually cause aging
2. telomere depletion hypothesis
a. telomeres - repeats of sequence TTAGGG at the ends of chromosomes
b. every time DNA is replicated for cell division, telomeres become shorter
c. after so many divisions, a cell can no longer divide
d. telomerase - an enzyme that "rebuilds" telomeres; only produced in certain cells that divide constantly (e.g., bone marrow cells)
e. telomerase is expressed in many cancer cells
f. cells that artificially caused to produce telomerase will divide many more times than cells of the same type without telomerase
3. wear and tear hypothesis
a. cells accumulate damage over time
b. partially due to harmful substances produced during metabolism
c. reduced flexibility at joints can be attributed to reactions involving these harmful substances

4. immunological exhaustion hypothesis
a. as we get older we become susceptible to infectious disease
b. fewer committee T cells
c. produce less interleukin-2 (a growth factor that stimulates production of T cells)
5. gene clock hypothesis
a. some genes are involved in regulating the aging process
b. Hutchison-Gilford Syndrome - extremely rapid aging in children (a very rare genetic disorder)
c. a combination of mutations in C. eligans can increase its life-span five fold
6. Is aging regulated by genes or does the body simply wear out over time?