Biology 191 - Lecture 24

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Fertilization is in interface between...

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reproduction and development

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Acrosomal reaction
(3)

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1. sperm head is capped by enzyme-filled acrosome
2. enzymes digest protective jelly coat of egg
3. bindin proteins join with species-specific egg receptors

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Acrosomal reacton uses a ...

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fast block to polyspermy: membrane depolarization
*polyspermy: egg penetrated by more than 1 sperm

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Slow block to polyspermy

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(20-30s)
1. Enzymes remove sperm-binding receptors
2. cortical vesicles fuze with plasma membrane of egg cell; constructe mote (fluid-filled cavity) that is a barrier to other sperm getting in

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What happens after sperm penetration in mammals?

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In mammals, thre is a 12 hr delay between egg penetration and fusion of nuclei

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After the 12 hour delay,
(2)

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1. egg nucleus enters second meiotic division
2. sperm and egg nuclei then fuse

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Consequence of unequal distribution of cytoplasmic components in the zygote

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Unequal distribution of cytoplasmic components in the zygote sets the state for an unfolding of positional information that orchestrates determination, morphogenesis, and differentiation

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How does positional information arise?

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1. nuclei in early embryo are exposed to different concentrations of cytoplasmic determinants
2. as a consequence, they express different genes

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2 Theories of Animal Development

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1. Preformation theory
2. epigenesis

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Preformation theory
(2)

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1. sperm contained pre-formed infant which simply grows during development
2. the pre-formed infant was called Homunculus
*theory now discarded

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Epigenesis

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Animal form emerges gradually from formless egg

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Model organisms in developmental biology
(6)

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1. sea urchin
2. xenopus laevis (frog)
3. C. elegans (nematode)
4. Drosophila melanogaster (fruit fly)
5. mouse
6. zebrafish

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Sea urchin
(3)

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1. external fertilization
2. large number of eggs, sperm, and embryos
3. transparent/translucent

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Xenopus laevis
(3)

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(African clawed frog)
1. external fertilization
2. large, translucent eggs
3. sensitive to environmental toxins

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C. elegans

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(nematode)
1. small animal with short life cycle
2. cheap and easy to keep large numbers
3. first multicellular organism whose genome was completely sequenced
4. can manipulate genes with RNAi

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RNAi

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1. RNAi = interfefence RNA
2. make double strand construct. taken up by organism through food. organism thinks its a virus. If sequence of RNA is complimentary to a gene in the organism, it turns off that gene. allows you to turn off any gene you want.

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Drosophila melanogaster
(4)

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(fruit fly)
1. small animal with short life cycle
2. mutant flies, with defects in any of several thousand genes, are available
3. entire genome recently sequenced
4. can manipulate genes with p-elements

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P-elements
(df)

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transposable element. insert gene sequence in transposable element. inject into fruit fly genome. Manipulate gene --> look at phenotype

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Mouse
(4)

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1. short life cycle with large offspring numbers
2. mus musculus is the classic model vertebrate
3. many inbred strains exist, as well as lines selected for particular traits
4. viviparous development (produce live young and multiple fetuses)

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Zebrafish
(4)

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1. small fish is a relatively new model
2. vertebrate model for aspects of human biology
3. cheaper and easier to handle than mice
4. transparent and readily accessible embryo (oviparous)

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Reporter genes

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1. used in zebra fish
2. male genetic construct. insert it into gene. when gene is active, it fluoreceses.

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Recap:
(3)

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1. RNAi used in C. elegans (nematodes)
2. p-elements used in fruit flies
3. reporter genes used in zebra fish

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Key stages in animal development
(6)

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1. determination
2. morphogenesis
3. differentiation
4. blastula
5. gastrula
6. adult

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Determination
(2)

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1. commitment of cells to a particular fate
2. fully specified in round worm
*fate maps needed for determination

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Morphogenesis
(2)

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1. gives rise to shapre of the multicellular body i.e. development o adult form.
2. It involves movement of cells and tissues

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Differentiation

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development of cellular speficity i.e. tissue-specific patterns of gene expression

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Blastula

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hollow ball of cells

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Gastrula

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region of the blastula folds inward, forming gut cavity

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Number of cells in male sea star vs. number of cells in hermaphrodite sea star

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male: 1050 cells. hermaphrodite: 960 cells. Reduction in hermaphrodite is due to apoptosis.

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Epigenetics
(df)

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Epigenetics refers to that there is more to gene expression that DNA

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DNA methylation modifications

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CH3 can attach to cytosine. Its presence or absence determines if it is transcribed

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Histone modifications

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DNA is wrapped in histones. Determines if DNA is accessible.

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Vegetal hemisphere

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In an unfertilized frog egg, dense nutrients settle to the bottom, which is called the vegetal hemisphere

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Animal hemisphere

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The haploid nucleus is located at the opposite end of the egg, which is called the animal hemisphere

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Gray crescent

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The gray crescent is the interface between the vegetal pole and the animal pole. It's exposed nonpigmented cytoplasm.

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Sperm always enters at the ...

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animal pole.

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Sperm entering at the animal pole causes...

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rotation of cytoplasm and iniation of bilateral symmetry

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Spemann's experiment

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1. Spemann used a baby's hair to constrict the zygote

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Results of Spemann's experiment
(2)

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1. If hair bisects the gray crescent, development is normal in both derivative embryos (both embryos get some of the gray crescent)
2. If the hair does not bisect the gray crescent, the blastomere without the gray crescent develops abnormally. It can't develop into an individual.

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Conclusion of Spemann's experiment

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Cytoplasmic factors in the gray crescent are crucial for normal development

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Cleavage
(df)

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Cleavage is a rapid series of ell divisions which differentially distribute nutrients and information molecules such as mRNA

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DNA synthesis and cell divisions proceed with ....

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little growth and gene expression. As a result, cleavage tends to produce a hollow ball of cells called a blastula.

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Different taxa differ substantially in their...

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cleavage patterns

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Factors influencing cleavage patterns
(2)

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1. amount of yolk
2. orientation of mitotic spindles
a. radial (right angles)
b. spiral (not at right angles)

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Types of Cleavage
(3)

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1. complete cleavage
2. incomplete cleavage
3. superficial cleavage

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Complete cleavage in sea urchin

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Sea urchin: yolk platelets evenly distributed

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Complete cleavage in frog

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Frog: yolk concentrated at vegetal pole

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Incomplete cleavage in chick

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Chick: embryo develops on top of yolk as disc of cells

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Superficial cleavage in fruit flies

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Fruit flies:
1 multiple nuclear divisions without cytokinesis produce syncytium
2. nuclei migrate to periphery and plasma membranes form
*ie. nuclei divide, but cells don't

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Syncytium
(df)

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multiple nuclei

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Gastrulation
(df)

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Gastrulation is the dramatic rearrangement of cells of a blastula nto a three-layered embryo which has a primitive gut

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Frog blastula fate map includes
(3)

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1. ectoderm
2. mesoderm
3. endoderm

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Frog blastula fate map: ectoderm

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The ectoderm forms outer (epidermal) layer

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Frog blastula fate map: mesoderm

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The mesoderm forms muscle, bone, blood, and connective tissue

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Frog blastula fate map: endoderm

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The endoderm forms the lining of the gut, liver, and lungs

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Thus, gastrulation gives rise to three cell layers

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1. ectoderm
2. mesoderm
3. endoderm

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Developmentally important genes in Fruit Flies
(4)

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1. maternal effect genes
2. gap genes
3. pair-rule genes
4. homeotic genes

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Maternal effect genes
(2)

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1. bicoid mRNA in egg
2. embryo relies on mRNA expressed by mother in early stages

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Gap genes
(3)

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1. now expressed by embryo
2. basic anterior/posterior subdivisions
3. mutants have gaps in segmentation

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Pair-rule genes

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Mutants have 1/2 normal segment number

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Homeotic genes
(df)

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Homeotic genes are a set of genes expressed in different combinations along the length of the body (anterior-posterior axis) that dictate fate of each segment

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Homeotic mutants

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Mutants in which normal body parts are formed in inappropriate segments (wrong region of body)

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Bithorax mutant

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Bithorax mutant transforms 3rd thoracic segment, which normally bears haltares, into form of 2nd thoracic segment, which bears wings

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Halteres

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2nd pair of wings in flies; very small. Involved in hovering. Mutation converts them to a normal sized set of wings

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Antennapedia mutation

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Antennapedia mutation causes flies to grow legs in place of antennae

Front 66

Hox genes
(df)

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Hox genes are a class of homeotic genes extensively studied in nematodes, insects and mice (they are regulatory genes)

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Hox genes contain a ...

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Hox genes contain a homeobox sequence that codes for a homeodomain protein

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Homeodomain binds to ...

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DNA and acts as a transcription factor

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Hox genes exhibit ...

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spatial colinearity

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Way in which hox genes exhibit spatial colinearity

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Hox genes are arranged in the same order on each chromosome as they are expressed along the anterior to posterior axis in the developing axis

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In vertebrates, Hox genes also exhibit...

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temporal colinearity

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temporal colinearity in Hox genes

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In vertebrates, Hox genes exhibit temporal colinearity: anterior genes are expressed earlier than posterior genes

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Mammalian development is unique because of ...

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genomic imprinting

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Most significant discovery in developmental genetics of the 20th century

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genomic imprinting

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Genomic imprinting is also called

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parent-of-origin gene expression

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Genomic imprinting
(df)

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Genomic imprinting is a process by which an allele is silenced or expressed in an individual, depending on whether the allele is transmitted through sperm or egg

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Transmitted through sperm or egg

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Some genes are only expressed from allele inherited from mother, while others are solely expressed from allele inherited from father

Front 78

Example of genomic imprinting

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epigenetic form of gene expression

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Epigenetic form of gene expression
(df)

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Epigenetics refers to heritable changes in gene function that occur without a change in DNA sequence

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In epigenetics, alleles are ...

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methylated differently (i.e. imprinted) during male and female gametogenesis

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There are known to be ___ imprinted loci in humans

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There are known to be about 80 imprinted loci in humans

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Nourishment of embryo by mother creates...

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Nourishment of embryo by mother creates post-fertilization arena for genomic conflict absent in species that lay eggs

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Conflict over maternal resources can arise between:
(3)

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1. mother and developing embryos
2. sibling embryos within womb
3. maternal and paternal genomes within individual embryos

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Function of viviparity

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Viviparity provides direct conduit for manipultion of mother's physiological system by paternal genes in embryo

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The placenta is derived from

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fetal tissue

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Placenta
(df)

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The placenta is an interface between fetal and maternal tissues

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The placenta acquires ...

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nutrients from the mother

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Function of the placenta

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The placenta transports hormones from fetus that influence maternal physiology

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Placenta and invasiveness

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The placenta is a highly invasive organ

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The placenta develops from...

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embrionic trophoblast cells that invade endometrium and remodel maternal spiral arteries into distended vessels that are unable to constrict

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What controls growth of placenta

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Paternal genes control growth of placenta

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From an evolutionary perspective, maternal-fetal interactions involve...

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complex interplay between mutual and conflicting interests

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Epigenetic machinery of DNA methylation provides...

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molecular mechanism through which conflict between mother and paternal genome in fetus can be played out
*this mechanism is called genomic imprinting

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Opposing contributions of maternal and paternal genomes by nuclear transplantation experiments on

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mice

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Experiment in mice

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Created diploid embryos with either 2 paternal or 2 maternal genomes

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Androgenic embryos

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Androgenic embryos (2 paternal genomes) had growth-without-form development of fetus and overgrowth of placenta (unsuccessful development)
*also happens when 2 sperm fertilize empty egg

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Gynogenic embryos

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Aynogenic embryos (2 maternal genomes) had form-without-growth development of fetus and gross underdevelopment of placenta (unsuccessful development)

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Maternal/paternal chromosome imbalances

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Same patterns of aberrant development occur naturally in fetuses with maternal/paternal chromosome imbalances
*called hydatidiform moles