Cleavage of Frog and Chick Eggs Chapter VI

Cleavage of Frog and Chick Eggs Chapter VI

4 Total Development = cleavage, morphogenesis, differentiation, growth 4 Cleavage-immediately after fertilization 4 Two kinds of divisions to convert egg to mass of cells – mitosis and cytokinesis 4 First cleavage in frog egg results in two equal size cells

4 2 nd cleavage division in frog is also complete and equal 4 3 rd division in frog is complete and unequal. Cells are different sizes 4 After many divisions the animal pole has more and smaller cells 4 Egg yolk has effect on cleavage divisions. Slows them down

4 Blastocoel-internal cavity forms near the animal pole after many cleavage divisions 4 Blastula--The developmental stage when the blastocoel forms 4 Bird eggs--99% of egg is yolk, yolk segregated from the cytoplasm 4 Most of the cytoplasm is in the cytoplasmic cap on top of the yolk

4 Cleavage happens in the cytoplasmic cap 4 Don’t have cleavage in entire egg-- Incomplete 4 The cytoplasmic cap is called the blastoderm 4 Under the blastoderm is a space called the subgerminal space 4 Blastoderm is not a blastula because there is no cavity

4 Cleavage in chicken egg produces a blastoderm located at the animal pole 4 Incomplete cleavage occurs

Morphogenesis in Frog and Chick Development Chapter VII

Morphogenesis 4 Animals acquire form and structure during this developmental stage 4 Movement of cells to specific locations is characteristic of morphogenesis 4 Organism takes on a specific shape 4 Gastrulation--happens in morphogenesis and results in three germinal layers. 4 Also results in the formation of gastrula

4 Three layers in gastrula are ectoderm, mesoderm and endoderm

Frog Gastrulation 4 Begins with the blastula 4 First part of gastrulation is the formation of a blastopore on surface of blastula 4 The cells begin to fold inward 4 Further folding of the blastopore results in a cavity called the archenteron. The future gut 4 Blastopore becomes anus 4 The blastocoel is being displaced

4 Continued morphogenic movements result in enlarging the archenteron, reducing the blastocoel and forming a yolk plug over the blastopore 4 In mature frog gastrula there are three germ layers 4 Ectoderm which are the outer cells, Endoderm which line the archenteron, and mesoderm which is in between endo and ecto

4 Mature gastrula is foundation for more development to occur 4 Doesn’t stop morphogenesis 4 Others follow to form cell masses called primary organ rudiments 4 Neurula-stage where the neural plate develops and forms the neural tube which will develop into the future nerve cord

Chick Gastrulation 4 Begins with the blastoderm 4 Forms a new cavity below the blastoderm 4 When cavity is formed its called the blastocoel. 4 The subgerminal space is now called the archenteron or future gut 4 Stage is not yet a gastrula because no germ layers

4 Primitive groove forms on surface of blastoderm due to cells moving into the blastocoel 4 Related to the blastopore in frog development 4 After cells move into the blastocoel there are three distinct germ layers and can now be called the gastrula

4 Continuation of morphogenesis results in the neural plate and development of other primary organ rudiments 4 In both the frog and chick the mature gastrula provides the structural foundation forming more organs

Differentiation in Animal Development Chapter VIII

4 Differentiation--where cells take on specific structures and functions 4 Experiment--Future eye region was transplanted into different areas and a salt water solution. – The regions differentiated into different structures when transplanted – Salt water-did not differentiate

4 Potency-the ability of an embryonic part to develop into different kinds of structures 4 Eye region of frog has many potencies 4 Future eye region of frog was undifferentiated 4 Differentiation of eye depends on its location based on experiment

4 Eye regions in neurula were placed at different locations in a later embryo 4 All developed into an eye 4 Eye region of neurula has only one potency 4 In neurula stage the future eye region has become differentiated and can only form an eye 4 The primary organ rudiment in neurula is more determined than in the gastrula

4 As the embryo ages the number of potencies possible for an area becomes less 4 Morphogenesis continues along with the processes of differentiation and growth

Factors Controlling Animal Development Chapter IX

Nuclear Factors 4 Experiments are presented that demonstrate possible control mechanisms of the events of animal development 4 Experiment with needle into frog eggs. – With nucleus---no effect on embryo – Without nucleus--abnormal or no embryo 4 Results indicate that nucleus controls embryo development 4 All cells result from mitosis and cytokinesis

Cytoplasmic Factors 4 All cells should have the same set of genes 4 They will differentiate into different structures 4 How can they develop differently if they have the same set of instructions? 4 Look at sea urchin development

4 Experiment-pigment band develops into the endoderm of gastrula 4 Egg cut in half through the animal and vegetal. Results animal forms blastula with ectoderm only and dies-Vegetal forms incomplete embryo with all 3 germs layers and dies 4 Results--endoderm and mesoderm formation are contained in the vegetal

4 Exp. 2 --Sea urchin cut in half through both animal and vegetal axis-length wise – Forms two small complete embryos 4 Results of both experiments--Germ formation for three germ layers has instructions in the cytoplasm

4 Frog egg experiments-p 120 – Hair used to constrict fertilized egg into two halves – Halves connected by bridge of cytoplasm – Gray crescent area of cytoplasm equally divided – Half with nucleus divides first. Both halves form normal embryos

4 2 nd frog experiment. Hair again only constricts with part of grey crescent on one half with nucleus 4 The side with only part of the grey crescent didn’t develop properly while the half with the gray crescent did 4 These experiments indicate that the control of nucleus is regulated by the cytoplasmic environment

Extracellular Factors 4 Experiments so far show what happens with unequal distribution of materials within the egg 4 Are there other factors at play in the development of the embryo? 4 In the experiment given on p 123 tissue was transplanted into the blastocoel of a gastrula stage embryo

4 From this transplanted tissue a 2 nd head formed where belly tissue should have formed 4 This means that transplanted tissue affects the normal development of tissue 4 Exp. On p 125 ectoderm placed in culture = no differentiation on tissues – Ectoderm and mesoderm in culture = differentiation of ectoderm but not mesoderm – Why? Mesoderm stimulated differentiation in ectoderm

4 Induction—name given to process where one tissue induces the differentiation of another 4 This induction can explain how the differentiation of different cells occur at different times. – As new tissue is formed it induces other tissues to form

4 Other experiments indicate that induction is not the only influence on development 4 When a frog gastrula was placed in culture with pieces of adult heart-no normal heart produced in embryo 4 Differentiated adult heart produce certain chemicals into culture that inhibited the development of heart tissue in embryo 4 Inhibition—differentiated cells produce substances that inhibit cells from developing in the same way. Don’t get two hearts this way

4 Interactions occurring between groups of tissues also help development of certain structures – Exp. Limb bud of chick forms when ectoderm and mesoderm work together. – When they are grafted separately they don’t form the limb properly 4 Also has to be limb bud ectoderm and mesoderm. Can’t be any ectoderm and mesoderm 4 Also wing and leg bud experiments performed

4 The mesoderm taken from wing or leg will develop into wing or leg if combined with ectoderm from the other limb 4 Mesoderm controls development of limb but may be affected by kind of ectoderm it is in contact with

4 Differential gene Action—not all genes are turned on at the same time in the cells nucleus 4 Cells differentiate at different rates and in different directions depending on which genes are turned on when

The Operon Theory of Gene Control Chapter X

4 How are the genes controlled so that some are on and some are off? 4 Operon Theory—developed with research on bacteria 4 Enzyme-controls rate of chemical reaction 4 Structural genes-control enzymes 4 Operon is operator gene and structural genes

4 Operator gene controls the structural genes 4 Regulatory gene also present to make repressor which deactivates the operator gene and turns of enzyme function of structural genes 4 When enzymes are produced, if high enough concentrations are reached some of them combine with the repressor to trigger the shut off of the operator gene

4 When substrate is high enough it deactivates the repressor and allows the operator gene to turn on the structural genes to function and make enzymes 4 Regulatory gene has indirect effect on the operator gene