Chapter 7 u How Do Embryos Develop?

Do Sperm and Egg Contain Tiny Preformed Individuals? u Preformation—until 18th century; miniature infant in sperm and egg u If... human sperm and eggs contain tiny preformed individuals and... a sperm and egg combine during fertilization then... twins should be produced But... twins are not often produced Therefore... sperm and eggs do not contain tiny preformed individuals

Just like Needham, some people modified the theory u Individual existed in the sperm OR the egg u Called a homunculus u Dismissed because each homunculus would then need to have a homunculus inside a homunculus inside a homunculus, etc.

At fertilization the sperm and nuclei fuse together and make a zygote

What Is the Pattern of Embryonic Development? u Embryonic development is the result of cell divisions, cell differentiations, and cell growth.

Cleavage –Fertilization can be internal or external (sperm and egg join to make a zygote) –Early cell division (mitosis) is called cleavage because cells cut in half, but embryo doesn’t get bigger. –Forms a hollow ball of cells called a blastula

Gastrula –Cells form three layers in a gastrula –The three layers start to form different kinds of cells—cell differentiation.

Differentiation –Groups of differentiated cells form tissues –Differentiation and growth continue to make organs with specific forms and functions –Eventually the organs are integrated to work together within organ systems t e.g. digestive, circulatory, respiratory, etc.

How Do New Embryonic Structures Develop? –Each cell should have every gene, but then how can the cells act differently?

Is Cell Differentiation Controlled by the External Environment? u If... environmental factors control embryos and... embryos are constantly rotated so that such factors are disrupted then... the embryos should not develop normally but... Roux’s experiment showed normal development Therefore... it is likely that embryos are controlled by internal factors

Is Cell Differentiation Controlled by the Egg’s Cytoplasm? u If... newly fertilized eggs contain differentiated cytoplasm that cause the cells to develop differently and... one cell of a two-celled frog embryo is killed with a hot needle then... the remaining cell should develop into half an embryo and... surviving embryos developed into half embryos Therefore... cell differentiation seems to be caused by cytoplasm differences

Does Cytoplasm also Control Cell Differentiation in Sea Urchin Embryos? u Hans Driesch did a variation of Roux’s experiment where instead of keeping the living and the dead cells connected, he separated the two cells u Driesch observed all the separated cells developing into complete, but smaller, embryos

What Pattern of Development Occurs in Amphioxus? u If... environmental factors control embryos and... embryos are constantly rotated so that such factors are disrupted then... the embryos should not develop normally but... Roux’s experiment showed normal development Therefore... it is likely that embryos are controlled by internal factors

What Did People Know by the Turn of the Century? ♦Cell Differentiation Theory (1900) ♦Chromosome Theory (1903)

Cell Differentiation Theory u Hereditary instructions in nucleus guide embryonic development u The instructions initiate a series of changes in newly fertilized eggs to produce differentiated cytoplasm u The degree of cytoplasmic differentiation varies from species to species u As cell division occurs, hereditary instructions duplicate so that identical instructions are passed down u Each cell division splits parent cell’s cytoplasm into two regions, with differentiated cytoplasm u The differentiated cytoplasm modifies, but does not destroy the instructions u The modified instructions produce cells with different characteristics in different embryonic regions

Which Cells Move During Development and Where Do They Go? u Gastrula—1920’s Walter Vogt tracked cell movements in embryos u Stained part of embryo u Held embryos in place with bent glass u Mapped where the stain went u Some cells moved inward forming the dorsal lip u Cavity inside was formed that became the gut

Which Cells Move During Development and Where Do They Go? u Three distinct layers formed the gastrula u Endoderm-became stomach, intestines, bladder, respiratory tract u Ectoderm-became nervous system and outer layer of skin u Mesoderm-became connective tissues, muscles, blood, kidneys, etc.

Does One Embryo Part Influence Development of Another? u If... neural-tube development is induced by notochord cells u and... the ectoderm cells are removed before the notochord cells appear u then... the removed ectoderm cells should not develop into a neural tube unless the notochord cells are nearby

Does One Embryo Part Influence Development of Another? u If... neural-tube development is controlled by something inside the ectoderm cells u then... removed ectoderm cells should still develop into a neural tube u But... the removed ectoderm cells did not u Therefore... embryonic induction is supported

Do Other Embryonic Parts Develop Due to Induction? u If... induction of the neural tube is caused by notochord cells that come from the dorsal lip region u and... the dorsal lip cells are grafted into another embryo that already has dorsal lip cells u then... both sets of dorsal lip cells should move into the embryo and two neural tubes should develop

Do Other Embryonic Parts Develop Due to Induction? u On the other hand, u If... the bottom half is the inducer u then... only one neural tube should develop. u but... two neural tubes developed u Therefore... embryonic cells differentiated because they are induced by nearby cells

Do Eyes Also Develop Due to Induction? –Lens and cornea turn out to be induced by the optic cup (Figure 7.10)

How Does Induction Work? –Some kind of molecule –Inducer cells make a lot of molecules –These chemical signals are the HOTTEST area of research in biology right now!!!!

How Does Regeneration Occur? u Some adult animals can grow back damaged tissues and organs –Regeneration

How Does Acetabularia Regenerate a Lost Part? If... genes in the nucleus guide regeneration u and... stalks of several Acetabularia cells are cut u then... a piece containing a foot with a nucleus should regenerate a new umbrella, but a piece containing an umbrella without a nucleus should not generate a new foot u and... the cells with a nucleus regenerated the lost part u Therefore... nuclear gene control is supported

How Does Acetabularia Regenerate a Lost Part? –This was contradicted later with other experiments, so probably the chemicals made by genes can control regeneration even without the genes being present.

Do All Cells Contain the Same Genes? –Looking at chromosomes from same organism, but different cells under a microscope show the same banding patterns (Figure 7.14)

Do All Cells Contain the Same Genes? –If... all genes are retained during embryonic development u and... nuclei from a many-celled albino frog are inserted into eggs from a green frog that lack nuclei u then... each egg should develop into a complete albino frog u and... they did u Therefore... embryo cells (and presumably all cells) have the same genes

Box 7.1Can We Someday Clone Humans? u How to clone –Cell from adult taken –Activate inactive genes –Take an egg from adult female and remove nucleus –Join the adult cell with a nucleus and the egg without the nucleus and activate with a spark –Genes duplicate and cell divides (embryonic development) –Implant embryo into surrogate mother to grow through to birth

Box 7.1Can We Someday Clone Humans? u What Are Some Problems of Cloning? –Clone mortality is high –Perhaps damage to genes in adult donor

Box 7.1Can We Someday Clone Humans? u What Is a Benefit of Cloning? –Transgenic animals important for pharmaceuticals –Can grow human proteins in high quantities for those in need