6 Animal Development

Question 1

What are the 3 key events that happen during animal development?

Answer 1

1) Cell division 2) Cell differentiation (cells become specialized); 3) Morphogenesis (different kinds of cells are organized into tissues & organs)

Question 2

What is the structure of the Oocyte before fertilization?

Answer 2

Oocyte has 2 hemispheres: Animal Pole (where nucleus is located) and Vegetal Pole (where yolk/nutrients lie). Oocyte is radially symmetrical, but becomes bilaterally symmetrical upon fertilization

Question 3

What are morphogens?

Answer 3

Signaling molecules that can diffuse along an axis, creating a concentration gradient. Direct differentiation of cells based on the level of concentration of the morphogens

Question 4

Is the egg homogeneous?

Answer 4

No, the “front” of the egg is different from the “back” and as development progresses cells sense their “position” in the embryo based on morphogen concentrations

Question 5

Where do morphgens come from?

Answer 5

Initial morphogen usually provided by mom within Oocyte via “maternal effect genes”. These genes are deposited by mom before Oocyte is fertilized

Question 6

What is the purpose of morphogens?

Answer 6

Morphogens set the stage or “axes”: animal and vegetal poles prior to fertilization - gives fertilization initial context. Morphogens set initial polarity across embryo and this initial gradient is in place before fertilization occurs, but gets more precise with future rounds of signaling - codes for genes that can act as txn factors and turn on genes that cause cells regions to differentiate. E.g. Bicoid is located to anterior end by cytoskeleton and turns on genes involved in anterior development; other maternal effect genes localized to posterior end and code for genes that are expressed at posterior end

Question 7

What is an example of a maternal effect gene?

Answer 7

Bicoid - Localized to anterior end by cytoskeleton; immediately after fertilization translated into protein - high concentrations of this protein signal anterior end of egg- protein acts as a txn factor that turns on certain genes involved in anterior development and serve as more specific morphogens

Question 8

What general changes take place after fertilization?

Answer 8

Fertilization stimulates rearrangements of egg cytoplasm. 1) Sperm binding creates a “dorsal-ventral” axis 2) Cytoplasm rotates 3) Fertilization creates “Gray Crescent” 4) Establishes polarity of zygote - molecules within the egg are organized with respect to this polarity

Question 9

What is the Gray Crescent?

Answer 9

Organizational region in the egg opposite the site of sperm entry

Question 10

How do morphogens and fertilization work together to arrange the egg?

Answer 10

Morphogens give polarity that can help set differences between both “anterior/posterior” as well as “dorsal/ventral” axis while fertilization fine-tunes where morphogens sit in the embryo so that they are oriented with the Gray Crescent region

Question 11

How are morphogens passed along during mitosis? Why?

Answer 11

Different morphogens that guide development will not be divided evenly among daughter cells - present in gradients and thus will not get passed on evening to dividing cells - this uneven distribution of signally molecules sets the stage for differential into different cell types

Question 12

What role do cytoplasmic chemicals play during development?

Answer 12

Different cytoplasmic chemicals can initiate different patterns of gene expression which leads to differential of cells

Question 13

What role do extracellular signals play during development?

Answer 13

Chemical signals at different concentrations can alter gene transcription and cell differentiation - cells’ neighbors can help control the cell’s differentiation through secreting these chemical signals i.e. inducers and repressors

Question 14

What are the major development stages?

Answer 14

Fertilization, Cleavage, Blastula, Gastrulation, Organogenesis (includes neurulation)

Question 15

What happens during the fertilization stage of development?

Answer 15

Sperm entry establishes site of Gray Crescent which will serve an important organizing region for future development

Question 16

What happens during the cleavage stage of development?

Answer 16

Rapid series of cell divisions (mitosis) but no cell growth which means the daughter cells will be smaller than the parent - results in small ball of cells - division not homogenous so there will be a different distribution of nutrients and cytoplasmic determinants in early embryo

Question 17

What happens during the blastula stage of development?

Answer 17

Cleavage forms a ball with a central fluid-filled blastocoel (cavity). Individual cells are called blastomeres at this point. In mammals blastula is called the blastocyst - fate map formed

Question 18

What is the fate map of the blastula?

Answer 18

After division and uneven distribution of cytoplasmic and signally components, the cell of the blastula undergo predictable patterns of migration and differentiation depends on concentrations based on location in blastula

Question 19

Are cells in the blastula stage differentiated or undifferentiated?

Answer 19

Undifferentiated - cells are pluripotent and still considered embryonic stem cells, but will differentiate soon based on location in embryo

Question 20

What happens during the Gastrulation stage of development?

Answer 20

Transition from ball of cells to complex embryo through 1) cell movement and new cell-cell contacts which sets up signaling cascade, 2) initiation of differentiation of cells, 3) setting stage for emergence of body plan, 4) development of multiple tissue layers and distinct body axes

Question 21

What does the gastrulation stage give rise to?

Answer 21

3 germ layers: ectoderm, mesoderm, endoderm

Question 22

What happens during Gastrulation in Sea Urchin embryo?

Answer 22

Cells at vegetal pole begin to invaginate into blastocoel. These cells: 1) differentiate 2) become endoderm 3) form primitive gut called “archenteron” 4) mouth forms where archenteron makes contact with overlying ectoderm 4) opening on invaginated cavity blastopore) will become anus 5) other cells break free and move into blastocoel cavity (mesenchyme - cells of the mesoderm)

Question 23

What happens during Gastrulation in frog embryo?

Answer 23

More complex gastrulation - amphibian blastulas are more than one cell thick. 1) Certain cells in Gray Crescent region change shape and cell adhesion properties 2) these cells bulge inwards towards blastocoel (the opening is called blastopore) 3) as cells move inward form dorsal lip 4) involution occurs - sheet of cells moves inwards - involuting cells become endoderm & archenteron 5) another group of cells moves between endoderm & outer layer to form mesoderm 6) cells from animal hemisphere flatten and move toward site of involution 7) Archenteron grows displacing blastocoel 8) 3 germ layers created (differentiation)

Question 24

What is an archenteron? Which cell layer makes up the archenteron?

Answer 24

Primitive gut found in sea urchins and frogs - the endoderm

Question 25

When is the differentiation of embryo axes visible in frog development?

Answer 25

at the end of gastrulation

Question 26

How many lips does the blastopore have during frog development?

Answer 26

2 - the dorsal lip which becomes the archenteron, endoderm and the ventral lip which gives rise to the endoderm

Question 27

What was the name of the biologist who performed experiments on salamanders in early 1900s? What did he do? What were the findings?

Answer 27

Hans Spemman. Found that cell differentiation happens between early and late gastrulation. Performed experiments on salamanders where tissue was transplanted from the gastrula of one salamander to another. Found that in early gastrula tissue took on identity of region in which they had been transplanted - in late gastrula differentiation had already occurred and tissue continued differentiated role in new host

Question 28

What did Spemann discover when he look at Gastrulation and Gray Crescent?

Answer 28

If bisection of embryo during gastrulation left parts of grey crescent in both new embryos then both would develop into normal embryos. If only one embryo got the Gray Crescent then the one with the crescent would develop normally while the other would stay a clump of undifferentiated cells. Spermann found that the Gray crescent and dorsal lip is crucial for organizing embryo formation and that the dorsal lip of the blastopore organizes the development of surrounding cells

Question 29

What is the Spermann Organizer?

Answer 29

The dorsal lip of the blastopore - Spemann found in an experiment that when the dorsal lip of a salamander embryo was transplanted into another embryo that another salamander would form, and the two would be attached at the belly. He determined that the dorsal lip was capable of inducing differentiation of surrounding tissue that otherwise would have had different fate - shows that some cells develop relative to their position to other cells rather than lineage history

Question 30

What is the purpose of the dorsal lip?

Answer 30

Organizes the development of the surround cells during gastrulation

Question 31

What is the molecular mechanism underlying the Spemann Organizer?

Answer 31

1) Unequal distribution of cytoplasmic components from cleavage 2) Transcription factor B-catenin is distributed highly in the cells that correspond to the location of the organizer 3) B-catenin initiates organizer activity 4) Initiates complex signaling pathways between txn factors and growth factors that control gene expression -

Question 32

What happens is b-catenin is missing? Overexpressed?

Answer 32

without it gastrulation does not occur - if it is overexpressed in another region of embryo this induces a second axis of embryo formation

Question 33

What structure is similar to the Spemann Organizer in mammals and birds?

Answer 33

The anterior node of primitive streak (Hensen’s node in bird’s)

Question 34

What is the equivalent to the dorsal lip in mammals and birds?

Answer 34

The primitive streak

Question 35

What is the function of the primitive streak?

Answer 35

Organizes development of the rest of the body - cells that move through it become endoderm and mesoderm and notochord

Question 36

How are identical twins formed?

Answer 36

If blastocyst splits in two after fertilization

Question 37

How are conjoined twins formed?

Answer 37

If blastocyst splits partially

Question 38

What is the blastocyst?

Answer 38

blastula in mammals

Question 39

What happens during the Organogenesis stage of development?

Answer 39

The development of many organs and organ systems simultaneously and in coordination with one another - including neurulation

Question 40

What is neurulation?

Answer 40

Early and important process of organogenesis; initiation of the nervous system. Ectodermal tissue overlying the mesoderm folds in upon itself to create the nervous system.

Question 41

What role does the notochord play?

Answer 41

Structural support for the developing embryo. Organizes functions - directs overlying ectoderm to become neural ectoderm by expressing signaling molecules

Question 42

What is the notochord?

Answer 42

Rod of mesodermal tissue forms the notochord at the end of gastrulation.

Question 43

How is the inner neural tube created during Neurulation?

Answer 43

1) thickening of ectoderm overlying the notochord (the neural plate); 2) edges of neural plate continue to thicken to form ridges, 3) Forms middle groove that deepens as folds roll over it; 4) Folds converge in midline and fuse forming a cylinder - the neural tube. Continuous layer of epithelial ectoderm over top

Question 44

What is the neural plate?

Answer 44

The thickened ectoderm overlying the notochord that will be used to form neural tube

Question 45

How does the brain develop from the neural tube?

Answer 45

The anterior end of the neural tube develops bulge which becomes major division of the brain.

Question 46

What does the neural tube develop into?

Answer 46

The brain (anterior end) and spinal cord

Question 47

What is Spina Bifida?

Answer 47

Failure of the neural folds to fuse at the posterior end

Question 48

What is Anencephaly?

Answer 48

Failure of the neural folds to fuse at the anterior end

Question 49

What are Somites?

Answer 49

Separate, segmented blocks of mesodermal tissue that become vertebrae, ribs, muscles of trunk and limbs

Question 50

How does body segmentation arise during neurulation?

Answer 50

As neural tube forms the mesodermal tissues gather along the sides of the notochord and form Somites. As development progresses different segments of the body change and become specialized

Question 51

How do different parts of the developing embryo know which parts to develop? (pattern formation)

Answer 51

Through morphogens - initially set anterior-posterior axis and now direct differentiation based on concentration (along gradient). Different concentration gradients of different morphogens activate release of other morphogens - signaling gets more and more precise with each subsequent round of signaling - multiple morphogens provide unique combination of signals

Question 52

What are Hox Genes?

Answer 52

Group of related genes that control development of the body segments along the anterior posterior axis (developmental specificity) - different embryonic segments express different Hox genes - Hox genes make Hox txn factors that turns on (or repress) specific genes to transcribe different proteins in different segments of the body

Question 53

How are Hox Genes turned on?

Answer 53

By a cascade of morphogens (morphogens which turn on other morphogens) and ultimately activate Hox genes in specific regions of embryo.

Question 54

What happened during the hox mutation in Drosophila experiment?

Answer 54

Leg Hox gene misexpressed in head - antennae took on identity of legs

Question 55

How do Hox genes change across species?

Answer 55

Hox genes show similar expression Patterns Across Species as they are important regulatory genes to control development and differentiation

Question 56

What are the stages of Human Development?

Answer 56

9 months - 3 trimesters

Question 57

What happens in the first trimester of human development?

Answer 57

1) Rapid cell division and tissue differentiation, implantation occurs about 6 days after fertilization - this is also when gastrulation begins. 2) Initial development of all major organ systems begin 3) Week 9 embryo is now a fetus and cell differentiation mostly complete

Question 58

When is the embryo most sensitive to pathogens that causes birth defects?

Answer 58

During the 1st Trimester

Question 59

When does neurulation occur? Heart begins beating? Limbs form?

Answer 59

Neurulation ~day 17-22; Heart begins beating during week 4, Limbs formed by week 8

Question 60

What happens during the second trimester of human development?

Answer 60

Fetus grows rapidly in weight. Limbs elongate, fingers, toes facial features become well formed and nervous system continue to develop and does so rapidly. Fetal movements first felt by mother.

Question 61

What happens during the third trimester in human development?

Answer 61

Fetus continues to grow rapidly and internal organs mature. Digestive enzymes begin to function, liver stores glycogen, kidneys produce urine and brain undergoes cycles of sleep/wake. Fetus is born when critical organs (lungs) mature.