Lecture 6 - Development

Question 1

3 Key Events of Animal Development

Answer 1

1. Cell Division
2. Cell Differentiation (cells become specialized in structure and function)
3. Morphogenesis (Different kinds of cells are organized into tissues and organs)

Question 2

When is directionality of the axes determined?

Answer 2

Before fertilization even occurs

Question 3

Structure of Oocyte Before Fertilization

How does it change once fertilization occurs?

Answer 3

• 2 hemispheres (animal, vegetal)

Animal: where nucleus is located
Vegetal: where yolk/nutrients are

Radially symmetrical
Becomes bilateral upon fertilization

Question 4

What is a Morphogen?

Significance of Gradient:

Answer 4

Initial Pattern Formation

Morphogen

• signaling molecule that can diffuse along an axis, creating a concentration gradient
• direct differentiation based on their concentration

Egg cell is not homogenous

• “front” different from “back”
• as development progresses, cells sense their “position” in the embryo

Question 5

Where does the initial morphogen come from?

Answer 5

Initial morphogen usually provided by mom within the oocyte

• maternal effect eners
• set initial polarity across embryo
• initial gradiant in place before fertilization occurs
• gets more precise with future rounds of signaling

Question 6

How does the morphogen establish polarity?

Answer 6

• MATERNAL EFFECT GENES are deposited by mom before oocyte is fertilized
• after fertilization it is translated inot a protein
• high conc of protein signal anterior end
• acts as transcription factor that turns on certain genes involved in anterior development and serves as more specific morphogens
• different maternal effect genes are localized to posterior end and code for proteins that are highly exressed at posterior end

Question 7

How do axes develop/arise?

Answer 7

• morphogens establish polarity in the embryo before fertilization happens (maternal effect genes)
• ex: give animal vegetal poles
• gives fertilization a context when it happens

Question 8

How does fertilization affect the axes?

Answer 8

• initially have anterior/posterior (front/back)

- fertilization determines dorsal/ventral (top/bottom)

Question 9

Fertilization Stimulation of rearrangements in cytoplasm

Answer 9

• sperm binding creates a dorsal-ventral axis
• cytoplasmic rotation
• creates an organizational region opposite the site of sperm entry called the gray crescent
• establishes polarity of zygote
• molecules within egg are organized with respect to this polarity

Question 10

Morphogen Shifting Based on Fertilization

Where are the morphogens?

Answer 10

Morphogens shift based on where fertilization occurs to “fine tune” axes

• give polarity that help set the differences between ant/pos, dors/vent
• fertilizaction fine tunes where they sit in the embryo so that they are oriented in grey crescent

Question 11

What happens to morphogens as the daughter cells divide?

Answer 11

• they are NOT divided evenly
• remain in gradients
• uneven distribution of signaling molecules sets the stage for differentiation into different cell types

Question 12

Other Extracellular Signaling (besides morphogens)

Answer 12

• neighbor cells help control differentiation
• secrete chemical signals
• chemical signals at different concentrations alter gene transcription and ell differentiation

Question 13

Major developmental stages?

Answer 13

1. fertilization
2. cleavage
3. blastula
4. gastrulation
5. organogenesis
   - -> nerulation

FCBGO

Question 14

Fertilization and the grey crescent

Answer 14

• establishes where grey crescent will form
• opposite sperm entry site
• will serve as an important organizing region for future dev

Question 15

Cleavage

what happens during cleavage?

Answer 15

Repackages cytoplasm

• rapid series of cell division
• because cytoplasm not homogenous, first divisions result in differential distribution of nutrients and cytoplasmic determinants in early embryo
• during cleaveage - rapid DNA replication and mitosis but no cell growth
• solid balls of smaller and smaller cells

Question 16

Blastula

Answer 16

• when cleavage forms a ball with a central fluid-filled avity
• cavity = blastocoel
• individual cells = blastomeres
• in mammals, blastula is called blastocyst

Question 17

Fate Maps of Blastula

Answer 17

• development is not random
• due to dist of cytoplasmic and signal components in the cells of blastula, cells undergo patterns of migration and differentiation
• at this point cells have not differentiated yet
• still considered embryonic stem cells pluripotent
• can PREDICT where they will differentiate based on location

Question 18

Gastrulation

Answer 18

How a ball of cells becomes a complex embryo

• cell movements set up new stem cell contacts
• sets up signaling cascades
• initiate differentiation of cells
• set the stage for the emergence of a body plan
• develop multiple tissue layers and distinct body axes

Question 19

Three Germ Layers

Answer 19

Ectoderm, Mesoderm, endoder,

Question 20

Ectoderm gives rise to…

Answer 20

• epidermis of skin and its derivatives (glands, hair follicles)
• epithelial lining of mouth and rectum
• cornea and lens of eye
• nervous system
  adrenal medula
• toothe enamel
• epithelium or pineal and pituitary glands

Question 21

Mesoderm gives rise to…

Answer 21

• notochord
• skeletal system
• muscular system
• muscular layers of stomach, intestine
• excretory system
• circulatory system
• lymphatic system
• reproductive system
• dermis of skin
• lining of body cavity
• adrenal cortex

Question 22

Endoderm gives rise to…

Answer 22

• epithelial lining of digestive tract
• epithelial lining of respiratory system
• lining of urethra, rinary bladder, and reproductive system
• liver
• pancreas
• thymus
• thyroid and parathyroid glands

Question 23

Gastrulation in sea urchin

Answer 23

• cells at vegetal pole begin to invaginate into blastocoel
• differentiate
• become endoderm
• form primitive gut - archenteron
• mouth forms where archenteron makes contact with overlying ectoderm
• opening = blastopore –> will become anus
• other cells break free and move into blastocoel cavity
• mesenchyme - cells of mesoderm

Question 24

primitive gut

Answer 24

archenteron

Question 25

gastrulation in frog | beginning

Answer 25

- ampibian blastulas are more than one cell thick - sperm is actually idrected to "animal pole" side of the egg - certain cells in gray crescent region change their shapes and cell adhesion properties - bulge inwards towards blastocoel - opening = blastopore - as they move inwards dorsal lip forms

Question 26

gastrulation in frog | later part

Answer 26

- involution - sheet of cells moves inwards - group of involuting cells becomes endoderm (form primitive gut) - another group of cells moves between endoderm and outer layer to form mesoderm - cells from animal hemisphere flatten and move toward site of involution - archenteron grows displacing blastocoel - three germ layers are created (differentiation) - differences among the axes now visible - chemical differences had been present, but now physiological differences - dorsal/ventral, anterior/posterior

Question 27

Blastocoel vs blastopore

Answer 27

Blastocoal - Blastula stage - ball of cells with fluid in center - ffluid in center = blastocoel blastopore - gastrulation stage - blastopore forms - invagination at dorsal lip that will become the archenteron and ultimately the gut - pore forming into what was previously the blastula

Question 28

Gastrulation and cell differentiation (Spemann Organizer) What did Spemann determine?

Answer 28

*cells at the two stages were transplanted to a different salamander - EARLY GASTRULAS: cells take on identity of region - are pluripotent - respond to cues of neighbors LATE GASTRULAS: differentiationha d occured - cells took on fate of their initial region * at some point during gastrulation, differentiation had occurred and fates of enbryonic cels had been determined

Question 29

Gastrulation and Grey Crescent What did Spemann determine?

Answer 29

If grey crescent not evenly divided between cells, they will be different - half with grey crescent would be normal - half without would stay a clump of undifferentiated cells

Question 30

Importance of grey crescent and dorsal lip

Answer 30

crucial for organizing embryo formation

Question 31

The spemann organizer

Answer 31

* dorsal lip of blastopore "organizes" the development of the surrounding cells* * small region capable of inducing differentiation of surrounding tissues that otherwise would have had different fates discovery: took dorsal lip of the blastopore of one salamander and put it in belly area of another gastrula. A complete second embryo formed in the belly area of the original

Question 32

Molecualr mechanisms underlying the spemann organizer | What molecule is important?

Answer 32

- unequal distribution of cytoplasmic components from cleavage - transcription factor B-catenin is distributed highly in the cells that correspond to the location of the organizer - Initiates organizer activity - Without b catenin, gastrulation does not occur - if B-catenin is over-expressed in a different region of the embryo, it induces a second axis of embryo formation - initiates complex signaling pathways beween transcription factors and growth fators that control gene expression ** B catenin alone would do the same thing as if a spemann organizer were present

Question 33

The primitive streak

Answer 33

- birds and mammals have *primitive streak* rather than dorsal lip - midline ridge tha invaginates - cells migrate through it to become endoderm and mesoderm - some cells become notochord and organize development similar to Spemann organizer - anterior node is equivalent to spemann organizer

Question 34

Primitive streaks and twins

Answer 34

- if blastocyst splits in two - identical twins - cells are still pluripotent so can produce two embryos - if blastocyst splits partially - conjoined twins - multiple or divided primitive streaks

Question 35

Organogenesis

Answer 35

development of many organs and organ systems simultaneously and in coordination with one another

Question 36

Neurulation

Answer 36

- initiation of nervous system - at the end of gastrulation, mesodermal layer is underneath the ectodermal layer - rod of mesodermal tissue --> notochord 1. structural support for developing embryo 2. organizing functions (directs overlying ectoderm to become neural ectoderm through signalling molecules) - ectodermal tissue overlying the mesoderm folds in upon itseld to create the nervous system

Question 37

Stages of Neurulation

Answer 37

*formation of an inner neural tube from an external sheet of cells 1. thickening of ectoderm overlying the notochord - 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 - neural tube continuous layer of ectoderm over the top

Question 38

From tube to brain

Answer 38

- on anterior end neural tube bulges - becomes major divisions of brain - rest of tube becomes spinal cord * failure of these processes results in various diseases

Question 39

spina bifida

Answer 39

failure of neural folds to fuse at the posterior end

Question 40

ancephaly

Answer 40

failure of neural folds to fuse at anterior end

Question 41

Body segmentation during neurulation

Answer 41

- as neural tube forms, mesodermal tissues gather along the sides of the notochord - form somites - seperate, segmented blocks of mesodermal tissue - become vertebrae, ribs, muscles of trunks, limbs - as development progresses, different segments of the body change and become specialized

Question 42

Morphogens

Answer 42

signaling molecules that diffuse along an axis and direct differentiation based on their concentration - set the initial anterior-posterior axis in the embryo - different concentration gradients of different morphogens activaate release of other morphogens - signaling gets more and more precise with each subsequent round of division

Question 43

Significance of Morphogen Patterns

Answer 43

- subsequent patterns of morphogens allow more and more refinement to a particular pattern - cells sense their "position" in the embryo - a morphogen sets the positional value of a cell by forming a concentration gradient across the developmental field in which the cell resides - sense and respond to position based on gradient - increasingly refined position - positional info guides pattern formation - different concentrations of the morphogen cause different effects - multiple morphogens provide unique combinations of skills

Question 44

Cascade of Morphogens

Answer 44

*morphogens ultimately effect development of specific body regions by turning on the gene transcription - one morphogen acts as a transcriptino factor regulating synthesis of other morphogens only in that specific area - increasingly specfic responses to different morphogen gradients - different morphogens ultimately activate different hox genes in specific regions of embryo

Question 45

Hox genes

Answer 45

*make transcription factors that direct development of specific body segments - group of related genes that control development of the body segments slong the anterior-posterior axis - different embryonic segments express different hox genes each encodes a *transcription factor* that determines form and function of each segment * one hox txn factor can activate one gene and express a different gene * which hox genes are turned on is based on signaling events that preceded it

Question 46

Drosophila Hox Mutation

Answer 46

- leg hox gene mis-xpresssed in head | - head takes on thoracic identity - legs develop in place of antennae

Question 47

Timing of human development

Answer 47

9 months 3 trimesters

Question 48

1st trimester

Answer 48

1. rapid cell division and tissue differentiation - embryo most sensitive to pathogens that can cause birth defects 2. implantation ~ 6 days after fertilization - gastrulation begins 3. initial development of all major organ systems begins - neurulatinon ~day 17-22 - heart begins to beat during week 4 - limbs formed by week 8 4. week 9: embryo now called FETUS - cell diff mostly complete

Question 49

2nd trimester

Answer 49

- grows rapidly in weight - limbs elongate - fingers, toes, facial features become well formed - nervous system continues to develop and does so rapidly - fetal movements first felt by mother

Question 50

3rd trimester

Answer 50

- continues to grow rapidly - at the end internal organs mature - digestive system begins to function - lover stores glycogen - kidneys produce urine - brain undergoes cycles of sleep/wake - born as soon as last critical organ (lungs) mature

Question 51

Hox genes and their transcription factors

Answer 51

hox transcription factors transcribe specific genes that encode specific proteins that dictate development of different structures (legs, antennae, etc) different segments of embryo have different combinations of proteins that confer their identity

Question 52

Example process of Hox Txn Genes (Arms)

Answer 52

1. specific morphogen turns on txn of hox gene 4 2. hox 4 protein (a txn factor) is released 3. hox 4 txn factor induces txn of many genes that regulate dev of arms 4. proteins that regulate development of arms are synthesized 5. arms are formed

Question 53

Hox gene - body region

Answer 53

1. hox gene leads to txn of multiple genes that encode proteins that are necessary to make that particualr structure 2. provides into about body positioning olong ant/pos axis 3. helps determine cell fate

Question 54

When does implantation happen? what happens then?

Answer 54

~ 6 days after fertilization | - gastrulation begins (cleavage and blastula have already happened)

Question 55

When does neurulation begin?

Answer 55

- neurulatinon ~day 17-22

Question 56

When does the heart begin beating?

Answer 56

- heart begins to beat during week 4

Question 57

When are the limbs finished forming?

Answer 57

- limbs formed by week 8

Question 58

When is the baby most susceptible to pathogens that cause birth defects?

Answer 58

1st trimester

Question 59

blastocoal vs blastopore

Answer 59

Blastocoel - fluid in the center of blastula Blastopore - pore that forms/invaginates into what was previously the blastula

Question 60

Anterior node

Answer 60

Equivalent of spemann organizer in primitive streak of mammals called hensen's node in birds

Question 61

Grey crescent vs dorsal lip vs organizer

Answer 61

Grey crescent: region that organizes, before involution Dorsal Lip: Lip - physical point of the involution Organizer: molecular basis, concept that there is an organizing region