Fertilization (fusion of 2 gametes) 

•  2 cell stage (Zygote) 

•  After multiple cleavages, a solid 

ball of cells (called a Morula) 

•  Blastula is a hollow ball of cells; 

lumen is blastocoel or blastocyst 

cavity 

•  Gastrula is formed by involution of 

cells through the “blastopore”

•  Neurula(formation of neural 

plate); neurulationincludes neural 

tube closure 

Totipotent:

- become any cells of the body

- have capacity to renew themselves

become any organ

- they are the true stem cells

Progenitors:

- Can't differentiate to be any cells

- Can't renew themselves indefinitely 

- They become more restricted through development

1. Human 2cell (zygote)

2. Human 4-cell

3. Human 16-cell (morula)

4. Human Blastula 

1.  Ectoderm – e.g. nervous system , skin 

2.  Mesoderrm– e.g. muscle (heart), blood vessels, skeleton 

3.  Endoderm – e.g. gut, lungs, kidneys 

In Deuterostomes (vertebrates), archenteron will become 

digestive tract and blastopore will become the anus. In 

Protostomes (invertebrates), blastopore will become the mouth. 

bottled cells important to the evolution process. critical to transforming these tissues into the three tissue types. Critical part of development. 

- they start to become a pocket called the archenteron. 

bottled cells important to the evolution process. critical to transforming these tissues into the three tissue types. Critical part of development. 

- they start to become a pocket called the archenteron. 

Transplanting organizer (bottle cells) to other parts of gastrula 

will start another blastopore (i.e. another axis of the body) 

1. Blastula flattens to blastodisc which contains cells of varying size large, yolk-rich cells accumulate at lower and posterior surface, small yolk-poor cells at upper surface

2. separaion gradually forms between large lower and small upper cells (=blastocoel), forms epiblast (dorsally) and hypoblast (ventrally)

- upper epiblast forms prospective ectoderm and mesoderm, lower hypoblast forms prospective endoderm 

-hypoblast spreads anterolaterally to form endoderm layer. 

- mesoderm is critical to the initial differentiation of the 3 tissues. Get the formation of the 3 different layers

3. Mesoderm infolding occurs in central dorsal region of embryo

- infolding begins at posterior end of blastodisc; progresses anteriorly forming a groove bounded by parallel ridges = primitive streak 

- at anterior end of this groove, there exists a raised node of tissue with a pit extending down and forward beneath it = henson's Node (site of the brain)

4. Mesodermal tissue migrates downwar along primitive streak expanding laterally beneath ectoderm and above the endoderm (ingresses from the ventral hypoblast)

When does the dorsal ectoderm acquire neural fate? 

i) what is animal cap assay? 

ii) What are the two different results of animal cap assay? 

i)Isolate animal caps from 

frog embryos of different 

stages. (embryo cut) 

ii)• Animal caps isolated from 

pre-gastrula embryos (i.e. 

blastula) differentiate into 

epidermis. 

• Animal caps isolated from 

gastrulaembryos 

differentiate into neural 

tissue. 

Involution of mesoderm is source of the neural induction signal.

•  Harvested the dorsal blastopore 

(mesodermal tissue) from a pigmented 

donor frog blastula 

•  Transplanted the dorsal blastopore to the 

ventral side of a non-pigmented host frog 

blastula 

Results: 

•  Formation of an ectopic neuraxis (another 

neuraxis) 

•  Donor tissue became notochord (note: 

notochord is mesoderm and will become 

bony spinal column) 

transplanted it to a different part of the embryo. it became a mirror image. created another invagination zone so result into 2 individuals.

- embryo can self differentiate, in which cells communicate with each other over great distances.

- ex. Agrican clawed frog embryo was cut in halves at blastula stage.

- if both halves have the dorsal organizer region, 2 perfect identical twins result

- in humans, 3 of 1000 live births arise frequently by the spontaneous separation of the inner cell mass of the mammalian blastula into two, followed by self regulation. 

- dorsal lip of the blastopore (Spemann's organizer) 

- Following exposure to 

involuting mesoderm 

(bottle cells), animal 

cap differentiates into 

neural tissue (cell-cell 

signaling or release of 

diffusible signal?

However, if you take saline from the 

dish containing gastrula, animal cap 

tissue will also differentiate into 

neural tissue. Thus, the inducing 

signal was suggested not to be due 

to cell-cell interaction, but some 

contaminating chemical? 

This controversial finding slowed developmental biology research for years

- the outcome of the animal cap assay changes with dissociation 

- When plated directly, animal 

caps isolated from pre-gastrula 

embryos differentiate into 

epidermis. 

BUT if the same animal cap is 

dissociated (cells separated) 

prior to plating, many of the 

dissociated cells will develop 

into neural tissue. 

Thus, neural fate is actively 

suppressed by cell-cell 

interactions within the 

ectoderm.

-Something expressed 

throughout the ectoderm inhibits 

the formation of neural tissue. 

What is the role of the TGFbeta family? (Transforming Growth Factor beta) 

• Activin monomer units are normally 

assembled into active receptors by the 

presence of their ligands (members of 

TGFβfamily, such as BMP4

• an experimentally truncated activin 

monomer will disrupt normal inhibition 

of neural tube formation by mesodermal 

cells (BMP4 cannot function) 

• Animal caps with truncated activin 

receptors become neural tissue even 

without dissociation

• Thus, TGFβfamily members, such as 

BMP4inhibit neural tissue formation via 

activin receptors

Animal caps isolated from pregastrula embryos (i.e. blastula 

stage) differentiate into 

epidermis. 

If the same animal cap is 

dissociated prior to plating, it 

develops into neural tissue. 

BUT if the same animal cap is 

dissociated + exposed to BMP4, 

the cells become epidermis. 

Noggin, Chordin & Follistatin are neural inducers 

(proteins that block binding of TGFβto activin receptors) 

Chordin, Noggin, and Follistatin 

are sufficient to: 

•  Neuralize intact (pre-gastrula) 

animal caps 

•  Rescue a ventralized embryo 

(created by UV light damage)

Expression: 

Found at the 

dorsal lip of the 

blastopore 

(pre-gastrula) 

so BMP4 binds to activin to cause formation of epidermal tissues and inhibits neural tissue formation. So Noggin, Chordin, Follistatin inhibits this inhibitor so BMP4 doesnt function so neural tissues are formed. 

Thus, the neural inducer(s) produced by Spemann’s 

Organizer (Chordin or Noggin) inhibit BMP4 which results 

in neural induction (or is it that simple)? 

Molecular basis of Spemann's Organizer

- in absence of neural inducers (noggin etc),BMP signaling pathway leads to phosphorylation of Smad complex

- smad activates DNA binding proteins that in turn activate the expression of epidermal genes and repress transcription of neural genes

- neural inducers such as noggin prevent the binding of BMP to its receptors leading to disinhibition of BMP and thus neural tissue. 

Sox family of 

transcription factors 

lies upstream of 

proneural genes 

Sox= The Soxgenes are 

named for a shared motif called 

the SRY box, a region 

homologous to the DNA-binding 

domain of SRY, the mammalian 

sex determining gene. There are 

over 20 Sox genes. 

bHLH = basic helix–loop–helix 

(transcription factors) 

- for neural differentiation

- Overexpression of 

SoxD in one half or the 

embryo results in the 

formation of ectopic 

neural tissue.  

a gain of function experiment. 

PRedict the outcome of the animal cap assay. 

1. intact animal cap from blastula

Predict outcome 

2. dissociated animal cap from blastula

predict the outcome

3. Intact animal cap from gastrula 

predict the outcome 

4. dissociated animal cap from blastula + BMP4

Predict the outcome 

5. intact animal cap from blastula and noggin 

predict the outcome 

6. Dissociated animal cap from the gastrula and BMP4

What happens to neural tissue in a Noggin KO mouse? As a result of this 

finding, what other signaling pathway has been implicated in neural 

induction? 

Describe the more complicated signaling system currently proposed for 

neural induction. 

There's always multiple of molecules so if one goes down there are backup molecules to take over the process. So processes are multigenic. This keeps everything highly accurrate in terms of getting to end point ie. a baby born to be healthy. 

show that bmp and noggin are both involved but are they the only ones involved? NO 

but if you KO both noggin and another, the mouse dies. they arent viable. 

invagination that will form the neural tube. 

notochord is a mesoderm tissue and becomes the vertebrae (becomes ossified). 

In neurulation, the 

ectoderm is divided into 

three types of cells: 

1. neural tube (which will 

form brain and spinal 

cord). some of that tissue should stay as skin (epidermal cells stay as skin) 

FGF expressions is higher in posterior neuraal tube resulting in spinal cord as opposed to brain tissue. 

2. neural crest (which 

will form peripheral 

and autonomic 

nervous systems, 

pigment cells, other 

tissues) 

3. epidermis (skin)

a part that give rise all autonomic, parasymptatheic syst, neural cells, all important cells in heart, for kidney and gut function. 

- can be diff into both mesodermal and neural tissues. 

- form all the bone and cartilage of your face. if have neural crest deformation get cleft disorder. problems of differentiation of neural crest. 

teratogens doesnt take place until the third week 

teratogen is a name given to a drug that gives birth defects. dont have proper extensions. hands were attached to the shoulder. 

teratogens doesnt take place until the third week 

teratogen is a name given to a drug that gives birth defects. dont have proper extensions. hands were attached to the shoulder.