Gastrulation thru Organogenesis

Question 1

Gastrulation General

Answer 1

•Blastula to second stage called gastrulation where end up w/gastrula

• Goal - form 3 primary germ layers

•in-folding; few cells break off and accumulate inside; pushed way in – push thumb into a tennis ball, creating new space which is the blastopore which becomes the anus of the embryo; mouth is second in deuterostomes

• Few more cells break off and replicate
• End up w/3 primary germ layers of cells in gastrula
• Endoderm surrounds primitive gut area called archenteron, ectoderm, mesoderm
• Gastrulation – infolding into gastropore or primitive grove the cells move into w/3 germ layers

Question 2

Fates for Primary Germ Layers

Answer 2

• Gastrula stage when embryo has folded in and formed 3 primary germ layers
• Couple cells set aside for germ cells which are saved until gamete time; most other cells form these 3 types
• Endoderm cells are structures of digestive tract /structures– throat, respiratory structures, digestive tube (internal structures)
• Mesoderm cells are notochord, bone tissue, muscle, blood, kidney
• Ectoderm cells are skin/outer surface and central nervous system
• Significant differentiation

Question 3

Gastrulation in Frogs

Answer 3

• 1st week is in fallopian tube and then 2nd week is implantation; embryo interacts w/wall of uterus and produces particular hormone that shows up in pregnancy tests
• Blastula where blastocoel is offset; normally vegetal pole has invagination but yolky cells so invagination is on edge of blastocoel, cells moving around and folding in which cretates a space which is primitive gut called archenteron

•Blastocoel is displaced and eventually goes away and archenteron gets bigger and is only space left

• Rounded opening and real blastopore which is filled w/yolk called yolk plug
• As cells move in the archenteron created and then start forming the 3 primary germ layers
• Some mesoderm will form notochord

Question 4

Gastrulation in Chick

Answer 4

• Groove forms on the surface of the disc which is called the primitive groove, sometimes becomes pigmented and then called primitive streak which indicates where groove is forming and axis where embryo is about to form
• Where head of embryo will develop is deeper groove called Hensen’s node
• The the groove and streak will appear in middle and move up a bit and as process of gastrulation cont and finishes then Hensen’s node will pull back and embryo’s dev happens faster in head end then tail end; head’s more developed then tail
• As groove appears then cells move across surface and into the groove like infolding in spherical embryos; cells from surface move across and into the groove and those in middle that break off become mesoderm
• Whole disc is sort of the animal pole there

Question 5

Amnion formation prior to gastrulation in humans

Answer 5

• Mammalian embryo – inner cell mass squashed up and dropped down and created a space that will create amnion
• Embryonic epiblast become embryo proper and external cells form external structures
• Some cells on outside are fused together and are no longer trophoblasts but are syncytiotrophoblasts since multi nuclei cells
• Inner cell mass now does same primitive groove, hensen’s node that saw in chicken
• Space down there become yolk sac – in humans we have little yolk so fluid filled sac
• Amnion is space > which creates bag of water around the embryo; mammals have portable bag of water where embryo develops
• Chorion is membrane around entire thing; synchiotrophoblast becomes outer layer w/multinuclear cells and invade wall of uterus to establish placenta
• Allantois develops near anus end of embryo to become sac that is used to store waste

Question 6

Gastrulation in human embryo

Answer 6

• Human embryo, and see yolk sac even though hardly any yolk in it, then drops down to form amnion space
• Look at embryonic disc and see some pigment and hensen’s node and cells go into the groove that forms
• Yellow is endoderm, mesoderm is pink and blue for ectoderm
• Hensen’s node was previously up and then receded since, no more groove for cells coming in
• Head is ahead of process of gastrulation then tail end
• Blastopore is the groove here ; anus will be back end of the streak
• Gastrulation still occurring at one end but already done on other end and estab 3 layers including some of mesoderm which is forming a notochord – stiff rod made of connective tissue which helps support embryo along its axis

Question 7

General Neuralation

Answer 7

• mesoderm (right down middle will form notochord) and will send signals to overlying ectoderm which is called primary induction – when a primary germ layer sends a signal to another primary germ layer to differentiate into something else
• Ex. Mesoderm is inducing overlying ectoderm to become the nervous system (dorsal hallow nerve chord which becomes our spinal chord in humans)
• Signals tell ectoderm to thicken and start infolding and cells start differentiating a bit and then signal causes thickening and folding
• Folds and pinches off so skin on top and dorsal hallow nerve chord formed from ectoderm due to signal from underlying mesoderm from primary induction if signal between 2 primary germ layers
• If signal from organ to non primary germ layer/structure then called secondary induction
• Neural tissue from ectoderm tissue and now neural tube but skin layer above is all ectoderm still

Question 8

Amphibian Neuralation (frog)

Answer 8

• Ectoderm got signal from mesoderm, starts to fold in and make groove which starts to fold and close and then closes so from outside you don’t see the grove anymore; see flattening of tissue called neural plate
• See neural fold and folds all around and see neural tube
• Under neural tube you see the notochord is below that
• See dorsal hallow nerve chord and under it is the notochord which is important landmark; be able to label a drawing
• Sea urchins never get dorsal hallow nerve cord or anything else anymore
• Mesoderm induces overlying ectoderm to form this nervous system

Question 9

Spemann Mangold Experiment (Primary Induction)

Answer 9

• Spemann Mangold used Newt (amphib)
• Embryo – take out some mesoderm, take another embryo and located a yolk plug and took out mesoderm from there where notochord would form and transferred to another
• Mesoderm induces 2 nervous systems ; forms a notochord
• Ex. Primary induction in embryos
• Induction is considered major mechanism of forming organs and structures in embryo

Question 10

Induction Leads to Cell Differentiation (eg. sea squirts tunicate)

Answer 10

•differentiation into 4 unique cell types by induction

• Not a deuterstome but nice induction effect – called sea squirt
• Dorsal hallow nerve chord, notochord and then a gut
• Section off center you get area of muscle and then general mesoderm cells
• Center produces fitroblast growth factor (FGF) in early embryo and also a signal factor called macho1
• Some cells have macho-1 and some not and some have FGF and some don’t
• After 32 cell to 64 cell stage ; purple cells in front become the nerve chord since no FGF and no Macho1 ; and those w/FGF and no Macho become notochord
• Green cells get macho-1 and FGF and become
• No FGF but have Macho-1 they become muscle cells

•w/2 signals you can make diff kinds of cells

• very diff fates depending on if you have FGF, Macho-1 (one, both or none of these signal factors)

Question 11

Head-to-tail progression of development in chick embryo

Answer 11

• Developing disc of chicken embryo – sliced 4 times
• Head is more developed and nervous system forming w/notochord

•Infolding about to close since head is ahead of tail end developmentally; begins to fold under and on the sides too so separate from disc its rest on too ; some cells included in embryo some are not

• But in back, still at primitive gastrulation and hensen’s node earlier was up at top pics and went shrinking back as gastrulation continues and finishes up front
• Notochord forming higher up and inducing overlying ectoderm to come up
• Somites (muscle blocks) form on either side of groove and become muscles and vertebrate as development keeps going
• Somites tell how far chick embryogenesis is
• Dev much farther advanced at head than tail w/gradient in between

Question 12

Fate of Notochord and Surroundings

Answer 12

• Somites (block of cells that will become muscle and some bone and stuff) contribute to vertebrate
• Muscle forms and some cells combine w/cells in notochord and become vertebrate And make ribs
• Area where notochord was become centrum of vertebrate and vertebral column; centrum of each vertebrate is where notochord was
• Spinal chord is dorsal hallow nerve chord we develop through development

Question 13

Neural Crest Cell Formation & Migration in Vertebrates

Answer 13

• Neural chord about to round up and close some cells break off and that region is called neural crest and they are called neural crest cells that migrate into embryo and become useful things like cells that make pigments, nerve cell bodies, ganglia, relay station, sensory structures, parts of the skull in mammals for enlarged brain’s, become parts of the face
• Neural crest cells make vertebrates better (better predators, better escapees, better sensory – they don’t make the animal)

Question 14

Neural Crest Cells Followed w/Fluorescent “reporter”

Answer 14

• Neural crest cells move through embryo itself
• Chicken embryo that was transformed w/gene to make green fluorescent protein
• Make protein produced in the chicken – way to follow the cells because they fluoresce ; developing chicken embryo w/spine forming and transplant neural tissue into another chicken embryo
• Cut out section and transplant it and look at it a day later and embryo and see green spots where neural crest cells move down through embryo and take up positions to make new nerves and new pigment cells

•As embryo gets older the neural crest cells move all around

•Neural crest cells, contribute to forming brain, face,

Question 15

Folding up to produce Embryonic Body shape

Answer 15

• To make organs, disc folds up on ends and sides
• Amnion folds up around and embryo has yolk sac hanging out; folds at head end and eye ball develops and heart and tail will develop too
• Further along you see brain - Dorsal hallow tube will start forming the brain chambers
• Yolk sac is smaller now since embryo is using up nutrition in that yolk sac
• Humans have nutrition from body stalk

Question 16

Secondary Induction

Answer 16

• Head region of vertebrate part that becomes brain is head end of neural tube and differentiates into optic stalk – not just primary germ layer but sticks out from sides of brain and these cells are signal molecule to tell ectoderm to do infolding and pinches off and forms the lens of the eye
• Lens of eye is formed from ectoderm layer under influence of signal from optic stalk from dorsal hallow neural tube which is 2ndary induction (tissue or organ signaling another tissue or primary germ layer) as long as not primary and primary germ layer

•Rest of optic stalk becomes optic nerve back to brain and lens becomes the retina in the eye

Question 17

More 2ndary induction

Answer 17

• Outer surface of body signaled to by various locations in the dermis – mesodermally derived thing of skin
• Derma where form hair; causes epidermal layer to in-fold and surround mesodermally derived tissue which signals epidermis to form hair
• Similar with feathers and scales
• Epidermis caused to in fold and make enamel parts of teeth from epidermis ; detin layer thought from neural crest cells

Question 18

Stem Cells

Answer 18

•Dermis from mesoderm which induces ectoderm to make skin layer

• stem cells are below basement membrane where make product that turns into epidermal layer for ectoderm
• desquamation > means loss of flat skin cells

•Push up cells w/bunch of desmosomes in them and then nucleus breaks down and lots of keratin produced and outer layers of skin are cornified when outer layers of keratin (former cells)