Note: read sec 1-4 of embryology lab guide in preparation for the lab 
next week!

GASTRULATION:

at the end of the blastula stage you are left w/a bunch of cells poised 
to start moving around and associating w/each other.

Recall the outer, middle, and inner layer of the early embryo...
these are the

ECTODERM (outer)
ENDODERM (inner)
MESODERM (middle)

Gastrulation creates these three layers of cells, which then interact 
w/each other to form the organ rudiments.  How are these formed so they 
can undergo these movements?

GERM LAYERS

ECTODERM gives rise to Nervous system and several layers of skin

ENDODERM gives rise to digestive system and associated organs

MESODERM gives rise to those structures which give rigidity to embryo, eg 
bone, muscle.

note that in the adult, it is highly irregular to find a concentration of 
just ONE of these types in one area. All the germ layers are used in most 
of the systems.

Gastrulation is the process of displacement of parts of the early embryo. 
That is, a removal into the interior of future endodermal and mesodermal 
material from the embryo's surface.

Ok, that sounds good. So, you have formless,shapeless,nameless blastula, 
sitting there. It looks kind of like a balloon.  So, if you have a 
balloon, and you push in your finger into the side, you create a two 
layer structure!

The indentation is called a BLASTOPORE.

        _______
	   /       \ -ectoderm
       |    ____|
       | *  |---------endoderm      This is NOT pacman.
       |    |____
       |         /
       \________/ 
*mesoderm forms in here

[slide- frog gastrulation shown - blastula only has very tiny blastoceole 
at one end. bird just has thin cell layer on end of yolk sac.]


[slide- amphibian egg photographs showing certain mechanisms of 
gastrulation] including invagination, as an inherent quality of the cell! 
also involution, a rolling over - the second group of cells to go in, eg, 
the cells attached to the invaginating cells, are seen to involute, roll 
over, and go in. as cells start moving from exterior to interior, you 
need more cells, so you have epibaly, which by mitotic division helps to 
repopulate the surface. this happens very site-specifically at the future 
caudal end of the animal, called the blastopore. on the dorsal lip of the 
blastopore lies a population of cells which have the inherent 
invagination quality. would invaginate even if moved elsewhere. So, you 
get a little slit where the invagination occurs; the ingression point 
gets larger, cells go in, yolk plug gets smaller and smaller, and at the 
end of the gastrula stage, part of the embryo flattens out - starting to 
form neural plate.

FATE MAPS

you can map the embryo to the adult.

the dorsal lip is the edge of he blastopore. just think about it. first 
parts to go IN through the pore HAVE to be cranial structures, since the 
pore is the caudal part....

first cells to go in become head endoderm - mouth, pharynx. then 
presumptive prechordal plate (head mesoderm - facial muscles, etc.), 
presumptive notochord (rod running cranial-caudal, main craniocaudal axis 
of vertebrates, scaffold for vertebrae), presumptive neural system (stays 
on outside and notochord runs just underneath it. there is an association 
betw notochord and overlying ectoderm.)

REmember that these cranial structures obviously go in first, others 
follow, it's pretty obvious.

[slide- gastrulation in the chicken embryo]

yolk is a real impediment in the migration of cells to the inside...

How the chicken solves this (and how mammals solved it too) is by having 
another slightly different method of gastrulation. We don't have 
invagination in birds and mammals...the cells don't go into a "hole" and 
do not have invaginative properties. THey do have involution and 
epiboly...and they also have DELAMINATION- a layer delaminates and 
breakes off from the heretofore single layer, from the bottom of the 
layer, and then drops down and reforms right on the surface of the yolk, 
as a separate layer. This happens on the blastoderm of the chick/mammal 
embryo. So, then you have epiblast layer on top of blastocoele and 
hypoblast underneath blastocoele.

in the early blastula, you have the area pellucida in the middle and the 
area opaca cranially and caudally sitting over the yolk directly.

primitive streak in chick/mammal homologous to blastopore in frog/amphib.

instead of going through a hole, the cells move through a trough and a 
knot of cells at the very cranial end called hensens's node. hensen's 
node, because of what goes through it, especially initially, is 
homologous to the dorsal lip of the blastopore.

[slide-schematic of primitive streak moving through hansens's node]

you can grow two embryos of the same age, and label one with radioiodide 
or something, transplant and see where the radioactive tags are.

Fate map of bird embryo similar to frog fatemap, although not exactly 
same.
look for this in the book.
when primitive streak about 2/3 length of embryo, cells moving cranial to 
it become notochord.Then the node starts moving caudally until it 
virtually disappears. The notochord follows the node caudally. cells 
going through the node at this time make lateral mesoderm.

mammal gastrulation takes place almost identically to bird gastrulation. 
mammal embryos act as if there were a huge yolk sac, but there isn't. 
There are a few modifications to the procedure, mainly due to nutritional 
considerations. mammalian embryo has to attach to uterine wall for 
nutrients, metabolic waste disposal, etc.

[slide-cleaving mammalian embryo, trophoblast]

source of trophoblast completely extraembryonic. will not be contained 
w/in embryo. inner cell mass equivalent to blastoderm of chick. 
delamination will occur w/blastoderm, primitive streak will appear. some 
cells will form extraembryonic structures (eg, placenta etc). 

so, trophoblast delaminates. inner cell mass bottom layer falls down to 
be entoderm (extraembryonic endoderm). THen you have inner cell mass 
delaminated from entoderm, and it's now called the embryonic disk. (inner 
cell mass minus delaminated layer= embryonic disk). This is the cell mass 
that will gastrulate and become the body and will contribute to 
extraembryonic structures as well.

blastocyst, fluid filled vesicle, inner cell mass is small and oriented 
at one pole of the fluid filled blastocyst. SHeep and pigs have HUGE 
blastocysts. Carnivores not too big. Horse pretty big.