9.19.96
Embryology


announcement: in Patten chapter on gastrulation there is a description of 
rodent gastrulation which is atypical for mammals. Disregard it. Just be 
sure to know typical mammalian gastrulation.

suggestion: read article which has been put on reserve in library on 
goosecoid gene and the origin of organizer cells in the early chick 
blastoderm.  This paper is one of the so called seminal papers of the 
past three years in this field, and is mentioned in Patten's 6th ed in 
several places.
read chapter on basic body plan of mammalian embryos and neural and axis 
formation in embryos. Will go VERY fast today, for an overview, and then 
come back to it, so understand that it will be hard to keep up and that's 
ok.

so, once gastrulation sorts out the germ layers and they are ready to 
interact w/eachother, the first of these interactions occurs, and it is 
called PRIMARY INDUCTION. This primary induction is of the NERVOUS 
SYSTEM. INDUCTION = the dependence of one tissue's association with an 
adjoining tissue for the fulfillment of its developmental endpoint. 
Earlier we talked about an induction, when the forebrain and the 
overlying skin(ectoderm) needed to touch to develop the eye.

The Organizer, known all the way back in the 1920s...dorsal lip of 
blastopore and cells surrounding it, if it was moved to another spot on 
the embryo, would organize or induce an entirely well integrated nervous 
system of a second embryo, and even some sense organs and urogenital 
system. If you remember how prospective notochord is the inner layer of 
the dorsal lip, you can kind of understand. The adjacency of tissues to 
reach enpoint is necessary, because if you put embryo in solution of the 
wrong isotonicity, they will exogastrulate, and then they don't underlie 
the ectoderm, and the nervous sytem isn't formed.

In the chick, Hensen's node and the primitive streak are homologous to 
the dorsal lip of the blastopore in the amphibian. Hensens node is the 
part that is the primary organizer of the embryo, homologous to dorsal 
lip. 

Now, the goosecoid gene is a homeobox gene, responsible for patterning, 
and this particular gene has been found to be involved with primary 
induction, because before the experiment, it was found to be localized in 
dorsal lip of blastopore, and now its transcripts were found in the chick 
embryo, and we've found the expression of goosecoid in the mouse embryo 
and in hansens node and the primitive streak of the mouse embryo.

some of the most telling evidence that goosecoid is involved is that they 
have isolated mRNA from it, injected the mRNA on the opposite side of an 
amphibian egg, and just like in old dorsal lip experiments, this resulted 
in formation of a secondary axis and nervous system. This happened in 
frog eggs injected w/chick goosecoid. So, chick goosecoid has potential 
to organize frog tissue similarly to how dorsal lip organizes frog 
embryo.

ORGANOGENESIS IN MAMMAL

organs of ectodermal derivation:
neural tube (around mesodermic notochord), formed from flattened or 
palisaded area of ectodermal cells called the neural plate. This plate 
curves into the neural fold, and then completes curving into neural tube. 
This change in the ectoderm is induced by the presence of the underlying 
notochord and mesoderm which induce ectodermal cells to palisade, curve 
and fold. The cranial end of the tube becomes expanded as the brain 
grows, and caudally becomes the spinal chord. Very soon the forebrain 
(telencephalon [olfactory] and diencephalon [associated w/eye])  
hindbrain is source of roots of most of cranial nerves, and can be seen 
in 10 mm pig. 

Another ectodermal derivative is the population of cells at the neural 
crest, called neural crest cells, which form at the edges where the 
neural groove meets and comes together as the neural tube. These cells 
kind of detach and sit between the neural tube and the overlying ectoderm 
(epidermis). These neural crest cells are wandering. They go all over the 
embryo, and make contributions to the developing organs - pigmented cells 
of skin and hair, some elements of spinal and cranial ganglia, adrenal 
medulla, pulp of teeth, always discovering new things they do. Usually 
work with other tissues to form structure. 

PResence of ectodermal structure called a PLACODE (thickening) occurs 
regionally when developing brain is close to overlying ectoderm. So you 
get a placode involved in forming the inner ear, the otic/auditory 
placode. you get an olfactory placode, a lens of the eye placode, and 
finally you have some placodes that contribute to part of the cranial 
nerve ganglia. so the cranial nerve ganglia come from the placodes + the 
neural crest cells.

Ectoderm also of course becomes cutaneous tissue. skin.

MESODERM: contributes by far most to shape and bulk of body. Is that 
layer whih interpolates itself initially between ectoderm and endoderm. 
Somites, on either side of the neural tube, also sometimes called 
segmental plate, paraxial mesoderm or axial mesoderm because of their 
periodicity, are made of mesoderm. The somites stretch laterally to form 
intermediate mesoderm, which splits into two layers - the "top" closer to 
the ectoderm is the somatic layer, and the bottom closer to the endoderm 
is the splanchnic layer. The intermediate mesoderm is responsible for 
forming almost all of the urogenital system. The lateral mesoderm is made 
of the somatic and splanchnic mesoderms. What is nice to see is that the 
space between the layers of lateral mesoderm is the operational 
definition of the coelom. The coelom becomes divided into the pleural, 
pericardial, and abdominal cavities. note that at 10mm stage, somite 
develops into three stages: myotome, dermatome, and scleratome. 
scleratome is closest to the notochord - develops from cells which 
migrate from somite and move toward notochord, and which develop into 
vertebrae and ribs. The dermatome is the most exterior part of the former 
somite,and mainly becomes the dermis/CT layer of skin. the myotome has a 
fate which is seeming more and more important as research continues. It 
contributes to skeletal muscle of the trunk and the limbs.  Somatic and 
splanchnic mesoderm form as the somatic mesoderm splits to cover the 
exterior side of the coelum and the interior of the gut; so it turns into 
peritoneum; and splanchnic mesoderm ends up being smooth muscle of GI 
tract and mesenteric membranes, and is also responsible for heart 
formation.

ENDODERMAL DERIVATIVES:

In the developing embryo there is a folding of the sheets of endoderm, 
forming an actual tubelike structure, and in the cranial part of the 
embryo it makes a depression which will be the future mouth cavity, 
currently called stomadeum, which is bounded and cut off from rest of GI 
tube by its own stomadeal or oral membrane or plate. This membrane, and 
also the cloacal membrane which separates the cloaca from the hindgut, 
are unique in that they are double layered. These membranes are double 
layers of endoderm and ectoderm. The cranial membrane breaks through 
first, and the cloacal membrane remains closed until a later stage. The 
stomadeal membrane is broken through in the 10mm pig, and the cloacal 
membrane is still intact, indicating the proctedaeum region. So, after 
the foldings take place, the membranes of the GI tract fold and meet 
cranially and caudally and form the anterior intestinal portal and the 
posterior intestinal portal - so the midgut is open, but as the membranes 
draw together it forms these cavities called portals, like mouths of 
caves. As development continues, membranes come together at "yolk stalk" 
into umbilicus. This open part of midgut is contiguous with the vestigial 
yolk sac, which is source of early blood cells, and is used for 
apposition to uterine lining and development of placenta, waste transfer 
bet. mom and fetus, etc. Right in the area between the yolksac and the 
hindgut is another population of endodermal cells which arises at the 
confluence of the yolk sac with the hindgut and the allantoic stalk, 
which are bizarre in that they are endodermal, but they are primordial 
germ cells. These cells have o migrate from that area to the area of the 
primitive gonad. These primordial germ cells migrate along the primitive 
mesentery until they lodge themselves in the primitive gonad. So the 
gonad has an endodermal component.

Accessory organs form from GI tract as well...pharynx, esophagus, gill 
clefts - no such structures in the mammal, but there are outpouches of 
the cranial endodermal tube, called pharyngeal pouches or sometimes 
called gill pouches - they are endoderm coming out and touching ectoderm, 
generating thymus, eustachian tube, parathyroids, and in the middle, the 
thyroid diverticulum develops into the thyroid. Further caudally is the 
lung bud bifurcation, it isn't a lung yet, but is the beginning of 
bronchi formation. further caudally is the stomach region - in ruminants, 
all four sections are derived from an embryonic simple stomach, not from 
the esophagus. further past stomach, past duodenum region, are the 
pancreatic rudiments. In the mammal you have a dorsal and a ventral 
pancreatic rudiment, which generally grow and intertwine into one mass. 
The ductal system differs between species. The dorsal pancreas empties 
into duodenum directly; the ventral pancreas empties into the bile duct; 
and either one of these or both of these can persist in the adult 
depending on species. You also have the sinusoidal liver region in two 
parts - pars hepatica and pars cystica (gall bladder).  Now, at about 
10mm stage we have all this rudimentary organ formation, andyou can see 
this under the microscope as we have done in lab.