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Early Craniate Morphogenesis
Why study this material?
Phylogeny
Morphology
Ontogeny
What is Life History?
Distinct periods or times?
How should we refer to them?
generate list(s) on board
Craniate Eggs
Types of eggs:
Microlecithal – iso-lecithal
distribution of yolk… found in
placental mammals and
amphioxus
Mesolecithal – telolecithal
distribution of yolk concentrated
at the vegetal pole… found in
lampreys, bony fish, amphibians
http://www.bio.unc.edu/faculty/harris/Courses/biol104/
frog.jpg
Macrolecithal – telolecithal
eggs… found in marine lampreys,
cartilaginous fish, reptiles,
monotremes
Craniate Eggs
Oviparity & Viviparity:
Animals that “lay” their eggs are
considered oviparous.
Animals that give birth are
considered viviparous.
If the embryo could develop
without maternal tissue then
ovoviviparous, while dependent
strategies are euviviparous
No viviparous turtles, crocodiles,
or birds
http://www.nationalaquarium.ie/images/dogfishEgg.jpg
Craniate Eggs
Viviparity:
Histotrophic vs. placental
nourishment
Life in cold blood caecilian clip
http://www.biologie.unihamburg.de/zim/herpe/bilder/Ichthyophis_Embryo.jpg
Craniate Eggs
Fertilization:
Internal and external modes exist
In which type of “–parity” are eggs
by necessity internally fertilized?
Usually external fertilization
requires millions and millions of
sperm
Urodeles (salamanders) may use
spermatophores and spermatheca
http://www.amphibiainfo.com/gallery/caudata/salamandrid
ae/triturus/cristatus/triturus_cristatus_mazzei.jpg
Early Development
Cleavage and the blastula:
As fertilized egg cells divide this is called
cleavage.
What happens to cell size initially?
Hollow sphere is called the blast-ula
and it contains a hollow space… the
blastocoel.
Excessive yolk impedes cell division…
such that a blastoderm develops on
an otherwise undivided yolk.
What taxonomic group(s) would
utilize this mode?
Early Development
Let’s review what we can infer since we know this
is “Chordate” development…
What type of cleavage pattern?
What does the 1st opening become?
How many germ layers do we have?
Early Development
Gastrulation is the process in
which 3 primitive germ layers are
formed
Amphioxous eggs lack yolk and
provide a clear picture
Involution of blastula
Resultant opening is the
blastopore
Early Development
Mesoderm forms from the
endoderm and then creates
pouches (coelomic or
mesodermal)
Lateral/superficial layer becomes
somatic mesoderm
Inner/deep layer becomes
splanchnic mesoderm
What becomes of the ectoderm?
How about the endoderm?
Early Development
Gastrulation:
Frog – because the yolk
cells are slow… involution
is delayed.
Consequently, slightly
different process
accomplishes same ends.
Lateral mesoderm moves
in from sides and ventral
area of blastopore
heading for the head.
Early Development
Gastrulation:
Chick – Special consideration required when we
have a marolecithal egg.
Blastoderm (epiblast and hypoblast)
Hypoblast continues around the yolk to become
part of the yolk sac
Epiblast has cells migrating inward that become
endoderm
Cells destined to become the mesoderm migrate
in between the other 2 “derms”
Early Development
What regulates differentiation of
these cells into various germs,
derms, tissues and organs?
Organizer area
Homeotic genes (sonic hedgehog
gene)
Proteins involved in signaling called
morphogens
http://health.yahoo.com/media/mayoclinic/images/image_p
opup/r7_spinabifida.jpg
Defective homeotic genes can have
severe developmental
consequences… ex. spina bifida
http://www.humanillnesses.com/images/hdc_0000_0001_0_img0024.jpg
Early Development
What is the
fate of the
different
“derms”?
Germ Layers
Placental mammals have
varying approaches to
gastrulation
However, generally a
neural plate forms
Neural folds develop
resulting in a neural
groove
Neural folds unite forming
a neural tube
Process called Neurulation
http://www.youtube.com/watch?v=UgT5rUQ9
EmQ&NR=1
Extraembryonic Membranes
Extraembryonic membranes include:
Yolk sac, amnion, chorion and allantois
http://embryology.med.unsw.edu.au/Movies/larsen/fetalmembranes.jpg
Extraembryonic Membranes
What does a yolk sac accomplish? How?
Yolk sac is a highly vascular membrane that
surrounds the yolk.
Empties into the midgut
Can secrete enzymes to digest yolk
Can serve as respiratory organ
in viviparous amphibians/fish
Can absorb nutrients from mother…
functions as a simple
yolk sac placenta or
a “pseudoplacenta”
http://www.minkhollow.ca/HatchingProgram/Re
sources/Pictures/embryo-1-wk.JPG
Extraembryonic Membranes
Reptiles and mammals develop inside
2 sacs… Amnion and Chorion:
Amnion surrounds the embryo
Chorion surrounds the amnion and
the yolk sac
Important feature that allows eggs to
be laid on land (with less dependence
on water)
Amniotic fluid surrounds the embryo
and is contained by the amnion
Where does this water come
from?
Extraembryonic Membranes
Allantois is an evagination of the cloaca
Communicates with the inner surface of the
chorion forming the chorioallantoic
membrane
Reptiles and monotremes aids in
transferring gases (respiration)
In most mammals serves as a membrane of
the placenta… transferring nutrients and
wastes.
Base of this sac becomes the urinary
bladder
http://www.youtube.com/watch?v=lXN_sDnd1ng
Extraembryonic Membranes
Placenta generally is any
place embryonic and
maternal tissues come
together for exchange
More
specifically/restricted
definition…
organ containing highly
vascular region of
extraembryonic
membrane in
communication with
highly vascular region of
maternal tissue
http://www.acmc.uq.edu.au/images/projectimages/Fresh_Placenta.jpg
Extraembryonic Membranes
Placentas can have a
simple communication
with maternal tissue…
nondeciduous placenta
Or
They can have chorionic
villi that invade
endometrium and pull
part of this lining off with
birth… deciduous
placenta
Villi can be positioned
variously on the chorion
http://upload.wikimedia.org/wikipedia/commons/thumb/1/13/Human
_placenta_uterine_side.jpg/799px-Human_placenta_uterine_side.jpg