Early Craniate MorphogenesisWhy study this material?

Phylogeny

OntogenyMorphology

What is Life History?

Distinct periods or times?

How should we refer to them?

generate list(s) on board

Craniate EggsTypes 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

Macrolecithal – telolecithal eggs… found in marine lampreys, cartilaginous fish, reptiles, monotremes

http://www.bio.unc.edu/faculty/harris/Courses/biol104/frog.jpg

Craniate EggsOviparity & 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 EggsViviparity:

Histotrophic vs. placental nourishment

http://www.biologie.uni-hamburg.de/zim/herpe/bilder/Ichthyophis_Embryo.jpg

Life in cold blood caecilian clip

Craniate EggsFertilization:

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/salamandridae/triturus/cristatus/

triturus_cristatus_mazzei.jpg

Early DevelopmentCleavage and the blastula:

As fertilized egg cells divide this is called cleavage.

What happens to cell size initially?

Hollow sphere is called the blast-ulaand 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 DevelopmentLet’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 DevelopmentGastrulation 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 DevelopmentMesoderm 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 DevelopmentGastrulation:

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 DevelopmentWhat 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

Defective homeotic genes can have severe developmental consequences… ex. spina bifida

http://health.yahoo.com/media/mayoclinic/images/image_popup/r7_spinabifida.jpg

http://www.humanillnesses.com/images/hdc_0000_0001_0_img0024.jpg

Early DevelopmentWhat is the fate of the different “derms”?

Germ LayersPlacental 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=UgT5rUQ9EmQ&NR=1

Extraembryonic MembranesExtraembryonic membranes include:

Yolk sac, amnion, chorion and allantois

http://embryology.med.unsw.edu.au/Movies/larsen/fetalmembranes.jpg

Extraembryonic Membranes

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/Resources/Pictures/embryo-1-wk.JPG

What does a yolk sac accomplish? How?

Extraembryonic MembranesReptiles 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 MembranesAllantois 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 MembranesPlacenta 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 MembranesPlacentas 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