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 Macrolecithal – telolecithal eggs… found in marine lampreys, cartilaginous fish, reptiles, monotremes http://www.bio.unc.edu/faculty/harris/Courses/biol104/ frog.jpg
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 http://www.biologie.uni- hamburg.de/zim/herpe/bilder/Ichthyophis_Embryo.jpg Life in cold blood caecilian clip
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 1 st 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 Defective homeotic genes can have severe developmental consequences… ex. spina bifida http://health.yahoo.com/media/mayoclinic/images/image_p opup/r7_spinabifida.jpg 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 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 What does a yolk sac accomplish? How?
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