Developmental Biology (BS 6th Ex. PPP)
Compare embryology of sea urchin, frog, bird and mammal
similarities and differences
Despite the differences in the adult form of animals across the vertebrates, their embryology is quite similar. What are the groups of vertebrates?
Anamniotes
1. Fish
2. Amphibians
Amniotes
3. Reptiles
4. Birds
5. Mammals
Superphylum: Deuterostomia (have 2 openings- mouth and anus)
Phyla: Echinodermata Chordata (FYI: Chaetognatha Hemichordata)
We'll look at sea urchins too because their embryology is so similar to early stages of vertebrates
Karl Ernst Von Baer: The general characters of the group to which an embryo belongs appear in development earlier than the special characters. Similarities are seen early, differences later.

Vertebrates (and the sea urchin) undergo similar transition from zygote to blastula

Many apparent differences seen among vertebrates are thought to be due to the amount of yolk in the zygote.

Basic terminology
Zygote - single celled embryo
cleavage furrow - cleft formed by cytokenesis
Blastomeres - cells resulting from cleavage
Blastula - embryo composed of many Blastomeres and a hollow center
Sea Urchin embryology: diagram
Microlecithal egg - very little yolk
Cleavage plane is perpendicular to previous cleavage (called radial cleavage)
Animal pole - contains mesomeres
Vegetal pole - contains macromeres and micromeres
Blastocoel - cavity of the blastula
Amphibian Embryology: diagram
Mesolecithal egg - moderate amount of yolk
Radial cleavage
Animal pole is pigmented;
unpigmented vegetal pole contains most of the yolk
Gray crescent - forms by the displacement of pigment following fertilization
Blastocoel is displaced toward the animal pole because of the presence of the large yolk-laden macromeres
The blasotcoel is surrounded by multiple layers of cells, unlike the sea urchin
Avian Embryology: diagram
Macrolecithal egg - large amount of yolk
Blastodisc - a small region of cytoplasm containing the nucleus and free of yolk
Cleavage does not divide yolk, as in amphibians
Discoidal cleavage - occurs only through blastodisc
Blastoderm - formed from the blastodisc as it acquires several layers
Subgerminal cavity - fluid filled cavity forms beneath blastoderm; it's not the blastocoel
Multicellular blastoderm layer above the subgerminal cavitiy becomes a single layer! Epiblast. Hypoblast cells come from the periphery, and migrate underneath epiblast forming a second cavity between epi- and hypo-blast, the blastocoel
Mammalian Embryology: diagram
Mammals are a diverse group. There are mammals that lay eggs! We'll study the embryology common to mice and humans.
Microlecithal egg, but embryo forms on one side as if it had a large yolk like a bird
Blastomeres undergo compaction to eliminate gaps, tight junctions form
Ions are pumped inward by the outer cells, causing osmosis to form a cavity - this embryo is now called a blastocyst (not a blastula)
Inner cells stick to one pole and are called the inner cell mass
Outer cells are called trophoblast
The inner cell mass divides into epiblast and hypoblast layers: delamination
Sea Urchin Gastrulation
Ingression: micromeres pass into the blastocoel
Invagination: the vegetal pole folds inward forming a pocket
Archenteron: the pocket (endoderm) continues to expand as a tube toward the animal pole
Outer cells: ectoderm Inner cells: endoderm Micromeres: mesoderm

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