Fertilized zygote Pronuclei (n=2) 1. Fertilization: 12-24 hours after ovulation 2. One-cell stage: two pronuclei fuse (zygote) 3. First cleavage : two-cell stage ≈ 30 hours post ovulation
2 cell stage During the 1st week, cells continue to undergo cleavage* Cells become smaller, form into a ball - morula (12-32 cells) - blastula (128 cells), around a central cavity - blastocyst (700-800 cells) morula blastula 4 cell stage blastocyst *using up energy reserves!
inner cell mass --> embryoblast (future organism) outer cell mass --> trophoblast (supporting structures)* Cells in blastocyst have already begun to change (differentiate) *more on the trophoblast next week
Blastocyst implants into uterine wall ( ≈ 50% chance...) WEEK TWO The embryoblast divides into two layers: Epiblast (top): future embryo Hypoblast (below): yolk sac
From here on, we will focus on just the embryo … How do we go from a bunch of cells, the epiblast, to forming all the structures and tissues of the embryo?
GASTRULATION! (Gastrula is Latin for ² stomach ³ )
Gastrulation is the formation of the three germ layers (cell lineages) common to all animals A process of migration and differentiation -Creates an embryo with three layers : ectoderm mesoderm endoderm -Creates a gut tube ( endoderm ) -Creates a central rod ( notochord )
Gastrulation in amphibians:
caudal (tail) cephalic (head) If you were to view gastrulation from the top (in mammals), you ± d see the primitive streak grow from the caudal (tail) end toward the middle
Cells in midline of epiblast dive down & spread out As they migrate, they form two new layers: mesoderm endoderm The epiblast layer becomes ectoderm Side view Top view
Notochord (stiffened rod of mesoderm; future ² core ³ of axial skeleton) IN ALL VERTEBRATES (actually chordates), another important event happens during gastrulation:
Here ± s what the embryo looks like in a section after gastrulation:
A movie of gastrulation (courtesy Univ. of Montréal):
(Cranial end of embryo) SO, what is the embryo like after three weeks?
- Fall '10
- Developmental Biology, Neural tube, Mesoderm