Blastocyst formation and gastrulation

Mammals follow a different route to become blastocysts not blastula. Cleavage is slow (12-24 hours) Gene transcription begins at the 2-cell stage. Cleavage takes place within the ZONA PELLUCIDA to form a MORULA (solid ball of cells). From the 8 cell-stage, cells undergo COMPACTATION forming tight junctions,

Internal and external Cells of the morulaBy the 16 cell-stage, the morula consists of a small group of internal cells surrounded by a larger groups of external cells. The external cells will become the TROPHOBLASTS (placenta food) that give rise to the placenta. Internal cells group at one pole to form the INNER CELL MASS that gives rise to the embryo, yolk sac, allantois and amnion. When the inner cell mass is isolated and cultured they give rise to embryonic stem cells which can become any cell type in the embryo proper.Whether a cell ends up in the inner cell mass or not depends on its position in the morula: cells on the inside become part of the inner cell mass.Allocation to the Inner Cell Mass:If you move a cell from the inside of one morula to the outside of another morula, it will become part of the trophectoderm of the second morula.

Forming the BlastocystAt the 32-cell stage the trophoblasts secrete fluid into the morular creating a blastocoel. The embryo is now called a blastocyst. By the 64-cells stage there is NO mixing between cells of the trophoblasts and cells of the inner cell mass. The blastocyst hatches from the zona pellucida after ~ 5 days and implants itselfs in the uterus. Fertilization occurs in the oviduct.

Early Mammalian DevelopmentIt is flexible as the embryo can adjust. Half embryos develop normally and embryos can twin up to at least day 9.Two 8-cell morulas can be combined to generate chimeric animalsInjecting cells into the inner cell mass can create transgenic mice in which the injected cells can participate in all adult structures including the germline

Fate MapsA fate map is a diagram which shows what will become of each region of the embryo in the course of normal development.To draw a fate map, the cells in a region are labeled with a dye. At a later stage in development the distribution of the label is re-measured to see where the daughter cells are. The cells of different regions must not randomly mix. The stronger the colour the higher proportion of that cell progeny contributed to that germ layer

GastrulationIt is the cell movement that gives rise to: ectoderm, mesoderm and endoderm. The cells that will form the endodermal and mesodermal organs are brought inside the embryo. The ectodermal cells that will form the skin and nervous system are spread over the outside surface. The topology of gastrulation varies depending on whether the embryo starts as a spherical blastula((sea urchins and frogs) or sheets of cells (bird and mammals). The basic outcome of is the same with: Endoderm on the inside Mesoderm in the middle and ectoderm on the outside. Many of the genes and mechanisms are conserved.InitiationIn xenopus it is initiated at the future dorsal side of the embryo, just below the equator in the region of the gray cresent. This is the marginal zone where animal and vegetal poles meet, the vegetal cells have very little yolk. Steps: The prospective endodermal cells invaginate to form a slit like blastopore a the dorsal equator. At the dorasl lip of the blastopore, the sheet of deep marginal zone cells involutes. At the same time the animal cells spread as a unit to enclose the deeper layers of the embryo InvolutionThe inward movement of an expanding outer layer so that it spreads over the internal surface of the remaining external cells.The first cells to involute will form the most anterior endodermal and mesodermal cells, the last will form the posterior structures. The cells are patterned as they involute.