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A. Morula and B. cross section of a blastula displaying the blastocoel and blastoderm of early animal embryonic development. Blastulation is the stage in early animal embryonic development that produces the blastula. In mammalian development, the blastula develops into the blastocyst with a differentiated inner cell mass and an outer trophectoderm.
The blastocyst is a structure formed in the early embryonic development of mammals. It possesses an inner cell mass (ICM) also known as the embryoblast which subsequently forms the embryo , and an outer layer of trophoblast cells called the trophectoderm .
The division of blastomeres from the zygote allows a single fertile cell to continue to cleave and differentiate until a blastocyst forms. The differentiation of the blastomere allows for the development of two distinct cell populations: the inner cell mass, which becomes the precursor to the embryo, and the trophectoderm, which becomes the precursor to the placenta.
The blastocoel further expands and the inner cell mass becomes positioned on one side of the trophoblast cells forming a mammalian blastula, called a blastocyst. The axis formed by the inner cell mass and the blastocoel is the first axis of symmetry of mammalian embryo and determines its attachment point to the uterus.
The blastula is usually a spherical layer of cells (the blastoderm) surrounding a fluid-filled or yolk-filled cavity the blastocoel. [citation needed] Mammals at this stage form a structure called the blastocyst, characterized by an inner cell mass that is distinct from the surrounding blastula.
The difference between a mammalian embryo and an embryo of a lower chordate animal is evident starting from blastula stage. Due to that fact, the developing mammalian embryo at this stage is called a blastocyst, not a blastula, which is more generic term. There are also several other differences from embryogenesis in lower chordates.
Cleavage ends with the formation of the blastula, or of the blastocyst in mammals. Depending mostly on the concentration of yolk in the egg, the cleavage can be holoblastic (total or complete cleavage) or meroblastic (partial or incomplete cleavage).
Bilaterians likely evolved from an ancestor that was radially symmetrical. There have been suggestions that the blastopore started out as the digestive surface on a radial organism that became elongated (and thus bilaterally symmetrical) before its sides closed over to leave a mouth at the front and an anus at the rear. [1]