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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 .
In the mammalian blastocyst there are three lineages that give rise to later tissue development. The epiblast gives rise to the fetus itself while the trophoblast develops into part of the placenta and the primitive endoderm becomes the yolk sac. [6] In the mouse embryo, blastocoel formation begins at the 32-cell stage.
Diagram of stages of developing blastocyst in the human until implantation. There are two stages of migration involved in implantation, the first is the migration of the zygote, and the second is the migration of the trophoblast. [13]
The morula is now watertight, to contain the fluid that the cells will later pump into the embryo to transform it into the blastocyst. In humans, the morula enters the uterus after three or four days, and begins to take in fluid, as sodium-potassium pumps on the trophoblasts pump sodium into the morula, drawing in water by osmosis from the ...
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 structure is then termed a blastula, or a blastocyst in mammals. The mammalian blastocyst hatches before implantating into the endometrial lining of the womb . Once implanted the embryo will continue its development through the next stages of gastrulation , neurulation , and organogenesis .
The trophoblast (from Greek trephein: to feed; and blastos: germinator) is the outer layer of cells of the blastocyst. Trophoblasts are present four days after fertilization in humans. [1] They provide nutrients to the embryo and develop into a large part of the placenta.
Once the blastocyst is formed, it undergoes implantation into the endometrium. [4] During implantation the blastocyst, which contains the inner cell mass, undergoes cellular differentiation into the two layers of the bilaminar embryonic disc. One of which is the epiblast, also known as the primitive ectoderm.