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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 .
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.
After hatching, the embryonic pole of the blastocyst faces the uterine endometrium. Once they make contact the trophoblast begins to rapidly proliferate. The cytotrophoblast secretes proteolytic enzymes to break down the extracellular matrix between the endometrial cells to allow finger-like projections of trophoblast to penetrate through.
Blastocyst with an inner cell mass and trophoblast. Cleavage itself is the first stage in blastulation, the process of forming the blastocyst. Cells differentiate into an outer layer of cells called the trophoblast, and an inner cell mass. With further compaction the individual outer blastomeres, the trophoblasts, become indistinguishable.
It is the mass of cells inside the blastocyst that will eventually give rise to the definitive structures of the fetus. The inner cell mass forms in the earliest stages of embryonic development, before implantation into the endometrium of the uterus. [1] The ICM is entirely surrounded by the single layer of trophoblast cells of 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.
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.
Cavitation is the formation of the blastocoel, a fluid-filled cavity that defines the blastula, or in mammals the blastocyst. [1] After fertilization , cell division of the zygote occurs which results in the formation of a solid ball of cells ( blastomeres ) called the morula .