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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 has a diameter of about 0.1–0.2 mm and comprises 100-200 cells following 7-8 rounds of cleavage (cell division without cell growth). About seven days after fertilization, [ 6 ] the blastocyst undergoes implantation , embedding into the endometrium of the uterine wall where it will undergo further developmental processes ...
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.
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.
Beard believed that the following about the trophoblast: 1) The trophoblast represented the larval phase of human development 2) the embryo and trophoblast competed and were at odds of each other 3) at birth placenta dies and was ejected 4) Proteins in the trophoblast are "stereotactically different" from those in embryo. [2] [4] [5]
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.
Fluid collects between the trophoblast and the greater part of the inner cell-mass, and thus the morula is converted into a vesicle, called the blastodermic vesicle. The inner cell mass remains in contact, however, with the trophoblast at one pole of the ovum; this is named the embryonic pole, since it indicates the location where the future ...