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Embedding of the blastocyst into the endometrium requires that it hatches from the zona pellucida, the egg coat that prevents adherence to the fallopian tube as the pre-embryo makes its way to the uterus. The use of blastocysts in in vitro fertilization (IVF) involves culturing a fertilized egg for five days before transferring it into
The blastocyst is still enclosed in the egg-coat known as the zona pellucida, and for it to be able to implant into the uterine wall it must rid itself of this covering. This stage is known as zona hatching, and when there is sufficient dissolution the blastocyst is able to initiate the apposition stage of implantation.
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.
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 ]
A blastocyst is then formed and implants in the uterus. Embryogenesis continues with the next stage of gastrulation , when the three germ layers of the embryo form in a process called histogenesis , and the processes of neurulation and organogenesis follow.
A blastoid is an embryoid, [1] a stem cell-based embryo model which, morphologically and transcriptionally resembles the early, pre-implantation, mammalian conceptus, called the blastocyst. The first blastoids were created by the Nicolas Rivron laboratory [2] [3] by combining mouse embryonic stem cells and mouse trophoblast stem cells.
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.
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.