Blastocyst: Definition, Implantation, And Gastrulation

Blastocyst Definition

A blastocyst is a ball of cells that forms early in pregnancy, about five to six days after a sperm fertilizes an egg. It is a distinctive stage of mammalian embryo development and develops from a berry-like cluster of cells called the morula.

The blastocyst consists of two groups of cells: the inner cell mass (ICM) which will become the embryo, and the outer group which will become the cells that nourish and protect it.

Passage to the Uterine Wall

Implantation is the process by which a fertilized egg attaches to the lining of the uterus. The blastocyst, which is a ball of cells that will eventually form the embryo and placenta, attaches to the top of the uterine wall about 6 days after fertilization.

Researchers believe that as the blastocyst travels along the uterine wall, L-selectin on its surface binds to carbohydrate molecules on the uterine wall until it gradually slows to a complete stop. Once attached, trophoblast cells secrete enzymes that digest endometrial tissue.

The endometrium is one of the few uterine surfaces to which a blastocyst cannot always implant. The properties of the endometrium change, and only in a brief window can implantation occur.

In humans, this window includes days six through ten after ovulation. Trophoblast cells invade the arterial walls of the uterus, displacing the mother’s own cells.

5-day blastocyst implantation timeline

After a blastocyst transfer, human blastocysts should hatch from the shell and begin to implant 1-2 days later. Implantation should finish around day nine of the process.

In a natural situation (not IVF), the blastocyst should hatch and implant at the same time – about 6 to 10 days after ovulation. Implantation takes place 1 to 5 days after a blastocyst transfer.

A healthy blastocyst should hatch from its shell (zona pellucida) by the end of six days or earlier and is then ready to begin to implant within the lining of the uterus. Blastocysts have survived an important “survival test”.

During the first few days, the embryo relies on the egg cell (from the mother) for all its growing nutrients. However, in order to survive post-day three or four, the embryo must activate its genes so that it can carry on growing and dividing.

Every woman’s body is different, and implantation may occur at different times for each individual. The timeline provided is an average estimate based on research.

Gastrulation of the blastula

Gastrulation is a process that occurs during embryonic development, transforming the embryo from a blastula with a single layer of cells to a gastrula with three layers of cells. Before gastrulation, a single-celled zygote must divide many times to form a ball of cells called a blastula through cleavage.

The blastulas of different organisms can take many different shapes and have different patterns of cleavage. There are two types of blastulas: coeloblastula and stereoblastula. Coeloblastula is a hollow ball of cells, one cell thick, while stereoblastula exists as a solid mass of cells.

During gastrulation, the cells in the blastula rearrange themselves spatially to form three layers of cells: ectoderm, mesoderm, and endoderm. The ectoderm forms the outer layer and gives rise to skin and nervous system tissues.

Mesoderm forms the middle layer and gives rise to muscle, bone, blood vessels, and other connective tissues. Endoderm forms the innermost layer and gives rise to internal organs such as the lungs, liver, pancreas, and digestive tract.

Gastrulation involves invagination or folding in on itself creating an opening known as the archenteron or primitive gut. Invagination involves the blastula folding in on itself creating a pocket with an opening.

These are known as the archenteron and blastopore which will become the digestive tract opening. Gastrulation is followed by organogenesis when individual organs begin to develop from these three germ layers.

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