Human embryogenesis is an extremely complex process. In the first 8 weeks after the creation of a zygote, a single cell will develop into a complex organism with a multi-body plan. This process occurs in stages. In the context of developmental biology, the blastocyst is a cellular complex that forms early in the embryonic development of mammals. A blastocyst contains 2 major kinds of cells, the inner cell mass (ICM) and the trophoblast. The ICM is the cells that will eventually develop into the embryo. The trophoblast is a layer of cells that will eventually develop into the placenta. The trophoblast encloses the ICM, forming a fluid-filled cavity the blastocoel.
In humans, the blastocyst forms approximately 5 days after fertilization. Blastocysts are small (0.1-0.2 mm) and contain roughly 200-300 cells. Once the blastocyst forms, it will anchor itself to the uterine wall where it will undergo the next stages of embryonic development. the formation of the blastocyst is the first point in embryogenesis where one can see cells beginning to differentiate into distinct kinds.
Let’s take a step back and view the entire process of embryogenesis, so we can see where the blastocysts stage fits in and why it is an important developmental stage.
Process of Embryogenesis
The process of embryogenesis begins immediately after an egg cell is fertilized. The cell made by the fertilization of an egg cell by sperm is called a zygote. A zygote contains a mix of DNA from both parents. Immediately after the fertilization of the zygote, it begins dividing rapidly into new cells. This process is called cleaving. During cleaving, the cell undergoes mitotic division at an exponential rate as each round of division effectively doubles the number of cells.
After dividing for a while the zygote has developed into a morula. Because this division happens so quickly, the embryo does not really have time to grow and the morula is about the same size as the initial zygote. The size of the morula is also constrained by the zona pellucida, a membrane of glycoproteins that surround the egg cell.
The next stage of embryogenesis is called blastulation and it is the stage when the blastocyst forms. During blastulation, the mass of cells continue to divide but they also differentiate to begin developing into certain kinds of cells. 2 layers of cells form. The trophoblast and the inner cell mass. The trophoblast surrounds the inner cell mass and contains the cells that will eventually develop into the placenta. The inner cell mass (sometimes called the embryoblast) is what will eventually grow into the embryo. Instead of being located directly in the center of the trophoblast sphere, the inner cell mass migrates to one wall of the trophoblast, leaving an open fluid-filled cavity called the blastocoel. The blastocoel contains the amino acids, growth hormones, and nutrients needed for development. Around this point, the zona pellucida begins to disappear so the blastocysts can begin to grow larger.
A blastocyst before implantation of the uterine wall. Credit: SPB via WikiCommons CC BY-SA 3.0
The cells in the embryoblast are pluripotent stem cells, meaning that they could potentially develop into any kind of cell; skin, muscle, bone, etc. Just before anchoring to the uterine wall, the embryoblast begins to differentiate into two structures called the epiblast and the hypoblast. These two structures together form the bilaminar disc. The hypoblast faces the blastocoel and the epiblast is on the interior. As the two structure grow, they begin to inflate, creating two new cavities within the blastocyst. The primitive yolk sac faces the hypoblast and the amniotic cavity faces the epiblast. The amniotic cavity becomes the space that surrounds the fetus later in development.
As a quick recap: the fertilization of an egg by sperm creates a zygote. The zygote undergoes mitotic division until it becomes a morula. Cells in the morula differentiate, creating a blastocyst that is made out of two layers of cells; the embryoblast and the trophoblast. Once the blastocyst is anchored in the uterine wall, the cells in the embryoblast differentiate to form the epiblast and the hypoblast, forming two new cavities within the blastocyte.
The whole process of zygote to blastocyst takes about 2 weeks. Next, the embryo enters a phase called gastrulation. During gastrulation, the embryoblast develops 3 germ layers of cells called the ectoderm, mesoderm, and endoderm. At its simplest, the human body is a single inner tube (digestive tract) surrounding by other tubes. The 3 layers of cells formed in the embryoblast correspond to these organizational tubes. The ectoderm (ecto- “outer”) ends up forming the skin, hair, nails, an peripheral nervous system, the mesoderm (meso- “middle”) forms the muscles, bone, connective tissues, and the endoderm (endo- “inner”) ends up forming the digestive tract, stomach, and colon. In other words, the layered organization of the germ layers reflects the layered organization of the organs in the human body.
The 3 germ layers and the structures they go on to become. Credit: CNX via WikiCommons CC BY-SA 3.0
The first point of gastrulation involves the formation of a primitive streak. The primitive streak is a line that forms through the epiblast that forms the opening that will eventually become the anus and the opening that will become the head. The primitive streak also sets the left-right axis for the human body. Directly below this primitive streak in the middle of the mesoderm are a rod of cells called the notochord. The notochord is a unique structure found only in the phylum Chordata. The notochord is the main structure that initiates the next stage of development called neuralation, in which the body starts to construct the tubes specified by the ectoderm, mesoderm, and endoderm.
The notochord forms a hardened structure of cells called the neural plate. The neural plate extends the entire length of the primitive streak and gets folded back on itself to create a closed neural tube which gives rise to the spinal cord and brain.
The process of neurulation. Credit: OpenStax via WikiCommons CC-BY 3.0
While neuralation is occurring in the mesoderm, the ectoderm and endoderm are rolling into tubes as well. The endoderm gets divided into three sections called the foregut, midgut, and hindgut. The foregut will turn into the esophagus, stomach, and lungs. The midgut will form the majority of the intestine and the hindgut will create the colon and rectum. The formation of these tubes will be complete in about 8 weeks, after which the newly formed embryo will continue to grow.
Problems In Embryogenesis
Embryogenesis is an extraordinarily complex process that requires the synchronization of several different cellular mechanisms. If just one of these mechanisms goes awry, the entire process can be thrown off balance, leading to developmental issues and birth defects. For example, failure of the neural tube to properly close during neuralation leads to a condition called Spinal Bifida. Spinal Bifida can cause a number of problems, including problems walking, incontinence, build up of fluid in the brain and spinal cord (hydrocephaly) and malformation of the spinal cord.
These sorts of errors in embryonic development can be caused by a number of environmental factors. Spinal Bifida, for example, can be caused by genetics, or by a lack of folic acid in the mother. Others can be caused by environmental factors, such as communicable illness, substance use while pregnant, or other hazards. Still, other times they occur without any specified cause: a random error in cellular machinery.
Blastocytes And In Vitro Pregnancy
Blastocysts are also used in in vitro fertilization. Traditional in vitro techniques involved fertilizing an egg cell outside of the body, then directly implanting the zygote into the uterus. Early zygotes can be finicky though, and many do not latch on and begin to develop properly. This is why most of the time in vitro techniques involve the implantation of multiple fertilized zygotes, to up the chances one of them will take hold.
Because of these issues, many clinicians have moved to implant blastocysts instead of newly formed zygotes. Blastocysts that have been given a few more days to develop are typically more robust than early zygotes and likely to survive. Upping this likelihood allows only one blastocyst to be implanted which minimizes complications that can arise from multiple fertilized zygotes. The need for only a single blastocyst instead of multiple fertilized zygotes also makes the in vitro procedure much less expensive.
To summarize, a blastocyst is a cellular structure that forms early in the embryonic development of mammals. The blastocyst develops from a zygote and contains two primary layers of cells, the inner cell mass and the trophoblast. The inner cell mass will go on to become the embryo and the trophoblast anchors the blastocyst to the uterine wall and will eventually become the placenta. The inner cell mass then differentiates into the three germ layers that form the basic organizational structure of the body.
The formation of the blastocyst represents the first point in embryonic development where cells differentiate into distinct structures. As such, the blastocyst has the most basic form of organization that eventually develops into a complex organism. Blastocysts are the first step in constructing the human body.