The trophoblast does not contribute to the formation of the embryo proper; the embryo develops from the inner cell mass. Fluid collects between the trophoblast and the greater part of the inner cell mass, and thus the morula, is converted into the blastodermic vesicle also called the blastocyst or blastula. The inner cell mass remains in contact with the trophoblast at one pole of the ovum.
This is named the embryonic pole, since it indicates the location where the future embryo will develop. In the case of monozygotic twins derived from one zygote , a zygote divides into two separate cells embryos at the first cleavage division. Monozygotic twins can also develop from two inner cell masses. A rare occurrence is the division of a single inner cells mass giving rise to twins. However, if one inner cell mass divides incompletely, the result is conjoined twins. Dizygotic twins is the development of two embryos from two different zygotes.
The blastocyst forms early in embryonic development and has two layers that form the embryo and placenta. In humans, the blastocyst is formed approximatelyy five days after fertilization. This stage is preceded by the morula. The morula is a solid ball of about 16 undifferentiated, spherical cells. As cell division continues in the morula, the blastomeres change their shape and tightly align themselves against each other. This is called compaction and is likely mediated by cell surface adhesion glycoproteins.
The blastocyst possesses an inner cell mass ICM , or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast, which later forms the placenta. The trophoblast surrounds the inner cell mass and a fluid-filled, blastocyst cavity known as the blastocoele or the blastocystic cavity.
The embryoblast is the source of embryonic stem cells and gives rise to all later structures of the adult organism. The trophoblast combines with the maternal endometrium to form the placenta in eutherian mammals.
Blastocyst : The blastocyst possesses an inner cell mass from which the embryo will develop, and an outer layer of cells, called the trophoblast, which will eventually form the placenta. Before gastrulation, the cells of the trophoblast become differentiated into two strata.
The outer stratum forms a syncytium, which is a layer of protoplasm studded with nuclei that shows no evidence of subdivision into cells termed the syncytiotrophoblast. The inner layer, the cytotrophoblast or layer of Langhans, consists of well-defined cells. As already stated, the cells of the trophoblast do not contribute to the formation of the embryo proper; they form the ectoderm of the chorion and play an important part in the development of the placenta. On the deep surface of the inner cell mass, a layer of flattened cells, called the endoderm, is differentiated and quickly assumes the form of a small sac, called the yolk sac.
Spaces appear between the remaining cells of the mass and, by the enlargement and coalescence of these spaces, a cavity called the amniotic cavity is gradually developed. The floor of this cavity is formed by the embryonic disk, which is composed of a layer of prismatic cells called the embryonic ectoderm.
Contact between the egg and sperm is random. Once the egg arrives at a specific portion of the tube, called the ampullar-isthmic junction, it rests for another 30 hours. Fertilization — sperm union with the egg — occurs in this portion of the tube. The fertilized egg then begins a rapid descent to the uterus. The period of rest in the tube appears to be necessary for full development of the fertilized egg and for the uterus to prepare to receive the egg.
Defects in the fallopian tube may impair transport and increase the risk of a tubal pregnancy, also called ectopic pregnancy. A membrane surrounding the egg, called the zona pellucida, has two major functions in fertilization. First, the zona pellucida contains sperm receptors that are specific for human sperm. Second, once penetrated by the sperm, the membrane becomes impermeable to penetration by other sperm. Following penetration, a series of events set the stage for the first cell division.
The single-cell embryo is called a zygote. Over the course of the next seven days, the human embryo undergoes multiple cell divisions in a process called mitosis. At the end of this transition period, the embryo becomes a mass of very organized cells, called a blastocyst. It's now believed that as women get older, this process of early embryo development is increasingly impaired due to diminishing egg quality.
Once the embryo reaches the blastocyst stage, approximately five to six days after fertilization, it hatches out of its zona pellucida and begins the process of implantation in the uterus. In nature, 50 percent of all fertilized eggs are lost before a woman's missed menses. In the in vitro fertilization IVF process as well, an embryo may begin to develop but not make it to the blastocyst stage — the first stage at which those cells destined to become the fetus separate from those that will become the placenta.
The blastocyst may implant but not grow, or the blastocyst may grow but stop developing before the two week time at which a pregnancy can be detected. The receptivity of the uterus and the health of the embryo are important for the implantation process. UCSF Health medical specialists have reviewed this information. Home pregnancy tests can detect hCG in the urine about 12—15 days after ovulation, in people with day menstrual cycles. A positive pregnancy test result may be possible on the day a missed period is due.
However, the tests are not so reliable that a negative result at this stage definitely means that the person is not pregnant. The American Association for Clinical Chemistry suggest that in this case, a person should test again about 10 days after the period was due.
It is sometimes accompanied by spotting or light bleeding, and the blood may be pale pink or rusty brown.
Some people experience cramping. However, bleeding and cramping can indicate a range of issues, including pregnancy loss, and are not a reliable indicator of implantation.
For greater certainty, use a home pregnancy test at least 10 days after a missed period, though some tests can detect pregnancy earlier, on the first day a period was due. While an impending pregnancy can be associated with feelings of excitement or anxiety, it is often accompanied by harmless bleeding that can cause….
How long implantation bleeding lasts can differ from person to person. It is usually lighter and shorter than a typical period. Learn more here. Pregnancy symptoms before a missed period can include nausea, headaches, fatigue, and breast changes. However, these are also signs of other…. Nausea, a missed period, and breast changes are all common early signs of pregnancy. Learn about these and other signs that a pregnancy test is a good….
Pregnancy tests measure levels of a specific pregnancy hormone. Merck and Co. From developing new therapies that treat and prevent disease to helping people in need, we are committed to improving health and well-being around the world. The Manual was first published in as a service to the community. Learn more about our commitment to Global Medical Knowledge. This site complies with the HONcode standard for trustworthy health information: verify here. Common Health Topics.
Development of the Blastocyst. Development of the Embryo. Development of the Fetus and Placenta. Normal Pregnancy. Test your knowledge. A breast lump mass is a thickening or bump that feels different from surrounding breast tissue. Which of the following statements about breast lumps is NOT correct? More Content. From Egg to Embryo Once a month, an egg is released from an ovary into a fallopian tube.
Placenta and Embryo at About 8 Weeks At 8 weeks of pregnancy, the placenta and fetus have been developing for 6 weeks. Allows materials to be exchanged between the blood of the mother and that of the embryo. The embryo floats in fluid amniotic fluid , which is contained in a sac amniotic sac. Provides a space in which the embryo can grow freely. By 12 weeks of pregnancy: The fetus fills the entire uterus.
By about 24 weeks: The fetus has a chance of survival outside the uterus. Was This Page Helpful? Yes No. Physical Changes During Pregnancy.
0コメント