Source: Video and Images Created by Amanda Soderlind
Welcome to this lesson today on development, weeks one through eight.
Today we will be talking about what happens as an embryo develops from one week through eight weeks of pregnancy.
Basically, during these weeks one through eight, the basic body plan of an embryo will began to develop. The first thing that happens is an embryonic disk will form. This embryonic disk is a thin disk of cells. It's basically the first stage of setting up a body plan of this embryo. And then from there, the neural tube will form. This neural tube forms from within the embryonic disk, and it will actually become the brain and spinal cord.
This neural tube, as I mentioned, is formed within the embryonic disk, will become the brain and spinal cord, and it's actually formed from the ectoderm, which is one of the three layers of germ cells.
Following this, other germ cell layers-- so you have three layers of germ cells-- those other germ cell layers will also begin to specialize, and will give rise to certain body features, and organs and organ systems and such.
By the end of week eight, the embryo will actually begin to resemble a human being. Prior to that, it doesn't quite resemble a human being yet, but by the end of week eight, if you are able to look at an embryo at the end of week eight, it actually does start to resemble a human being.
During these weeks, one through eight, the reproductive structures will also begin to develop, depending on the inheritance of the sex chromosomes. The sex chromosomes are the chromosomes that determine the sex of the individual. If the individual inherited an X and a Y, it will become male, and male reproductive structures will form. If the individual inherited an X and an X, it will become a female, and female reproductive structures will then form.
At the end of week eight, the embryo is then called a fetus. So it's an embryo prior to week eight, and then, by the end of week eight, we call it a fetus.
During these weeks one through eight, there is always a chance that a miscarriage can occur. Miscarriages can also occur leader in development, although it's much less common. A miscarriage is when an embryo or fetus will be spontaneously expelled from the uterus. So basically, the cause for this-- there can be many factors that caused this-- but often times the factor that causes it is a genetic abnormality. So as this body plan is being laid out, and as this embryo is starting to develop into a fetus and taking on a human form, if there's some sort of genetic abnormality, that embryo will not be able to properly develop. And therefore, the body will expel it from the uterus because of these genetic abnormalities. So that's what a miscarriage is. It's when the embryo or fetus is spontaneously expelled from the uterus.
This lesson has been an overview on development of an embryo and a fetus from weeks one through eight.
Source: Video and Images Created by Amanda Soderlind
Welcome to this lesson today on fetal development. Today, we will be discussing fetal development during the second and third trimesters of pregnancy. So during the second and third trimesters, the embryo that's developing in the mother's uterus is referred to as a fetus. So we no longer called it an embryo, but we call it a fetus.
So we're going to discuss how this fetus is developing during the second and third trimesters. So the second trimester of pregnancy is from the start of the fourth month to the end of the six month. So the start of the fourth month to the end of the sixth month is considered the second trimester.
So basically during this time of fetal development, organ and organ systems are maturing. So organ and organ systems are maturing. And also at this point, fetal movement can be felt. So the mother is actually able to feel the fetus moving inside of her.
During the third trimester, the third trimester is marked as the seventh month until birth. So from the seventh month until birth, which is around the ninth month. So during this time, organ and organ systems are continuing to mature, and the fetus is preparing for birth.
So babies who are born before the third trimester will generally have a low survival rate, because their organ systems at that point are too underdeveloped. So this time that it's inside the mother is very important for the organ and organ systems in order to be able to properly develop. So if it's born before that third trimester, it will have a lower survival rate because those organ systems will not be well enough developed for the baby to be able to survive independently.
OK, and the circulatory system of a fetus is composed of temporary vessels and lungs that do not function while it's in the uterus. So while the fetus is within the uterus, gas exchange is not occurring. So the lungs at that point are not functioning because of the placenta is playing the role of delivering oxygen to the fetus and removing carbon dioxide. So the fetus is not actually breathing. Its lungs are not functioning while it's in the uterus.
But once the baby is born, the circulatory system then becomes independent and the lungs will begin gas exchange when the first breath is taken. So while it's in the-- while the fetus is within the mother, the circulatory system is kind of depending on the mother and the lungs are not functioning. But once the baby is born, that circulatory system becomes independent, and then the lungs are able to actually begin gas exchange at that point as well. So this lesson has been an overview on fetal development during the second and third trimesters of pregnancy.