They are already nine months old, and what happened to them during those nine months will help shape the rest of their life. Some of the shortest — but most important — life stages occur before birth. These stages include the germinal, embryonic, and fetal stages. This concept focuses on the earliest of all human life stages: the germinal stage.
The germinal stage of development is the first and shortest of the stages of the human lifespan. The germinal stage lasts a total of eight to nine days. It begins in a Fallopian tube when an ovum is fertilized by a sperm to form a zygote day 0.
The germinal stage continues as the zygote undergoes several initial cell divisions to form a solid ball of cells called a morula days It then continues as the morula undergoes additional changes to become a hollow ball of cells called a blastocyst days The germinal stage ends when the blastocyst implants in the endometrium of the uterus days In the germinal stage, however, nutrients must be obtained from cell cytoplasm or secretions in the Fallopian tube or uterus.
The germinal stage involves several different processes that change an egg and sperm first into a zygote, and then into an embryo. The processes include fertilization, cleavage, blastulation, and implantation. Many sperm travel towards the egg due to chemical attraction. However, only one sperm will succeed in fertilizing the ovum egg , by penetrating its cell membrane and depositing the genetic material into the egg, where the two nuclei fuse.
The fertilized ovum zygote immediately becomes resistant to penetration by any other sperm arriving later. After fertilization occurs, the zygote remains in the fallopian tube for about 72 hours, and during this time it develops rapidly.
By the second day after fertilization, the single-celled zygote undergoes mitosis to form two daughter cells. Mitosis continues to take place every 12 to 24 hours to produce the first four cells, then eight, and as many as sixteen cells by day 4. These early mitotic divisions are called cleavage. Although cleavage results in more cells, the overall mass of cells making up the morula is still the same size as the initial zygote because the cells are confined within the zona pellucida.
A large amount of cytoplasm in the original zygote becomes subdivided among the multiple cells of the morula. The embryo is approximately 1 inch in length and weighs about 4 grams at the end of this period.
The embryo can move and respond to touch at this time. About 20 percent of organisms fail during the embryonic period, usually due to gross chromosomal abnormalities. As in the case of the germinal period, often the mother does not yet know that she is pregnant.
It is during this stage that the major structures of the body are taking form making the embryonic period the time when the organism is most vulnerable to the greatest amount of damage if exposed to harmful substances.
Potential mothers are not often aware of the risks they introduce to the developing child during this time. When the organism is about nine weeks old, the embryo is called a fetus. From 9—12 weeks, the sex organs begin to differentiate. By the 12th week, the fetus has all its body parts including external genitalia.
In the following weeks, the fetus will develop hair, nails, teeth and the excretory and digestive systems will continue to develop. At the end of the 12th week, the fetus is about 3 inches long and weighs about 28 grams. At about 16 weeks, the fetus is approximately 4. Fingers and toes are fully developed, and fingerprints are visible. During the th months, the eyes become more sensitive to light and hearing develops.
The respiratory system continues to develop. Reflexes such as sucking, swallowing and hiccuping develop during the 5th month. Cycles of sleep and wakefulness are present at that time as well.
Throughout the fetal stage, the brain continues to grow and develop, nearly doubling in size from weeks 16 to The majority of the neurons in the brain have developed by 24 weeks although they are still rudimentary and the glial or nerve cells that support neurons continue to grow. At 24 weeks the fetus can feel pain Royal College of Obstetricians and Gynecologists, By the time the fetus reaches the sixth month of development 24 weeks , it weighs up to 1.
The hearing has developed, so the fetus can respond to sounds. Between the 7th and 9th months, the fetus is primarily preparing for birth. It is exercising its muscles, its lungs begin to expand and contract. It is developing fat layers under the skin. The fetus gains about 5 pounds and 7 inches during this last trimester of pregnancy which includes a layer of fat gained during the 8th month. This layer of fat serves as insulation and helps the baby regulate body temperature after birth.
The embryonic stage plays an important role in the development of the brain. Approximately four weeks after conception, the neural tube forms. This tube will later develop into the central nervous system including the spinal cord and brain. The neural tube begins to form along with an area known as the neural plate. The earliest signs of development of the neural tube are the emergence of two ridges that form along each side of the neural plate.
Over the next few days, more ridges form and fold inward until a hollow tube is formed. Once this tube is fully formed, cells begin to form near the center.
These vesicles will eventually develop into parts of the brain , including the structures of the forebrain, midbrain, and hindbrain. Around the fourth week, the head begins to form, quickly followed by the eyes, nose, ears, and mouth. The blood vessel that will become the heart start to pulse. During the fifth week, buds that will form the arms and legs appear. By the eighth week of development, the embryo has all of the basic organs and parts except those of the sex organs.
At this point, the embryo weighs just one gram and is about one inch in length. By the end of the embryonic period, the basic structures of the brain and central nervous system have been established. At this point, the basic structure of the peripheral nervous system is also defined. The production of neurons, or brain cells, begins around day 42 after conception and is mostly complete sometime around the middle of pregnancy.
As neurons form, they migrate to different areas of the brain. Once they have reached the correct location, they begin to form connections with other neural cells, establishing rudimentary neural networks. Once cell differentiation is mostly complete, the embryo enters the next stage and becomes known as a fetus. The fetal period of prenatal develop marks more important changes in the brain.
This period of development begins during the ninth week and lasts until birth. This stage is marked by amazing change and growth. The early body systems and structures established in the embryonic stage continue to develop. The neural tube develops into the brain and spinal cord and neurons continue to form. Once these neurons have formed, they begin to migrate to their correct locations. Synapses, or the connections between neurons, also begin to develop.
Between the ninth and twelfth week of gestation at the earliest , reflexes begin to emerge. The fetus begins to make reflexive motions with its arms and legs. During the third month of gestation, the sex organs begin to differentiate. By the end of the month, all parts of the body will be formed. At this point, the fetus weighs around three ounces. The fetus continues to grow in both weight and length, although the majority of the physical growth occurs in the later stages of pregnancy.
The end of the third month also marks the end of the first trimester of pregnancy. During the second trimester, or months four through six, the heartbeat grows stronger and other body systems become further developed. Fingernails, hair, eyelashes, and toenails form. So what's going on inside the brain during this important period of prenatal development? The brain and central nervous system also become more responsive during the second trimester.
Around 28 weeks, the brain starts to mature faster, with an activity that greatly resembles that of a sleeping newborn. During the period from seven months until birth, the fetus continues to develop, put on weight, and prepare for life outside the womb. The lungs begin to expand and contract, preparing the muscles for breathing. While development usually follows this normal pattern, there are times when problems with prenatal development occur.
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