Embryo
An embryo is the early developmental stage of a multicellular organism, particularly in humans, where it develops from conception until the eighth week of pregnancy. This critical embryonic period, which begins about two weeks after conception, is characterized by rapid growth and the formation of body systems, internal organs, and external structures. Initially starting as a zygote, the embryo is formed from a blastocyst that implants into the uterine wall. During this time, the embryo develops three primary layers: the ectoderm, mesoderm, and endoderm, each responsible for different body systems and organs.
As the embryo grows, it is housed in a protective amniotic sac filled with amniotic fluid, which supports its development. Vulnerable to external factors, such as medications and environmental toxins, the embryo can be at risk of birth defects during this delicate stage. By the end of the eighth week, the embryo has developed all essential organs and begins to resemble a recognizable human form. It transitions into the fetal stage, where development continues until birth, typically after a total gestation period of around 37 to 42 weeks. Understanding embryonic development is essential for appreciating human growth and the factors that can influence health outcomes during pregnancy.
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Embryo
An embryo is an early developmental stage of a multicelled animal, which in humans begins two weeks after conception and lasts until the eighth week (tenth week of pregnancy). During the embryonic period, also called the embryonic stage, the embryo goes through rapid developmental changes. The body systems, internal organs, and external body structures develop, form, and grow at this time. The embryo is very delicate and can be damaged by a variety of factors, such as medications and drugs, which can lead to birth defects. Prior to the embryonic period, the developing human is known as a zygote and blastocyst, and after this stage, it is known as a fetus until it is born.
Background
The normal gestation period, or the amount of time a human fetus is developing, is around thirty-seven to forty-two weeks. This time is split into three trimesters of about three months, or twelve weeks. Obstetricians calculate the length of pregnancy and estimated due date (EDD) from the first day of the last menstrual cycle. The embryo forms during the last few weeks of the first trimester.
During the first two weeks of pregnancy, the body begins to prepare to become pregnant (a woman is not yet pregnant at this time). The first week starts with the first day of a woman's menstrual period. The next week, the body prepares for ovulation, or the release of an egg.
The third week begins as the ovaries (female reproductive organs) release an egg into a fallopian tube. This period is known as ovulation, and a woman is most likely to become pregnant during this time. For fertilization to occur, the sperm must meet the egg in the fallopian tube. Conception occurs when the sperm enters the egg; it becomes a zygote, which is a single cell. The zygote then travels through the fallopian tube and attaches to the uterus, the organ that will house the developing baby. This process takes up to five days.
During this time, the zygote divides and becomes a blastocyst, or hollow ball of cells. The inner cells of the blastocyst will become the embryo, which will develop into a fetus. The outer cells will become the placenta, the membranes that will nourish and protect the fetus until birth. The placenta allows nutrients, oxygen, and water to pass from the mother to the unborn baby through the umbilical cord. The placenta also removes the unborn baby's waste.
The blastocyst reaches the uterus where it stays for several days before it implants in the uterine wall's inner lining. If implantation occurs around the fourth week, or about two weeks after conception occurred, pregnancy occurs. At this time, the body secretes a hormone called human chorionic gonadotrophin (hCG), which means that pregnancy can be detected by a urine or blood test. The embryonic stage starts at week five of the pregnancy or about twelve days after fertilization.
Overview
The embryonic period is an important developmental stage for the blastocyst, which is now known as an embryo. The embryo is very tiny at this point, only about the size of the tip of an ink pen. The embryo consists of three layers. The ectoderm, or top layer, will become the connective tissue, eyes, inner ears, nervous system, and skin. The mesoderm, or middle layer, is where the bones, kidneys, muscles, and reproductive system will form. The last layer, or the endoderm, is responsible for the formation of the bladder, intestines, and lungs. The embryo resides in a protective and strong amniotic sac, which is filled with a clear liquid known as amniotic fluid. It gives the embryo room to grow and protects it.
The embryo elongates, and its cells rapidly multiply and differentiate, or prepare to take on specific functions. The blood cells, kidney cells, and nerve cells evolve, and the body's different systems begin to develop. The external body structures and features emerge. The brain, heart, spinal cord, and gastrointestinal tract form. The embryo is very delicate and vulnerable at this time and at a very high risk of damage. The use of medications, drugs, cigarettes, and alcohol can negatively affect the embryo and cause malformations, or birth defects. Infections, radiation (such as X-rays), and other factors can damage the developing embryo.
During the sixth week, the region that will become the heart begins to flutter. Sometimes an ultrasound can detect this electrical signal. The facial features and arm and leg buds begin to emerge. At week seven, five areas of the brain develop, and the cranial nerves emerge. The head continues to grow and tissue that will become the baby's spine and other bones develops. The arm and leg buds extend, and the nostrils, ears, and lenses of the eyes form. The heart and other organs form. The embryo is the size of a pencil eraser at this time.
The eyelids and ears form during week eight, though sound will not be detected until about the eighteenth week. The nose and upper lip begin to form. The arms and legs continue to extend, and the feet and hands emerge. The lungs start to develop, and the brain continues to grow.
Its arms continue to grow, and it gains the ability to bend its arms at the elbows. The toes begin to appear. Nipples and hair follicles emerge. The embryo has all its essential organs, which will continue to grow throughout the rest of the gestation period.
The embryo, which is barely two inches long, begins to look like a baby by the tenth week of a pregnancy—the last week of the embryonic period. The features, limbs, and body systems continue to develop. It has eyelids, the ears take shape, and the facial features become more pronounced. The embryo can move, but it is too early for the mother to feel the movements.
The embryo becomes a fetus eight weeks after conception. The fetus continues to rapidly grow and develop until its birth at the end of the third trimester.
Bibliography
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