Development of the embryo
The zygote goes through a series of changes before it reaches the uterus. In the uterus, the zygote develops into a form called the embryo. The embryo develops rapidly. Within two months, all the tissues and organs of the body have begun to form.
The first days of pregnancy. After fertilization, the zygote travels through the fallopian tube toward the uterus. Along the way, the zygote begins to divide rapidly into many cells with no increase in overall size. The resulting cell mass is called a morula. By the third or fourth day after fertilization, the morula enters the uterus. At that time, the morula is still surrounded by the zona pellucida and consists of about 12 to 16 cells.
The embryo develops from the central cells of the morula. These cells are called the inner cell mass. The outer cells of the morula are called the outer cell mass. They develop into the placenta, a special organ that enables the embryo to obtain food and oxygen from the mother.
After the morula enters the uterus, it continues to divide. A fluid-filled cavity forms between the inner cell mass and the outer cell mass, and the zona pellucida begins to disintegrate. At this stage, the ball of cells is called a blastocyst or blastula. The cells of the blastocyst divide as it floats in the uterus for one or two days.
About the fifth or sixth day of pregnancy, the blastocyst becomes attached to the internal surface of the uterus. The outer cells of the blastocyst, called the trophoblast, secrete an enzyme that breaks down the lining of the uterus. The trophoblast begins to divide rapidly, invading the uterine tissue. The process of attachment to the uterine wall is called implantation. By the 11th day of the pregnancy, the blastocyst is firmly implanted in the uterus.
Nourishing the embryo. Various structures develop in the uterus to help the embryo grow. These structures include the placenta and certain membranes.
By the 13th day of pregnancy, a space called the chorionic cavity has formed around the embryo. Two membranes surround the chorionic cavity. The outer membrane is called the chorion, and the inner membrane is called the amnion. The chorion interacts with tissues of the uterus to form the placenta. The chorion pushes into the wall of the uterus with fingerlike projections called chorionic villi. The chorionic villi contain the embryo's first blood vessels. The chorion is attached to the embryo by a structure called the body stalk. The body stalk develops into the umbilical cord, which joins the embryo to the placenta.
The amnion forms a sac around the embryo and is filled with fluid. The embryo floats in this fluid, called amniotic fluid. The amniotic fluid protects the embryo by absorbing jolts to the uterus. It also allows the embryo to move without damaging the amnion and other tissues.
About the 21st day of pregnancy, blood begins to circulate between the placenta and the embryo. The blood vessels of the mother and those of the embryo exchange substances through a thin layer of cells called the placental barrier. Waste products from the embryo are carried away through the barrier. Likewise, nutrients and oxygen from the mother's blood pass through the thin walls of the barrier and enter the embryo's blood. However, such organisms as viruses and bacteria, as well as chemical substances, including drugs, also may cross the placental barrier and harm the embryo.
Origin of tissues and organs. At about the same time that the placenta begins to form, the inner cell mass flattens and develops into three layers of cells in what is called the embryonic disc. The three types of cell layers are the ectoderm, the mesoderm, and the endoderm. In a process called differentiation, cells from each layer move to certain areas of the embryonic disc and then fold over to form tubes or clusters. These tubes and clusters develop into various tissues and organs of the body.
Cells from the ectoderm form the brain, nerves, skin, hair, nails, and parts of the eyes and ears. Cells from the mesoderm form the heart, muscles, bones, tendons, kidneys, glands, blood vessels, and reproductive organs. The linings of the digestive and respiratory systems develop from cells of the endoderm.
Development of organs and organ systems. The body's organs and organ systems grow rapidly from the third through eighth weeks of pregnancy. The major structures include the central nervous system and the circulatory system, as well as such organs as the eyes, ears, and limbs. Defects in the development of these structures often occur during these weeks. Such defects sometimes are caused by substances introduced from the mother's body through the placental barrier. These substances are called teratogens. They include medications taken by the mother, as well as viruses, bacteria, and other infectious organisms. Other teratogens include nonmedicinal drugs, alcoholic beverages, and cigarette smoke.
The central nervous system, which consists of the brain and spinal cord, starts to develop in the middle of the third week of pregnancy. It begins as a flattened strip of cells within a long cylinder of cells called the neural tube. At about the 25th day of pregnancy, one end of the neural tube closes. The brain develops from three sacs formed in this end of the tube. The other end of the tube closes two days later. Failure of the tube to close can result in birth defects, especially spina bifida, a disorder of the spine.
The circulatory system also begins to develop in the third week of pregnancy. Two tubes of cells combine to form a single tube that becomes the heart. By the fourth week, a simple circulatory system is functioning and the heart has begun to pump blood. During the fourth to seventh weeks of pregnancy, the heart tube divides into four chambers. Any irregularity in the normal pattern of development during this period can produce a defect in the heart.
The eyes and ears begin to develop in the fourth week of pregnancy. Both these organs form rapidly. The external parts of the ears appear by the sixth week. Defects in the eyes or ears often stem from abnormalities that occur during the fourth to sixth weeks.
The arms and legs appear as buds of tissue during the fifth week of pregnancy. The arms develop a few days ahead of the legs. The fingers and toes become recognizable in the sixth week. They form when certain cells die and leave spaces in the remaining tissue.
The structures of the mouth, such as the lips and palate, begin to form during the fourth and fifth weeks of pregnancy. The lips and palate form during the sixth to ninth weeks. Each forms from paired structures that gradually move from the sides toward the middle of the face and fuse (join). If anything interferes with normal development during this period, a split in the upper lip or palate may develop. Such a defect is called cleft lip or cleft palate. See CLEFT PALATE.
Growth of the fetus
From the ninth week of pregnancy until birth, the developing baby is called a fetus. In the first three months of this period, the fetus increases rapidly in length. It grows about 2 inches (5 centimeters) in each of these months. In the later months of pregnancy, the most striking change in the fetus is in its weight. Most fetuses gain about 25 ounces (700 grams) in both the eighth and ninth months of pregnancy.
Stages of growth. Physicians commonly divide pregnancy into three, three-month parts called trimesters. At the end of the first trimester, the fetus weighs about 1 ounce (28 grams) and is about 3 inches (7.6 centimeters) long. At the end of the second trimester, the fetus weighs about 30 ounces (850 grams) and measures about 14 inches (36 centimeters) long. At the end of the third trimester, the fetus measures about 20 inches (50 centimeters) and weighs about 7 pounds (3.2 kilograms).
The mother can feel movements of the fetus by the fifth month of pregnancy. By this time, fine hair called lanugo covers the body of the fetus. Hair also appears on the head. Lanugo disappears late in pregnancy or shortly after birth. The eyelids open by the 26th week of pregnancy. By the 28th week, the fingernails and toenails are well developed.
Until the 30th week of pregnancy, the fetus appears reddish and transparent because of the thinness of its skin and a lack of fat beneath the skin. In the last six to eight weeks before birth, fat develops rapidly and the fetus becomes smooth and plump.
The mother also experiences many physical changes during pregnancy. For example, a pregnant woman gains weight and her breasts increase in size. For more information on such changes, see PREGNANCY.
Checking the fetus. Physicians can use several procedures to monitor the development of the fetus in the mother's uterus. Two of the most commonly used techniques are ultrasonography and amniocentesis.
Ultrasonography, also called ultrasound, involves the use of high-frequency sound waves to produce an image of the fetus on a screen. By viewing the shape and body features of the fetus, a physician can measure its growth and detect malformations. Fetal abnormalities also can be detected through amniocentesis. This technique involves the removal of a sample of the amniotic fluid, which contains cells of the fetus. The fluid and cells are then analyzed and examined. See AMNIOCENTESIS; ULTRASOUND. |
