What is embryology?

Science and Profession

The study of human embryology is the study of human prenatal development. The three stages of development are cleavage (the first week), embryonic development (the second through eighth weeks), and fetal development (the ninth through thirty-eighth weeks).

After an egg is fertilized by sperm in the fallopian tube, the resulting zygote begins to divide rapidly. This period of rapid cell division is known as cleavage. By the third day, the zygote has divided into a solid ball containing twelve to sixteen cells. The small ball of cells resembles a mulberry and is called the morula, which is Latin for “mulberry.” The morula moves from the uterine tube into the uterus.

The morula develops a central cavity as spaces begin to form between the inner cells. At this stage, the developing human is called a blastocyst. The ring of cells on the outer edge of the hollow ball is called the trophoblast and will form a placenta, while the cluster of cells within becomes the inner cell mass and will form the embryo. By the end of the first week, the surface of the inner cell mass has flattened to form an embryonic disc, and the blastocyst has attached to the lining of the uterus and begun to embed itself.

During the second week of development, the trophoblast makes connections with the uterus into which it has burrowed to form the placenta. Blood vessels from the embryo link it to the placenta through the umbilical cord, through which the embryo receives food and oxygen and releases wastes. Two sacs develop around the embryo: the fluid-filled amniotic sac that surrounds and cushions the embryo and the yolk sac that hangs beneath to provide nourishment. Finally, a large chorionic sac develops around the embryo and the two smaller sacs.

During the third week, the cells of the embryo are arranged in three layers. The outer layer of cells is called the ectoderm, the middle layer is the mesoderm, and the inner layer is the endoderm. The ectoderm gives rise to the epidermis (outer layer) of the skin and to the nervous system; the mesoderm gives rise to blood, bone, cartilage, and muscle; and the endoderm gives rise to body organ linings and glands.

Other significant events of the third week are the development of the primitive streak and notochord. The primitive streak is a thickened line of cells on the embryonic disk indicating the future embryonic axis. Development of the primitive streak stimulates the formation of a supporting rod of tissue beneath it called the notochord. The presence of the notochord triggers the ectoderm in the primitive streak above it to thicken, and the thickened area will give rise to the brain and spinal cord. Later, when vertebrae and muscles develop around the neural tissue, the notochord will disappear, leaving the center of the vertebral disk as its remnant.

The important event of the fourth week is the formation of the neural tube. After the thickened neural plate tissue has formed, an upward folding forms a groove, finally closing to form a neural tube. Closure begins at the head end and proceeds backward. The neural tube then sinks beneath surrounding ectodermal surface cells, which will become the skin covering the embryo.

Blocks of mesoderm cells line up along either side of the notochord and neural tube. These blocks are called somites, and eventually forty-two to forty-four pairs will form. They give rise to muscle, the cartilage of the head and trunk, and the inner layer of skin. At the same time, embryonic blood vessels develop on the yolk sac. Because the human embryo is provided little yolk, there is need for the early development of a circulatory system to provide nutrition and gas exchange through the placenta.

The heart is formed and begins to beat in the fourth week, though it is not yet connected to many blood vessels. During the fourth through eighth weeks, all the organ systems develop, and the embryo is especially vulnerable to teratogens (environmental agents that interfere with normal development). A noticeable change in shape is seen during the fourth week, because the rapidly increasing number of cells causes a folding under at the edges of the embryonic disk. The flattened disk takes on a cylindrical shape, and the folding process causes curvature of the embryo and it comes to lie on its side in a C-shaped position.

The beginnings of arms and legs are first seen in the fourth week and are called limb buds, appearing first as small bumps. The lower end of the embryo resembles a tail, and the swollen cranial part of the neural tube constricts to form three early sections of the brain. The eyes and ears begin to develop from the early brain tissue.

During the fifth through eighth weeks, the head enlarges as a result of rapid brain development. The head makes up almost half the embryo, and facial features begin to appear. Sexual differences exist but are difficult to detect. Nerves and muscles have developed enough to allow movement. By the end of the eighth week the limb buds have grown and differentiated into appendages with paddle-shaped hands and feet and short, webbed digits. The tail disappears, and the embryo begins to demonstrate human characteristics. By convention, the embryo is now called a fetus.

The fetal stage of development is the period between the ninth and thirty-eighth weeks, until birth. Organs formed during the embryonic stage grow and differentiate during the fetal stage. The body has the largest growth spurt between the ninth and twentieth weeks, but the greatest weight gain occurs during the last weeks of pregnancy.

In the third month, the difference between the sexes becomes apparent, urine begins to form and is excreted into the amniotic fluid, and the fetus can blink its eyelids. The fetus nearly doubles in length during the fourth month, and the head no longer appears to be so disproportionately large. Ossification of the skeleton begins, and by the end of the fourth month, ovaries are differentiated in the female fetus and already contain many cells destined to become eggs.

During the fifth month, fetal movements are felt by the mother, and the heartbeat can be heard with a stethoscope. Movements until this time usually go unnoticed. The average length of time that elapses between the first movement felt by the mother and delivery is twenty-one weeks.

During the sixth month, weight is gained by the fetus, but it is not until the seventh month that a baby usually can survive premature birth, when the body systems, particularly the lungs, are mature enough to function. During the eighth month, the eyes develop the ability to control the amount of light that enters them. Fat accumulates under the skin and fills in wrinkles. The skin becomes pink and smooth, and the arms and legs may become chubby. In the male fetus, the testes descend into the scrotal sac. Growth slows as birth approaches. The usual gestation length is 266 days, or thirty-eight weeks, after fertilization.

Diagnostic and Treatment Techniques

Knowledge of normal embryonic development is very important both in helping women provide optimal prenatal care for their children and in promoting scientific research for improved prenatal treatment, better understanding of malignant growths, and insight into the aging process.

Environmental stress to the embryo during the fourth through eighth weeks can cause abnormal development and result in congenital malformation, which may be defined as any anatomical defect present at birth. Environmental agents that cause malformations are known as teratogens. Malformations may develop from genetic or environmental factors, but most often they are caused by a combination of the two. Some of the common teratogens are viral infections, drug use, a poor diet, smoking, alcohol consumption, and irradiation.

The genetic makeup of some individuals makes them particularly sensitive to certain agents, while others are resistant. The abnormalities may be immediately apparent at birth or hidden within the body and discovered later. Embryos with severe structural abnormalities often do not survive, and such abnormalities represent an important cause of miscarriages. In fact, up to half of all conceptions spontaneously abort, with little or no notice by the prospective mother.

Genetic birth defects are passed on from one generation to another and result from a gene mutation at some time in the past. Mutations are caused by the accidental rearrangement of deoxyribonucleic acid (DNA), the material of which genes are made, and range in severity from mild to life-threatening. They may cause such conditions as extra fingers and toes, cataracts, dwarfism, albinism, and cystic fibrosis. Gene mutations on a sex chromosome are described as sex-linked and are usually passed from mother to son; these include hemophilia, hydrocephalus (an excessive amount of cerebrospinal fluid), color blindness, and a form of baldness.

Abnormalities in the embryo may result because of an unequal distribution of chromosomes in the formation of eggs or sperm. This imbalance can cause a variety of problems in development, such as Down syndrome and abnormal sexual development. The normal human cell contains twenty-three chromosomes, twenty-two pairs of which are nonsex chromosomes, or autosomes. The last pair consists of the two sex chromosomes. Females normally have two X chromosomes, and males have an X and a Y chromosome.

When females have only one X, a set of conditions known as Turner syndrome results. The embryo will develop as a normal female, though ovaries will not fully form and there may be congenital heart defects. Because the single X chromosome does not cause enough estrogen to be produced, sexual maturity will not occur. If a male embryo should receive only the Y chromosome, it cannot survive. Sometimes a male will receive two (or more) X chromosomes along with a Y chromosome (XXY), producing Klinefelter syndrome. The appearance of the child is normal, but at puberty the breasts may enlarge and the testes will not mature, causing sterility. Males receiving two Y chromosomes (XYY) develop normally, but they may be quite tall and find controlling their impulses to be difficult.

Viral infections in the mother during the embryonic stages can cause problems in organ formation by disturbing normal cell division, fetal vascularization, and the development of the immune system. The organs most vulnerable to infection will be those undergoing rapid cell division and growth at the time of infection. For example, the lens of the eye forms during the sixth week of development, and infection at this time could cause the formation of cataracts.

While most microorganisms cannot pass through the placenta to reach the embryo or fetus, those that can are capable of causing major problems in embryonic development. Rubella, the virus that causes German measles, often causes birth defects in children should infection occur shortly before or during the first three months of pregnancy. The developing ears, eyes, and heart are especially susceptible to damage during this time. When a rubella infection occurs during the first five weeks of pregnancy, interference with organ development is most pronounced. After the fifth week, the risks of infection are not as great, but central nervous system impairment may occur as late as the seventh month.

The most common source of fetal infection may be the cytomegalovirus (CMV), a form of herpes that causes abortion during the first three months of pregnancy. If infection occurs later, the liver and brain are especially vulnerable and impairment in vision, hearing, and mental ability may result. Evidence has also suggested that the immune system of the fetus is adversely affected.

Other viruses may affect fetal development as well. When herpes simplex infects the fetus several weeks before birth, blindness or developmental disabilities may result. Toxoplasma gondii, a parasite of animals often kept as pets, may adversely affect eye and brain development without the mother having known that she had the infection. Syphilis infection in the mother leads to death or serious fetal abnormalities unless it is treated before the sixteenth week of pregnancy; if it is untreated, the fetus may possess hearing impairment, hydrocephalus, facial abnormalities, and developmental disabilities. Women infected with human immunodeficiency virus (HIV) or who have acquired immunodeficiency syndrome (AIDS) may transmit the virus to their infants before or during birth.

Certain chemicals can cross the placenta and produce malformation of developing tissues and organs. During an embryo’s first twenty-five days, damage to the primitive streak can cause malformation in bone, blood, and muscle. While bones and teeth are being formed, they may be adversely affected by antibiotics such as tetracycline.

At one time, thalidomide was widely used as an antinauseant in Great Britain and Germany and to some extent in the United States. Large numbers of congenital abnormalities began to appear in newborns, and the drug was withdrawn from the market after two years. Thalidomide caused the failure of normal limb development and was especially damaging during the third to seventh weeks.

Exposure to other chemicals causes central nervous system disorders when the neural tube fails to close. When the anterior end of the tube does not close, development of the brain and spinal cord will be absent or incomplete and anencephaly results. Babies can live no more than a few days with this condition because the higher control centers of the brain are undeveloped. If the posterior end of the tube fails to close, one or more vertebrae will not develop completely, exposing the spinal cord; this condition is called spina bifida. This condition varies in severity with the level of the defect and the amount of neural tissue that remains exposed, because exposed tissue degenerates.

It has been long believed that neural tube disorders accompany maternal depletion of folic acid, one of the B vitamins, and research has substantiated that relationship. Anencephaly and spina bifida rarely occur in the infants of women taking folic acid supplements. One of the harmful effects of alcohol and anticonvulsants is their depletion of the body’s natural folic acid. A decrease in the mother’s folic acid levels in the first through third months of pregnancy can cause abortion or growth deformities.

Maternal smoking is strongly implicated in low infant birth weights and higher fetal and infant mortality rates. Cigarette smoke may cause cardiac abnormalities, cleft lip and palate, and a missing brain. Nicotine decreases blood flow to the uterus and interferes with normal development, allowing less oxygen to reach the embryo.

Alcohol use may be the number one cause of birth defects. Exposure of the fetus to alcohol in the blood results in fetal alcohol syndrome. Symptoms may include growth deficiencies, an abnormally small head, facial malformation, and damage to the heart and the nervous and reproductive systems. Behavioral disorders such as hyperactivity, attention deficit, and an inability to relate to others may accompany fetal alcohol syndrome.

Radiation treatments given to pregnant women may cause cell death, chromosomal injury, and growth retardation in the developing embryo. The effect is proportional to the dosage of radiation. Malformations may be visible at birth, or a condition such as leukemia may develop later. Abnormalities caused by radiation include cleft palate, an abnormally small head, developmental disabilities, and spina bifida. Diagnostic x-rays are not believed to emit enough radiation to cause abnormalities in embryonic development, but precautions should be taken.

Oxygen deficiency to the embryo or fetus occurs when mothers use cocaine. Maternal blood pressure fluctuates with the use of this drug, and the embryonic brain is deprived of oxygen, resulting in vision problems, lack of coordination, and developmental disabilities. Too little oxygen to the fetus may also cause death from lung collapse soon after birth.

Obvious physical malformations resulting from embryonic exposure to drugs have been recognized for a number of years, but recent investigators have found that there are more subtle levels of effect that may show up later as behavioral problems. Physical abnormalities have been easily documented, but more attention is needed regarding the behavioral effects caused by teratogens.

Perspective and Prospects

The first recorded observations of a developing embryo were performed on a chick by Hippocrates in the fifth century BCE. In the fourth century BCE, Aristotle wondered whether a preformed human unfolded in the embryo and enlarged with time, or whether a very simple embryonic structure gradually became more and more complex. This question was debated for nearly two thousand years until the early nineteenth century, when microscopic studies of chick embryos were carefully conducted and described.

Understanding human embryology is foundational for recognizing the relationships that exist between the body systems and congenital malformations in newborns. This field of study takes on new importance in light of advances in modern technology, which have made prenatal diagnosis and treatment a reality.

The study of embryology is also making contributions toward finding the causes of malignant growth. Malignancy is a breakdown in the mechanisms for normal growth and differentiation first seen in the early embryo. Questions about uninhibited malignant growth may be answered by studying embryonic tissues and organs.

The study of old age is another area in which embryological research is valuable. Understanding the clock mechanisms of embryonic cells has led to greater understanding of the “winding down” of cells in old age. It is also important that researchers discover how environmental conditions modify rates of growth and affect the cell’s clock. The degree to which the human life span can be expanded remains one of the most challenging questions in the area of aging.

In addition to the health benefits that may be derived from embryological research, this field is an important source of insight into some of the moral and ethical dilemmas facing humankind. Artificial insemination, contraception, and abortion regulations are some of the problems that should require close collaboration between ethicists and scientists, especially embryologists.

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