Embryology

Embryology is the study of the development of organisms. This is as true of plants as it is of animals.

Seed formation proceeds following fertilization in higher plants. The seed consists of the embryo, the seed coat, and another part sometimes called the endosperm. While plants are extraordinarily important for survival of animal life, animal embryology is described here.

The dictionary definition limits the meaning of the term "embryo" to developing animals that are unhatched or not yet born. Human embryos are defined as developing humans during the first eight weeks after conception. The reason that many embryologists have difficulty with this terminology is that it is purely arbitrary. It would be difficult indeed, if not impossible, to discriminate a human embryo nearing the end of the eighth week from a developing human during the ninth week after conception. Correspondingly, there are no morphological events that distinguish a pre-hatching frog tadpole from a post-hatching tadpole (hatching never occurs synchronously in an egg mass—there are always those that hatch early and those larvae which are dilatory).

Embryologists consider development from a zygote to a multicellular organism. In the particular case of humans, development does not even stop at birth. Note that teeth continue to develop and sex glands with sexual differentiation mature long after birth. For a number of years, many embryologists have referred to their discipline as developmental biology to escape from the need to confine their studies to earlier stages. Embryology in the modern sense is the study of the life history of an animal and human embryology considers developmental aspects of life as a whole and not just the first eight weeks.

The study of embryology, the science that deals with the formation and development of the embryo and fetus, can be traced back to the ancient Greek philosophers. Originally, embryology was part of the field known as "generation," a term that also encompassed studies of reproduction, development and differentiation, regeneration of parts, and genetics. Generation described the means by which new animals or plants came into existence. The ancients believed that new organisms could arise through sexual reproduction, asexual reproduction, or spontaneous generation. As early as the sixth century B.C., Greek physicians and philosophers suggested using the developing chick egg as a way of investigating embryology.

Aristotle (384–322 B.C.) described the two historically important models of development known as preformation and epigenesis. According to preformationist theories, an embryo or miniature individual preexists in either the mother's egg or the father's semen and begins to grow when properly stimulated. Some preformationists believed that all the embryos that would ever develop had been formed by God at the Creation. Aristotle actually favored the theory of epigenesis, which assumes that the embryo begins as an undifferentiated mass and that new parts are added during development. Aristotle thought that the female parent contributed only unorganized matter to the embryo. He argued that semen from the male parent provided the "form," or soul, that guided development and that the first part of the new organism to be formed was the heart.

Aristotle's theory of epigenetic development dominated the science of embryology until the work of physiologist William Harvey (1578–1657) raised doubts about A human two-cell embryo 24 hours after fertilization. Photograph by Richard G. Rawlins. Custom Medical Stock Photo. Reproduced by permission. many aspects of classical theories. In his studies of embryology, as in his research on the circulation of the blood, Harvey was inspired by the work of his teacher, Girolamo Fabrici (ca.1533–1619). Some historians think that Fabrici should be considered the founder of modern embryology because of the importance of his embryological texts: On the Formed Fetus and On the Development of the Egg and the Chick. Harvey's On the Generation of Animals was not published until 1651, but it was the result of many years of research. Although Harvey began these investigations in order to provide experimental proof for Aristotle's theory of epigenesis, his observations proved that many aspects of Aristotle's theory of generation were wrong.

Aristotle believed that the embryo essentially formed by coagulation in the uterus immediately after mating when the form-building principle of the male acted on the material substance provided by the female. Using deer that had mated, Harvey dissected the uterus and searched for the embryo. He was unable to find any signs of a developing embryo in the uterus until about six or seven weeks after mating had taken place. In addition to his experiments on deer, Harvey carried out systematic studies of the developing chick egg. His observations convinced him that generation proceeded by epigenesis, that is, the gradual addition of parts. Nevertheless, many of Harvey's followers rejected epigenesis and turned to theories of preformation.

Naturalists who favored preformationist theories of generation were inspired by the new mechanical philosophy and by the microscope, a device that allowed them to see the embryo at earlier stages of development. Some naturalists produced very unreliable observations of early embryos, but Marcello Malpighi (1628–1694) and Jan Swammerdam (1637–1680), two pioneers of microscopy, provided observations that seemed to support preformation. Based on Swammer dam's studies of insects and amphibians, naturalists suggested that embryos preexisted within each other like a nest of boxes. However, given such a theory, only one parent can serve as the source of the sequence of preformed individuals. At the time, the egg of many species was well known, but when the microscope revealed the existence of "little animals" in male semen, some naturalists argued that the preformed individuals must be present in the sperm.

Respected scientists of the time, including Albrecht von Haller (1708–1777), Charles Bonnet (1720–1793), Lazzaro Spallanzani (1729–1799), and René Antoine Ferchault de Reaumur (1683–1757), supported preformation. Bonnet's studies of parthenogenesis in aphids were regarded as strong support of ovist preformationism. Thus, some naturalists argued that the whole human race had preexisted in the ovaries of Eve, while others reported seeing homunculi (tiny people) inside spermatozoa. Other eighteenth century naturalists rejected both ovist and spermist preformationist views. One of the most influential was Casper Friedrich Wolff (1733–1794), who published a landmark article in the history of embryology, "Theory of Generation," in 1759. Wolff argued that the organs of the body did not exist at the beginning of gestation, but formed from some originally undifferentiated material through a series of steps. Naturalists who became involved in the movement known as nature philosophy found Wolff's ideas very attractive. During the nineteenth century, cell theory, the discovery of the mammalian ovum by Karl Ernst von Baer (1792–1876), and the establishment of experimental embryology by Wilhelm Roux (1850–1924) and Hans Driesch (1867–1941) transformed philosophical arguments about the nature of embryological development.

About a century ago, careful observations were made of a number of developing organisms. By this time, there was a cell theory and good microscopes were available. Next came a causal analysis. For instance, it was known that the dorsal ectoderm of all vertebrate embryos rolls up into a tube to form the central nervous system. What factors control the very regular appearance of the nervous system and subsequent differentiation into the various parts of the brain and the spinal cord? It was hypothesized that the underlying chordamesoderm cells of the gastrula signaled the ectoderm to become neural. The signal was referred to as induction. Other embryonic organs also seemed to appear as a result of induction. Chemical embryology sought to characterize the nature of inducing signals. Now, modern molecular embryology seeks to examine on the level of the gene what controls differentiation of specific tissue and cell typed of a developing organism.

There are practical considerations that drive some embryologists. The causes of developmental abnormalities (congenital malformations) in humans becomes more understandable with a consideration of embryology. The human embryo is extraordinarily vulnerable to drugs, viruses, and radiation during the first several months of development when many critical organ systems are developing.