Embryology
Embryology is the study of the early development of organisms. With regard to genetics, molecular biologists have long sought to discover the mechanisms by which a fertilized egg, the recipient of genetic information from its parents, is able to give rise to the various types of cells that comprise an adult organism. The mechanisms of transcription and translation, the essential processes of protein synthesis, are much more completely understood than are the mechanisms of cell differentiation that regulate the synthesis of specific proteins in specific tissues at certain times during embryonic development.
Beyond the continued study of birth defects, clinical embryologists also study the interplay between genetic and environmental factors in such increasingly critical fields as cryoembryonics (processes by which frozen embryos are stored for later implantation and development), in vitro fertilization, and a number of other fertility related areas. Embryologists are also concerned with the similarity between embryonic cells, and dedifferentiated cells often associated with various cancers. Many tumor type cells, for example, lose some of their specialized nature and character, including the ability to regulate cell growth based upon cell density (e.g., contact inhibition), and so in some ways appear similar to embryonic cells.
Strictly defined, an embryo is an organism in the early stages of development. In humans, this term is genrally applied to development during the second to seventh weeks post fertilization. Embryogenesis is the process of embryonic development. Critical areas of study related to embryogeneis include the mechanisms of embryonic induction (a process whereby the development of cells is affected by nearby developing cells), and research on embryonic stem cells.
The study of embryology is also important in the botanical world. Seed formation proceeds following fertilization in higher plants. The seed consists of the embryo, the seed coat and another part sometimes termed the endosperm. The mature gametophytes of higher plants contain an embryo sac (a space within the ovule of the seed plant) in which fertilization and early development can take place. Haploid cells in anthers can produce a mature haploid plant form embryoid cells.
Human embryos are defined as developing humans during the first eight weeks after conception. It is, at best, often difficult to discriminate a human embryo nearing the end of the eighth week from a developing human during the ninth week after conception. Likewise, there are no dramatic 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). Instead of attempts to define embryological processes according to developmental stages or horizons, emphasis in the last few decades, especially among molecular embryologists turned toward the mechanisms of 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. Accordingly, 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.
While modern embryology, which seeks to know developmental mechanisms in molecular terms, dates from after World War II, embryology has its roots over two millennia ago. The Greek philosopher Aristotle (384-322 B.C.) scrutinized developing chick embryos. He argued correctly that development of an embryo was not simply growth from a minute preformed organism derived from either the egg or sperm, but was a process of form acquisition from a formless precursor. While it is recognized that the zygote is not a simple cell, it certainly does not contain within its plasma membrane a perfect and preformed organism. The embryo becomes progressively more complex and this correct view of Aristotle is now known as epigenesis.
Approximately a century ago, careful observations were made of a number of developing organisms. By this time, there was a workable cell theory and good microscopes were available. Causal analysis allowed, for example, an understanding of the formation of the three principal germ layers (ectoderm, mesodernm and endoderm). More specifically it allowed embryologists to describe developmental processes in terms movements of those germ layers (e.g., that the dorsal ectoderm of all vertebrate embryos rolls up into a tube (neural tube) to form the central nervous system). It was hypothesized that the certain cells (e.g., the underlying chordamesoderm cells of the gastrula) signaled the ectoderm to become neural. The process 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 types of a developing organism.
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.
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