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The study of the development of an organism, commencing with the union of male and female gametes. Embryology literally means the study of embryos, but this definition is restrictive. An embryo is an immature organism contained within the coverings of an egg or within the body of the mother. Strictly speaking, the embryonic period ends at metamorphosis, hatching, or birth. Since developmental processes continue beyond these events, the scope of embryology is customarily broadened to encompass the entire life history of an organism. Embryology may, in this wider context, consider the mechanisms of both asexual reproduction and regeneration.
The production of male and female gametes is commonly considered to be the first phase in animal development. The differentiating gametes arise from diploid stem cells in the gonads. Cell division by meiosis reduces the number of chromosomes carried by a mature gamete to one-half that present in the stem cell. See Gametogenesis
The union of gametes (spermatozoon and ovum), representing the second phase of development, creates a diploid zygote with the potential to form an entire organism. Two events must occur for successful fertilization: the ovum must respond to contact with the spermatozoon by making preparations for further development, an event called activation, and the haploid nucleus of the spermatozoon must combine with the haploid nucleus of the ovum, an event called amphimixis.
Fertilization is the typical method to initiate development, but it is not the only method. In a few animals, the ovum develops independently by parthenogenesis, that is, without the participation of a spermatozoon.
A period of cell proliferation, converting the unicellular zygote into a multicellular embryo, represents the third phase of development. Cleavage is a modified form of cell division by mitosis, distinguished by little or no growth between the divisions. The cells of the embryo, or blastomeres, become progressively smaller at the end of each division, so the embryo maintains the relative size and shape of the zygote. Small, fluid-filled spaces form between the cleaving blastomeres, and these spaces eventually coalesce to create an internal cavity, or blastocoele. Upon the appearance of a blastocoele, the cells of an embryo are referred to collectively as the blastoderm. See Blastulation
The fourth phase of development is poorly delineated from cleavage, because the cells of the embryo continue to divide. Gastrulation is distinguished from cleavage by extensive cell rearrangements that lead, in most animals, to the establishment of three germ layers: an outer ectoderm, a middle mesoderm, and an inner endoderm. Endodermal and mesodermal cells of the blastoderm migrate to the inside of the embryo, while ectodermal cells remain on the surface, where they spread to completely cover the body.
Control of development passes from the cytoplasm to the nucleus immediately prior to gastrulation. Responding to cytoplasmic cues, the nuclei begin to specify the production of proteins that make the cells qualitatively different from one another. In a few invertebrates, the transfer of control from cytoplasm to nucleus actually fixes the developmental fate of a cell. In most other organisms, and particularly in vertebrates, the determination of cell fate is not finalized until the blastoderm has rearranged into the three germ layers. See Cell lineage, Gastrulation, Germ layers
The organization of cells into the tissues and organs of the body, constituting the fifth phase of development, is closely allied with gastrulation. Blastodermal rearrangements during creation of the germ layers shift cells into new positions and bring about new intercellular relationships. The developmental fate of a cell can, to a considerable degree, be the consequence of its new position. The influence exerted by one group of cells over the developmental fate of a neighboring group is called induction. Induction occurs by the transmission of chemical substances, called inducing agents.
Differentiation, or the process by which a cell becomes specialized, correlates to a reduction in the amount of genetic information that is expressed. Determination, or the fixation of a developmental fate, occurs when a cell has such a limited amount of usable genetic information that it must commit to a terminal pathway of differentiation. See Cell differentiation
Cellular differentiation is just one aspect of morphogenesis, or the development of form. Morphogenesis must be considered at all levels of organization, ranging from the individual cell to the whole organism. Such a broad perspective complicates the formulation of general theories of development. Presently, no comprehensive theory exists, but there are some embryologists who anticipate that a theory is possible once activities of the DNA molecule have been fully integrated into the topic of development. See Animal morphogenesis, Reproduction (animal)
Reproductive development in multicellular plants is generally divided into three phases: gametogenesis, fertilization, and embryogenesis. The zygote produced by the fusion of male and female gametes divides to form a multicellular embryo with meristematic regions that ultimately produce the adult plant.
Development of the cell in flowering plants begins with a diploid megasporocyte located within the nucellar tissue of an immature ovule. This megasporocyte undergoes meiosis to form a tetrad of four haploid megaspores. In the most common pattern of development, three of these megaspores degenerate, leaving a single functional megaspore that undergoes several postmeiotic mitoses to form a mature megagametophyte (embryo sac) composed of seven cells and eight haploid nuclei. One of these haploid cells is the egg cell.
Development of the male gametes begins with numerous diploid cells (microsporocytes) located within the anthers of an immature flower. Each microsporocyte undergoes meiosis to form a tetrad of four haploid microspores, which then separate and enlarge to form mature pollen grains. Each microspore divides unequally to form a large vegetative cell, and a small generative cell located within the cytoplasm of the vegetative cell. The generative cell divides again, in either the maturing pollen grain or the elongating pollen tube, to form two genetically identical male gametes, the sperm cells.
The zygote is produced as part of a unique process known as double fertilization. One of the male gametes fuses with the egg cell to form the diploid zygote, while the other male gamete fuses with two polar nuclei, located near the center of the embryo sac, to form a triploid endosperm nucleus. Following double fertilization, the zygote develops into an embryo composed of two parts, the embryo proper and the suspensor. The embryo proper ultimately differentiates into the mature embryo, whereas the suspensor degenerates during later stages of development and is not usually present at maturity.
Flowering plants can be divided into two groups, monocots and dicots. In most dicots, the endosperm tissue is gradually absorbed by the developing embryo and is not present in the mature seed. Nutrients required for the germination of dicot seeds are generally stored in the embryonic leaves known as cotyledons. In contrast, most mature monocot seeds contain a significant amount of starchy endosperm tissue that serves as a source of nutrients for the germinating seedling.
Two important regions of the mature embryo are the root and the shoot apical meristems. The entire shoot system (stems, leaves, and flowers) of the adult plant forms from cells that are located in the shoot apical meristem of the mature embryo. The root apical meristem that is formed during embryogenesis becomes active during the early stages of germination and ultimately produces the entire root system of the adult plant. See Apical meristem, Root (botany)
The final stages of embryogenesis in angiosperms include maturation, desiccation, and preparation for seed dormancy.
Different patterns of embryo development are found in gymnosperms and in the more primitive vascular and nonvascular plants. Double fertilization and the development of a nutritive endosperm tissue are features unique to the angiosperms. The haploid microgametophyte (germinating pollen grain) in most gymnosperms contains two male gametes, but only one of these participates in fertilization. The nutritive function served by the endosperm tissue in angiosperms is served in gymnosperms by the large haploid megagametophyte. Early divisions of the zygote are also different in gymnosperms; the zygote typically undergoes a series of free nuclear divisions during the earliest stages of embryogenesis, and multiple embryos often arise from a single zygote through a process known as polyembryony. Even more striking differences in embryogenesis are found in ferns and mosses, where the haploid or gametophytic phase of the life cycle is much more extensive.
Several major differences also exist between embryogenesis in plants and animals. Plant cells are surrounded by a cell wall that limits the contact and movement between adjacent cells. Embryogenesis in plants therefore proceeds without the morphogenetic movements that are characteristic of animal development. Morphogenesis in plants is also not limited to embryo development, but occurs throughout the life cycle. The mature plant embryo is therefore not simply a miniature version of the adult plant. See Plant morphogenesis
literally, the science of the embryo, but embracing a broader range of subject matter. The term usually refers to animal and human embryology, as distinguished from plant embryology.
In animals and man. Animal and human embryology is the study of preembryonic development (oogenesis and spermatogenesis), fertilization, embryonic development (the embryo’s development within the egg or embryonic membranes), and the larval (in many invertebrates and amphibians), postembryonic (in fishes, reptiles, and birds), or postnatal (in mammals) period of development, which continues until the developing organism is transformed into an adult that is capable of reproducing. A distinction is made between general, comparative, experimental, and ecological embryology, depending on its objectives and research methods. Biochemical embryology is being successfully developed. The broader science of the patterns of individual development—that is, developmental biology, or the study of ontogeny—arose at the meeting point of embryology with various other sciences, such as cytology, genetics, biochemistry, and molecular biology.
All branches of embryology are closely related to general biology, and particularly to the theory of evolution. Morphological embryology is the basis of comparative anatomy. The natural system of animals, especially in its major divisions, rests largely on embryological data. Embryology is closely associated with histology, cytology, physiology, and genetics.
History. Embryological studies in India, China, Egypt, and Greece before the fifth century B.C. largely reflected religious and philosophical teachings. The views developed at that time, however, had a definite effect on the subsequent history of embryology, which may be properly said to have been founded by Hippocrates (together with his associates and contributors to the work known as the Hippocratic Collection) and by Aristotle. Hippocrates and his followers were chiefly interested in the human embryo, and it was only for the purpose of comparison that they recommended studying the chick’s development in the egg.
Aristotle relied to a large extent on observation; his extant works The History of Animals and On the Origin of Animals contain information on the development of man, mammals, birds, reptiles, fishes, and many invertebrates. His most detailed study is that of the development of the chick embryo. Aristotle’s theory of the successive formation of organs in embryogenesis is connected with epigenetic notions. He contrasted these ideas to those set forth in the Hippocratic Collection with respect to the preexistence of the future offspring, in all its parts, in the paternal or maternal “seed.” Aristotle’s views persisted throughout the Middle Ages, remaining substantially unchanged until the 16th century.
New observations on the development of the chick embryo appeared in works by the Dutch scientist V. Coiter (1573) and the Italian scientist Fabricius of Aquapendente (1604), marking a new stage in the history of embryology. It was only in the middle of the 17th century that a substantive step forward was made in embryology with the appearance of W. Harvey’s treatise On the Generation of Living Creatures (1651), which was based on his study of the development of chicks and mammals. Although he generalized the notion of the egg as the source of development of all animals, Harvey believed, as Aristotle did, that vertebrates develop primarily by epigenesis, arguing that no part of the future offspring “actually exists in the egg; rather, all parts are potentially found in it.” On the other hand, Harvey conceded that the bodies of insects are formed by the “metamorphosis” of previously existing primordial parts.
Like the Dutch scientist R. de Graaf (1672), who mistook for eggs the ovarian follicles later called Graafian follicles, Harvey had never seen a mammalian ovum. The Italian scientist M. Malpighi (1672), by means of a microscope, identified the chick’s organs at certain stages of embryonic development— stages in which the formed parts of the embryo had been previously indistinguishable. Malpighi supported the notion of pre formation, which prevailed in embryology almost until the end of the 18th century and whose chief defenders were the Swiss scientists A. Haller and C. Bonnet.
C. F. Wolff’s dissertation Theory of Conception (1759; Russian translation, 1950) dealt a decisive blow to the concept of preformation, which was inseparably linked to the idea of the immutability of living things. In Russia, Wolffs ideas influenced the thinking of L. Tredern, Kh. I. Pander, and K. M. Ber. In 1817, Pander published his study of certain details of the early stages of chick embryogenesis in which he set forth his ideas about the germ layers.
The ovum in the mammalian and human ovary was discovered and described in 1827 by Ber, the founder of modern embryology. In his classic work On the History of Animal Development, Ber was the first to describe in detail the main features of embryogenesis in various vertebrates. He amplified the concept of the germ layers, viewing them as the fundamental embryonic organs and clarifying their subsequent development. Ber’s observations, which compared the embryonic development of birds, mammals, reptiles, amphibians, and fishes, led him to draw some theoretical conclusions, the most important of which is the law of the similarity of embryos belonging to different classes of vertebrates; the younger the embryo, the closer the resemblance. Ber linked this phenomenon to the fact that the properties of the phylum are the first to appear in the course of embryonic development, to be followed by the properties of the class, order, and so forth, with the properties of the species and individual traits appearing last. In spite of its rather sketchy nature, this idea played an important role in the development of comparative vertebrate embryology.
The German scientist R. Remak made a significant contribution to embrology—specifically, by establishing the cellular structure of the germ layers. The start of research in invertebrate embryology dates back to the mid-19th century. A. Grube studied the development of leeches (1844) and N. A. Varnek the embryogenesis of gastropods (1850). Many other scientists subsequently collected data on the development of various forms from other invertebrate phyla.
A. O. Kovalevskii, E. Metchnikoff, and their many followers both in Russia and abroad laid the foundations of comparative evolutionary embryology, which is based on Darwin’s theory and which in turn furnishes the latter with convincing proof of the kinship of animals belonging to different phyla. Kovalevskii and Metchnikoff found that all the various invertebrate phyla go through a developmental stage of separation of the germ layers that are homologous to the germ layers of vertebrates. This fact was the basis of Kovalevskii’s theory of germ layers (1871), which holds that the principal organ systems in all multicellular animals are established in the form of layers of cells—an indication of the common origin of all the phyla of multicellular animals. The theory was later used by E. Haeckel in support of his hypothesis on the origin of multicellular animals (the gastraea theory), and it also served as the basis of O. Hertwig’s and R. Hertwig’s views on the origin and significance of the mesoderm, or middle germ layer.
Major contributions to comparative embryology were made by various Russian scientists, including A. N. Severtsov (as well as several members of his school), V. V. Zalenskii, V. M. Shimke-vich, P. P. Ivanov, N. V. Bobretskii, A. A. Korotnev, N. F. Kashchenko, M. I. Usov, E. A. Meier, and S. M. Pereiaslavtse-va. The “cell tracing” method played an important role in establishing the patterns of embryonic development; “cell tracing” serves to determine the lineage of blastomeres—that is, the subsequent development of the first cells formed by the cleavage of the egg.
Descriptive studies in embryology were accompanied by experimental work. As early as 1820, E. Geoffroy Saint-Hilaire had performed experiments to determine the role played by oxygen in the development of hens’ eggs. Research studies by the German scientists W. Roux and H. Driesch, as well as later studies by H. Spemann and by the Soviet scientist D. P. Filatov, played an important part in substantiating the principles of experimental embryology, which was initially called the mechanics of development. Experimental embryology became the arena of heated arguments on questions of general biological significance—namely, the various conflicting interpretations of embryonic development, such as the mechanistic approach of W. Roux and of the American scientist J. Loeb and the vitalistic concepts of H. Driesch. It should also be noted that for a long time no links were established between experimental embryology and the theory of evolution.
Methods. Embryological research methods vary considerably. All the different types of light and electron microscopy are used in morphological studies. Methods of observation in vivo are especially important—specifically, tracing the movements of embryonic material (morphogenetic movements); this is done by means of vital staining (the injection of tracer dyes in the living embryo), by histochemical methods, and by such other means as the use of radioactive isotopes. Experimental methods involve the removal and transplantation of different parts of the embryo. Biochemical methods have been favored since the 1950’s.
Modern developments. The objective of embryology today is the continuing study of preembryonic development, fertilization, division, formation of germ layers, organogenesis, histogenesis, the function of provisional organs, and the various manifestations of pathological development. A great deal of research is directed toward stimulating development by means of chemical agents, identifying the moving force of embryonic morphogenesis, and discovering the genetic and cytological bases of cell differentiation.
The studies of H. Spemann and his school, which were concerned with the influence exerted by certain parts of the embryo on other parts, played a major role in the development of embryology from the 1920’s to the 1940’s. The concept of the “inductor” and of the “organizer” were introduced. D. P. Filatov and other Soviet investigators developed and made substantive corrections in Spemann’s ideas; specifically, they pointed out the fallacy of the notion that embryonic material is indifferent and that its development into one or another organ is caused by contact with the inductor. Filatov linked experimental embryology to the doctrine of evolution and advanced the idea of a morphogenetic apparatus consisting of the “inductor” and the embryonic tissues that react to it—that is, those parts of the embryo whose interaction (rather than the one-sided influence of one part on another) gives rise to the specific stages of development. Filatov outlined the pathways of evolutionary change in the morphogenetic apparatus.
P. P. Ivanov’s theory of larval segments, explaining how the bodies of metameric animals are formed, represented an important advance in comparative embryology. In addition to the theory of embryonic induction, other explanations were proposed for the mechanisms that control embryonic development. For example, the American biologist C. Child held that the changes in functional differentiation along the axes of the body of the developing embryo—that is, the physiological gradient—are the decisive factor in development. A. G. Gurvich and some of his followers maintained that the orderly development of structures and processes in the embryo is determined by the “biological field.”
Soviet biologists have made substantive contributions to our understanding of the laws governing individual development. N. K. Kol’tsov hypothesized a synthesis of embryology and genetics. P. G. Svetlov proposed an original version of the theory of “critical” periods in the development of organisms. B. P. Tokin was among those who investigated somatic embryogenesis, or the development of organisms from somatic cells. O. M. Ivanova-Kazas did research in comparative invertebrate embryology and polyembryony. Some of Filatov’s students, including T. A. Detlaf, carried out numerous studies on organogenesis. I. I. Shmal’gauzen made a significant contribution to modern embryology, particularly with his research on those correlations and new interactions of parts of the organism that arise in ontogeny and that determine the development processes.
According to most geneticists, embryonic morphogenesis depends on the presence of hereditary information in the fertilized egg, such information being contained in the molecules of deoxyribonucleic acid (DNA) of the nucleus and consisting of discrete parts—namely, genes. The genes control protein synthesis by means of messenger, ribosomal, and transfer ribonucleic acid (RNA) and, ultimately, they control the development of the morphological features of the developing organism. The embryonic genome is already functioning in the fertilized egg, but initially only part of the genetic information is transcribed. The other part remains inactive and is used in the succeeding stages of development. Beginning in the gastrula stage, the genetic information shows a marked degree of variation, resulting in the specific differentiation of the various types of cells. The totipotency of nuclei in the early stages of development was demonstrated in experiments by the American scientists R. Briggs and T. King (1952 and later), who showed that the transplantation of nuclei from the cells of an embryo into the enucleated egg of a frog results in the development of a full-fledged organism.
Practical significance. The uterine phase of human development is of considerable interest to medicine. Embryology is related to the physiology and pathology of pregnancy, and thus to various aspects of obstetrics, such as hygiene in pregnancy, prevention of stillbirths, and the control of intrauterine asphyxia and developmental anomalies.
Agriculture makes extensive use of embryological data; for example, such data serve as a basis for improving the breed of farm animals by acting on the embryonic development of their offspring. Proper incubation of poultry eggs and fish breeding are also based on the findings of embryology. Embryological studies of useful and harmful insects (for example, the common honeybee, the Asiatic silkworm, the oak silkworm, and the locust) are of practical significance. Embryological data are also needed for the proper control of parasites and animals that transmit the causative agents of epidemic diseases (such as the Anopheles mosquito and various mites, ticks, and rodents).
The leading embryological research center in the USSR is the N. K. Kol’tsov Institute of Biology of Development, of the Academy of Sciences of the USSR. Embryology is taught in the universities and pedagogical institutes, and it is included in the courses of anatomy, histology, and general biology taught in the medical institutes. There is a society of anatomists, histologists, and embryologists; the Moscow Society of Naturalists has a cytology, histology, and embryology section, and the Leningrad Society of Naturalists has a developmental biology section.
Periodical publications play a major role in the development of embryology. Those issued in the USSR include Arkhiv anatomii, gistologii i embriologii (since 1916), Ontogenez (since 1970), and Uspekhi sovremennoi biologii (since 1932). Foreign journals include Archiv für Entwicklungsmechanik der Organismen (Berlin-Heidelberg-New York-Munich, since 1894; founded by Roux and renamed W. Roux’s Archives after his death), Biological Bulletin (Lancaster, since 1898), Journal of Experimental Zoology (Philadelphia, since 1904), Journal of Embryology and Experimental Morphology (London-New York, since 1953), and Developmental Biology (New York, since 1959). International embryological congresses and conferences have been regularly held since 1949.
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L. IA. BLIAKHER