embryology (redirected from embryologists)

embryology

Study of the formation and development of an embryo and fetus. Before widespread use of the microscope and the advent of cellular biology in the 19th century, embryology was based on descriptive and comparative studies. From the time of Aristotle it was debated whether the embryo was a preformed, miniature individual or an undifferentiated form that gradually became specialized. The latter theory was proved in 1827 when Karl Ernst Baer discovered the mammalian ovum (egg). The German anatomist Wilhelm Roux (1850–1924), noted for his pioneering studies on frog eggs (from 1885), became the founder of experimental embryology.

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embryology

1. the branch of science concerned with the study of embryos

2. the structure and development of the embryo of a particular organism

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embryology [em·brē′äl·ə·jē]

(biology)

The study of the development of the organism from the zygote, or fertilized egg.

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Embryology

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.

Animals

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)

Plants

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