Developmental biology

Developmental biology

Developmental biology is the field of biology that attempts to understand how organisms change from single cell to multicellular organisms. This course considers the development of animals only.It includes development of the embryo, as well as processes that occur postembryonically.Modern developmental biology is primarily experimental; biologists intervene in the developmental process, observe the outcome, and attempt to deduce the mechanisms of development from these experiments. Multicellular organisms arise by a relatively slow process of progressive change that we call development; The animal development has traditionally been called embryology,But development does not stop at birth, or even at adulthood.Most organisms never stop developing. Each day we replace more than a gram of skin cells and our bone marrow sustains the development of millions of new rbc’s every minute of our lives,Therefore recently scientists started to speak of developmental biology as the discipline that studies embryonic and other developmental processes.Is one of the fastest growing and most exciting fields in biology .creates a framework that integrates molecular biology, physiology, cell biology, anatomy, cancer research, neurobiology, immunology, ecology, and evolutionary biology.The study of development has become essential for understanding any other area of biologyThus, there are two fundamental questions in developmental biology:1- How does the fertilized egg give

       rise to the adult body?2- How does that adult body produce

      yet another body? Between fertilization and birth, the developing organism is known as an embryo  Almost all higher animals start their lives from a single cell, the fertilized ovum (zygote).  The zygote has a dual origin from two gametes- a spermatozoon from the male parent and an ovum from the female parent. 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.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.

Classical concept of embryology

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.In 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.) was the first embryologist known to history ,The development of an animal from an egg has been a source of wonder throughout history;He performed a simple expt.. Cracking open chick’s egg for each successive day of 3-weeks of incubation and observed the dvt processes,He noted the formation of the major organs,Aristotle 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 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 the Harvey investigations was experimental proof of the 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

Naturalists such as Marcello Malpighi (1628–1694) and Jan Swammerdam (1637–1680), two pioneers of microscopy, provided observations that seemed to support preformation.Based on Swammerdam'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. 

Special fields in embryology

oDescriptive embryology

oComparative embryology

oExperimental embryology

oChemical embryology

oDevelopmental biology

Descriptive embryology

Over the yrs, the science of embryology has evolved in response to new modes of thought and the availability of new techs.The earliest studies going back to the time of the ancient Greeks were concerned with understanding, the basic structural pattern of the embryonic body.Btn 1880 and 1890,  the new techs. of serial sections and of making three dimensional wax plate reconstructions from them provided the basis for descriptive embryology.More than a century later, the availability of supercomputers and the appropriate software allows the construction of digitized images.Comparative embryologyHaving its roots in the same type of descriptive work, the field of comparative embryology arose late in the 19thC.A driving force behind the dev. of this field was a great interest in evolution.Studies of dev of many spps, especially marine invertebrates, led to the recognition of different modes of dev and adoption of number of spps. as model systems for experimental studies by subsequent generations of embryologists. 

Experimental embryology

The acquisition of detailed structural information on embryos paved the way for the growth of experimental embryology.Experimental embryologist seek to understand the causative agent in dev by posing hypotheses and testing them by manipulating the embryos. 

Chemical embryology

During the 1930s and 1940s newly emerging chemical and biochemical techniques led to the establishment of chemical embryology, which provided descriptive information about chemical and physiological events in the embryo.More recent biochemical and molecular studies are revolutionizing our understanding of the manner in which different components of embryos interact and how the basic body pattern of the embryo is laid.

Developmental biology

A currently popular way of looking at embryonic development is through the approach known as developmental biology. This field includes not only embryonic dev, but also postnatal processes such as normal and abnormal growth, metamorphosis, regeneration, and tissue repair at levels of complexity ranging from molecular to the organismal. 

Embryology in current society

“Test-tube baby”;Transfer of genes from one sp into the egg of another sp. This tech. has a potential to be applied in the diagnosis and/or treatment of genetic diseases.Application of ultrasound and new x-ray imaging allows the diagnosis of many anatomical defects in fetuses.

Methods used in the study of embryonic development

Over the years many methods have been devised for studying various aspects of embryonic dev.The choice of techniques is determined by the question that is being asked.These methods ranged from using naked eyes or simple lenses to sophisticated molecular probes.

1.Direct observation of living embryos

This is the earliest technique used in embryology using naked eyes or lenses.Direct observation, particularly of living embryo, provides one with good overall view of the embryo.Application of vital dyes to cells or group of cells allows these structures to be traced over a period of time.

2. Examination of fixed material

Most morphological analysis of embryos is done on materials that have been subjected to fixation-the preservation of structures by treating the tissues with substances like formalin, glutaraldehyde that preserve structures without causing any distortion or other artifacts in the tissues. The fixed materials then cut into serial sections, which will later be observed under microscope after staining.Nowadays with computers, serial sections can be digitized and three-dimensional reconstructions made through techniques of image analysis.Scanning electron microscopy is another technique of producing 3D view of the surface of the embryos from fixed tissue.

3.Histochemical methods

Histochemistry is a method of localizing specific chemical substances or sites of chemical activity on morphological structures that are disturbed as little as possible.The tissue or embryo is rapidly frozen in a liquid nitrogen and sectioned with special low-temperature microtome called a cryostat.The tissue is then placed on the glass slide and subjected to a specific chemical reaction that leads to the deposition of a coloured product at the site of enzymatic activity or at place where certain molecules are concentrated.Through this is possible to obtain very fine resolution of histochemical reactions at the electron microscopic level.Disadvantage of histochemistry at light microscopic level is that the staining methods often do not allow specific structural features of the embryo or organ with great clarity.

4. Autoradiography

This is a technique that allows the localization of a radioactive isotope within cells or tissues.Typically, a radioactively labeled amino acid or precusor of DNA or RNA is administered to an embryo and shortly thereafter the embryo is fixed and sectioned for microscopic examination.In addition to the usual histological procedures the tissue sections are covered with a photographic emulsion and kept in dark for several weeks.Radioactive emissions from the isotope, which has been incorported into the proteins or nucleic acid of the embryo, impinge upon the emulsion during the period of exposure.The emulsion is then developed in much the same manner as photographic film, and tiny grains of silver are deposited in the emulsion over the cells containing the radioactive label.Autoradiography has been used in embryological studies to localize sites of nucleic acid and protein synthesis in embryos.A technique that allows the localization of specific types of RNA molecules is insitu hybridization.Radioactively labeled complementary DNA or RNA molecules are added to tissue sections suspected of containing the mRNA in question. If that mRNA is present, the labeled complementary nucleic acid hybridizes with the corresponding nucleotides of the mRNA.After standard autoradiographic processing, the presence of silver grains tells where in the cell or tissue the RNA molecules are located.

5. Tracing methods

Using markers to trace cell movements in embryo.Normally these markers (vital dyes) are harmless e.g. Nile blue sulfate and neutral red, can be applied to living cells without harming them.Changes of position of cells treated with these dyes can be followed through an extensive period of growth before the dye becomes so diffused that identification is no longer possible.Another tracing method consists of injecting tiny amounts of horseradish peroxidase (HRP) into cells. This enzyme is distributed throughout the cell, it is very useful in studying nerve cells.It becomes distributed throughout the long processes of the cell, enabling the investigator to determine where the processes go and with what other cells they are connected.The most recent tracing methods involve the introduction of viruses into cells. Retroviruses, engineered to contain a reporter gene, e.g. beta-galactosidase which become incorporated into the DNA of the cellular host, and the reporter gene is expressed in that cell, where the gene product can be demonstrated with a histochemical reaction.Retroviral markings has the advantage of not being diluted with cell divs. of the infected cell.

6. Immunological methods

Cells of different types, or even diff dev. stages of the same cell type, contain diff proteins and polysaccharides in the cytoplasm or on their surface membranes. These macromolecules are antigenic;ie. When injected into another animal, they provoke the immune cells of the animal to form antibodies against them.If prepared properly against specific antigens, antibodies can be valuable in studies of dev because they can be used To probe for  specific presence or absence of the antigenic molecule in question in a tissue or organ.Normally it is common to take a section of a tissue suspected of containing an antigen and cover it with a fluid containing an antibody against that antigen.Antigen-antibody complex formed cannot be  detected as it stands.Therefore it is common to add another antibody which is against the first one.The second antibody will be complexed to a marker molecule, commonly one with fluorescent properties.The marker then can be detected with fluorescent microscopy, and its location on the tissue indicates the presence of the antigen in question. The name commonly given to the localization of specific molecules in tissues by means of antibodies is immunocytochemistry.

7.Microsurgical techniques

Microsurgical techniques are used in many types of experiments. One of the simplest is ablation, or remove of part of an embryo to determine what effect the absence of that structure will have on the remainder of the embryo.Transplantation:Tissues are sometimes transplanted to other sites on the same embryo (autografting). 

8.Culture techniques

The embryonic material is placed into dishes or tubes of glass or plastic and surrounded by an artificial culture medium designed to resemble as closely as possible the environment surrounding the material in its normal site in the embryo.The major advantage is that surrounding medium can be altered in defined ways that would never be possible invivo.

9.Molecular techniques

A number of highly specialized techs. Have been devised for demonstrating particular species information-containing nucleic acids.These techs. take advantage of the unique sequence of bases that constitute a strand of DNA or RNA.They involve hybridization of simple strand of DNA with a strand of DNA or RNA so that there is matching up of complementary sets of bases.

Polymerase chain reaction (PCR)

PCR-is a more recent tech. that uses hybridization to amplify the amount of a specific nucleic acid sequence, e.g. mRNA message that may be present in very low abundance in an embryonic tissue.1st double stranded DNA is denatured (separated) by heating and small primer segments (synthetic oligonucleotides complementary to the region to be amplified) are added.In the presence of nucleotides and DNA polymerase in the solution, the primers initiate the formation of a new compementary DNA strand. The double-stranded DNA thus formed is again denatured by heating, and the process is repeated, with ever-amplifying amounts of DNA.A modification that allows cDNA’s to be formed and amplified from mRNA molecules is proving to very useful for studies of development.Up to a millionfold amplification can be made available to the investigator from even a single DNA sequence, provided that at least part of the sequence is known.The cell and its environmentA working knowledge of dev biology requires a firm background in cell and molecular biology.From the moment of fertilization, embryonic dev at all levels is a direct or indirect result of synthetic activities within cells.DNA within the nucleus is the repository of much of the genetic information within the cell.During cell divisions certain portions of the nuclear DNA molecule are free of restricting proteins that bind to the DNA and can direct the synthesis of mRNA. This process is known as transcription.The newly formed mRNA molecule contains regions (exons) that code for specific segments of a protein molecule.mRNA migrate from the nucleus into the cytoplasm of the cell via pores in the  nuclear membrane.The synthesis of intracellular proteins is accomplished by polysomes,  which consists of molecules of mRNA associated with ribosomes to form polyribosomes.Synthesis of proteins for export from the cell is accomplished on the rough endoplasmic reticulum.From there they are transported to the Golgi apparatus where they may be complexed with newly synthesized polysaccharides then,The small membrane-bound vesicles containing the proteins leave the GA.When they reach the cell membrane, fuse with it and release the protein molecules by a process called exocytosis.

 REFERENCES

Gilbert,S.F(2013).Develepmental Biology,Sanderland Mass,Sineaur Association Inc

Slack,J.M.W(2013).Essential Develepmental Biology OXFORD, Willey-Blackwell

Wolpert.l and Tickle.c (2011),Principles of Develepmental Oxford and New York University Press