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In this article we will discuss about the definition and mechanism of embryology.
Definition of Embryology:
The science of embryology is usually defined as the process of development of an individual from the zygote to a stage resembling the adult form. But in recent years, the science of embryology has achieved a new meaning.
It not only involves the changes through which the young passes but also it includes such events as growth, repair, maintenance of pattern, aging and death. Thus the science of development includes all the events, which begin from the initiation of germ cells to the death of the individual. The old term “Embryology” has thus got a new name, “Developmental Biology”.
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Historical Review:
The origin of the complex adult organism from a zygote has drawn the keenest attention of exponents of natural sciences since earliest times. The earliest philosophers tried to reason out the dynamics of development without performing any experiment. The science of development is as old as Biology itself.
The correct interpretation of developmental dynamics rests on the cell theory advocated by M.J. Schleiden (1804-1881) and T. Schwann (1810-1882) in the year 1839. Most of the embryological observations prior to 1839 were erroneous and grotesque.
Two distinct schools of developmental Biologists approached to solve the problems of development with eagerness.
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They established:
(i) The Theory of Preformation and
(ii) The Theory of Epioenesis.
Theory of Preformation:
This theory postulated that the gamete (ovum) contains a more or less perfect miniature of an adult animal in its substance and development involves mere growth and unfolding of predetermined pattern, i.e., growth and unfolding of the miniature form into an adult stage. The miniature human form was designated as homunculus (little man).
Every ovum in the ovary was assumed to contain such homunculus. The dealers of this concept (Preformationists) speculated that an ovum in the ovary of a female must contain a homunculus, the homunculus itself must contain a secondary homunculus in the ovum of the homunculus and this again tertiary homunculus, and so on until the homunculi become too small to exist.
The preformationists have estimated that mother Eve carried about two hundred million generations of homunculi. It was also believed that when the homunculi become used up, no more human beings could be born.
With the discovery of human spermatozoon by Antonj van Leeuwenhoek (1632- 1723), an additional complication was added to the preformation concept. Leeuwenhoek named the spermatozoon as the animalcule. With the establishment of significance of fertilization, the question arose as to whether the ovum or animalcule contained the homunculus.
A micro- scopist, Hartsoeker described human spermatozoon by newly invented microscope and showed the existence of homunculus within the human spermatozoon.’ (Fig. 5.1). Since then the preformationists became divided into two rival groups— Animalcultists (Spermists) and Ovulists. August Weismann (1834-1914), the discoverer of the continuity of germplasm, was the champion amongst ovulists.
Theory of Epigenesis:
The theory of Epigenesis relates that development involves differentiation which originates in a homogeneous mass of living material. There is no preformed tissue or organ at the beginning. The developmental changes are epigenetic, which mean that the determination is established subsequently. This was first advanced by Aristotle who was aware about the transformation of chick embryo.
William Harvey was an ardent supporter of epigenetic concept along Aristotelian lines. Harvey and Kaspar F. Wolff (1733-1794) added additional concepts to this theory. Besides his physiological contributions, Harvey worked on the development of mammalian and avian embryos. To Harvey we owe the famous revolutionary aphorism in embryology, “Omne vivum ex ovo”.
Although the epigenetic theory was initiated by Aristotle, K. F. Wolff (1733-1794) and Johann Friedrich Meckel (1781-1833) placed the epigenetic concept on a logical platform. We accept the theory of Epigenesis as true but we do not accept it in the form advanced by Wolff.
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With the advancement of biological science the epigenetic theory also underwent modifications. With the advent of twentieth century, numerous experimental methods have been applied to find the causal mechanism of development. Wilhelm Roux (1850-1924), one of Haeckel’s students, interpreted development through experimental methods.
His experimentations and the experiments and work of Endres (1895), Spemann (1901-12) proved that there is no preformation in development, on the contrary, the developmental changes are epigenetic, which mean that at the beginning, cells are indeterminate in nature and determination is established subsequently.
Spemann’s discovery of ‘organizer’ and its role in embryonic development added new dimension to this field. Spemann was awarded the Nobel Prize in 1935 for his discovery.
Since then all eyes were focused to search the answer of a simple question, how determination occurs. But unfortunately, despite extensive experimentations by men of different disciplines, a clear-cut answer was not available.
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But this search placed Embryology to the most important position in Biology, where number of disciplines reconciled together to realise the question of development. The answer to this question will not only solve the mystery of determination hut also will help to understand various other problems related to life itself.
Mechanism of Embryology:
The prerequisite of development is reproduction. Asexual or sexual reproduction, whatever it may be, it denotes the beginning of development. This is followed by three distinct steps, growth, interaction and differentiation.
An important realisation of developmental biology is that all developmental steps are essentially cellular processes which include: Growth, involves cellular synthesis and cellular division. Interaction, includes cell movement, cell contact and cellular induction. Differentiation, causes cells to transform into a heterogeneous state from homogeneous condition.
All these steps lead towards the attainment of a specific pattern. This is called morphogenesis. The attainment of pattern and birth of the young were formerly thought to be the termination of development, but the realisation of modern biology is that, this is the midway, developmental activities operate even after birth, when maintenance of the attained morphogenetic pattern takes place.
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This maintenance involves many complicated events like synthesis, growth, repair and death, all of which are essentially cellular processes.
