This essay provides information about the History of Embryology!
Life and death have been fascinating subjects and have remained mysteries from time immemorial.
How living beings came into existence? What is death? What happens after death? All these have been interesting subjects to scientists and have remained unsolved mysteries.
Fragmentary knowledge was available during different stages of development of animals and birth of a baby from very ancient times.
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Aristotle (384-322 B.C), a Greek philosopher studied embroys of animals and many of his observations were accurate. Anaximander (600 B.C) was perhaps the first to conceive the idea that living creatures arose from moist element and man was like fish in the beginning.
The Greek philosopher Empedocles was aware of the fact that foetus arose partly from male and party from females semen. He also had knowledge regarding the different stages of development and that heart formed first and nails later. The credit of establishing Embryology as a separate and independent branch of Biology goes to Aristotle.
He believed that the soul or form of the embryo was derived from the father and mother supplies the matter or soil in which the embryo grows. Aristotle’s views were acceptable through most of the medieval period, and his views appeared in the writings of Fabricius (1537-1619) and Harvey (1578-1657).
These scientists stated that the soul or the determining cause for development existed in the blood of animals. With the invention of the microscope by Robert Hooke in 1665 the sperm and ovum were discovered but their importance in development was not established.
Contents
[I] Preformation theories:
According to the preformation doctrine, the human being is preformed either in the ovum or the sperm. The ovarian follicles were described by De Graff (1672) while Antonii van Leeuwenhock (1679), a Dutch scientist, described the sperm of a dog. However, the significance of the sex cells was not known to them.
Haller (1708-1777), Bonnet (1720-1793) and Malpighi (1673) believed that the ovum contained a small miniature of the adult. The semen from the male stimulated the growth of this miniature. This view was called as the ovist view.
In contrast to this view, the homunculist or spermist view is that it is not the ovum but the sperm which contains a miniature which grows into a fully formed baby by obtaining nourishment from the ovum.
Hartsoeker (1656-1725) drew a picture of the sperm with a miniature human being located in the head of the sperm. These two views are known as the preformation theories as both of them believe that the human being is fully formed and resides in a miniature form in the sperm or ovum.
One extreme view of the preformation doctrine is the encasement or emboitement theory propounded by Bonnet and Swammerdam. According to this theory, successive generations of individual organisms pre-existed one inside other of the germ cells of the mother. In other words, the ovum of the mother contained a miniature fully formed individual of the successive generation.
The ovum of the miniature individual in turn contained a miniature fully formed individual of the second generation and so on. Thus, it was believed that the ovaries of Eve, contained as many as 200 million human beings already delineated and belonging to many successive generations.
The preformation theories persisted upto the eighteenth century. A German scientist C.F. Wolff (1759) and Spallanzani (1729-1799) showed that both sperm and ovum were necessary for the development of the individual and there was no miniature fully formed individual either in the ovum or in the sperm, thus disproving the prefromation theory.
[II] Theory of epigenesist:
The epigenetic concept was proposed by C.F. Wolff (1759). According to this theory, there is no preformation of the individual in the sperm or ovum but the embryo develops by progressive growth and differentiation.
The material from which the embryo formed already existed in the ovum and during development; this material rearranges itself into layers which fold in various ways to form the embryo. This process of progressive development of the embryo from simple to more complex form is called as epigenesis. Wolff’s work was mostly theoretical.
[III] Baer’s law:
Karl Ernst Von Baer (1792-1876) considered to be father of modem embryology, was the first to present embryological data in a coherent form. In 1828 he proposed his famous law known as Baers law based on the extensive research work on developing embryos.
According to this law during the development of an animal from ovum, the more general features of the group to which it belongs develop first and the special features of the species appear later. For example, during development of chick, the generalized chordate characters such as notochord, dorsal tubular nerve cord and gill slits develop on the first and second days followed by the specialized characters of birds, like feathers, claws and beak on the fifth day.
At the time when Von Baer postulated his law, the theory of evolution was not firmly established. According to the theory of evolution that came into existence in the middle of the nineteenth century, animals and plants have slowly evolved over millions of years from simple unicellular organisms.
First the invertebrates came into existence followed by vertebrates. Among the chordates, the sequences of evolution was cyclostomes, fishes, amphibia and reptiles. Reptiles gave rise to birds and mammals.
[IV] Biogenetic law:
Fritz Muller, a German scientist (1864) after studying the process of evolution of animals stated that during the development of animals, the characteristic features of ancestral animals appear in earlier stages than the characters of more recent origin. In other words, during the ontogenic development of an animal, the entire racial history or phylogenetic relationship is repeated— ontogeny repeats phylogeny.
During the development of chick, for example, the characteristic features of fish such as gill slits appear before the appearance of lungs and claws which are characters of amphibia and reptiles. Feathers and beak characteristic features of the birds appear in the last. Thus, developmental stages of the chick encompass all the evolutionary stages in the history of birds. Ernst Haeckel (1866) another noted German biologist coined the term Biogenetic theory or Recapitulation theory.
According to the recapitulation theory, chick during its development, should first resemble a fish, later an amphibian followed by a reptile. However, this does not occur. The chick embryo resembles a fish embryo, an amphibian embryo and a reptilian embryo in the last. Therefore, the recapitulation theory in its modern modified form can be stated as “ontogeny of an animal repeats the fundamental steps of the ontogeny of ancestral forms”.
Until the time of Wilhelm Roux (1888), embryology remained a descriptive subject, based on observations only. Wilhelm Roux, a German scientist, performed experiments on frog’s eggs and introduced the modem era of experimental embryology.
This field has been supported in the last part of the nineteenth century and early twentieth century by the works of Weismann (1883), Driesh (1891), Endres (1895), Spemann (1903), Morgan (1908) and others. Their work helped to solve many of the unexplained aspects of embryology and contributed in propounding certain landmark theories in embryology.
[V] Germ plasm theory of Weismann:
The germ plasm theory proposed by A.Weismann in 1883 can be considered as a major landmark in developmental biology because he was the first to distinguish germ plasm as a seperate entity from somatoplasm. According to him, the germ plasm is a self-perpetuating cellular entity which continues its existence through ages.
The child inherits characters of the parents through the sex cells of the parents and not from other parts of the body. The germ cells acquire the characters from preexisting germ cells. In each new generation, a temporary soma or body is built around the germ plasm that descended from the parents.
According to Weismann, the embryo remains already organized in the chromosomes of the nucleus and during early development these characters are unpacked in an orderly manner. He recognized units of heredity as “determinants” and these are segregated during the early cell divisions of the embryo making these division differential.
In the first cell division, the determinants are distributed into right and left sides of the future embryo and later divisions into anterior and posterior parts and so on. Later, interaction between them enable epigenetic development possible.
[VI] Mosaic theory of Roux:
In 1888, Wilhelm Roux performed an experiment on frog’s egg. He killed the nucleus of frog’s blastula at two-celled stage, by touching one of the two blastomeres with a red hot needle. He later observed that the embryo developed with one half part defective. Thus it was concluded that certain areas of the egg were predetermined to develop into specific parts of the body.
The animal pole is predetermined to develop into anterior side of the embryo and the vegetal pole into the posterior part. The grey crescent develops into the blastspore. During cleavages, these predestined regions are segregated into the blastomeres that develop into specific tissues and organs. The blastula thus is a mosaic of blastomeres with potencies to develop into different parts of the body, and as development proceeds, these are differentiated and segregated into specific cells.
[VII] Regulative theory of Driesh:
The regulative theory is in contradiction to the mosaic theory. Hans Driesh (1891), a German scientist, observed that the blastomeres of sea urchin at two-celled stage, when separated from each other, grow to develop into two young embryos of comparatively smaller size. At four cell stage, separation of the blastomeres led to the formation of four distinct embryos of even still smaller size. If intact, all the four blastomeres formed only one embryo.
However, this pattern continued only for a few cleavages, later cleavages could not produce separate fully formed embiyos upon isolation of the blastomeres. Therefore, Driesh concluded that the early cleavages of the egg were all equal. The homogenous material of the ovum was quantitatively equally distributed into the blastomeres. In early cleavages all the blastomeres have equal potencies to develop into a whole embryo.
However, if not separated, their fate in the whole is determined by the position occupied by them in the whole blastula. This type of development is known as regulative development. Each cleaving egg, according to Driesh, was a harmonious and equipotent entity. Each blastomere has potentially the properties of the whole. At present, it is known that every egg is both regulative and mosaic in development. The extent of regulation is dependent upon the mRNA content of the cytoplasm.
[VIII] Gradient theory of Child:
The experiments conducted by Boveri (1901), Child (1940), Horstadius (1955), Runnstrom (1967) and Josefsson (1969) on sea urchin egg have given a new concept to experimental embryology. Boveri (1901) noted that the vegetal pole region of sea urchin egg controlled the activities of the regions situated towards animal pole. In fact, the vegetal pole region had a dominant effect over the animal pole areas. No reason was given by him. Child (1940) proposed the metabolic axial gradient theory.
According to this theory, the single physiological gradient namely metabolic axial gradient controlled the process of morphogenesis during the development of sea urchin. He observed that there was vast difference in the rate of general oxidative metabolism in the eggs of sea urchin between the vegetal pole and animal pole. It was greater in the animal pole region and gradually decreased towards the vegetal pole in a graded fashion.
Similar gradient existed in the developing eggs of starfish. The animal pole region being more active metabolically, divides at a higher frequency as compared to the less active vegetal pole region. Thus, a metabolic gradient is found along the animal-vegetal axis with high metabolic rate in the animal pole region, gradually decreasing towards the vegetal pole.
Child’s original concept of matabolic gradient has been modified in later years by Horstadius (1955) and Runnstorm (1967). These biologists observed a double gradient system extending along the animal-vegetal axis.
These are the animal gradient and vegetal gradient. The animal gradient is most active at the animal pole region and gradually decreases in intensity towards vegetal pole. The vegetal gradient, on the other hand, is most active at the vegetal pole region and decreases towards the animal pole.
At the animal pole, the power to form structures characteristic of this region are most pronounced. Similarly, at the vegetal pole, the power to form structures like archenteron, mesenchyme, etc. is greatest. As both the gradients extend along the animal-vegetal axis, they overlap each other and differ quantitatively.
The animal gradient is believed to be cortical in location while the vegetal is located more internally. Later work has shown that the gradients are due to the distribution of certain chemical substances like tryptophan and nucleotides. The animal gradient promotes protein synthesis while the vegetal gradient inhibits this process so that protein synthesis may occur at different times along the animal-vegetal axis.
[IX] Organizer theory of Spemann:
Spemann performed experiments on amphibian eggs during 1901-1912 and observed that the formation of the lens of eye in grass frog embryo is dependent upon the presence of the optic vesicle. When the optic vesicle was transplanted into the belly region, the overlying ectoderm differentiated into a lens. Removal of the presumptive eye rudiment resulted in the failure of lens formation. In 1918 Spemann transplanted the dorsal lip of the blast pore of one frog embryo into another.
In the new site, the dorsal lip induced the formation of a second embryo. The dorsal lip induced the tissues surrounding it to differntiate into a second embryo, thus some influence was passed on from the dorsal lip to other tissues determining the development of the ectoderm of the gastrula. It was the chorda mesoderm situated in the roof of the archenteron which had an organizing effect on the over-lying tissues.
Transplantation of skin ectoderm over the chorda mesoderm, the neural plate area resulted in the differentiation of the skin ectoderm into neural tissue. On the other hand, if a piece of neural ectoderm is transplanted into the skin area, it lost its identity and differentitated into skin ectoderm.
After this breakthrough, many workers subsequently reported the organizing influence of chorda mesoderm of many other animals and efforts were made to identify and isolate the chemical responsible for the inducing effect. Different workers reported different substances. Waddington and his coworkers noted that even a chemical (dye) could function as an inducer.
For his landmark discovery of dorsal lip of amphibian blastula acting as a primary organizer, Spemann received Nobel Prize in 1935. Organizers at present are recognized as an embryonic tissues that influence and organize other tissues to differentiate and produce a tissue or structure that in normal course should not have been formed. This process is known as induction and the tissue producing this effect as the inductor or organizer.