What is an organizer?
The effect of embryonic interaction or organizer is a morphogenetic effect by which one organic tissue transmits a chemical substance that influences other embryonic part to produce a structure that otherwise could not come into existence. The embryonic tissue which exerts such an influence is called an inductor and the chemical substance secreted by an inductor is known as evocators. The tissue on which evocator works and the tissue responses is known as responsive tissue. The action of the indicator through evocator is known as induction action or organizer action. This process of induction influences greatly the protein synthesis mechanism of responsive tissues as a result of which definite structure forming cells become very active.
Origin of the concept of the organizer
Spemann’s experiment (1924): A German embryologist Hans Spemann and his student Hilde Mangold (1924) performed transplantation experiment on a newt Triturus cristatus, an Urodela of class Amphibia. Spemann grafted a piece taken from the dorsal lip of early gastrula of Rana sp. to the lateral lip region of the early gastrula of Triturus cristatus. The embryo of Rana sp. is donor and the embryo of Triturus is the host. They observed that the cells of the grafted piece enter into the gastrula and form notochord and somites. In this embryo its own dorsal lip of blastopore forms neural groove, notochord, mesoderm etc. Similarly the grafted tissue influences to form notochord, neural groove and mesoderm. That is in the same embryo double set of notochord, nerve cord and mesoderm are produced. In this case donor tissue has secreted some chemical substances which has induced to form neural groove, notochord etc. in the host embryo. The donor tissue had pigments and the induced neural groove has also coloured pigments. After the completion of the gastrulation they observed that a larva has developed with two heads. One head is due to normal development and the other head production has been induced by donor tissue.
They examined the larva under the microscope and found that notochord, renal tubules, gut etc. have been formed by the tissue of the host embryo as a secondary set. If the donor tissue would not have been grafted such secondary structures wound not develop. From this experiment they concluded that dorsal lip of the donor had influenced greatly the tissue and thus has brought about change in the host tissue development. If it is not the fact then how a head had developed in the abdomen of the host. This secondary head formation is due to induction effect of donor tissue. This process of influencing other tissue was termed as induction by Spemann and the tissue that induced the tissue was known as the inductor or organizer.
Spemann continued his grafting experiments taking tissues from different zones of the gastrula and observed that except dorsal lip of the early gastrula other zone of tissue can not create any induction effect but when dorsal lip is grafted a complete embryo is formed. He named the dorsal lip as organizer as this dorsal lip organizes the developmental process of the embryo. According to him this dorsal lip induces to form neural tube and the neural tube then induces to form the eyes. The dorsal lip is composed of chorda-mesoderm and as it primarily acts as inducer so he named the dorsal lip or chordamesoderm as primary organizers.
Secondary, tertirary and quaternary organizers:
As the gastrulation proceeds due to primary organizer’s induction primary organs begin to form and the early stages of organ development are known as organ rudiments. These organ rudiments themselves may act as organizer and then they are known as secondary organizer. Tissues formed by the action of secondary organizer may in turn induce further development. Then they are known as tertiary organizer. These successive stages of organizer activities start from the primary organizer.
How these organizers act in succession can clearly be understood from the examples of the development of eye in amphibian, chick etc. First of all due to induction effect of the primary organizer forebrain and within the forebrain eye forming cells are produced. These cells push out as a vesicle outside the forebrain. These vesicles are known as optic vesicle. This vesicle grows through the lateral mesenchyme and reaches the epidermis.
As soon as the vesicle comes in contact with the epidermis the outer layer of the vesicle invaginates to form a double layered optic cup. The inner layer of the optic cup is formed of sensory cells and the outer layer is formed of pigmented cells. They two together form the retina. The chemical substances secreted by the optic cup induce to form the lens between the optic cup and the epidermis. The peculiar thing is that if the optic vesicle is prevented from coming in contact with the epidermis there will be no lens formation. So the optic cup acts as secondary organizer. Similarly lens and retina together induce to form cornea so lens and retina together act as tertiary organizer and so on.
Classification of induction:
Lovtrup (1974) classified induction into two principal classes.
Endogenous induction: Shapes and sizes of some of the embryonic cells changes after secreting inducting substances and this induction brings about differentiation of cells. As for example small cells of the dorsal lip carrying yolk granules act as endogenous induction.
- Exogenous induction: When either by external influence or by contact any cell or tissue induces nearby tissue to differentiate, then it is known as exogenous induction. Exogenous induction may again be of two types. As-
- Homotypic: When the contact induction induces to form same types of cells, it is known as homotypic.
- Heterotypic: When the contact induction induces different types of cell differentiation, it is known as heterotypic induction.
Embryonic induction in vertebrates:
Spemann observing the induction effect of dorsal lip named it as primary organizer but Ebert and Sussex (1974) said the formation of secondary embryo is due to cell differentiation of both the donor as well as of the host. They preferred to call the primary organizer of Spemann as embryonic inductor. As the primary organizer induces the epidermis for the formation of neural tube so now a days the primary organizer has been renamed as primary inductor or neural inductor.
Morphology of Neural inductor:
Vogt (1924) has shown by vital staining technique that cells of the dorsal lip of blastopore of a newt’s gastrula, move interior and form the roof of the archenteron. If a block of tissue from archenteron roof is transplanted to the abdomen of another gastrula then from the abdomen created by the host gastrula tissue, a secondary larva is formed. All parts of the dorsal lip can not induce such induction. If only endodermal cells are grafted it will give rise to a partial embryo. If the anterior part is grafted it will induce to form the mouth, sensory organs head with the brain of the partial embryo. If the middle part is grafted it will give rise to eye and nasal cavities, lateral side induces to form posterior part of the head and if the posterior part is grafted then it will induce to form spinal cord, trunk and tail mesenchyme. From these experiments it can be concluded that the dorsal lip possess the regionality of its induction activity
Types of inductors:
On the basis of various experimental evidences Lehmon (1945) said that specific regionality of induction effects present in the dorsal lip of the blastopore. He further said that the roof of the archenteron definitely possess specific induction activities for the differentiation of head and trunk regions. On the basis of the regional specificity he classified the inductors into three groups. They are:
Archenocephalic inductor: Due to induction effect of this inductor partial head, fore-brain, eye, nasal cavities are formed.
- Deuterencephalic inductor: By its induction effect posterior portion of the head, ear cavities etc. are formed.
As arechenocephalic and deuterencephalic inductors induce the formation of different parts in the head region so they together are known as cephalic or head inductors.
- c) Spino-caudal inductor: Their inductive influence leads to the formation of spinal cord and different structures of the tail region.
Development of Eye in Chick
The first sign of the development of the eyes is a bulging at the lateral sides of the prosencephalon. These are the rudiments of the optic vesicles which lie beneath the head ectoderm. Meanwhile, the distal part of each optic vesicle (the future sensory layer) invaginates and presses against the proximal part (the future pigment layer of the retina, iris and ciliary body). This results in the formation of the optic cup, the elimination of the original lumen of the optic vesicle and the formation of a new lumen, the future vitreous chamber.
The lens is formed from the lens placode, a thickening of the ectoderm formed in response to an inductive signal from the optic cup. The lens sinks beneath the surface of the ectoderm, the latter becoming the cornea.
As the lens continues to grow, the cells in the thickened region lose their ability to divide and become converted into fibres that will become the core of the adult lens. New fibres are formed from the cells at the periphery of the lens which divide rapidly and become arranged in concentric circles around the original core. By the time of hatching there are three concentric layers of fibres, the core, the intermediate layer of irregularly arranged fibres, and the radial layers which continue to grow after hatching. The lens capsule, which is an extracellular material with a high collagenous component, starts to form about day 7. The ciliary body develops close to the lens, its role being to secrete the fluid of the vitreous chamber.
As the lens loses contact with the ectoderm a space is formed, the anterior chamber of the eye. The corneal epithelium develops from the ectoderm covering the anterior chamber, whilst the corneal stroma forms from the mesenchyme and becomes visible on day 4 as a thin layer beneath the epithelium. It becomes thicker as mesenchyme cells migrate into it during day 7.
The iris arises from cells at the margin of the anterior chamber at about day 7. Removal of the lens results in disorganization of the components of the anterior chamber.
The retina is formed from the optic cup. Its inner layer becomes the neural retina and its outer layer the pigmented retina.
The choroid and sclera differentiate from the mesenchyme around the optic cup, forming the inner pigmented vascular layer, and the outer, fibrous layer, respectively. The melanophores of the choroids are derived from cells of the neural crest that reach the eye during day 2 and develop pigment on day 7. Cartilage starts to form in the sclera on day 8.
The eyelids start to form at about 7 days from a circular fold of skin surrounding the eye.
The choroid fissure usually begins lo close in the region near the lens about day 4. At this time a ridge of mesoderm, carrying with it a blood vessel, migrates along the choroid fissure into the posterior chamber of the eye and enlarges during day 5 to form the pectin. The pigment cells of the pecten are derived from the pigmented retina. The pecten is a structure characteristic of birds, and it is thought that it acts not only by bringing oxygen and nutritive materials to the eye but that it may also play a role in vision.
The vitreous humour is secreted by the cells of the optic cup.