Fundamental properties of individual development
The process of individual development of each organism relies on a strict set of rules. The main characteristics of ontogenesis include:
- Initial genetic program. This is the starting blueprint formed as a result of complex processes of cell division (meiosis) and subsequent fertilization.
- Irreversibility of ontogenesis. Biological time moves strictly forward. Upon implementing the embedded genetic program, the organism is physically incapable of returning to previous, already completed stages of development.
- Specialization (restriction of potencies). As the embryo develops, the likelihood of a drastic change in the course of ontogenesis rapidly decreases. Cells choose their pathway, and their plasticity declines.
- Adaptive nature. Development is not a blind process. It exhibits polyvariance, which provides the organism with the ability to flexibly adapt to diverse and changing environmental conditions.
- Uneven rate. The speed of growth and maturation processes is not a constant; it changes dynamically over time.
- Integrity and continuity. No stage arises in isolation. Any morphological or functional features appearing at later stages of development are strictly based on structures laid down at early stages.
- Presence of cyclicity. Development is subject to certain internal rhythms.
- Presence of critical periods. In the life of an organism, there are moments of maximum vulnerability when structural primordia are especially sensitive to external influences.
The concept of the morphogenetic field: historical aspect
Attempts to explain how disparate cells form a complex organism led to the emergence in the 1920s–1930s of the concept of the morphogenetic field. Its authors were prominent scientists: P. Weiss in Austria, as well as A.G. Gurwitsch and N.K. Koltsov in Russia.
The essence of the concept is that the embryo in its early stages (or its separate region in later stages) is viewed as an absolute integral whole. It was believed that the development process is governed by a certain field of this integral formation, generated by the totality of all its elements.
However, scientists' views on the nature of this field differed substantially:
- The approach of P. Weiss and A.G. Gurwitsch: They believed that the morphogenetic field fundamentally lacks conventional physicochemical characteristics. A.G. Gurwitsch introduced the concept of the biological field, actively participating in morphogenesis. The presumed source of such a field was the cell nucleus, or more specifically, the chromosomes. According to this model, cells remotely influence each other through their local fields, and the general field of the embryo is a global unification of the fields of all its cells.
- The approach of N.K. Koltsov: In contrast to his colleagues, Koltsov insisted that the force field governing embryonic development is purely physical. He believed that this exact field generates potentials of various natures, under the strict influence of which all morphogenetic processes are triggered.
According to the classical concept, development dynamics looks like a cycle: Initial field action $\rightarrow$ Execution of the morphogenetic process (e.g., formation of an organ primordium) $\rightarrow$ Modification of the field itself $\rightarrow$ Further morphogenesis.
Modern definition of organ fields
With the development of embryology, historical views transformed into more precise biological terms. Currently, the concept of the field has acquired a specific structural meaning.
Today, organ fields are referred to as specific embryonic territories over which the state of integral determination extends. This means that the cells of this region have already decided their fate and possess a confirmed ability to develop into the primordium of a strictly specific organ.
Briefly: answers to topical questions
What specific critical periods are distinguished in human ontogenesis?
In human ontogenesis, periods of embryonic sensitivity to teratogens are distinguished depending on the stage of intrauterine development. The main periods are:
- Embryogenesis — weeks 1–8, the period of maximum sensitivity due to organogenesis.
- Organogenesis — weeks 9–38, when sensitivity decreases but persists for certain systems.
The following extremely important periods for the formation of malformations of individual organs are distinguished:
- CNS — from week 3 throughout development.
- Heart — weeks 3–6.
- Upper limbs — weeks 4–5.
- Lower limbs — weeks 4–6.
- Eyes — weeks 4–8.
- Teeth — weeks 6–8.
- Palate — weeks 6–9.
- Hearing — weeks 4–9.
- External genitalia — weeks 7–9.
To easily recall the key properties of ontogenesis on the exam, use the acronym "GNS ANTS TsK" (in Russian): Genetic program, Irreversibility, Specialization, Adaptability, Unevenness, Integrity, Cyclicity, Critical periods.
Class and exam questions
How is the adaptive nature of ontogenesis manifested?
It manifests as the polyvariance of individual development, meaning the organism's ability to alter the trajectory of trait formation to adapt to various environmental conditions.
How does the concept of the morphogenetic field explain the dynamics of organ formation?
The process proceeds cyclically: the initial field acts on cells, causing morphogenesis (e.g., the appearance of an organ primordium), which alters the structure of the field itself, which in turn triggers the next stage of morphogenesis.
What is the key difference between Gurwitsch's and Koltsov's views on the nature of the field?
Gurwitsch considered the field a special "biological" phenomenon without classical physicochemical properties, originating from chromosomes. Koltsov, however, defined it as a physical force field generating potentials for development.
What else to study on this topic
- Mechanisms of genetic program formation during meiosis and fertilization
- The biological significance of potency restriction and mechanisms of cell specialization
- The significance of critical periods in human embryogenesis
- Molecular basis of integral determination of embryonic territories
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