The role of apoptosis and the fate of embryonic primordia
During embryonic development, an excessive number of tissue primordia is often formed. Their final number and shape are strictly regulated by apoptosis—genetically programmed cell death.
Classic examples of structure reduction via apoptosis:
- Vertebral column: the embryo initially develops a long tail. Normally, its tissues undergo mass apoptosis, leaving only 4–5 vertebrae that form the coccyx.
- Hair coat: most primary hair primordia undergo reduction. Disruption of this delicate mechanism leads to the development of pathologies such as hypertrichosis (excessive hair growth).
- Limbs: at early stages, 6–7 digit primordia are formed. The classic pentadactyl limb results exclusively from targeted cell apoptosis in the interdigital spaces.
A natural question arises: why do structures destined to vanish form at all? The fact is that even if an organ is completely reduced in the adult organism, its initial primordium in embryogenesis is critically important. The embryonic primordium acts as a vital element of the correlative system. It serves as a specific inducer that sets the vector and determines the correct development of other, essential body systems.
Main modes of phylembryogenesis
Evolutionary changes in individual development can occur at different time intervals. Depending on the stage at which deviations arise, three main pathways are distinguished: arhallaxis, deviation, and anaboly.
1. Arhallaxis (changes at early stages)
Arhallaxis represents a developmental deviation at the very earliest stages of embryogenesis, directly during the initial formation of an organ or tissue.
Arhallaxis is based on the following mechanisms:
- Alteration of the initial mass of the embryonic primordium.
- Alteration of the direction of cell differentiation within the primordium.
- Alteration of topography (the site of organ formation).
- Alteration of chronology (the timing of primordium appearance).
Striking examples of this evolutionary pathway include an increase in the number of germ layers in multicellular organisms, changes in the baseline number of digits or vertebrae, as well as the formation of an increased number of feathers in birds or hair follicles in mammals.
2. Deviation (changes at intermediate stages)
Deviation is characterized by the fact that the deviation from the ancestral developmental pathway occurs at intermediate stages of embryogenesis.
A classic example of deviation is the evolution of the middle ear bones in vertebrates. In amphibians, reptiles, and birds, only a single auditory ossicle—the stapes—forms in the middle ear apparatus. In mammals, the development of this apparatus up to a certain stage proceeds identically to that of lower vertebrates. However, a deviation then occurs—the pathway becomes more complex, and three auditory ossicles ultimately form: the malleus, incus, and stapes.
3. Anaboly (changes at late stages)
Anaboly affects the very final stages of ontogeny. This mode of phylembryogenesis can be implemented in two opposite ways:
- Prolongation of development (addition): completely new stages are added to the end of ontogeny. This is how the four-chambered heart and the complex metanephric kidney of higher vertebrates appeared in the course of evolution.
- Truncation of development (neoteny): the final stages of ontogeny are omitted, and the organism reaches sexual maturity while remaining at the larval stage. A textbook example is the axolotl. This is a larva of the ambystoma, which in the course of evolution acquired the ability to reproduce without transforming into the adult terrestrial form, effectively becoming an independent species.
Briefly: answers to topical questions
What human developmental pathologies arise from impaired apoptosis of embryonic primordia?
When apoptosis of embryonic primordia is impaired in humans, hair coat pathologies and limb development anomalies occur.
- Hypertrichosis develops as a result of impaired reduction of the majority of primary hair primordia.
- Limb anomalies arise in the absence of interdigital apoptosis, which is normally necessary for the formation of the pentadactyl limb from the initial 6–7 digit primordia.
Who is the founder of the theory of phylembryogenesis?
The founder of the theory of phylembryogenesis is A.N. Severtsov.
Within the framework of his theory, he identified three main modes of evolutionary change in the course of individual development (ontogeny):
- Arhallaxis—developmental deviation at early stages of embryogenesis (during organ or tissue formation).
- Deviation—developmental deviation at intermediate stages of embryogenesis.
- Anaboly—deviation at the final stages of development (prolongation or truncation of development).
What types of ontogenetic correlations ensure the integrity of the developing organism?
Types of ontogenetic correlations include:
- genomic;
- morphogenetic;
- ergontic.
Ergontic correlations manifest at later stages of ontogeny and are driven by the interdependence of functions of various organs or body parts.
From which embryonic primordia do the auditory ossicles of mammals form?
The auditory ossicles of mammals form from the embryonic primordia of the first and second pharyngeal arches.
- Malleus (malleus)—develops from the material of the first pharyngeal arch.
- Incus (incus)—also forms from the first pharyngeal arch.
- Stapes (stapes)—develops from the second pharyngeal arch.
The complexification of the middle ear apparatus in mammals occurs via deviation—a developmental deviation at intermediate stages of embryogenesis.
The acronym ADA can help remember the three types of phylembryogenesis by the time of their occurrence: Arhallaxis (early) — Deviation (intermediate) — Anaboly (late).
Class and exam questions
Why does the embryo develop structures that subsequently disappear?
Temporary primordia function as elements of the correlative system. They induce and direct the development of other vital organs, without which normal fetal formation is impossible.
How is the normal pentadactyl limb formed?
Initially, 6–7 digits are laid down, but excess cells and interdigital webs are eliminated through programmed cell death—apoptosis.
What is neoteny and what type does it belong to?
Neoteny is the attainment of sexual maturity at the larval stage (e.g., in the axolotl). It is a variant of anaboly involving the truncation of ontogeny with the omission of final developmental stages.
What else to study on this topic
- Molecular mechanisms of apoptosis in embryogenesis
- Embryonic induction and organ correlation systems
- Causes of hypertrichosis development from an embryological perspective
- Evolutionary origin of mammalian auditory ossicles
- Stages of pelvic kidney and four-chambered heart formation
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