Developmental biology - Introduction and History | BSc

Описание: Developmental biology is a branch of life sciences that aims to understand how organisms grow and develop. It provides insights into cell differentiation, tissue formation, and the development of the whole organism from a single fertilized egg.

Aristotle

According to the epigenesis theory, the single-celled embryo gradually progresses through different stages to form the complete organism. He cracked open the chick eggs on successive days of their 21-day incubation period and observed what was present in them. Aristotle believed that the white of the egg forms the embryo and the yellow provides the nutrients. He also observed a speck of blood that forms the heart, followed by the veins and other organs. By 3 weeks, the complete bird was formed. He delineated the developmental stages and proved that it was epigenesis and not preformation that occurs wherein a single cell gradually develops into a complete organism.

Karl Ernst von Baer
Karl Ernst von Baer published four laws of animal development, which were a response to the recapitulation theory of Johann Friedrich Meckel. Meckel believed that the different developmental stages of highly complex animals were the adult forms of less complex animals. von Baer believed that animal embryos shared only a few basic forms, and subsequently branched out into organisms with a completely different appearance.

Four laws of von Baer
The general features of an organism appear before the specialized features.
The development of an organism starts with a simple and uniform structure that gradually becomes complex and diverse. For example, the presence of the vertebral column appears early in embryonic development, and the specialized features, such as fur on mammals, appear in the later stages.
Different species of animals appear similar only in the initial few embryonic forms; they diverge as ontogeny progresses. He discussed the embryonic forms of humans, chicks, and fish where they appeared similar in the early stages but looked increasingly dissimilar with continued growth.
He believed that the embryos of complex animals resemble only those of less complex animals, not their adult forms.

Ernst Haeckel

Ernst Haeckel is known for the biogenetic law, which states that an organism’s developmental phases (ontogeny) follow that of its evolutionary history (phylogeny). He supported this theory using the drawings of embryonic fish, chick, pig, and man showing the embryos of these organisms looked very similar in the early developmental stages. The three assumptions of the biogenetic law are as follows:

The law of correspondence states that every stage of development in higher animals resembles those of the adult stages of lower animals. For example, the gill slits observed in human embryos resembled the adult form of the fish.
Phylogenesis occurs toward the later stages of the developmental process. Haeckel believed that the similarity observed in the embryos of the different animals in the early stages was caused by developmental constraints. These limitations were absent toward the later stages of development, adding new characters.
The principle of truncation states that the early developmental phases occur rapidly in higher organisms compared with lower organisms. Otherwise, embryonic development would become much longer than the gestation period due to the addition of new characters when normal ontogeny ends in the extant species.

Wilhelm Roux

He pricked one of the two blastomeres in a 2-cell stage egg only once with a hot needle for a lengthier duration than his previous experiment. A light brown color appeared in the area surrounding the egg. When he observed the punctured eggs under the microscope, he noticed that most of them were destroyed, very few undamaged blastomeres survived and only some underwent normal development. The destroyed cells appeared grey indicating cell death.

Roux believed that the cells died due to needle puncture and not because of their inability to interact with the other cells. The half embryo in the eggs that survived developed normally, with the region next to the punctured cell slightly affected. He concluded that a 2-cell or 4-cell stage egg can develop independently in the formation of an embryo, suggesting a predetermined developmental path owing to internal factors operating on the cell.

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