ANIMAL AND PLANT ORGANOGENESIS AND THEIR CHARACTERISTICS - BIOLOGY - 2025

The organogenesis in biology development, is a time of change where the three layers constituting the embryo to become the number of bodies found in fully developed individuals.

Placing ourselves temporarily in the development of the embryo, the process of organogenesis begins at the end of gastrulation and continues until the birth of the organism. Each germ layer of the embryo is differentiated into specific organs and systems.

Source: Anatomist90

In mammals, the ectoderm gives rise to external epithelial structures and nervous organs. The mesoderm to the notochord, cavities, organs of the circulatory, muscular system, part of the skeleton and urogenital system. Lastly, the endoderm produces the epithelium of the respiratory tract, pharynx, liver, pancreas, the lining of the bladder, and smooth muscle.

As we can infer, it is a finely regulated process where the initial cells undergo a specific differentiation where specific genes are expressed. This process is accompanied by cell signaling cascades, where the stimuli that modulate cell identity consist of both external and internal molecules.

In plants, the process of organogenesis occurs until the death of the organism. Vegetables generally produce organs throughout their life - such as leaves, stems, and flowers. The phenomenon is orchestrated by plant hormones, their concentration and the relationship between them.

What is organogenesis?

One of the most extraordinary events in the biology of organisms is the rapid transformation of a small fertilized cell into an individual that is made up of multiple and complex structures.

This cell begins to divide and there comes a point where we can distinguish the germ layers. Organ formation occurs during a process called organogenesis and takes place after segmentation and gastrulation (other stages of embryonic development).

Each primary tissue that has formed during gastrulation differentiates into specific structures during organogenesis. In vertebrates this process is very homogeneous.

Organogenesis is useful to determine the age of embryos, using the identification of the developmental stage of each structure.

Organogenesis in animals

Embryonic layers

During the development of organisms the embryonic or germ layers are generated (not to be confused with germ cells, these are the ovules and sperm), structures that will give rise to the organs. A group of multicellular animals have two germ layers - endoderm and ectoderm - and are called diploblastic.

Sea anemones and other animals belong to this group. Another group has three layers, those mentioned above, and a third that is located between them: the mesoderm. This group is known as triploblastic. Note that there is no biological term to refer to animals with a single germ layer.

Once all three layers have been established in the embryo, the process of organogenesis begins. Some very specific organs and structures are derived from a specific layer, although it is not surprising that some are formed starting from two germ layers. In fact, there are no organ systems that come from a single germ layer.

It is important to highlight that it is not the layer that by itself decides the fate of the structure and the differentiation process. In contrast, the determining factor is the position of each of the cells with respect to the others.

How does organ formation occur?

As we mentioned, organs are derived from specific regions of the embryonic layers that make up your embryos. The formation can occur by formation of folds, divisions and condensations.

The layers can begin to form folds that later give rise to structures that resemble a tube - later we will see that this process gives rise to the neural tube in vertebrates. The germ layer can also divide and give rise to vesicles or extensions.

Next we will describe the basic plan of organ formation starting from the three germ layers. These patterns have been described for model organisms in vertebrates. Other animals can show substantial variations in the process.

Ectoderm

Most of the epithelial and nervous tissues come from the ectoderm and are the first organs to appear.

The notochord is one of the five diagnostic characteristics of chordates - and that's where the group's name comes from. Below this there is a thickening of the ectoderm that will give rise to the neural plate. The edges of the plate are raised, then bent, creating an elongated, hollow interior tube, called a hollow neural dorsal tube, or simply a neural tube.

The neural tube generates most of the organs and structures that make up the nervous system. The anterior region widens, forming the brain and cranial nerves. As development proceeds, the spinal cord and spinal motor nerves are formed.

The structures corresponding to the peripheral nervous system are derived from the cells of the neural crest. However, the crest not only gives rise to the nervous organs, it also participates in the formation of pigment cells, cartilage and bone that make up the skull, the autonomic nervous system ganglia, some endocrine glands, among others.

Endoderm

Branch Organs

In most vertebrates, the feeding canal is formed from a primitive intestine, where the final region of the tube opens to the outside and lines up with the ectoderm, while the rest of the tube lines up with the endoderm. From the anterior region of the intestine arise the lungs, liver and pancreas.

Respiratory tract

One of the derivatives of the digestive tract includes the pharyngeal diverticulum, which appear at the beginning of embryonic development of all vertebrates. In fish, the gill arches give rise to the gills and other supporting structures that persist in adults and allow the extraction of oxygen from bodies of water.

In evolutionary development, when amphibian ancestors begin to develop a life outside of water, the gills are no longer necessary or useful as air-breathing organs and are functionally replaced by the lungs.

So why do terrestrial vertebrate embryos possess gill arches? Although they are not related to the respiratory functions of animals, they are necessary for the generation of other structures, such as the jaw, inner ear structures, tonsils, parathyroid glands and thymus.

Mesoderm

The mesoderm is the third germ layer and the additional layer that appears in triploblastic animals. It is related to the formation of skeletal muscle and other muscle tissues, the circulatory system, and the organs involved in excretion and reproduction.

Most muscle structures are derived from the mesoderm. This germ layer gives rise to one of the first functional organs of the embryo: the heart, which begins to beat at an early stage of development.

For example, one of the most used models for the study of embryonic development is the chicken. In this experimental model, the heart begins to beat on the second day of incubation - the entire process takes three weeks.

The mesoderm also contributes to the development of the skin. We can think of the epidermis as a kind of developmental "chimera", since more than one germ layer is involved in its formation. The outer layer comes from the ectoderm and we call it the epidermis, while the dermis is formed from the mesoderm.

Cell migration during organogenesis

A prominent phenomenon in the biology of organogenesis is the cell migration that some cells undergo to reach their final destination. That is, the cells originate in one place in the embryo and are capable of moving long distances.

Among the cells that are capable of migrating, we have blood precursor cells, cells of the lymphatic system, pigment cells and gametes. In fact, most of the cells that are related to the bony origin of the skull migrate ventrally from the dorsal region of the head.

Organogenesis in plants

As in animals, organogenesis in plants consists of the process of formation of the organs that make up plants. There is a key difference in both lineages: while organogenesis in animals occurs in the embryonic stages and ends when the individual is born, in plants organogenesis stops only when the plant dies.

Plants show growth during all stages of their life, thanks to regions located in specific regions of the plant called meristems. These areas of continuous growth are regularly producing branches, leaves, flowers and other lateral structures.

Role of phytohormones

In the laboratory, the formation of a structure called callus has been achieved. It is induced by applying a cocktail of phytohormones (mainly auxins and cytokinins). The callus is a structure that is not differentiated and is totipotential - that is, it can produce any type of organ, such as the well-known stem cells in animals.

Although hormones are a key element, it is not the total concentration of the hormone that directs the organogenesis process but the relationship between cytokinins and auxins.

References

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