Organ - Anatomy

An organ is a group of tissues that carry out similar functions. Organ biology consists of the formation and the structural organization of those groups of tissues that are destined to carry out similar functions. Both plant and animal life relies on the day-to-day co-ordination of such physiological processes in the body that are carried out by the organ systems. The organ systems are essential for carrying out various individual functions that sustain the life of the plant and animal body. Examples of organ biology in humans include the digestive system, respiratory system, muscular system, cardiovascular system, lymphatic organ system, etc. 

What are Different Organ Systems Made up of?

Organ definition biology states that when a group of tissues arising from the same germ layer during embryonic development are destined to perform similar functions, it forms an organ and in turn an organ system. The tissue systems in organ biology can be broadly classified into the parenchyma and the stroma. The parenchyma is the tissues that are specific to the organ and are responsible for carrying out the organ’s desired function whereas the stroma are the ones that carry out different functions such as supportive, structural and ancillary functions. Considering the example of a gland, the tissue that makes and produces hormones is the parenchyma whereas the example of stroma includes the nerves that innervate the parenchyma. Other functional support provided by the stroma includes the blood vessels that oxygenate the parenchyma and nourish it and the connective tissues that provide a suitable space for holding and anchoring the parenchyma inside the body. 

As mentioned in the organ definition biology, the main tissues that make up a particular organ usually tend to have common origins as they arise from the same germ layer during embryonic development. 

When two or more functionally related organs work together towards achieving the same goal, it gives rise to the organ systems inside the body. Most of the multicellular organisms consist of organs and organ systems. Such specialized organs also exist in unicellular organisms. But in a unicellular organism such as the bacteria, the functional analogue of the organs is known as the organelle which is usually a cell body present inside the cytoplasm. 

In plants as well such organizational structures exist and the main organs in the plant body are three which are: the root, stem and leaves. There are several organs and organ systems present in the animal body as well. By the organ definition biology classification of 79 different organ types has been done for the human body. Some of these organs include the hollow organs as well which are structurally identified by hollow tubes or pouch such as the stomach, intestine or bladder.

Origin and Evolution of the Organs

The first detected level of organ organization in animals is in flatworms and the other derived phyla from the flatworms. Less advanced organisms of the taxa such as Placozoa, Sponges and Radiata, do not show any such consolidation of tissues into organs. 

Complex organisms such as human beings are composed of different organs which have further evolved over time. The liver is one of the best examples of such a case. The liver evolved in the stem vertebrates over 500 million years ago while the gut and brain are even more ancient having been evolved and developed in the vertebrates almost 600 million years ago. 

Structural Organisation of Different Organs and Organ Systems

The structural organisation of different organs can be categorized into tissue systems and organ systems. They are both briefly described below:

Tissue Systems

The biological definition of tissue places it in-between the cells and organs in the organizational level of the cell. Thus, tissues are the in-between stage of cells and complete organs and organ systems. A tissue is a collection of cells and their extracellular matrix arising from the same origins to carry out a specific function. The functions carried out by the cells in a tissue system are usually the same. From such tissue systems, organs arise when multiple tissues performing the same function come together. Thus, organ biology consists of the functional grouping together of the multiple tissues.

The study for the understanding of the tissues in animals is known as histology and in plants, it is known as plant morphology. In order to understand the structural formations of the different organs, the understanding of the tissue organization plays an important role. The classical tools involve the paraffin block or the agarose gel depending on whether the tissue sample is from an animal or plant, in which the tissue is embedded and sectioned as per the requirement, and then after the application of histological stain, it is observed under the microscope. With the development of electron microscopy, immunofluorescence techniques, and the use of frozen sectioned tissue samples, a detailed understanding of the tissue organization has been achieved. These tools also help in a better and faster medical diagnosis and prognosis. 

Organ Systems

An organ system is formed by two or more organs that work together to perform and execute a specific function. This organ system is also known as the biological system or the body system. An example of a very simple organ system is the digestive system in any animal body or human body which comprises the different organs in human beings such as the stomach, oesophagus, pancreas, liver, intestines, etc. This example clearly demonstrates that the functions of the organs in the organ system are towards one specific goal that is digestion. 

Another interesting example of an organ system is the nervous and endocrine system. Both of them are operated by a shared organ - the hypothalamus. Both the systems combined together is known as the neuroendocrine system. A similar case is for the musculoskeletal system because of the relationship between the muscular system and the skeletal system. 

In plants, the common organ system is the differentiation of shoot and root systems. All parts of the plant above the ground also include the functionally distinct leaf and flower organs as well which are classified as the shoot organ system.