Tissues and Organs

What do we mean by level of organization of life? What does it say about the relationship between cells, tissues, organs, and organ systems?

Let's start by talking about the levels of organization of life.

Cells, which are the fundamental units that make up all living organisms, are at the base of this hierarchy.

What is the hierarchical relationship between cells, tissues, organs, and organ systems?

Figure 1 below illustrates the hierarchical levels of organization.

• Cells make up all living organisms.

• Tissues are formed when cells with similar or related features and functions join together.

• Organs are made up of different types of tissues that work together. They perform specific and specialized functions within the body.

• Organ Systems are made up of different organs that perform coordinated functions.

• An organism may have one or more organ systems.

Tissue and Organ Definition and Types

As implied in the previous section, the definition of tissue and organ are as follows:

In the following section, we will look into the different types of tissues and organs that can be found in Animals and Plants.

What are the types of tissues found in animals, and how do they function in organs?

There are four major types of tissues found in multicellular animals: epithelial, connective, muscle, and nervous. In the following section, we will describe each type of tissue and discuss how they function in organs and organ systems.

Epithelial tissues

Epithelial tissues are sheets of cells that cover the exterior surface of organs and structures in the body and line the internal cavities and passageways, and form specific glands.

Epithelial tissues can consist of a single layer (called simple epithelia) or multiple layers of cells (called stratified epithelia).

There are four major types of epithelial tissues which are illustrated in Figure 2 and summarized in Table 1 below.

Table 1. This table describes and cites examples of each major type of epithelial tissue.

Connective tissues

Connective tissues bind the cells and organs of the body together and protect, support, and integrate all parts of the body. These consist of extracellular matrix, living cells, and a nonliving material called ground substance. The ground substance is made up of an organic substance like protein and an inorganic substance like water.

Connective tissues are made up primarily of fibroblasts, cells that produce the fibers found in nearly all connective tissues. The types of fibers found in connective tissue are elastic, collagen, or reticular fibers.

• Elastic fibers can stretch up to 150% of its original length and return to its original size and shape. These fibers allow tissues to be flexible.

• Collagen fibers provide the tissue strength and keep it from tearing or separating from the surrounding tissues.

• Reticular fibers support the tissue and other organs to which it is attached.

Some of the tissues also contain macrophages, neutrophils, lymphocytes, and leukocytes. Macrophages and neutrophils are types of white blood cells that enclose and kill Microorganisms, remove dead cells, and promote the action of other Immune System cells. Lymphocytes and leukocytes are types of immune system cells that are produced in the bone marrow and are found in blood and lymph tissue.

Some connective tissues have specialized cells–such as chondrocytes in cartilage and osteocytes in bones–that are not found in other tissues.

The cells or fibers and the Extracellular Matrix in connective tissue have an inversely proportional relationship : the more cells or fibers in a connective tissue, the less dense its matrix, and vice versa.

The six major types of connective tissue are illustrated in Figure 3 and summarized in Table 2 below. Specialized cells are defined in the second column.

Table 2. This table describes the different types of connective tissue.

Muscle tissue

Muscle tissues respond to stimulation and contract to provide movement. There are three major types of muscle tissues: skeletal, smooth, and cardiac. These can be controlled either voluntarily or involuntarily. Some of these have striations or bands.

The types of muscle tissue are illustrated in Figure 4 and summarized in Table 3.

Table 3. This table summarizes the different types of muscle tissues.

Nervous tissues

Nervous tissues are made up of neurons and glial cells (Fig. 5).

Neurons (or nerve cells) make up most of nervous tissue, and these receive and transmit electrical signals, enabling communication between different parts of the body.

Glial cells do not transmit signals; rather, they support the activities of the neurons in various ways. Types of glial cells include:

• Astrocytes, which regulate the chemical environment of the neuron.

• Oligodendrocytes, which bind together and insulate the neurons so that the nerve impulse is transmitted more efficiently.

Some glial cells also provide protection against bacterial invasion, while others provide nutrients by attaching neurons to blood vessels.

Is there a difference between an animal and a plant’s tissue, organ, and organ system?

Like animals, most plants follow the hierarchical level of organization: its tissues make up different organs. Here, we will discuss the basic parts of Vascular Plants, plants with specialized tissues that transport water and nutrients.

Vascular Plants have two major organ systems (Fig. 6):

• The root system, which absorbs water and minerals from below the ground.

• The shoot system, which consists of the stems and leaves that absorb carbon dioxide and light from above the ground surface.

The roots, stems, and leaves are the basic organs of vascular plants.

Three major tissues make up these organs: dermal, vascular, and ground:

• Much like our skin, dermal tissue is a continuous layer of cells that serves as the plant’s protective covering.

• Vascular tissues transport substances throughout the plant and provide mechanical support. Vascular tissues also connect the root and shoot systems. There are two major types of vascular tissues:

  • Phloem, which transports sugars from the site of Photosynthesis to roots, leaves, and fruits, where they are needed or stored.

  • Xylem, which transports water and nutrients from the roots to the shoots.

• Ground tissues specialize in storage, Photosynthesis, support, short-distance transport, and regeneration.

What are some examples of living organisms that do not have tissues, organs, or organ systems?

While we have discussed the different Biological Structures that make up multicellular organisms, it is important to note that not all organisms have the same set of structures.

Sponges and corals are examples of animals that lack organs or even well-defined tissues. Mosses and liverworts, on the other hand, are examples of Nonvascular Plants, which do not have specialized vascular tissue (Xylem or Phloem), and as such also lack true leaves, stems, and roots. Multicellular protists like brown Algae also lack true tissues.