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Tissues and Organs

You might know that your lungs are in charge of bringing oxygen into your body and carbon dioxide out. You might also know that your heart pumps Blood throughout your body. But have you ever thought about what these organs are made of? Have you ever wondered what type of tissues and organs can be found in other life forms?

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Tissues and Organs

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You might know that your lungs are in charge of bringing oxygen into your body and carbon dioxide out. You might also know that your heart pumps Blood throughout your body. But have you ever thought about what these organs are made of? Have you ever wondered what type of tissues and organs can be found in other life forms?

Here, we will discuss how Cells, tissues, organs, and Organ Systems work together in a hierarchical level of organization in a living organism. We will also identify and describe the different types of tissues and organs found in Animals and Plants. Finally, we will go through examples of living organisms that do not have organs or even well-defined tissues.

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.

The levels of organization of life represent the hierarchical relationship among the Biological Structures that make up a living organism.

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:

A tissue is a group of cells that share morphological features and perform similar functions.

An organ is a group of tissues that make up a functional unit that is specialized to execute specific functions within the body.

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.

Type of Epithelial Tissue

Cell shape

Location and function in organs and organ systems

Squamous

Flat and round, with an irregular cell outline and a nucleus at the center.

Simple squamous epithelia facilitate diffusion: Gas Exchange in lung alveoli and nutrient and waste exchange in Blood capillaries.

Stratified squamous epithelia in the skin, mouth, and vagina provide protection from outside abrasion and damage.

Cuboidal

Cube-shaped with a nucleus at the center.

Simple cuboidal epithelia are commonly found in glands where they prepare and secrete glandular substances and in walls of tubules and in the ducts of the Kidney and liver where they help filter the blood.

Columnar

Tall, narrow, with the nucleus closer to the base.

Simple columnar epithelia in the digestive tract absorb material from the lumen and help them enter the body through the circulatory and lymphatic systems.

Pseudostratified (exist in one later but the nuclei are arranged at different levels) columnar epithelia help mucus and trapped particles move out of the respiratory tract.

Transitional

Round and simple but can pile on top of each other, making them appear stratified.

Transitional epithelia in the bladder and ureter thin out and expand to hold urine.

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.

Type of Connective tissue

Cells

Fibers and matrix

Location and function in organs and organ systems

Loose (or areolar)

Fibroblasts

Macrophages

Lymphocytes

Neutrophils

Contains collagen, elastic, and reticular fibers.

The matrix fills the spaces between the tissue’s components.

Loose connective tissue in Blood vessels help to keep them in place. It is also found around and between many organs.

Fibrous (or dense)

Fibroblasts

Macrophages

Contains large amounts of collagen.

Fibrous connective tissues are irregularly arranged in the dermis layer of the skin, where stress can come from various directions.

Fibrous connective tissues are arranged regularly, where strands are parallel to each other, in tendons and ligaments.

Cartilage

Chondrocytes make up the fibers and matrix of the tissue.

Chondroblasts are immature chondrocytes.

Hyaline cartilage consists of a matrix with chondrocytes embedded on it. It contains few collagen and elastic fibers.

Elastic cartilage contains a large amount of elastic fibers.

Fibrocartilage has a large amount of collagen fibers.

Hyaline cartilage can be found at the outer portion of the human nose and at the end of long bones, reducing friction and cushioning movement.

Elastic cartilage can be found in the ears and larynx of most vertebrates.

Fibrocartilage makes up the intervertebral discs of vertebrates.

Bone

Osteoblasts make bones for growth and remodeling by depositing bone material into the matrix.

Osteocytes make up most of bone tissue and derive from osteoblasts.

Osteoclasts break down bone for remodeling and make calcium stored in tissues accessible.

Contains some collagen and elastic fibers, which provide flexibility, as well as some mineral salts like calcium, which give the tissue hardness.

Bone tissue makes up the internal skeleton of vertebrates, giving structure and attachment points for tendons.

Adipose (or fat tissue)

Adipocytes collect and store fat.

Does not contain fibroblasts and only has a few fibers.

Adipose tissues store fat as a source of energy, insulation, and cushioning against damage to body organs.

Blood

Red blood cells

White blood cells

Although blood does not contain fibers, it is considered connective tissue because it has a matrix, a fluid called plasma.

Blood is found all over the body and its primary function is to transport materials such as oxygen, nutrients, and waste.

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.

Type of Muscle

Strations

Control

Location and function in organs and organ systems

Skeletal

Striated

Voluntary

Skeletal muscles are found in muscles that move bones.

Smooth

Nonstriated

Involuntary

Smooth muscles are a major component of the walls of Blood vessels as well as the tubes of the digestive and reproductive system.

Cardiac

Striated

Involuntary

Cardiac muscles can be found only in The Heart.

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.

Tissues and Organs - Key takeaways

  • The levels of organization of life represent the hierarchical relationship among the biological structures that make up a living organism.
  • A tissue is a group of cells that share morphological features and perform similar functions. Together, tissues make up an organ, a functional unit that is specialized to execute specific functions within the body.
  • There are four major types of tissues found in multicellular animals: epithelial, connective, muscle, and nervous.
  • Three major tissues make up roots, stems, and leaves in vascular plants: dermal, vascular, and ground.
  • It is important to note that some organisms lack organs or even well-defined tissues.

References

  1. Zedalis, Julianne, et al. Advanced Placement Biology for AP Courses Textbook. Texas Education Agency.
  2. Reece, Jane B., et al. Campbell Biology. Eleventh ed., Pearson Higher Education, 2016.
  3. “Definition of Lymphocytes - NCI Dictionary of Cancer Terms.” National Cancer Institute, www.cancer.gov/publications/dictionaries/cancer-terms/def/lymphocyte. Accessed 14 Sept. 2022.
  4. “Definition of Macrophage - NCI Dictionary of Cancer Terms.” National Cancer Institute, www.Cancer.gov/publications/dictionaries/cancer-terms/def/macrophage. Accessed 14 Sept. 2022.
  5. “Organ.” Genome.Gov, 12 Sept. 2022, www.genome.gov/genetics-glossary/Organ.
  6. “Definition of Neutrophil - NCI Dictionary of Cancer Terms.” National Cancer Institute, www.cancer.gov/publications/dictionaries/cancer-terms/def/neutrophil. Accessed 14 Sept. 2022.
  7. “Definition of Leukocyte - NCI Dictionary of Cancer Terms.” National Cancer Institute, www.cancer.gov/publications/dictionaries/cancer-terms/def/leukocyte. Accessed 14 Sept. 2022.
  8. “Nervous Tissue | SEER Training.” Nervous Tissue | SEER Training, training.seer.cancer.gov/anatomy/cells_tissues_membranes/tissues/nervous.html. Accessed 14 Sept. 2022.
  9. OpenStaxCollege. “Types of Tissues – Anatomy and Physiology.” Types of Tissues – Anatomy & Physiology, pressbooks-dev.oer.hawaii.edu/anatomyandphysiology/chapter/types-of-tissues. Accessed 14 Sept. 2022.

Frequently Asked Questions about Tissues and Organs

Tissue can exist without an organ system. Some organisms such as sponges and corals have tissues (although not well-defined) but lack organs and organ systems.

Similar cells make up tissues, and different tissues make up organs. The organ performs specific functions on its own or in coordination with other organs to ensure that the needs of the organism are met.

Different types of tissues make up organs. For example, the digestive tract is mainly composed of smooth muscle tissues, but it is also lined with epithelial tissues.

Yes, multiple tissues combine to form functional organs. 

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

An organ is a group of tissues that make up a functional unit that is specialized to execute specific functions within the body.

Test your knowledge with multiple choice flashcards

A/n ____ is a group of cells that share morphological features and perform similar functions. 

______ 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. 

_____ tissues bind the cells and organs of the body together and protect, support, and integrate all parts of the body. 

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