What are Tissues?

The smallest, basic, and structural component of the body is the cell. It is the functional unit of life. No organism can exist on this Earth without a cell. An organism's body, whether unicellular or multicellular, is made of cells. In unicellular organisms such as amoeba and protozoa, all the body functions are performed by a single cell. The multicellular organisms have collective cells which perform a specific function in the body. Tissues are a group of cells that work together to perform a particular function in the body. They make a series of networks or a system to coordinate with the different tissues in the body, forming a tissue system. The word tissue is derived from the Latin word 'weave.' Plants and animals have various kinds of tissue systems that differ in their work and composition according to the different requirements.

Types of Tissues

The tissues make the cellular organization between cells and organs. Their study is called histology. The layers of organs can be stained and observed under a microscope that enhances the visualization of tissues.

Animal Tissues

Animals have four types of tissues: connective, epithelial, nervous, and muscle. 

1. Connective Tissue

The connective tissue is made of an extracellular matrix, just like blood has plasma. Its function is to hold the organs in place. Examples of connective tissue are ligament, tendon, bone, adipose, and areolar. Ligaments do the bone-to-bone attachment. Their rigidity helps maintain the body structure. The bones store minerals and need calcium to maintain their structure.

Further classification of the connective tissue includes fibrous, skeletal, and fluid. The cells found in the connective tissue are mast cells, white blood cells, macrophages, adipocytes. Cartilage makes the skeletal or supportive connective tissue. Lymph and blood make the fluid connective tissue. Lack of vitamin C in the body causes scurvy as it is used to make collagen, a major part of the connective tissue.

2. Epithelial tissue

The epithelial tissue covers the skin surface of various organs. They have semi-permeable tight junctions that provide a barrier between the organ and the external environment. The epithelial tissues are specialized to perform excretion, absorption, and secretion. Certain epithelial tissues are simple squamous, simple cuboidal, ciliated, stratified keratinized, stratified non-keratinized. The epithelial tissues form a layer of sheets of cells on placing them alongside and sometimes at the top of each other. During wound healing, the epithelial cells migrate at the site of injury and accumulate to form tissues.  The epithelial cells have a diameter of 10-20 microns.

3. Nervous tissue

The nervous tissue is a part of the central nervous system (CNS) and peripheral nervous system (PNS). The brain and the spinal cord forms the CNS. The cranial and spinal nerves form the peripheral nervous system and transmit information to and from the brain. The nervous tissue function is to maintain the signaling of the transmitters and information from the brain to the body's reflexes. It has two components: neuron and neuroglia. The neurons are nerve cells that conduct nerve impulses. The different types of neuroglial cells in the nervous system are oligodendrocytes, microglial cells, ependymal cells, and astrocytes. These cells provide support to the neurons.

4. Muscle tissue

The muscle forms the contractile tissue of the body. Its function is to produce motion and force. It is categorized into skeletal, cardiac, and smooth tissues. The cardiac muscle is found in the heart. The contraction of this muscle helps pump blood throughout the body. The smooth muscle is found in the inner linings of visceral organs. The skeletal tissue is attached to the bone (providing structure, stability, and rigidity) and generates movement.

Plant Tissues

Plants have three types of tissues: epidermis, vascular, and ground.

1. Epidermis

It is the outer layer of tissue present on the leaves and young plants, making their structure. The epidermis is like the epithelial tissue in the human body, which lines the outer surface of the cells to provide a barrier. The epidermis protects the leaves, flowers, fruits from mechanical injury. It also protects against sudden water loss and maintains gas exchange. In the roots, the epidermis helps absorb water and nutrients. The epidermis is a tightly packed system that does not allow any material from the external environment to enter the cytoplasm.

2. Vascular

The vascular tissue makes the plant's vascular system through xylem and phloem that help transport fluids and food throughout the plant. These tissues form the conducting vascular system in the plants. The xylem transports water and nutrients from roots to all the parts of the plant. The phloem transports food from the leaves to the rest of the plant body. The xylem also provides structural support to the stem.

3. Ground

The ground tissue helps make the nutrients by photosynthesis and store them in the leaves. These tissues are less differentiated. They make the bulk of the plant's primary body. The ground tissue directly plays a role in photosynthesis. It has various cells whose functions are based on their type and location. These cells include the parenchyma cells, collenchyma cells, and sclerenchyma cells. They form permanent tissue that does not divide.

The plants have two other types of tissues: meristematic and permanent.

1. Meristematic

These tissues have actively dividing cells that increase the thickness and length of the plant. These tissues are present at the extremities of stems and roots. Certain meristematic cells are apical meristem, lateral meristem, and intercalary meristem.

2. Permanent

These tissues have a specialized function and cannot divide further. They are of three types: parenchyma, sclerenchyma, and collenchyma.

1. Parenchyma is an unspecialized living cell. It is present in between the interstitial spaces of cells. It makes the bulk of leaves, inner stem, and roots. In addition, it aids in photosynthesis, gas exchange, sap secretion, and food storage.
2. Collenchyma is a living cell that provides strength, structure, and support to the plant. This cell has a thick cellulose deposit in the cell wall, thus appear polygonal in appearance. This thickened cell wall, along with the longitudinal interlocking of the cells, provides strength to the plant.
3. Sclerenchyma is composed of thick and dead cells. It provides mechanical support and structure through fibers and sclereids. It has a high amount of lignin and 80% of cellulose content that makes the cell wall. The sclerenchyma provides extra structural support to the plant. It can exist as aggregates like sclereids and is found in the cortex of stems and leaves. It has two types of walls: primary and secondary.

Common Mistakes

• Histology includes microanatomy and biopsy. These two terms are not different.
• Cells are undifferentiated, but a cluster of differentiated cells perform a specific task.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelor of Science in Biology
• Master of Science in Zoology
• Bachelor of Medicine and Bachelor of Surgery
• Master of Science in Biotechnology

Related Concepts

1. Digestive system
2. Excretory system
3. Histopathology