What is meant by tissue?

In multicellular organisms, tissue is a level of structure that consists of a group of physically and functionally identical cells and their intercellular substances. Unicellular organisms do not have tissues. Tissues are absent or poorly differentiated even in the simplest multicellular animals, such as sponges. However, more evolved multicellular animals and plants have specialized tissues that may organize and govern an organism's reaction to its surroundings. 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 tissue of the body is divided into three groups, they are:

Continuously dividing tissue

Labile tissues also known as continuously dividing tissues are made up of cells that are constantly multiplying to replace dead or sloughed-off cells. Epithelial tissues such as the epidermis, gastrointestinal epithelium, and salivary gland tissue, and hematopoietic tissues are examples of such tissues.

Stable tissues

Some tissues also called stable tissues, are made up of cells that are generally non-dividing but may enter the cell cycle in response to certain stimuli, such as cell damage. Cardiac and skeletal muscle are examples of non-dividing tissues.

Permanent tissue

Permanent tissues are those that have reached full maturity and have lost their ability to divide. The permanent tissues are formed when the meristematic tissues divide and differentiate.

Tissue Regeneration

Regeneration defines as the proliferation of cells and tissues to replace lost structures. Regeneration is thus also a developmental process that involves growth, morphogenesis, and differentiation. The ability to regenerate lost part is present throughout the animal kingdom but to various extents. Regeneration is carried out by specialized cells called stem cells.

What is meant by stem cells?

Stem cells are the body's raw materials, they are the cells that give rise to all other cells with specialized functions. Differentiated cells constitute the majority of the body's cells. These cells can only fulfill a single function in a specific organ. Every human being begins as a single cell. A zygote, or fertilized egg, is the name for this cell. After then, the zygote splits into two cells, four cells, and so on. The cells eventually start to differentiate, taking on a specific function in a certain portion of the body. Differentiation is the term used for this process. Stem cells are cells that have not yet differentiated into other types of cells. They have the power to divide and reproduce themselves in an infinite number of ways. Other cells in the body have a limited number of times they can replicate before they start to break down. When a stem cell divides, it can either remain the same or become a differentiated cell like a muscle cell or a red blood cell.

Where do stem cells come from?

Different types of stem cells can be used for different purposes, they are:

Embryonic stem cells

Embryonic stem cells are derived from three to five-day-old human embryos. They're extracted during an in-vitro fertilization procedure. Instead of fertilizing an embryo inside the female body, this method entails fertilizing it in a laboratory. Pluripotent stem cells are derived from embryonic stem cells. These cells can give rise to almost any other cell type in the body.

Non-embryonic (adult) stem cells

Adult stem cells are also found in newborns and children; therefore, their name is misleading. These stem cells are derived from the body's developing organs and tissues. The body uses them to repair and replace damaged tissue in the same spot where they're discovered.

Induced pluripotent stem cells

Adult stem cells can now be converted into pluripotent stem cells, according to researchers. Induced pluripotent stem cells (iPSCs) are a novel type of cell. They can differentiate into a variety of specialized cells throughout the body. This implies that they may be able to generate new cells for any organ or tissue. Adult stem cells are genetically reprogrammed to act like embryonic stem cells to make induced pluripotent stem cells.

Potential uses of stem cells

  • Repair damaged organs or tissues with new cells grown in a lab.
  • Research causes of genetic defects in cells.
  • Test new drugs for safety and effectiveness.

Physiological regeneration

There is a constant loss of many kinds of cells due to wear and tear caused by day-to-day activities. The replacement of these cells is called physiological regeneration. An example of physiological regeneration is the replacement of red blood cells and replacement of epidermal cells of the skin.

Reparative regeneration

This is the replacement of lost parts or repair of damaged body organs. In this type of regeneration, the wound is repaired or closed by the expansion of the adjoining epidermis over the wound. Renewal of limbs in salamanders and healing of wounds is an example of this.

Types of regeneration based on the cellular mechanism

Based on the cellular mechanism renewal can be classified into two types, they are:


In this type, regeneration occurs mainly by the remodeling of existing tissues and the reestablishment of boundaries, thus involving very little new growth. As a result, the renewed individual is much smaller initially. It subsequently increases its size and becomes normal after feeding. This type of regeneration is known as morphallaxis or morphallactic regeneration. It Doesn’t include the formation of blastemal and there is no proliferation.


It is defined as the regeneration of some lost or damaged part. This type of regeneration occurs by the proliferation of new cells from the surface of the injured part.  In this type of regeneration, first, the adult structure at the lost or damaged part dedifferentiates, and then proliferate to increase the number of dedifferentiated cells and form blastema. This blastema grows and forms distal structures. Cells in the blastema redifferentiation from the rudiment of the lost part.

Historical process during regeneration

Wound healing

When an organ is injured, the internal tissues are exposed to the outer world. Some cells in the wounded area are killed, and there may be bleeding at the site of the injury. The blood clots rapidly and prevents the flow of blood to the outside. The epidermal cells then proliferate and migrate from all directions towards the wound's core. As a result, the epithelium covers the wounded tissue completely under the blood clot. The length of time it takes for a wound to heal is determined by the size of the wound, the size of the regenerating creatures, and the outside temperature.

Demolition and defense

The next event is the destruction and removal of the damaged tissue and foreign elements such as germs. These are removed by autolysis and phagocytosis by the blood cells. This is favored by the increased blood supply to this area.


It refers to the reversion of differentiated cells to the embryonic totipotent condition. Cells from the adjacent epidermis, muscles, nerves, cartilages, connective tissue, etc. undergo dedifferentiation. The intracellular matrix of bones and cartilages becomes dissolved and the osteoblasts and chondroblasts are set free They dedifferentiate into totipotent cells. Similar de differentiation occurs in muscle cells, connective tissue fibers, and nerve cells. All these cells revert to the embryonic totipotent state.

Blastema formation

The next step is the formation of blastema or regeneration bud. This is due to the accumulation of the dedifferentiated cells under the epithelial covering of the wound. As more and more cells aggregate below the epidermis, it becomes pushed out and a conical projection appears. This bud-like region consists of an outer cap of the epidermis and the central core of the dedifferentiated cells is called blastema or regeneration bud.


The blastema grows in size. This is caused by mitotic divisions of the blastema cells.


After attaining sufficient size, the blastema passes into the redifferentiation stage. The bone cells, cartilage cells, muscle cells, nerve cells, and connective tissue cells are again re differentiated. They give rise to the bones, muscles, nerves, and connective tissue of the organ which is to be regenerated. A blastema undergoes redifferentiation like a limb – bud in the embryo.

Factor influencing regeneration

The factors influencing regeneration are:

  • Growth factor
  • Epidermal growth factor
  • Fibroblast growth factor
  • Polarity in regeneration

Context and Applications

This topic is significant for the professional exam, especially for

  • Bachelor of Science in Biology
  • Bachelor of Science in Zoology
  • Bachelor of Science in Botany

Practice Problems

Question 1: What is a stem cell?

  1. A cell is only found in the stem of plants.
  2. As specialized cells that can only generate cells of the same type.
  3. An unspecialized cell with the ability to create specialized cells.

Answer: Option 3 is correct.

Explanation: Stem cells are separate human cells that can differentiate into a variety of cell types. From muscle cells to brain cells, this can happen. They can also repair damaged tissues in some circumstances. Stem cell-based therapies, according to researchers, could one day be used to cure major disorders including paralysis and Alzheimer's disease.

Question 2: What is the process of cell specialization called?

  1. Proliferation
  2. Cryopreservation
  3. Differentiation

Answer: Option 3 is correct.

Explanation: Stem-cell differentiation is the process of transforming a stem cell into a more specialized cell while losing some of the stem cell's developmental potential.

Question 3: Adult stem cells are described as______-

  1. somatic
  2. specialized
  3. syngeneic

Answer: Option 1 is correct.

Explanation: A stem cell is a type of cell that has the unique potential to differentiate into different cell types in the body.

Question 4:  What is the least invasive source of stem cells from the human body?

  1. Adipose tissue
  2. Bone marrow
  3. Cord blood

Answer: Option 1 is correct.

Explanation: The blood left in the umbilical cord after birth is called cord blood. It has stem cells in it.

Question 5: Where can scientists obtain stem cells?

  1. Only from an embryo
  2. Only from the brain
  3. Only from tissue in the body
  4. All of the above.

Answer: Option 3 is correct.

Explanation: Stem cells can also be obtained from a fetus, umbilical cord blood, or by ‘reprogramming' specialized body cells to act like embryonic stem cells, according to a recently developed technology.

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