Introduction

The human body is structured into systems. The figure below illustrates the structural organization of the human body from the microscopic (cells) to the macroscopic level (organ system).

Structural Organization of the Human Body

Cell	Tissue	Organ	Organ System
Nerve cells	Nerve tissue	Brain	Nervous system

The cell is the smallest and most basic unit of life. Tissues are a group of cells that are similar in shape that work together to perform a specialized function. The human body is made up of four primary types of tissue: epithelial, connective, muscle, and nervous tissue.

However, before you can learn about the different types of tissues, you need to learn a little about stem cells. Stem cells can be considered the precursor to tissues. They have the remarkable potential to develop into many different cell types in the body during early life and growth. In addition, in many tissues they serve as an internal repair system, dividing essentially without limit to replenish other cells as long as the person or animal is alive. When a stem cell divides, each new cell has the potential either to remain a stem cell or become another type of cell with a more specialized function, such as a muscle cell, a red blood cell, or a brain cell. This is why embryonic stem cells generated through therapeutic cloning have been proposed as promising candidates for future therapies.

1- Epithelial tissues are a thin covering that protects organs, lines body cavities, and covers the surface of the body. Some examples of epithelial tissue are the outer layer of the skin, the inside of the mouth and stomach, and the tissue surrounding the body's organs.

This is a cross-section through the small intestine. The tall, vertical cells seen at the top of the tissue are columnar cells (one is shown - red arrow), which make up simple columnar epithelium (the cells at the bottom of the image are connective tissue, which will be discussed later). The black arrow is indicating a goblet cell. These cells show a triangular basal nucleus and a large amount of clear apical cytoplasm. The stain is clear due to the large amount of mucus within it that the cell secretes to protect the intestinal cells from digestion by the digestive enzymes (your body wants to digest the food, not itself!). Image courtesy of the University of Guelph, Development Online Biology.

This is a cross-section through the small intestine. The tall, vertical cells seen at the top of the tissue are columnar cells (one is shown - red arrow), which make up simple columnar epithelium (the cells at the bottom of the image are connective tissue, which will be discussed later). The black arrow is indicating a goblet cell. These cells show a triangular basal nucleus and a large amount of clear apical cytoplasm. The stain is clear due to the large amount of mucus within it that the cell secretes to protect the intestinal cells from digestion by the digestive enzymes (your body wants to digest the food, not itself!).

2- Connective tissue provides support and holds various parts of the body together. Some examples of connective tissue include the inner layers of skin, tendons, ligaments, cartilage, bone, and fat tissue. In addition to these more recognizable forms of connective tissue, blood is also considered a form of connective tissue.

Image courtesy of the University of Guelph, Development Online Biology.

Loose connective tissue is found just about everywhere in the body, as it provides support for structures passing through it, such as blood and lymph vessels, and nerves. As well, it also serves to bind together with other tissues, including organs and their components. In the slide preparation above, both collagen fibres (1 - a bundle of fibres circled) and elastic fibres (2 - each a single fibre) can be seen. The darker stained blobs are fibroblast nuclei.

3- Muscle tissues contain sheets or bundles of muscle cells that contract to produce movement, both involuntary like the cardiac muscles (which circulates the blood throughout your body) and voluntary muscles (like your leg and calf muscles which allow you to walk). Smooth muscle is found in the walls of the digestive tract, uterus, bladder, blood vessels, and other internal organs.

Image courtesy of the University of Guelph, Development Online Biology.

Smooth muscle features long, narrow "spindle" shaped cells with a single central, somewhat elongated, nucleus. These cells are arranged parallel to one another. This type of muscle is called "involuntary" because it acts to contract and relax without conscious thought (i.e., you can't will your intestines to contract, they just do - this is what causes that embarrassing grumbling halfway through class when you're hungry!).

4- Nervous tissue provides communication between all body structures. Nerve tissue has the ability to generate and conduct electrical signals in the body. These electrical messages are managed by nerve tissue in the brain and transmitted down the spinal cord to the body.

Image courtesy of the University of Guelph, Development Online Biology.

This slide is a smear from a mammalian spinal cord showing an isolated neuron (large arrow) and the nuclei of the surrounding neuroglial cells (small arrows). Note the numerous cytoplasmic extensions coming out from the neuronal cell body and the size of the neuron compared with the neuroglial cells.

Types of Tissue in Plants

A cross-section showing the three plant tissue types. Image courtesy of The Science Creative Quarterly.

Plants have only three types of tissue systems. These systems are called dermal tissue, ground tissue, and vascular tissue. However, before you can learn about the different types of tissues, you need to learn a little about meristem cells. Meristem cells are responsible for the increase in a number of cells; cell division occurs solely in meristematic regions usually at the tips of branches or roots. However, like the stem cells in animals, meristem cells are not specialized and therefore not considered tissue. However, the cells that are produced by the meristem become specialized tissues of the plant body mentioned below.

1- Dermal tissue system is the group of cells that covers leaves, flowers, roots, and stems. Dermal tissue is composed of epidermal cells, closely packed cells that act as a buffer between the environment and the internal plant tissues. They have a variety of functions including the absorption of water and minerals primarily in the root region; in the regions of the stem and leaves, these areas are generally covered by a waxy cuticle that aids in the prevention of water loss. The dermal tissue can form a barrier of resistance to bacteria and fungi. In woody plants, they provide the exterior bark or cork. On roots cells, they have hairs that help absorb water and minerals from the soil. This tissue is considered to be the skin of the plant.

Image courtesy of Progressive Gardens.

This image shows the dermal (epidermis) layer protecting the plant. Ground tissues are between the dermal and vascular tissues.

2- The ground tissue system comprises the bulk of the primary plant body. These tissues are made up of three cell types called parenchyma, collenchyma, and sclerenchyma cells. These tissues are the filler between the dermal and vascular tissues. They have a variety of functions; in green plants, they manufacture nutrients through photosynthesis, in the roots they store food, and in the stem, they provide structure and support.

3- The vascular tissue system transports food, water, hormones, and minerals within the plant. Vascular tissue includes the xylem and phloem. These two tissues transport fluid and nutrients internally. The xylem's main functions are to transport water and dissolved substances, support the physical structure of the plant, and act in food storage. The phloem's function is to transport dissolved organic materials throughout the plant. It is not surprising that their form should be similar to pipes.

ePortfolio Submission

This is a voice recording task. Provide verbal explanations to the following questions using a voice recording device. Add examples where needed. add your audio file to your portfolio.

Name the four general types of tissues in animals.
What are the three general types of tissues in plants?
What are the functions of the vascular tissues?