Living organisms move, feed, respire (burn food to make energy), grow, sense their environment, excrete and reproduce. These seven characteristics are sometimes summarized by the words “MRS GREN”. functions of:
Living organisms are made from cells which are organised into tissues and these are themselves combined to form organs and systems.
Skin cells, muscle cells, skeleton cells and nerve cells, for example. These different types of cells are not just scattered around randomly but similar cells that perform the same function are arranged in groups. These collections of similar cells are known as tissues.
There are four main types of tissues in animals. These are:
- Epithelial tissues that form linings, coverings and glands,
- Connective tissues for transport and support
- Muscle tissues for movement and
- Nervous tissues for carrying messages.
Epithelium (plural epithelia) is tissue that covers and lines. It covers an organ or lines a tube or space in the body. There are several different types of epithelium, distinguished by the different shapes of the cells and whether they consist of only a single layer of cells or several layers of cells.
Squamous epithelium consists of a single layer of flattened cells that are shaped rather like ‘crazy paving’. It is found lining the heart, blood vessels, lung alveoli and body cavities (see diagram 4.1). Its thinness allows molecules to diffuse across readily.
Diagram 4.2: Cuboidal epithelium
Cuboidal epithelium consists of a single layer of cube shaped cells. It is rare in the body but is found lining kidney tubules (see diagram 4.2). Molecules pass across it by diffusion, osmosis and active transport.
Diagram 4.3: Columnar epithelium
Columnar epithelium consists of column shaped cells. It is found lining the gut from the stomach to the anus (see diagram 4.3). Digested food products move across it into the blood stream.
Diagram 4.4: Columnar epithelium with cilia
Transitional epithelium - with a variable number of layers
The cells in transitional epithelium can move over one another allowing it to stretch. It is found in the wall of the bladder (see diagram 4.5).
Stratified epithelia - with several layers of cells
Diagram 4.6: Stratified squamous epithelium
Epithelia with several layers of cells are found where toughness and resistance to abrasion are needed.
Stratified squamous epithelium
Stratified squamous epithelium has many layers of flattened cells. It is found lining the mouth, cervix and vagina. Cells at the base divide and push up the cells above them and cells at the top are worn or pushed off the surface (see diagram 4.6). This type of epithelium protects underlying layers and repairs itself rapidly if damaged.
Keratinised stratified squamous epithelium
Keratinised stratified squamous epithelium has a tough waterproof protein called keratin deposited in the cells. It forms the skin found covering the outer surface of mammals. (Skin will be described in more detail in Chapter 5).
Blood, bone, tendons, cartilage, fibrous connective tissue and fat (adipose) tissue are all classed as connective tissues. They are tissues that are used for supporting the body or transporting substances around the body. They also consist of three parts: they all have cells suspended in a ground substance or matrix and most have fibres running through it.
Blood consists of a matrix - plasma, with several types of cells and cell fragments suspended in it. The fibres are only evident in blood that has clotted. Blood will be described in detail in chapter 8.
Lymph is similar in composition to blood plasma with various types of white blood cell floating in it. It flows in lymphatic vessels.
Connective tissue ‘proper’
Diagram 4.7: Loose connective tissue
Connective tissue 'proper' consists of a jelly-like matrix with a dense network of collagen and elastic fibres and various cells embedded in it. There are various different forms of ‘proper’ connective tissue (see 1, 2 and 3 below).
Loose connective tissue
Loose connective tissue is a sticky whitish substance that fills the spaces between organs. It is found in the dermis of the skin (see diagram 4.7).
Dense connective tissue
Dense connective tissue contains lots of thick fibres and is very strong. It forms tendons, ligaments and heart valves and covers bones and organs like the kidney and liver.
Adipose tissue consists of cells filled with fat. It forms the fatty layer under the dermis of the skin, around the kidneys and heart and the yellow marrow of the bones.
Diagram 4.8: Cartilage
Cartilage is the ‘gristle’ of the meat. It consists of a tough jelly-like matrix with cells suspended in it. It may contain collagen and elastic fibres. It is a flexible but tough tissue and is found at the ends of bones, in the nose, ear and trachea and between the vertebrae (see diagram 4.8).
Bone consists of a solid matrix made of calcium salts that give it its hardness. Collagen fibres running through it give it its strength. Bone cells are found in spaces in the matrix. Two types of bone are found in the skeleton namely spongy and compact bone. They differ in the way the cells and matrix are arranged. (See Chapter 6 for more details of bone).
Muscle tissue is composed of cells that contract and move the body. There are three types of muscle tissue:
Diagram 4.9: Smooth muscle fibres
Smooth muscle consists of long and slender cells with a central nucleus (see diagram 4.9). It is found in the walls of blood vessels, airways to the lungs and the gut. It changes the size of the blood vessels and helps move food and fluid along. Contraction of smooth muscle fibres occurs without the conscious control of the animal.
Diagram 4.10: Skeletal muscle fibres
Skeletal muscle (sometimes called striated, striped or voluntary muscle) has striped fibres with alternating light and dark bands. It is attached to bones and is under the voluntary control of the animal (see diagram 4.10).
Diagram 4.11: Cardiac muscle fibres
Cardiac muscle is found only in the walls of the heart where it produces the ‘heart beat’. Cardiac muscle cells are branched cylinders with central nuclei and faint stripes (see diagram 4.11). Each fibre contracts automatically but the heart beat as a whole is controlled by the pacemaker and the involuntary autonomic nervous system.
Diagram 4.12: A motor neuron
Nervous tissue forms the nerves, spinal cord and brain. Nerve cells or neurons consist of a cell body and a long thread or axon that carries the nerve impulse. An insulating sheath of fatty material (myelin) usually surrounds the axon. Diagram 4.12 shows a typical motor neuron that sends messages to muscles to contract.