Structural Organisation in Animals Animal Tissues
A tissue is a group of similar cells specialised for performing of a common function. The term ‘Tissue’ was introduced by Bichat.
The branch of biological science which deals with the study of tissue is called Histology. Term ‘Histology’ was coined by Mayer (1819). Marcello Malpighi is known as founder of Histology.
Animal tissues are classified as epithelial (i.e. covers body surfaces, lines body cavities and forms glands), connective (i.e. protects and supports the body and its organs and binds organs together), muscular (i.e. responsible for movements) and nervous (i.e. initiates and transmits nerve impulses that coordinate body activities) tissues.
- Epithelial Tissue
It covers the outer surface of all body organs and also lines the cavities of all hollow organs of body. Cells are compactly arranged and are held together by intercellular junctional complexes. Cells of the lower most layer rest on basement membrane. Depending upon the number of layers of cells, epithelial tissues are of two types, i.e. simple or unstratified and compound or stratified or multilaminar epithelial.
Some columnar or cuboidal cells get specialised for secretion are called glandular epithelium. They may be unicellular (e.g. goblet cells of alimentary canal) and multicellular
(e.g. salivary gland).
Based on the mode of pouring of their secretions, glands may be Exocrine (i.e. sefcretion releases through ducts or tubes) or endocrine (i.e. hormones secreted directly into blood).
In animal tissues, specialised junctions provide both structural and functional links between its individual cells.
Three types of junctions found in epithelium, which are given below:
- Tight junctions which help to stop substances from leaking across a tissue.
- Adhering junctions perform cementing to keep neighbouring cells together.
- Gap junctions facilitate the cells to communicate with each other by connecting the cytoplasm of adjoining cells, for rapid transfer of ions, small molecules and sometimes big molecules.
- Connective Tissue
Connective tissue is the most abundant and widely distributed tissue of the body. It is mesodermal in origin. It binds together the various tissues of an organ to support different parts of the body and involve in packaging around different organs.
Hertwig (1883) coined the word mesenchyma for the mesodermal tissue present between ectoderm and endoderm. Connective tissue constitutes about 30% of the body mass. It has both cellular and’extracellular components. Connective tissue plays important role in body defence, tissue repair, fat storage, transport, of materials, support and insulation, etc.
These are properties of some important connective tissues are described below:
- Tendons and ligaments are dense connective tissues. Tendons attach skeletal muscles to bones, whereas ligaments attach one bone to another bone.
- Bones have a hard and non-pliable ground substance rich in calcium salts and collagen fibres, which give bone its strength. The bone cells (osteocytes) present in the space called lacunae. The bone marrow in some bones is the site of production of blood cells.
- Cartilage is a specialised connective tissue consists of cells called chondrocytes.