Friday, March 27, 2009

Know the Animal Tissues

In animals, there are four basic types of tissues: epithelial or linings, connective or supporting, muscular, and nervous. An organ of the body may have all the four types of tissues. For example, the stomach, an organ of the digestive system has all the four types of tissues.

Epithelial Tissues:
The cells are arranged in single or multilayered sheets. They basically form the covering on the external and internal surfaces of the organs and body parts. Epithelial cells are not supplied with blood vessels. They protect the internal tissues from physical injury and infection. The free surface of the epithelial tissue may be of different types depending on its special function such as secretory, absorption, or excretory functions. Epithelial cells are basically classified according to their shapes.

There are three basic cell shapes in epithelial tissues: columnar, cubical, and squamous (scale-like).

The deep columnar cells often have a secretory function, and the nucleus is pushed to the bottom by the made and stored secretions near the surface from which they will exit (e.g., the cells lining the stomach, which secrete mucus). Cubical cells form the walls of small ducts as from salivary glands. Squamous cells are very flat, and the nucleus may form a bulge; they look something like a fried egg. The thinness permits diffusion of molecules across membranes (e.g., alveolar walls in lungs). Thick layers of cells (e.g., skin) prevent diffusion. In addition to the above three basic types there are some modified forms of these tissues. They are the following. Ciliated epithelium, the columnar cells with numerous cilia on their free surface, which lines the respiratory passage. Psudostratified epithelium, which forms a single layer of cells but on sectional view appears to be multilayered. The last one is the stratified epithelium, which are multilayered and form a very tough and impervious barrier.

The secretory or glandular cells may be present individually as in the case of goblet cells or in groups forming multicellular glands. An epithelial tissue having many goblet cells that secrete mucus is called mucus membrane. If the glandular cells or glands discharge their secretion on the surface of the cells or through a duct, they are called exocrine glands. But there are glands that discharge their secretion directly into the bloodstream and do not have any ducts. They are called the ductless glands.

Connective Tissues:
This includes the various types of supporting tissues in the body. Connective tissues are cells in a matrix. The matrix may be a fluid, semi-fluid, or a composite structure made up of secretory products of cells such as fibrous proteins. Blood is a connective tissue in which cells are embedded in a fluid matrix. In fibrous connective tissue cells are scattered among the collagen fibers (fibrous protein) they secrete. In bone and cartilage, cells are scattered throughout the hard or pliable matrix. In cartilage, the cells known as chondroblasts deposit in the matrix. The cell, along with the matrix, forms the chondrocytes. The cartilage is hard but flexible because the matrix is compressible and elastic. Bone is a calcified connective tissue. The cells are embedded in a hard matrix. The cells in the bone tissue are called osteoblasts, which are present in lacunae. Lacunae are present throughout the tissue. The main inorganic component of bone is hydroxyapatite.

Muscle Tissues:
These are made up of highly differentiated contractile cells or fibers held together by connective tissues. Muscle tissues are of three types. Striated muscle cells are large, multinucleate, and column-shaped cells; they are chiefly attached to the skeleton and are known as skeletal muscles or voluntary muscles. Voluntary muscles are under the control of the voluntary nervous system. They show powerful rapid contractions. They are attached to the bones in the trunk, limbs, and head. Smooth muscle cells are small and mononucleate; they are found in the walls of tubes such as blood vessels, glandular ducts, and the digestive system. They are also known as unstriated or involuntary muscles. The involuntary muscles are under the control of the autonomic nervous system and show sustained rhythmical contraction and relaxation movements. Cardiac muscle cells of the heart are small, striated, and branched. They are present only in the heart. They show rapid rhythmical contractions and relaxation movements with long refractory periods and do not show any fatigue.

Nervous Tissues:
Nervous tissues consist of nerve cells, the neurons and associated neuroglial cells. Neurons are capable of generating and transmitting electrical impulses. These cells also act as supporting connective tissue in the brain and spinal cord. The neurons transmit the stimuli from receptors such as skin to the effectors such as muscles and glands that then react to the stimuli.

Tags: Bio Technology, Bio Genetics , Animal Tissues

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