The important groups under which the animal tissues are divided are given below:
Histology is the study of tissues. All multicellular organisms possess group of cells of similar structure and function assembled together to form tissues. A tissue can be defined as a group of physically linked cells that is specialized for a particular function. Animal tissues can be divided into 4 groups – 1. Epithelial 2. Connective 3. Muscle 4. Nervous.
Groups
1. Epithelial tissue:
These are arranged in single or multilayered sheets and cover the internal and external surfaces of the body of an organisms. Epithelial cells are not supplied with blood vessels; they rely on diffusion of oxygen and nutrients from lymph vessels which run through intercellular spaces.
Classification of Epithelial Tissues:
Squamous epithelium:
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Cells are thin and flattered. Occurs in renal capsules of the kidney, alveoli of lungs, blood capillary walls.
Cuboidal epithelium:
Cells are cube shaped and possess a central spherical nucleus. Form lining of many ducts and line the proximal and distal convoluted tubules and collecting ducts of kidney.
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Columnar epithelium:
Cells are tall and quite narrow and the nucleus is situated at its basal end. It lines the stomach.
Ciliated epithelium:
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Cells bear numerous cilia at their free surfaces. They are always associated with mucus secreting goblet cells. It lines the insides of the oviducts, ventricles of brain, spinal carb and respiratory passages.
Pseuedostratified epithelium:
In a section, it appears to be in layers. But it is due to the different levels of cells and it found lining the urinary tract and the respiratory passages.
Stratified epithelium:
The tissue is made up of a number of layers of cells. So it forms a tough, impervious layer. They are abundant on external skin surfaces, lining the buccal cavity and vagina.
Transitional epithelium:
Consists of 3-4 layers of cells all of similar size and shape except at the free surface where they are flattered. Useful in structures which undergo stretching like bladder, ureter, and pelvic region of kidney.
2. Connective Tissue:
It is the supportive tissue of the body. It includes the skeletal tissue, bone and cartilage, and in addition in binds other tissues together. It can be divided into the following types.
Bone:
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It is the most abundant of all skeletal material providing support, protection and some metabolic functions. Cells are embedded in a firm, calcified matrix. About 30% of the matrix is composed of organic material, consisting of collagen fibres and glycoproteins. Whilst 70% is in organic bone salts. (Hydroxy apatite, a form of calcium phosphate).
Bone cells are called osteoblasts. Haversion canal is cylindrical structure of compact bone. Canaliculi are fine channels linking up with Haversion canal. Covering the bone is a layer of dense connective tissue called periosteum. Spongy bones (occurring in embryo, swollen end of long bones) consist of a mash work of thin, interconnecting bony streets called ‘ Trebeculae’.
Cartilage:
It is a connective tissue consisting of cells embedded in a matrix of chindrin. Matrix is deposeted by cells called chondroblasts. Cartilage is a hard but flexible tissue. The matrix is compressible and elastic an is able to absorb mechanical shocks.
Hyaline cartilage (glassy) present in air passages of respiratory system. Yellow elastic cartilage is semiopaque and is present in external ear and the epiglottis. White fibrous cartilage is present in discs between adjacent vertebrae.
3. Muscle Tissue:
It makes up 40% of a mammals body weight. 3 types of muscles are present namely striated, smooth and cardiac.
Striated muscle (Skeletal, striped or voluntary muscle):
Muscle which is attached to bone. It is concerned with locomotion, contracts quickly, and fatigues quickly. It receives nerves from voluntary nervous system.
Smooth muscle (unstriated, unstriped, involuntary):
Muscle which is found in the walls of organs of the body such as the gut and bladder and is concerned with movement of materials through them. It contracts slowly and fatigues slowly. It receives nerves from autonomous nervous system.
Cardiac muscle:
It receives nerves from the autonomous nervous system.
4. Nervous Tissue:
It contains densely packed nerve cells called neurons, which are specialized for conduction of nerve impulses. Neurons are the functional units of the nervous system.
They are capable of transmitting electrical impulses, providing means of communication between cells or transmitting electrical impulses, providing means of communication between cells or organs. Each neuron possesses a cell body and a variable number of nerve fibres extending from it.
Nissle’s granules are group of ribosome’s and rough endoplasmic reticule present in the cell body. Nerve fibres which conduct impulses away from the cell body are termed axons.
The end of an axon break up into many fine branches with swollen endings called ‘synoptic knobs’. Nerve fibres are completely surrounded and insulated by a fatty myelin sheath. This is formed by another type of cell called ‘Schwann cell’.