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- All higher multicellular animals, called eumetazoans, exhibit a tissue and higher levels of organization
- There are four primary tissues in animals epithelial, muscular, connective and nervous
- Epithelial tissue (epithelium) is the simplest. It is avascular and develops from all the three primary germ layers.
- Epithelial cells are almost always compactly arranged and have abundant cytoplasm with prominent nucleus.
- Epithelium can be distinguished into simple, stratified and pseudostratified
- Simple epithelium consists of a single layer of cells on a basement membrane. It is further distinguished into squamous, cuboidal, columnar, ciliated and glandular
- Stratified epithelium has more than one layer of cells on a basement membrane. It can be further differentiated into stratified squamous, stratified cuboidal, stratified columnar and transitional epithelium
- In the skin of vertebrates stratified squamous epithelium undergoes a process called keratinization.
- Pseudostratified epithelium has a single layer of cells on a basement membrane giving a false appearance of many layers
- Muscular tissue (muscle) is responsible for movements in the body
- Muscular tissue is formed by muscle fibres in which the cytoplasm is specialized (sarcoplasm) by enclosing contractile units called myofibrils
- Muscular tissue is distinguished into smooth muscle, striated muscle and cardiac muscle
- Smooth muscle has spindle shaped uninucleate muscle fibres without a sarcolemma
- Smooth muscle is involuntary in nature. It does not easily experience fatigue
- Smooth muscle occurs in almost all the hollow visceral organs of the body
- Striated muscle is characterised by elongated cylindrical, multinucleate muscle fibres with a distinct sarcolemma
- Striated muscle is voluntary in nature. It experiences fatigue very easily
- Striated muscle is usually formed attached to bores
- Cardiac muscle is characterised by long, cylindrical, branched multinucleate muscle fibres with an indistinct sarcolemma
- Cardiac muscle is involuntary in nature. It never experiences fatigue
- Cardiac muscle occurs exclusively in the heart
- Connective tissue is the most abundant tissue in the animals body. It is characterised by the presence of a large amount of matrix, few cells and supporting fibres
- Connective tissue can be distinguished into three types connective tissue proper, supporting tissue and fluid connective tissue
- Connective tissue proper is typical. It includes Areolar tissue, fibrous tissue, elastic tissue and adipose tissue
- Areolar tissue is most abundant. It has a matrix containing white fibres, yellow fibres and different types of cells
- Supporting tissue includes cartilage and bone
- Cartilage has a matrix (chondrin) rich in organic substances. It encloses chondriocytes inside spaces called lacunae
- Cartilage forms the embryonic endoskeleton in most vertebrates. In sharks and rays it forms the adult endoskeleton
- Bone has a matrix (ossein) rich in calcium phosphate. In mammals it is deposited in the form of concentric rings called lamellae, together forming a Haversian system
- Fluid connective tissue includes blood and lymph
- Blood is the chief circulating fluid in the body. It consists of a plasma enclosing three types of cells RBC (erythrocytes) WBC (leukocytes) and platelets (thrombocytes)
- Lymph differs from blood in the absence of RBC
- Nervous tissue is the most highly specialized animal tissue
- The functional units of nervous tissue are called neurons
- A typical neuron is multipolar. It has a cell body (cyton) and a long fibre (axon)
- The axon has a noncellular covering of myelin sheath and a cellular covering called neurilemma
- The axon ends in branched projections called telodendrons.