Structural Organisation in Animals

Epithelial Tissues

• An epithelium is a tissue composed of one or more layers of cells that cover the body surface and lines its various cavities.

• It serves for protection, secretion and excretion.

• The word ‘epithelium’ was introduced by Ruysch.

• Epithelial tissue evolved first in animal kingdom.

• It originates from all the three primary germ layers. e.g. Epidermis arises from ectoderm, Coelomic epithelium from the mesoderm and epithelial lining of alimentary canal from the endoderm.

Glands

• Multicellular exocrine glands are classified by structure, using the shape of their ducts and the complexity (branching) of their ducts system as distinguishing characteristics.

• Shape include tubular and alveolar (Sac like).

• Simple exocrine glands e.g. intestinal glands, mammalian sweat glands, cutaneous glands of frog etc. have only one duct leading to surface.

• Compound exocrine glands have two or more ducts e.g. liver, salivary glands etc.

Muscle Tissues

• Muscle cells are highly contractile (contracting to 1/3 or 1/2 the resting length).

• Muscle cells lose capacity to divide, multiply and regenerate to a great extent. Study of muscle is called myology.

• About 40% to 50% of our body mass is of muscles.

• The muscle cells are always elongated, slender and spindle-shaped, fibre-like cells, These are, therefore called muscle fibres.

• These possess large numbers of myofibrils formed of actin and myosin.

Connective Tissues

• It connects and supports all the other tissues, the intercellular element predominating.

• The cellular element is usually scanty. In function this tissue may be mechanical, nutritive and defensive.

• It is a tissue made up of matrix (abundant intercellular substance or ground substance) and living cells that connects and support different tissues.

• Connective tissue was called mesenchyme by Hertwig (1893).

Important Points

• Argentaffin cells which produce a precursor of serotonin, a potent vasoconstrictor hormone, occurs in intestinal cells.

• The brown adipose tissue in human is restricted till third month of post natal life.

• White fibres yield gelatin on boiling and are digestible with enzyme pepsin but yellow (elastic) fibres are not digestible by enzyme trypsin.

• The fat in the globules is stored in the form of triglycerides.

• The Cytoplasmic granules basophils contain histamine.

=> Sprain – Excessive pulling of ligaments.

=> Plasma cells - are also called as “Cart wheel cells”.

=> Collagen constitutes about 33% of total body protein.