Revision Notes: Structural Organization in Animals | NCERT Exemplar & Revision Notes for NEET PDF Download

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.
• Types of Epithelium
  

Glands

• Multicellular exocrine glands are classified by structure, using the shape of their ducts and the complexity (branching) of their duct 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.
• Structural classification of exocrine glands:

Important Tips

• Study of tissue outside the body in a glass tube is known as in vitro, while study of living tissue in situ is known as in vivo.
• Among epithelium, simple epithelium were first to evolve.
• Transitional epithelium also called plastic epithelium or urothelium. It lacks basement membrane
• False epithelium derived from mesenchymal a diffuse network of tissue derived from embryonic mesoderm) and lining the synovial cavities.
• Mammary glands without teats are present in prototheria.
• A malignant tumors arising from an epithelium is called a carcinoma. If it arises from a squamous epithelium it is a squamous cell carcinoma and if it arises from glandular epithelium it is called an adenoma.
• The epithelial lining of brain ventricles and central canal of spinal cord is known as ependyma.
• Stereocilia are elongated membrane outgrowths found in certain parts of male reproductive tract.

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.

Difference between three types muscle fibres

Connective Tissues

• It connects and supports all the other tissues, the intercellular elements 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).
• Types of connective tissues

(1) On the basis of their texture:
The bones are divided into two categories spongy or cancellous or tubecular bones and compact or periosteal bones

(2) On the basis of origin of bone:
Ossification or osteogenesis is the process of bone formation. A bone is classified into four categories.

(3) On the basis of treatment:
These are of two types :-

(6) Number of RBC: The number of RBCs is counted by instrument haemocytometer. The total number of RBC per cubic mm of blood is called RBC count. RBC count is slightly lower in women than a man and number of RBC is more in people who live on mountains because there is less oxygen. RBC are absent in cockroach.

(7) Life span of RBC: The life span of red blood corpuscles circulating in the blood stream varies in different animals. RBCs have longest life span in blood. The mammalians RBC have short life span due to absence of nucleus, which is disappeared during development.

(8) Function of RBCs: The major function of erythrocytes is to receive O₂ of respiratory surfaces and then transport and readily deliver it to all cells of body. This important function is performed by haemoglobin which has a great ability to combine loosely and reversibly with O₂ and is, hence, called “respiratory pigment”. Haemoglobin, in annelids, is dissolved in the plasma because of absence of red blood corpuscles. In mollusc and some arthropods, etc., a different respiratory pigment, haemocyanin is found dissolved in the plasma. This pigment is bluish due to presence of copper in place of iron.

(9) Comparison Between Blood and Lymph

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.

Nervous Tissues

• A most complex tissue in the body, composed of densely packed interconnected nerve cells called neurons (as many as 10¹⁰ in the human brain).
• It specialized in communication between the various parts of the body and in integration of their activities.
• Nervous tissue is ectodermal (from neural plate) in origin.
• Difference between axon and dendron
  

Classification of nervous tissues