Structural Organisation in Animals Class 11 Notes

Animal Tissues — Overview

Animal tissues are groups of cells with similar structure and function. They are classified into four types: epithelial, connective, muscular and nervous (neural). Unlike plants, animals do not have meristematic or permanent tissue categories. The levels of organisation in animals are: cell → tissue → organ → organ system → organism.

Epithelial Tissue

Epithelial tissue covers body surfaces, lines cavities and forms glands. Cells are closely packed with little intercellular matrix and rest on a basement membrane. Epithelia are classified by number of layers and cell shape.

Simple Epithelia (single layer)

• Simple squamous: single layer of flat cells — lines blood vessels (endothelium) and body cavities (mesothelium). Allows diffusion and filtration.
• Simple cuboidal: cube-shaped cells — lines kidney tubules, ducts of glands. Functions in secretion and absorption.
• Simple columnar: tall cells — lines stomach and intestine. May have microvilli (brush border for absorption) or cilia (e.g. fallopian tubes).
• Ciliated columnar: cilia on the free surface move mucus or ovum — e.g. trachea, fallopian tubes.
• Glandular columnar: cells modified for secretion — form glands (sweat, salivary, mucous glands).

Stratified Epithelia (multiple layers)

• Stratified squamous: many layers; outermost cells flat — skin (keratinised, dead outer layer) and lining of mouth and oesophagus (non-keratinised). Protective function.
• Stratified cuboidal and columnar: rare, found in ducts of large glands.
• Transitional epithelium (urothelium): cells change shape from cuboidal to squamous as organ stretches — lines urinary bladder.

Connective Tissue

Connective tissue is the most abundant and widely distributed tissue. It has cells separated by an abundant extracellular matrix (fibres + ground substance). The matrix determines the tissue type.

Loose Connective Tissue

• Areolar: cells (fibroblasts, macrophages, mast cells) loosely arranged in gel-like matrix with all three fibre types (collagen, elastic, reticular). Found under skin and between organs — packing material.
• Adipose: cells (adipocytes) with large fat droplets — found under skin and around kidneys. Functions: insulation, energy storage, shock absorption.

Dense Connective Tissue

• Dense regular: collagen fibres in parallel bundles — tendons (muscle to bone) and ligaments (bone to bone).
• Dense irregular: collagen in all directions — dermis of skin, organ capsules.

Specialised Connective Tissues

• Cartilage: cells (chondrocytes) in lacunae, firm matrix (chondroitin sulphate). Hyaline (most common — trachea, nose), elastic (external ear), fibrocartilage (intervertebral discs — strongest).
• Bone: hardest connective tissue; matrix mineralised with calcium phosphate. Osteocytes in lacunae, Haversian canals run through osteons. Functions: support, protection, mineral storage, blood cell production.
• Blood: fluid matrix (plasma) with cells — RBC (haemoglobin, gas transport), WBC (immunity), platelets (clotting). Blood is a connective tissue because it has cells in a non-living matrix.
• Lymph: similar to blood but without RBCs and platelets; less protein.

Muscular Tissue

Muscular tissue is made of elongated cells (muscle fibres) specialised for contraction using actin and myosin filaments. It produces movement, maintains posture and generates heat.

• Skeletal (striated, voluntary): long, cylindrical, multinucleate fibres with clear cross-striations. Attached to bones by tendons — under voluntary control. Functions: locomotion, breathing, facial expression.
• Smooth (visceral, involuntary): spindle-shaped, uninucleate fibres, no striations. Found in walls of gut, blood vessels, uterus — involuntary control. Functions: peristalsis, vasoconstriction.
• Cardiac: branched, striated, uninucleate (sometimes binucleate) fibres joined by intercalated discs (gap junctions for rapid electrical coupling). Found only in the heart — involuntary, autorhythmic. The heart beats ~72 times/min throughout life without fatigue.

Neural Tissue

Neural tissue consists of neurons (nerve cells) and neuroglial cells (support cells). A neuron has a cell body (soma) with Nissl bodies, a long axon, and many short dendrites. Axons may be myelinated (schwann cells form myelin sheath in PNS) for fast conduction. Neuroglia outnumber neurons and provide nutrition, insulation and protection.

• Sensory (afferent) neurons: carry impulses from receptors to CNS.
• Motor (efferent) neurons: carry impulses from CNS to effectors (muscles, glands).
• Interneurons (association): connect sensory and motor neurons within the CNS.
• Synapse: junction between two neurons where neurotransmitters cross the synaptic cleft.

The Frog — Morphology and Organ Systems

The frog (Rana tigrina) is a representative amphibian studied in this chapter. Body is streamlined, divided into head and trunk (no neck). Skin is moist, glandular, and used for cutaneous respiration. Hind limbs are adapted for jumping and swimming; forelimbs are smaller.

Organ Systems of the Frog

• Digestive: complete alimentary canal — mouth, buccal cavity, oesophagus, stomach, small intestine (duodenum, ileum), large intestine, cloaca. Liver and pancreas are digestive glands. Teeth (maxillary + vomerine) are for gripping, not chewing.
• Respiratory: both lungs (pulmonary) and skin (cutaneous). Buccal cavity lining also helps. No ribs — ventilation by buccal cavity pumping.
• Circulatory: double circulation. Heart is 3-chambered (2 atria + 1 ventricle) with a sinus venosus receiving deoxygenated blood. Mixing of blood occurs but is minimised by the truncus arteriosus.
• Excretory: kidneys (mesonephric), ureters, urinary bladder (not connected to kidneys in males). Nitrogenous waste: urea (uricotelic in tadpoles, ureotelic in adults).
• Nervous: CNS (brain + spinal cord), PNS (cranial + spinal nerves), ANS (sympathetic + parasympathetic). Eyes have nictitating membrane for underwater vision.
• Reproductive: unisexual. Males have vocal sacs for croaking and nuptial pads. Fertilisation is external (in water). Development: egg → tadpole → metamorphosis → adult frog.

Solved Examples

Example: A patient's blood smear shows an abnormally high count of white blood cells. What tissue is affected and what function might be altered?

Solution: Blood is a specialised connective tissue. White blood cells (leucocytes) are part of the immune system. An abnormally high WBC count (leukocytosis) suggests an active immune response — infection, inflammation, or in severe cases, leukaemia. The affected function is the body's defence against pathogens. The physician would examine the differential count (neutrophils, lymphocytes, eosinophils etc.) to pinpoint the cause.

Example: Why is the frog's heart 3-chambered rather than 4-chambered, and what is the consequence?

Solution: A 3-chambered heart (2 atria + 1 ventricle) allows some mixing of oxygenated and deoxygenated blood in the single ventricle. Frogs are amphibians — they supplement pulmonary breathing with cutaneous respiration, so the slightly less efficient double circulation is adequate. In contrast, 4-chambered hearts (birds, mammals, crocodiles) completely separate the two circuits for full separation of oxygenated and deoxygenated blood, supporting higher metabolic rates.