Animal Tissues & Frog Anatomy

Animal tissues are organized into four principal types: epithelial, connective, muscular, and neural. Each performs specialized functions that NEET frequently tests through identification-based questions.

Epithelial tissue forms covering and lining layers with tightly packed cells resting on a basement membrane. Simple epithelium has a single cell layer: squamous (flat, thin cells lining blood vessels and alveoli for diffusion), cuboidal (cube-shaped cells in kidney tubules for secretion and absorption), columnar (tall cells lining the stomach and intestine), ciliated (bearing cilia in the trachea and oviducts for particle and ovum movement), and glandular (secretory — unicellular goblet cells or multicellular salivary glands). Compound epithelium is multi-layered and primarily protective, found in skin, pharynx, and buccal cavity, with limited secretory or absorptive function. All epithelial tissue is avascular and depends on underlying connective tissue for nourishment.

Connective tissue, the most abundant tissue type, is characterized by an extensive extracellular matrix. Loose connective tissue includes areolar tissue (found beneath the skin, containing fibroblasts, mast cells, and macrophages) and adipose tissue (stores fat, provides insulation). Dense connective tissue occurs as regular (tendons connecting muscle to bone with parallel collagen fibres, and ligaments connecting bone to bone with slightly elastic fibres) and irregular (skin dermis with randomly oriented collagen). Specialized connective tissues include cartilage (semi-rigid, chondrocytes in lacunae, chondroitin sulfate matrix — hyaline in trachea, elastic in ear pinna, fibrocartilage in intervertebral discs), bone (hard matrix of calcium phosphate, osteocytes arranged in the Haversian system with concentric lamellae around a central canal), and blood (fluid connective tissue with plasma as the matrix, containing RBCs, WBCs, and platelets).

Muscular tissue exists in three forms. Skeletal muscle is striated, voluntary, multinucleated, cylindrical, unbranched, and fatigable — it attaches to bones for locomotion. Smooth muscle is non-striated, involuntary, uninucleated, spindle-shaped, found in the walls of hollow organs, and capable of slow sustained contraction. Cardiac muscle is striated yet involuntary, uninucleated or binucleated, branched, and features intercalated discs (gap junctions) that allow synchronized contraction. Cardiac muscle is autorhythmic and found exclusively in the heart wall.

Neural tissue comprises neurons (cell body/cyton, dendrites receiving impulses, axon transmitting impulses) and neuroglia (non-excitable support cells including Schwann cells for peripheral myelination, astrocytes, oligodendrocytes, and microglia).

Frog (Rana) anatomy is a classic NEET topic. The frog possesses a three-chambered heart (two atria, one ventricle), moist skin enabling cutaneous respiration alongside buccopharyngeal and pulmonary routes, mesonephric kidneys (ureotelic), ten pairs of cranial nerves, and undergoes external fertilization. Males are identified by vocal sacs and nuptial pads.

The key testable concept is the comparative properties of the three muscle types — particularly that cardiac muscle is striated but involuntary, with intercalated discs as its distinguishing feature.