Animal tissues form the structural and functional foundation of all multicellular organisms. NEET Biology tests this topic through identification-based MCQs, frequently trapping students on fine distinctions between tissue types. There are four principal tissue categories: epithelial, connective, muscular, and neural.

Epithelial Tissue lines and covers body surfaces. All epithelial cells rest on a basement membrane, are tightly packed, and are avascular — they depend entirely on the underlying connective tissue for nourishment. Simple epithelium consists of a single cell layer and is subdivided by cell shape and specialization. Squamous epithelium has flat, scale-like cells optimized for passive diffusion and filtration; it lines blood capillaries and pulmonary alveoli. Cuboidal epithelium has cube-shaped cells performing secretion and absorption; it is found in kidney tubules and salivary ducts. Columnar epithelium has tall, pillar-like cells that line the stomach and intestinal wall for secretion and absorption. Ciliated columnar epithelium bears hair-like cilia on the apical surface; it lines the trachea (to trap and expel particles) and the oviducts (to propel ova toward the uterus). Glandular epithelium is specialized for secretion — unicellular goblet cells secrete mucus, while multicellular salivary glands secrete digestive enzymes. Compound (stratified) epithelium is multi-layered and primarily protective; it lines the skin, pharynx, and buccal cavity, with minimal secretory or absorptive capacity.

Connective Tissue is the most abundant tissue type in the body and is defined by an extensive extracellular matrix that separates the cells. Loose connective tissue includes areolar tissue (found under the skin and between organs; contains fibroblasts, mast cells, and macrophages embedded in a semi-fluid matrix of collagen and elastin fibres) and adipose tissue (adipocytes packed with fat droplets; insulates the body and cushions organs). Dense connective tissue is fibre-rich. Dense regular connective tissue has parallel collagen bundles and forms tendons (muscle-to-bone junctions) and ligaments (bone-to-bone junctions, with slightly elastic fibres). Dense irregular connective tissue has randomly oriented collagen fibres and forms the dermis of the skin. Specialized connective tissues include cartilage (semi-rigid; chondrocytes sit in fluid-filled spaces called lacunae within a matrix of chondroitin sulfate; three subtypes — hyaline in the trachea and embryonic skeleton, elastic in the ear pinna, fibrocartilage in intervertebral discs), bone (hard matrix of calcium phosphate and collagen; osteocytes occupy lacunae arranged in concentric lamellae around a central Haversian canal, forming the Haversian system), and blood (the only fluid connective tissue; plasma is the matrix; formed elements are RBCs for $O_{2}$ transport, WBCs for immunity, and platelets for clotting).

Muscular Tissue enables movement. Three types exist, each with distinct microscopic features. Skeletal (striated voluntary) muscle attaches to bones via tendons and produces locomotion. It is cylindrical, unbranched, and multinucleated (nuclei at periphery); striations arise from the regular arrangement of actin and myosin filaments; it contracts rapidly but fatigues. Smooth (non-striated involuntary) muscle lines the walls of hollow visceral organs — digestive tract, blood vessels, urinary bladder, and uterus. Cells are spindle-shaped with a central nucleus; contractions are slow and sustained. Cardiac muscle is unique: it is striated like skeletal muscle but involuntary like smooth muscle. Cells are branched, uni- or binucleated, and joined end-to-end by intercalated discs that contain gap junctions enabling the synchronized spread of electrical impulses across the entire myocardium. Cardiac muscle is autorhythmic — it generates its own contraction rhythm without external nervous input.

Neural Tissue is composed of neurons (the functional units) and neuroglia (support cells). Each neuron has a cell body (cyton), multiple branching dendrites that receive impulses, and a single axon that transmits impulses away from the cell body. Schwann cells in the peripheral nervous system produce the myelin sheath around axons. Central nervous system glial cells include astrocytes (structural support and blood–brain barrier maintenance), oligodendrocytes (CNS myelination), and microglia (immune surveillance).

Frog Anatomy (Rana tigrina) is a dedicated NEET subtopic. The frog has a three-chambered heart: two atria and one ventricle, plus a sinus venosus receiving deoxygenated blood and a truncus arteriosus distributing blood to the body. Respiration occurs through three routes simultaneously: the skin (cutaneous — requires the skin to remain moist), the buccal cavity (buccopharyngeal), and the lungs (pulmonary, which are simple sac-like structures). Excretion is ureotelic; mesonephric kidneys produce urea, which exits through ureters into the urinary bladder and then the cloaca. The nervous system includes a brain with olfactory lobes, cerebral hemispheres, diencephalon, optic lobes (midbrain), cerebellum, and medulla oblongata, and issues ten pairs of cranial nerves. The reproductive system involves external fertilization in water; males are distinguished by vocal sacs (for calling) and nuptial pads (for gripping the female during amplexus).

The most heavily tested NEET distinctions from this session are: (1) cardiac muscle is striated but involuntary with intercalated discs; (2) tendons connect muscle to bone while ligaments connect bone to bone; (3) blood is a connective tissue; (4) the frog has a three-chambered heart and is ureotelic.