Animal Tissues
Epithelial Tissue
- Epithelial Tissue: Features a free surface facing either a body fluid or the outside environment, providing a covering or lining; cells are compactly packed with minimal intercellular matrix.
- Simple Squamous Epithelium: Consists of a single thin layer of flattened cells with irregular boundaries; located in blood vessel walls and lung alveoli, functioning as a diffusion boundary AIPMT 2010 NEET 2022.
- Simple Cuboidal Epithelium: Made of a single layer of cube-like cells; commonly found in ducts of glands and tubular parts of nephrons, functioning in secretion and absorption AIPMT 2010.
- Exception/Crucial detail: The epithelium of the proximal convoluted tubule (PCT) in the nephron specifically features microvilli to massively increase the surface area for absorption AIPMT 2011 NEET 2020.
- Simple Columnar Epithelium: Composed of a single layer of tall and slender cells with basally located nuclei; lines the stomach and intestine, aiding in secretion and absorption.
- Ciliated Epithelium: Cuboidal or columnar cells bearing cilia on their free surface; functions to move particles or mucus in a specific direction over the epithelium; found in the inner lining of hollow organs like bronchioles and fallopian tubes AIPMT 2008 AIPMT 2009 AIPMT 2011 NEET 2022.
- Glandular Epithelium: Specialized for secretion; categorized into unicellular (e.g., isolated goblet cells of the alimentary canal) NEET 2024 and multicellular (e.g., salivary glands). Exocrine glands have ducts, whereas endocrine glands are ductless and secrete hormones directly into the fluid.
- Compound Epithelium: Multi-layered (stratified) with a limited role in secretion and absorption; primary function is protection against chemical and mechanical stress; covers the dry surface of the skin, moist surface of the buccal cavity, pharynx, and inner lining of salivary and pancreatic ducts AIPMT 2012 NEET 2022.
Cell Junctions
| Junction Type | Core Function | PYQ Relevance |
|---|---|---|
| Tight Junctions | Help to stop substances from leaking across a tissue. | AIPMT 2009 NEET 2024 |
| Adhering Junctions | Perform cementing to keep neighboring cells together. | AIPMT 2009 |
| Gap Junctions | Facilitate communication by connecting cytoplasm for rapid transfer of ions and small molecules. | AIPMT 2009 AIPMT 2015 NEET 2021 |
Connective Tissue
- Connective Tissue: The most abundant and widely distributed tissue in complex animals; named for its special function of linking and supporting other tissues/organs.
- Exception/Crucial detail: In all connective tissues except blood, the cells secrete structural proteins called collagen or elastin, which provide strength, elasticity, and flexibility NEET 2018.
- Areolar Tissue: A loose connective tissue present beneath the skin; serves as a support framework for epithelium and contains fibroblasts, macrophages, and mast cells AIPMT 2012 NEET 2024.
- Adipose Tissue: A loose connective tissue located primarily beneath the skin; specialized to store fats AIPMT 2011.
- Dense Regular Connective Tissue: Collagen fibres are present in rows between many parallel bundles of fibres; includes tendons (attach skeletal muscles to bones) AIPMT 2011 and ligaments (attach one bone to another).
- Dense Irregular Connective Tissue: Contains fibroblasts and many randomly oriented fibres (mostly collagen); present in the skin.
- Cartilage: A specialized connective tissue with a solid, pliable intercellular material that resists compression; cells (chondrocytes) are enclosed in small cavities (lacunae) within the matrix; found in the tip of the nose, outer ear joints, and between vertebrae AIPMT 2009.
- Bone: Hard, non-pliable ground substance rich in calcium salts and collagen fibres; provides structural frame to the body; bone cells (osteocytes) are present in spaces called lacunae.
- Blood: Fluid connective tissue containing plasma, red blood cells (RBCs), white blood cells (WBCs), and platelets; functions as the main circulating fluid for transport.
Muscle Tissue
| Muscle Type | Core Features | Location | PYQ Relevance |
|---|---|---|---|
| Skeletal | Striated, voluntary, multinucleated, parallel bundles wrapped in a connective tissue sheath. | Attached to skeletal bones. | |
| Smooth | Non-striated, involuntary, fusiform (tapering at both ends), uninucleate. | Walls of internal organs (blood vessels, stomach, intestine). | AIPMT 2013 AIPMT 2014 NEET 2022 |
| Cardiac | Striated, involuntary, branched; features intercalated discs (communication junctions) allowing cells to contract as a unit. | Exclusively in the heart. | AIPMT 2011 NEET 2023 |
Neural Tissue
- Neural Tissue: Exerts the greatest control over the body's responsiveness to changing conditions; neurons are the excitable functional units.
- Neuroglia: Non-excitable cells that protect and support neurons; they make up more than one-half the volume of neural tissue in the human body NEET 2021.
Cockroach (Periplaneta americana)
Morphology and Exterior Features
- Class Insecta, Phylum Arthropoda; nocturnal omnivores measuring 34-53 mm in length.
- Exoskeleton: The entire body is covered by a hard, brown, chitinous exoskeleton. In each segment, hardened plates called sclerites (dorsal tergites and ventral sternites) are joined by a thin, flexible articular membrane known as the arthrodial membrane AIPMT 2015.
- Body Division: Segmented into three distinct regions: Head, Thorax, and Abdomen.
- Head: Triangular, formed by the fusion of 6 segments, and shows great mobility due to a flexible neck. Features a pair of compound eyes and thread-like antennae with sensory receptors. Mouthparts are of the biting and chewing type AIPMT 2006. They consist of a labrum (upper lip), paired mandibles, paired maxillae, labium (lower lip), and a median flexible lobe acting as a tongue (hypopharynx).
- Thorax: Divided into prothorax, mesothorax, and metathorax. Each segment bears a pair of walking legs.
- Wings:
- Tegmina (Forewings): Arise from the mesothorax. They are opaque, dark, leathery, and cover the hindwings when at rest AIPMT 2015 NEET 2022.
- Hindwings: Arise from the metathorax. They are transparent, membranous, and used in flight.
- Abdomen: Consists of 10 segments in both sexes.
- Exception/Crucial detail: Sexual Dimorphism is distinct. Males bear a pair of short, thread-like anal styles, which are completely absent in females AIPMT 2012 NEET 2018. Both sexes possess jointed filamentous structures called anal cerci on the 10th segment.
Internal Anatomy
| System | Core Features & Mechanisms | PYQ Relevance |
|---|---|---|
| Digestive | Foregut: Mouth → pharynx → oesophagus → Crop (stores food) → Gizzard/Proventriculus (grinds food via 6 highly chitinous plates/teeth formed by thick inner cuticle) NEET 2020. Midgut: At the foregut-midgut junction is a ring of 6-8 blind tubules called hepatic (gastric) caeca, which secrete digestive juices NEET 2020. | NEET 2020 |
| Excretory | Main organs are Malpighian tubules: a ring of 100-150 yellow, thin, filamentous tubules at the midgut-hindgut junction. They absorb nitrogenous waste and convert it into uric acid. Cockroaches are uricotelic AIPMT 2015 NEET 2020. Assisted by fat body, nephrocytes, and urecose glands. | AIPMT 2015 NEET 2020 |
| Circulatory | Open type. Blood vessels are poorly developed and open into the haemocoel. Visceral organs are bathed in haemolymph (colourless plasma + haemocytes) AIPMT 2013 NEET 2019. The heart is an elongated muscular tube differentiated into 13 funnel-shaped chambers with ostia on either side NEET 2019. | AIPMT 2013 NEET 2019 |
| Respiratory | Network of trachea opening through 10 pairs of lateral holes called spiracles. Branching tracheoles carry oxygen to all parts. Gas exchange occurs by diffusion. | |
| Nervous | Segmentally arranged fused ganglia (3 in thorax, 6 in abdomen). The head holds a small portion; the rest is ventral (a headless cockroach survives for a week). The brain is the supra-oesophageal ganglion. | |
| Sensory | Compound eyes have ~2000 hexagonal ommatidia each. Multiple images of an object result in mosaic vision (more sensitivity, less resolution, adapted for nocturnal vision) AIPMT 2001 AIPMT 2012. | AIPMT 2001 AIPMT 2012 |
Reproductive System & Development
- Male Reproductive System: Pair of testes in the 4th-6th abdominal segments. A characteristic mushroom-shaped gland in the 6th-7th abdominal segments functions as an accessory reproductive gland AIPMT 1994 NEET 2022. External genitalia are represented by asymmetrical chitinous phallomeres. Sperms are glued into bundles called spermatophores.
- Female Reproductive System: Two large ovaries in the 2nd-6th abdominal segments, each with 8 ovarioles. A pair of spermatheca is present in the 6th segment which opens into the genital chamber NEET 2019.
- Development: Fertilized eggs are encased in dark brown capsules called oothecae (females drop 9-10 oothecae, each with 14-16 eggs). Development is paurometabolous (nymphal stage looks like the adult). The nymph undergoes moulting about 13 times to reach the adult form AIPMT 2013 NEET 2021.
Frog (Rana tigrina)
Morphology and Exterior Features
- General Characteristics: Frogs are cold-blooded (poikilotherms) and exhibit camouflage to hide from predators (mimicry). They undergo aestivation (summer sleep) and hibernation (winter sleep) to escape extreme temperatures.
- Skin & Body Division: The skin is smooth, slippery (due to mucus), and highly vascularized. The body is divided only into a head and trunk; neck and tail are strictly absent.
- Exception/Crucial detail: Frogs never drink water; they absorb it directly through their moist skin.
- Sensory Organs: Eyes are bulged and covered by a nictitating membrane to protect them while underwater. A membranous tympanum on either side of the eyes represents the ear and receives sound signals AIPMT 2012.
- Sexual Dimorphism: Male frogs are distinctly identified by the presence of sound-producing vocal sacs and a copulatory pad on the first digit of the forelimbs. Both of these structures are completely absent in female frogs AIPMT 2011.
Internal Anatomy & Physiology
| System | Core Features & Mechanisms | PYQ Relevance |
|---|---|---|
| Digestive | The alimentary canal is uniquely short because frogs are carnivores, which necessitates a reduced length of the intestine NEET 2024. Path: Mouth → buccal cavity → pharynx → oesophagus → stomach → intestine → rectum → cloaca (a common exterior opening for digestive, excretory, and reproductive tracts). Bile emulsifies fats; pancreatic juices digest carbs and proteins. | NEET 2024 |
| Respiratory | Aquatic: Cutaneous respiration (exchange of dissolved oxygen via moist skin). Terrestrial: Pulmonary respiration (lungs, buccal cavity, skin). Exception: During aestivation and hibernation, gaseous exchange takes place exclusively through the skin. | |
| Circulatory | Closed type. Features a 3-chambered heart (two atria, one ventricle) covered by a protective pericardium NEET 2024. The heart is myogenic (auto-excitable) and will continue to beat for some time even when taken out of the body NEET 2017. Special portal systems: Hepatic (liver-intestine) and Renal (kidney-lower body). | NEET 2017 NEET 2024 |
| Blood Components | Composed of plasma, RBCs, WBCs, and platelets. Crucial detail: Unlike mammals, frog RBCs (erythrocytes) are nucleated and contain the red pigment haemoglobin AIPMT 2010. | AIPMT 2010 |
| Excretory | Main organs are compact, bean-like kidneys containing numerous uriniferous tubules (nephrons). Frogs excrete urea and are thus ureotelic. In males, ureters act as a urinogenital duct opening into the cloaca AIPMT 2011. In females, ureters and oviducts open separately into the cloaca. | AIPMT 2011 |
| Neural & Endocrine | Brain is enclosed in a bony cranium. 10 pairs of cranial nerves arise from the brain. The medulla oblongata passes out through the foramen magnum and continues into the spinal cord. Endocrine coordination is via pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals, and gonads. |
Reproductive System & Development
- Male Reproductive System: Consists of a pair of yellowish ovoid testes adhered to the upper part of the kidneys by a double fold of peritoneum called the mesorchium.
- Core Mechanism - Sperm Pathway: 10 to 12 vasa efferentia arise from the testes → enter the kidneys → open into Bidder's canal → communicate with the urinogenital duct (ureter) → finally emerge from the kidneys to open into the cloaca AIPMT 1993 AIPMT 2000 NEET 2017.
- Female Reproductive System: Consists of a pair of ovaries situated near the kidneys. Crucial detail: There is no functional connection between the ovaries and the kidneys. The oviduct arises from the ovaries and opens separately into the cloaca. A mature female can lay 2500 to 3000 ova at a time.
- Development: Fertilization is external and takes place in water. Development is indirect, involving a larval stage called a tadpole, which undergoes metamorphosis to form the adult.