Structural Organisation in Animals

Story: Imagine bathroom tiles.

Simple = single layer tiles: Squamous flat like pavement tiles thin flat lung alveoli for diffusion, Cuboidal cube like dice kidney tubules with microvilli for absorption secretion, Columnar tall like pillars stomach intestine with nucleus at base, Ciliated with cilia tiny hairs beating like broom moving mucus in trachea fallopian tube eggs move, Pseudostratified looks double layered but single all cells touch basement membrane nuclei at different heights like people of different heights standing — trachea.

Compound = multilayer tiles for protection like skin many layers.

Glandular = factories: Goblet single cell unicellular mucus like single shop, multicellular many cells salivary glands like factory with many workers. Exocrine with duct like factory with pipe delivering product outside, Endocrine without duct hormones directly into blood like secret delivery.

Junctions = cement: Tight junction prevents leakage like waterproof cement, Adhering junction cements like glue, Gap junction communication like small windows talking. Once you see tiles, you remember epithelium.

Loose: Areolar under skin fills space like cotton filling in pillow. Contains fibroblasts make fibres like factory workers, mast cells release heparin histamine like alarm, macrophages eat germs like security guards, collagen white fibres strength like steel wires, elastin yellow fibres stretch like rubber band.

Adipose fat storage signet ring fat globule pushes nucleus to side like ring, white fat one big droplet energy storage, brown fat many droplets heat in babies hibernating animals like heater.

Dense Regular: Collagen parallel in bundles like parallel ropes. Tendons attach skeletal muscle to bone white fibrous strong inelastic like white rope muscle-bone, Ligaments bone to bone yellow elastic stretchy like yellow rubber band you can stretch slightly. That is why sprain ligament stretch painful.

Irregular random collagen like random net in skin.

Specialized: Cartilage chondrocytes in lacunae small houses in matrix solid pliable like firm jelly, types hyaline elastic fibrous calcified. Bone osteocytes in lacunae Haversian system lamellae rings canaliculi canals hard with calcium salts like concrete with steel rods.

Plasma 55% water proteins albumin globulin fibrinogen clotting, salts, glucose like river water with minerals. RBC erythrocytes biconcave disc no nucleus adult mammals like donut without hole, haemoglobin red carries oxygen, 4.5-5.5 million per mm3, life 120 days like 4 months duty then spleen graveyard. WBC leucocytes 5-8k soldiers less number: Granulocytes with granules — Neutrophils 60-65% multilobed 2-5 lobes phagocyte first soldiers, Eosinophils bilobed allergy 2-3% increase in allergy asthma, Basophils S-shaped nucleus heparin histamine 0.5-1% alarm. Agranulocytes no granules — Lymphocytes B T 20-25% immunity like special forces, Monocytes kidney shaped nucleus 6-8% biggest WBC phagocyte like big security. Platelets thrombocytes 1.5-3.5 lakh no nucleus cell fragments clotting like bandage. Blood groups A B AB O clotting cascade stops bleeding like plaster. Remember: Never Let Monkeys Eat Bananas = Neutro Lympho Mono Eosino Baso percentages 60 20 6 2 1.

Muscle: Skeletal striated voluntary you control cylindrical multinucleated peripheral unbranched with sarcomere actin thin myosin thick light I dark A bands like zebra crossing, moves bones you lift dumbbells. Smooth non-striated involuntary spindle single central nucleus wall of viscera gut blood vessel peristalsis like you cannot control gut movement. Cardiac striated branched involuntary intercalated discs discs like steps connecting cells, heart never fatigue beats lifelong like tireless worker.

Neural: Neuron unit cell body soma with Nissl granules RER, dendrites receive like antennas, axon long carries impulse like wire, myelin sheath by Schwann cells insulating like electric tape with nodes of Ranvier gaps saltatory fast jumping like jumping stones. Types multipolar many dendrites brain spinal cord like tree with many branches, bipolar one dendrite one axon retina nose, unipolar embryo dorsal root single process. Neuroglia support cells astrocytes oligodendrocytes microglia cleaners. Synapse pre + post + cleft like two people talking with gap, neurotransmitter message.

External: Skin smooth moist with mucus cutaneous respiration in water and hibernation winter sleep under mud like you breathe through skin in water, color olive dorsal camouflage pale ventral countershading, body head trunk no neck tail absent adult, eyes bulged with nictitating membrane 3rd eyelid like goggles protection in water, tympanum ear membrane behind eye like drum hears, forelimbs 4 digits hindlimbs 5 webbed swimming like flippers, male larger vocal sacs croaking rain sound + nuptial pads rough on first finger for amplexus holding female tightly during mating.

Digestive: Mouth wide buccal cavity bifid sticky tongue protrusible catches insects like sticky tape flicks out, maxillary vomerine teeth hold prey, oesophagus short stomach J-shaped, liver bilobed gall bladder stores bile green, pancreas mixed gland, small intestine duodenum ileum coiled large intestine rectum cloaca common opening for digestive excretory reproductive like single exit.

Respiratory 3 ways: Cutaneous skin in water and hibernation moist must be moist, Buccopharyngeal lining of buccal cavity on land mouth moves, Pulmonary lungs pink thin-walled sacs adult air breathing.

Circulatory: Heart 3 chambered 2 atria 1 ventricle + sinus venosus receives blood + truncus arteriosus distributes, closed type blood in vessels, single circuit mixed blood but okay for slow life, RBC nucleated unlike mammals, lymphatic, hepatic portal renal portal.

Excretory: Mesonephric kidneys 2 dark red bean shaped, ureters open into urinogenital duct to cloaca, ureotelic adult excrete urea less toxic, ammonotelic tadpole ammonia highly toxic water easily removed.

Nervous: Brain forebrain olfactory smell cerebrum thinking diencephalon midbrain optic lobes vision hindbrain cerebellum balance medulla vital centres, 10 pairs cranial nerves.

Endocrine: Pituitary master thyroid parathyroid thymus pineal pancreatic islets adrenal gonads.

Reproductive: Male testes yellow attached to kidney by mesorchium membrane, Bidder's canal connects to kidney, ureter urinogenital duct common. Female ovaries, oviduct coiled ovisac uterus opens to cloaca. Fertilization external in water amplexus male holds female releases sperms on eggs in water, eggs in jelly mass, development tadpole larva fish-like tail gills herbivore then metamorphosis by thyroxine hormone iodine needed → froglet with limbs → adult carnivore. If iodine less, tadpole never becomes frog — experiment.