Comparative Histology of Organs | Animal Husbandry & Veterinary Science Optional for UPSC PDF Download

Vascular System

Cardiovascular System:

• Components: The circulatory apparatus includes the cardiovascular system and the lymphatic system.
• Cardiovascular System: Comprises the heart, arteries, capillaries, and veins.
• Lymphatic System: Includes lymph nodes, lymphatic capillaries, and lymphatic vessels draining into the thoracic and right lymphatic ducts.

Blood Vascular System:

• Common Lining: The entire blood vascular system is lined by a single layer of cells called endothelium.
• Heart Structure: The heart, arteries, and veins have additional layers of muscle and connective tissue.

Capillaries:

• Characteristics: Capillaries are thin tubes with an average diameter of 7-9 microns.
• Network Variation: The density and size of capillary networks vary in different organs based on their metabolic activity.
• Capillary Wall: Consists of a single layer of flat endothelial cells with narrow spaces filled with intercellular cement.
• Cell Arrangement: Cells are either narrow with pointed ends or oval with blunt ends, arranged parallel to the tube's long axis.
• Cytoplasm and Nucleus: Cytoplasm is clear or finely granular, and the nucleus is central and elongated.
• Sinusoids: Liver and other organs have larger sinusoidal capillaries (25-30 microns) with lining cells including phagocytic and non-phagocytic cells.
• Function: Capillary walls facilitate the processes of diffusion, filtration, and osmosis of fluids to and from the bloodstream.

Arteries:

• Composition: Artery walls have three layers – the innermost layer called Tunica intima, the middle layer or Tunica media made mostly of smooth muscle with elastic and collagenous tissue, and the outer layer known as Tunica adventitia made of loose connective tissue.
• Size Variation: Arteries are categorized into small arterioles, medium-sized arteries, and large elastic arteries, each with distinct structures based on their size.

Veins:

• Structure: Veins generally have larger diameters than arteries but thinner walls due to reduced muscle and elastic elements. The walls are primarily made up of collagenous connective tissue, making them flabbier and prone to collapse when not filled with blood.
• Coats Variation: The boundaries of the three coats in veins are often unclear, and the histological structure varies not only with vessel size but also based on local mechanical conditions.

Valves in Veins:

• Presence: Veins over 2 mm in diameter have valves at intervals to prevent backflow.
• Structure: These valves are semilunar flaps derived from the intima, consisting of connective tissue covered by a layer of endothelium.

Lymphatic System:

• Structure: The lymphatic system consists of channels lined by endothelium that collect tissue fluid and return it to the bloodstream.
• Direction of Circulation: Lymph circulates from the periphery towards the heart in a one-way direction.
• Components: Lymphatic capillaries begin blindly in tissues, collect lymph, and connect along their course. Lymph passes through lymph nodes before entering the bloodstream.
• Distribution: Lymphatic capillaries are found in most tissues and organs, excluding the central nervous system, bone marrow, eye ball, internal ear, and fetal placenta.

Blood Vascular Endothelium:

• Permeability Barrier: The endothelium of blood vessels acts as a barrier, preventing platelets from sticking and promoting a negative charge to prevent red cell aggregation.
• Functions: It synthesizes prostacyclin to inhibit platelet adhesion, contains thromboplastic substances for clotting, and contributes to smooth blood flow, preventing eddying and turbulence.

The Digestive Systerm

• Alimentary Tract: Main digestive tube extending from the mouth to the anus, including associated structures like the tongue, teeth, salivary glands, liver, and pancreas.

Salivary Glands:

• Types: Classified as serous, mucous, and mixed glands based on secretions and epithelium.
• Structure: True compound tubuloalveolar glands with large and small lobules, surrounded by connective tissue.

Tongue:

• Composition: Consists of layers of stratified squamous epithelium and striated muscle.
• Papillae: Different types include filiform, fungiform, circumvallate, and foliate, housing taste buds.
• Taste Buds: Made up of supporting and neuroepithelial cells, located in various papillae.

Alimentary Canal:

• Layers: Four layers from inside out – mucous membrane, submucosa, muscularis, and serosa.
• Mucous Membrane: Comprises epithelium, lamina propria, and muscularis mucosae, with varying types of epithelium based on canal function.

Esophagus:

• Wall Structure: Mucous membrane lined by stratified squamous epithelium, muscular coat with striated muscle, and fibrous tunic binding to surrounding structures.

Stomach:

• Wall Composition: Mucosa, submucosa, muscularis, and serosa layers. Mucosa has surface epithelium, glandular lamina propria, and muscularis mucosae. Glands include fundic, pyloric, and cardiac glands.

Intestine:

• Similar Layers: Same layers as the stomach. Epithelium is simple columnar with goblet cells. Villi have microvilli on the striated free border. Intestinal glands or crypts of Lieberkühn are tubular glands in the lamina propria.

Liver:

• Size and Function: Largest gland with exocrine function.
• Structure: Hepatic lobules separated by fibrous bands. Central vein, hepatic cells, sinusoids, and bile canaliculi present. Kupffer cells line the sinusoids.

Pancreas:

• Exocrine and Endocrine Parts: Covered by connective tissue with septa dividing it. Acinar cells are serous, and Islets of Langerhans represent the endocrine part.
• Islets of Langerhans: Three types – alpha cells (produce glucagon), beta cells (produce insulin), and delta cells. Pancreatic juice contains enzymes essential for digestion.

Nervous System

• Components: The nervous system is a complex network consisting of neurons, nerve fibers, ganglia, neuroglia, and other specialized cells, along with connective tissue and blood vessels.
• Function: It processes information, perceives stimuli, and generates appropriate responses in the body, based on the properties of protoplasm—irritability and conductivity.

Divisions of the Nervous System:

• Central Nervous System (CNS): Comprising the brain, its extensions, and the spinal cord.
• Peripheral Nervous System (PNS): Includes nerve trunks, nerve cell bodies, and nerve terminals. Further divided into the somatic nervous system and autonomic nervous system.

Neurons:

• Basic Unit: Neuron is the basic unit of the nervous system.
• Structure: It consists of a nucleus, surrounding cytoplasm, and one or more processes (dendrons or dendrites).
• Classification: Neurons vary in size, shape, and the number and length of processes. Classified as unipolar, bipolar, multipolar, or pseudo-unipolar.

Nerve Fibers:

• Structure: Nerve fibers consist of axons and their sheaths, covered by neurolamma or the sheath of Schwann.
• Myelin Sheath: Some fibers have a myelin sheath (medullary sheath) between the axis cylinder and neurolemma.
• Classification: Nerve fibers are categorized as myelinated or amyelinated based on the presence of myelin.

Nerve Terminations:

• Axons: Carry nerve impulses to peripheral structures, classified as efferent (outgoing) or afferent (incoming).
• Receptors: Different receptors respond to stimuli, categorized as exteroceptors (external stimuli), proprioceptors (body movement and posture), and visceroceptors (internal stimuli).

Ganglia and Neuroglia:

• Ganglia: Aggregations of neurons outside the CNS.
• Neuroglia: Interstitial tissue in the nervous system, divided into astroglia, oligodendroglia, microglia, and ependyma.

Protection of CNS:

• Cranial and Spinal Protection: The CNS is safeguarded within the skull and spinal column.
• Membranes: Fibrous membranes—dura mater, arachnoid, and pia mater—enclose the CNS.

Autonomic Nervous System (ANS):

• Function: Regulates the internal environment (homeostasis).
• Components: Includes sensors, afferent pathways, central integration and control centers, efferent pathways, and effectors.
• Subdivisions: Sympathetic and parasympathetic nervous systems.

Respiratory System

• Function: The respiratory system primarily serves for the intake of oxygen and the elimination of carbon dioxide.
• Components: Main parts include the nose, pharynx, larynx, trachea, and bronchi, which further divide into smaller branches ending in alveoli for gas exchange.

Trachea:

• Structure: A thin-walled, rigid tube continuing from the larynx, ending in two chief bronchi.
• Layers: Includes mucosa, submucosa, fibroelastic membrane with hyaline cartilage rings, and adventitia.
• Mucosa: Lined by pseudo-stratified-columnar ciliated epithelium with goblet cells, supported by elastic fibers and forming the tracheal annular ligament.
• Adventitia: Contains collagenous and elastic tissues, adipose tissue, blood vessels, and nerves.

Lungs:

• Pair of Organs: Lungs are paired organs covered by a serous membrane called pleura.
• Internal Structure: Divided into lobes and further into lobules, with each lobule having a pyramidal shape.
• Alveolar Sacs: Thin-walled sacs made up of numerous alveoli for gas exchange.
• Alveoli: Cup-shaped structures where the interchange of gases between blood and air occurs.
• Wall Composition: Contains elastic and reticular fibers, with a rich network of capillaries. Lined by simple squamous epithelium.

Function of Alveoli:

• Gas Exchange: Alveoli walls facilitate the exchange of gases between the air and blood.
• Network in Walls: Elastic and reticular fibers create a framework, while a dense capillary plexus ensures effective gas exchange.
• Lumen Lining: Lined by a thin layer of cytoplasm in simple squamous epithelial cells.

Musculo-Skeletal System

• Diverse Form: The musculoskeletal system's form is as varied as the muscles and bones it comprises.
• Tissue Contribution: All basic tissues contribute to this system, including bone, skeletal muscle, dense white fibrous connective tissue, and cartilage.
• Primary Functions: Provides support and enables movement of the body.

Bone Structure:

• Specialized Connective Tissue: Bone is a specialized connective tissue with cells embedded in a gel-like substance that mineralizes over time.
• Mineral Component: Comprised of amorphous calcium phosphate and hydroxyapatite crystals.
• Osteocytes: Bone cells in contact through cellular processes in canals (canaliculi) within the matrix.
• Bone Growth: Osteoblasts secrete osteoid, undergoing mineralization. Stem cells allow for appositional growth.
• Covering Membranes: Periosteum and endosteum cover bones.

Types of Bones:

• Bones can be long, short, flat, irregular, or sesamoid. Birds have pneumatic bones.
• Histologically, bones are either woven (immature) or lamellar (mature) and can be cancellous (spongy) or compact (dense) based on arrangement.

Muscle Tissue:

• Structural Element: Muscle cells or fibers are elongated and may be uninucleated or multinucleated.
• Components: Enclosed by sarcolemma, containing reticular fibers, and cytoplasm (sarcoplasm) with organelles and myofilaments (actin and myosin).
• Functional Types: Voluntary (skeletal), involuntary (smooth and cardiac).
• Structural Types: Smooth muscle (non-striated), skeletal muscle (striated, voluntary), and cardiac muscle (striated, involuntary).

Joints:

• Definition: Points where two or more bones are joined, facilitating stability or movement.
• Morphologic Classification: Synarthrosis, synchondrosis, synostosis, symphysis, and diarthrosis.
• Functions: Stabilize and unite bones, enable movement, facilitate bone growth, and permit weight-bearing.

Urogenital System

Urinary System:

• Components: Includes glandular organs - kidneys, and excretory passages - ureters, urinary bladder, and urethra.
• Kidney Structure: Compound tubular glands with a cortex (outer zone) and medulla (inner region).
• Parenchyma: Comprises uriniferous tubules, blood vessels, and minimal interstitial connective tissue.
• Nephron: Functional unit, starts with Bowman's capsule enclosing capillaries, forming the renal or Malpighian capsule.
• Nephron Parts: Proximal and distal convoluted tubules, descending and ascending limbs of Henle's loop.

Bowman's Capsule:

• Layers: Visceral and parietal layers of simple squamous epithelium.
• Cell Types: Podocytes in the visceral layer.
• Excretory Passages: Lined by transitional epithelium.

Genital System:

• Male Genital Organs: Include testis, epididymis, ductus deferens, penis, and accessory genital glands.
• Testis Function: Produces sperm and male sex hormones.
• Testis Structure: Compound tubular glands enveloped by Tunica albuginea and Tunica vaginalis.
• Spermatogenesis: Occurs in seminiferous tubules' germinal epithelium, dependent on testosterone.
• Germinal Epithelium Cells: Spermatogenic cells and supporting cells (Sertoli cells).
• Sertoli Cells: Maintain environments for stem cells and spermatogenesis in basal and adluminal compartments.
• Interstitial Cells of Leydig: Secrete testosterone.
• Epididymis: Contains ciliated epithelium and smooth muscles in a collagenous tunic.
• Ductus Deferens: Mucosa lined by pseudostratified columnar cells, with a rich elastic lamina propria.

Accessory Genital Glands (Male):

• Components: Seminal vesicles, prostate, and Cowper's (bulbourethral) gland.
• Prostate Gland: Compound tubulo-alveolar gland with columnar epithelium in alveoli.
• Cowper's Gland: Lobulated compound tubuloacinar gland with acidophilic cytoplasm.

Female Genital Organs:

• Ovary: Cytogenic gland producing ova and hormones. Divided into cortex (parenchymatous zone) and medulla (vascular zone).
• Tunica Albuginea: Surrounds the ovary, covered by germinal epithelium.
• Ovarian Follicles: Structures with an ovum surrounded by follicular epithelial cells (primary, secondary, Graafian).
• Corpus Luteum: Forms after follicle rupture, transforms into corpus albicans if not fertilized.
• Oviduct (Fallopian Tube):
  • Composed of mucosa, muscularis, and serosa.
  • Mucosa has ciliated columnar epithelium, more secretory during estrus.
  • Changes gradually from ovarian to uterine end.

Uterus:

• Components: Mucosa, muscularis, and fibrosa.
• Mucosa: Columnar epithelium, undergoes changes during estrous cycle and pregnancy.
• Muscularis: Three layers of plain muscle (longitudinal, circular, mixed).

Vagina:

• Structure: Mucosa, muscularis, and fibrosa.
• Mucosa: Non-glandular, thrown into folds, lined by stratified squamous epithelium.
• Lamina Propria: Rich in elastic tissue, serous layer at the anterior part.

Mammary Gland:

• Structure: Compound tubulo-alveolar gland during lactation.
• Parenchyma: Secretory tubules with alveoli lined by columnar/cuboidal epithelium.
• Ducts: Lined by secretory or columnar epithelium, terminal end has stratified squamous epithelium.