(a) Define Placenta. Classify Placenta on the basis of histological structure with example. (20 marks) (b) Classify the drugs…

(a) Placenta: Definition and Histological Classification The placenta is the specialized fetomaternal organ formed by the union of fetal membranes (chorion) and maternal uterine tissue (endometrium). Its primary function is the physiological exchange of gases, nutrients, waste products, and hormones between the fetus and the dam. Histologically (Grosser's classification), placentas are classified by the layers of maternal tissue separating the fetal chorion from maternal blood, whereas the morphological classification (diffuse, cotyledonary, zonary, discoid) is based on the distribution of chorionic villi.

The four types are:

1. Epitheliochorial: The chorion is separated from maternal blood by the intact uterine epithelium, connective tissue, and endothelium. This is seen in the mare and sow. Grossly, both are diffuse (horse and pig).
2. Syndesmochorial: The uterine epithelium is eroded, but the connective tissue (stroma) remains intact. This is characteristic of ruminants (cow, ewe), now more precisely termed synepitheliochorial because fetal binucleate cells fuse with uterine epithelium rather than eroding it. In cattle, this correlates with the cotyledonary gross structure, where discrete placental units (cotyledons) interdigitate with maternal caruncles.
3. Endotheliochorial: Both the uterine epithelium and connective tissue are eroded, leaving only the endothelium of maternal blood vessels between the chorion and maternal blood. This is typical of carnivores (dog, cat), which have a zonary band of attachment.
4. Hemochorial: All maternal tissues (epithelium, stroma, and endothelium) are eroded, allowing direct contact between fetal chorion and maternal blood. This is seen in primates and rodents, corresponding to a discoidal gross structure.

(b) Drugs Acting on the Autonomic Nervous System Pharmacological agents acting on the autonomic nervous system (ANS) are classified by their site of action and receptor specificity.

Cholinergic Agents act at muscarinic (M) or nicotinic (N) receptors.

• Direct-acting Muscarinic Agonists: Drugs like methacholine and bethanechol bind specifically to muscarinic receptors (G-protein coupled), causing parasympathomimetic effects such as bradycardia and increased secretions. They do not significantly activate nicotinic receptors.
• Indirect-acting Cholinesterase Inhibitors: Drugs like neostigmine and physostigmine inhibit acetylcholinesterase, prolonging the action of endogenous acetylcholine at both muscarinic and nicotinic sites.
• Cholinergic Antagonists: Atropine blocks muscarinic receptors, reducing parasympathetic tone.
• Ganglionic Agents: Hexamethonium blocks nicotinic receptors in autonomic ganglia, inhibiting both sympathetic and parasympathetic transmission.

Adrenergic Agents act on alpha (α) and beta (β) adrenergic receptors.

• Alpha Agonists: Phenylephrine stimulates α1 receptors, causing vasoconstriction and increased blood pressure, while xylazine, an α2 agonist, reduces noradrenaline release and is widely used for sedation in cattle.
• Beta Agonists: Isoproterenol stimulates both β1 (cardiac) and β2 (smooth muscle) receptors, causing tachycardia and bronchodilation.
• Adrenergic Antagonists: Propranolol is a non-selective β-blocker that inhibits β1 and β2 receptors, reducing heart rate and contractility. Prazosin is an α1-blocker used for vasodilation.

(c) Diagnosis and Surgical Management of Choke and Hernia

Choke (Esophageal Obstruction) Diagnosis is based on history of sudden cessation of eating, regurgitation of feed, excessive salivation, and ruminal tympany. Confirmation is achieved by passing a stomach tube; if it cannot pass beyond the obstruction, the diagnosis is confirmed. Surgical management depends on location. For distal (thoracic) obstructions, rumenotomy may be performed to remove large boluses. For proximal (cervical) obstructions that fail conservative treatment (sedation, lubrication), esophagotomy is performed to remove the foreign body, followed by layered closure of the esophagus.

Hernia Diagnosis involves identifying a reducible or irreducible swelling. In cattle, umbilical hernia in calves is the commonest, ventral abdominal hernias follow trauma such as horn injury, and inguinal/scrotal hernias occur in bulls, presenting as scrotal swelling. Palpation reveals the hernial ring. Ultrasonography helps identify the contents (intestine, omentum) and assess for strangulation. Surgical management involves herniorrhaphy. For inguinal hernias, the inguinal canal is exposed, the contents are reduced, and the internal ring is closed with non-absorbable sutures. For large ventral or umbilical defects, mesh repair may be used to reinforce the abdominal wall. Postoperative care includes broad-spectrum antibiotics and analgesics to prevent infection and manage pain.