📝 STANDARD DESCRIPTION:
RESPIRATORY:
• Laryngotracheal groove on ventral foregut → Lung Bud → divides into 2 primary bronchi → secondary + tertiary → bronchioles → alveoli.
• Alveoli develop late (Type I + II pneumocytes); fluid-filled in utero.
• Surfactant from Type II pneumocytes appears late in gestation.
URINARY:
• Develops in 3 stages from intermediate mesoderm:
1. Pronephros — vestigial in mammals.
2. Mesonephros — temporary functional kidney; mesonephric duct persists in male as ductus deferens.
3. Metanephros — definitive adult kidney; from ureteric bud + metanephric mesenchyme.
• Ureteric bud → ureter + renal pelvis + collecting ducts.
CARDIOVASCULAR:
• Heart develops from cardiogenic mesoderm as a single tube → loops into S-shape → septates into 4 chambers.
• Fetal shunts include the foramen ovale, ductus arteriosus, ductus venosus and umbilical vessels. Functional closure begins around birth, whereas anatomical obliteration occurs later and varies by structure and species.
📚 DETAILED DESCRIPTION:
SYSTEM-WISE DEVELOPMENT (continued)
I. RESPIRATORY SYSTEM:
1. Origin: A ventral midline outgrowth of the FOREGUT endoderm → laryngotracheal groove (~day 22 in human; analogous early in domestic species).
2. Stages:
• Groove deepens → respiratory diverticulum (lung bud).
• Lung bud splits into LEFT + RIGHT primary bronchial buds.
• Each primary bud subdivides → secondary (lobar) bronchi → tertiary (segmental) bronchi → repeated branching (~17 rounds) → terminal + respiratory bronchioles → alveolar ducts → alveoli.
3. Lung histodifferentiation (4 stages):
• Embryonic (early branching).
• Pseudoglandular (looks like tubular glands, no alveoli).
• Canalicular (vascularization; primitive respiratory units).
• Terminal sac/alveolar (Type I + II pneumocytes; surfactant; capillary contact).
4. Surfactant production begins late in gestation. Premature foals/calves often have surfactant deficiency → respiratory distress syndrome.
5. At birth: Lungs go from fluid-filled to air-filled with the first breath. Surfactant prevents alveolar collapse.
II. URINARY SYSTEM:
Develops in 3 successive stages from intermediate mesoderm:
1. Pronephros — most cranial; vestigial; non-functional in mammals; gone by week 4 in human.
2. Mesonephros — middle; transiently functional in mammals; produces urine briefly. The mesonephric (Wolffian) duct persists in males as the epididymal duct + ductus deferens + species-appropriate derivative glands; efferent ductules arise from mesonephric tubules. In females it regresses but leaves vestiges.
3. Metanephros — definitive adult kidney. Two origins:
• Ureteric bud (outgrowth of mesonephric duct) → ureter + renal pelvis + calyces + collecting ducts.
• Metanephric mesenchyme (blastema) → glomeruli + convoluted tubules + Loop of Henle + connecting tubules.
• Reciprocal induction is critical — failure causes renal aplasia.
4. Bladder:
• Cloaca splits into urogenital sinus (ventral) + rectum (dorsal) by urorectal septum.
• Urogenital sinus develops into bladder + urethra.
• Allantoic stalk → urachus → median ligament of bladder. Patent urachus = newborn anomaly.
5. Genital ducts:
• Mesonephric (Wolffian) duct → male tract (epididymis, vas deferens, vesicular gland).
• Paramesonephric (Müllerian) duct → female tract (uterine tubes, uterus, cervix, cranial vagina). Anti-Müllerian hormone in male foetus causes regression.
III. CARDIOVASCULAR SYSTEM:
1. Origin: Cardiogenic mesoderm in the cardiogenic plate (just cranial to neural plate).
2. Heart Tube Formation:
• Two endothelial tubes fuse → single primitive heart tube.
• Tube has 5 dilatations cranial → caudal: Truncus arteriosus → Bulbus cordis → Primitive ventricle → Primitive atrium → Sinus venosus.
• Beats by ~day 22 in human (early in domestic species).
3. Cardiac Looping:
• Tube grows faster than its space → loops into S-shape (around day 23).
• Bulbus cordis + ventricle move ventrally + caudally; atrium moves dorsally + cranially.
4. Septation:
• Atria: Septum primum + septum secundum form, leaving foramen ovale open for fetal blood shunt.
• Ventricles: Interventricular septum grows up + down + meets endocardial cushions.
• Outflow tract: Spiral aorticopulmonary septum divides truncus arteriosus into aorta + pulmonary trunk.
5. FETAL CIRCULATION SHUNTS (functional closure begins around birth; anatomical closure follows later):
• Ductus venosus — bypasses liver; closes → ligamentum venosum.
• Foramen ovale — atrial-level R-to-L shunt; closes → fossa ovalis.
• Ductus arteriosus — pulmonary trunk-to-aorta shunt; closes → ligamentum arteriosum.
• Umbilical arteries + vein — close after delivery → round ligaments of bladder + ligamentum teres of liver.
6. Common Defects:
• Patent Ductus Arteriosus (PDA) — one of the common congenital heart defects in dogs, with breed and population variation (Poodle, Pomeranian); continuous murmur.
• Ventricular Septal Defect (VSD) — hole in IVS.
• Tetralogy of Fallot — pulmonary stenosis + RV hypertrophy + VSD + overriding aorta.
• Persistent Right Aortic Arch (PRAA) — vascular ring entrapping oesophagus → regurgitation in young dogs.
// COMPARATIVE ANALYSIS
| ALL |
Common cardiac development plan. Mid- to late-gestation morphogenesis is similar across mammals. |
| BIRD |
Right aortic arch is dominant in birds, whereas the left is dominant in mammals. The early avian heart passes through common-chamber stages but the adult avian heart has four chambers. |
CLINICAL RELEVANCE
PDA: Most common cardiac defect in dogs. Surgical ligation curative. VSD, PRAA, ToF: Other common congenital cardiopathies. Renal Aplasia/Dysplasia: Failed reciprocal induction. Hypospadias / Persistent Urachus: UG defects. Surfactant Deficiency: Premature foals/calves.
