Fiche de révision : Embryology and Anatomy of Development

📋 Course Outline

1. Embryonic development and fertilization
2. Placentation and placental barriers
3. Thoracic cavity and mediastinum
4. Pharyngeal arches and tongue development
5. Salivary glands and oral anatomy
6. Horse teeth and temporomandibular joint
7. Larynx and vocal cavity
8. Pancreas and fetal vasculature
9. Urogenital anatomy and bird respiration
10. Eye anatomy and visual structures
11. Ear anatomy and hearing
12. Spinal cord and cranial nerves

📖 1. Embryonic development and fertilization

🔑 Key Concepts & Definitions

• Embryonic development : Embryonic development is the series of processes where a fertilized egg forms an embryo, then a fetus capable of adapting to postnatal life.
• Embryonic period : The embryonic period is the phase during which the major organ systems are established.
• Fertilization site : The fertilization site is the location where sperm contact the oocyte, mainly the ampullary region of the oviduct.
• Zona pellucida : The zona pellucida is the oocyte covering the fertilizing sperm must interact with during the first steps of fertilization.
• Polyspermy block : The polyspermy block is the cortical response after gamete fusion that prevents further sperm from entering the oocyte.

📝 Essential Points

• Sperm reach the fertilization site mainly in the ampullary region of the oviduct through successive rapid transport phases.
• During transport, sperm undergo capacitation so they achieve fertilizing capability.
• The first division usually occurs about 24 hours after ovulation.
• Cleavage occurs without cellular growth, so the embryo develops by increasing cell number before later uterine entry.
• The zygote first forms maternal and paternal pronuclei that move together, fuse, and enter prophase of the first mitotic division.

💡 Memory Hook

Zona-pellucida contact → sperm fusion → cortical polyspermy block; then pronuclei fuse and mitosis starts (≈24 h after ovulation).

📖 2. Placentation and placental barriers

🔑 Key Concepts & Definitions

• Histotrophe : Histotrophe is uterine-gland secretion that initially nourishes the embryo after it enters the uterine cavity.
• Haemotrophe : Haemotrophe is maternal blood nutrients imported through the developing fetal–maternal vascular exchange at the placenta.
• Embryotrophe : Embryotrophe is the combined nutrient supply provided by histotrophe and haemotrophe during placentation.
• Chorioallantoic placenta : Chorioallantoic placenta is the primary functional placenta in all domestic species, formed by chorion and allantois contributions.
• Placental barrier layers : Placental barrier layers describe the maternal and fetal tissue strata that separate maternal blood from fetal blood for exchange.

📝 Essential Points

• Placenta formation requires synchrony between uterine receptivity and embryonic stage.
• In chorioallantoic placenta, the fetal barrier always includes endothelium in chorioallantoic blood vessels plus chorioallantoic mesenchyme plus chorionic epithelium.
• Maternal–fetal contact patterns define epitheliochorial, synepitheliochorial, endotheliochorial, and haemochorial placentas across species.
• Chorioallantoic placenta types by chorionic distribution are diffuse, folded diffuse, villous diffuse, cotyledonary, zonary, and discoid according to species patterns.
• Deciduate placentas include carnivores, rodents, and humans with endometrial shedding at parturition, while adeciduate placentas (ruminants, pigs, horses) show no endometrial shedding.

💡 Memory Hook

Think “Epi→Syn→Endo→Hae”: epitheliochorial has no maternal loss, then tissue crossing/fusion, then loss of epithelium/connective, ending with near-complete barrier reduction (haemochorial).

📖 3. Thoracic cavity and mediastinum

🔑 Key Concepts & Definitions

• Thoracic cavity portions : Anatomical division of the thorax into regions distinguished by their location along the cranio-caudal axis: pectoral and abdominal intrathoracic portions.
• Pleura layers : Serous membranes covering the thoracic wall and the lungs, forming the pleural cavity between parietal and visceral layers.
• Pleural recesses : Expansions formed by pleura reflections that create specific spaces, including the mediastinal and costodiaphragmatic recesses and the pleural cupula.
• Mediastinum parts : Midline thoracic compartments separated craniocaudally into cranial, middle, and caudal mediastinum for organizing thoracic organs.
• Thoracic duct : Major lymph-collecting channel that drains from the cisterna chyli into the mediastinum and terminates in a cranial vein.

📝 Essential Points

• The thoracic cavity has two cranio-caudal portions: pectoral and abdominal intrathoracic.
• The pleural cavity lies between parietal (costal, mediastinal, diaphragmatic) and visceral (pulmonary) pleura.
• The costodiaphragmatic recess is very large compared with other pleural recesses listed.
• The cranial mediastinum includes esophagus, trachea, cranial vena cava, brachiocephalic trunk, and vagus plus phrenic nerves.
• The thoracic duct passes through the aortic hiatus into the mediastinum and usually terminates in the left jugular vein or the cranial vena cava.

💡 Memory Hook

Mediastinum is a craniocaudal “stack”: cranial = trachea/vessels, middle = heart in pericardium region, caudal = thoracic aorta + caudal cava/azygos region.

📖 4. Pharyngeal arches and tongue development

🔑 Key Concepts & Definitions

• Pharyngeal (branchial) arches : Pharyngeal arches are mesenchymal bars in the pharyngeal region that form multiple head and neck structures.
• Pharyngeal clefts and pouches : Pharyngeal clefts and pouches are external clefts and internal pouches that develop alongside the arches and contribute to derivatives.
• Pharyngeal arch layers : Pharyngeal arches have three layers with ectoderm externally, endoderm internally, and mesenchyme in between.
• Lingual septum fusion : Lingual septum formation results from fusion of medial and lateral tongue swellings along the midline.
• Lingual papillae : Lingual papillae are surface projections on the tongue that include mechanical and gustatory types.

📝 Essential Points

• Six pharyngeal arches form initially, but the fifth is rudimentary and the sixth fuses with the fourth, leaving four clearly visible arches.
• The first pharyngeal cleft develops into the external auditory meatus.
• Mesenchyme in each pharyngeal arch forms specific cartilage or bone, muscles, an associated aortic arch artery, and a particular nerve.
• Tongue musculature develops from myoblasts that invade the tongue primordium, forming all skeletal tongue muscles.
• Mechanical papillae are cornified (filiform most numerous in all species; conical in cats; marginal in newborn dogs and pigs), while gustatory papillae include fungiform, vallate, and foliate.

💡 Memory Hook

4 visible arches because 5 is rudimentary and 6 fuses with 4; first cleft becomes the ear canal (external auditory meatus).

📖 5. Salivary glands and oral anatomy

📖 6. Horse teeth and temporomandibular joint

📖 7. Larynx and vocal cavity

📖 8. Pancreas and fetal vasculature

🔑 Key Concepts & Definitions

• Avian pancreas : A gland lying between the limbs of the duodenal loop that releases pancreatic juice into the distal duodenum.
• Dorsal and ventral lobes : The pancreas is built from dorsal and ventral lobes that are connected distally to keep the gland continuous.
• Pancreatic ducts : The pancreas uses two or three ducts to convey pancreatic juice into the distal end of the duodenum.

📝 Essential Points

• The avian pancreas is elongated and positioned between the limbs of the duodenal loop.
• The pancreas connects dorsal and ventral lobes distally.
• Pancreatic ducts (2–3) drain pancreatic juice into the distal end of the duodenum.

💡 Memory Hook

Think pancreas = “2–3 ducts to distal duodenum,” built from dorsal+ventral lobes connected distally.

📖 9. Urogenital anatomy and bird respiration

🔑 Key Concepts & Definitions

• Cloaca compartments : A cloaca is divided into cranial coprodeum, middle urodeum, and caudal proctodeum by complete annular folds.
• Renal portal system : A renal portal system is a venous pathway in birds where blood from the posterior body passes through kidney tissue before reaching the heart.
• Hepatic portal system : A hepatic portal system is a venous pathway in birds that sends nutrient-rich blood from the gut, spleen, and pancreas to the liver first.
• Air sacs : Air sacs are blind, thin-walled expansions of the bronchial system that extend beyond the lungs and are essential for avian airflow.

📝 Essential Points

• The cloaca receives colorectum plus ureters and deferent ducts (or the left oviduct), integrating digestive and urogenital outflow.
• Renal portal effects can reduce drug delivery from the hindlimb because blood may be filtered by the renal parenchyma before systemic distribution.
• Bird hepatic portal blood reaches the liver before entering general circulation, enabling liver processing of toxins and pathogens.
• Birds have eight air sacs in the chicken: 1 cervical, 1 clavicular, paired cranial thoracic, caudal thoracic, and abdominal sacs.
• In birds, fresh air moves through the lung on expiration as well as inspiration, with caudal sacs receiving relatively fresh air.

💡 Memory Hook

Cloaca is “UD-to-out” (urodeum in the middle), and air sacs keep airflow “through-expiration too” like a two-way lane.

📖 10. Eye anatomy and visual structures

🔑 Key Concepts & Definitions

• Vision pathway : Afferent vision pathway transmits light information from the retina through optic pathways to the occipital visual cortex.
• Embryological lens dual origin : The cornea develops from both surface ectoderm and mesoderm, giving it a dual embryological origin.
• Orbit : The orbit is a bony cavity that holds the eyeball and associated structures.
• Fibrous tunic of eyeball : The fibrous tunic is the external protective layer that gives shape to the eyeball, consisting of sclera and cornea.
• Uvea : The uvea is the vascular tunic comprising choroid, ciliary body, and iris.

📝 Essential Points

• Directional ophthalmology terms map anterior to rostral, posterior pole to caudal, medial to nasal, and lateral to temporal.
• The vision afferent route is retina → optic nerve (CN II) → optic chiasm → optic tract → lateral geniculate nucleus → optic radiation → occipital lobe (visual cortex).
• The corneal limbus is the corneoscleral junction surrounding the scleral groove, with ciliary muscles and the scleral venous plexus at this border.
• The lens is solid, biconvex, elastic, and lies in a depression of the vitreous humor.
• Accommodation for near vs distant vision differs because ciliary muscle and zonular ligaments change lens shape and thickness.

💡 Memory Hook

Retina to occipital: retina → chiasm → tract → LGN → radiation → occipital; “see = chiasm-LGN-occipital”.

📖 11. Ear anatomy and hearing

🔑 Key Concepts & Definitions

• Vestibulocochlear organ : The vestibulocochlear organ is the ear system that provides both balance and hearing functions through distinct receptor regions.
• Membranous labyrinth : The membranous labyrinth is the inner-ear duct and cavity system that contains endolymph and stimulates sensory cells.
• Organ of Corti : The organ of Corti is the cochlear structure that converts sound-related fluid vibrations into nerve stimuli.
• Tympanic membrane : The tympanic membrane is the thin eardrum suspended in the tympanic ring that receives sound vibrations and relays them to ossicles.
• External acoustic meatus : The external acoustic meatus is the ear canal that conducts sound from the auricle to the tympanic membrane.

📝 Essential Points

• The tympanic cavity has 3 openings: auditory (Eustachian) tube to the nasopharynx, round window to the cochlea, and oval window to the vestibule.
• The oval window releases inner-ear pressure, decompresses acoustic energy, enables cochlear fluid movement, and stimulates basilar membrane hair cells.
• The round window is enclosed by the stapes plate (smallest ossicle) and transmits middle-ear vibration into the cochlea.
• Tensor tympani tensions the ossicle chain and tympanic membrane, while stapedius moves the stapes away from the vestibular window to attenuate sound transmission.
• Carnivores have a vertical and a horizontal EAM (other species are straight), and the horse Eustachian tube communicates with the guttural pouch.

💡 Memory Hook

Round window → cochlea; Oval window → vestibule.

📖 12. Spinal cord and cranial nerves

🔑 Key Concepts & Definitions

• Central canal : The central canal is the hollow core of the spinal cord that contains cerebrospinal fluid.
• Grey and white matter : Grey matter consists of neuronal cell bodies, while white matter consists of myelinated axon bundles grouped into cords.
• Cauda equina : Cauda equina is the long sacral and coccygeal spinal nerves with an extended course because the spinal column grows faster than the nerves in embryos.
• Cranial pairs : Cranial pairs are the twelve nerve pairs that arise from the brain and brainstem and exit the skull to distribute mainly to the head.

📝 Essential Points

• The spinal cord extends from the medulla oblongata (caudal brainstem) to the lumbar region within the vertebral canal and is covered by meninges.
• Grey matter is organized into paired dorsal horns (sensory relay neurons) and paired ventral horns (motor neurons), connected across the midline by the commissure of grey matter.
• The lateral horn is present only in thoracic, lumbar and sacral regions and belongs to the autonomic nervous system.
• The cervical intumescence is associated with brachial plexus spinal nerves, while the lumbar intumescence is associated with pelvic cavity and hindlimb innervation.
• Cranial pairs are 12 pairs and include sensory, motor, and mixed nerves such as CN I Olfactory and CN X Vagus, which innervates thoracic and abdominal viscera.

💡 Memory Hook

Dorsal horns relay sensory; ventral horns drive motor—think “Dorsal = Feel, Ventral = Move.”

📊 Synthesis Tables

Placental barrier types (maternal–fetal layers)

⚠️ Common Pitfalls & Confusions

1. Confusing the embryonic period with the fetal period: embryonic period establishes major organ systems, while fetal period is mainly growth and refinement.
2. Thinking cleavage occurs with cellular growth: cleavage and subsequent divisions occur without cellular growth; expansion by uterine entry comes later.
3. Mixing up meiosis II timing: oocyte activation continues meiosis II; the first mitotic division then occurs about 24 hours after ovulation.
4. Assuming the 5th pharyngeal arch is present like the others: it is rudimentary, leaving four clearly visible arches because the 6th fuses with the 4th.
5. Reversing placental barrier logic: epitheliochorial is higher barrier with no maternal tissue loss; haemochorial is the most reduced (complete maternal barrier reduction).
6. Forgetting tympanic cavity openings: round window transmits to the cochlea, while oval window stimulates/permits pressure relief to move cochlear fluid and stimulates basilar membrane hair cells.
7. Assuming cauda equina is “a separate spinal cord part”: it consists of long sacral and coccygeal spinal nerves with an extended course because the spinal column grows faster than the nerves.

✅ Exam Checklist

1. State the embryonic development definition and distinguish embryonic period (major organ systems established) from fetal period (growth and organ refinement).
2. Describe fertilization events: ampullary fertilization site, capacitation during transport, zona pellucida interaction, cortical polyspermy block, pronuclei approach/fuse, and first mitotic division timing (~24 h after ovulation).
3. Explain cleavage essentials (no cellular growth) and the entry to uterus after cleavages; define morula as 16-blastomere cluster and blastocele (fluid cavity inside trophectoderm).
4. Outline gastrulation to implantation: hypoblast and epiblast formation, bilaminar disc, implantation with syncytiotrophoblast growth/junctions and syncytiotrophoblastic lacunae, plus primitive streak (caudal groove).
5. List germ-layer derivatives and notochord derivative nucleus pulposus, and link neurulation to neural tube formation, neuropores, and spina bifida/anencephaly causes.
6. Classify pharyngeal arches (six initially; 5th rudimentary; 6th fuses with 4th) and give the key cleft derivative (1st cleft → external auditory meatus).
7. Identify tongue development result of lingual septum fusion and the mechanical vs gustatory papillae categories (filiform most numerous; fungiform/vallate/foliate).
8. Describe chorioallantoic placentation with embryo/maternal nutrient sources (histotrophe/haemotrophe → embryotrophe) and the barrier-layer progression epitheliochorial → synepitheliochorial → endotheliochorial → haemochorial, then state deciduate vs adeciduate species pattern.
9. In thorax, state pleural cavity location between parietal and visceral pleura, list mediastinum parts (cranial/middle/caudal) with the cranial mediastinal contents, and trace thoracic duct origin (cisterna chyli) and usual termination.
10. For bird urogenital/respiration, define cloaca compartments (coprodeum/urodeum/proctodeum) and list hepatic vs renal portal system significance, plus enumerate the eight air sacs and the “fresh air moves on expiration too” principle.
11. For eye and hearing, reproduce the vision afferent pathway to occipital visual cortex and the ear tympanic cavity openings/functions (auditory tube, round window → cochlea, oval window → vestibule), then name ossicles and the species rule for carnivore EAM orientation.
12. For nervous system/spinal cord, state grey vs white matter organization, dorsal horns vs ventral horns functions, lateral horn presence (thoracic/lumbar/sacral) as autonomic-related, cauda equina cause, and cranial nerve XII and X targets/types (motor vs mixed).