Chapter 29 Embryonic Stage 02

Embryo-Level Morphological Landmarks

• 35 dpf35\ \text{dpf}
• Upper limb sequence: arm ➔ forearm ➔ hand.
• Lower limb sequence: thigh ➔ leg ➔ foot.
• 50 dpf50\ \text{dpf}
• Rapid “morphological change” phase—overall body proportions begin to resemble post-natal pattern (larger head, discernable limb regions, emerging facial contour).

Organogenesis I (≈16−20 dpf16{-}20\ \text{dpf})

• Somitogenesis
• Serial paired blocks (somites) derived from paraxial mesoderm.
• Give rise to: axial muscle, vertebrae, ribs, part of dermis.
• Process essentially complete by ≈30 dpf\approx30\ \text{dpf}.
• Other concurrent events
• Neural plate visible—first ectodermal specialization toward nervous system.
• Thyroid primordium appears (future endocrine regulation).
• Heart exists as 2 parallel endocardial tubes (later fuse).

Neural Tube Formation & Early Neurogenesis

• Neuroectoderm thickens into neural plate by ≈18 dpf\approx18\ \text{dpf}.
• Lateral plate borders elevate, fold, and fuse ➔ neural tube.
• Neural tube closes cranio-caudally; complete closure by ≈26 dpf\approx26\ \text{dpf}.
• Fate map
• Neural tube ➔ CNS (brain + spinal cord).
• Neural crest cells (at the dorsal margin of closing tube) migrate widely ➔ PNS & multiple ectomesenchymal derivatives.

Organogenesis II (≈21−25 dpf21{-}25\ \text{dpf})

• Neural crest cell emigration begins—forms craniofacial mesenchyme, sensory ganglia, melanocytes, etc.
• Primordia starting/visible
• Eyes (optic vesicles), ears (otic placodes).
• Heart now a single tube; intrinsic contractions start (“heart beats”).
• Diaphragmatic septum begins partitioning thorax/abdomen.
• Tooth dentin template laid by neural-crest-derived odontoblasts.
• Gut regions delineated: foregut & hindgut pockets; midgut still open to yolk-sac cavity.
• Pronephros (first kidney) forms; allantois stalk projects into connecting stalk (future umbilical cord).
• Primordial germ cells specified, migrate along hindgut.

Neural Crest Derivatives (Head & General)

• Cranial crest ➔ skull bones, dentin of teeth, extra-ocular skeletal muscles, craniofacial connective tissue.
• Term “mesenchyme” = loosely organized embryonic CT derived either from mesoderm or neural crest.

Digestive Tract Formation (Fig. 29.9)

• Longitudinal & lateral body-folding transform the flat trilaminar disc into a tube-within-a-tube.
• Key landmarks (sagittal)
• Foregut: enclosed after cranial fold; ends blindly at oropharyngeal membrane (future mouth).
• Hindgut: enclosed after caudal fold; ends at cloacal membrane (future anus/urethra).
• Midgut: remains temporally open to yolk sac via yolk stalk.
• Allantois: endodermal diverticulum extending into connecting stalk—future urinary bladder apex.
• Cross-sectional changes
• Ectoderm (outer), mesoderm (middle; splits around coelom), endoderm (inner gut lining).
• Pharyngeal structures
• Branchial arches (mesoderm core + neural crest) flank pharynx.
• Pharyngeal pouches yield auditory tube, tonsils, thymus, parathyroids.
• Lung buds sprout ventrally from foregut.
• Liver & pancreas buds sprout caudally from foregut.

Organogenesis III (≈26−30 dpf26{-}30\ \text{dpf})

• Limb buds (paddle-shaped) emerge—upper limb usually ∼1 day\sim1\ \text{day} ahead of lower.
• Endodermal organs: parathyroid, thymus, trachea, lung buds, liver, tongue, pancreas all recognizable.
• Mesonephros (intermediate kidney) begins filtering embryonic plasma.
• Genital tubercle apparent (undifferentiated external genitalia).

Organogenesis IV (≈31−35 dpf31{-}35\ \text{dpf})

• Hand & foot “plates” form with digital rays.
• Cutaneous sensory receptors begin differentiating.
• Motor & sensory nerve processes grow into limb buds.
• Pituitary gland (adenohypophysis from oral ectoderm, neurohypophysis from diencephalon) takes shape.
• Respiratory: secondary bronchi bifurcate.
• Metanephros (definitive kidney) forms ureteric bud and nephrogenic blastema.

Organogenesis V (≈36−40 dpf36{-}40\ \text{dpf})

• Crown-rump length ≈15 mm15\ \text{mm}.
• Fine anatomy
• Fingers, toes separate; lips defined.
• Cartilaginous models for axial/appendicular bones, mandible/jaw.
• Adrenal cortex & medulla differentiated.
• Spleen from dorsal mesogastrium.
• Tertiary bronchi form.
• Gonadal ridges enlarge beneath mesonephros.

Organogenesis VI (≈41−45 dpf41{-}45\ \text{dpf})

• Length ≈20 mm20\ \text{mm}.
• Limb cartilage further condenses; joints becoming distinct.
• Special senses: olfactory nerves connect nasal epithelium ➔ forebrain, external ear hillocks visible.
• Cardiovascular: interventricular septum closes.
• Craniofacial: secondary palate shelves elevate; tooth buds in alveolar ridges.
• Primordial germ cells invade gonadal ridges.

Organogenesis VII (≈46−50 dpf46{-}50\ \text{dpf})

• Length ≈25 mm25\ \text{mm}.
• Hand & finger cartilage; muscular tissues contract (functional myotomes).
• Pineal gland evaginates from diencephalon roof.
• Interatrial septum completed (foramen ovale still patent).
• Tracheal cartilage rings appear.

Organogenesis VIII (≈51−55 dpf51{-}55\ \text{dpf})

• Length ≈35 mm35\ \text{mm}.
• Limb elongation continues; elbows, knees defined.
• Increased cutaneous sensory innervation.
• Vestibular: semicircular canals finished.
• Lymphatic system gains adult-like pattern of major vessels.
• Mesonephros regresses as metanephros assumes filtration.

Organogenesis IX (≈56−60 dpf56{-}60\ \text{dpf}) — End of Embryonic Stage

• Face acquires distinctly human profile (eyes migrate medially, nose projects, ears ascend).
• Bone ossification centers appear in many cartilaginous templates.
• Secondary palate fusion complete ➔ separates oral & nasal cavities.
• Skeletal muscles largely arranged as in adult.
• Special senses: eyelids meet & fuse; cochlear turns complete.
• External genitalia sexually dimorphic—penile/clitoral structures, labioscrotal swellings.
• Internal genitalia: uterus begins forming in females; ductus deferens, seminal vesicles in males.
• Respiratory tree branching
• Bronchiole branching reaches ≥17\ge17 levels by ≈180 dpf\approx180\ \text{dpf} (adult ≈2424 levels).

Heart Development Highlights (YouTube sequence)

• Early looping converts straight heart tube ➔ S-shaped.
• TA (Truncus Arteriosus) & TH (possibly Truncus + Bulbus “Trunco-Bulbar” region) partition into aorta & pulmonary trunk via spiral septum.
• Septation of atria & ventricles synchronized with valve formation (AV & semilunar).

Face Development Overview (YouTube sequence)

• Five facial prominences: frontonasal, 2 maxillary, 2 mandibular.
• Medial nasal processes merge ➔ philtrum, primary palate, central upper lip.
• Maxillary processes fuse with lateral nasal ➔ nasolacrimal groove closure.
• Failure of fusion ➔ cleft lip/palate (clinical note).

Digestive System Development (YouTube addendum)

• “Rotation” events
• Stomach rotates ≈90∘\approx90^\circ clockwise ➔ left vagus becomes anterior.
• Duodenum swings right & becomes secondarily retroperitoneal.
• Pancreatic buds (ventral + dorsal) fuse; head/uncinate from ventral, body/tail from dorsal.

External Genitalia Development (YouTube sequences)

• Indifferent stage: genital tubercle, urogenital folds, labioscrotal swellings.
• Male pathway (DHT dependent)
• Tubercle elongates ➔ glans penis.
• Urogenital folds fuse ➔ penile urethra.
• Labioscrotal swellings fuse ➔ scrotum.
• Female pathway (absence of DHT)
• Tubercle ➔ clitoris.
• Urogenital folds ➔ labia minora.
• Labioscrotal swellings ➔ labia majora.

Reproductive System Differentiation (Fig. 29.16/29.23)

• Gonadal ridge + mesonephric/paramesonephric ducts determine internal ducts.
• Male (testis, XYXY)
• Mesonephric (Wolffian) duct ➔ epididymis, ductus deferens, seminal vesicle.
• Paramesonephric duct regresses (Anti-Müllerian hormone).
• Female (ovary, XXXX)
• Paramesonephric (Müllerian) duct ➔ uterine tubes, uterus, upper vagina.
• Mesonephric duct regresses.
• Metanephros (definitive kidney) common to both sexes.

Numerical / Statistical References

• Somite generation rate ≈q.i.d.\text{q.i.d.} (4 pairs/day; implied but useful for staging).
• Embryo lengths used clinically in mm align with Carnegie stages.

Integrative / Clinical Correlations

• Neural-tube closure failure ➔ spina bifida, anencephaly; folate supplementation critical pre-28 dpf28\ \text{dpf}.
• Cardiac septation defects (ASD, VSD) trace to 35−50 dpf35{-}50\ \text{dpf} window.
• Limb teratogens (e.g.
thalidomide) exert peak risk during limb-bud differentiation 24−38 dpf24{-}38\ \text{dpf}.
• Palatal fusion failure ➔ cleft palate around 50−60 dpf50{-}60\ \text{dpf}.
• Sex differentiation disorders influenced by hormone exposure 50−70 dpf50{-}70\ \text{dpf}.