Respo – Embryo

✅ Correct answer: Esophageal atresia and tracheoesophageal fistula

Explanation:

The clinical scenario describes a newborn with:

• Continuous coughing and choking during feeds
• Excessive saliva and mucus
• Difficulty passing a nasogastric tube into the stomach

These are classic signs of esophageal atresia, often accompanied by a tracheoesophageal fistula (TEF). Most commonly, the proximal esophagus ends blindly, and the distal esophagus connects to the trachea, allowing air into the stomach and gastric contents to reflux into the lungs.

This condition prevents normal feeding and can lead to aspiration pneumonia if not corrected surgically.

Why the other options are incorrect:

• Hyaline membrane disease: ❌
  → Causes respiratory distress in preterm infants due to surfactant deficiency; does not cause choking or inability to pass a catheter.
• Tracheal stenosis: ❌
  → Leads to stridor or respiratory distress, but the esophagus is patent.
• Tracheal atresia: ❌
  → Rare; results in severe cyanosis and immediate respiratory failure.
• Duodenal atresia: ❌
  → Causes bilious vomiting and a “double bubble” on X-ray; feeding tube can pass into the stomach.

✅ Deficiency of surfactant due to underdeveloped alveolar type II cells — Correct.
In premature infants, alveolar type II pneumocytes are immature and produce insufficient surfactant, leading to neonatal respiratory distress syndrome (NRDS). The lack of surfactant causes high alveolar surface tension, resulting in alveolar collapse (atelectasis) and the characteristic “ground-glass” appearance on chest X-ray.

❌ Excessive production of surfactant — Would improve compliance, not cause distress.
❌ Overdevelopment of type I cells — These form the gas exchange surface, not surfactant.
❌ Increased pressure of elastic fibers — Not the cause of respiratory distress; this describes recoil mechanics, not surfactant deficiency.
❌ Deficiency due to underdeveloped type I cells — Type I cells don’t secrete surfactant; they cover the alveolar surface for gas exchange.

✅ Canalicular period — Correct.
During the canalicular period (16–25 weeks), the respiratory bronchioles and alveolar ducts begin to form. The lung tissue becomes highly vascularized, and primitive type II pneumocytes start secreting surfactant toward the end of this phase — making limited respiration possible by around 20–24 weeks.

❌ Pseudoglandular period (6–16 weeks) — Lung resembles exocrine glands; no respiratory bronchioles or alveoli yet, so gas exchange is impossible.
❌ Terminal sac period (26 weeks to birth) — Terminal sacs (primitive alveoli) develop and mature surfactant production begins.
❌ Alveolar period (birth to 8 years) — Alveoli increase in number and size postnatally.
❌ Embryonic period (4–6 weeks) — Only the laryngotracheal diverticulum and primary bronchi form; far too early for respiration.

✅ Notochord and sclerotome — Correct.
The intervertebral discs develop from two sources:

• The notochord forms the nucleus pulposus, the gelatinous core.

• The sclerotome (part of the somite derived from paraxial mesoderm) forms the annulus fibrosus, the tough outer ring.

❌ Paraxial mesoderm — Gives rise to somites, but the disc specifically arises from sclerotome + notochord, not the whole paraxial mesoderm.
❌ Neural crest cells — Contribute to peripheral nerves and ganglia, not the vertebral column.
❌ Intermediate mesoderm — Forms urogenital structures, not the spine.
❌ Somatic mesoderm — Contributes to body wall and limbs, not vertebral discs.

✅ Correct answer: Canalicular period

Explanation:

The respiratory bronchioles — the first parts of the airway involved in gas exchange — appear during the canalicular period of lung development.

This stage marks the beginning of differentiation between the conducting and respiratory portions of the lungs.

🫁 Stages of Lung Development:

• Embryonic period (4–7 weeks)
  → Formation of lung bud, trachea, and main bronchi.
• Pseudoglandular period (5–16 weeks)
  → Formation of terminal bronchioles; lungs resemble exocrine glands; no respiratory bronchioles yet, so no gas exchange possible.
• ✅ Canalicular period (16–26 weeks)
  → Formation of respiratory bronchioles and alveolar ducts. Vascularization increases. Type II pneumocytes begin to appear around week 20.
• Terminal sac period (26 weeks–birth)
  → Formation of terminal sacs (primitive alveoli). Type I and II pneumocytes develop further.
• Alveolar period (birth–8 years)
  → Mature alveoli form by septation; number increases from ~20 million at birth to ~300 million.

The diaphragm develops from four key embryological structures, each contributing to distinct parts of its anatomy and explaining its innervation and function.

1. Septum Transversum

• A thick plate of mesoderm located between the thoracic cavity and the yolk sac stalk in early development.

• It forms the central tendon of the diaphragm.

• Initially lies opposite the third to fifth cervical somites, which explains why the diaphragm later receives phrenic nerve (C3–C5) innervation.

2. Pleuroperitoneal Membranes

• Two posterolateral membranes that grow inward from the body wall.

• Fuse with the septum transversum and mesentery of the esophagus, sealing the pericardioperitoneal canals.

• Their fusion completes the partition between the thoracic and abdominal cavities.

3. Mesentery of the Esophagus

• Contributes to the crura of the diaphragm, which anchor it to the vertebral column.

• Ensures that the esophageal hiatus remains open for the esophagus and vagal trunks.

4. Myoblasts from Cervical Somites (C3–C5)

• Myogenic cells migrate into the developing diaphragm from cervical somites.

• Form the muscular part of the diaphragm.

• This migration explains the diaphragm’s phrenic nerve innervation (C3–C5) — summarized in the classic phrase:

  “C3, 4, 5 keep the diaphragm alive.”

During lung development, there are five key stages:

1. Embryonic stage (4–7 weeks)

   • Formation of the respiratory diverticulum (lung bud) from the foregut.

   • Division into bronchial buds, forming primary, secondary, and tertiary bronchi.

2. Pseudoglandular stage (5–16 weeks)

   • Terminal bronchioles form; lungs resemble exocrine glands.

   • No respiratory bronchioles or alveoli yet → no gas exchange possible.

3. Canalicular stage (16–25 weeks) ✅

   • Terminal bronchioles enlarge to form two or more respiratory bronchioles.

   • Respiratory bronchioles then divide into primordial alveolar ducts.

   • Vascularization increases greatly, and the epithelium thins, allowing potential gas exchange by the end of this stage.

   • By around 24 weeks, some type II pneumocytes begin to appear and start producing surfactant.

4. Terminal sac stage (26–birth)

   • Formation of terminal sacs (primitive alveoli).

   • Differentiation of type I and type II pneumocytes.

   • Surfactant production increases significantly.

5. Alveolar stage (36 weeks to 8 years postnatally)

   • Mature alveoli form through secondary septation.

   • Number of alveoli increases until early childhood.

❌ Why the Other Options Are Incorrect:

• 5 to 10 weeks:
  Corresponds to the pseudoglandular stage, where terminal bronchioles form but no respiratory bronchioles or alveolar ducts exist yet.

• 11 to 16 weeks:
   Still within the pseudoglandular stage — lungs appear glandular, no structures for gas exchange.

• 28 to 32 weeks:
  Falls within the terminal sac stage, when primitive alveoli form and surfactant production increases.

• 33 to 40 weeks:
  Part of the alveolar stage, characterized by alveolar maturation and expansion, not the formation of respiratory bronchioles.

✅ Correct Answer: Fertilization

Explanation:

Trisomy 21 (Down syndrome) results from nondisjunction — the failure of homologous chromosomes or sister chromatids to separate properly during meiosis.

This error occurs in the gametes (egg or sperm) before or at the time of fertilization, leading to a zygote with an extra copy of chromosome 21.

Once fertilization happens, this chromosomal abnormality is present in every cell of the developing embryo, causing the characteristic features of Down syndrome.

❌ Just before childbirth – Too late; by this time, the baby’s genetic makeup is already established.

❌ First hour after birth – No chromosomal errors occur postnatally; the disorder is genetic, not acquired.

❌ Placenta development – The placenta develops from the blastocyst after fertilization; chromosomal errors happen earlier.

❌ Organ development – Organogenesis occurs during the embryonic period, but it’s the result of genetic instructions, not their cause.

The vertebrae develop from the sclerotome portion of somites, which arise from paraxial mesoderm. During development, each sclerotome splits into a cranial (loosely packed) and caudal (densely packed) part.

The caudal half of one sclerotome fuses with the cranial half of the next — a process known as resegmentation, which allows spinal nerves to exit between vertebrae.
This migration of sclerotomal cells around the notochord and neural tube occurs around the 4th week of development, not the 6th week.

The notochord largely degenerates within the vertebral bodies but persists as the nucleus pulposus within the intervertebral discs. The mesenchymal cells remaining between vertebrae contribute to the annulus fibrosus portion of the intervertebral disc.

❌ Why the Other Options Are Incorrect

• Mesenchymal cells between sclerotomes form intervertebral discs:
  ✅ True — these form the annulus fibrosus, surrounding the nucleus pulposus.

• Notochord remnants are in the form of nucleus pulposus:
  ✅ True — the notochord persists in the center of the disc as the gelatinous core.

• Vertebrae develop from sclerotome portions of somites:
  ✅ True — sclerotomes give rise to vertebrae and ribs.

• Caudal and cranial ends of adjacent sclerotomes fuse:
  ✅ True — this resegmentation creates intersegmental vertebrae.

The respiratory diverticulum (lung bud) arises as a ventral outgrowth from the foregut during the 4th week of embryonic development.

• This marks the beginning of the lower respiratory system.

• It later branches to form the trachea, bronchi, and lung buds.

Other options:

• 5th–8th weeks → period of branching morphogenesis and formation of bronchial tree and alveoli, not initial appearance.

• 6th–7th weeks → further airway branching and differentiation

Each somite (derived from paraxial mesoderm) differentiates into three parts:

1. Dermatome → forms dermis of skin

2. Myotome → forms skeletal muscles

3. Sclerotome → forms vertebrae and ribs

🦴 Formation of Vertebrae:

• Sclerotomal mesenchyme migrates around two central structures:

  1. Notochord — central rod

  2. Neural tube — dorsal hollow structure

These migrating sclerotomal cells surround both structures in different directions:

• Around the notochord → vertebral body

• Around the neural tube → vertebral arch

• Into the body wall → ribs (in thoracic region)

🩻 Therefore:

➡️ The mesenchyme surrounding the neural tube forms the vertebral arch (including the pedicles, laminae, and spinous process).
Meanwhile, the mesenchyme surrounding the notochord forms the vertebral body.

⚖️ Why Other Options Are Incorrect:

During embryonic development, the notochord appears in the 3rd week as a midline rod of mesodermal cells that serves as a primitive axial skeleton and a signaling center for the surrounding tissues.
It plays a critical role in:

• Inducing the formation of the neural tube (future CNS).

• Defining the vertebral axis.

• Organizing somite differentiation.

🦴 Fate of the Notochord:

• Most of the notochord degenerates as the vertebral column forms around it.

• However, a small remnant persists in the center of each intervertebral disc as the nucleus pulposus — the gelatinous, shock-absorbing core that helps maintain flexibility of the spine.

❌ Why Other Options Are Incorrect:

The term for the 8th to the 16th week of respiratory system development is the Pseudoglandular stage.

During this stage:

• The major parts of the conducting airways, up to the terminal bronchioles, are formed through extensive branching.
• The lung tissue has a glandular, or duct-like, appearance, which gives this stage its name (pseudo-glandular, meaning “false gland”).
• There are no respiratory bronchioles or alveoli yet, meaning gas exchange is not possible, and a fetus born at this time would not survive.

❌Why the other options are incorrect

Lung development occurs in distinct, sequential stages, each characterized by specific structural changes and functional maturation. Let’s go through them to identify which fits this description:

1. Embryonic stage (Weeks 4–7)

• Formation of lung bud from the foregut.

• Division into primary, secondary, and tertiary bronchi.

• Establishes the bronchial tree, but no terminal bronchioles yet.

2. Pseudoglandular stage (Weeks 5–17)

• Extensive branching of the bronchial tree occurs, forming terminal bronchioles.

• The lung tissue appears gland-like (hence “pseudoglandular”) on histological slides.

• No respiratory bronchioles or alveoli are present yet — therefore, no gas exchange is possible at this stage.

• A fetus born during this period cannot survive, as the lungs are nonfunctional for respiration.

This exactly matches the histological description in the question:

“Branching had continued to form terminal bronchioles. There were no respiratory bronchioles or alveoli present.”

Hence, the correct stage is Pseudoglandular.

3. Canalicular stage (Weeks 16–25)

• Formation of respiratory bronchioles and alveolar ducts.

• Vascularization increases, and gas exchange becomes possible near the end of this stage.

• The presence of respiratory bronchioles distinguishes it from the pseudoglandular stage.

4. Terminal sac stage (Weeks 26–birth)

• Development of terminal sacs (primitive alveoli).

• Type I pneumocytes (gas exchange) and Type II pneumocytes (surfactant production) differentiate.

• Fetal viability increases as functional gas exchange begins.

5. Alveolar stage (Late fetal period to 8 years postnatally)

• Maturation and multiplication of alveoli.

• At birth, only about 20–30 million alveoli are present, increasing to about 300 million by 8 years of age.

❌ Why Other Options Are Incorrect:

• Canalicular stage: Has respiratory bronchioles and alveolar ducts; not present in this case.

• Terminal sac stage: Has terminal sacs (primitive alveoli); absent in this description.

• Alveolar stage: Involves mature alveoli and septation; much later in development.

• Postnatal: Alveolar multiplication continues after birth — beyond the findings in this question.

✅ Correct Answer:

7th week

Around the 7th week of embryonic development, the intraembryonic coelom (a continuous cavity initially extending from the thorax to the pelvis) becomes clearly divided into separate cavities.

Before this separation, it exists as a large continuous space — the primitive coelomic cavity — that connects the thoracic, abdominal, and pelvic regions.

By the 7th week, partitioning occurs through the formation of the:

• Septum transversum (forms part of the diaphragm),

• Pleuropericardial folds (separate the pleural and pericardial cavities), and

• Pleuroperitoneal membranes (separate thoracic and abdominal cavities).

Thus, the large thoracoabdominal cavity develops and begins to separate into distinct compartments around this time.

❌ Incorrect Options:

• 8th week — ❌ By this time, the diaphragm and cavity divisions are mostly complete.

• 9th week — ❌ The separation is well established before this stage.

• 10th week — ❌ Organ development occurs within the already formed cavities.

• None of them — ❌ Incorrect; the correct developmental milestone occurs during the 7th week.

✅ Correct Answer:

Neural crest cells

While neural crest cells play important roles in the development of many head and neck structures — such as craniofacial bones, cartilage, and connective tissue — they do not directly participate in forming the upper respiratory tract epithelium.

The upper respiratory tract (nasal cavity, nasopharynx, larynx) develops primarily from:

• Pharyngeal arches → contribute to skeletal and muscular structures of the larynx and pharynx.

• Pharyngeal pouches → form epithelial linings of the middle ear, tonsils, thymus, and parathyroids.

• Pharyngeal clefts → form external features like the external auditory meatus.

• Nasal placodes → give rise to the nasal pits, nasal cavities, and olfactory epithelium.

Thus, neural crest cells, although crucial for supporting tissues of the face, do not form part of the epithelial or luminal components of the upper respiratory tract.

❌ Incorrect Options:

• Pharyngeal pouches — ❌ Contribute to epithelial derivatives of the upper respiratory tract.

• Pharyngeal clefts — ❌ Help form external structures and lining related to the upper airway.

• Pharyngeal arches — ❌ Provide the framework and muscles of the larynx and pharynx.

• Nasal placodes — ❌ Directly involved in nasal cavity and olfactory epithelium formation.

✅ Correct Answer:

Nucleus pulposus

During the third week of embryonic development, the notochord forms from mesodermal cells along the midline of the trilaminar disc.
It acts as the primary axial support and provides important inductive signals for the development of the neural tube and vertebral column.

In the adult, the notochord largely disappears but persists as the nucleus pulposus — the gelatinous central core of the intervertebral disc, which provides cushioning and flexibility between vertebrae.

❌ Incorrect Options:

• Transverse process — ❌ Formed from the vertebral arch and body, derived from sclerotome, not notochord.

• Vertebral body — ❌ Develops from sclerotomal mesenchyme that surrounds the notochord, not the notochord itself.

• Vertebral arch — ❌ Derived from the dorsal part of sclerotome, not from notochord.

• Sternum — ❌ Arises from somatic mesoderm in the ventral body wall, unrelated to notochordal development.

The canalicular period (16–25 weeks) is the stage of lung development during which respiratory bronchioles and alveolar ducts begin to form.

This marks the transition from conducting to respiratory portions of the airway — meaning gas exchange becomes possible near the end of this period.

❌

Incorrect options:

• Period between 5–16 weeks / Pseudoglandular period:
  Only terminal bronchioles form — no respiratory bronchioles yet.
• Terminal sac period:
  Formation of terminal sacs (primitive alveoli), not bronchioles.
• Alveolar period:
  Formation and maturation of alveoli, not bronchioles.

✅ Correct Answer: Tracheo-esophageal fistula

Explanation:

A tracheo-esophageal fistula (TEF) is a congenital anomaly in which there is an abnormal connection between the trachea and the esophagus. It usually occurs due to incomplete separation of the foregut into respiratory and digestive tubes during embryonic development (around the 4th week).

The most common type (≈85%) involves a blind-ending upper esophageal pouch and a lower segment connected to the trachea.

Key clinical features include:

• Excessive drooling of saliva (due to inability to swallow secretions)
• Coughing, choking, and cyanosis on feeding (“three Cs”)
• Immediate regurgitation of milk
• Abdominal distension due to air entering the stomach via the fistula
• Polyhydramnios in the mother during pregnancy (since the fetus cannot swallow amniotic fluid properly)

❌ Laryngo-tracheoesophageal cleft:

A rare defect where the trachea and esophagus fail to completely separate, forming a cleft. It may cause aspiration and respiratory distress, but the symptoms are not limited to drooling and regurgitation — there’s also severe respiratory distress from birth.

❌ Congenital lung cysts:

These are developmental cystic spaces within the lung parenchyma. They present with respiratory distress, infection, or air trapping, not drooling or regurgitation after feeding.

❌ Tracheal bronchus:

An anomalous bronchus arising directly from the trachea, often asymptomatic or presenting with localized pneumonia, not feeding difficulties or regurgitation.

❌ Tracheal stenosis:

A narrowing of the trachea that causes stridor, dyspnea, or respiratory distress, but not drooling or regurgitation after feeding.

✅ Correct Answer: 24 weeks to late fetal period

Explanation:

This corresponds to the saccular stage of lung development. During this phase:

• The terminal sacs (primitive alveoli) begin to form.
• The cuboidal type II pneumocytes start differentiating into thin, flat type I pneumocytes, which form the blood-air barrier—essential for gas exchange.
• Surfactant production increases significantly, reducing surface tension and allowing the alveoli to remain open.
• The lungs become sufficiently developed for limited respiration, making fetal viability possible around 26–28 weeks.

❌ Late fetal period to one year:

This describes the alveolar stage, during which the number and size of alveoli increase postnatally. However, the blood-air barrier is already established before birth, so this phase focuses on alveolar multiplication, not initial barrier formation.

❌ 2 years to 8 years:

This is a continuation of the alveolar stage, where alveolar growth and capillary development continue, but no new structural phase of differentiation occurs.

❌ 16 to 25 weeks:

This corresponds to the canalicular stage, where bronchioles become vascularized and respiratory bronchioles form, but the cells are still cuboidal, and the blood-air barrier is not yet functional.

❌ 5 to 17 weeks:

This is the pseudoglandular stage, where the developing lungs resemble exocrine glands. The structures present are bronchi and terminal bronchioles, but no respiratory bronchioles or gas exchange structures exist yet.

As the vertebral column forms, most of the notochord degenerates in the regions where vertebral bodies develop. However, in the regions between vertebrae — where intervertebral discs form — the notochord persists as the nucleus pulposus.

The nucleus pulposus is a soft, gelatinous central portion of the intervertebral disc that allows flexibility and acts as a cushion between vertebrae.

❌ Why the Other Options Are Incorrect:

• Notochordal plate:
  This is a transient embryonic structure formed during gastrulation before the definitive notochord develops. It does not persist in adults.

• Annulus fibrosus:
  This is the tough, fibrous outer ring of the intervertebral disc, derived from sclerotome (mesoderm), not the notochord.

• Prechordal plate:
  A separate structure located anterior to the notochord; it contributes to the development of the forebrain and midface, not the spine.

• Blastula:
  Refers to an early stage of the embryo (a hollow ball of cells) before gastrulation — unrelated to the notochord’s fate.

Sclerotomes are the ventromedial portions of somites, which arise from the paraxial mesoderm on either side of the neural tube. They give rise to vertebrae and ribs and play a key role in forming the axial skeleton that supports the thoracic cavity and, indirectly, the respiratory system (since ribs and vertebrae form the framework protecting the lungs).

The somites begin forming by the third week, but sclerotome migration and vertebral development occur later, around the fourth week.

Therefore, the statement claiming that sclerotome moves around the spinal cord during the 3rd week is inaccurate, as this movement actually occurs in the 4th week.

❌ Why the Other Options Are Correct:

• Notochord is completely regressed in the vertebral region: True. The notochord disappears where vertebral bodies form, persisting only as the nucleus pulposus of intervertebral discs.

• Somites are derived from the paraxial mesoderm: True. Somites form from paraxial mesoderm and segment along the neural tube.

• They undergo process of resegmentation: True. Each vertebra forms from the fusion of the caudal half of one sclerotome and the cranial half of the next.

• Somites form the vertebrae: True. Sclerotome components of somites give rise to vertebrae and ribs.

🩺Clinical Correlation:

Improper resegmentation or migration of sclerotomes can lead to congenital vertebral anomalies such as hemivertebrae or spina bifida, which can distort the thoracic cage and compromise respiratory function in severe cases.

✅ Correct answer: Splanchnic mesoderm

Explanation:

The splanchnic (visceral) mesoderm forms the cartilaginous, muscular, and connective tissue components of the trachea, bronchi, and lungs. It surrounds the foregut endoderm and provides the supportive framework and smooth muscle for the developing respiratory system. The endoderm of the foregut, on the other hand, gives rise to the epithelial lining and glands of the trachea and lungs.

Why others are incorrect:

• ❌ Neural crest cells: Contribute to nerve ganglia and parts of the autonomic nervous system, not to the structural components of the trachea or lungs.
• ❌ Somatic mesoderm: Forms the parietal pleura, body wall, and limbs — not the internal organs’ connective tissues.
• ❌ Endoderm: Forms the epithelial lining of the respiratory tract, not the connective tissue, cartilage, or muscle.
• ❌ Ectoderm: Gives rise to the epidermis and nervous system, not respiratory structures.

✅ Correct Answer:

Endoderm of foregut

The epithelium of the lower respiratory system — including the larynx, trachea, bronchi, and alveoli — arises from the endoderm of the foregut during embryonic development.

Around the 4th week, a respiratory diverticulum (lung bud) forms as an outpouching from the ventral wall of the foregut.

• The endoderm gives rise to the epithelial lining and glands of the lower respiratory tract.

• The surrounding splanchnic mesoderm forms the cartilage, smooth muscle, and connective tissue.

❌ Incorrect Options:

• Endoderm of neural crest cells — ❌ Neural crest cells contribute to nerves and connective tissues, not respiratory epithelium.

• Endoderm of nasopharynx — ❌ This gives rise to upper respiratory structures, not the lower respiratory tract.

• Endoderm of midgut — ❌ Forms structures like the small intestine and part of the large intestine, not the lungs or trachea.

• Ectoderm — ❌ Forms the epidermis and nervous system, not the respiratory epithelium.

The lateral plate mesoderm is part of the intraembryonic mesoderm, which lies on each side of the developing embryo. During embryonic folding and the formation of the intraembryonic coelom, this mesoderm splits into two distinct layers:

🔹 1. Somatic (Parietal) Layer

• Lies adjacent to the ectoderm.

• Together with the ectoderm, it forms the somatopleure, which contributes to the body wall and limb buds.

🔹 2. Splanchnic (Visceral) Layer

• Lies adjacent to the endoderm.

• Together with the endoderm, it forms the splanchnopleure, which gives rise to the gut wall and associated visceral structures.

Between these two layers lies the intraembryonic coelom, which later forms the pericardial, pleural, and peritoneal cavities.

❌ Why the Other Options Are Incorrect:

• Single layer:
  The lateral plate mesoderm begins as a single sheet but later splits into two layers — so this is incomplete.

• Three / Four / Five layers:
  There’s no further subdivision beyond the two functional layers (somatic and splanchnic). These numbers don’t align with embryological reality.

✅ Correct Answer:

Nucleus pulposus

During the 3rd week of embryonic development, the notochord forms from mesodermal cells that migrate through the primitive node. It acts as the primary axial support of the embryo and serves as an inductive signaling center for the development of the neural tube and vertebral column.

In adults, the notochord largely disappears, but its remnants persist as the nucleus pulposus, the gelatinous core of the intervertebral disc. This central part provides cushioning and flexibility to the vertebral column.

❌ Incorrect Options:

• Transverse process — ❌ Formed from the vertebral arch and costal processes derived from sclerotome, not from the notochord.

• Vertebral arch — ❌ Develops from posterior extensions of sclerotome cells, forming the bony ring around the spinal cord.

• Vertebral body — ❌ The notochord contributes transiently to the center of vertebral bodies during formation, but it is replaced by bone, not retained.

• Sternum — ❌ Arises from somatic mesoderm (sternal bars), completely unrelated to the notochord.

The primitive body cavity (intraembryonic coelom) appears during the 3rd week as a single horseshoe-shaped cavity. Initially, it is continuous, but as development progresses, partitions form to divide it into separate cavities.

• By the 5th week, the process of partitioning is well-advanced, and the coelom becomes separated into the pericardial cavity, pleural cavities, and peritoneal cavity.

• This partitioning involves the septum transversum, pleuropericardial folds, and pleuroperitoneal membranes, which help establish the definitive cavities.

❌ Why the other options are incorrect:

• 3rd week: The intraembryonic coelom appears, but it is still a single primitive cavity, not yet divided.

• 4th week: The body folding process occurs, but cavities are still continuous.

• 6th week: Cavities are already separated by this time; the division was established earlier.

• None of these: Incorrect, because the division does occur at a definite time (5th week).

Correct Answer:
✅ Bridge of the nose

• Explanation: The frontonasal prominence contributes to the formation of the forehead, dorsum (bridge) of the nose, and the medial/lateral nasal prominences. These prominences later give rise to specific nasal structures. Thus, the bridge (dorsum) of the nose directly develops from the frontonasal prominence.

Incorrect Options:

❌ The intermaxillary segment

• Derived from the fusion of the two medial nasal prominences, not the frontonasal prominence itself.

❌ Sides of the nose

• Formed by the lateral nasal prominences, which come from the frontonasal prominence secondarily, but the direct structure is the lateral nasal prominence, not the frontonasal prominence as a whole.

❌ The nasal septum

• Formed by the medial nasal prominences, not directly from the frontonasal prominence.

❌ Tip of the nose

• Derived from the medial nasal prominence, not the entire frontonasal prominence.

• During gastrulation (which occurs in the third week of embryonic development), the mesoderm forms between the ectoderm and endoderm.

• Shortly after it forms, the paraxial mesoderm (the part closest to the midline) begins to segment into somites.

• This process — called segmentation or somitogenesis — starts near the cranial end and progresses caudally.

🧬 Timeline:

• Week 3: Mesoderm forms and starts segmenting into somites.

• By end of week 5: About 42–44 pairs of somites have formed.

❌ 7th week

• By this stage, organogenesis is well underway, limb buds are forming, and many somites are already developed. Segmentation started much earlier.

❌ 6th week

• Neural tube is closed, upper limbs show hand plates, and intestines begin herniation — segmentation is already long established.

❌ 4th week

• By the 4th week, segmentation is well advanced, and somites are visible as paired blocks along the neural tube. But the process began earlier in week 3.

❌ 5th week

• Growth and differentiation of structures continues; again, segmentation is not starting here — it has already occurred.

✅ Correct Answer:

Week 16 – Week 26

• The canalicular stage of lung development lasts from 16 to 26 weeks of gestation.
• Key features:
  • Formation of respiratory bronchioles and early alveolar ducts.
  • Lumen of airways becomes larger (“canalized”).
  • Vascularization increases, bringing capillaries close to developing epithelium.
  • By the end of this stage, limited gas exchange becomes possible, so survival outside the womb (though poor) may begin around week 24.

❌ Week 36 – Week 40

• This corresponds to the alveolar stage, when true alveoli form and mature until after birth.

❌ Week 26 – Week 36

• This is the saccular stage, characterized by terminal sacs forming and surfactant production increasing.

❌ Week 6 – Week 16

• This is the pseudoglandular stage, when the lung looks like an exocrine gland and only conducting airways are formed (no gas exchange possible).

❌ Week 4 – Week 8

• This is the embryonic stage, when the lung bud branches into primary, secondary, and tertiary bronchi.

Correct Answer:
✅ Nucleus pulposus

• Explanation: The notochord is a midline structure formed during the 3rd week of development, important for signaling and inducing the development of the neural tube and axial skeleton. In adults, most of the notochord disappears, but it persists as the nucleus pulposus of the intervertebral disc — the gelatinous central portion that acts as a shock absorber between vertebrae.

Incorrect Options:

❌ Transverse process

• This is derived from the vertebral arch and mesenchymal condensations, not the notochord.

❌ Sternum

• Develops from somatic mesoderm, not the notochord.

❌ Vertebral arch

• Derived from the sclerotome of somites, not from the notochord.

❌ Vertebral body

• The vertebral body develops around the notochord from the sclerotome, but the notochord itself regresses here and does not persist.