Embryology and Development — MCQs

Embryology and Development Indian Medical PG Practice Questions and MCQs

Question 781: Anal opening is formed in which week of embryonic development?

A. 5th
B. 7th (Correct Answer)
C. 10th
D. None of the above

Explanation: The development of the anal opening is a critical event in the partitioning of the hindgut. During the **7th week** of embryonic development, the **urorectal septum** (a wedge of mesoderm) grows caudally to fuse with the **cloacal membrane**. This fusion divides the cloaca into two distinct parts: the primitive urogenital sinus (anteriorly) and the anorectal canal (posteriorly). Simultaneously, the cloacal membrane is divided into the urogenital membrane and the **anal membrane**. By the end of the 7th week, the anal membrane ruptures, creating a patent communication between the rectum and the exterior, thus forming the **anal opening** [1]. **Analysis of Options:** * **Option A (5th week):** During this stage, the urorectal septum is just beginning to divide the cloaca; the membranes are still intact and the openings are not yet formed. * **Option B (7th week):** **Correct.** This is the definitive period when the anal membrane ruptures to establish the anal orifice [1]. * **Option C (10th week):** By this time, the gastrointestinal tract is undergoing rotation and physiological herniation return; the anal opening is already well-established. **High-Yield Clinical Pearls for NEET-PG:** * **Dual Origin of Anal Canal:** The upper 2/3rd (above the pectinate line) is derived from **endoderm** (hindgut), while the lower 1/3rd (below the pectinate line) is derived from **ectoderm** (proctodeum). * **Imperforate Anus:** This clinical condition occurs if the anal membrane fails to perforate during the 7th-8th week. * **Pectinate Line:** This represents the site of the former anal membrane and serves as a landmark for differences in blood supply, nerve supply, and lymphatic drainage.

Question 782: By which day after fertilization is placental circulation established?

A. 11th day
B. 13th day
C. 15th day
D. 17th day (Correct Answer)

Explanation: ### Explanation The establishment of placental circulation is a multi-step process involving the development of the chorionic villi and the fetal cardiovascular system. **Why 17th Day is Correct:** By the **17th day** after fertilization, the fetal blood vessels (derived from the extraembryonic mesoderm) within the **tertiary villi** become functional [1]. These vessels connect with the developing embryonic heart tube. At this stage, the fetal blood begins to circulate through the capillaries of the villi, and the **placental (fetoplacental) circulation** is officially established [1]. This allows for the exchange of nutrients and gases between the maternal blood (in the intervillous spaces) and fetal blood. **Analysis of Incorrect Options:** * **11th Day:** This marks the formation of **lacunae** within the syncytiotrophoblast [1]. While maternal blood begins to flow into these lacunae (primordial uteroplacental circulation), the fetal component is not yet developed. * **13th Day:** This is characterized by the appearance of **primary villi** (proliferations of cytotrophoblast covered by syncytiotrophoblast) [1]. No blood vessels are present in the villi yet. * **15th Day:** This corresponds to the formation of **secondary villi**, where extraembryonic mesoderm invades the core of the primary villi. However, these mesenchymal cells have not yet differentiated into blood vessels. **NEET-PG High-Yield Pearls:** * **Primary Villi:** Trophoblast only (Day 13) [1]. * **Secondary Villi:** Trophoblast + Mesoderm core (Day 15). * **Tertiary Villi:** Trophoblast + Mesoderm + **Capillaries** (Day 17-21). * **Heart Beat:** The embryonic heart starts beating around **Day 21-22**, which is when the circulation becomes fully effective. * **Uteroplacental vs. Fetoplacental:** Maternal blood enters lacunae by Day 11-12 (Uteroplacental), but the fetal circuit (Fetoplacental) requires tertiary villi (Day 17) [1].

Question 783: Muscles of facial expression develop from which embryonic arch?

A. First Arch
B. Second Arch (Correct Answer)
C. Third Arch
D. Fourth Arch

Explanation: The muscles of facial expression develop from the **mesoderm of the Second Pharyngeal Arch** (also known as the Hyoid Arch). In embryology, each pharyngeal arch is associated with a specific cranial nerve that supplies all the structures derived from that arch. The nerve of the second arch is the **Facial Nerve (CN VII)**; therefore, all muscles supplied by the facial nerve—including the muscles of facial expression, stapedius, stylohyoid, and the posterior belly of the digastric—originate from this arch. **Analysis of Options:** * **First Arch (Mandibular Arch):** Associated with the **Trigeminal Nerve (V3)**. It gives rise to the muscles of mastication (tensor tympani, tensor veli palatini, mylohyoid, and anterior belly of digastric). * **Third Arch:** Associated with the **Glossopharyngeal Nerve (CN IX)**. It gives rise to a single muscle: the **Stylopharyngeus**. * **Fourth Arch:** Associated with the **Superior Laryngeal branch of the Vagus Nerve (CN X)**. It gives rise to the cricothyroid muscle and the levator veli palatini. **High-Yield Clinical Pearls for NEET-PG:** * **Skeletal Derivatives:** The second arch also forms the stapes, styloid process, stylohyoid ligament, and the lesser cornu (and upper body) of the hyoid bone. * **Golden Rule:** If you know the nerve supply of a muscle, you can deduce its arch of origin. * CN V = 1st Arch * CN VII = 2nd Arch * CN IX = 3rd Arch * CN X = 4th & 6th Arches * **Treacher Collins Syndrome:** Results from the failure of first arch neural crest cells to migrate, leading to mandibular hypoplasia and facial abnormalities.

Question 784: The vitelline vein develops into which of the following vascular structures?

A. Hepatic vein (Correct Answer)
B. Azygos vein
C. Inferior mesenteric vein
D. Splenic vein

Explanation: **Explanation:** The vitelline veins (omphalomesenteric veins) are responsible for carrying blood from the yolk sac to the sinus venosus. As the liver develops within the septum transversum, the vitelline veins form a complex anastomotic network around the duodenum [1], which eventually gives rise to: 1. **Hepatic Sinusoids:** Formed by the fragmentation of the veins by the growing liver cords [1]. 2. **Hepatic Veins:** Derived from the proximal portion of the right vitelline vein [4]. 3. **Portal Vein:** Formed from the anastomotic network around the duodenum [3]. 4. **Superior Mesenteric Vein:** Derived from the distal part of the right vitelline vein [3]. **Analysis of Incorrect Options:** * **B. Azygos vein:** This is derived from the **Right Supracardinal vein** (part of the cardinal venous system), which drains the body wall. * **C. Inferior mesenteric vein:** This develops from the **subcardinal/cardinal system** and vitelline tributaries, but it is not a direct derivative of the vitelline trunk itself in the same way the hepatic/portal systems are. * **D. Splenic vein:** This develops from the dorsal mesentery veins and joins the portal system; it is not a primary derivative of the vitelline veins [3]. **NEET-PG High-Yield Pearls:** * **Left Vitelline Vein:** Mostly regresses; the persistent part contributes to the hepatic sinusoids. * **Right Vitelline Vein:** Forms the **Inferior Vena Cava (Hepatic segment)**, the Portal vein, and the Superior Mesenteric vein [2]. * **Umbilical Veins:** The **Right** umbilical vein disappears completely; the **Left** umbilical vein persists to carry oxygenated blood from the placenta to the liver [2] (becoming the *Ligamentum teres* after birth).

Question 785: The primitive streak is initiated and maintained by which factor?

A. Nodal gene (Correct Answer)
B. BMP4
C. FGF
D. Brachyury gene

Explanation: **Explanation:** The formation of the primitive streak marks the beginning of **gastrulation** (the conversion of the bilaminar disc into a trilaminar disc) during the third week of development. [1] **1. Why Nodal gene is correct:** The primitive streak is initiated and maintained by the expression of the **Nodal gene**, a member of the Transforming Growth Factor-β (TGF-β) family. Nodal is secreted by cells in the primitive node and the posterior epiblast. It acts as a key signaling molecule that induces the epiblast cells to proliferate and migrate toward the midline, establishing the cranio-caudal axis and the streak itself. **2. Why the other options are incorrect:** * **BMP4 (Bone Morphogenetic Protein 4):** In the presence of FGF, BMP4 ventralizes the mesoderm into intermediate and lateral plate mesoderm. It is not the initiator of the streak. * **FGF (Fibroblast Growth Factor):** While FGF8 is crucial for the migration of cells *through* the streak (by downregulating E-cadherin), it is not the primary factor responsible for the initiation and maintenance of the streak itself. * **Brachyury (T) gene:** This gene is essential for the formation of the **notochord** and the specification of dorsal mesoderm. It is expressed *after* the streak is established. **Clinical Pearls for NEET-PG:** * **Site:** The primitive streak forms at the **caudal end** of the epiblast. * **Fate:** If the primitive streak fails to regress and persists in the sacrococcygeal region, it leads to a **Sacrococcygeal Teratoma** (the most common tumor in newborns). * **Situs Inversus:** Disruption of Nodal signaling or cilia movement at the primitive node can lead to defects in left-right asymmetry.

Question 786: Which of the following statements is false regarding the notochord?

A. Prenotochordal cells intercalate with the hypoblast.
B. The cranial end forms last. (Correct Answer)
C. The neurenteric canal communicates the amniotic and umbilical vesicle cavities.
D. It extends from the oropharyngeal membrane to the primitive node.

Explanation: ### Explanation The notochord is a cellular rod that defines the primordial longitudinal axis of the embryo and serves as the basis for the development of the axial skeleton. **Why Option B is False (The Correct Answer):** The development of the notochord follows a **cranio-caudal** sequence. Prenotochordal cells migrate through the primitive pit and move cranially until they reach the prechordal plate. Therefore, the **cranial end forms first**, and the caudal end forms last as the primitive streak regresses. **Analysis of Other Options:** * **Option A:** During development, prenotochordal cells invaginate and intercalate into the **hypoblast** to form the temporary notochordal plate. * **Option C:** The **neurenteric canal** is a temporary communication between the amniotic cavity and the yolk sac (umbilical vesicle) at the site of the primitive pit. * **Option D:** The definitive notochord is a solid rod of cells extending from the **oropharyngeal membrane** (cranially) to the **primitive node** (caudally). **High-Yield NEET-PG Pearls:** 1. **Inductive Role:** The notochord is the primary inducer of the overlying ectoderm to differentiate into the **neural plate** (neuroectoderm). 2. **Adult Remnant:** In adults, the notochord disappears except for the **nucleus pulposus** of the intervertebral discs. 3. **Clinical Correlation:** **Chordoma** is a rare, slow-growing malignant tumor that arises from remnants of the notochord, most commonly found in the cranial (clivus) or sacrococcygeal regions. 4. **Molecular Marker:** **Sonic Hedgehog (SHH)** is the key signaling molecule secreted by the notochord for ventral patterning of the neural tube.

Question 787: What is the probable diagnosis for a cyst in a child that is located at and associated with vertebral defects?

A. Myelocele
B. Bronchogenic cyst
C. Neuroenteric cyst (Correct Answer)
D. Neuroblastoma

Explanation: ### Explanation **Correct Answer: C. Neuroenteric cyst** **Mechanism and Pathophysiology:** A **neuroenteric cyst** (also known as a gastrocystoma) results from the failure of the **notochordal canal** to separate completely from the primitive foregut during the 3rd week of development. Normally, the neurenteric canal is a temporary communication between the amnion and the yolk sac. Persistence of this canal leads to a cyst lined by endoderm-derived epithelium (respiratory or gastrointestinal) located within the spinal canal or mediastinum. Because the notochord serves as the induction template for the vertebral column, its abnormal development invariably leads to **vertebral defects** (e.g., hemivertebrae, butterfly vertebrae, or clefts). **Analysis of Incorrect Options:** * **A. Myelocele:** This is a form of open neural tube defect (spina bifida aperta) where the neural plate is exposed [1]. While associated with vertebral arch defects, it presents as a flat, raw area rather than a discrete internal cyst and is usually diagnosed at birth [1]. * **B. Bronchogenic cyst:** These are foregut duplication cysts caused by abnormal budding of the tracheal bud. While they occur in the mediastinum, they are **not** typically associated with vertebral anomalies. * **D. Neuroblastoma:** This is a solid malignant tumor derived from neural crest cells (sympathetic chain). While it can cause vertebral erosion or "dumbbell" extensions into the spinal canal, it is a solid mass, not a developmental cyst associated with primary vertebral malformations. **NEET-PG High-Yield Pearls:** * **Classic Triad:** Mediastinal mass + Intraspinal mass + Vertebral anomalies (like hemivertebrae). * **Lining:** These cysts are uniquely lined by **mucus-secreting epithelium** (gastric or intestinal). * **Location:** Most commonly found in the **posterior mediastinum** and the cervical/upper thoracic spinal cord. * **Key Embryological Structure:** Persistent **Kovalevsky’s canal** (neurenteric canal).

Question 788: The epiglottis is derived from which pharyngeal arch?

A. 1st arch
B. 2nd arch
C. 3rd arch
D. 4th arch (Correct Answer)

Explanation: **Explanation:** The development of the larynx and epiglottis is a high-yield topic in embryology. The respiratory system begins as an outgrowth from the ventral wall of the foregut. The internal lining of the larynx originates from endoderm, but the cartilages and muscles develop from the **mesenchyme of the pharyngeal arches.** **Why the 4th Arch is Correct:** The epiglottis specifically develops from the **hypobranchial eminence** (also known as the copula). While the anterior part of this eminence (formed by the 3rd arch) contributes to the tongue, the **posterior part** of the hypobranchial eminence, derived from the **4th pharyngeal arch**, gives rise to the epiglottis. **Analysis of Incorrect Options:** * **1st Arch (Mandibular):** Gives rise to the muscles of mastication, malleus, incus, and the anterior two-thirds of the tongue. * **2nd Arch (Hyoid):** Gives rise to the muscles of facial expression, stapes, and the styloid process. * **3rd Arch:** Contributes to the posterior one-third of the tongue and the stylopharyngeus muscle. It does not form laryngeal cartilages. **Clinical Pearls & High-Yield Facts:** * **Laryngeal Cartilages:** The thyroid, cricoid, and arytenoid cartilages are derived from the **4th and 6th arches**. * **Nerve Supply:** Because the epiglottis is derived from the 4th arch, its sensory innervation is provided by the **superior laryngeal nerve** (a branch of the Vagus nerve, which is the nerve of the 4th arch). * **Laryngotracheal Diverticulum:** The opening of this diverticulum into the pharynx is the primordial laryngeal orifice, bounded by the epiglottis and arytenoid swellings.

Question 789: Ciliary muscles develop from which germ layer/cell type?

A. Surface ectoderm
B. Neural crest cells (Correct Answer)
C. Mesoderm
D. Neuroectoderm

Explanation: ### Explanation The development of the eye is a complex process involving multiple embryological origins. The correct answer is **Neural Crest Cells (NCCs)**. **1. Why Neural Crest Cells are correct:** While most muscles in the body are mesodermal in origin, the intraocular muscles are unique exceptions. The **ciliary muscle** and the **ciliary stroma** develop from the mesenchymal condensation of neural crest cells surrounding the optic cup. These cells migrate to the region of the future ciliary body to differentiate into smooth muscle fibers [1]. **2. Analysis of Incorrect Options:** * **Surface Ectoderm:** This layer gives rise to the **lens**, the corneal epithelium, and the lacrimal apparatus. * **Mesoderm:** In the eye, mesoderm contributes primarily to the **extraocular muscles** (derived from pre-otic somites) and the vascular endothelium [2]. It does *not* form the ciliary muscle. * **Neuroectoderm:** This layer forms the retina, the posterior layers of the iris, and the optic nerve. Crucially, it also gives rise to the **sphincter and dilator pupillae** muscles—making them the only muscles in the body derived from the nervous system. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "Rule of Two" for Intraocular Muscles:** * Ciliary Muscle = **Neural Crest Cells** * Iris Muscles (Sphincter/Dilator) = **Neuroectoderm** * **Sclera & Choroid:** Also derived from Neural Crest Cells. * **Vitreous:** Dual origin (Mesenchyme and Neuroectoderm). * **Key Concept:** If a question asks about the *connective tissue* or *stroma* of the eye, the answer is almost always Neural Crest Cells [1].

Question 790: In which of the following conditions is a Barr body absent?

A. Klinefelter syndrome
B. Turner syndrome (Correct Answer)
C. Super female syndrome (Triple X syndrome)
D. None of the above

Explanation: ### Explanation The **Barr body** (sex chromatin) represents an inactivated X chromosome. According to the **Lyon Hypothesis**, in individuals with more than one X chromosome, all except one are randomly inactivated during early embryonic development to ensure dosage compensation [1]. The number of Barr bodies is calculated using the formula: **Number of Barr bodies = (Total number of X chromosomes) – 1**. **1. Why Turner Syndrome is the correct answer:** In **Turner syndrome (45, XO)**, there is only one X chromosome present [2]. Applying the formula (1 – 1 = 0), there are no extra X chromosomes to inactivate. Therefore, a Barr body is **absent**. This is the only condition among the options where the individual is chromatin-negative. **2. Analysis of Incorrect Options:** * **Klinefelter Syndrome (47, XXY):** Despite being phenotypically male due to the Y chromosome, these individuals have two X chromosomes [3]. One X chromosome undergoes inactivation, resulting in **one Barr body**. * **Super Female Syndrome (47, XXX):** These individuals have three X chromosomes [3]. Following the formula (3 – 1 = 2), they possess **two Barr bodies**. **3. NEET-PG High-Yield Pearls:** * **Sample Source:** Barr bodies are most commonly studied using a **Buccal Smear** (squamous epithelial cells) or by looking for **"Drumsticks"** in polymorphonuclear leukocytes (neutrophils). * **Mary Lyon’s Principle:** Inactivation occurs around the 16th day of embryonic life (blastocyst stage). * **Clinical Correlation:** Barr body testing was historically used for quick sex determination, but definitive diagnosis now requires **Karyotyping** [2]. * **Rule of Thumb:** If the X chromosome count is $n$, the Barr body count is $n-1$.

Practice by Chapter

Gametogenesis and Fertilization

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Early Embryonic Development

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Placentation

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Development of Nervous System

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Development of Cardiovascular System

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Development of Gastrointestinal System

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Development of Urogenital System

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Development of Musculoskeletal System

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Development of Head and Neck

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Congenital Anomalies

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Teratology

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Molecular Mechanisms in Development

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