Embryology and Development — MCQs

Embryology and Development Indian Medical PG Practice Questions and MCQs

Question 441: The gut tube develops from which germ layer?

A. Ectoderm
B. Mesoderm
C. Endoderm (Correct Answer)
D. Neuroectoderm

Explanation: **Explanation:** The development of the gastrointestinal system is a high-yield topic in embryology. The correct answer is **Endoderm** because the epithelial lining of the entire digestive tract—from the pharynx to the upper two-thirds of the anal canal—is derived from the primitive gut tube [1], which forms from the dorsal part of the yolk sac lined by endoderm. * **Why Endoderm is correct:** While the gut tube is a composite structure, the **parenchyma** (lining and glands) of the digestive system, including the liver and pancreas, originates solely from the endoderm [2]. * **Why Ectoderm is incorrect:** Ectoderm gives rise to the epidermis and the nervous system. In the GI tract, it only contributes to the extreme ends: the epithelium of the mouth (stomodaeum) and the lower one-third of the anal canal (proctodeum). * **Why Mesoderm is incorrect:** Mesoderm does contribute to the gut, but it forms the **stroma** (connective tissue, muscular layers, and visceral peritoneum), not the gut tube lining itself [2]. * **Why Neuroectoderm is incorrect:** This specialized layer forms the brain, spinal cord, and neural crest cells. While neural crest cells migrate to the gut to form the enteric nervous system (Auerbach’s and Meissner’s plexuses), they do not form the tube itself. **Clinical Pearls for NEET-PG:** 1. **Dual Origin of the Anal Canal:** The junction between the endoderm (hindgut) and ectoderm (proctodeum) is the **Pectinate Line**. This is a landmark for different blood supply, nerve supply, and lymphatic drainage. 2. **Derivatives:** * **Foregut:** Pharynx to proximal duodenum (Celiac trunk) [1]. * **Midgut:** Distal duodenum to proximal 2/3 of transverse colon (Superior Mesenteric artery) [1]. * **Hindgut:** Distal 1/3 of transverse colon to upper anal canal (Inferior Mesenteric artery) [1]. 3. **Hirschsprung Disease:** Results from the failure of neural crest cells to migrate into the distal colon, leading to an aganglionic segment.

Question 442: Rohr's stria found in the placenta are due to:

A. Deposition of fibrin (Correct Answer)
B. Blood vessels
C. Langhan's cells accumulation
D. Hofbauer cells accumulation

Explanation: **Explanation:** **Rohr’s stria** refers to a layer of **fibrinoid deposition** found at the bottom of the intervillous space, specifically on the maternal side of the placenta [2]. It is located within the **decidua basalis**, just superficial to the basal plate. 1. **Why Option A is Correct:** During placental development, fibrinoid material (a mixture of fibrin, placental proteins, and degenerating cells) accumulates at the feto-maternal interface [2]. There are two major fibrinoid layers: * **Rohr’s Stria:** Located in the intervillous space, covering the basal plate [2]. * **Nitabuch’s Layer:** A deeper layer of fibrinoid degeneration between the trophoblast and the decidua. It is clinically significant as it prevents overly deep penetration of the placenta into the myometrium. 2. **Why Other Options are Incorrect:** * **Option B (Blood vessels):** While the placenta is highly vascular, Rohr’s stria is an acellular, proteinaceous deposit, not a vascular structure [2]. * **Option C (Langhan’s cells):** Also known as cytotrophoblasts, these are the inner layer of the trophoblast. Their accumulation does not form these specific striae [1]. * **Option D (Hofbauer cells):** These are specialized macrophages found in the stroma of the chorionic villi [1]. They serve an immunological function and are not involved in fibrinoid layer formation. **High-Yield Clinical Pearls for NEET-PG:** * **Nitabuch’s Layer:** Absence of this layer is associated with **Placenta Accreta**, where the placenta adheres directly to the myometrium. * **Location Tip:** Remember **"R"** for Rohr’s is more **"R"**elated to the intervillous space (superficial), while Nitabuch’s is deeper. * **Composition:** Both layers consist of **Fibrinoid** (Type X collagen, laminin, and fibronectin).

Question 443: What is the approximate number of primordial follicles in an ovary at birth?

A. 2 million (Correct Answer)
B. 6 million
C. 5 million
D. 10 million

Explanation: ### Explanation The correct answer is **A. 2 million**. Oogenesis is a finite process that begins during fetal life and involves a significant reduction in the germ cell population through atresia before birth [1]. **1. Why Option A is Correct:** During fetal development, oogonia undergo mitosis to reach a peak population of approximately **6–7 million** by the 5th month of gestation [1]. Following this peak, a massive process of programmed cell death (atresia) begins. By the time a female infant is born, the number of primordial follicles has decreased to approximately **2 million** in both ovaries combined [1]. **2. Why Other Options are Incorrect:** * **Option B (6 million):** This represents the peak number of oogonia present at the **20th week (5th month)** of intrauterine life, not at birth. * **Option C (5 million):** This is an intermediate number during the decline from the 5th-month peak but does not represent the standard clinical value for the birth population. * **Option D (10 million):** This exceeds the maximum number of germ cells ever recorded during human oogenesis. **3. High-Yield Facts for NEET-PG:** * **At Puberty:** The number of follicles further depletes to approximately **300,000 to 400,000** [1]. * **Reproductive Life:** Only about **400–500** follicles will actually ovulate during a woman's lifetime [1]. * **Meiotic Arrest:** Primordial follicles are arrested in the **Diplotene stage of Prophase I** (also known as the Dictyate stage) until puberty [1]. * **Clinical Correlation:** The rapid decline in follicle count (atresia) is a natural process; however, an accelerated loss before age 40 leads to **Premature Ovarian Insufficiency (POI)**.

Question 444: Which layer of the placenta disappears at the end of pregnancy?

A. Decidua parietalis
B. Decidua capsularis
C. Decidua basalis (Correct Answer)
D. Chorion frondosum

Explanation: The correct answer is **Decidua basalis**. This question focuses on the fate of the maternal components of the placenta during the progression of pregnancy. **1. Why Decidua Basalis is correct:** The decidua is the modified endometrium of pregnancy [2]. It is divided into three parts based on location. The **decidua basalis** lies deep to the conceptus and forms the maternal component of the placenta [4]. As pregnancy reaches full term, the decidua basalis undergoes significant thinning and degenerative changes [1]. By the end of pregnancy, it largely disappears or becomes incorporated into the placental membranes, allowing for the eventual separation of the placenta during the third stage of labor. **2. Why the other options are incorrect:** * **Decidua capsularis:** This layer covers the abembryonic pole. As the fetus grows, it expands and eventually fuses with the decidua parietalis around the 20th week, obliterating the uterine cavity [3]. It disappears much earlier than the end of pregnancy. * **Decidua parietalis:** This is the remaining lining of the uterus. It does not disappear; it fuses with the chorion laeve and decidua capsularis to form the fetal membranes [3]. * **Chorion frondosum:** This is the "bushy" fetal portion of the placenta containing the villi [4]. It persists and functions until birth as the primary site of nutrient exchange. **High-Yield Clinical Pearls for NEET-PG:** * **Nitabuch’s Layer:** A zone of fibrinoid degeneration between the trophoblast and decidua basalis that prevents over-invasion of the placenta. * **Placental Membrane (Barrier):** Initially consists of four layers. After the 4th month, the cytotrophoblast and connective tissue disappear to facilitate faster diffusion [1]. * **Decidua Reaction:** The transformation of endometrial stromal cells into large, lipid/glycogen-filled cells, a key histological marker of pregnancy [2].

Question 445: Hypoblast forms what structure?

A. Notochord
B. Apical ligament of atlas
C. Endoderm
D. Prochordal plate (Correct Answer)

Explanation: ### Explanation The **hypoblast** (primitive endoderm) is the ventral layer of the bilaminar germ disc [1]. At the beginning of the third week of development, the hypoblastic cells in the cephalic (cranial) region become columnar and firmly adhere to the overlying epiblast. This localized thickening is known as the **Prochordal Plate** (or Prechordal Plate). **Why the correct answer is right:** * **Prochordal Plate:** It is the first sign of the cranio-caudal axis and marks the future site of the mouth (buccopharyngeal membrane). It also serves as an important signaling center for organizing the forebrain. **Analysis of Incorrect Options:** * **A. Notochord:** This is derived from **epiblast** cells that invaginate through the primitive node (mesodermal origin). * **B. Apical ligament of atlas:** This is a remnant of the **notochord**, which, as stated above, is derived from the epiblast/mesoderm, not the hypoblast. * **C. Endoderm:** While the hypoblast was historically thought to form the definitive endoderm, we now know that the **definitive endoderm** is formed by epiblast cells that migrate through the primitive streak and displace the hypoblast [1]. The hypoblast primarily contributes to the extraembryonic mesoderm and the yolk sac lining. **High-Yield Facts for NEET-PG:** * **Fate of Hypoblast:** It forms the **Heuser’s membrane** (lining the primary yolk sac) and the **prochordal plate** [1]. * **The "Rule of 2s" (2nd Week):** Trophoblast differentiates into 2 layers (Syncytio & Cytotrophoblast); Embryoblast into 2 layers (Epiblast & Hypoblast); 2 cavities form (Amniotic & Yolk sac) [1]. * **Clinical Pearl:** The prochordal plate is essential for head development; defects in this area can lead to severe holoprosencephaly or facial midline defects.

Question 446: The embryonic development of the vestibule of the vagina is from which of the following developmental structures?

A. Urogenital sinus (Correct Answer)
B. Genital ridge
C. Wolffian duct
D. Mullerian duct

Explanation: **Explanation:** The **vestibule of the vagina** is the cleft between the labia minora into which the urethra and vagina open [1]. Its development is tied to the **Urogenital Sinus (UGS)**, specifically the lower, phallic part. 1. **Why Urogenital Sinus is correct:** During the 6th week of development, the cloaca is divided by the urorectal septum into the anorectal canal and the urogenital sinus. In females, the **distal (phallic) part** of the UGS expands to form the vestibule. While the upper 2/3rd of the vagina is derived from the fused Mullerian ducts, the lower 1/3rd (and the vestibule) arises from the UGS via the formation of the sinovaginal bulbs [2]. 2. **Why other options are incorrect:** * **Genital ridge:** This is the precursor to the **gonads** (testes or ovaries), formed by the proliferation of intermediate mesoderm and overlying epithelium. * **Wolffian (Mesonephric) duct:** In females, these ducts largely regress due to the absence of testosterone. Remnants may persist as **Gartner’s cysts**, Epoophoron, or Paroophoron [3]. * **Mullerian (Paramesonephric) duct:** These form the **Fallopian tubes, uterus, and the upper 2/3rd of the vagina** [2]. They do not contribute to the external genitalia or the vestibule. **High-Yield NEET-PG Pearls:** * **Hymen:** Formed at the junction where the sinovaginal bulbs (UGS) meet the Mullerian tubercle [2]. * **Skene’s glands:** Female homologue of the prostate; derived from the UGS [1]. * **Bartholin’s glands:** Female homologue of Cowper’s (bulbourethral) glands; derived from the UGS [1]. * **Dual Origin of Vagina:** Remember the **2/3 - 1/3 rule**: Upper 2/3 (Mullerian) vs. Lower 1/3 (UGS).

Question 447: A harelip is due to the non-fusion of which structures during embryonic development?

A. Medial nasal process with lateral nasal process
B. Lateral nasal process with maxillary process
C. Medial nasal process with maxillary process (Correct Answer)
D. Lateral nasal process with palatal process

Explanation: ### Explanation **Concept Overview:** The development of the face occurs between the 4th and 8th weeks of gestation through the fusion of five facial primordia: the single frontonasal process, two maxillary processes, and two mandibular processes. **Why Option C is Correct:** A **harelip (cleft lip)** occurs due to the failure of fusion between the **medial nasal process** (a derivative of the frontonasal process) and the **maxillary process** [1]. * The medial nasal processes fuse in the midline to form the **intermaxillary segment**, which gives rise to the philtrum of the upper lip, the premaxilla (carrying the four incisors), and the primary palate. * The maxillary processes grow medially to fuse with the intermaxillary segment [1]. Failure of this fusion on one or both sides results in a unilateral or bilateral cleft lip [1]. **Analysis of Incorrect Options:** * **Option A:** Fusion of the medial nasal processes with each other forms the bridge of the nose and the philtrum. Failure here leads to a rare **median cleft lip**. * **Option B:** Failure of fusion between the lateral nasal process and the maxillary process results in an **oblique facial cleft**, exposing the nasolacrimal duct. * **Option D:** The palatal processes (palatine shelves) are internal extensions of the maxillary processes. Their failure to fuse with each other or the primary palate results in a **cleft palate**, not a cleft lip [1]. **High-Yield Clinical Pearls for NEET-PG:** * **Unilateral Cleft Lip:** Most common congenital facial abnormality [1]. * **Cleft Palate:** Failure of fusion of **lateral palatine shelves** with each other or with the **primary palate** (median palatine process). * **Macrostomia:** Failure of fusion between the maxillary and mandibular processes. * **Rule of 10s for Surgery:** Cleft lip repair (Cheiloplasty) is usually done at 10 weeks of age, 10 lbs weight, and 10 g/dL hemoglobin.

Question 448: The sinus venosus receives blood from all except:

A. Vitelline vein
B. Umbilical vein
C. Common cardinal vein
D. Subcardinal vein (Correct Answer)

Explanation: In the middle of the 4th week of development, the **sinus venosus** (the venous end of the heart tube) consists of a central body and two lateral extensions called **sinus horns**. Each sinus horn receives blood from three major paired veins: 1. **Vitelline Veins:** Carry oxygen-poor blood from the yolk sac. 2. **Umbilical Veins:** Carry oxygenated blood from the placenta [1]. 3. **Common Cardinal Veins:** Formed by the union of the anterior and posterior cardinal veins; they drain the body of the embryo proper. **Why Option D is Correct:** The **Subcardinal veins** are part of a complex system (along with supracardinal and sacrocardinal veins) that develops later to form the **Inferior Vena Cava (IVC)**. They do not drain directly into the sinus venosus; instead, they drain into the posterior cardinal veins. Therefore, they are not primary tributaries of the sinus horns. **Why Other Options are Incorrect:** * **A & B:** The vitelline and umbilical veins are the primary extracardiac vessels entering the sinus venosus during early development [1]. * **C:** The common cardinal vein is the primary systemic vessel entering the sinus venosus before the definitive venae cavae are formed. **High-Yield Facts for NEET-PG:** * **Fate of Sinus Venosus:** The right horn is incorporated into the right atrium to form the **smooth part (Sinus Venarum)**. The left horn regresses to form the **Coronary Sinus** and the **Oblique vein of the left atrium (of Marshall)**. * **Sinoatrial Orifice:** Guarded by right and left venous valves. The right valve forms the **Crista Terminalis**, the valve of the IVC (Eustachian), and the valve of the coronary sinus (Thebesian) [1]. * **Vitelline Vein Fate:** The right vitelline vein forms the terminal portion of the IVC and the portal vein.

Question 449: The target gene for SRY which favors testis differentiation is:

A. SOX-9 gene (Correct Answer)
B. RSPO1 gene
C. Dax1 gene
D. WNT-4 gene

Explanation: **Explanation:** The determination of fetal sex is a complex genetic cascade. The **SRY gene** (Sex-determining Region on Y chromosome) acts as the master switch for male development. **1. Why SOX-9 is the correct answer:** The SRY protein acts as a transcription factor that directly binds to the enhancer region of the **SOX-9 gene**. Once activated, SOX-9 induces the differentiation of bipotential gonadal cells into **Sertoli cells**. These cells then secrete Anti-Müllerian Hormone (AMH), leading to the regression of paramesonephric ducts [1], and trigger the development of Leydig cells to produce testosterone [2]. SOX-9 is considered the "effector" gene for testis determination; even in the absence of SRY, experimental upregulation of SOX-9 can induce testis formation. **2. Why other options are incorrect:** * **WNT-4 and RSPO1 (Options B & D):** These are "pro-ovary" genes. They act synergistically to promote ovarian differentiation and inhibit the male pathway. WNT-4 upregulates **DAX1**, which antagonizes SOX-9. * **DAX1 (Option C):** Located on the X chromosome, DAX1 acts as an anti-testis gene. While it plays a role in adrenal development, in the context of gonadal differentiation, it functions to repress SOX-9. High levels of DAX1 can inhibit the SRY-mediated induction of SOX-9, potentially leading to sex reversal. **Clinical Pearls for NEET-PG:** * **Master Switch:** SRY gene (on the short arm of the Y chromosome). * **Primary Target of SRY:** SOX-9. * **Sertoli Cells:** Derived from the surface epithelium of the genital ridge; they produce AMH (inhibits Müllerian ducts) [1]. * **Leydig Cells:** Derived from the mesenchyme of the genital ridge; they produce Testosterone (stimulates Wolffian ducts) [1]. * **Campomelic Dysplasia:** Caused by mutations in the SOX-9 gene, characterized by skeletal abnormalities and XY sex reversal (phenotypic females).

Question 450: Midline caecum results from which of the following abnormalities?

A. Malrotation
B. Mixed rotation (Correct Answer)
C. Reverse rotation
D. Non-rotation

Explanation: ### Explanation The development of the midgut involves a complex **270° counter-clockwise rotation** around the Superior Mesenteric Artery (SMA). This process occurs in three stages: the first 90° occurs during herniation, and the remaining 180° occurs during return to the abdominal cavity. **1. Why Mixed Rotation is Correct:** Mixed rotation occurs when the **pre-arterial segment** (small intestine) fails to rotate properly, while the **post-arterial segment** (large intestine) undergoes only partial rotation (usually 90° instead of 180°). This results in the caecum failing to reach the right iliac fossa, instead becoming fixed in the **midline** [1] or sub-hepatic position. This is clinically significant as it often leads to the formation of **Ladd’s bands**, which can cause duodenal obstruction [1]. **2. Analysis of Incorrect Options:** * **Non-rotation:** The midgut fails to rotate after the initial 90°. This results in "Left-sided colon," where the small intestine lies on the right and the entire colon lies on the left side of the abdomen [1], [2]. * **Reverse rotation:** The midgut rotates **clockwise** instead of counter-clockwise. The primary consequence is that the **transverse colon lies posterior to the SMA**, potentially leading to colonic obstruction. * **Malrotation:** This is a broad umbrella term for any deviation from normal rotation [1]. While "mixed rotation" is a type of malrotation, "Mixed Rotation" is the specific anatomical cause for a midline caecum. ### High-Yield Clinical Pearls for NEET-PG: * **Ladd’s Bands:** Peritoneal bands connecting a malpositioned caecum to the posterior abdominal wall, crossing and obstructing the duodenum [1]. * **Volvulus:** Malrotation results in a narrow mesenteric base, making the gut prone to midgut volvulus (twisting), a surgical emergency [1]. * **Radiology Sign:** On a barium meal, the "Corkscrew sign" is characteristic of midgut volvulus. * **Normal Rotation:** 270° Counter-clockwise (90° in the physiological umbilical hernia, 180° upon return).

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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