Embryology and Development Practice Questions

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

Deposition of fibrin. **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).

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

2 million. ### 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)**.

Q: Which layer of the placenta disappears at the end of pregnancy?

Decidua basalis. 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].

Q: Hypoblast forms what structure?

Prochordal plate. ### 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.

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

Urogenital sinus. **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).

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

Medial nasal process with maxillary process. ### 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.

Q: The sinus venosus receives blood from all except:

Subcardinal vein. 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.

Q: The target gene for SRY which favors testis differentiation is:

SOX-9 gene. **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 1

The gut tube develops from which germ layer?

Accepted Answer

Endoderm

Answer

Ectoderm

Answer

Mesoderm

Answer

Neuroectoderm

Question 2

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

Accepted Answer

Deposition of fibrin

Answer

Blood vessels

Answer

Langhan's cells accumulation

Answer

Hofbauer cells accumulation

Question 3

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

Accepted Answer

2 million

Answer

6 million

Answer

5 million

Answer

10 million

Question 4

Which layer of the placenta disappears at the end of pregnancy?

Accepted Answer

Decidua basalis

Answer

Decidua parietalis

Answer

Decidua capsularis

Answer

Chorion frondosum

Question 5

Hypoblast forms what structure?

Accepted Answer

Prochordal plate

Answer

Notochord

Answer

Apical ligament of atlas

Answer

Endoderm

Question 6

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

Accepted Answer

Urogenital sinus

Answer

Genital ridge

Answer

Wolffian duct

Answer

Mullerian duct

Question 7

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

Accepted Answer

Medial nasal process with maxillary process

Answer

Medial nasal process with lateral nasal process

Answer

Lateral nasal process with maxillary process

Answer

Lateral nasal process with palatal process

Question 8

The sinus venosus receives blood from all except:

Accepted Answer

Subcardinal vein

Answer

Vitelline vein

Answer

Umbilical vein

Answer

Common cardinal vein

Question 9

The target gene for SRY which favors testis differentiation is:

Accepted Answer

SOX-9 gene

Answer

RSPO1 gene

Answer

Dax1 gene

Answer

WNT-4 gene

Question 10

Midline caecum results from which of the following abnormalities?

Accepted Answer

Mixed rotation

Answer

Malrotation

Answer

Reverse rotation

Answer

Non-rotation

Embryology and Development — MCQs

Embryology and Development — MCQs

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