Embryology and Development Practice Questions

Q: Which substance is secreted by Sertoli cells of the fetal testis?

Mullerian inhibiting substance. **Explanation:** The differentiation of the male reproductive system is an active process driven by hormones secreted by the fetal testes. The correct answer is **Mullerian Inhibiting Substance (MIS)**, also known as Anti-Müllerian Hormone (AMH) [2]. 1. **Why it is correct:** In a male fetus (XY), the *SRY* gene on the Y chromosome triggers the differentiation of primitive gonads into testes. Within the testes, **Sertoli cells** secrete MIS [1]. The primary function of MIS is to cause the regression of the Paramesonephric (Müllerian) ducts, which would otherwise develop into the uterus, fallopian tubes, and upper vagina [2]. 2. **Why other options are incorrect:** * **hCG (Human Chorionic Gonadotropin):** Secreted by the syncytiotrophoblast of the placenta to maintain the corpus luteum. * **LH (Luteinizing Hormone):** Secreted by the anterior pituitary gland. * **Testosterone:** Secreted by the **Leydig cells** (interstitial cells) of the fetal testis, not Sertoli cells. Testosterone is responsible for the development of Mesonephric (Wolffian) ducts into the epididymis, vas deferens, and seminal vesicles [2]. **High-Yield Clinical Pearls for NEET-PG:** * **Sertoli Cells:** Produce MIS (inhibits Müllerian ducts) and Inhibin [1]. * **Leydig Cells:** Produce Testosterone (stimulates Wolffian ducts) and Dihydrotestosterone (DHT) via 5α-reductase (develops external genitalia) [2]. * **Persistent Müllerian Duct Syndrome:** Occurs due to a deficiency of MIS or its receptors, resulting in a male phenotype with a rudimentary uterus and tubes. * **Mnemonic:** **S**ertoli cells **S**top Müllerian ducts; **L**eydig cells **L**ead to Wolffian development.

Q: The hyoid bone is a derivative of which of the following?

Second and third pharyngeal arches. The hyoid bone is a unique structure in the neck that develops from the cartilaginous elements of the **second and third pharyngeal arches**. ### **Explanation of the Correct Answer** The hyoid bone is formed through the fusion of multiple components derived from different arches: * **Second Pharyngeal Arch (Reichert’s Cartilage):** Gives rise to the **lesser cornu (horn)** and the **upper part of the body** of the hyoid. * **Third Pharyngeal Arch:** Gives rise to the **greater cornu (horn)** and the **lower part of the body** of the hyoid. ### **Analysis of Incorrect Options** * **Option A (First Arch):** The first arch (Mandibular arch/Meckel’s cartilage) forms the malleus, incus, and the mandible (via membrane bone formation around the cartilage). It does not contribute to the hyoid. * **Option B (First and Second):** While the second arch contributes, the first arch does not. * **Option D (Fourth Arch):** The fourth and sixth arches contribute to the **laryngeal cartilages** (thyroid, cricoid, arytenoid, corniculate, and cuneiform), not the hyoid bone. ### **High-Yield Clinical Pearls for NEET-PG** * **Nerve Supply:** Because the hyoid is derived from the 2nd and 3rd arches, the muscles attaching to it are supplied by the **Facial nerve (CN VII)** and **Glossopharyngeal nerve (CN IX)** respectively. * **Stylohyoid Ligament:** This is also a derivative of the **second arch** and connects the styloid process to the lesser cornu of the hyoid. * **Fracture Significance:** A fractured hyoid bone is a classic forensic finding in cases of **strangulation or throttling**. * **Ectopic Tissue:** The **Thyroglossal duct** descends from the foramen caecum to the thyroid gland, passing either anterior to or through the hyoid bone. This is a common site for thyroglossal cysts.

Q: Which of the following is NOT true about the primitive streak?

It appears at the cranial end.. ### Explanation The **primitive streak** is a pivotal structure in embryology, marking the beginning of **gastrulation** (the process by which the bilaminar disc transforms into a trilaminar disc) [1]. **1. Why Option C is the correct answer (The False Statement):** The primitive streak appears at the **caudal (posterior) end** of the dorsal aspect of the epiblast, not the cranial end. It elongates cranially, and its appearance establishes the cranio-caudal axis, the medial-lateral axis, and the dorsal-ventral symmetry of the embryo. **2. Analysis of Incorrect Options:** * **Option A:** This is a true statement. The formation of the primitive streak on day 15 is the **first morphological sign of gastrulation**. * **Option B:** This is a true statement. The streak is formed by the proliferation and migration of **epiblast cells** toward the median plane of the embryonic disc [1]. These cells then undergo an epithelial-to-mesenchymal transition to form the intraembryonic mesoderm and endoderm. **3. NEET-PG High-Yield Facts & Clinical Pearls:** * **Primitive Node (Hensen’s Node):** The cephalic end of the streak expands to form this node, which acts as the "organizer." * **Fate of the Streak:** Under normal conditions, the primitive streak regresses and disappears by the end of the 4th week. * **Clinical Correlation (Sacrococcygeal Teratoma):** If the primitive streak fails to regress and remnants persist in the sacrococcygeal region, it can lead to a **Sacrococcygeal Teratoma**. This is the most common tumor in newborns and contains tissues from all three germ layers. * **Situs Inversus:** Abnormalities in the signaling molecules at the primitive streak/node (like *SHH* and *FGF8*) can lead to defects in left-right asymmetry.

Q: At what age does the fetal circulation first become intact and functional, separate from maternal circulation?

21 days. ### Explanation The development of the cardiovascular system is a critical milestone in embryology, as it is the first functional organ system to develop in the human embryo. **Why 21 Days is Correct:** By the beginning of the **third week** of development, the embryo can no longer satisfy its nutritional and oxygen requirements by simple diffusion alone. Vasculogenesis and angiogenesis begin in the extraembryonic mesoderm (yolk sac and stalk) and the intraembryonic mesoderm. By **Day 21**, the primitive heart tubes fuse and begin to beat. At this point, a continuous circuit is established between the heart, the intraembryonic vessels, and the extraembryonic vessels (umbilical and vitelline systems), making the fetal circulation **intact and functional** [1]. **Analysis of Incorrect Options:** * **A. 8 days:** This corresponds to the early implantation phase (blastocyst stage). The embryo is a bilaminar disc, and no vascular system exists. * **B. 12 days:** This is the stage of primary villi formation and the development of the extraembryonic mesoderm. Circulation has not yet begun. * **C. 17 days:** At this stage, blood islands begin to appear in the yolk sac and the primitive streak is active (Gastrulation), but the heart has not yet started pumping. **High-Yield NEET-PG Pearls:** * **First system to function:** Cardiovascular system (Week 3). * **Heartbeat begins:** Day 21–22. * **First site of hematopoiesis:** Yolk sac (extraembryonic mesoderm) starting around Day 17. * **Fetal Hemoglobin (HbF):** Has a higher affinity for oxygen than adult hemoglobin (HbA), facilitating oxygen transfer across the placenta [2]. * **Cardiac Progenitor Cells:** Derived from the **epiblast** (lateral plate mesoderm).

Q: What is the most common site of fertilization?

Ampulla. **Explanation:** Fertilization is the process where the male and female gametes fuse to form a zygote. In humans, this occurs within the **Fallopian tube (uterine tube)** [1]. **Why Ampulla is the Correct Answer:** The **Ampulla** is the widest and longest part of the fallopian tube (approximately 5 cm long). Due to its spacious lumen and its location distal to the isthmus, it serves as the most favorable environment for the sperm to meet the oocyte. Statistically, it is the site of fertilization in over 90% of cases [1]. **Analysis of Incorrect Options:** * **Isthmus (A):** This is the narrow, thick-walled medial part of the tube. While sperm pass through it, its narrow lumen is not the primary site for fertilization. * **Infundibulum (C):** This is the funnel-shaped lateral end with fimbriae that "catch" the ovum from the ovary [1]. It is the entry point for the egg, not the site of fusion. * **Interstitial/Intramural (D):** This is the segment that pierces the uterine wall. It is the narrowest part of the tube and is not involved in fertilization. **NEET-PG High-Yield Pearls:** 1. **Ectopic Pregnancy:** The **Ampulla** is also the most common site for an ectopic pregnancy. 2. **Narrowest Part:** The **Interstitial (Intramural) part** is the narrowest portion of the fallopian tube (0.7 mm). 3. **Fertilization Timing:** Fertilization typically occurs within **12–24 hours** after ovulation. 4. **Implantation:** While fertilization occurs in the ampulla, implantation usually occurs in the **posterior wall of the body of the uterus** on approximately the 6th day after fertilization [1].

Q: How many vessels are typically seen in a cross-section of the umbilical cord?

Three. **Explanation:** The umbilical cord typically contains **three vessels**: **two umbilical arteries** and **one umbilical vein** [1], all embedded in a gelatinous substance called **Wharton’s jelly**. 1. **Why Three is Correct:** During early development, there are initially four vessels (two arteries and two veins). However, the **right umbilical vein** undergoes regression (disappears) around the 8th week of gestation, leaving only the **left umbilical vein** and the two umbilical arteries. The vein carries oxygenated blood from the placenta to the fetus, while the arteries carry deoxygenated blood from the fetus back to the placenta [1]. 2. **Why Other Options are Incorrect:** * **One:** A single vessel is never normal and is incompatible with life. * **Two:** A **Single Umbilical Artery (SUA)** occurs in about 1% of pregnancies. While it can be an isolated finding, it is highly associated with congenital anomalies (renal or cardiac) and chromosomal trisomies, making it a significant clinical finding rather than the "typical

Q: The upper three-fourths of the vagina develops from which embryonic structure?

Mullerian duct. **Explanation:** The development of the female reproductive tract is a high-yield topic in embryology. The vagina has a **dual origin**, arising from two distinct embryonic structures: 1. **Mullerian (Paramesonephric) Ducts:** The caudal ends of the Mullerian ducts fuse to form the **uterovaginal canal**. This canal gives rise to the uterus, cervix, and the **upper 3/4th (upper portion)** of the vagina [1]. 2. **Urogenital Sinus (Sinovaginal Bulbs):** The lower part of the uterovaginal canal meets the urogenital sinus, inducing the formation of the **sinovaginal bulbs**. These bulbs proliferate and canalize to form the **lower 1/4th** (or 1/5th) of the vagina [1]. **Analysis of Options:** * **Option A (Correct):** As stated, the Mullerian ducts form the upper 3/4th of the vagina, along with the fallopian tubes and uterus [1]. * **Option B (Incorrect):** The **Wolffian (Mesonephric) ducts** regress in females due to the absence of testosterone. Remnants may persist as **Gartner’s cysts** in the lateral wall of the vagina. * **Option C (Incorrect):** The sinovaginal bulbs form the **lower 1/4th** of the vagina, not the upper portion [1]. * **Option D (Incorrect):** While the urogenital sinus is endodermal, the upper 3/4th of the vagina is derived from the Mullerian duct, which is **mesodermal** in origin [1]. **High-Yield Clinical Pearls for NEET-PG:** * **Mayer-Rokitansky-Küster-Hauser (MRKH) Syndrome:** Congenital absence of the uterus and the upper 2/3rd to 3/4th of the vagina due to Mullerian agenesis. * **Hymen:** Formed at the junction where the sinovaginal bulbs meet the urogenital sinus. * **Epithelium:** Initially, the entire vagina is lined by columnar epithelium (Mullerian), which is later replaced by **stratified squamous epithelium** from the urogenital sinus.

Q: All of the following are components of the placental barrier, EXCEPT:

Amnion. The **placental barrier** (placental membrane) is a composite structure that separates the maternal blood in the intervillous spaces from the fetal blood within the villi [1]. Its primary function is to prevent the mixing of blood while allowing the exchange of nutrients, gases, and waste [3]. ### Why Amnion is the Correct Answer The **Amnion** is the innermost fetal membrane that encloses the amniotic cavity and the fetus [1]. While it is part of the fetal membranes, it **does not contribute to the structure of the chorionic villi** where gas and nutrient exchange occurs [1]. Therefore, it is not a component of the placental barrier. ### Explanation of Other Options The placental barrier is composed of four layers (from external to internal): * **Trophoblast (Option A):** Specifically the outer **Syncytiotrophoblast** (which is in direct contact with maternal blood) and the inner **Cytotrophoblast** [5]. * **Mesoderm (Option C):** The extraembryonic mesoderm forms the connective tissue core of the villus, providing structural support [4]. * **Fetal capillary endothelium (Option B):** The innermost layer that lines the fetal blood vessels within the villi [4]. ### High-Yield Facts for NEET-PG * **Thinning of the Barrier:** In early pregnancy, all four layers are present. In late pregnancy (after the 4th month), the barrier thins to facilitate faster exchange; the **cytotrophoblast** and **mesoderm** largely disappear, leaving only the syncytiotrophoblast and fetal endothelium. * **The "Barrier" is not absolute:** Most drugs with a molecular weight <600 Da, viruses (e.g., Rubella, CMV, HIV), and certain antibodies (IgG) can cross this barrier [3]. * **IgG Transfer:** IgG is the only immunoglobulin that crosses the placenta, providing passive immunity to the newborn [2].

Question 1

Which substance is secreted by Sertoli cells of the fetal testis?

Accepted Answer

Mullerian inhibiting substance

Answer

hCG

Answer

LH

Answer

Testosterone

Question 2

The hyoid bone is a derivative of which of the following?

Accepted Answer

Second and third pharyngeal arches

Answer

First pharyngeal arch

Answer

First and second pharyngeal arches

Answer

Fourth pharyngeal arch

Question 3

Which of the following is NOT true about the primitive streak?

Accepted Answer

It appears at the cranial end.

Answer

It is the first sign of gastrulation.

Answer

It is derived from the epiblast.

Answer

All of the above are true.

Question 4

At what age does the fetal circulation first become intact and functional, separate from maternal circulation?

Accepted Answer

21 days

Answer

8 days

Answer

12 days

Answer

17 days

Question 5

What is the most common site of fertilization?

Accepted Answer

Ampulla

Answer

Isthmus

Answer

Infundibulum

Answer

Interstitial

Question 6

How many vessels are typically seen in a cross-section of the umbilical cord?

Accepted Answer

Three

Answer

One

Answer

Four

Answer

Two

Question 7

In renal agenesis, where is the adrenal gland typically located?

Accepted Answer

Present at the usual location

Answer

Absent

Answer

Present on the contralateral side

Answer

Ectopic in the iliac fossa

Question 8

The upper three-fourths of the vagina develops from which embryonic structure?

Accepted Answer

Mullerian duct

Answer

Wolffian duct

Answer

Sinovaginal bulb

Answer

Endoderm

Question 9

All of the following are components of the placental barrier, EXCEPT:

Accepted Answer

Amnion

Answer

Trophoblast

Answer

Fetal capillary endothelium

Answer

Mesoderm

Question 10

Which of the following procedures is used as a routine technique for karyotyping using light microscopy?

Accepted Answer

G-banding

Answer

C-banding

Answer

Q-banding

Answer

V-staining

Embryology and Development — MCQs

Embryology and Development — MCQs

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