Embryology and Development Practice Questions

Q: By which stage does the zona pellucida disappear?

5th day after fertilization. ### Explanation The **Zona Pellucida (ZP)** is a specialized glycoprotein shell surrounding the oocyte and the early embryo. Its disappearance, known as **"Hatching,"** is a critical step for successful implantation. **Why Option C is Correct:** After fertilization, the zygote undergoes cleavage while traveling through the fallopian tube. By the **4th day**, it becomes a **morula** and enters the uterine cavity [1]. By the **5th day**, fluid accumulation transforms it into a **blastocyst**. At this stage, the blastocyst must "hatch" from the rigid zona pellucida to allow for rapid growth and to expose the underlying trophoblast cells for attachment to the uterine endometrium [2]. If the ZP does not disappear by the 5th day, implantation cannot occur. **Why Other Options are Incorrect:** * **Option A & B:** The ZP is essential during these stages. It contains species-specific sperm receptors (ZP3) that facilitate fertilization and undergoes the **"Zona Reaction"** immediately after the first sperm enters to prevent polyspermy [1]. It also prevents the blastomeres from dissociating during early cleavage [1]. * **Option D:** By the 7th day, the blastocyst has already begun the process of **implantation** (which typically starts on day 6) [2]. If the ZP were still present on day 7, the embryo would be unable to adhere to the decidua. ### NEET-PG High-Yield Pearls: * **Function of ZP:** Prevents **ectopic implantation** by ensuring the embryo does not stick to the fallopian tube walls. * **Hatching Mechanism:** Occurs due to the secretion of proteases (trypsin-like enzymes) by the trophoblast and the rhythmic expansion-contraction of the blastocyst. * **Clinical Correlation:** In IVF, "Assisted Hatching" (using lasers or chemicals to thin the ZP) is sometimes performed to improve implantation rates in older women or those with thickened ZP.

Q: Which of the following events is involved in cleavage of the zygote during week 1 of development?

A decrease in size of blastomeres. Cleavage is a series of rapid mitotic divisions that occur as the zygote travels through the uterine tube toward the uterus. The defining characteristic of cleavage is that it occurs within the confines of the rigid **Zona Pellucida** [1]. **1. Why the Correct Answer is Right:** During cleavage, the total volume of the embryo remains constant because the Zona Pellucida prevents expansion [1]. Consequently, with each successive mitotic division, the resulting daughter cells (**blastomeres**) become progressively **smaller** [1]. This increases the surface-area-to-volume ratio, facilitating efficient nutrient exchange before implantation. **2. Analysis of Incorrect Options:** * **Option A:** While cleavage *is* a series of mitotic divisions, the question asks for the specific *event* or consequence involved. While technically true, "decrease in size" is the more specific physiological hallmark of cleavage emphasized in embryology. * **Option B:** Blastomeres are not "highly undifferentiated" in the sense of being random; they undergo **compaction** (at the 8-cell stage) where they maximize contact and begin the first differentiation into the inner cell mass and trophoblast [1]. * **Option C:** The *total* cytoplasmic content of the embryo remains relatively stable; it is partitioned into smaller units [1]. The nuclear-to-cytoplasmic ratio actually **increases** as DNA replicates while the total cytoplasm does not. **Clinical Pearls for NEET-PG:** * **Compaction:** Occurs at the **8-cell stage**, mediated by **E-cadherin**. This is the first step in segregation of cell lineages. * **Morula:** Formed at the **16-cell stage** (approx. Day 3) [1]. * **Blastocyst:** Formed when a cavity (blastocele) appears (approx. Day 4) [2]. * **Hatching:** The blastocyst must "hatch" from the Zona Pellucida to implant; failure to hatch results in infertility, while premature hatching can lead to ectopic pregnancy [2].

Q: Lens develops from which germ layer?

Surface ectoderm. **Explanation:** The development of the eye is a complex process involving multiple germ layers. The **Surface Ectoderm** is the correct answer because the lens originates from the **lens placode**, a thickening of the surface ectoderm that occurs when the underlying optic vesicle (from the neuroectoderm) comes into contact with it. This placode eventually invaginates to form the lens vesicle, which detaches to become the crystalline lens. **Analysis of Options:** * **Surface Ectoderm (Correct):** Besides the **lens**, it gives rise to the corneal epithelium, conjunctival epithelium, lacrimal apparatus, and the epithelium of the eyelids/eyelashes. * **Neuroectoderm (Incorrect):** This layer forms the "neural" components of the eye, including the **retina**, posterior layers of the iris, ciliary body epithelium, and the **optic nerve**. * **Mesoderm (Incorrect):** It contributes primarily to the extraocular muscles, vascular endothelium, and the temporal portion of the sclera. * **Endoderm (Incorrect):** The endoderm does not contribute to the formation of any ocular structures. **High-Yield Clinical Pearls for NEET-PG:** * **Neural Crest Cells:** These are vital for the "structural" framework. They form the **corneal stroma/endothelium**, most of the **sclera**, and the trabecular meshwork. * **Lens Nutrition:** In fetal life, the lens is nourished by the **hyaloid artery** (tunica vasculosa lentis). Failure of this system to regress leads to Persistent Hyperplastic Primary Vitreous (PHPV). * **Aphakia:** The absence of the lens (congenital or surgical) results in a high degree of hypermetropia.

Q: Anencephaly occurs due to the inability of the neural tube to close at which week of intrauterine life?

4th week. ***4th week***- The development of the **neural tube** occurs during the 4th week of intrauterine life, with the **anterior neuropore** normally closing around day 25 after conception. - Failure of this anterior closure, specifically at the cranial end during the 4th week, results in the severe congenital anomaly known as **anencephaly** [1].*3rd week*- The 3rd week is when **neurulation** (the folding of the neural plate) is initiated and the neural groove is formed, not when the final critical closure occurs.- The developing embryo is undergoing **gastrulation** during the 3rd week (formation of the three germ layers), preceding the completion of neural tube closure.*5th week*- By the 5th week, the neural tube is typically completely closed, and the process of **primary brain vesiculation** (forebrain, midbrain, hindbrain) is actively underway.- A closure defect leading to anencephaly would have necessarily occurred earlier, during the 4th week.*2nd week*- The 2nd week of development is characterized by the formation of the **bilaminar germ disc** (epiblast and hypoblast) and implantation.- **neurulation** (the process that forms the neural tube) does not begin until the start of the 3rd week, making the 2nd week too early for this specific closure defect.

Q: What is the status of the marked individual?

Adopted from another family. ***Adopted from another family*** - In pedigree analysis, square **brackets `[ ]`** placed around an individual's symbol (a circle for female, a square for male) specifically denote that the person has been **adopted** into the family. - The relationship lines connecting an adopted individual to their adoptive parents are typically represented as **dashed lines** to distinguish them from biological parent-offspring lines, which are solid. *Has a disease of chronic origin* - An individual affected by a genetic disease or trait is represented by a **shaded or filled-in** symbol, not by brackets. The circle in the image is not shaded, indicating an unaffected status. - Pedigree symbols do not typically differentiate between chronic and acute diseases; they primarily indicate the **presence or absence** of the specific trait being studied. *Curable disease* - The **prognosis or curability** of a disease is not represented by a standard pedigree symbol. An affected status is shown by **shading the symbol**, irrespective of the disease's nature. - The symbol for adoption (`[ ]`) is distinct and unrelated to any health status or medical condition. *Twin* - **Twins** are indicated by two individual symbols branching from the same point on the parental line. A horizontal line connecting the symbols signifies **monozygotic (identical) twins**. - The symbol shown represents a single individual, and the brackets denote adoption, not a twin relationship.

Q: Which structure is formed by the structure indicated by the arrow?

Nucleus pulposus. ***Nucleus pulposus*** - The arrow indicates the **notochord**, a midline mesodermal rod that serves as the basis of the axial skeleton. Its postnatal remnant is the **nucleus pulposus**. - The **nucleus pulposus** is the gelatinous inner core of the intervertebral disc, responsible for providing shock absorption and flexibility to the vertebral column. *Annulus fibrosus* - The **annulus fibrosus** is the strong, fibrocartilaginous outer layer of the intervertebral disc that surrounds the nucleus pulposus. - It is derived from the **sclerotome** portion of the somites, not the notochord. *Neural tube* - The **neural tube**, shown as the large purple structure dorsal to the notochord, develops into the central nervous system (brain and spinal cord). - It is formed from the folding of the **neuroectoderm**, a process induced by the underlying notochord, but it is a distinct structure. *Septum transversum* - The **septum transversum** is a mass of mesodermal tissue that develops more cranially and ventrally in the embryo. - It is a major embryonic precursor to the central tendon of the **diaphragm** and does not originate from the notochord.

Q: What is the embryological basis for the uterine anomaly shown in the images?

Non-fusion of paramesonephric ducts. ***Non-fusion of paramesonephric ducts*** - The image shows a **uterus didelphys**, which is a complete duplication of the uterus and cervix. This anomaly occurs due to a complete failure of the two **paramesonephric (Müllerian) ducts** to fuse medially during embryogenesis. - The **paramesonephric ducts** are the embryological precursors to the fallopian tubes, uterus, cervix, and the upper one-third of the vagina. Their proper fusion is essential for forming a single uterine cavity. *Non-fusion of mesonephric duct* - The **mesonephric (Wolffian) ducts** are precursors to male internal genitalia (e.g., epididymis, ductus deferens, seminal vesicles) and largely regress in females. - Remnants of the mesonephric duct in females may form **Gartner's cysts**, but they do not contribute to the formation of the uterus. *Failure of fusion of metanephric duct* - The **metanephric duct**, or **ureteric bud**, is involved in the development of the urinary system, specifically the ureters, renal pelves, calyces, and collecting ducts of the kidneys. - This structure is entirely unrelated to the embryological development of the female reproductive tract. *Complete agenesis of Müllerian structures* - Complete agenesis of the **Müllerian structures** results in the congenital absence of the uterus, fallopian tubes, and upper vagina, a condition known as **Mayer-Rokitansky-Küster-Hauser (MRKH) syndrome**. - The image clearly depicts the presence of uterine horns and a cervix, which contradicts a diagnosis of agenesis (complete absence).

Q: What stage of embryonic development is shown in the following image?

Blastocyst. ***Blastocyst*** - The image shows a structure with a distinct fluid-filled cavity (the **blastocoel**, labeled X), an inner cell mass (**embryoblast**), and an outer cell layer (**trophoblast**), which are the defining features of a blastocyst. - This stage occurs around day 5 post-fertilization and is responsible for implantation into the **endometrium** of the uterine wall, which is depicted superior to the blastocyst in the illustration. *Morula* - A morula is a solid, compacted ball of 16-32 cells (**blastomeres**) that precedes the blastocyst stage; it does not have the large internal cavity shown in the image. - The morula forms around day 3-4 after fertilization, and it is the entry of fluid into the morula that transforms it into a blastocyst. *Ova* - An ovum (plural: ova) is a single, unfertilized female gamete (egg cell). - The image shows a multicellular embryo that is the result of fertilization and several rounds of cell division, not a single cell. *Gastrula* - Gastrulation is the process that follows the blastocyst stage, where the embryo organizes into three primary germ layers: **ectoderm**, **mesoderm**, and **endoderm**. - The structure shown is a pre-implantation embryo and has not yet undergone gastrulation, which is characterized by the formation of the **primitive streak**.

Q: What is the most common type of ventricular septal defect?

Membranous defect. ***Membranous defect***- This type of defect, also known as **perimembranous VSD**, is the most common form, accounting for approximately 70-80% of all ventricular septal defects [1].- It is located just below the aortic valve, involving the **proximal third** of the septum and is defined by its relationship to the fibrous trigones [1].*Muscular defect*- **Muscular VSDs** (or trabecular VSDs) are less common than membranous defects and can be found anywhere within the thick muscular portion of the septum.- These defects often have a higher rate of **spontaneous closure**, especially the small, multiple defects referred to as 'Swiss cheese' VSDs.*Non-development of the perimembranous part of the septum*- While the failure of complete septation of the **membranous portion** is the underlying embryological cause, the specific anatomical diagnosis defining the common VSD type is the *membranous defect*. - This phrasing is descriptive of the etiology but **membranous defect** identifies the most common resulting structure.*Incomplete fusion of the septum*- This is a generic description of the mechanism leading to VSD development (failure of the septal components to fuse correctly), but it does not specify the **most common anatomical location**.- The most critical and common fusion failure involves the development of the **membranous septum**, which results in the named *membranous defect*.

Q: Which of the following represents the most severe form of neural tube defect where neural tissue is completely exposed without overlying skin, vertebral arches, or meninges?

Open Neural tube defect. ***Open Neural tube defect*** - The image displays a severe congenital anomaly where the neural tube has failed to close, leaving the neural tissue completely exposed. This specific presentation is consistent with **rachischisis**, the most severe form of spina bifida. - Characteristic features visible are the complete absence of overlying skin, vertebral arches, and meninges, exposing the flattened, malformed spinal cord (neural placode) along the entire spine. *Meningocele* - A **meningocele** is a less severe defect where a sac containing only meninges and cerebrospinal fluid protrudes through an opening in the spine; the spinal cord itself is not displaced. - This is incorrect as the image clearly shows exposed **neural tissue**, which is not a feature of a simple meningocele. *Spina bifida* - **Spina bifida** is a broad term for a range of defects involving incomplete closure of the spine. It includes mild forms like **spina bifida occulta** and severe forms like myelomeningocele, which are typically localized. - The defect in the image is far more extensive than a typical localized spina bifida, affecting a large portion or the entire spine, making it a severe type of open neural tube defect known as rachischisis. *Myelomeningocele* - In a **myelomeningocele**, both the meninges and the spinal cord protrude through the defect, typically forming a sac on the infant's back. - The image does not show a contained sac but rather a completely open, flattened neural plate, which is characteristic of **rachischisis**, a more severe condition than a typical myelomeningocele.

Question 1

By which stage does the zona pellucida disappear?

Accepted Answer

5th day after fertilization

Answer

Before fertilization

Answer

During fertilization

Answer

7th day after fertilization

Question 2

Which of the following events is involved in cleavage of the zygote during week 1 of development?

Accepted Answer

A decrease in size of blastomeres

Answer

A series of mitotic divisions forming blastomeres

Answer

Production of highly undifferentiated blastomeres

Answer

A decreased cytoplasmic content of blastomeres

Question 3

Lens develops from which germ layer?

Accepted Answer

Surface ectoderm

Answer

Endoderm

Answer

Mesoderm

Answer

Neuroectoderm

Question 4

Anencephaly occurs due to the inability of the neural tube to close at which week of intrauterine life?

Accepted Answer

4th week

Answer

2nd week

Answer

3rd week

Answer

5th week

Question 5

What is the status of the marked individual?

Accepted Answer

Adopted from another family

Answer

Has a disease of chronic origin

Answer

Curable disease

Answer

Twin

Question 6

Which structure is formed by the structure indicated by the arrow?

Accepted Answer

Nucleus pulposus

Answer

Annulus fibrosus

Answer

Septum transversum

Answer

Neural tube

Question 7

What is the embryological basis for the uterine anomaly shown in the images?

Accepted Answer

Non-fusion of paramesonephric ducts

Answer

Failure of fusion of metanephric duct

Answer

Non-fusion of mesonephric duct

Answer

Complete agenesis of Müllerian structures

Question 8

What stage of embryonic development is shown in the following image?

Accepted Answer

Blastocyst

Answer

Gastrula

Answer

Morula

Answer

Ova

Question 9

What is the most common type of ventricular septal defect?

Accepted Answer

Membranous defect

Answer

Incomplete fusion of the septum

Answer

Non-development of the perimembranous part of the septum

Answer

Muscular defect

Question 10

Which of the following represents the most severe form of neural tube defect where neural tissue is completely exposed without overlying skin, vertebral arches, or meninges?

Accepted Answer

Open Neural tube defect

Answer

Meningocele

Answer

Spina bifida

Answer

Myelomeningocele

Embryology and Development — MCQs

Embryology and Development — MCQs

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