201. An embryo with \(16\) blastomeres is moving toward the uterus. Its cells have not yet differentiated into the outer and inner cellular arrangement characteristic of the next major stage. What is the most appropriate expected transition?
ⓐ. It should undergo meiosis and return to a haploid condition
ⓑ. It should continue development from the morula into a blastocyst
ⓒ. It should implant immediately without further cellular reorganisation
ⓓ. It should transform into a secondary oocyte before entering the uterus
Correct Answer: It should continue development from the morula into a blastocyst
Explanation: An embryo with about \(8\text{–}16\) blastomeres fits the morula stage. The description also specifies that it has not yet acquired the differentiated outer and inner cellular arrangement associated with the subsequent blastocyst, confirming that it remains pre-blastocyst. As development continues during movement toward the uterus, the morula undergoes further division and organisation and becomes a blastocyst. This transition does not involve a return to meiosis or haploidy, since the embryonic cells are undergoing mitotic development after fertilisation. Implantation also requires the later blastocyst organisation and interaction with the endometrium. The expected next transition is continued development from morula to blastocyst.
202. Determine the correct combination of the following statements about a human blastocyst.
I. Its outer cellular layer is the trophoblast.
II. The inner cell mass lies within and is attached to the trophoblast.
III. The inner cell mass contributes to formation of the embryo.
IV. Trophoblast and inner cell mass are simply two names for the same group of cells.
ⓐ. I and IV only
ⓑ. I, II and III only
ⓒ. II, III and IV only
ⓓ. I, III and IV only
Correct Answer: I, II and III only
Explanation: During the morula-to-blastocyst transition, the embryonic cells become organised into two recognisable cellular groups. The trophoblast forms the outer layer of the blastocyst, whereas the inner cell mass is positioned internally and remains associated with that outer layer. These two groups subsequently perform different developmental roles. The inner cell mass gives rise to the embryo, while the trophoblast participates importantly in attachment to the uterine lining and later contributes to placental development. They cannot be treated as interchangeable names for the same cells. The first three statements correctly combine position and developmental role, while the fourth removes the cellular differentiation that distinguishes a blastocyst from the less specialised morula stage.
203. A blastocyst is described as having region P forming the outer cellular layer and region Q appearing as an internal cluster of cells attached to P. P later participates in uterine attachment, whereas Q contributes to formation of the embryo. P and Q are respectively:
ⓐ. inner cell mass and trophoblast
ⓑ. endometrium and trophoblast
ⓒ. chorionic villi and inner cell mass
ⓓ. trophoblast and inner cell mass
Correct Answer: trophoblast and inner cell mass
Explanation: The outer position of P identifies it as the trophoblast, and its later participation in attachment to the uterine lining provides an independent functional confirmation. Q is the inner cell mass because it lies as an internal cellular group associated with the trophoblast and provides the cells that form the embryo. The description uses both spatial organisation and developmental role. Chorionic villi arise later from trophoblastic tissue after implantation-related development has progressed, while the endometrium belongs to the maternal uterine lining rather than to the blastocyst itself. The correct interpretation preserves the major blastocyst distinction: outer trophoblast for attachment-related functions and inner cell mass for embryonic development.
204. A blastocyst has a normal inner cell mass, but its trophoblast is severely defective and cannot perform its normal attachment-related role. Which early consequence is most directly expected?
ⓐ. Implantation into the uterine endometrium is impaired despite preservation of the embryonic cell mass
ⓑ. Implantation proceeds normally because the inner cell mass alone mediates attachment
ⓒ. The inner cell mass substitutes for trophoblast and attaches directly as the normal implantation layer
ⓓ. Placental development proceeds normally even though trophoblast cannot attach to endometrium
Correct Answer: Implantation into the uterine endometrium is impaired despite preservation of the embryonic cell mass
Explanation: The trophoblast is the outer cellular component of the blastocyst and participates directly in its interaction with the uterine endometrium. A selective trophoblastic defect threatens attachment and implantation even when the inner cell mass remains structurally normal. The inner cell mass represents the embryonic cell population, but its preservation does not substitute for the outer layer needed to establish the blastocyst-endometrium interaction. Earlier events such as fertilisation, zygote formation, cleavage and morula development must already have occurred for a blastocyst to exist. The defect should be interpreted at the next developmental transition rather than projected backward. Normal embryonic cells can be present while implantation still fails since the trophoblast cannot perform its required attachment-related function. Cleavage increases blastomere number by mitosis while preserving the diploid chromosome complement established at fertilisation.
205. Match each early developmental component with its most appropriate relation. A Column II entry is used once.
| Column I | Column II |
|---|
| P. Morula | 1. Outer cellular layer of blastocyst |
| Q. Trophoblast | 2. Embryonic cell group within blastocyst |
| R. Inner cell mass | 3. Approximately \(8\text{–}16\)-blastomere stage |
| S. Endometrium | 4. Maternal uterine tissue involved in implantation |
ⓐ. P-1, Q-3, R-4, S-2
ⓑ. P-3, Q-2, R-1, S-4
ⓒ. P-3, Q-1, R-2, S-4
ⓓ. P-4, Q-1, R-2, S-3
Correct Answer: P-3, Q-1, R-2, S-4
Explanation: The morula is the compact early embryo containing approximately \(8\text{–}16\) blastomeres, so P matches the third relation. Further development produces a blastocyst in which the trophoblast forms the outer cellular layer, giving Q-1. The inner cell mass is the internal embryonic cell group, so R-2. The endometrium is not part of the blastocyst; it is the maternal uterine lining with which the trophoblast interacts during implantation, establishing S-4. The mapping follows a developmental transition from morula to blastocyst while also separating fetal-derived blastocyst components from maternal uterine tissue. This distinction becomes important when later considering implantation and placental formation. The morula-to-blastocyst transition adds cellular organisation, including outer trophoblast and an internal cell mass with different later roles.
206. Which sequence correctly represents the principal events of implantation after a blastocyst reaches the uterus?
ⓐ. Uterine-cell proliferation around blastocyst → trophoblast attachment to endometrium → embedding of blastocyst
ⓑ. Trophoblast attachment to endometrium → uterine-cell proliferation around blastocyst → embedding of blastocyst
ⓒ. Trophoblast attachment to endometrium → embedding of blastocyst → uterine-cell proliferation around blastocyst
ⓓ. Embedding of blastocyst → uterine-cell proliferation around blastocyst → trophoblast attachment to endometrium
Correct Answer: Trophoblast attachment to endometrium → uterine-cell proliferation around blastocyst → embedding of blastocyst
Explanation: Implantation begins with interaction between the trophoblast and the uterine endometrium. After the blastocyst attaches, uterine cells proliferate around it, and the blastocyst becomes progressively embedded in the endometrial tissue. The sequence moves from initial attachment to increasing maternal-tissue association and finally to embedding. Fertilisation and morula formation occur earlier and cannot appear inside the implantation sequence. The inner cell mass also remains the embryonic component rather than leaving the blastocyst to create trophoblast. Understanding the dependency among these events prevents implantation from being reduced to a single moment: it is a coordinated blastocyst-endometrium process that establishes the developing conceptus within the uterine lining. Implantation requires a blastocyst capable of interacting with receptive endometrium; uterine arrival alone is not equivalent to successful implantation.
207. A morphologically normal blastocyst reaches the uterus, but the endometrium is severely damaged and cannot provide the normal tissue response around the attaching blastocyst. Which outcome is most directly predicted?
ⓐ. Implantation is impaired even though a normal blastocyst has already formed
ⓑ. Fertilisation is prevented because it normally occurs in the endometrium
ⓒ. The morula cannot undergo any cleavage within the oviduct before reaching the uterus
ⓓ. Spermatogenesis stops because uterine tissue regulates Sertoli cells
Correct Answer: Implantation is impaired even though a normal blastocyst has already formed
Explanation: The case separates embryo formation from implantation. A normal blastocyst has already formed and reached the uterus, so fertilisation, cleavage and the morula-to-blastocyst transition must have occurred upstream. Implantation now depends on interaction between trophoblast and a responsive endometrium, including proliferation of uterine cells around the blastocyst and subsequent embedding. Severe endometrial damage compromises this uterine stage even when the blastocyst itself is structurally normal. Fertilisation occurs earlier in the fallopian tube, not in the endometrium, and spermatogenesis is a testicular process unrelated to the uterine defect. The predicted failure lies specifically at the blastocyst-endometrium interface required for establishment of pregnancy. Trophoblast and inner cell mass share one blastocyst but have different developmental roles, so loss of one cannot be replaced automatically by the other. Upstream events that have already occurred remain established; the defect should be traced forward only through processes that require the impaired structure or signal.
208. Assertion: Implantation establishes pregnancy by embedding the blastocyst within the uterine endometrium.
Reason: Implantation involves trophoblastic attachment followed by proliferation of uterine cells around the blastocyst and progressive embedding.
ⓐ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓑ. Assertion is true, but Reason is false; therefore Reason cannot explain Assertion
ⓒ. Assertion is false, but Reason is true; therefore Reason cannot explain Assertion
ⓓ. Both Assertion and Reason are true, and Reason correctly explains Assertion
Correct Answer: Both Assertion and Reason are true, and Reason correctly explains Assertion
Explanation: The assertion accurately identifies implantation as the process by which the blastocyst becomes established within the uterine endometrium. The reason describes the cellular sequence that produces this state. The trophoblast first interacts with and attaches to the endometrial surface, uterine cells proliferate around the blastocyst, and the blastocyst becomes embedded in the maternal lining. These events explain how the free blastocyst is transformed into an implanted conceptus associated with uterine tissue. The reason provides the mechanism supporting the assertion rather than merely stating another unrelated fact. Fertilisation created the zygote earlier, whereas implantation establishes the subsequent uterine association needed for continuation of pregnancy.
209. Equal numbers of normal blastocysts are placed under two experimental conditions. In P, trophoblast cells can attach normally to endometrial tissue. In Q, trophoblast attachment is selectively blocked, while the inner cell mass remains viable. Embedding occurs only in P. What is the strongest inference?
ⓐ. Inner cell mass alone is sufficient to embed without trophoblastic participation
ⓑ. Implantation begins before blastocyst formation
ⓒ. Trophoblast attachment is an essential early step for normal implantation
ⓓ. Endometrial tissue is unnecessary when the embryo has a viable inner cell mass
Correct Answer: Trophoblast attachment is an essential early step for normal implantation
Explanation: The two conditions differ specifically in the ability of trophoblast cells to attach to the endometrium. The inner cell mass remains viable in both, so failure in Q cannot reasonably be attributed to loss of the embryonic cell group. Embedding occurs only where trophoblast attachment is permitted, supporting the conclusion that this attachment is an essential early event in the implantation sequence. The result does not show that trophoblast is the only requirement, because responsive uterine tissue is also involved. It does demonstrate that a viable inner cell mass cannot bypass the need for the blastocyst's outer trophoblastic layer to establish initial contact with the endometrium before progressive embedding can occur.
210. A blastocyst is shown in words as structure P lying on the uterine lining. Its outer layer Q contacts tissue R, and cells of R begin to proliferate around P until P becomes embedded. P, Q and R are respectively:
ⓐ. blastocyst, trophoblast and endometrium
ⓑ. morula, inner cell mass and myometrium
ⓒ. zygote, chorionic villi and perimetrium
ⓓ. blastocyst, inner cell mass and myometrium
Correct Answer: blastocyst, trophoblast and endometrium
Explanation: P is identified as the blastocyst given that it has reached the uterus and is undergoing the attachment and embedding process characteristic of implantation. Q must be the trophoblast, the outer blastocyst layer that directly contacts the maternal uterine lining. R is the endometrium, whose cells proliferate around the attaching blastocyst during implantation. The myometrium is the deeper smooth-muscle layer of the uterus and does not form the implantation surface described. A zygote and morula are earlier developmental stages that precede this uterine embedding event. The three labels reflect one continuous interaction: blastocyst as the developmental stage, trophoblast as its attaching outer layer, and endometrium as the maternal tissue receiving it.
211. Evaluate these statements about chorionic villi and placental formation.
I. Chorionic villi arise from trophoblastic tissue after implantation-related development begins.
II. The villi are finger-like projections associated with maternal uterine tissue.
III. Chorionic villi and uterine tissue together contribute to formation of the placenta.
IV. The placenta consists only of the inner cell mass and contains no maternal tissue.
ⓐ. I and IV only
ⓑ. II and IV only
ⓒ. I, III and IV only
ⓓ. I, II and III only
Correct Answer: I, II and III only
Explanation: After implantation, trophoblastic tissue develops finger-like chorionic villi. These villi interact closely with the maternal uterine tissue and are associated with maternal blood, creating the structural basis for the placental interface. Placenta formation involves a relationship between fetal-derived chorionic villi and maternal uterine tissue rather than being produced by only one side. The inner cell mass contributes primarily to the embryo and should not be substituted for the chorionic-villus component of the placenta. The first three statements preserve the correct origin and tissue relationship. The fourth removes the maternal contribution and misidentifies the relevant embryonic component, so it is inconsistent with the described placental organisation. Placental endocrine activity and maternal-fetal exchange should be evaluated as distinct outputs when one of those outputs is selectively preserved.
212. Match each structure with the relation most directly associated with it. A Column II entry is used once.
| Column I | Column II |
|---|
| P. Trophoblast | 1. Maternal tissue contributing to placental formation |
| Q. Chorionic villi | 2. Outer blastocyst layer from which villous structures develop |
| R. Uterine tissue | 3. Finger-like projections interacting with maternal tissue |
| S. Placenta | 4. Composite exchange and endocrine interface formed through fetal-maternal association |
ⓐ. P-3, Q-2, R-4, S-1
ⓑ. P-2, Q-3, R-1, S-4
ⓒ. P-2, Q-4, R-3, S-1
ⓓ. P-1, Q-3, R-2, S-4
Correct Answer: P-2, Q-3, R-1, S-4
Explanation: The trophoblast forms the outer cellular component of the blastocyst and later gives rise to villous structures, so P-2. Chorionic villi are the finger-like projections that interact with maternal uterine tissue, giving Q-3. Uterine tissue supplies the maternal contribution to the developing placental interface, establishing R-1. The placenta is the resulting fetal-maternal organ that supports exchange and also performs endocrine functions, giving S-4. The mapping follows developmental origin and structural integration. It avoids treating trophoblast, chorionic villi and placenta as identical terms: trophoblast gives rise to the villous component, while the mature placental relationship incorporates those villi together with maternal uterine tissue. Implantation establishes the association that permits later placental development; placental villi are not a prerequisite for the earlier gamete-fusion event.
213. Chorionic villi form, but they fail to establish their normal close interaction with maternal uterine tissue. Which consequence is most directly expected?
ⓐ. Placental exchange remains normal because chorionic villi can function without maternal tissue association
ⓑ. The maternal uterine component alone fully replaces the failed villous interaction
ⓒ. Normal placental formation and maternal-embryonic exchange through the placenta are compromised
ⓓ. Normal placental formation proceeds independently of contact between chorionic villi and uterine tissue
Correct Answer: Normal placental formation and maternal-embryonic exchange through the placenta are compromised
Explanation: Chorionic villi are fetal-derived projections that normally interact closely with maternal uterine tissue and maternal blood as the placenta develops. If the villi form but fail to establish this relationship, the structural basis for a normal placenta is impaired. As the placenta later supports exchange of oxygen, nutrients, carbon dioxide and wastes between maternal and embryonic compartments, poor villous-maternal association also threatens exchange. Earlier processes such as sperm transport and morula formation are already far upstream and are not reversed by this later defect. The inner cell mass also does not transform into maternal tissue. The consequence is a combined placental-formation and exchange problem arising specifically from failure of the fetal villous component to associate normally with the maternal side.
214. Four descriptions of placental formation are compared.
| Record | Fetal-side feature | Maternal-side feature | Interpretation |
|---|
| P | Inner cell mass | Myometrium | Complete placenta |
| Q | Morula blastomeres | Perimetrium | Complete placenta |
| R | Chorionic villi | No uterine tissue | Complete placenta |
| S | Chorionic villi | Uterine tissue | Placental interface |
Which record best represents the normal fetal-maternal structural relation?
ⓐ. P
ⓑ. Q
ⓒ. R
ⓓ. S
Correct Answer: S
Explanation: The normal placental interface involves chorionic villi on the fetal side interacting with maternal uterine tissue. Record S is the only description that includes both of these required components. Record R contains the fetal villous structure but removes the maternal contribution, so it cannot represent the complete fetal-maternal relation. The inner cell mass primarily forms the embryo rather than serving as the placental villous component, and morula blastomeres represent an earlier developmental stage before implantation and placental organisation. The table requires recognition of developmental identity as well as tissue origin. Placenta should be understood as a composite interface formed through association between fetal chorionic villi and maternal uterine tissue, not as fetal tissue alone. Maternal and fetal contributions meet at the placenta without their tissues becoming identical, preserving a specialised exchange interface.
215. Assess the following statements about placental exchange.
I. Oxygen and nutrients pass from the maternal side toward the embryo.
II. Carbon dioxide and embryonic metabolic wastes pass toward the maternal side for removal.
III. The placenta participates in exchange between maternal and embryonic compartments.
IV. The placenta transfers only wastes and has no role in supplying substances required for embryonic growth.
ⓐ. I and IV only
ⓑ. I, II and III only
ⓒ. II, III and IV only
ⓓ. I, III and IV only
Correct Answer: I, II and III only
Explanation: The placenta serves as a major exchange interface between maternal and embryonic compartments. Oxygen and nutrients required for growth are supplied across the placental relation toward the developing embryo. In the opposite functional direction, carbon dioxide and metabolic wastes generated by the embryo are transferred toward the maternal side for removal. The exchange is bidirectional in terms of substance movement even though different materials move in different directions. The fourth statement is incomplete because it recognises waste removal while denying the equally important supply function. Placental exchange should be viewed as a coordinated transfer system that supports embryonic metabolism by both delivering needed substances and removing products that must not accumulate.
216. Use the arrangement described below. Finger-like fetal projections P are closely associated with maternal uterine tissue and maternal blood. A cord Q connects the developing embryo to this exchange region. P and Q are respectively:
ⓐ. chorionic villi and umbilical cord
ⓑ. inner cell mass and cervical canal
ⓒ. mammary alveoli and lactiferous duct
ⓓ. trophoblast and vas deferens
Correct Answer: chorionic villi and umbilical cord
Explanation: P is identified as chorionic villi since the structures are finger-like fetal projections associated with maternal uterine tissue and maternal blood at the placental interface. Q is the umbilical cord since it provides the connection between the developing embryo and the placenta. These structures participate in the same transport system but are not synonymous. Chorionic villi contribute to formation of the placental exchange surface, whereas the umbilical cord links the embryo with that placenta so substances can be transported between them. The description combines a fetal-maternal exchange region with an embryo-placenta connection. Recognising this distinction prevents the common error of calling the umbilical cord itself the placenta.
217. Placental structure remains attached to the uterus, but exchange of oxygen and nutrients from the maternal side toward the embryo is severely reduced. Which consequence is most directly supported?
ⓐ. Maternal attachment alone guarantees normal oxygen and nutrient delivery despite reduced exchange
ⓑ. Only waste removal is affected; maternal-to-embryo transfer remains normal
ⓒ. Embryonic supply of essential respiratory and nutritional materials is compromised
ⓓ. Placental endocrine secretion can fully replace reduced transfer of oxygen and nutrients
Correct Answer: Embryonic supply of essential respiratory and nutritional materials is compromised
Explanation: The case preserves placental attachment but selectively reduces one of its central physiological functions: transfer of oxygen and nutrients from the maternal side toward the embryo. The direct consequence is inadequate delivery of substances needed for embryonic metabolism and growth. The umbilical cord can transport material between embryo and placenta, but it does not independently manufacture maternal nutrients or replace the placental exchange surface. Reduced incoming oxygen also does not imply that embryonic carbon dioxide production instantly stops; rather, normal metabolism becomes poorly supported. The strongest inference remains tied to the altered exchange process. A placenta can remain anatomically present while its physiological exchange capacity is seriously impaired. Chorionic villi provide a fetal-side placental component that interacts closely with maternal uterine tissue to support exchange.
218. Four substances are considered at the placental interface.
| Substance | Normal functional direction |
|---|
| P. Oxygen | Maternal side toward embryo |
| Q. Nutrients | Maternal side toward embryo |
| R. Carbon dioxide | Embryo toward maternal side |
| S. Embryonic metabolic wastes | Embryo toward maternal side |
If maternal-to-embryo transfer is selectively impaired while embryo-to-maternal removal remains relatively preserved, which substances are most directly affected first?
ⓐ. R and S only
ⓑ. P, R and S only
ⓒ. P and Q only
ⓓ. Q, R and S only
Correct Answer: P and Q only
Explanation: The altered condition specifically affects transfer from the maternal side toward the embryo. In the table, oxygen and nutrients are the substances moving in that direction, so both P and Q are directly affected. Carbon dioxide and embryonic metabolic wastes move functionally in the opposite direction, from the embryo toward the maternal side, and the description states that this removal pathway remains relatively preserved. The reasoning requires sorting substances by direction before applying the changed condition. Placental exchange is not a single undirected process; different materials cross the interface according to different physiological needs. Selective impairment of one directional function can reduce supply without initially abolishing the removal function described. The umbilical cord links the developing embryo or foetus with the placenta; it does not replace the placental exchange surface.
219. Assertion: The umbilical cord should not be considered synonymous with the placenta.
Reason: The umbilical cord connects the embryo with the placenta, whereas the placenta is the fetal-maternal exchange and endocrine interface involving chorionic villi and uterine tissue.
ⓐ. Both Assertion and Reason are true, and Reason correctly explains Assertion
ⓑ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓒ. Assertion is true, but Reason is false; therefore Reason cannot explain Assertion
ⓓ. Assertion is false, but Reason is true; therefore Reason cannot explain Assertion
Correct Answer: Both Assertion and Reason are true, and Reason correctly explains Assertion
Explanation: The assertion is true considering that the umbilical cord and placenta are anatomically and functionally distinct structures. The cord provides the connection between the developing embryo and the placenta, allowing transport to occur through that relationship. The placenta itself is the specialised fetal-maternal interface formed through association of chorionic villi with uterine tissue and supports both exchange and endocrine functions. The reason states precisely this difference and explains the assertion. Confusing the two would collapse a connecting structure and an exchange organ into one term. Their functions are coordinated, but the cord links the embryo to the placenta rather than replacing the placenta or independently performing all placental functions. Oxygen and nutrients move toward the developing embryo, while carbon dioxide and metabolic wastes move toward the maternal side.
220. Identify the valid set among these statements about the placenta.
I. It supports exchange of gases, nutrients and wastes.
II. It also functions as endocrine tissue.
III. Placental endocrine products include hCG and hPL.
IV. Its only function is mechanical attachment of the embryo to the uterus.
ⓐ. I and IV only
ⓑ. II and IV only
ⓒ. I, III and IV only
ⓓ. I, II and III only
Correct Answer: I, II and III only
Explanation: The placenta is multifunctional. Its exchange role provides oxygen and nutrients to the developing embryo while allowing carbon dioxide and metabolic wastes to move toward the maternal side. In addition to this transport-related function, the placenta acts as endocrine tissue. The placental hormonal output includes hCG and hPL, along with estrogens and progestogens within the stated treatment. Describing the placenta only as a mechanical attachment structure ignores both of these major physiological roles. Attachment and implantation are important earlier relationships involving the trophoblast and endometrium, but the established placenta develops into an organ that supports the pregnancy through exchange as well as endocrine activity. The first three statements give the complete supported functional picture.
