Pregnancy and Embryonic development Questions in English

(B) $(i)$ Progesterone is a steroid hormone secreted by the corpus luteum.
$(ii)$ Progesterone is responsible for the maintenance of pregnancy by maintaining the endometrium wall,hence it is called the pregnancy hormone.
$(iii)$ Hyposecretion of progesterone results in abortion; therefore,it is also known as the anti-abortion hormone.
$(iv)$ During pregnancy,progesterone helps in the attachment of the embryo to the uterine wall,the development of the placenta,and the growth of secretory alveoli in the mammary glands.

(B) Prolactin is secreted by the anterior pituitary gland,which stimulates mammary gland development during pregnancy and lactation after childbirth.
Placenta is a temporary endocrine organ that forms a connection between the uterine wall of the mother and the foetus.
It facilitates the exchange of materials between the mother and the foetus.
The placenta secretes hormones such as Human chorionic gonadotrophin $(hCG)$,Human placental lactogen $(hPL)$,oestrogen,and progesterone.

(D) In females,$LH$ (Luteinizing Hormone) acts on the corpus luteum.
It stimulates the corpus luteum to secrete the hormone progesterone.
$FSH$ (Follicle Stimulating Hormone) is responsible for the development of the Graafian follicle.

(A) Human chorionic gonadotropin $(HCG)$ is a glycoprotein hormone produced by the placenta during pregnancy.
It acts on the corpus luteum in the ovary to stimulate the secretion of progesterone,which is essential for maintaining the pregnancy.
Chemically,$HCG$ is proteinaceous in nature,as it consists of protein subunits.
Other examples of proteinaceous hormones include human growth hormone,$TSH$,$FSH$,$LH$,and $PTH$.
Steroid hormones include testosterone and estrogen,while amine hormones include epinephrine and norepinephrine.

(A) Human chorionic gonadotrophin $(hCG)$ is a hormone secreted by the placenta during pregnancy.
It acts as a luteotropic agent,meaning it maintains the corpus luteum in the ovary,preventing its involution (degeneration) and ensuring the continued production of progesterone.
This hormone is structurally and functionally similar to the luteinizing hormone $(LH)$ secreted by the anterior pituitary gland.
Therefore,both the Assertion and the Reason are correct,and the Reason is the correct explanation for the Assertion.

(A) The period of pregnancy, which is the time from conception to birth, is known as the $Gestation \text{ period}$.
During this time, the fetus develops inside the mother's uterus.
$Incubation \text{ period}$ refers to the time between exposure to a pathogen and the appearance of symptoms.
$Pre-patent \text{ period}$ is the time between infection and the appearance of the parasite in the blood or feces.
$Blastulation$ is the process of formation of a blastula from a morula.

(D) After the formation of the zygote,it undergoes successive mitotic divisions known as cleavage to form a mass of cells.
Cleavage is essentially mitosis without a resting phase ($G_1$ or $G_2$ phases are absent or very short).
Since mitosis maintains the same genetic constitution,all resulting cells are genetically identical.
Cleavage is subsequently followed by cell differentiation processes,such as gastrulation,which eventually lead to the formation of different tissues and body parts.

(B) The chances of survival of young ones are greater in $Eutherian$ mammals because they exhibit internal development of the embryo within the mother's uterus.
This provides a protected environment,constant nutrition,and stable conditions for the developing fetus.
Furthermore,$Eutherian$ mammals provide significant parental care after birth,which increases the probability of the offspring surviving to adulthood.
In contrast,fishes,amphibians,and most birds lay eggs in the external environment,where the offspring are exposed to predators and environmental fluctuations,leading to higher mortality rates.

(A) The notochord,skeletal system (bones and cartilage),and the dermis of the skin are derived from the mesoderm germ layer during embryonic development.
Other structures derived from the mesoderm include connective tissues,muscles,the circulatory system,and the excretory system.

(B) The hormone responsible for preparing and maintaining the uterus during pregnancy is progesterone.
This hormone is primarily secreted by the $Corpus \ luteum$,which is a temporary endocrine structure formed from the ruptured $Graafian \ follicle$ after ovulation.

(B) During embryonic development,the blastocyst consists of an outer layer called the trophoblast and an inner group of cells called the inner cell mass $(ICM)$. The inner cell mass contains the pluripotent stem cells that differentiate into the three primary germ layers: ectoderm,mesoderm,and endoderm,which eventually give rise to all tissues and organs of the body.

(C) Relaxin is a hormone that is secreted by both the ovary (specifically the corpus luteum) and the placenta during pregnancy.
It plays a crucial role in childbirth by increasing the flexibility of the pubic symphysis and the ligaments of the sacroiliac and sacrococcygeal joints.
This action helps to dilate the uterine cervix,thereby facilitating the process of parturition (labour).

(A) The placenta produces the hormone Human Chorionic Gonadotropin $(HCG)$.
$HCG$ is secreted by the trophoblastic cells of the developing embryo.
This hormone has properties similar to the Luteinizing Hormone $(LH)$ of the pituitary gland.
$HCG$ maintains the corpus luteum in the ovary and stimulates it to secrete progesterone throughout the early stages of pregnancy.
Progesterone is essential for maintaining the endometrium of the uterus,which supports the developing fetus and prevents menstruation.
Because $HCG$ is produced only during pregnancy,its presence in the urine is used as a diagnostic marker for pregnancy tests.

(D) The placenta acts as an endocrine tissue and produces several hormones essential for pregnancy. These include Human Chorionic Gonadotrophin $(HCG)$,Human placental lactogen $(hPL)$,oestrogens,and progestogens (progesterone). Additionally,in the later phase of pregnancy,the placenta also secretes relaxin. Therefore,all the listed options are correct.

(A) The $Trophoblast$ is an epithelium surrounding the mammalian blastocyst. It forms the outer layer of the chorion and becomes a significant part of the embryonic component of the extra-embryonic membranes.

(A) According to the $NCERT$ textbook on human reproduction:
$1$. After one month of pregnancy,the embryo's heart is formed.
$2$. By the end of the second month of pregnancy,the foetus develops limbs and digits.
$3$. By the end of the first trimester ($12$ weeks),most of the major organ systems are formed,for example,the limbs and external genital organs are well-developed.
$4$. By the end of the second trimester ($24$ weeks),the body is covered with fine hair,eyelids separate,and eyelashes are formed.
Therefore,$A$ refers to the heart and $D$ refers to the second trimester.

(A) In the process of development,the morula continues to divide and transforms into a blastocyst.
As the blastocyst forms,the cells arrange themselves into an outer layer called the trophoblast and an inner group of cells called the inner cell mass.
The blastocyst also contains a fluid-filled cavity known as the blastocoel.
During this stage,the zona pellucida begins to degenerate or break,allowing the blastocyst to implant in the uterine wall.
Based on the provided figure:
$A$ points to the breaking zona pellucida.
$B$ points to the inner cell mass.
$C$ points to the blastocoel (cavity).
$D$ points to the outer layer of cells,the trophoblast.

(B) The human placenta is derived from the $Trophoblast$ (also known as $Trophoectoderm$).
This is the outermost layer of cells of the $Blastocyst$.
It is responsible for forming the foetal component of the placenta.
It does not contribute to the formation of the embryo proper.

(A) The placenta acts as an endocrine gland because it produces several hormones essential for the maintenance of pregnancy.
These hormones include human chorionic gonadotropin $(HCG)$,human placental lactogen $(HPL)$,estrogens,and progesterone.
In the later phase of pregnancy,it also produces relaxin.

(B) During the embryonic development of humans,cleavage refers to a series of rapid mitotic cell divisions of the zygote.
In the second cleavage division,one of the two blastomeres usually divides a little sooner than the second.
Cleavage increases the number of cells (blastomeres) without increasing the total mass of the cytoplasm,as the size of the embryo remains constant during these early stages.

(B) The correct identification is $A-$Chorion,$B-$Amnion,$C-$Yolk sac,$D-$Allantois.
Extraembryonic or Foetal Membranes:
The growing embryo/foetus develops four membranes called the extraembryonic or foetal membranes. These include chorion,amnion,allantois,and yolk sac.
$(i)$ Chorion: It is made up of trophoblast outside and somatopleuric extraembryonic mesoderm inside. It completely surrounds the embryo and protects it. It also takes part in the formation of the placenta.
$(ii)$ Amnion: It is composed of trophoblast inside and somatopleuric extraembryonic mesoderm outside. The space between the embryo and the amnion is called the amniotic cavity,which is filled with a clear,watery fluid. The amniotic fluid prevents desiccation of the embryo and acts as a protective cushion that absorbs shocks.
$(iii)$ Yolk Sac: The primary yolk sac consists of endoderm inside and splanchnopleuric extraembryonic mesoderm outside. The yolk sac is non-functional in human beings except that it functions as the site of early blood cell formation.
$(iv)$ Allantois: The allantois is composed of endoderm inside and splanchnopleuric extraembryonic mesoderm outside. It is a sac-like structure that arises from the gut of the embryo. In humans,the allantois is small and non-functional except for furnishing blood vessels to the placenta.

(D) The placenta acts as an endocrine tissue and produces several hormones essential for pregnancy.
These hormones include Human chorionic gonadotropin $(hCG)$,Human placental lactogen $(hPL)$,estrogens,and progesterone.
$hCG$ is responsible for maintaining the corpus luteum,while $hPL$ is involved in maternal metabolism.
Estrogens and progesterone are crucial for maintaining the uterine lining and supporting fetal development.
Therefore,all the listed hormones ($A, B, C,$ and $D$) are produced by the placenta.

(A) During pregnancy,the placenta acts as an endocrine tissue and produces several hormones that are exclusive to the pregnant state.
$hCG$ (Human Chorionic Gonadotropin) maintains the corpus luteum to ensure the continued secretion of progesterone.
$hPL$ (Human Placental Lactogen) is involved in the growth of the mammary glands and metabolic changes in the mother.
Relaxin is also secreted during the later stages of pregnancy to facilitate parturition by softening the pubic symphysis.
Therefore,the group consisting of $hCG, hPL$,and relaxin is produced only during pregnancy.

(C) After implantation,the inner cell mass,also known as the embryoblast,differentiates into the epiblast and hypoblast. These layers eventually give rise to the three germ layers: ectoderm,mesoderm,and endoderm,which form all the tissues and organs of the developing embryo.

(C) The blastocyst consists of an outer layer called the trophoblast and an inner group of cells attached to the trophoblast called the inner cell mass. The inner cell mass contains stem cells which have the potency to give rise to all the tissues and organs of the body. These are known as pluripotent stem cells.

(C) During human embryonic development,the zygote undergoes cleavage to form a solid ball of $8-16$ cells known as the Morula $(A)$.
As the Morula continues to divide and move into the uterus,it transforms into a Blastocyst $(B)$.
The Blastocyst consists of an outer layer of cells called the Trophoblast $(C)$ and an inner group of cells attached to the Trophoblast known as the Inner cell mass $(D)$.

(C) According to human developmental biology,the heart of the developing embryo is formed after one month of pregnancy. This is the first organ to develop and can be detected by listening to the heartbeat through a stethoscope.

(B) The gestation period for a dog is approximately $60-65$ days.
The gestation period for an elephant is approximately $607-641$ days.
The gestation period for a cat is approximately $58-65$ days.
The gestation period for a cow is approximately $279-292$ days.
Therefore,the correct match is $B$ (Elephant: $607-641$ days).

(C) The modified endometrium of the uterus after implantation is known as the decidua. It is shed during parturition.
$1$. $\text{Decidua basalis}$: This is the part of the decidua that lies between the chorionic villi and the myometrium.
$2$. $\text{Decidua parietalis}$ ($=$ $\text{Decidua vera}$): This part lines the rest of the uterine cavity, excluding the site of embryo attachment.
$3$. $\text{Decidua capsularis}$: This layer grows over the developing embryo, effectively forming a partition between the embryo and the uterine lumen.
$4$. $\text{Contra-deciduate}$: This refers to a condition where the fetal part of the placenta is retained and absorbed to provide nourishment, as seen in $Talpa$.

(C) The umbilical cord is a conduit that connects the fetus to the placenta,facilitating the transport of oxygen,nutrients,and waste products between the mother and the fetus. It does not possess endocrine functions. The production of hormones such as $hPL$ (human Placental Lactogen),estrogen,and progesterone is a function of the placenta,not the umbilical cord. Therefore,option $C$ is the incorrect statement.

(A) During the first trimester of pregnancy,the $corpus \ luteum$ in the ovary is the primary source of progesterone,which is essential for maintaining the pregnancy. If both ovaries are removed during this period,the progesterone levels drop significantly,leading to the shedding of the uterine lining and resulting in abortion. After the first trimester,the placenta takes over the role of producing sufficient progesterone to maintain the pregnancy.

(A) The decidua is the modified endometrium of the uterus during pregnancy.
It is divided into three regions based on its position relative to the embryo:
$1$. $Decidua$ $basalis$: This is the part of the decidua that lies between the embryo and the stratum basalis of the uterus. It contributes to the formation of the maternal component of the placenta.
$2$. $Decidua$ $capsularis$: This is the part of the decidua that covers the embryo and separates it from the uterine cavity.
$3$. $Decidua$ $parietalis$: This is the remaining part of the decidua that lines the rest of the uterine cavity.
Therefore,the $Decidua$ $basalis$ is the correct answer as it directly participates in the formation of the maternal portion of the placenta.

(C) In the $Haemoendothelial$ type of placenta,all maternal tissue layers and the connective tissue of the foetal villi are eroded,leaving only the endothelium of the foetal blood vessels in direct contact with the maternal blood. This results in the minimum number of barriers between the foetal and maternal blood. Examples include rats and rabbits.

(C) The umbilical cord acts as a vital connection between the foetus and the placenta.
It contains umbilical arteries and an umbilical vein,which transport blood exclusively between the foetus and the placenta.
Therefore,the blood circulating within the umbilical cord is $100\;\%$ foetal in origin,as it is part of the foetal circulatory system.

(C) The human placenta is classified as follows:
$1$. Hemochorial: The maternal blood comes in direct contact with the chorionic villi because the maternal endothelium,connective tissue,and uterine epithelium are eroded.
$2$. Metadiscoidal: Initially,the villi are distributed over the entire surface of the blastocyst,but later they are restricted to a disc-shaped area.
$3$. Deciduate: During parturition,a portion of the maternal uterine tissue (decidua) is shed along with the placenta.
$4$. Chorionic: It is formed primarily by the chorionic villi.
Therefore,the correct classification is Hemochorial,metadiscoidal,deciduate,and chorionic.

(A) The human placenta is classified as metadiscoidal. In the early stages of development,the chorionic villi are distributed over the entire surface of the blastocyst (diffuse),but as development progresses,they become restricted to a disc-shaped area,resulting in a discoidal placenta.

(A) $hCG$ (human chorionic gonadotropin) is produced by the developing placenta after implantation.
It acts on the corpus luteum to prevent its regression,thereby maintaining it as the corpus luteum of pregnancy.
The corpus luteum continues to secrete progesterone,which is essential for maintaining the endometrium during the first trimester of pregnancy.
After the first trimester,the placenta takes over the role of producing sufficient progesterone to maintain the pregnancy.

(A) During the first trimester of pregnancy,the corpus luteum in the ovary is the primary source of progesterone,which is essential for maintaining the pregnancy.
If both ovaries are removed during the first trimester,the source of progesterone is lost,leading to the termination of pregnancy (abortion).
However,after the first trimester,the placenta takes over the role of producing sufficient progesterone to maintain the pregnancy.
Therefore,if the ovaries are removed after the first trimester,the foetus continues to grow normally because the placenta provides the necessary hormones.
Thus,both the Assertion and the Reason are correct,and the Reason correctly explains the Assertion.

(B) The Assertion is correct: The placenta is connected to the embryo through an umbilical cord,which facilitates the transport of oxygen,nutrients,and waste products between the mother and the fetus.
The Reason is also correct: The placenta acts as an endocrine tissue and produces several hormones like human chorionic gonadotropin $(hCG)$,human placental lactogen $(hPL)$,estrogens,and progestogens,which are essential for maintaining pregnancy.
Since both statements are factually correct but the Reason does not explain why the umbilical cord is used for transport,the correct option is $B$.

(C) The Assertion is incorrect because the first sign of a growing foetus is usually noticed by listening to the heart sound,but this is typically done after the first month of pregnancy,not as the 'first sign' in a general developmental context compared to other milestones. More importantly,the heart sound is best heard via a stethoscope later in development. However,the statement in the Assertion is technically considered false because the first sign of pregnancy is often the missed period or pregnancy test,and the heart sound is a clinical sign.
The Reason is correct because,by the end of the second month of pregnancy,the foetus does indeed develop limbs and digits.
Therefore,the Assertion is incorrect and the Reason is correct.

(C) The morula undergoes compaction to form the blastocyst,which consists of two distinct cell types: the outer layer called the trophoblast and the inner group of cells known as the inner cell mass (embryoblast).
The trophoblast cells eventually form the extra-embryonic membranes (like the chorion) and the placenta,while the inner cell mass differentiates into the embryo proper.
Therefore,the Assertion is correct as it describes the formation of these two cell types,but the Reason is incorrect because the inner cell mass forms the embryo,not the trophoblast.

(A) Gastrulation is the process in embryonic development during which the blastula (or blastocyst in mammals) is reorganized into a multilayered structure known as the gastrula,which contains the three germinal layers: ectoderm,mesoderm,and endoderm.
This process is driven by coordinated cell movements known as morphogenetic movements,which include processes like epiboly,emboly,involution,invagination,and delamination.
These movements allow cells to migrate to their specific positions,thereby establishing the body plan and the final morphology of the embryo.
Therefore,both the Assertion and the Reason are correct,and the Reason provides the correct explanation for the process described in the Assertion.

(B) During pregnancy,the $Corpus \ luteum$ is the primary structure responsible for secreting the hormone $Relaxin$ in the later stages.
$Relaxin$ helps in the relaxation of the pelvic ligaments and widens the birth canal to facilitate parturition (childbirth).
While the placenta also produces various hormones like $hCG$,$hPL$,$estrogen$,and $progesterone$,$Relaxin$ is specifically secreted by the $Corpus \ luteum$ in humans.

(D) Progesterone is known as the pregnancy hormone.
It is secreted by the corpus luteum after ovulation.
Its primary function is to maintain the endometrium of the uterus,which is essential for the implantation of the embryo and the successful maintenance of pregnancy.
If the level of progesterone drops,it leads to the shedding of the endometrium,resulting in menstruation.

(A) The development of an embryo from a zygote involves two primary processes:
$1$. $I -$ Cell division (Mitosis): The zygote undergoes repeated mitotic divisions to increase the number of cells,forming the embryo.
$2$. $III -$ Cell differentiation: These cells undergo differentiation to form specialized tissues and organs.
Meiosis $(II)$ does not occur during embryogenesis because the zygote is already diploid $(2n)$ and needs to maintain this ploidy level in the developing organism. Therefore,the correct processes are $I$ and $III$.

(C) In organisms where the development of the zygote takes place inside the female body, the organism is known as $Viviparous$.
In $Viviparous$ animals, the zygote develops into a young one inside the female body, and after attaining a certain stage of growth, the young ones are delivered out of the body of the female organism.
This is in contrast to $Oviparous$ animals, where the development occurs outside the female body in eggs.

(C) After ovulation,the ruptured Graafian follicle transforms into a temporary endocrine structure known as the $Corpus \ luteum$.
This structure primarily secretes large amounts of $Progesterone$,which is essential for the maintenance of the endometrium for implantation of the fertilized ovum.
Therefore,the correct option is $C$.

(B) The endometrium is the inner lining of the uterus which is essential for the implantation of the fertilized ovum and other events of pregnancy.
Progesterone is a hormone secreted by the corpus luteum after ovulation.
It is primarily responsible for the maintenance of the endometrium,which is necessary for the support of pregnancy.
If fertilization does not occur,the corpus luteum degenerates,leading to a decrease in progesterone levels,which causes the breakdown of the endometrium (menstruation).

(A) The mitotic division starts as the zygote moves through the isthmus of the oviduct towards the uterus and forms $2, 4, 8,$ and $16$ daughter cells called blastomeres.
The embryo with $8-16$ blastomeres is called a morula.
The morula continues to divide and transforms into a blastocyst as it moves further into the uterus.

(A) The process of human embryonic development involves the formation of a $16$-celled stage known as the $Morula$.
Following fertilization,the $Zygote$ undergoes mitotic divisions called cleavage to form $2, 4, 8,$ and $16$ daughter cells called blastomeres.
The embryo with $8-16$ blastomeres is called a $Morula$.
The $Morula$ continues to divide and transforms into a $Blastocyst$ as it moves further into the uterus.