Test: Embryology - NEET PG MCQ

• The first pair of somites arises in the occipital region of the embryo at approximately the 20th day of development.
• From here, new somites appear in craniocaudal sequence at a rate of approximately three pairs per day until, at the end of the fifth week, 42 to 44 pairs are present.
• The first occipital and the last five to seven coccygeal somites later disappear (now the total is 37), these remaining somites form the axial skeleton.
• The age of an embryo can be accurately determined by counting number of somites

• Embryonic period extends from beginning of the third week to the end of eighth week of intrauterine life.
• Some authors consider the embryonic period from fertilization to the end of eight week.

• Embryonic period extends from beginning of the third week to the end of eighth week of intrauterine life.
• There is no correct answer, the most appropriate option has been chosen as the answer.
• Total number of somites developing in fetus are 42 pair

• During prophase of meiosis-I, pairing of maternal and paternal (homologous) chromosomes (synapsis) occurs, followed by chiasma (site for genetic exchange) formation and crossover of genetic material between non-sister chromatids. There is no change of ploidy status, i.e., the diploid cell remains diploid. Maternal and paternal (homologous) chromosomes separate during Anaphase I of Meiosis I.
• During the synthesis (S) phase of interphase, chromosomes in a cell are replicated and sister chromatids are created.
• A sister chromatid is ‘one-half ’ of the duplicated chromosome. Sister chromatids separate during anaphase II of meiosis II.

There are three subtypes of spermatogonia in humans:

• Type A (dark) cells, which are reserve spermatogonial stem cells which do not usually undergo active mitosis.
• Type A (pale) cells, which are the spermatogonial stem cells that undergo active mitosis to produce Type B cells.
• Type B cells, which divide (mitosis) to give rise to primary spermatocytes.

Primary spermatocyte subsequently enters meiosis cell division.

• Spermatid is the haploid male gamete that results from division of secondary spermatocytes.
• As a result of meiosis, each spermatid contains only half of the genetic material present in the original primary spermatocyte.
• Spermatids spermiogenesis to form the spermatozoa. They undergo no cell division (mitosis or meiosis).

• Spermatogonia are diploid (2n) cells containing 46 chromosomes, which divide (mitosis) to give rise to primary spermatocytes.
• Primary spermatocytes are diploid (2n) cells, which undergo meiosis I, to give two secondary spermatocytes.
• Secondary spermatocytes are haploid (n) cells that contain 23 chromosomes.

• Spermatogenesis is the process in which spermatozoa are produced from spermatogonial stem cells by way of mitosis and meiosis and takes 74 days to complete.
• Earlier editions of standard textbooks used to mention it as 64 days (2 months).

• Primary spermatocyte changes to secondary spermatocyte during meiosis I.
• Maternal and paternal chromosomes separate during Meiosis - I by independent assortment.
• During meiosis, the pairs of homologous chromosome are divided in half to form haploid cells, and this separation, or assortment, of homologous chromosomes is ‘random’.
• This means that all of the maternal chromosomes will not be separated into one cell, while the all paternal chromosomes are separated into another. Instead, after meiosis occurs, each haploid cell contains a ‘mixture’ of genes from the individual’s mother and father, and that too at random levels, i.e., its not predetermined.

• Chromosome number gets reduced to half during meiosis- I (reduction division).
• Maternal and paternal chromosomes separate during meiosis- I.
• Meiosis- II doesn’t reduce the number of chromosomes (hence, it is not a reduction division).

• Secondary oocyte is a haploid cell (n) with two units of DNA (2N).
• Key: n- number of chromosomes and N- amount of DNA.

• Primordial germ cell is a derivative of epiblast, earlier they were believed to arise from endoderm of yolk sac (endodermal sinus)
• They are derivatives of epiblast cells, developing in primitive streak and later migrate to the endodermal yolk sac and pass through the dorsal mesentery of hindgut to finally reach the genital ridge.

• Primordial germ cell is a derivative of epiblast, earlier they were believed to arise from endoderm of yolk sac (endodermal sinus)
• They become evident at the distal end of primitive streak by the 2nd week of development.
• These cells are migratory cells and reach the endodermal wall of yolk sac (fourth week).
• They reach the indeterminate gonad by the end of the fifth week, to differentiate into gametes (gametogenesis).
• Note: Aberrant migration may lead to germ cell tumours (e.g., teratoma).

• Oocyte lies eccentrically in the ovarian (Graafian) follicle surrounded by some granulosa cells called cumulus oophorus.
• Discus proligerus is the attachment point of the cumulus oophorus to the most peripheral granulosa cells of an antral follicle.

At fertilization, the secondary oocyte completes meiosis II to form a mature oocyte (23, 1N) and a second polar body.

• After ovulation has occurred, the oocyte (ovum) remains fertilizable for 48 hours, although the chance is mostly lost by 18–24 hours.
• If no fertilization occurs, the oocyte will degenerate between 12 and 24 hours after ovulation.
• Sperm is capable of fertilization for 48 hours, once ejaculated in the female genital tract.

• The majority of oogonia continue to divide by mitosis, but some of them arrest their cell division in prophase of meiosis I and form primary oocytes.
• After first meiotic division secondary oocyte is formed and after second meiosis mature oocyte (ovum) is formed.

• Meiosis is a reduction division, since it reduces the chromosome number to half. Primary gametocyte (diploid) undergoes meiosis-I to become secondary gametocyte (haploid).
• Male produces four functional sperm for each spermatocyte that enters meiosis, but the female produces only one functional ovum for each oocyte that completes meiosis.

Although sperms remain alive and motile in the cervical canal and uterus for 5–7 days and in the fallopian tubes for 85 hours, they usually do not retain their power of fertilization after 24–48 hours of coitus.

• After ovulation has occurred, the oocyte (ovum) remains fertilizable for 48 hours, although the chance is mostly lost by 18–24 hours.
• If no fertilization occurs, the oocyte will degenerate between 12 and 24 hours after ovulation.

• The most appropriate option has been taken as the answer.
• An acute rise of LH triggers ovulation and development of the corpus luteum.
• LH surge occurs 34–36 hours before ovulation. Some authors mention a wider range: 24–48 hours before ovulation.
• Ion responsible to prevent polyspermy at the time of fertilization is Ca (Calcium) 
• Correct sequence of the embryonic events is Acrosome reaction → Cortical reaction → Zona reaction 

Trophoblast differentiates into cytotrophoblast and syncytiotrophoblast at 6th day post-ovulation (or post-fertilization). 

Fertilized ovum forms the morula, which enters uterine cavity on day 4, post ovulation (or post-fertilization).

• Conceptus enters the uterine cavity at advanced morula stage (more than 16 cells) at day 4.
• It gets converted into blastocyst same day and later attaches to endometrium on day 6 (implantation).

• Implantation is a week-long process, beginning at day 5 (post-ovulation, or post-fertilization) and is completed at day 12.
• It is 20th to the 23rd day after the last menstrual period.