General Embryology Question And Answers

General embryology

Define embryology

The embryology is study of the prenatal development of an individual. The developmental events begin with the fertilization of an ovum and culminate with the birth of a baby.

General embryology Generally, this phase of development is divided into two periods:

General embryology Embryonic period: From fertilization to the end of the 8th week.

General embryology Fetal period: From the beginning of the 9th week till the birth of a baby.

Question 1. What is fertilization? Give steps and results of the fertilization.
Answer:

Fertilization:

The fertilization is a process of union of male and female gametes (pronuclei).

Fertilization Steps of fertilization

Fertilization These are:

• Penetration of corona radiata by a spermatozoa
• Penetration of zona pellucida
• Fusion of cell membranes of male germ cell (sperm) and female germ cell (secondary oocyte)
• Completion of second meiotic divisions of the secondary oocyte
• Formation of male and female pronuclei
• Fusion of male and female pronuclei

Read And Learn More: Anatomy Notes And Important Question And Answers

Fertilization Results of fertilization (i.e., events occurring just after fertilization):

Fertilization These are:

• Formation of the zygote (transient)
• Restoration of diploid number of chromosomes
• Determination of genetic sex
• Initiation of cleavage
• Variation of species

Question 2. Write a short note on in vitro fertilization (IVF).
Answer:

Vitro fertilization:

It is the fertilization of ova (secondary oocytes) in vitro* (i.e. in a petri dish or test tube).

Vitro fertilization The steps involved in IVF (in vitro fertilization) are as follows:

• Stimulation of ovarian follicles by gonadotrophins viz. FSH, progesterone etc.
• Aspiration of secondary oocytes from mature ovarian follicles.
• Oocytes are placed in a petri-dish containing a culture medium.
• Sperms are added to the petri dish containing ova.
• Fertilization and cleavage of zygote monitored by microscope till it reaches 8–16 cell stage (blastocyst).
• About 4–8 blastocysts are transferred into the uterus.

Question 3. What is a zygote?
Answer:

Zygote:

• It is a transient unicellular stage of developing humans formed as a result of fertilization.
• It is diploid in number.
• It is multipotent.

Question 4. Write a short note on spermatogenesis.
Answer:

Spermatogenesis:

The spermatogenesis is a process of events by which spermatogonia are transformed into spermatozoa.

Stages of spermatogenesis:

• Spermatogonia (diploid) divides mitotically into two primary spermatocytes.
• Each primary spermatocyte (diploid) undergoes the first meiotic division to form two secondary spermatocytes.
• Each secondary spermatocyte (haploid) undergoes a second meiotic division to form two spermatids (haploid).
• Each spermatid (haploid) is transformed into a spermatozoon by a process called spermiogenesis.

Question 5. Write a short note on spermiogenesis.
Answer:

Spermiogenesis:

• It is a process by which spermatids are transformed into spermatozoa.
• The spermatid is a small circular cell with a spherical nucleus, golgi complex, mitochondria, and a centriole.

Spermiogenesis The following changes take place in the spermatid:

• The nucleus becomes denser and more ovoid to form the head.
• The Golgi complex is converted into an acrosomal cap (a dense staining body) that caps the head.
• Centrosome contains two centrioles (proximal and distal).
  • The proximal centriole moves to the neck and gives rise to the axial filament.
  • Distal centriole moves at the distal end of the body to form an annulus (i.e., ring around the distal end of the middle piece).
• The axial filament between the head and annulus becomes surrounded by mitochondria to form the middle piece of the spermatozoa.
• The axial filament elongates to form a tail (principal piece).
• The cytoplasm is extruded/shed off, and only the cell membrane remains as the covering of spermatozoa.

Question 6. Give the anatomical/embryological basis of semen analysis in male infertility.
Answer:

Male infertility:

The semen analysis is done to see the following factors responsible for male infertility.

Male infertility Low sperm count

• Oligospermia <15 million per ejaculate
• Azoospermia, i.e. zero sperm count (i.e. absence of sperms in semen)

Male infertility Abnormal sperm/gametes

• Chromosomal abnormalities, viz. monosomy or trisomy
• Morphological abnormalities, viz. sperms with double tail, sperms with double head and a common tail, etc.

Male infertility Sperm motility < 40% is abnormal sperm motility

Question 7. Write a short note on oogenesis
Answer:

Oogenesis:

It is a process by which oogonia are transformed into oocytes (female gametes).

Oogenesis Stages of oogenesis

• Oogonium enlarges to form a primary oocyte (diploid).
• Primary oocytes undergo first meiotic division to form a secondary oocyte (haploid) and the 1st polar body (haploid).
• The secondary oocyte (haploid) undergoes a second meiotic division to form an ovum (mature secondary oocyte) and the second polar body (haploid).

Oogenesis Note:

The first meiotic division of the ovum completes at its formation, while its second meiotic division completes only if the ovum is fertilized.

Question 8. Write a short note on the Graafian follicle.
Answer:

Graafian follicle:

• It is a mature ovarian follicle that lies in the ovarian cortex near the surface.
• It is a fluid-filled follicle and contains a mature oocyte (secondary oocyte).
• It ruptures during ovulation to release the ovum.
• Empty follicle collapses to form corpus luteum which acts as an endocrine gland and secrete progesterone.

Graafian follicle Functions:

• Provides maturation and release of a mature oocyte.
• Forms corpus luteum which secretes progesterone a hormone essential to maintain early pregnancy.

Question 9. Write a short note corpus luteum.
Answer:

Iuteum:

After ovulation, the empty Graafian follicle, under the influence of LH, shrinks and is transformed into a yellowish glandular structure called corpus luteum.

luteum Steps of Formation

• First, the granular cells and theca internal cells get vascularized by the surrounding vessels and become polyhedral.
• Later, a yellow pigment develops in these cells, and they are called luteal cells.

luteum The fate of corpus luteum

• If fertilization occurs, the corpus luteum persists for 3–4 months (under the influence of HCG) and is called a corpus luteum pregnancy. It secretes progesterone that maintains pregnancy for the initial 3–4 months.
• If fertilization does not occur, the corpus luteum lasts only for 10–14 days. Thereafter, it degenerates and transformed into a mass of fibrous tissue called corpus albicans. It is also known as the corpus luteum of menstruation.

Question 10. Write a short note on implantation.
Answer:

Implantation:

It is the process of embedding the blastocyst (i.e. conceptus) in the endometrium of the uterus. It occurs 6–10 days after fertilization.

Stages of implantation:

• Zona pellucida disappears.
• Trophoblast adheres to the endometrium.
• Trophoblast differentiates into cytotrophoblast and syncytiotrophoblast.
• Syncytiotrophoblast penetrates the endometrium.
• Migration of blastocyst into the endometrium.

Site of implantation:

The endometrium of the posterior wall of the uterine cavity near the fundus.

Implantation Applied anatomy:

If the implantation is deep, it may lead to postpartum bleeding.

Question 11. Enumerate abnormal sites of the implantation.
Answer:

Implantation:

Implantation The abnormal sites of implantation are:

• In the uterine tube (tubal implantation): The order of frequency is
• ampulla,
• infundibulum, and
• interstitial part.
• In the abdomen (abdominal implantation): The order of frequency is
• pouch of Douglas (rectouterine pouch) and mesentery.
• In the ovary (ovarian implantation).

Question 12. Write a short note on ectopic pregnancy.
Answer:

Ectopic pregnancy:

It is a pregnancy in which the fetus develops outside the uterus. It occurs due to the implantation of a blastocyst outside the uterus.

Ectopic pregnancy The common sites of ectopic pregnancies, in order of frequency, are:

1. uterine tube,
2. abdominal cavity,
3. ovary.

The Tubal pregnancy is the commonest (95%), and if it is allowed to progress, the uterine tube generally ruptures in 2nd month of the pregnancy, leading to severe internal bleeding which may be fatal.

Ectopic pregnancy Clinically it presents as:

• Pelvic pain
• Vaginal bleeding

Question 13. Write a short note on decidua. 
Answer:

Decidua:

The endometrium of the uterus undergoes significant changes in preparation for implantation to occur (decidual reaction). After implantation, this changed endometrium is termed decidua (decidua = shedding off).

Decidual reaction:

It occurs under the influence of the HCG. The following changes occur in the endometrium.

• Cells of the endometrium (i.e. stromal cells) swell due to the accumulation of glycogen and lipid in their cytoplasm (now called decidual cells).
• Nuclei become rounded.
• The number of cytoplasmic organelles increases.

Decidua Functions:

• To provide a suitable site for implantation.
• To provide nutrition for the early embryo.
• To provide the immunologically privileged site for conceptus.

DeciduaParts of decidua:

• Decidua basalis, part towards the embryonic pole, deep into the embryo.
• Decidua capsularis, part toward the embryonic pole uterine cavity. It surrounds the embryo.
• Decidua parietalis, the remaining part of the decidua.

Decidua Fate:

• Decidua basalis undergoes development to form a decidual plate, which gives rise to the maternal component of the placenta.
• Decidua capsularis disintegrates and fuses with the decidua parietalis, and obliterates the uterine cavity.
• At the time of delivery, the placenta separates with decidua and is expelled.

Question 14. Write a short note on blastocyst. 
Answer:

Blastocyst:

The morula (consisting of 16 blastomeres) is transformed into a blastocyst shortly after its entry into the uterine cavity (4th day after fertilization). The fluid-filled spaces appear inside the morula and coalesce to form a cavity called a blastocyst cavity.

Blastocyst As the fluid level increases in this cavity, the blastomeres are separated into two parts:

• An outer cell layer called trophoblast (G. trophic = nutrition).
• An inner cell mass of remaining blastomeres at one side is called an embryoblast.

Blastocyst Fate of blastocyst:

• Embryoblast gives rise to the embryo proper.
• Trophoblast gives rise to the embryonic part of the placenta.
• Fluid in the blastocyst provides nutrition to the blastomeres.

Question 15. Write a short note on zona pellucida.
Answer:

Zona pellucida:

• The zona pellucida (ZP) is an extracellular matrix surrounding the developing oocyte.
• It is formed by the secretions from the oocyte and surrounding granulosa cells of the follicle.
• It consists of 4 special types of zona pellucida glycoproteins, viz. ZP1, ZP2, ZP3 and ZP4.

Zona pellucida Functions:

• Protection of oocytes during development.
• Growth and transport of oocyte.
• Prevention of polyspermy.
• Prevention of ectopic pregnancy.

Question 16. Write a short note on extraembryonic/fetal membranes.
Answer:

Fetal membranes:

All structures derived from the zygote and yet not forming any part of the embryo are called extraembryonic/fetal membranes.

Fetal membranes These are:

• Amnion
• Chorion
• Yolk sac
• Allantois
• Placenta
• Umbilical cord

Fetal membranes The roles played by these membranes are given as follows:

• Amnion forms an amniotic sac filled with fluid. It protects the fetus from injury.
• Chorion forms the fetal part of the placenta.
• The Yolk sac forms a primitive gut tube and allantois.
• Allantois forms the apex of the urinary bladder and median umbilical ligament.
• Placenta provides exchange, respiratory gases (O2 and CO2) nutrition, and waste products between mother and fetus.
• The umbilical cord provides passages for blood to pass to and fro between mother and fetus to subserve the functions of the placenta hence, it is also called the lifeline between mother and fetus.

Question 17. Write a short note on the yolk sac.
Answer:

Yolk sac:

The yolk sac develops from the blastocyst cavity and thus lies ventral to the embryonic disc.

Yolk sac The functions of the yolk sac are:

• Providing nutrition to the development of the embryo (early stage).
• Hemopoiesis until the liver is formed.
• Formation of primordial germ cells.

Yolk sac Stages of development:

Yolk sac The yolk sac undergoes 3 stages of development as follows:

• Primary Yolk Sac: At the end of 2nd week, the cavity of the blastocyst becomes lined by Heuser’s membrane and becomes the primary yolk sac. The Heuser’s membrane is made up of flattened cells derived from the endoderm of the embryonic disc.
• Secondary Yolk Sac: With the appearance of extraembryonic coelom, the primary yolk sac becomes much smaller, and flattened cells lining it become cuboidal. The primary yolk sac is now called secondary yolk sac.
• Tertiary Yolk Sac (Remnant ofSecondary Yolk Sac): The dorsal part of the secondary yolk sac becomes incorporated in the embryo while the ventral part which is not incorporated in the embryo, communicates with the midgut by the vitellointestinal duct.

Yolk sac Thus, the secondary yolk sac is divided into 3 parts:

• Intraembryonic part
• The intermediate connecting part called the vitellointestinal duct
• The extraembryonic part called the tertiary yolk sac

Yolk sac Fate:

• The intraembryonic part gives rise to the gut tube (foregut, midgut, and hindgut) and allantoic diverticulum.
• The intermediate part or Vitello intestinal duct atrophies and gets detached from the midgut loop.
• The extraembryonic part atrophies and disappears.
• Vitelline vessels supplying the yolk sac give rise to celiac superior mesenteric and inferior mesenteric vessels.

Yolk sac Applied anatomy:

If vitello-intestinal duct fails to atrophy it persists as Meckel’s diverticulum, sinus, or fistula.

Question 18. Write a short note on Allantois. 
Answer:

Allantois:

Allantois is a diverticulum, which arises from the caudal end of the yolk sac. It grows into connecting stalk. After the folding of the embryo, it becomes connected to the dilated terminal part of the hindgut, the cloaca, and passes from the ventral side of the cloaca into the connecting stalk. The allantois is vascularized by allantoic vessels.

Allantois Fate:

The developing urinary bladder is continuous with the allantois.

• Allantois atrophies and is seen in postnatal life as a fibrous band – the urachus, which extends from the apex of the urinary bladder to the umbilicus. It is also called as median umbilical ligament. It may contribute to the formation of the apex of the urinary bladder.
• The allantoic vessel becomes an umbilical vessel and gets connected to the placenta.

Allantois Congenital anomalies:

If allantois fails to fibrosis, it persists as urachal fistula, urachal sinus, or urachal cyst.

Question 19. Write a short note on amnion and discuss its applied anatomy.
Answer:

Anatomy:

The amnion is a thin, tough extraembryonic membrane that forms an amniotic sac filled with fluid called amniotic fluid. Initially, the sac lies dorsal to the embryonic disc but as it enlarges, it envelops the embryo’s future umbilical cord and fetal part of the placenta. It develops during 10–12 weeks of IUL.

Anatomy Formation:

The amnion consists of two layers: an inner layer of angiogenic/amniotic cells and an outer layer of somatopleuric mesoderm. The angiogenic layer is derived from the edges of the epiblast of the embryonic disc, while the somatopleuric layer is derived from the extraembryonic mesoderm.

Applied anatomy:

• Amniotic cells lack major histocompatibility complex antigens, which enable these cells to be exposed to the maternal immune system without any adverse reaction.
• Nowadays, the amnion is being used:
  • In the repair of corneas.
  • As a graft material for restructuring the vagina in women with cloacal abnormalities.

Question 20. Describe in brief the amniotic fluid and its applied importance.
Answer:

Importance:

It is a fluid present in the amniotic sac of the developing embryo.

Importance Formation:

Importance It is formed by:

• Filtration of fluid from maternal and fetal vessels
• Urine secreted by the fetus

Importance The volume of amniotic fluid:

• At week 10 of gestation = 30 ml
• At week 20 of gestation = 350 ml
• At week 37 of gestation = 800–1000 ml

Importance Composition:

• Metabolites
• Hormones (HCG and HPL)
• Desquamated cells of the fetal epithelium
• Fetal urine

Importance Circulation:

Importance Part of it:

• Goes into the maternal blood
• Swallowed by the fetus

Importance Functions:

• Protects embryo from injury by acting as a water cushion (shock absorber)
• Permits symmetrical external growth of the embryo
• Regulates body temperature of the embryo
• Forms hydrostatic bag (bag of water), which helps in dilatation of the cervix during birth
• Allows free movements of the fetus for proper development of the musculoskeletal system

Importance Congenital anomalies:

Importance Oligohydramnios:

A condition in which the amount of amniotic fluid is less than 400 ml. It is associated with renal agenesis. This condition may cause pulmonary hypoplasia, facial defects, limb defects, and compression of the umbilical cord.

Importance Polyhydramnios:

A condition in which the amount of amniotic fluid is more than 2000 ml. It is usually
associated with severe anomalies of the central nervous system and esophageal atresia.

Importance Applied anatomy

Importance Amniocentesis:

It is the aspiration of amniotic fluid to estimate the level of alpha-fetoprotein to detect fetal defects and to do chromosomal analysis for antenatal sex determination. It is done at the 14th or 15th week of gestation when the amount of fluid is about 200 ml.

Question 21. Write a short note on chorion.
Answer:

Chorion:

The chorion is an extraembryonic membrane that envelops the developing embryo and plays a key role in the development of the placenta.

Chorion Formation:

It is formed by an outer layer of trophoblast and an inner layer of somatopleuric extraembryonic mesoderm.

Chorion Fate:

• It gives rise to numerous fingers-like projections called chorionic villi, which grow, i.e. invade the surrounding endometrium (decidua) and allow the transfer of nutrients from maternal blood to fetal blood.
• Later the chorion differentiation into two parts chorion leave and chorion frondosum.

Chorion Chorion leave:

On the side of the decidua capsularis, the chorionic villi degenerate, leaving behind a smooth surface called chorionic leave.

Chorion Chorionic frondosum:

On the side of decidua basalis, the chorionic villi grow further, giving rise to a leafy appearance called chorionic frondosum. This forms the fetal part of the placenta.

Chorion Applied anatomy

Chorion Chorionic villi biopsy:

It is done to detect genetic disorders at week 8, i.e., much earlier than the amniocentesis
which is performed after week 14 of gestation.

Question 22. Write a short note on the umbilical cord.
Answer:

Umbilical cord:

It is a long cord with a length of 2 ft (50 cm) and a diameter of 1–2 cm, which extends from the umbilicus of the fetus to the placenta. It is considered a lifeline connecting the fetus to the mother.

Umbilical cord Formation:

After the folding of an embryo, the connecting stalk elongates to form the umbilical cord. It is covered by a glistening membrane, the amnion.

Umbilical cord Contents:

• Two umbilical arteries
• One umbilical vein (left)
• Wharton’s jelly (formed due to mucoid degeneration of intraembryonic mesoderm of the connecting stalk)
• Remnants of vitellointestinal duct
• Remnants of allantoic diverticulum

Umbilical cord Functions:

• It provides passage to transfer deoxygenated blood of the fetus to the placenta.
• It provides passage to transfer oxygenated blood from the placenta to the fetus.
• It suspends the fetus into the amniotic cavity.

Umbilical cord Applied anatomy:

• Cord prolapse: The umbilical cord may prolapse through the uterus during childbirth and may be compressed between the fetal head and pelvic wall of the mother. This may cause fetal hypoxia.
• During delivery, it may encircle the neck of the fetus and cause fetal strangulation.
• Nowadays, the umbilical cord is being cryopreserved for stem cells for future clinical use.

Question 23. Describe the placenta in brief and discuss its applied anatomy.
Answer:

Anatomy:

The placenta is a feto-maternal organ that develops within the uterus. It is a highly vascular disc-like structure, by which the fetus is intimately connected to the mother (uterus).

Anatomy Functions:

In general, it provides oxygen and nutrients to the fetus and removes waste products from the fetus. The details are as under:

• Exchanges of respiratory gases, i.e., O₂ and CO₂ between fetus and mother.
• Transport of nutrients from mother to fetus.
• Transport of waste products from the fetus to the mother.
• Synthesis of hormones, such as HCG, progesterone, estrogens somatotropic hormone, and relaxin.
• Prevention of harmful microorganisms, drugs, and hormones (ACTH and TSH) to enter the fetal blood from the mother (barrier function).
• Storage glycogen, calcium, and iron in the early month of pregnancy.
• Transmission of antibodies from mother to fetus, viz. IgG, α-globulins, and immunoglobulins.

Anatomy Development/formation:

The placenta develops from two sources: fetal and maternal. The fetal source is chorion frondosum, and the maternal source is decidua basalis. (The chorion consists of trophoblast and extraembryonic mesoderm. The trophoblast differentiates into cytotrophoblast and syncytiotrophoblast. The chorion with villi is called chorion frondosum.)

Anatomy Chorion frondosum:

It forms villi (primary, secondary, and tertiary villi), cytotrophoblastic shells, and intervillous spaces.

• Primary villi: These are finger-like projections derived from cytotrophoblast covered by syncytiotrophoblast.
• Secondary villi: The mesodermal core penetrates the primary villi to form the secondary villi.
• Tertiary villi: The fetal blood vessels develop in the mesoderm of the secondary villi to form the tertiary villi.
• Cytotrophoblastic shell: The cytotrophoblast from the apical region of the villi penetrates the syncytiotrophoblast to reach the decidua basalis where it spreads out to form a layer called a cytotrophoblastic shell. Now, all the tertiary villi are anchored to the decidua basalis, and they are called anchoring villi.
• Intervillous spaces: These are lacunae in syncytiotrophoblast, which communicate with each other and are filled with maternal blood. The blood enters these spaces due to the erosion of blood vessels of the endometrium by the syncytiotrophoblast.

Anatomy Decidua basalis:

• The decidua basalis provides the site where chronic frondosum grows to form villi and intervillous spaces to allow maternal blood vessels to enter into the intervillous spaces.
• The decidua basalis also forms septa, which grow into intervillous spaces of the developing placenta and divides it into various lobules called cotyledons.

Anatomy Full-term placenta:

• It presents the following features:
• Shape: Disc-shaped.
• Dimensions
  • Weight: 500–600 gm
  • Diameter: 15–20 cm
  • Thickness: 3 cm
• Surfaces:
• These are two:
  • Maternal surface: Presents 15–20 lobes/cotyledons.
  • Fetal surfaces: Smooth, shining, and covered with amnion. Umbilical vessels ramify under amnion. The umbilical cord is attached to this surface.

Anatomy Note:

Anatomy Components of full-term placenta

Fetal components

• Amnion
• Chorion
• Choriodecidual spaces
• Cotyledons

Anatomy Maternal components

• Maternal blood
• Endometrial septae
• Decidua

Anatomy Applied anatomy

Anatomy Hydatidiform mole or vesicular mole:

• It occurs due to the excessive proliferation of the trophoblast to form a vesicular or polycystic mass called a hydatidiform mole.
• It resembles a bunch of grapes. It often leads to the death of the embryo. It is usually a noninvasive growth. But in about 3–5% of cases,
  the hydatidiform mole may undergo malignant change and form choriocarcinoma.

Question 24. Write a short note on the placental membrane or placental barrier.
Answer:

Placental barrier:

The placental membrane/barrier separates fetal blood from maternal blood within the placenta. It is across this membrane that the exchange of gases, nutrients, and waste products takes place. It measures about 14 m2.

Placental barrier Layers of placental membrane:

Placental barrier From the maternal to fetal side, these are:

• Syncytiotrophoblast
• Cytotrophoblast
• The basement membrane of cytotrophoblast
• Mesoderm of villus
• The basement membrane of fetal capillaries
• Endothelium of fetal capillaries

In the later part of pregnancy, the thickness of the placental membrane decreases from 0.25 mm to 0.002 mm to increase.

The efficiency of the transport of nutrients as a result of the following changes:

• Syncytiotrophoblast becomes thin.
• Cytotrophoblast disappears.
• Two basement membranes disappear.
• Endothelial cells of the capillaries become thin.

Question 25. Give a brief account of anomalies of the placenta according to its shape.
Answer:

Account of anomalies of the placenta:

Account of anomalies of the placenta These are:

Account of anomalies of the placenta Bilobed/discoidal:

Placenta consists of two lobes.

Account of anomalies of the placenta Multilobular:

Placenta is divided into more than two lobes.

Account of anomalies of the placenta Circumvallate:

The peripheral margin of the placenta is surrounded by a sulcus and overlapped by a circular fold of decidua.

Account of anomalies of the placenta Placenta succenturiata:

In this, a small placenta is connected to the main placenta by blood vessels and membranes.

Account of anomalies of the placenta Diffuse placenta:

Placenta is thin and not disc-shaped.

Account of anomalies of the placenta Placenta fenestrate:

In this, a hole is present in the placental disc.

Question 26. Give a brief account of anomalies of the placenta according to the sites of attachment of the umbilical cord.
Answer:

Umbilical cord:

Umbilical cord These are:

Umbilical cord Marginal (battledore) placenta:

The cord is attached close to the margin of the placenta.

Umbilical cord Furcate placenta:

Umbilical vessels get divided before reaching the placenta.

Umbilical cord Velamentous placenta:

The cord fails to reach the placenta and gets attached to the amnion at the periphery. The umbilical vessels ramify in the amnion.

Question 27. Write a short note on placenta previa.
Answer:

Placenta previa:

The attachment of the placenta in the lower uterine segment (i.e., lower 1/3rd of the body of the uterus) is called placenta previa.

Placenta previa Incidence

1:200.

Placenta previa  Degrees of placenta previa

Placenta previa There are 4 types:

Placenta previa First-degree:

The attachment of the placenta does not reach up to the internal OS.

Placenta previa Second-degree:

The margin of the placenta reaches the internal OS but does not cover it.

Placenta previa Third-degree:

The edge of the placenta covers the internal OS, but when the OS is dilated at birth, the placenta does not occlude it.

Placenta previa Fourth-degree:

Placenta completely covers the internal OS and keeps it occluded even after the OS is dilated at birth. It is also called central placenta previa.

Placenta previa Applied anatomy

The painless bleeding in the third trimester of pregnancy (usually in the 8th month) is a diagnostic feature of placenta previa. It commonly occurs in the 4th degree of the placenta previa.

Question 28. What is gastrulation?
Answer:

Gastrulation:

• It is the process of formation of the three germ layers. They are precursors of all embryonic tissues.
• It is the beginning of the morphogenesis to develop body form.
• It is the most important event during the third week of pregnancy.
• During this period, the embryo is referred to as gastrula.
• During gastrulation, the bilaminar embryonic disc is converted into trilaminar embryonic disc.
• The first sign of gastrulation begins with the formation of the primitive streak.

Question 29. Write a short note on a primitive streak
It is a thickened linear band of epiblast that appears caudally in the median plane on the dorsal aspect of the embryonic disc at the beginning of the 3rd week.
Answer:

Primitive streak:

The important features of primitive streak are :

• It is formed as a result of the proliferation and movements of the epiblast cells to the median plane of the embryonic disc.
• It elongates by the addition of cells to its caudal end, whereas its cranial end proliferates to form a rounded elevation called a primitive node.
• A narrow groove called a primitive groove is continuous with a small depression in the primitive node called a primitive pit/blastopore.
• The primitive groove and primitive pit are formed by the invagination (inward movement) of epiblastic cells.

Primitive streak Significance:

• To determine the craniocaudal axis of the embryo
• To determine the dorsal and ventral surfaces of the embryo
• To determine the right and left halves of the embryo

Primitive streak The fate of the Primitive streak:

Forms extraembryonic mesoderm by ingression of its cells into embryonic disc until the end of 3rd week of IUL.

Primitive streak Applied anatomy:

Primitive streak Sacrococcygeal teratoma:

Normally, the primitive streak undergoes degenerative changes and disappears by the end of the 4th week of IUL. But, the remnants of primitive streak may persist and give rise to sacrococcygeal teratoma, the most common tumor in newborns.

Question 3O. Write a short note on the development of trilaminar germ disc.
Answer:

Germ disc:

The cells of the embryonic disc differentiate into 3 layers placed one above the other. From superficial to deep, these are ectoderm, mesoderm, and endoderm. This is called trilaminar (three-layered) germ disc.

The process of formation of these layers is as follows:

• The cells of embryoblast first differentiate into two layers a superficial layer of columnar cells called epiblast and a deep layer of flattened cells called hypoblast.
• The cells of epiblast migrate toward the future primitive streak. As they reach this region, they become flask-shaped. These flask-shaped cells detach
  themselves from the epiblast and slip underneath it.
• Some of these cells replace the hypoblast cells to form the endoderm and others lie above the newly formed endoderm to form the mesoderm. The remaining cells of the epiblast form the ectoderm.

Germ disc Note:

All the tissues and organs of the body develop from these three primary germ layers.

Question 31. Enumerate the derivatives of three germ layers.
Answer:

Germ layers:

Germ layers Derivatives of ectoderm

• Epidermis of skin, hair, nails, sweat and sebaceous glands, and mammary gland
• Epithelium of lips, cheek, gums, the floor of the mouth, palate, nasal cavities, and paranasal air sinuses
• Epithelium of the lower part of the anal canal, the lower part of the vagina, and external urethral meatus
• The enamel of teeth, Rathke’s pouch gives rise to adenohypophysis (anterior part of the pituitary)
• The lens of the eye, anterior epithelium of the cornea, and outer layer of the tympanic membrane
• Central nervous system including the retina, the optic nerve, epithelium over the ciliary body and iris, and musculature of the iris
• Peripheral nervous system and adrenal medulla (derivatives of neural crest)

Germ layers Derivatives of mesoderm:

• Connective tissue, cartilage, bone, and dentine of teeth
• Muscles, smooth cardiac, and skeletal
• Heart, blood vessels, and blood cells
• Lymph glands, lymph vessels, and spleen
• Connective tissue sheaths of muscles, tendons, and nerve endings, synovial membranes of joints, and bursae
• Dermis of skin
• Pachymeninx or dura mater
• Urinary system, i.e., kidney, ureter, and urinary bladder (except the part of the urinary bladder)
• The cortex of suprarenal (adrenal) gland
• Pericardium pleura and peritoneum
• Testes and ovaries

Germ layers Derivatives of endoderm:

• Epithelium of gastrointestinal tract except for the lower end of the anal canal
• Epithelium of tongue, pharynx, and respiratory tract (i.e., larynx, trachea, bronchi, and alveoli of the lungs)
• Epithelium of pharyngotympanic tube, middle ear cavity, an inner layer of the tympanic membrane, and mastoid air cells
• Parenchyma of liver, pancreas, thyroid, parathyroid, and thymus
• Epithelium of urinary bladder (except trigone), most of the female urethra, part of male urethra, prostate, and vagina
• Epithelium of uterus and upper part of the vagina

Question 32. Write a short note on the notochord.
Answer:

Notochord:

The notochord is a solid rod of cells situated in the midline of the embryonic disc. It extends from the primitive knot to the prochordal plate. The cells of notochord are derived from the primitive knot of the primitive streak.

Notochord Formation:

• The cells of primitive knots proliferate and move inward to form a depression called the blastopore.
• The cells from the bottom of the blastopore migrate forward in the midline toward the prochordal plate called the notochordal process.
• The notochordal process gets canalized to form a notochordal canal, which is continuous within the blastopore.
• Cells at the floor of the notochordal canal break and form a communication between the amniotic cavity and yolk sac. This is called the eurocentric canal.
• The wall of the Eurocentric canal is flattened to form a notochord plate.
• The notochordal plate becomes curved to form a canal again.
• Cells of the tube proliferate to fill its lumen. Thus, a solid rod of cells is formed again. This is called the definitive notochord, which extends from the primitive knot to the prochordal plate.

Notochord Significance/functions:

• Forms the central axis of the embryo
• Supports the embryo
• Acts as a vertebral column of the embryo
• Induces the surface ectoderm to form the neural tube

Notochord Fate of notochord:

• It is an embryonic structure and disappears in later life.
• In the adult life, its remains are as follows:
  • Nucleus pulposus of intervertebral discs
  • Apical ligament of dens

Notochord Applied Anatomy:

Notochord Chordoma:

It is a tumor that may arise from the remains of notochordal cells. It commonly arises either in the cranial region or in the sacral region.

Question 33. Write a short note on the neural tube.
Answer:

 Neural tube:

• The neural tube develops from the neural plate. In early embryonic life, viz. on day 16, the ectoderm lying in front of the notochord thickens to form a neural plate.
• The later neural plate folds to form neural grooves with elevated margins called neural folds. The tips of neural folds are called neural crests.
• Now the neural folds join to form a neural tube in such a way that neural crests separate from neural folds and come to lie dorsolateral to the neural tube. Now neural tube differentiates into an enlarged cranial part and a narrow caudal part.

 Neural tube The fate of neural tube:

• The enlarged cranial part gives rise to the brain.
• The long and narrow caudal part forms the spinal cord.

Question 34. Write a short note on neural crest cells.
Answer:

Neural crest cells:

The neural crest cells are derived from tips of neural folds of surface ectoderm during the formation of the neural tube (i.e. neurulation). They lie dorsolateral to the neural tube as its roof plates and form many structures.

Derivatives (fate) of neural crest cells:

• Melanocytes
• Sensory ganglia (spinal as well as cranial)
• Chromaffin cells of the adrenal medulla
• Schwann cells
• Satellite cells

Question 35. Give a brief account of intraembryonic mesoderm.
Answer:

Intraembryonic mesoderm:

It is the intermediate layer of the three laminar embryonic discs, i.e., it is present between the ectoderm and endoderm.

Intraembryonic mesoderm Formation:

It is formed in the 3rd week of gestation from cells of primitive streak that migrate laterally between ectoderm and endoderm. These cells spread all over the embryonic disc except at two sites/areas – the prochordal plate (buccopharyngeal membrane) and cloacal membrane. With the development of the neural tube, it lies on either side of this tube.

Intraembryonic mesoderm Subdivisions:

On either side of the neural tube, the intraembryonic mesoderm is divided into 3 parts. From medial to lateral, these are:

• Paraxial mesoderm
• Intermediate cell mass (intermediate mesoderm)
• Lateral plate mesoderm

Question 36. Enumerate derivatives of the 3 subdivisions of intraembryonic mesoderm (paraxial mesoderm, intermediate cell column, and lateral plate mesoderm).
Answer:

Intraembryonic mesoderm:

• Paraxial mesoderm gives rise to somitomeres and somites.
• Intermediate cell mass gives rise to:
  • The connective tissue of the gonads
  • Metanephric blastema
  • Smooth muscle and connective tissue of the reproductive system
• Lateral plate mesoderm gives rise to:
  • Septum transversum
  • Intraembryonic coelom that later differentiates into pericardial and peritoneal cavities
  • Splanchnopleuric layer that forms smooth muscle and connective tissue of intestinal and respiratory tracts and their associated glands
  • Somatopleuric layer that forms the appendicular skeleton
  • Angiogenic mesoderm that forms the endocardium of the heart and epithelium of vessels

Question 37. Describe somites in brief.
Answer:

Brief:

• These are cubical blocks of the paraxial mesoderm, arranged in a vertical row on either side of the notochord.
• The thick longitudinal column of the paraxial mesoderm extends bilaterally from the cranial to the caudal end of the notochord. It undergoes transverse segmentation in a craniocaudal direction to form cubical blocks called somites.

Brief Formation of Somites:

A total of 42–44 somites are formed during the 20th to 30th day of gestation (somite period of human development) in a craniocaudal direction. These are:

• 4 occipital
• 8 cervical
• 12 thoracic
• 5 lumbar
• 5 sacral
• 8–10 coccygeal

Brief Note:

The occipital somites help in the formation of the skull and muscles of the tongue.

Brief Fate of Somites:

Each somite is triangular in cross-section with a small slit-like cavity in the center. It is subdivided into 3 parts.

Brief From medial to lateral, these are:

• Sclerotome
• Myotome
• Dermatome
  • Sclerotomes form vertebral columns and ribs.
  • Myotomes form skeletal/striated muscles in the cervical, thoracic, lumbar, and sacral regions.
  • Dermatomes form the dermis of the skin on the back and front of the trunk and limbs.

Brief Applied anatomy

Brief Age determination:

The somites are a good index of gestational age. The age of the fetus therefore can be determined by counting the number of somites. The 1st somite is formed on day 20 and then approximately 3 pairs of somites are added each day until day 30.

Question 38. Write a short note on the septum transverse.
Answer:

Transverse:

The septum transversum is an unsolicited part of the lateral plate mesoderm cranial to the prochordal plate

Transverse Derivatives

The derivatives of septum transversum are:

• Epicardium and fibrous pericardium
• The fibrous stroma of the liver
• Part of diaphragm
• The dorsal mesentery of the esophagus

Question 39. Write a short note on intraembryonic coelom.
Answer:

Intraembryonic coelom:

It is a cavity present in the embryonic disc.

Intraembryonic coelom Formation

A large number of small cavities appear on each side in the lateral plate mesoderm and mesoderm anterior to the prochordal plate. These cavities coalesce to form a single large horseshoe-shaped cavity called intraembryonic coelom.

Intraembryonic coelom Subdivisions:

The intraembryonic coelom is divided into 3 parts as follows:

1. Pericardial cavity anterior to prochordal plate.
2. And
3. Right and left primitive peritoneal cavities.

The pericardial cavity communicates with the peritoneal cavities through pericardioperitoneal canals.

Intraembryonic coelom The fate of intraembryonic coelom:

As a result of the development of 4 partitions – the right and left pleuropericardial and pleuroperitoneal membranes, the intraembryonic coelom gives rise to pericardial, pleural
and peritoneal cavities.

Question 40. Enumerate derivatives of the wall of the coelomic epithelium.
Answer:

Coelomic epithelium:

Coelomic epithelium These are:

• Myocardium and parietal pericardium
• Visceral and parietal layers of the pleura
• Visceral and parietal layers of the peritoneum
• The epithelial lining of ductus deferens, epididymis, seminal vesicles, ejaculatory
  ducts ureters, and trigone of the urinary bladder
• Mullerian ducts epithelial lining of uterine tubes, the body, and cervix of the uterus, and the vagina.
• Germinal epithelium of gonads
• Germinal epithelium forming the adrenal cortex

Question 41. Briefly describe the folding of embryos. The folding of the embryo is a significant embryonic event to establish a primitive form of the human body.
Answer:

Human body:

• The folding of flat embryonic discs occurs both in longitudinal and horizontal planes.
• Longitudinal folding produces head and tail folds. Further during longitudinal folding part of the yolk sac is incorporated within the embryo to form a primitive gut.
• Transverse folding produces right and left lateral folds.
• As a result of longitudinal and transverse foldings, the embryo becomes cylindrical, and connecting stalk moves to lie on the ventral aspect embryo surrounding the yolk sac.
• Other changes taking place during folding are:
  • The buccopharyngeal membrane lies cranial to the foregut.
  • Septum transversum lies caudal to the pericardial cavity and heart tube