Conception, implantation and placentation

Question 1

How are gametes transported during fertilisation?

Answer 1

• At ovulation, the cervical mucus changes consistency, allowing easier sperm transportation
• Epithelial cells of the uterine tube are highly ciliated
• The ends of the uterine tube come into close contact with the ovary durinf ovulation as the fimbriae “sweep” the ovulated ovum into the tube.

Question 2

How are spermatozoa transported during fertilisation?

Answer 2

• Spermatozoa enter the uterine tube and wait for the oocyte. They become immotile and temporarily bind to epithelial cells
• Become active again after ovulation occurs
• Travel to ampullary-isthmic junction
• Dependent on chemoattractant release by oocyte

Question 3

What is capacitiation:

Answer 3

• Newly ejaculated spermatozoa are unable to fertilise ovum
• Glycoproteins removed from plasma membrane of the head of the sperm cell
• Increased cytoplasmic pH leads to increased Ca 2+ permmeability (motility)
• Process takes around 5-7 hrs
• Occurs once sperm cells have entered the uterine tube
• Only capacitated spermatozoa can pass through the corona radiata cells surrounding the oocyte
• Hyperactivated motility : increased strength and amplitude of flagellar beats
• Surface membrane changes : allow acrosome reaction

Question 4

What is fetilisation srage 1?

Answer 4

• The penetration of corona radiata and zona pellucida
• Flagellar action and release of enzymes from acrosome aids corona radiata penetration
• The acrosomal reaction : chemical changes that occur in the acrosome of the spermatozoa when it comes into contact with the zona pellucida of the oocyte
• Zona pellucida = glycoprotein shell surrounding oocyte
• ZP3 : Ligand which facillitates binding of sperm and acrosomal reaction
• Hyaluronidase enzymes within acrosome are exposed and released from the sperm - digests extracellular matrix around the corona radiata cells (rich in hyaluronic acid)

Question 5

What is Fertilisation stage 2?

Answer 5

• Cortical reaction
• release of cortical vesicles from oocyte plasma membrane after fusion
• Increased calcium levels causes cortical granules to fuse with the oocyte membrane and release their content s
• Enzymes destroy ZP receptors
• Tyrosine residues on adjacent ZPs are cross-linked- zona becomes non- dissolvables by proteolytic enzyme
• Prevents polyspermy = multiple sperm cells fertilising the same ovum
• Dispermy - 2
• Triploidy = 3
• Embryo/foetus usually not viable

Question 6

What is fertilisation stage 3?

Answer 6

• Fusion of plasma membranes and 2nd meiotic division
• Head and tail of sperm cell enter the cytoplasm of the oocyte
• Plasma membrane of spem cell remains on oocyte surface
• The oocyte was previously arrested in metaphase of the 2nd meiotic diviison
• Immediately completes meiosis after entry of sperm cell to form mature oocyte and second polar body (daughter cell)

Question 7

What is fertilisation stage 4?

Answer 7

• Two pronuclei each contains 23 chromosomes (haploid)
• Male and female pronuclei are indistinguishable
• Membranes of the pronuclei break down, the chromosomes become arranged for mitotic cell divison (the first cleavage division)
• Ferilisation is complete - zygote formed

Question 8

What are the results of fertilisation?

Answer 8

• Restoration of the diploid number of chromosomes (46)
• Determination of the chromosomal sex of the new individual (XX, XY)
• Initiation of cell cleavage

Question 9

What is the initiation of cell cleavage?

Answer 9

• Zygote divides via mitosis to form smaller cells known as blastomeres
• Totipotent stem cells : can become any kind of embryonic cell/ tissue
• Each blastomere has the potential to forma n entire embryo
• Blastomeres are in a loose clump until the 8-cell stage when they become compacted
• Still surrounded by the zona pellucida of the ovum
• A = 2 blastomeres
• B - 4 blastomeres
• C = 12 blastomeres
• D = morula (16 blastomeres)

Question 10

How is the blastocyst formed?

Answer 10

• Formed in week 1
• The morula befcomes a blastocyst around day 4 when a blastocele (fluid filled cavity) forms
• Blastocyst has 2 separate groups of cells:
• Inner cell mass or embryoblast : forms embryo
• Outer cell mass or trophoblast forms placenta
• Blastocyst loses its zona pellucida by day 6
• Can now implant on the uterine endometrium

Question 11

What are the stages of implantation?

Answer 11

• Blasyocyst adheres to the endometrium
• Trophoblast (non-embryo/) cells divide and differentiate into two seperate groups:
• An inner layer of cells called the cytotrophoblast
• An expanding multi-nucleated synctoum called the synctiotrophoblast releases enzymes to digest the endometrium
• Also releases human chorionic gonadotropin (HCG) - detected by pregnancy testing
• Cytotrophoblast :
• inner layer of trophoblast
• Forms the synctyiotriohoblast
• Forms the chronic villi (important for early placental development)
• What are synctiotrophoblast:
• Multi-nucleated invasive synctial cell mass
• Lacks independent proliferative capacity
• Secretes hormones (e.g. progesterone, (beta-) human chorionic gonadotropin (hCG), human placental lactogen (HPL)

Question 12

What is the prolongation of corpus luteum (luteal phase)?

Answer 12

• hCG produced by syncytiotrophoblast from approx, day 6-7 and passes into the maternal blood circulation
• hCG binds to LH receptors on luteal cells (corpus luteum) in ovary
• • Progesterone release from corpus luteum is maintained
• Progesterone actively promotes survival of corpus luteum by autocrine stimulation - positive feedback

Question 13

What are some abnormal implantation sites?

Answer 13

• Most common site of implantation is the upper posterior wall of the body of the uterus
• Abnormal sites include:
• Uterine tube (majority of ectopic pregnancies occur here- tubal pregancy)
• Rectouterine pouch (Pouch of Douglas)
• Intestinal mesenteries
• Ovary
• Implantation in the region of the cervical internal os : may result in placenta praevia

Question 14

What occurs during week 2 of implantation?

Answer 14

• In week 2 , the embryoblast
• (inner cell mass of blastocyst) splits into the two epiblast and hypoblast tissue layers
• Not known as the bilaminar disc
• Epiblast layer will form the embryo proper as well as the aminoblasts lining the amnion (amniotic sac)
• Amniotic cavity starts to form inside epiblast
• Amniotic cavity fills with amniotic fluid : important for physically supporting and protecting the embryo/ foetus
• Hypoblast layer form sthe primary yolk sac (exocoelomic cavity)
• Important initial source of nutrition for embryo before the placenta is functional

Question 15

What is the state of the embryo at the start of week 3?

Answer 15

• Embryo surrounded by a chorionic cavity and extra embryonic tissues of the developing placenta
• Primary yolk sac replaced by a secondary yolk sac
• A connecting stalk marks the future location of the umbilical cord
• Two distinct epiblast and hypoblast cell layers
• Epiblast and hypoblast cell layers
• Epiblast layer begins to undergo gastrulation ; formation of the three embryonic germ (tissue) layers

Question 16

What is gastrulation?

Answer 16

• Primitive streak appears in the midline at the caudal (tail) end of the epiblast
• First step of gastrulation
• Important for establishing left and right sides of the embryp (cranio-causal axis)
• Epiblast cells start to migrate towards the primitive streak

Question 17

How are the primary germ layers formed during gastrulation?

Answer 17

• Epiblast cells migrate towards the primitive streak and undergo an epithelial- to-mesenchymal transition. Celsl break away from epiblast, invaginate through the primitive streak and spread out underneath
• The first cells through the primitive streak will displace the hypoblast cell layer and form the endoderm
• Some cells lie in the middle and form the mesoderm
• Remaining epiblast layer cells form the ectoderm

Question 18

What happens to the ectoderm, medoderm and endoderm during gastrulation?

Answer 18

• Ectoderm : further differentiates into the :
• Neuroectoderm nervous system
• Surface ectoderm : epidermis of the skin
• The mesoderm is important for forming many body tissues such as the dermis of the skin, skeletal muscle, smooth muscle, bone, cartilage, blood vessels and the heart
• Some mesoderm cells migrate outside the embryo to help to form the placenta : extra-embryonic mesoderm
• The endoderm gives rise to epithelial linings of the digestive and respiratory systems and many internal organs (e.g. liver, pancreas, thyroid gland, urinary bladder)

Question 19

What occurs during week 4 of implantation?

Answer 19

• Folding of the embryo
• Longitudinal folding occurs due to rapid enlargement of the cranial end of the neural tube to form the brain
• Embryo bends had and tail ends are brought close together
• Amniotic cavity, expands and pushes in at either end of embryo, encouraging folding
• Lateral folding of the embryo occurs due to enlargement of the somites (mesoderm)
• Important for forming the abdominal wall
• The grwoing amniotic cavity pinches off the connection of the embryonic gut tube (endoderm) to the yolk sac
• Amniotic cavity now surrounds the embryo

Question 20

What is the placenta?

Answer 20

• formed by both foetal and maternal tissues
• Extensive exposure of maternal blood to non maternal trophoblst tissue
• Simulataneous execution of diverese tasks :
• Gaseous exchange
• Transport of nutrients
• Excretion
• Hormone and protein synthesis
• ## Limited lifespan/ organ (“the deciduous organ”)

Question 21

What is decidualisation?

Answer 21

• changes in the endometrium to prepare for blastocyst implantation
• Progesterone causes uterine stromal cells to swell up and accumulate glycogen and lipids = decidual cells
• Increased vascualrisation of endometrium
• Blastocyst triggers further decidualisation of uterine as the syncytiotrophoblast layer erodes the endometrium
• Endometrium now known as the decidua ( could fall off during mesnstruation)

Question 22

What are the 3 layers of the decidua?

Answer 22

• Decidua basalis :
• layer beneath the developing embryo
• Forms placenta with the trophoblasr
• Decidua capsularis
• Decidual layer covering the embryo
• Decidual parietalis
• Decidual lining elsewhere in the uterus away from the embryo
• DC and DP ultimately fuse together as gestational sac grows and fills uterine cavity

Question 23

What are fetal membranes?

Answer 23

• Amnion (inner membrane)
• Lines amniotic sac and protects embryo/fetus from physical damage
• Secreyes amniotic fluid
• Oligohyramnios : low volume of amniotic fluid resultinf in compression of fetus

Chorion (outer membrane)
- Formed by trophoblast and extra-embryonic mesoderm
- Gives rise to foetal part of placenta : chrorion frondosum
- Embryo suspended in chorionic cavity until amniotic sac expands and obliterates this space
- connecting stalk left behind - important for forming umbillical cord
- Yolk sac and allantois
- Yolk sac is an early source of embryonic nutrition
- Primitive yolk sac then secondaty yolk sac

Question 24

What are the trophoblast changes in week 2?

Answer 24

• Lacunae gaps form within synctiotrophoblast and fill with blood from maternal sinusoids (sinusoidal capillaries)
• Early source of nutrition for embryo
• Chrorioinic cavity forms and surrounds embryo
• Extra-embryonic mesoderm forms around chorionic cavity
• important for fetal membranes

Question 25

What are chorionic villi?

Answer 25

• Primary chorionic villi :
• Cells of the cytotrophoblast proliferate and grow into the synctiotophoblast : primitive uteroplacental circulation begins
• Secondary chorionic villi :
• Extra-embryonic mesoderm grows into the core of each primary chorionicvillus
• Tertiary chorionic villi
• extra-embryonic mesoderm differentiates into blood cells and small blood vessels

Question 26

What is uteroplacental circulation?

Answer 26

• From week 2, there is an increasing need for a circualtory system : more efficient gas and nutrient exchanhe
• Embryonic blood vessels in the tertiary chorionic villi come into contact with the intervillous spaces supplied by the maternal spinal arteries of the uterine endometrium
• Umbilical arteries start to form to allow deoxygenated blood to leave the embryo. Umbillical vein stats to form to allow oxygenated blood to return from chorionic villi to embryo
• Chorionic viili at embryonic pole (near umbillical cord) increase in size and number to become the chorion frondosum
• Placenta formed by both the decidua baslais and chorion frondosum
• Basal plat eof placenta= materna decidua basalis
• Chorionic plate of placenta = fetal chorionic frondosum
• Anchoring villi secure the two plates together
• Placenta fully functiional by week 12 (end of first trimester)
• Chronic villi at abembryonic pole (opposite side from umbillical cord) become the compressed and avascular chorion laeve (“smooth”)

Question 27

What is the placental membrane?

Answer 27

• site of gas/nutrient exchange
• 4 components at first:
  1. Fetal capillary endothelium , chorionic villus connective tissue, cytotrophoblast, syncytiotrophoblast
• From 4th month the placental membrane thins, connective tissue and cytotrophoblasr disappear
• Synctiotrophoblast persists as important barrier between maternal and fetal circulations
• Maternal antibodies can cross placental membrane, as can teratogenic substances e.g, alcohol

Question 28

What is the structure of placenta (2nd trimester onwards)?

Answer 28

• Placenta during the second half of pregnancy - purple or crimson colour, grows to around 22cm long and 2cm thick
• Cotyledons are lobules of the placenta which have been separated by septa (walls) from the decidua (maternal aspect of placenta)

Question 29

What are some placdental pathologies?

Answer 29

Placenta praevia: placenta is too close to or blocks the
internal os of the cervix
Risk of haemorrhage before or during childbirth
Placenta accreta. placenta grows too deep into uterine wall
myometrium and strongly attaches to myometrium
Risk of haemorrhage when placenta tries to detach from uterine
wall during delivery
Placenta increta: even deeper attachment to myometrium
Placenta percreta: placenta grows through myometrium, may
attach to other pelvic structures e.g. bladder
Placental abruption: early detachment of placenta from the
uterus
Risk of haemorrhage before or during childbirth
Placental insufficiency: unable to supply enough nutrients
and oxygen for fetal growth
Low birth weight
Risk factors include diabetes, pre-eclampsia, drug use, smoking

Question 30

Describe the fetal membrane of twins.

Answer 30

Twins and their
fetal membranes
Dizygotic twins (A): two fertilised zygotes implant in
the uterus at separate sites (non-identical twins)
Each has its own placenta and fetal membranes
(chorion and amnion)
Monozygotic twins: zygote splits into two (identical
twins)
May split at two cell stage: identical twins but
otherwise placenta and fetal membranes have the
same arrangements as dizygotic twins
Inner cell mass (embryoblast) may split (B): shared
placenta and chorion but two separate amniotic sacs
Bilaminar embryonic disc may split (C) e.g. due to developing two primitive streaks during
gastrulation: shared placenta, chorion & amniotic sac
* Twins may not survive due to risk of
entanglement of umbilical cords

Question 31

What are conjoined twins?

Answer 31

• occurs when inner cell mass does not separate completely .
• Extent of shared tissue may bary
• Thin tissye bridge of skin and muscle which can be surgically incised after birth e.g. the abdominal wall
• Complex fusion of body regions such as the head and thorax with shared organs - surgical separation may not be possible without the loss of lide
• Parasitic twins: less common form of conjoined twins
• much smaller but often complete portion of a body protudes from the otherwise normal host twin
• Only the host twin is viable