week 6

Question 1

The development of the morula then leads to the formation of a blastocyst with:

Answer 1

Inner cell mass = Embryoblast (future embryo)
Outer cell mass = Trophoblast = released enzymes to erode zona pellucida (hatching), critical for the implantation of blastocyst in the endometrium of uterus and will then differentiate into a large part of placenta
Cells of trophoblasts will display factors which are immunosuppressive (to facilitate implantation in uterine wall)
Blastocoele = fluid filled cavity “cell cave” essential for blastocyst development

Question 2

Cleavage

Answer 2

occur in the zygote, forming a morula(8 to 32 blastomeres joined by gap-junctions) between Day 3 to Day 5 (after fertilisation).

Question 3

Trophoblast will lead to

Answer 3

Future placenta
Display factors that are immunosuppressive, facilitating implantation

Question 4

Trophoblast differinciate into

Answer 4

Cytotrophoblast: inner layer of the trophoblast
Syncytiotrophoblast: outer layer of trophoblast invading into the maternal endometrium to facilitate implantation and secrete important hormones such as human chorionic hormone
second trimester secretion of placental growth hormone

Question 5

Ectoderm will generate

Answer 5

epidermis and nervous system

Question 6

Mesoderm will generate

Answer 6

notochord, musculoskeletal system, circulatory and reproductive systems

Question 7

Endoderm will generate

Answer 7

epithelial lining of GIT, reproductive and respiratory systems, as main organs for digestive system.

Question 8

SOMITES

Answer 8

differentiation of mesoderm into musculo-skeletal tissues including the vertebrae, rib cage; skeletal muscle, cartilage, tendons, and dermis.

Question 9

NOTOCHORD

Answer 9

guide the development of tissues

Question 10

Neurulation

Answer 10

stimulates synthesis of growth factors causing differential growth rates of ectoderm layer (forming NEURAL PLATE).

The thickening of the ectoderm (neural plate) becomes a NEURAL GROOVE. With differential division rates of the cells, the neural groove closes and forms a NEURAL TUBE (lateral border will become NEURAL CREST CELLS, giving birth to peripheral nervous system

Question 11

EMBRYO-FOLDING

Answer 11

cranio-caudally and laterally. This results into vesicles formed from the neural tube to be organised into a C-shape.

Question 12

Brief summary of the development of the limbs

Answer 12

Week 4: Apparition limb buds (through growth of mesoderm), note the upper limb precedes lower limb by a few days.
Week 5: The end of limb buds flattens and contains five condensations of mesoderm each, corresponding to primitive digits (looking like flippers or hand paddles).
Week 6: Separation terminal and proximal portions, forming hand/foot.
Week 7: Further muscle development (flexors and extensors) with nerve supply.
Week 8: Apparition elbow/knee, digits.

Question 13

Amelia

Answer 13

absence of a limb

Question 14

Phocomelia

Answer 14

terminated development of a limb at the limb bud stage, flipper like appearance

Question 15

Polydactyly

Answer 15

presence of extra digits or parts of digits.

Question 16

FORMATION OF THE HEART

Answer 16

the formation of the heart begins at Day 17 to Day 19 and originates from Splanchnic mesoderm layer.

Two longitudinal cell clusters (angioblastic cells) form at the cranial part of the embryo and then fuse to become paired endocardial heart tubes.

Between Day 18 to Day 22, both endocardial tubes fuse to form the Primitive Heart tube, which will move towards the future thoracic cage of the embryo due to the cranio-caudal embryo folding.

Question 17

The Primitive heart tube has originally 5 vesicles:

Answer 17

Truncus arteriosus, Bulbus cordis, Primitive ventricle, primitive atrium and sinus venosus.

Question 18

heart primitive tube then folds, and remodeling

Answer 18

The partitioning of the heart occurs with formation of septum primum and then septum secundum in the atria, forming two atria chambers connected by the foramen ovale. The ventricular septum appears and separates the two ventricles.

Question 19

Foramen ovale

Answer 19

an opening in the septum (between the two atria of the heart that is normally present only in the foetus (closing at birth).

Question 20

Ductus arteriosus

Answer 20

channel or connection between the pulmonary artery and the aorta in the foetus, which bypasses the lungs to distribute oxygen received through the placenta from the mother’s blood. It normally closes once the baby is born and the lungs inflate, separating the pulmonary and systemic circulations. If this ductus arteriosus doesn’t close properly at birth, there is irregular transmission of blood between the aorta and the pulmonary artery causing hypoxia.

Question 21

Umbilical vein/artery

Answer 21

the umbilical vein carries oxygenated, nutrient-rich blood from the placenta to the foetus, and the umbilical arteries carry deoxygenated, nutrient- depleted blood from the fetus to the placenta.

Question 22

Ductus venosus

Answer 22

describes the vitelline blood vessel lying within the liver that connects (shunts) the portal and umbilical veins to the inferior vena cava and also acts to protect the fetus from placental over-circulation. This shunt functionally closes after birth.

Question 23

Fertilisation

Answer 23

Both nuclei (male (n haploid) and female (n haploid) will then form the zygote (2n diploid, so 23 pairs of chromosomes).

Question 24

There 5 main phases in lung development in embryo/foetus

Answer 24

Phase 1: EMBRYONIC (from week 3 /4): Bronchi formation
Phase 2: PSEUDOGLANDULAR (from week 5) Bronchioles formation
Phase 3: CANALICULAR (from week 16) Formation of respiratory bronchioles
Phase 4: SACCULAR (from week 24) Formation of alveolar ducts and start of surfactant secretion
Phase 5: ALVEOLAR (from week 36) Formation of alveoli and increased secretion of surfactant

Question 25

Capacitation

Answer 25

Maturation of sperm requiring both exposure to seminal gland secretions and exposure to female tract (removal of the glycoprotein coat on sperm cell which will help with motility).

Question 26

CIRCULATORY SYSTEM in response to pregnancy

Answer 26

The maternal blood volume increases by 30 to 50% during pregnancy, leading to an increase of heart rate (by 15 beats per minute).

Foetal metabolic activity reduces PO2 and increases PCO2

Progesterone stimulates the increase of erythropoietin production and coagulation factors

Relaxin increases cardiac output, increased local nitric oxide and vasodilation to towards kidney and placenta due to high activity for embryonic/foetal proper development

Question 27

URINARY SYSTEM in response to pregnancy

Answer 27

The Glomerular Filtration Rate increases by 50% due to increased blood volume and the need for wastes from foetal metabolism to be excreted.

Pregnant women need to urinate frequently due to both extra urine production and growing foetus pressing on bladder.

Question 28

RESPIRATORY SYSTEM in response to pregnancy

Answer 28

Due to higher O2 demand with metabolic rate increasing, breathing rate need to adjust and maintain an appropriate level of CO2 in blood.

Increased tidal volume, deeper breathing occurring for pregnant women

Question 29

GASTRO-INTESTINAL TRACT in response to pregnancy

Answer 29

Enlarging uterus pressing upwards on stomach, can cause reflux and heartburn.

Nausea and vomiting (due to hCG secretion, stopping in second trimester usually).

Question 30

ENDOCRINE in response to pregnancy

Answer 30

Pituitary increases in size by 30-50%, largely due to the progesterone-stimulated increased synthesis of prolactin.

Question 31

1. FIRST STAGE: CONTRACTIONS

Answer 31

high oestrogen levels stimulate the synthesis of oxytocin receptor.

When the baby pushes against the cervix, the stretch stimulates further the oxytocin release.

Oxytocin stimulates smooth muscle contraction of myometrium layer of uterine wall (positive feedback!).

Contractions, sweep from top of uterus down, increase in length and frequency as getting closer to delivery.

Question 32

SECOND STAGE: BABY DELIVERY/ EXPULSION

Answer 32

usually lasts between 30 min to 1h30. ACTIVE LABOR

Stretch receptors activate neural reflexes triggering abdominal wall contractions. The contractions become more frequent and can be painful. The cervix will soften further and dilates.

When your cervix is fully dilated (around 10 cm), the baby will move further down the birth canal towards the entrance to the vagina. At this stage we have CROWNING: this is when you can see the top of your baby’s head through the opening of the vagina.

When baby is crowning, the mother has to push voluntarily by contracting abdominal muscles at this time to be able to deliver the baby.

When baby is delivered, the umbilical cord is tied and severed.

Question 33

THIRD STAGE: PLACENTA DELIVERY

Answer 33

Occurs usually within 15 to 30 min after baby is delivered.

In the first stage, the contractions separate the placenta from the myometrium. The uterine blood vessels tear → blood loss. However, the uterine contractions compress vessels and restrict blood flow limiting the blood loss during delivery.