Embryology

Question 1

What are the 3 main phases in development of a human?

Answer 1

Pre-embryonic (0-3 weeks), embryonic (4-8 weeks) and foetal (9-40 weeks).

Question 2

What are the egg and sperm formed by?

Answer 2

Egg formed from oogenesis from an oogonium and sperm from spermatogenesis from spermatogonium. 4 sperm made and 1 egg with 3 polar bodies.

Question 3

Describe the main events that occur during week 1 of the embryonic phase.

Answer 3

ZYGOTE divides into MORULA then BLASTOCYST. Moves through uterine tube and attaches in uterine cavity.

Zygote divides until a solid mass of cells is formed - MORULA. As this > in size, more difficult to get nutrition to core. Blastocystic cavity forms. Cells accumulate at 1 end to form inner cell mass and outside is called trophoblast. 1st cell division takes 36 hours, successive ones take less time. By day 5/6 blastula has formed and has reached the uterine cavity for implantation.

Question 4

Which parent are mitochondrial diseases inherited from?

Answer 4

Mother as mtDNA comes solely from mother.

Question 5

What is an ectopic pregnancy?

Answer 5

Can occur if cilia function is abnormal. Ovum is fertilised but gets stuck in uterine tubes. Can be due to PID which destroys epithelium.

Question 6

Describe the main events that occur during week 2 of the embryonic phase.

Answer 6

Implantation occurs in the uterine endometrial layer and the placenta begins to develop. Cells that will later form the embryo form the bilaminar disk. Sacs, membranes and the cord begin to form that will later nourish the baby.

Question 7

How does implantation occur?

Answer 7

At day 7 the blastocyst begins to burrow into the endometrium/uterine wall. Chorion is the trophoblastic layer which has villi and so plays an important role in burrowing. Lots of blood vessels in the endometrium and so very nutritious.

Question 8

What is the chorion?

Answer 8

Trophoblast divides into 2 layers which ultimately forms chorion and develops chroronic villi.

Question 9

What are the functions of the chorion?

Answer 9

Implantation (villi), forms part of the placenta and secretes human chorionic gonadotrophin (HCG) - pregnancy tests detect this. Keeps endometrium in place and tells body to keep secreting estrogen and progesterone during the first 12 weeks of pregnancy.

Question 10

After implantation, what are the layers of the endometrium?

Answer 10

From inside to outside: maternal blood vessels, decidua basalis, endometrium then myometrium.

Question 11

What happens to the inner cell mass?

Answer 11

Some of the cells form the 2 layered, flat, bilaminar disk. These 2 cavities begin to form the amniotic cavity (epiblast and hypoblast) and yolk sac.

Question 12

What forms the membranes, sacs and cords?

Answer 12

Trophoblast. Everything else is the chorionic cavity. The bag that surrounds the embryo is the allantoic cavity and this is where waste products go. Everything together called the placenta. Maternal and foetal blood should not mix unless there is a rupture.

Question 13

What are the main functions of the placenta?

Answer 13

Foetal nutrition, transport of waste/gases and immunity. Foetal side smooth with foetal blood vessels at the end of the umbilical cord. Maternal part is decidua basalis of endometrium, rough with maternal blood vessels. Placental starts to form at 12 weeks and matures by 18-20 weeks. Weighs 1/6 of foetus.

Question 14

Describe fraternal/dizygotic twins?

Answer 14

Can run in families. 2 ova and 2 sperm. 2 placenta and different genetic make up.

Question 15

Describe identical/monozygotic twins.

Answer 15

1 ova, 1 sperm and 1 zygote which then splits into 2 embryos. Same genetic make up and same placenta.

Question 16

Describe the main events that occur during week 3 of development.

Answer 16

Formation of germ layers (gastrulation), formation of neural tube (neurulation), development of somites and early developmetn of CVS.

Question 17

What is the primitive streak?

Answer 17

Formed in the midline of the epiblast by the invagination of cells. Cells now know R, L, top and bottom.

Question 18

Describe the process of gastrulation.

Answer 18

Epiblast cells migrate into the space betwwen epiblast and hypoblastic layers. 3 layers are formed: ectoderm, mesoderm and endoderm.

Question 19

Describe the process of neurulation.

Answer 19

Cells below primitive streak sink down and form a solid tube of cells - NOTOCORD. This secretes signals which promotes formation of neural plate and neural tube formed from ectoderm (brain top spinal cord bottom).

Question 20

What happens to mesoderm?

Answer 20

Mesoderm thickens and splits into 3 parts: paraxial, intermediate plate and lateral plate (splits into somatic and splanchnic). Paraxial forms somites, intermediate forms urogenital and lateral forms body cavities and coverings.

Question 21

What are the somites?

Answer 21

Blocks of tissue running across both sides of neural tube which form muscle and tissues of back. This becomes segmented which forms each segment of spinal cord.

Question 22

Describe events occurring in the embryonic period (4-8 weeks).

Answer 22

Lots happens here!

Heart starts to beat on day 24; lateral tube folding completes and body cavities develop; neural tube develops; gut forms from endoderm; urogenital system develops; somites differentiate; limb buds form; and neck develops from pharyngeal arches.

Question 23

What are the parts of a somite?

Answer 23

Each somite divides into 3: dermatome (epidermis of skin), myotome (muscles) and sclerotome (bones including vertebrae).

Question 24

What does the ectoderm form?

Answer 24

Epidermis if skin and neural tube.

Question 25

What does mesoderm form?

Answer 25

Paraxial, intermediate (urogenital system - kidneys and reproductive) and lateral (somatic and splanchnic - body coverings (pleura and peritoneum) and cavities).

Question 26

What does endoderm form?

Answer 26

Gut and respiratory system.

Question 27

What is a teratogen?

Answer 27

Environmental factors that cause abnormal development. 60% unknown, 10% environmental, 10% genetic and 20% multifactorial. Embryo most sensitive to teratogens during weeks 3-8.

Question 28

What does ToRCH stand for?

Answer 28

Toxoplasma, rubella, CMV and herpes. These are infectious agents that can cross the placenta and affect the developing embryo. Drugs, alcohol and radiation can also affect the embryo.

Question 29

When does the gut develop?

Answer 29

Week 4 from ventral wall of foregut. Develops from lateral folding of the endoderm which gives the primitive gut tube. Longitudinal folding then gives rise to foregut and hindgut.

Question 30

Describe the respiratory primordium.

Answer 30

Starts as a median outgrowth called the laryngotracheal groove. This develops behind the 4th pair of pharyngeal arches.

Question 31

How does the larynx develop?

Answer 31

Epithelial lining of larynx develops from ectoderm. Cartilage develops from 4th and 6th pairs of pharyngeal arches. Epiglottis develops from caudal part of hypopharyngeal eminence.

Question 32

What is laryngeal atresia?

Answer 32

Rare birth defect - failure of recanalisation. Produces obstruction of upper foetal airway –> congenital high airway obstruction syndrome.

Question 33

How does the lung bud form?

Answer 33

Laryngotracheal groove gives rise to laryngotracheal diverticulum. Ventral outgrowth from foregut. Diverticulum becomes invested with mesoderm and enlarges to form lung bud.

Question 34

Describe development of the trachea.

Answer 34

Endoderm of laryngotracheal tube forms pulmonary epithelium and glands of trachea. Splanchnic mesoderm (mesenchyme) forms cartilage, muscles and connective tissue.

Question 35

When does respiration/life become possible?

Answer 35

End of the cannalicular stage of development.

Question 36

What is surfactant made of?

Answer 36

Mixture of phospholipids and proteins. Production begins at 20-22 weeks. No surfactant? HMD/RDS - affects 2% of live-born infants and makes up 30% of neonatal disease.

Question 37

Which drugs can stimulate surfactant production?

Answer 37

Thyroxine and corticosteroids - can be given to prevent RDS pre-term labour.

Question 38

At which age does the alveolar stage of lung development end?

Answer 38

Around age 8 - alveoli is now around 300 million which is the same as an adult.

Question 39

Name some congenital lung conditions.

Answer 39

Lobe of azygous vein, accessory lung, agenesis of lungs (failure to develop), lung hypoplasia and congenital lung cysts.

Question 40

Which 4 embryonic components are used in diaphragm development?

Answer 40

Septum tranversum; pleuroperitoneal membranes; dorsal mesentery of oesophagus; muscular in-growth from lateral body walls.

Question 41

Describe postolateral defect of diaphragm.

Answer 41

Common birth defect of diaphragm that affects 1/2200 newborn infants. Associated with congenital diaphragmatic hernia.