S9-10) Embryology

Question 1

What is embryology?

Answer 1

Embryology is the study of the molecular, cellular and structural factors involved in the progression from a single cell to a baby in 9 months

Question 2

Describe the process of oogenesis

Answer 2

Question 3

Where does fertilisation normally occur?

Answer 3

The ampullary region of the fallopian tube

Question 4

Sperm must be introduced into the female reproductive tract within 5 days before ovulation.

How long are the sperm and secondary oocyte viable?

Answer 4

• Sperm are viable for 5 days
• Secondary oocyte is viable for 12-24 hours

Question 5

Why is there a need for capacitation?

Answer 5

• Sperm are initially incapable of fertilisation
• Capacitation allows the tail of the sperm to move faster, causing the oocytes plasma membrane to alter

Question 6

Describe the process of fertilisation

Answer 6

Question 7

Define the following terms:

• Embryonic age
• Gestational age

Answer 7

- Embryonic age: time since fertilisation

- Gestational age: time since last menstruation (embryonic age minus 2 weeks)

Question 8

Define the following terms:

• Germinal stage
• Embryonic period
• Foetal period

Answer 8

• Germinal stage: time from fertilisation to the end of the 2nd week

- Embryonic period: time from the 3rd to end of the 8th week

- Foetal period: time from the beginning of the 9th week to birth at 38 weeks

Question 9

Before the blastocyst can implant it has to lose the zona pellucida.

What is this process called and why does it happen?

Answer 9

Hatching is the process in which the blastocyst emerges from its zona pellucida as the zona pellucida prevents the morula/blastocyst adhering to the oviduct and enlarging

Question 10

Distinguish between histiotrophic nutrition and haemotrophic nutrition

Answer 10

• Histiotrophic nutrition is the nutrition provided to the embryo which is not from the maternal blood and is important in humans up to the 12th week

- Haemotrophic nutrition is the nutrition provided by the mother’s blood and is important in humans from 12th week on

Question 11

Where and when does the blastocyst implant?

Why is this important?

Answer 11

• Normal implantation of the blastocyst occurs in the posterosuperior wall of the uterine cavity on the 6th day
• The implantation site determines the site of formation of the placenta

Question 12

Identify 7 abnormal implantation sites

Answer 12

Question 13

Differentiate between the following terms:

• Trophoblast
• Synctiotrophoblast
• Cytotrophoblast

Answer 13

• Trophoblast is a layer of tissue on the outside of a mammalian blastula, supplying the embryo with nourishment

- Syncytiotrophoblast are cells that merge together in the trophoblast

- Cytotrophoblast are cells in the trophoblast

Question 14

What is the bilaminar disc?

Answer 14

• The bilaminar embryonic disc refers to the epiblast and the hypoblast, evolved from the embryoblast
• These two layers are sandwiched between the primitive yolk sac and the amniotic cavity

Question 15

Differentiate between the following terms:

• Hypoblast
• Epiblast

Answer 15

• Hypoblast is a tissue type, lying beneath the epiblast, that forms from the inner cell mass and consists of small cuboidal cells

- Epiblast is the outermost layer of an embryo before it differentiates into ectoderm and mesoderm

Question 16

Differentiate between the follow structures:

• Amniotic cavity
• Yolk sac
• Chorionic cavity

Answer 16

• Amniotic cavity is the closed sac between the embryo and the amnion, containing the amniotic fluid

- Yolk sac is a membranous sac attached to an embryo, formed by cells of the hypoblast adjacent to the embryonic disk

- Chorionic cavity is the space surrounding the primary yolk sac and amniotic sac (not around connecting stalk)

Question 17

Why is the second week of development called the week of 2’s?

Answer 17

• The trophoblast differentiates into 2 layers (cytotrophoblast & syncytiotrophoblast)
• The embryoblast forms 2 layers (epiblast & hypoblast)
• The hypoblast contributes to the formation of two cavities (yolk sac & chorionic cavity)

Question 18

Identify the 6 key events/processes in early development

Answer 18

• Fertilisation
• Implantation
• Gastrulation
• Neurulation
• Segmentation
• Folding

Question 19

Gastrulation happens in the 3rd week.

What occurs during gastrulation?

Answer 19

During gastrulation, the bilaminar disk is converted to a trilaminar disk consisting of three germ layers: ecotoderm, mesoderm and endoderm and the primitive streak forms in the caudal epiblast

Question 20

What is the importance of the primitive streak?

Answer 20

• The primitive streak forms in the caudal epiblast, leading to migration and invagination of epiblast cells
• This process ensures the correct placement of precursor tissues to allow sub sequent morphogenesis to occur

Question 21

What is the germ layer derivative of the ectoderm?

Answer 21

Ectoderm – organs and structures that maintain contact with the outside world e.g. nervous system, epidermis

Question 22

What is the germ layer derivative of the mesoderm?

Answer 22

Mesoderm – supporting tissues e.g. muscle, cartilage, bone, vascular system

Question 23

What is the germ layer derivative of the endoderm?

Answer 23

Endoderm – internal structures e.g. epithelium of GI tract, respiratory tract and parenchyma of glands

Question 24

Discuss the left-right asymmetry that arises from gastrulation

Answer 24

• Prior to gastrulation the embryonic disk is bilaterally symmetrical
• However, in the body there are clear left vs right differences:

I. Thoracic viscera – left lung has 2 lobes while the right has 3

II. Abdominal viscera – liver on right, stomach and spleen on the left

Question 25

What is neurulation?

Answer 25

**Neurulation** refers to the folding process in vertebrate embryos, which includes the thickening of the ectoderm to form the neural plate and its transformation into the neural tube (neurectoderm)

Question 26

What is the role of the notochord?

Answer 26

- The notochord is formed during gastrulation and **drives neurulation** - It has no function in adult life and forms the **nucleus pulposus** of the intervertebral disc

Question 27

Somites appear as regular block of mesoderm cells arranged around a small cavity. Explain the degeneration of somites into the myotome and dermatome

Answer 27

- The **ventral wall** of the somites break down and the **sclerotome** forms - Further **organisation** of the dorsal portion forms the **dermomyotome** - **Myotome** proliferates and migrates & **dermatome** disperses

Question 28

Differentiate between the dermatome, myotome and sclerotome

Answer 28

- **Dermatome** – skin section i.e. dermis - **Myotome** – muscle section i.e. muscles - **Sclerotome** – hard tissue section i.e. bones

Question 29

What are the implications of segmentation?

Answer 29

- Organisation of mesoderm into somites gives rise to **repeating structures** *e.g. vertebrae, ribs, spinal cord segments* - Guides **innervation**

Question 30

What does embryonic folding do?

Answer 30

- Folding draws together the **margins** of the disk - It creates a **ventral body wall** - It pulls the **amniotic membrane** around the disk - It pulls the **connecting stalk** ventrally

Question 31

Briefly described how the intraembryonic coelem is formed

Answer 31

- The **intraembryonic coelom** is the cavity between the **somatopleure** (dorsal) and the **splanchnopleure** (ventral) which forms during the 3rd week of development from the splitting of the **lateral mesoderm** - This space will give rise to the **thoracic** and **abdominal cavities**

Question 32

Why do the buccopharyngeal membrane and the cloacal membrane have no mesoderm?

Answer 32

- The **buccopharyngeal** and **cloacal membrane** have no mesoderm as these membranes form the upper and lower parts of the GI tract - The mesoderm derives into **muscle, cartilage and bone**; which are not part of the GI tract, hence not necessary

Question 33

Saccrococcgeal teratomas are common in newborns. The developmental basis for these tumours is the excessive persistence of the primitive streak. Why are these tumours composed of many tissue types?

Answer 33

- The primitive streak gives rise to the **mesoderm** which is a derivative of supporting tissues (muscle, cartilage & bone) - Hence, saccrococcgeal tumours would consist of **precursor cells of the mesoderm**

Question 34

What are embryonic stem cells?

Answer 34

**Embryonic stem cells** are pluripotent stem cells derived from the inner cell mass of a blastocyst

Question 35

What can umbilical cord stem cells be used for?

Answer 35

**Umbilical cord blood** collected at birth is a **rich source of stem cells** that can be used in **research** and in the **clinic** to treat diseases of the blood and immune system

Question 36

Where can adult stem cells be found?

Answer 36

**Adult stem cells** have been identified in many organs and tissues, including brain, bone marrow, blood, liver, gut, heart, skin and teeth

Question 37

What is placenta praevia?

Answer 37

**Placenta previa** is a condition wherein the placenta covers all or part of the cervix during the last months of pregnancy and can cause severe bleeding before/during labour (emergency C-section may be necessary)

Question 38

Why is it that ectopic pregnancy can quickly become a life-threatening emergency?

Answer 38

- The narrow **fallopian tube cannot distended largely** and with the progression of pregnancy, the fallopian tube ruptures - This can cause heavy internal bleeding and abdominal pain

Question 39

Identify 5 of commonest causes for maternal mortality globally

Answer 39

- Post-partum bleeding - Complications from unsafe abortion - Hypertensive disorders of pregnancy - Postpartum infection - Obstructed labour

Question 40

Identify 5 of the commonest causes for maternal mortality in the UK

Answer 40

- Thrombosis and thromboembolism - Antepartum and postpartum haemorrhage - Amniotic fluid embolism - Genital tract sepsis - Ectopic pregnancy