BL Session 12 - Embryology

Question 1

What is embryology?

Answer 1

• Embryology: The study of progression from a single cell to a baby in 9 months.
• It includes molecular, cellular and structural factors.
• The understanding embryogenesis has the potential to treat disease, particularly repairing damaged cells/tissues/organs.
• Research also involves studies around embryonic stem cells.

Question 2

Identify the features of the female reproduction system.

Answer 2

Question 3

Outline the steps of oogenesis

Answer 3

Question 4

Identify the necessary circumstances for fertilisation.

Answer 4

• Fertilisation normally occurs in the ampullary region of the Fallopian tube
• Sperm must be introduced into the female reproductive tract within 5 days before ovulation.
• Sperm are viable for 5 days
• Secondary oocyte viable for 12-24 hours
• Sperm initially incapable of fertilisation and thus requires capacitation, meaning the tail of the sperm moves faster, causing the plasma membrane to alter.

Question 5

Because 50% of the implanting embryo’s genome is derived from the father, sperm are a foreign bodies – should they be rejected?

Answer 5

• Interestingly, both men and women can produce antibodies against sperm, and this is one of the causes of infertility.
• The surface proteins in sperm cells are “non-self” even for the male who produces them.
• Antibodies against a foreign protein can only be formed if there is contact with the blood.
• In the testes, Sertoli cells form a barrier made of specialised cell membrane structures called tight junctions, to avoid contact between blood cells and sperm cells, but when the barrier is broken by different type of injury,

Question 6

Define the following terms:

• Embryonic age
• Gestational age
• Germinal stage
• Embryonic period
• Foetal period.

Answer 6

• Embryonic age: time since fertilisation.
• Gestational age: time since last menstruation; that is embryonic age minus 2 weeks.
• Germinal stage: time from fertilisation to the end of the second week
• Embryonic period: time from the third to end of the eight week
• Foetal period: time from the beginning of the ninth week to birth at 38 weeks

Question 7

Outline the features and steps in implantation.

Answer 7

• Before the blastocyst can implant it has to lose the zona pellucida. The zona pellucida is thought to prevent the morula/blastocyst adhering to the oviduct. The process of losing the zona pellucida is called hatching.

- It is mostly about (potential lack of) nutrition!

I. The zona pellucida stops the morula enlarging.

II. Morula and blastocyst use anaerobic and aerobic pathways.

III. The trophoblast becomes the yolk sac and placenta

• The implantation site determines the site of formation of the placenta. ​
• Normal implantation off the blastocyst occurs in the superior and posterior wall of the uterine cavity.
• The fertilised human oocyte migrates through the tube and implants itself on the 6^th day in the endometrium

Question 8

Distinguish between histiotrophic and haemotrophic nutrition.

Answer 8

• Histiotrophic nutrition: nutrition provided to the embryo not from the maternal blood. Important in humans up to the 12th week.
• Haemotrophic nutrition: nutrition provided by the mother’s blood. Important in humans from 12th week on.

Question 9

What are abnormal fertilisation sites?

Answer 9

• Ovarian
• Infundibular
• In the fallopian tube (most frequent ectopic),
• Interstitial
• In the lower uterus segment (placenta praevia)
• Abdominal
• In the pelvic area.

Question 10

Describe the role and properties of the following terms:

• Bilaminar embryonic disc
• Yolk sac
• Amniotic cavity
• Chorionic cavity

Answer 10

• Bilaminar embryonic disc: refers to the epiblast and the hypoblast, evolved from the embryoblast. These two layers are sandwiched between two balloons: the primitive yolk sac and the amniotic cavity.
• Amniotic cavity: the closed sac between the embryo and the amnion, containing the amniotic fluid.
• Yolk sac: a membranous sac attached to an embryo, formed by cells of the hypoblast adjacent to the embryonic disk.
• Chorionic cavity: the space surrounding the primary yolk sac and amniotic sac, except where the connecting stalk attaches to the cytotrophoblast of the blastocyst.

Question 11

Why is the second week of development known as the week of 2s?

Answer 11

• The trophoblast differentiates into 2 layers – the cytotrophoblast and syncytiotrophoblast.
• The embryoblast forms 2 layers – the epiblast and hypoblast – the bilaminar embryonic disc.
• The hypoblast contributes to the formation of two cavities – the yolk sac and chorionic cavity.

Question 12

What is the embryonic period?

Answer 12

• Weeks 3-8
• During this period:

I. Period of greatest change

II. All major structures and systems are formed

III. Greatest risk of major congenital malformation (i.e. teratogenesis) due to environmental exposure to drug therapy

Question 13

Describe the results and processes involved in early embryonic development

Answer 13

• Result – foundations laid for the development of the body systems
• Processes:

I. Sufficient cells

II. Of the right type

III. In the right place

Question 14

What are some key events and processes in early development?

Answer 14

Question 15

Outline the process of gastrulation.

Answer 15

• Occurs in the third week of development, marking the start of the embryonic period.
• The bilaminar disk is converted to a trilaminar disk consisting of three germ layers: ectoderm, mesoderm and endoderm
• 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 subsequent morphogenesis to occur.

Question 16

Identify and describe the germ layers derivatives.

Answer 16

• Ectoderm – organs and structures that maintain contact with the outside world e.g. nervous system, epidermis.
• Mesoderm – supporting tissues e.g. muscle, cartilage, bone, vascular system.
• Endoderm – internal structures e.g. epithelium of GI tract, respiratory tract and parenchyma of glands

Question 17

Outline the features of situs inversus.

Answer 17

• Complete mirror-image viscera
• Commonly results from immotile cilia
• No associated morbidity (usually)
• Problems arise if there is both normal and mirror-image disposition

Question 18

Discuss left-right asymmetry.

Answer 18

• 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 19

What is neurulation?

Answer 19

Neurulation: refers to the folding process in vertebrate embryos, which includes the transformation of the neural plate into the neural tube. The embryo at this stage is termed the neurula.

Question 20

What are somites?

Answer 20

Question 21

Identify and describe some somite derivatives.

Answer 21

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

Question 22

Discuss the implication of segmentation

Answer 22

• Organisation of mesoderm into somites gives rise to repeating structures

I. Vertebrae

II. Ribs

III. Intercostal muscles

IV. Spinal cord segments

• Guides innervation

Question 23

Outline embryonic folding.

Answer 23

• Folding draws together the margins of the disk

I. Creating a ventral body wall

II. Pulling amniotic membrane around the disk: embryo becomes suspended within the amniotic sac

III. Pulling connecting stalk ventrally

• At the end of the fourth week:

I. The nervous system has started to form

II. Segments have appeared, assigning specific tasks to specific cells

III. The embryo has folded, putting everything in the right place

Question 24

Suggest in general terms one adult derivative for the splanchic layer of the lateral plate mesoderm

Answer 24

• The lateral mesoderm will split into two layers: the somatic layer and the spanchnic layer.
• The splanchnopleuric layer forms the circulatory system. Spaces within the lateral plate are encolsed and forms the intraembryonic coelem.

Question 25

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

Answer 25

• The buccopharyngeal and cloacal membrane have no mesoderm as these membranes form the upper and lower parts of the GI tracct.
• Mesoderm derives into muscle, cartilage and bone, which are not part of the GI tract, hence, not necessary in essential membranes of the GI tract.

Question 26

Saccrococcygeal teratomas are large abnormal growths in the saccrococcygeal region and are the most common tumours in newborns. Given that the developmental basis for these tumours is persistence of the primitive streak beyond the time when it should normally regress, suggest why these tumours can be composed of many discrete tissues.

Answer 26

• The primitive streak is a transient structure whose formation, on day 15 of human development, marks the start of gastrulattion, the process in which the inner cell mass is converted into the trilaminar disc, which is comprised of the three germ layers.
• The primitive streak gives rise to the mesoderm which is a derivative pf supporting tissues e.g. muscle, cartilage and bone. Hence, if these sort of tumours involve the persistence of the primitive streak, then it consists precursor cells of the mesoderm, expaining why it can be composed of many discrete tissue types.

Question 27

What are embryonic stem cells? What ethical issues are posed by their use?

Answer 27

• Embryonic stem cells are pluripotent stem cells derived from the inner cell mass of a blastocyst.
• Many of the debates surrounding human embryonic stem cells concern issues such as what restrictions should be made on studies using these types of cells.
• At what point does one consider life to begin? Is it just to destroy an embryo cell if it has the potential to cure countless numbers of patients?

Question 28

What can umbilical cord stem cells be used for?

Answer 28

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

Question 29

Where can adult stem cells be found. You have already learnt about their use in helping treat the complications of an intestinal disease - which disease? Adult stem cells are increasingly used to treat diseases of another system. Which system?

Answer 29

• Adult stem cells have been identified in many organs and tissues, including the brain, bone marrow, peripheral blood, blood vessels, skeletal muscle, skin, teeth, etc
• Adult stem cells have been used to treat many diseases of the immune system such as MS, Diabetes Type I and RA

Question 30

Why is Caesarean section indicated in placenta praevia?

Answer 30

• Placenta previa is when the placenta covers part or all of the cervix during the last months of pregnancy.
• This condition can cause severe bleeding before or during labor.
• Bed rest is often the recommended treatment
• If heavy bleeding occurs, an emergency C-section delivery may be necessary.

Question 31

How can an ectopic pregnancy present clinically?

Answer 31

• The classical symptoms of ectopic pregnancy are abdominal pain and vaginal bleeding.
• The typical history is of an abnormal ‘period’ where the bleeding is prolonged with brown ‘prune juice’ spotting.
• The woman may not realise that she is pregnant if the bleeding strated around the time of her expected period. The patient may also complain of shoulder tip pain if the ectopic pregnancy is causing intraperitoneal bleeding.

Question 32

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

Answer 32

• The narrow Fallopian tube can only stretch a little.
• If the pregnancy grows further it will normally split (rupture) the Fallopian tube.
• This can cause heavy internal bleeding and pain

Question 33

What are the commonest causes of maternal mortality globally?

Answer 33

• Postpartum bleeding (15%)
• Compications from unsafe abortion (15%)
• Hypertensive disorders of pregnancy (10%)
• Postpartum infections (8%)
• Obstructed labour (6%)

Question 34

What are the commonest causes of maternal mortality in the UK?

Answer 34

• Thrombosis and thromboembolism
• Antepartum and postpartum haemorrhages
• Amniotic fluid embolism
• Genital tract sepsis
• Ectopic pregnancy

Question 35

Why are not all spermatozoa rejected?

Answer 35

Seminal fluid appears to contain a number of factors which suppress the immune response of the female from attacking the sperm:

• Certain cytokines, possibly TGF beta. This is a normal chemical of the imune system which can suppress T cell and macrophage responses.
• Prostaglandins, which may also suppress the immune response.
• Proteins specific to semen which block T cell responses
• Proteins which block complement-mediated destruction of sperm
• The sperms themselves appear to appear to be special in that they lack expression of MHC molecules, which are necessary for T cell mediated rejection. The surface of the sperm also has molecules which deactivate complement.

Question 36

Why is the foetus not routinely rejected?

Answer 36

• At sites of inflamamtion, proteins called chemokines recruit T cells as part of the immune response. In mice, the expression of genes that encode chemokines are switched off in the tissue around the foetus and placenta. This means that certain immune signals are turned off in the mother’s part of the placenta, which keeps her body from attacking the baby.
• Despite these protective mechanisms, pregnancies do not always reach full term (36 weeks). Some scientists attribute this to when the ‘tolerance process goes awry’, resulting in miscarriages, pre-eclampsia and preterm labour.

Question 37

Define the following terms:

• Trophoblast
• Synctiotrophoblast
• Cytotrophoblast
• Hypoblast

Answer 37

• Trophoblast: a layer of tissue on the outside of a mammalian blastula, supplying the embryo with nourishment and later forming the major part of the placenta.
• Syncytiotrophoblast: cells that merge together in the trophoblast
• Cytotrophoblast: cells in the trophoblast
• Hypoblast: a tissue type that forms from the inner cell mass. It lies beneath the epiblast and consists of small cuboidal cells.
• Epiblast: the outermost layer of an embryo before it differentiates into ectoderm and mesoderm.

Question 38

Briefly describe how the intraembryonic coelem is formed.

Answer 38

• During the 3rd week of development, the lateral mesoderm splits into a dorsal somatic mesoderm (somatopleure) and a ventral splanchnic mesoderm (splanchopleure).
• The resulting cavity between the somatopleure and splanchnopleure is called the intraembryonic coelem. This space will give rise to the thoracic and abdominal cavities.