Embryology 1 and 2

Question 1

Give a brief description of SPERMATOGENESIS

Answer 1

• spermatogonium (46 chromosomes)
• Meiosis occurs
• 4 sperm cells produced (22+X or 22+Y)

Question 2

Give a brief description of OOGENESIS

Answer 2

• Oogonium (46 chromosomes)
• Meiosis occurs
• 1 ovum (22+X) and 3 polar bodies

Question 3

How is a zygote formed?

Answer 3

Several sperms surround the ovum, only one penetrates.

(pro)Nucleus of sperm enters and fuses with the (pro)nucleus of ovum to give a diploid cell called the ZYGOTE.

Question 4

Week 1 pre-embryonic phase

Answer 4

• Zygote (diploid = 46 chromosomes) is formed
• Divides to form a blastocyst.
• Moves through uterine tube, to reach uterine cavity.

Question 5

How are mitochondrial diseases inherited from?

Answer 5

Mothers

Question 6

Why is a BLASTOCYSTIC CAVITY formed?

Answer 6

As the number (and size) of cells increase, getting nutrition to the central core of cells starts becoming difficult. Therefore a cavity (blastocystic cavity) develops.

• Cells accumulate at one end to form an inner cell mass.
• The outer lining of cells is called the trophoblast.

Question 7

What causes ectopic pregnancy?

Answer 7

abnormal function of cilia

Question 8

What happens in week 2 of pre-embryonic phase?

Answer 8

Implantation occurs in the uterine endometrial layer and placenta begins to develop (this may start by day 6 of 1st week)

Cells that later form the embryo form a bilaminar disc

Sacs, membranes and cord to nourish the human conceptus (baby) start to form

Question 9

What are trophoblasts?

Answer 9

Cells that form the outer cell mass
- divides to form 2 layers
- ultimately forms CHORION
- develops finger like processes: chorionic villi

Question 10

What do CHORION do?

Answer 10

1. Implantation process (chorionic villi)
2. Forms part of the placenta in due course
3. Secretes human Chorionic gonadotropin (HCG) (used to detect pregnancy)

Question 11

Describe briefly IMPLANTATION

Answer 11

At ~ 7 days the blastocyst begins to burrow into the uterine wall (endometrium)

The chorion plays an important role in burrowing.

Question 12

Describe what happens in the Endometrium After Implantation Has Occurred

Answer 12

Decidua basalis: part of the endometrium deep to the implanted conceptus.

hCG (Human Chorionic Gondaptropin) secreted by the chorion is going to help maintain the decidua

Maternal blood (and urine) levels of hCG increase till around 12 weeks gestation

Question 13

What happens to the inner cell mass?

Answer 13

The cells of the inner cell mass form a 2–layered flat disc called the BILAMINAR DISC

2 cavities now begin to form:

• Amniotic cavity (A)
• Yolk sac (Y)

Question 14

What is the main functions of the PLACENTA?

Answer 14

Main functions:

• Fetal nutrition
• Transport of waste and gases
• Immune etc

Question 15

What are the two parts of the placenta?

Answer 15

Foetal part and surface: smooth with foetal blood vessels and end of umbilical cord

Maternal part: Decidua basalis of endometrium
Rough and has maternal blood vessels

Question 16

Describe the formation of 3 germ layers in week 3

Answer 16

Epiblast cells migrate to lie inbetween epiblast and hypoblast layers

Cells then displace Hypoblast

Forms 3 germ layers :

1. Ectoderm
2. Mesoderm
3. Endoderm

Trilaminar disc is formed!

Cells have now become specialised!

Question 17

What is NEURULATION?

Answer 17

Neural tube formation

A solid tube of cells called the notochord is formed
Notochord INDUCES ectodermal cells in the midline to form a neural tube

Question 18

What induces the mesoderms to thicken?

Answer 18

The neural tube

Question 19

What are the 3 separate parts of the mesoderm?

Answer 19

• Paraxial mesoderm
• Intermediate plate mesoderm
• Lateral plate mesoderm

Lateral plate mesoderm splits

Question 20

Each somite (paraxial mesoderm) divides into 3, what are they?

Answer 20

Dermatome
Myotome
Sclerotome

Question 21

Define TERATOLOGY

Answer 21

Study of when things go wrong during development

Question 22

Define TERATOGENS

Answer 22

Environmental factors that cause abnormal development.

Question 23

Causes of abnormal development - ENVIRONMENTAL

Answer 23

• Drugs – prescription / other
• Alcohol/ tobacco
• Infectious agents: ToRCH (Toxoplasma, Rubella, Cytomegalovirus, Herpes ) can transfer through placenta and affect developing embryo
• Others – eg: radiation

Question 24

Causes of abnormal development - GENETIC FACTORS

Answer 24

• Too many/too few chromosomes
  
  • Turner’s syndrome – 45 chromosomes single X
  • Down’s syndrome – 47 chromosomes trisomy 21
• Structural changes – deletions of genes, segments of chromosomes
• Causes:
  
  • Increased maternal age
  • Damage from environmental factors such as radiation (eg: X-rays)

Question 25

What does the risk posed by teratogen dependent on?

Answer 25

• Exposure during critical periods of development
• Dosage of drug/chemical/factor
• Genetic constitution of embryo ie some more susceptible than others at equivalent doses etc

Question 26

Diagnosis of malformation - PRENATAL

Answer 26

• Blood - AFP
• Ultrasound scan – 12 week anomaly scan
• Invasive tests: chorionic villus sampling and amniocentesis

Question 27

Diagnosis of malformation - POSTNATAL

Answer 27

• Hip stability
• Testes (descent)
• Fingers and toes
• Hearing

Question 28

What happens from week 4-8

Answer 28

• Folding into a tube (lateral folds) which started in 3rd week completes
• Neural tube – forebrain, midbrain, hindbrain and spinal cord development
• Heart starts to beat on Day 24!
• Gut formation from endoderm
• Urogenital system formation from intermediate mesoderm
• Body cavities from lateral plate mesoderm
• 43 pairs of somites form in the paraxial mesoderm and differentiate further
• Limb buds form
• Neck development – pharyngeal arches