Embryology

Question 1

What are the stages of prenatal development?

Answer 1

• From conception to birth.
• There are 3 stages: germinal period, embryonic period, foetal period

Question 2

What happens in the germinal period of prenatal development?

Answer 2

• First two weeks post fertilisation of development
• The zygote divides to form two cells about 18-39 hours after fertilization (first cleavage)
• 2 cells divide to form 4, 8, 16….
• The first 6-7 days of early development usually take place in the fallopian tube as the ovum is being transported to the uterus by the ciliated cell lining.
• Result in formation of the primitive germinal layers

Question 3

What happens in the embryonic period of prenatal development?

Answer 3

• 2nd to end of the 8th period
• Organ systems develop
• Greatest potential for congenital defects to arise

Question 4

What happens during the foetal period of prenatal development?

Answer 4

• Last 30 weeks
• Organ systems grow and mature sufficiently to allow the newborn baby to survive outside of the womb.

Question 5

Describe the process of fertilisation

Answer 5

• Usually occurs in the distal portion of the fallopian tube
• Is the union of a sperm cell and oocyte
• Spermatozoa utilise a sac of enzymes (acrosome) to digest through the outer cells (corona) of the ovum resulting in a fertilised egg or zygote

Question 6

Describe the nature of the morula

Answer 6

Repeated cell divisions give rise to a solid ball of 12 or more cells called a morula - Day 3-5

Question 7

Describe the nature of the blastocyst

Answer 7

• Cells in the centre of the morula migrate from the centre towards the periphery leaving a hollow cavity called the blastocele.
• The now hollow ball of cells is now referred to as a blastocyst.

Question 8

Describe the two major regions of the blastocyst and what they develop into

Answer 8

• The trophoblast develops primarily into the placenta
• The inner cell mass develops into the embryo

Question 9

Explain the process of implantation

Answer 9

• Typically day 7-9
• Around day 5-7 blastocyst arrives at the uterus
• Progesterone stimulates the production of sticky mucus “uterine milk” which encourages adherence to uterine lining (endometrium)
• Following contact and adherence, trophoblast begins to invade endometrial tissue
• Uterine milk provides source of nutrition to blastocyst
• Implantation process can lead to the rupturing of blood vessels beneath the endometrium which may lead to a small amount of blood loss

Question 10

What happens during the late germinal period?

Answer 10

• Cells of the inner cell mass repeatedly divide and inner cell mass thickens
• The cells of the inner cell mass differentiate into two distinct layers which form a convex embryonic disc.

Question 11

What two layers form the convex embryonic disk?

Answer 11

• The ectoderm = outer layer
• The endoderm = inner layer

Question 12

Name the three layers of the trilaminar embryo

Answer 12

Ectoderm, endoderm, mesoderm

Question 13

What happens during the third week? (embryonic period)

Answer 13

• The two-layered embryonic disk gradually thickens
• Cells from ectoderm divide rapidly and divide rapidly forming a thickening called the primitive streak
• Lower ectoderm cells differentiate into a third (middle) layer = mesoderm
• 2 layered embryonic disk is now 3 layered
• The ectoderm, mesoderm and endoderm are traditionally referred to as germinal/germ layers.

Question 14

What is organogenesis?

Answer 14

Formation of body organs
- By 8th week all organ systems are recognisable

Question 15

What major organs does the ectoderm develop into?

Answer 15

• Nervous system
• Epidermis of the skin

Question 16

What major organs does the mesoderm develop into?

Answer 16

• Skeletal system
• Muscles
• Dermis of the skin
• Reproductive system
• Urinary system
• Cardiovascular system

Question 17

What major organs does the endoderm develop into?

Answer 17

• Gastrointestinal tract
• Epithelial linings of hollow organs and respiratory system

Question 18

Describe the origins of the central nervous system

Answer 18

• Neurulation = first stage of organogenesis and gives rise to the brain and spinal cord.
• Ectoderm thickens forming neural plate
• Neural plate folds inwards forming neural groove with two prominent neural folds and neural crests
• By 22nd day neural crests come together and fuse
• Results in neural tube structure which pinches off into the body
• Anterior end becomes brain, the rest becomes the spinal cord
• the closure of the neural tube is reliant on adequate levels of folic acid (vit B9) being available during pregnancy
• Failure of the neural tube closing can result in spina bifida. This is a relatively common congenital birth defect

Question 19

Genetic birth defects

Answer 19

• Inherited
• Usually follow predictable mendelian ratios
• e.g. polydactyly and albinism

Question 20

Congenital birth defects

Answer 20

• Developmental defects
• Difficult to predict, may arise spontaneously as a result of dietary deficiency or result of exposure to teratogens
• Many cases, no traceable cause
• e.g. cleft plate and cardiac septal defects (both can be treated by surgery)
• Large number are fatal and lead to miscarriage in first trimester (first three months of pregnancy)

Question 21

What is meant by the term teratogen?

Answer 21

Environmental factors which interfere with prenatal development and can cause birth defects

Question 22

Name some common teratogens

Answer 22

• Ionising radiations
• Noxious chemicals
• Certain drugs e.g. thalidomide
• Certain infectious diseases e.g. rubella

Question 23

Effects of Cytomegalovirus

Answer 23

• CMV transmitted by sexual contact and secretions, adult infections may be asymptomatic
• About 1% of pregnant women are infected and only a small number of babies will have clinical symptoms
• Symptoms including hearing loss (about 25% of deaf children), cerebral palsy, microcephaly, mental retardation, visual defects, dental abnormalities

Question 24

Effects of alcohol

Answer 24

• Features of feotal alcohol syndrome (FAS) occur in 30-40% of babies of chronic alcoholics another 50-70% of babies may suffer from foetal alcohol effects.
• To be diagnosed with FAS a baby may have the following clinical features:
  Growth retardation
  CNS problems
  Characteristic facial experience, small head, flat face, thin upper lip
  Children with FAS have failure to thrive, mild to moderate intellectual disability
  (used to be termed mental retardation) and behaviour problems.