Lecture notes Year 1 MBChB: Introduction to Medical Sciences (IMS)

E MBRYOLOGY
INTRODUCTION

Dating of pregnancy
 Menstrual age – clinicians
o Dates pregnancy from the woman’s last menstrual period
o Three equal trimesters
 Fertilisation age – Embryologists
o Age of the embryo from when the oocyte was fertilised by sperm
o Early development (cell division, pre-embryonic) period (ED)
o Embryonic (organogenesis) period (E)
o Foetal period (F)




Genetic causes:
 Monogenic- caused by a defective gene on a chromosome
 Chromosomal- numerical issue, having too many chromosomes
o E.g. Trisomy 21 (Downs syndrome) Pair 21 has an extra chromosome

Environmental problems: teratogens
 Infectious: Toxoplasmosis, Other (hepatitis B, Syphilis), Rubella (German measles), Cytomegalovirus (CMV), Herpes
o Diseases can cross the placenta and cause birth defects
 Chemical – Alcohol, Thalidimide/drugs
 Physical – Radiation (Chernobyl)
 Maternal diseases - Diabetes
 Deficiency- Folic acid


Description Symptoms
Toxoplasmosis  Parasite  Inflammation of retina and eye/
 Caused by: cat faeces and undercooked/raw meat micropthalmia – eye doesn’t form
 Usually asymptomatic properly
 Hearing loss
 Enlarged Liver & Spleen
 Hydrocephaly
 Microcephaly

Rubella  Infection passes over placenta in first 3 months of  Cloudy Cornea
pregnancy, when foetus is at most risk for  Intellectual disability
congenital malformations  Microcephaly
 Can be protected by having an MMR vaccine  Heart Defects
Cytomegalovirus  Virus that crosses the placenta  Inflammation of the retina
 Infection via bodily fluids  Enlarged spleen or liver
 Asymptomatic  Mineral deposits on the brain
 Microcephaly
 Psychomotor retardation

Herpes virus  Herpes Simplex and Herpes Zoster  Segmental skinloss/ scarring
 Varicella zoster virus – Chickenpox  Limb hypoplasia/paresis
o Most dangerous in between 13-20 weeks or  Microcephaly
just before birth to two days postpartum  Visual defects
Zika  Transmitted by mosquitoes, bodily fluids  Microcephaly
 Fever, rash, joint pain, red eyes,
 Cognitive disabilities
Radiation  Cell death or chromosome changes  Microcephaly

,  CNS is most sensitive foetus is most sensitive in  Mental and cognitive disabilities
first trimester  Haemopoietic malignancies and
leukaemia
Diabetes  Cellular structure defects  Spina bifida
 Changes in cellular physiology  Renal agenesis
 Macrosomia – can’t regulate glucose
levels properly, poor homeostasis
Folic acid  Dietary deficiency  Malformations in the central nervous
deficiency  Needed for first part of CNS development formation
 Spina Bifida
 Anencephaly

Thalidomide
 Prescribed for morning sickness
 But caused shortened/ absent limbs
 It is now used to treat leprosy and HIV

Foetal alcohol syndrome
 Clear relationship between alcohol consumption & congenital abnormalities
 Associated with: prenatal and postnatal growth retardation, intellectual disability, impaired motor ability and
coordination
 Characteristics: small eye openings, thin upper lip, smooth philtrum

Gametogenesis: Mitosis: diploid cells, meiosis: haploid cells

Fertilisation
 Fusion of male and female gametes to form zygote
 Capacitation of sperm, acrosome reaction, formation of zygote,
fusion of pronuclei
 Fertilisation usually takes place at the ampulla of the uterine tube
 Fimbriae sweep oocyte (egg) into the uterine tube
 Sperm undergo capacitation in female reproductive tract
 Cortical reaction occurs post-fertilisation and prevent polyspermy

Acrosome reaction
 Capacitated sperm pass through corona radiate
 Acrosome releases enzymes that allow sperm to penetrate zona pellucida
 Sperm penetration initiates cortical reaction
 Zona pellucida becomes impenetrable

Cleavage
 After fertilisation zygote cells divide
 No change in size of zygote, blastomeres get smaller

Formation of morula
 Forms after 8 cell divisions
 16- 32 cells
 Morula is important, differentiation of cells starts to occur
 Embyroblasts form the inner cell mass- from the embryo foetus
 Trophoblats form the outer cell mass, form support structures such as the placenta

Formation of blastocyst
 Formation of fluid cavity, ball of cells accumulates fluid by osmosis
 Composed of embryoblasts and trophoblasts
 Embryoblasts form compact
mass, trophoblast form thin
layer

Blastocysts hatching and initiating
implantation (days 5-6)



Sagittal Coronal