Introduction to Embryology Carolyn Joy Rodgers MD FACOG

AIM • Begin your study of embryology • Outline the early major developmental events and anatomical landmarks of a human embryo • Understand be able to use the terminology of early human development • Act as an introduction to the systems embryology you will encounter in the phase one modules • Begin to identify relevance of these processes to clinical practice

Learning Outcomes: lecture one • Describe simply the processes from fertilisation to implantation • Describe simply the second week of human development • Describe simply the early stages of placental development

What is embryology? • Development from single diploid cell…. to…. • Multicellular organism • Organised body structure • Specialised tissues

Development • Growth • Morphogenesis • Differentiation

Role and relevance of embryology • • Understand normal body structure Maturation of adult structures Anatomical relationships Links prenatal development to clinical medicine • Values in medicine

How is it studied? • • • Starting with anatomy Teratology Experimental Molecular / Cellular Genetic

The embryonic period • • • Conception Eight weeks Body axes Organ primordia Three layers Early placenta

First Week • • Fertilisation Cleavage Blastocyst formation Implantation

The first day • • • Gametes and fertilisation-zygote A unique new individual Ampulla Cleavage Blastomeres

Day 3 • Compaction • Morula

Entering the uterus: day 4 -5 • Blastocoele / Blastocyst • Outer cells-Trophoblast • Inner cell mass-embryoblast -pluripotent

Implantation: day 6 -7 • Hatching • Cytotrophoblast • Syncytiotrophoblast • Invasion

Implantation process: day 7 • Uterus has been prepared • Usually in upper part of uterus • Specific orientation and maternal/embryonic communication mechanisms • Immunologically complex • Very little information in humans • IVF- most likely stage of failure

Second week • Two layers • Amniotic cavity • Yolk sac • Extraembryonic mesoderm-splanchnic • - somatopleural

The two layers: Bilaminar disc: day 8 -9 • Epiblast and hypoblast • Amniotic cavity • Primitive yolk sac

Day 10 -12 • Utero-placental • circulation • Extra-embryonic • mesoderm • Chorionic cavity • (Extra-embryonic coelom)

Day 13 -14 • • • Implantation bleed Human chorionic gonadtrophin HCG Villous formation Secondary yolk sac Chorionic plate Connecting stalk