8. Early Embryology I Flashcards
How does one cell become a multicellular body?
Growth, morphogenesis, and differentiation.
Define morphogenesis
Development of form and structure
Define differentiation
Development and specialisation for function
What is the first two weeks following fertilisation?
The pre-embryonic period
What is weeks 3-9 following fertilisation?
Embryonic period
What is the develop post-9 weeks until birth
Foetal development
What occurs during the pre embryonic period?
Cleavage - formation of morula
Compaction - formation of blastocyst
Implantation
Where does fertilisation occur?
In the ampulla - fallopian tube
Oocyte fertilised to become zygote
What occurs in week 1 following fertilisation?
Cleavage, mitotic cell division begins, zona pellucida forms - glycoprotein shell around zygote so no more sperm can penetrate.
What potency are the cells within week 1?
Totipotent, have capacity to become anything
What occurs during assisted reproductive techniques for fertilisation? What testing can be done at this stage?
Oocytes fertilised in vitro and divide to 4/8 cell stage
Morula transferred to uterus
Pre-implantation genetic diagnosis; cell removed from morula, tested for heritable conditions before placed in mother
What happens during compaction in week 1?
Formation of the first cavity, first differentiation
Blastocyst with cavity, inner cell mass known as embryoblast which forms cells of the body
Outer cell mass trophoblast, cells that support fetus - placenta etc
Zona pellucida outside
What potency are cells after compaction?
Pluripotent, the capacity to become one of many cell types - multilineage potential
What occurs during hatching?
Blastocyst hatches from zona pellucida, no longer constrained free to enlarge and interact with uterine surface to implant
What occurs during implantation?
Conceptus now approx 100 cells, of these 8 form embryo remainder for fetal membranes.
What occurs during the second week?
Differentiation - two distinct cellular layers emerge.
From outer cell mass:
- syncytiotrophoblast
- cytotrophoblast
Inner cell mass becomes bilaminar disk - epiblast and hypoblast
What has occurred by the end of week 2?
The conceptus has implanted
Embryo has two cavities - amniotic and yolk sacs
Suspended in chorionic cavity with connecting stalk
(Yolk sac larger sac)
What does the bilaminar disc look like at 2 weeks?
Amniotic cavity surrounded by cytotrophoblast and syncytiotrophoblast, below is hypoblast, below that is epiblast making up bilaminar disc. Below this enclosed also by cytotrophoblast is blastocyst cavity.
What happens in implantation?
Complete at the end of week two.
Embedded within internal tissues, uterine epithelium is breached and conceptus implants in uterine stroma.
Establishes maternal blood flow with placenta and basic structural unit of materno-fetal exchange. Fibrin plug formed.
What are some conditions linked to implantation defects?
Ectopic pregnancy - implantation at site other than uterine body, peritoneal or ovarian, life-threatening
Placenta praevia - implantation in lower uterine segment, haemorrhage, c-section
What is important about Day 9 of week 2?
Primitive yolk sac formed, in contact with cytotrophoblast layer
What occurs at day 11 of week 2?
Primitive yolk sac pushed away from cytotrophoblast layer by acellular extraembryonic reticulum.
Reticulum layer converted to extraembryonic mesoderm by cell migration.
What occurs at day 12 of week 2?
Maternal sinusoids invaded by syncytiotrophoblasts,
Uteroplacental circulation begins, stroma prepares for support of embryo.
What occurs at day 13 of week 2?
Formation of secondary yolk sac - definitive.
Pinches off from primitive.
What occurs at day 14 of week 2?
Spaces within extraembryonic mesoderm merge, forms chorionic cavity.
Embryo and cavities suspended by connecting stalk forming future umbilical cord.
Some menstrual like bleeding.
What is the blastocoele?
First cavity formed as a result of compaction
What is the amniotic sac?
Formed from spaces in epiblast
What is the primitive yolk sac?
Exocoelomic cavity
Formed by hypoblast lining
What is the secondary yolk sac?
Formed within primitive yolk sac, is definitive one
What is the extraembryonic coelom?
Chorionic cavity, formed by spaces within extraembryonic reticulum and mesoderm
