Embryology Flashcards
What are the key processes by which one cell becomes a multicellular body?
Growth, morphogenesis (development of form and structure), and differentiation.
What are the three periods of embryonic development?
Weeks 1-2 are the pre-embryonic period, weeks 3-8 are the embryonic period, and week 9+ (~38) are the fetal period.
What happens during fertilisation in the pre-embryonic period?
The oocyte is released from the ovary and fertilised in the ampulla of the Fallopian tube, becoming a zygote.
What occurs after fertilisation?
Cleavage (the first mitotic division) occurs about 30 hours after fertilisation, resulting in two blastomeres.
What is the zona pellucida?
A glycoprotein ‘shell’ that forms to prevent access by any further sperm.
What is a morula?
A ball of about 16 cells, all of which are totipotent, and the same size as the original zygote.
What is compaction?
The process when the first cavity forms, and the zona pellucida still surrounds the cells.
What is the blastocyst cavity?
A large fluid-filled space created by fluid released from the cells.
What are the two masses of cells in the blastocyst?
The inner mass called the embryoblast (which will form the being) and the outer mass called the tropoblast (which forms supporting tissues).
What happens after compaction?
The cells become pluripotent.
When does the blastocyst hatch from the zona pellucida?
About day 5, as the risk of being fertilised by another sperm is low.
What is required for the blastocyst around day 6-7?
The blastocyst needs to implant itself in the uterine lining.
What does the tropoblast do during implantation?
It communicates with the endometrial cells.
What are the two cell layers generated by the tropoblast in week 2?
The syncytiotropoblast and cytotropoblast.
What does the embryoblast become by the end of week 2?
A two-layered disk called the bilaminar disk, comprised of the epiblast and hypoblast.
What cavities are present in the conceptus by the end of week 2?
The amniotic cavity and yolk sac, suspended by a connecting stalk within the chorionic cavity.
What occurs during implantation?
The blastocyst buries itself into the endometrium and establishes maternal blood flow within the placenta to access nutrients and remove waste.
What is ectopic pregnancy?
Implantation at another site, most commonly the Fallopian tube.
It can also occur in the ovaries and may become a life-threatening emergency.
What can occur when the embryo implants in the lower uterine segment?
Placenta praevia can occur, which may block a normal delivery and require a c-section.
What happens by day 9 of implantation?
The syncytiotrophoblast undergoes rapid development at the embryonic pole.
What forms at the abembryonic pole by day 9?
The primitive yolk forms, which is in contact with the cytotrophoblast.
What pushes the primitive yolk sac away from the cytotrophoblast?
An acellular extraembryonic reticulum at about day 11.
What occurs at day 12 of implantation?
Maternal sinusoids are invaded by the syncytiotrophoblast for blood supply, forming continuous lacunae.
What begins at day 12?
The uteroplacental circulation begins.
What forms at day 13?
A secondary yolk sac forms by pinching off the primitive yolk sac.
What is suspended by the mesoderm at day 14?
The embryo and its cavities.
What is the embryonic period?
Weeks 3 - 8, during which the greatest change occurs and the highest risk of major congenital malformation exists.
What happens in the 3rd week of the embryonic period?
A primitive streak with its pit and node appears, leading to the formation of the three germ layers.
What are the three germ layers formed during gastrulation?
Ectoderm, mesoderm, and endoderm.
What does the ectoderm develop into?
Organs and structures in contact with the outside world.
What does the mesoderm develop into?
Supporting tissues.
What does the endoderm develop into?
Internal structures.
What is the notochord?
A solid rod of cells formed during gastrulation with important signalling roles.
What role does the notochord play?
It directs the overlying ectoderm into the neuroectoderm.
Somite derivatives
dermatome, myotome, sclerotome
Cephalocaudal folding
Ectoderm folds over endoderm, this pushes the cardiogenic field down
