embryology Flashcards
what are three periods of embryology ?
- pre-embryonic period ( 1st week )
. fertilisation in ovary to implantation in uterus - embryonic period ( weeks 2-8 )
. multicellular blastocyst to recognisable vertebrate
. organogenesis - fatal period week 9-38 ( 3 trimesters )
. major growth of existing structures
When does embryology happen?
from conception to birth- 38 weeks
What are the general cellular events in embryonic development?
1- Cell addition by mitosis or cleavage- from a single cell to more than 5x10^12 cells
2- Cell movement and migration- in order to from layered, folded and or tubular structures
3- Programmed cell death or apoptosis- regression and elimination of tissues that are no longer required after it has served its function.
What happens at day zero?
The zygote
- Successfully fertilisation- sperm implanted inside ovary- creating a single cell (male sperm release nucleus)
- This one cell is called Zygote
- Then forms diploid chromosomes (23pairs)
What is the appearance of the zygote?
maternal and paternal pronuclei + cytoplasm
surrounded by a transparent capsule = the Zona Pellicuda
What happens at Day 1?
- Start to get mitosis- splitting of cell
- Divides to form 2 blastomeres (Cell)
- Still surrounded by capsule - splits inside capsule
- Doesn’t double in size yet as no replication etc
What happens now during Week 1: Pre Embryonic period?
Cleavage: daily series of binary cell divisions: Both blastomeres divide and increase to 4 then to 8 and 16 etc
- Ocytes moves down oviduct (lined with ciliated columnar epithelia) to uterine cavity , wafted by cilia (on oviduct epithelium ) , as they wanna move the egg down the oviduct to the uterine cavity
What happens at 3 days?
Produced a mass of ball of cells called Morula
What happens at 6 days?
Get rearrangement and migration of cells which are forming structures
-get a blastocyst structure- created this internal cavity which is filled with fluid.
What does the blastocyst do?
In uterus
And starts to implant itself on the uterine wall
What happens at Day 5& 6 of Blastocyst?
Starts to segregate into 2 separate regions of cell
1-Inner cell mass= Embryoblast
forms the embryo (blast means maker)
2- Outer layer of cells= Trophoblast
- forms the placenta (tropho means feeder) - separated by fluid (blastocele)
- placenta - organ that interphase between the maternal blood supply and the embryo blood supply -takes nutrients and oxygen from mother’s blood supply to embryo
What happens at days 7-10?
The membrane around blastocyst ruptures- Zona Pellucida
-This then allows the trophoblast cells to begin invading the epithelium and stroma of the maternal uterine wall.
What happens between days 5 and 7?
The blastocyst and initial stage of its implantation in the uterine wall
Trophoblast attach and start to migrate between the columnar epithelium of the uterine wall.
The embryo does not develop in the uterus cavity
Where does the embryo develop?
in the mothers tissue
-blastocyst migrates and works its way through the uterine epithelium and embeds in uterine stroma - inside the connective tissue
What happens in week 2?
Deep impact and Division of the blasts
- The trophoblasts cells go further into uterine wall until whole blastocyst is inside the epithelium.
The embryoblast splits into 2 layered disc:
1 -Epiblast, outer layer of columnar cells
- Hypoblast- inner layer of cuboidal cells
2The trophoblast splits into 2 divisions:
-Cytotrophoblast- inner cells remain as single layer around the blastocyst
- Syncytiotrophoblast- outer cells lose their membranes, coalesce and infiltrate deeper forms syncitium