Life Before Birth Flashcards
what is embryology?
the study of development of an embryo from the stage of fertilisation until the fetal stage
preimplantation stage
week 1
embryonic stage (organogenesis)
weeks 2-8 (where development of placenta happens)
fetal stage (growth and development)
weeks 9-38
cleavage
mitotic divisions of fertilised egg, overall size remains the same. It is allowed passage through the narrowest part of the uterine tube (ISTHMUS) and is surrounded by a tough glycoprotein coat (ZONA PELLUCIDA)
morula
around 4 days after fertilisation, cells maximise contact with each other, forming a cluster of cells held together by tight junctions which helps with communication and signalling between the cells. it enters the uterus
blastocyst formation
the first sign of cellular differentiation, inner cell mass goes on to form the embryo and extraembryonic tissues
outer cells trophoblasts contributes to the placenta
as the embryo enters the uterine cavity, fluid enters via the zone pellucid into the spaces of the inner cell mass. a fluid filled blastocyst cavity forms
what typically happens on day 7?
the blastocyte hatches out of the zone pollucida
why does the blastocyte need to hatch?
it begins to run out of nutrients and needs to implant
how does the blastocyte hatch?
Icm cells undergo proliferation and the fluid builds up in the cavity, eventually resulting in the blastocyte hatching from the zone pellucid to facilitate implantation
when does implantation occur?
in week 2 of development
what is implantation?
the interaction between the embryo and the endometrial layer of the uterus
what do inner cell mass cells form
the embryo
what do trophoblast cells form
the placenta
what is a blastocyst?
consists of two cell populations, inner cell mass and trophoblasts
when does decidualisation occur?
when the blastocyst makes contact with the endometrium of the uterus
what happens during decidualisation?
it is a process that results in several changes in the endometrium to prepare for pregnancy. it happens in the stromal cells of the uterus. these cells will form the maternal complex of the placenta. the process triggers the production of several molecules and promotes the trophoblast cells to become invasive.
what are cytotrophoblasts?
a single layer of cells closest to the inside of the embryo
what is the syncytiotrophoblast?
more invasive and extensive than the cytotrophoblasts. it is the outer layer of cells.
what is the bilaminar disc?
the 2 layers formed from the inner cell mass, the epiblasts and hypoblasts
how is the diffusion of oxygen, waste and nutrients via the blood supply established?
towards the end of week 2, the implanting syncytiotrophoblast cells communicate with the maternal side of the placenta and establish a connection to able this diffusion
what is hCG (human chorionic gonadotrophin)
a hormone produced by the end of week 3 by the syncytiotrophoblasts which is then secreted into the urine
what happens during week 2 of development?
implantation occurs, creating a connection between the embryo and the maternal endometrium.
the inner cell mass forms two laters making up the bilaminar disc, the epiblast and the hypoblast.
the trophoblast layer differentiates to form two placental layers, the cytoptrophoblast and the invasive syncytiotrophoblast
what is an ectopic pregnancy?
where implantation occurs in the wrong place
what can cause an eptopic pregnancy ?
clot transit through the uterine tube, premature hatching of the blastocyte or common implication of IVF
what are extra-embryonic membranes?
membranes that are derived from the blastocyte but do not contribute to the embryo
amnion
continuous with the epiblast of the bilaminar disc.
lines a structure called the amniotic cavity which is filled with fluid and acts to protect the developing embryo
this is present up until birth
chorion
double layered membrane formed by the trophoblast and the extra embryonic membranes
it lines a structure called the chorionic cavity which is seen in early pregnancy but disappears due to the expansion of the amniotic cavity.
it forms the fetal component of the placenta
yolk sac
continuous with the hypoblast of the bilaminar disc.
it is important in nutrient transfer in weeks 2-3 but disappears completely by week 20
important in blood cell formation and formation of the gut
allantois
forms as an outgrowth of the yolk sac
it contributes to the umbilical arteries and connects to the fetal bladder
in non human mammals it acts as a reservoir for fetal urine
what is gastrulation
the process of cell division and migration resulting in the formation of three germ layers
what is the primitive streak?
a thickened area of cells which appears at the tail end of the cells. a small bulge at the end called the primitive node
how are the 3 layers formed?
the epiblast evaginates (cells move towards the primitive streak and move down under the epiblast and spread out again) and creates 3 new layers of cells. this is controlled by fibroblast growth factor 8 which is made by primitive streak
what are the 3 layers (outside to inside)
ectoderm, mesoderm, endoderm
what produces the ectoderm?
the top layer of epiblasts
what are some examples of ectoderm derivatives?
- epidermis of skin
- nervous system
- tooth enamel
- epithelial lining of mouth and anus
- cornea and lease of eye
what are some examples of mesoderm?
- skeletal system
- muscular system
- dermis of skin
circulatory and lymphatic system
what are some examples of endoderm?
- epithelial lining of digestive track
- liver
epithelial lining of resp system - thymus
- Thyroid glands and parathyroid glands
totipotent cells?
total power, eg zygote has the ability to make all types of cells
pluripotent cells?
can make very many types of cells within the body. eg inner cell mass of blastocyte as these cells can give rise to three germ layers. another example is embryonic stem cells
multipotent cells?
can make multiple cell types, eg haematopoietic stem cells can differentiate into many types of stem cells
what cells form the primitive streak?
epiblast cells that migrate to the midline then form the primitive streak
what do the hypoblast cells surround?
the yolk sack
what do the epiblast cells form?
the cavity known as the amniotic cavity
what is the primitive streak?
elongates from caudal to cranial
what happens to epiblast cells that migrate beyond the primitive streak and into the hypoblast?
become the endoderm
what cells make up the ectoderm?
the remaining epiblast cells that do not enter the hypoblast
what cells make up the mesoderm?
the majority of epiblast cells that occupy the space between the epiblast and hypoblast layers differentiate to become the mesoderm
what is the first mesodermal tissue to form?
notochord
where are somites derived from?
paraxial mesoderm
what are the 3 segments of the mesoderm after it changes shape (medial to lateral)?
paraxial mesoderm
intermediate mesoderm
lateral plate mesoderm
what are the different parts of the lateral plate mesoderm?
the one closest to ectoderm?
- somatic layer of the lateral plate mesoderm
the one closest to the endoderm - sphlanic layer of the lateral plate mesoderm
what do the somites turn into?
dermatome myotome -top
sclerotome - bottom
what does the paraxial mesoderm eventually turn into (dorsal to ventral)?
dermatome
myotome
sclerotome
