Flashcards in Histology: Placental Development Deck (36):
What is the placenta?
A temporary structure which arises from both foetal and maternal cells
What is the role of the placenta?
Primary function is selective transport between mother and foetus but is also the site of synthesis for many molecules (hCG, relaxin, oestrogens and progesterone)
Outline the five key stages in the formation of the placenta
1. Implantation and invasion of uterine tissues
2. Differentiation of the trophoblast
3. Development of the villous structure
4. Remodelling of the spiral arteries
Where does fertilisation occur?
Ampulla of oviduct
Describe the implantation stage of placental development
When the endometrium mucosa is sufficiently prepared for implantation and it can provide a suitable cellular and nutrient environment for the environment, the trophoblast cells over the inner cell mass adhere to its surface
Describe the blastocyst at day 5
Has 'hatched' (removed zp), inner cell mass is known as the embryoblast and peripheral blastomeres constitute the trophoblast (gives rise to foetal component of placenta)
Describe the decidual reaction in placental development
Involves the thickening of the endometrium at site of implantation and local endometrial glands enlarging --> more vascularised and oedematous. The stromal cells of the endometrium differentiate into active secretory cells (decidual cells). Secretions of decidual cells and endometrial glands are thought to support growth of the implanting embryo. 14 days later, the stroma cell changes --> increased vascularisation spreads through endometrium, now known as the decidua.
How is the syncytiotrophoblast formed?
At about day 7, embryonic pole proliferates on contac with endometrium --> some cells lose their plasma membranes and join together to form a syncytium known as the syncytiotrophoblasts
How do the syncytiotrophoblasts and cytotrophoblasts invade the placenta?
Syncytiotrophoblasts start to invade and erode the underlying endometrium, cytotrophoblasts are rapidly proliferating and will continue to add to the non-dividing mass of the syncytiotrophoblast. Syncytiotrophoblast continues to erode endometrium and it's finger-like projections draw the blastocyst into the endometrium and then begins to develop vacuoles (trophoblastic lacuna) which maternal capillaries supply)
What two cell types does the trophoblast become?
Syncytiotrophoblasts and cytotrophoblasts
What two layers does the embryoblast differentiate into?
Epiblast and hypoblast (form bilayered disc)
What stimulates the placental villus invasion into the endometrium?
Hypoxia in the second week (need for more efficient nutrition)
What is the role of ectoderm in embryological development?
Forms part of amniotic cavity
What is the role of mesoderm in embryological development?
Forms structure separating the amnion from the yolk sac
What is the role of endoderm in embryological development?
Forms part of yolk sac
Where does the chorionic cavity develop at 16 days?
In the middle of the extraembryonic mesoderm
Outline the formation of the chorion
Extraembryonic mesoderm induces the overlying cytotrophoblasts to proliferate and form projections which extend into the trophoblastic lacunae --> the trophoblast layers and underlying extraembryonic mesoderm form the chorion
What are the primary chorionic stem volli?
The projections from the cytotrophoblast into the trophoblastic lacunae
What is the secondary chorionic stem villi?
When the extraembryonic mesoderm penetrates the core of the primary chorionic stem villi to convert them into secondary chorionic stem villi
What is the cytotrophoblast shell?
Where the columns of cytotrophoblast invading into the syncytiotrophoblast extend out to the periphery, forming the interface between the trophoblast and the endometrium
What are tertiary stem villi?
The extraembryonic mesenchymal core of the secondary chorionic villus differentiates into CT and blood vessels, --> thereafter known as tertiary stem villi which connect with the vessels forming in the embryo proper in order to establish uteroplacental circulation
What are the 4 layers that materials must pass through to get from maternal blood to the embryo?
1. Endothelium of the villus capillaries
2. The loose connective tissue in the core
3. A layer of cytotrophoblast
4. A layer of syncytiotrophoblast
What characterises the primary stem villus?
Only has cytotrophoblasts projecting towards the decidua
What characterises the secondary stem villus?
Has deep invasion of the cytotrophoblasts into the decidua
What characterises the tertiary stem villus?
Has foetal vessels present
Describe the remodelling of the spiral arteries
Extravillus cytotrophoblasts are drawn to the spiral arteries by an oxygen gradient, and invade the spiral arteries to replace the smooth muscle and endothelium there with trophoblasts
Why are the spiral arteries remodelled by the cytotrophoblasts?
To create high-flow, low-resistance vessels to supply the lacunae with a large increase in maternal blood.
What is the decidua basalis?
Where the embryo is implanted, the endometrium is known as the decidua basalis
What is the decidua capsularis?
The endometrium facing to the uterus
In the mature placenta, what barriers remain between the maternal blood in the lacunae and the foetal capillaries?
Reduced to a layer of syncytiotrophoblast and the endothelium of the foetal capillary, separated only by a shared basement membrane
Describe the histology of the placenta at 6 weeks
6 week placenta will have a core villus composed of mesenchymal tissue which has a mucoid connective tissue high in GAGs, erythrocytes stil have nucleus (obvious in capillaires), lacunae are empty spaces in histology slides
What is a syncytial knot?
A fully developed placenta has a syncytial knot, whereby the syncytiotrophoblast nuclei cluster together to leave zones of thin cytoplasm between them in order to reduce the diffusion barrier in these intervening areas
Describe the structure of the umbilical cord
Has two umbilical arteries and one umbilical vein that are supported by a mucoid connective tissue called Wharton’s jelly which is made mainly of ground substance with fibroblast-like precursor ‘mesenchymal cells’
Describe the umbilical arteries
Arteries have smaller diameter and thicker muscular walls and carry waste-laden blood from foetus to the placenta
Describe the umbilical vein
Carries oxygen and nutrient rich blood from placenta to the foetus