Microanatomy 3 Flashcards
what is the placenta
- this is a temporary structure which arises from both fetal and maternal cells
what is the function of the placenta
- nutrient trasnrpot
- waste removal
- gases transport
- site of synthesis for a variety of molecules including hCG and relaxin and steroid hormones particular oestrogen’s and progesterone
what week does the placenta assume the role of steroid hormone synthesis
12 week (between the 2nd and third months of pregnancy) - - takes over form the corpus luteum which degenerates and becomes the corpus albicans
what are the keys stages in placentation
- implantation and invasion into uterine tissues
- differentiation of the trophoblast
- develop of the villous structure
- remodelling of the spiral arteries
- maturation
describe the development of the blastocyst
- fertilising occurs in the ampulla of the uterine tube
- 3-4 days after fertilisation the morula arrives in the uterine body where implantation occurs
- by day 5 the blastocyst has formed and has hatched through the zone pelluida
- at the blastocyst stage the blastomeres of the inner cell mass are also called the embryo blast as they give rise to the embryo proper
- peripheral blastomeres constitute the trophoblast which gives rise to the fetal component of the placenta
when does implantation occur
- when the endometrium mucosa is sufficiently prepared for implantation in the secretory phase of the central cycle - days 22-24
- it can provide a suitable cellular and nutrient environment for the embryo, the trophoblast cells over the inner cell mass adhere to its surface
what is the stromal reaction
- this is also called the decidual reaction
- involves the thickening of the endometrium at the site of implantation
- local endometrial glands enlarge
- the area become more vascularised and oedematous
- inflammatory cells also invade
- secretions of the decidual cells and the endometrial glands are through to support the growth of implanting embryo
what is the stormal reaction also called
the decidual reaction
what do the stromal cells differentiate into
- the stromal cells of the endometrium differentiated into metabolically active secretory cells called decidual cells
- by the end of two weeks, storm cell changes and increased vascularisation spreads throughout the endometrium which is now known as the decidua
describe the differentiation of the trophoblast
- happens around day 7
- the trophoblast at the embryonic pole proliferates when it is contact with the endometrium
- some of the cell loose there plasma membranes and form a syncytium called the syncytiotrophoblast
- the cells lining the blastocyst are now called the cytotrophoblast
- the synctitrophoblast starts to invade and erode the underlying endometrium
- the rapidly proliferating cut-trophoblast cells will continue to add to the non dividing mass of the syncytiotrophoblast
what does the trophoblast divide into
- syncytiotrophoblast
- cytotrophoblast
when does trophoblast differentiation happen
day 7
what does the embryoblsat differntatie into
epiblast
hypoblast
what does the epiblast and hypoblast form
- bilayered embryonic disc
- and form two extraembyronic membranes that enclose the amnionic and yolk sac cavities
what draws the blastocyst into the endometrial mucosa
- the syncytiotrophoblast continues to erode into the endometrium and its fingerlike projects draw the blastocyst in to the endometrial mucosa
- by day 9 only a small fibrin clot marks the post the blastocyst implanted
by what day does the syntriotrophoblast cover the entire blastocyst
- around day 11
what happens once the synctiotrophoblasts cover the entire blastocyst
- one it covers the entire blastocyst it begins to develop vacuoles called trophoblastic lacunae
- the maternal capillaries widen to form sinusoids and are eroded by the syncytiotrophoblast causing the trophoblastic lacunae to fill with blood
- at the same time the extra embryonic mesoderm begins to form under the cytotrophoblast
how does the formation of chorionic villi take place
- the extra embryonic mesoderm induces the overlying cut-trophoblast to proliferate and form projects that extend into the trophoblastic lacunae
- the trophoblast layers with the underlying extra embryonic mesoderm forms the chorion
- these projects of the trophoblast are therefore called the primary chorionic stem vili
why do we form chorionic vili
- embryo is rapidly growing in week 2 therefore there is need for a more efficient nutritional and gas exchange
- hypoxia stimulates placental villus invasion into the endometrium
where and when does the chorionic cavity develop
- by 16 days
- develops in the middle of the extra embryonic mesoderm
what forms the three primary germ layers of the embryo
- gastrulation
what suspends the embryo in the chorionic cavity
- the embryo is now suspended in the chorionic cavity by a connecting stalk of extra embryonic mesoderm
what covers primary chorionic stem vili into secondary chorionic stem vili
- the extra embryonic mesoderm penetrates the core of the primary chorionic stem vili transforming them into secondary chorionic stem vili
how does the cytotrophoblast shell form
- the columns of cytotrophoblast invading into the synctiotrophoblast extend out to the periphery forming the interface between the trophoblast and the endometrium
- this is the cytotrophoblast shell
what does the secondary chorionic villus differentiate into
- by day 21 the extra embryonic mesenchymal core of the secondary chorionic villus differentiates into connective tissue and blood vessels
- this vili is called tertiary stem vili
what do the tertiary stem vili develop into
- these connect with vessels forming the embryo proper linked to the primitive heart
when does the primitive heart start beating
day 22
what are the four tissues materials must pass
- the endothelium of the villus capillaries
- the loose connective tissue in the core
- a layer of cytotrophoblast
- a layer of syncytiotrophoblast
how does remodelling of spiral arteries happen
- the extravillus trophophoblasts invade the spiral arteries and replace the smooth muscle and endothelium there with trophoblasts
- this creates high flow low resistance vessels which supply the lacunae with maternal blood
what do trophoblasts do in spiral arteries
- this creates high flow low resistance vessels which supply the lacunae with maternal blood
where the embryo is implanted what is the maternal endometrium called
the decidua basalis
on the opposite pole to the embryo what is the endometrium called
decidua capsularis
what happens to the placenta at 4 and 1/2 weeks
- the chorionic vili extend completely around the growing embryo into both decidua
what happens with further maturation to the decidua capsularis
- it looses its chorionic vili and vascular role of rate fetus
what does the decidua basalis do in further maturation
- with its chorionic vili it form the mature disc shaped placenta
when is the placenta fully developed
from the fourth month onwards
what happens to the placenta after it is fully developed
- it will only increase in diameter as uterine growth proceeds
- the cytotrophoblast layer within the villi becomes smaller
- the barrier between the maternal blood in the lacunae and capillaries can be reduced to a layer of synctiotrophoblasts and the endothelium of the foetal capillary is separated only by a shared basement membrane
describe the umbilical arteries
- have a smaller diameter and thicker muscular wall than the veins
what are the umbilical arteries and veins supported by
- they are supported by a mucoid connective tissue called Wharton’s jelly
describe the structure of Wharton’s jelly
it is mainly made of ground substance with fibroblasts - like precursors “mesenchymal cells”
