Histology: Placental Development

Question 1

What is the placenta?

Answer 1

A temporary structure which arises from both foetal and maternal cells

Question 2

What is the role of the placenta?

Answer 2

Primary function is selective transport between mother and foetus but is also the site of synthesis for many molecules (hCG, relaxin, oestrogens and progesterone)

Question 3

Outline the five key stages in the formation of the placenta

Answer 3

1. Implantation and invasion of uterine tissues
2. Differentiation of the trophoblast
3. Development of the villous structure
4. Remodelling of the spiral arteries
5. Maturation

Question 4

Where does fertilisation occur?

Answer 4

Ampulla of oviduct

Question 5

Describe the implantation stage of placental development

Answer 5

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

Question 6

Describe the blastocyst at day 5

Answer 6

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)

Question 7

Describe the decidual reaction in placental development

Answer 7

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.

Question 8

How is the syncytiotrophoblast formed?

Answer 8

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

Question 9

How do the syncytiotrophoblasts and cytotrophoblasts invade the placenta?

Answer 9

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)

Question 10

What two cell types does the trophoblast become?

Answer 10

Syncytiotrophoblasts and cytotrophoblasts

Question 11

What two layers does the embryoblast differentiate into?

Answer 11

Epiblast and hypoblast (form bilayered disc)

Question 12

What stimulates the placental villus invasion into the endometrium?

Answer 12

Hypoxia in the second week (need for more efficient nutrition)

Question 13

What is the role of ectoderm in embryological development?

Answer 13

Forms part of amniotic cavity

Question 14

What is the role of mesoderm in embryological development?

Answer 14

Forms structure separating the amnion from the yolk sac

Question 15

What is the role of endoderm in embryological development?

Answer 15

Forms part of yolk sac

Question 16

Where does the chorionic cavity develop at 16 days?

Answer 16

In the middle of the extraembryonic mesoderm

Question 17

Outline the formation of the chorion

Answer 17

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

Question 18

What are the primary chorionic stem volli?

Answer 18

The projections from the cytotrophoblast into the trophoblastic lacunae

Question 19

What is the secondary chorionic stem villi?

Answer 19

When the extraembryonic mesoderm penetrates the core of the primary chorionic stem villi to convert them into secondary chorionic stem villi

Question 20

What is the cytotrophoblast shell?

Answer 20

Where the columns of cytotrophoblast invading into the syncytiotrophoblast extend out to the periphery, forming the interface between the trophoblast and the endometrium

Question 21

What are tertiary stem villi?

Answer 21

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

Question 22

What are the 4 layers that materials must pass through to get from maternal blood to the embryo?

Answer 22

1. Endothelium of the villus capillaries
2. The loose connective tissue in the core
3. A layer of cytotrophoblast
4. A layer of syncytiotrophoblast

Question 23

What characterises the primary stem villus?

Answer 23

Only has cytotrophoblasts projecting towards the decidua

Question 24

What characterises the secondary stem villus?

Answer 24

Has deep invasion of the cytotrophoblasts into the decidua

Question 25

What characterises the tertiary stem villus?

Answer 25

Has foetal vessels present

Question 26

Describe the remodelling of the spiral arteries

Answer 26

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

Question 27

Why are the spiral arteries remodelled by the cytotrophoblasts?

Answer 27

To create high-flow, low-resistance vessels to supply the lacunae with a large increase in maternal blood.

Question 28

What is the decidua basalis?

Answer 28

Where the embryo is implanted, the endometrium is known as the decidua basalis

Question 29

What is the decidua capsularis?

Answer 29

The endometrium facing to the uterus

Question 30

In the mature placenta, what barriers remain between the maternal blood in the lacunae and the foetal capillaries?

Answer 30

Reduced to a layer of syncytiotrophoblast and the endothelium of the foetal capillary, separated only by a shared basement membrane

Question 31

Describe the histology of the placenta at 6 weeks

Answer 31

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

Question 32

What is a syncytial knot?

Answer 32

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

Question 33

Describe the structure of the umbilical cord

Answer 33

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’

Question 34

Describe the umbilical arteries

Answer 34

Arteries have smaller diameter and thicker muscular walls and carry waste-laden blood from foetus to the placenta

Question 35

Describe the umbilical vein

Answer 35

Carries oxygen and nutrient rich blood from placenta to the foetus

Question 36

Histologically, how does the endometrium appear before and during implantation?

Answer 36

During implantation: most of the space is taken up by the stromal cells, with their cytoplasm staining a light pink