Placentation and Implantation Flashcards
1
Q
What are the phases of development of the placenta?
A
- invasion (of conceptus to endometrium)
- decidualisation (endometrial remodelling to maintain embryo)
- placentation (placenta formation)
2
Q
How is the placenta supplied?
A
- embryonic portion: from outermost layers of trophoblast cells (chorion)
- maternal portion: endometrium underlying chorion
- chorionic villi extend from chorion to endometrium which have a network of capillaries
- is part of embryos circulatory system
- each villi surrounded by pool of maternal blood
3
Q
Describe placenta blood supply
A
Maternal blood:
- enters placental pools by uterine artery
- flows through sinuses
- exits by uterine veins
Foetus blood:
- flows into capillaries of choronic villi by umbilical arteries
- back to foetus by umbilical vein
4
Q
Describe the events that take place at towards the end of the first week after fertilisation
A
- attachment and implantation
- day 7 blastocyst leaves zona pelucids and is bathed by uterine secretion for 2 days
- progesterone prepares supportive uterine environment by increasing glandular tissue
- oestradiol is required to release glandular secretions
5
Q
Describe the process of attachment and implantation
A
- very limited time window
- complex interactions between trophoblast and maternal epithelial tissue
- causes syncytiotrophoblast cells to flow into endometrium
causes decidualisation: oedema, glycogen synthesis and increased vascularisation
(endometrium now termed decidua)
6
Q
Describe the events that take place during implantation
A
- day 13 (when expecting next period)
- syncytiotrophoblast cells erode through walls of large maternal capillaries causing them to bleed into the spaces
- forms primitive placental circulation (although nutrition is still dependent on uterine secretion and tissues)
- breakthrough bleeding may occur
7
Q
Describe how the placenta develops a basis for exchange between mother and child
A
- syncytiotrophoblast forms villi that porject into chorionic villi
- in core is foetal capillary loop which is dilated at the tip
- the embryonic placental structure develops over several weeks with villi becoming localised at embryonic pole
- presents huge surface area for exchange of nutrients, O2 and waste products
8
Q
What separates maternal and foetal circulations preventing mixing of blood?
A
placental membrane
9
Q
Summarise the first trimester
A
- limited embryonic growth
- nutrition of embryo based largely on uterine secretion and tissues
- lack of appropriate hormonal support
- endomertrium needs to be at least 8mm for successful implantation
10
Q
How is hormonal support in early development maintained?
A
- LH supports steroid secretion of corpus luteum
- hCG maintains progesterone secretion from corpus luteum until placenta can synthesise its own progesterone (syncytiotrophoblasts secrete hCG soon after implantation)
11
Q
What are the functions of hCG?
A
- prevents menstruation in the presence of an implanted embryo
- from day 6-7 can be detected
- mimics action of LH and supports steroid synthesis of corpus luteum preventing menstruation and further follicular development
- stimulates Leydig cells of male fetuses to produce testosterone which is important for development of male duct system
12
Q
What is the role of progesterone in pregnancy?
A
- needed to maintain pregnancy and increases throughout
- suppresses follicular growth and ovulation
- suppresses immune response
- maintains endometrium
13
Q
Describe the role of oestrogen in pregnancy
A
- oestriol is main oestrogen in pregnancy
- foetus and placenta cooperate to secrete oestrogens
- stimulates growth of uterine myometrium
- stimulates growth (with progesterone) of ductal tissue of breast
- (with relaxin) relaxes and softens maternal pelvic ligaments and symphysis pubis of pelvis bones to allow for uterus expansion
- stimulates LDL cholesterol uptake and activity of P450 enzymes (for progesterone synthesis)
- foetal well-being and placental function can be measured by monitoring oestrogen levels
14
Q
What are the functions of the placenta over the first 3 months?
A
- 1: villus formation
- 2: increasing surface area and circulation
- 3: growing, becoming increasingly efficient
15
Q
Describe nutrient exchange across the placenta
A
- rapid and increases as pregnancy advances
- water and electrolytes diffuse freely
- glucose passes by facilitated diffusion (as foetus has little capacity for gluconeogenesis)
- amino acids actively transported for foetal growth
- lipids cross as free fatty acids
- vitamins exchanged
