What takes precedence in early embryonic development?
The establishment of the placenta
Why does the establishment of the placenta take precedence in early embryonic development?
Ensures support for pregnancy
What happens in week 2 of embryonic development?
What is formed from differentation in week 2 of embryonic development?
Two distinct cellular layers, the outer cell mass and the inner cell mass
What is the outer cell mass formed from?
What does the inner cell mass become?
The bilaminar disc
What does the bilaminar disc consist of?
What happens on day 6 of embryonic development?
The synctiotrophoblast breaches the uterine epithelium, and the conceptus becomes embedded
What is the breaching of the uterine epithelium by the syncytiotrophoblast under the control of?
What has happened in embryonic development by the end of the second week?
- The conceptus has implanted, and is now embedded in endometrium
- Two cavities have been formed
When does the implantation process continue into?
Continues into the 1st trimester
What are the two cavities formed by the end of the second week of embryonic development?
- Amniotic cavity
- Yolk sac
What is the yolk sac suspended within?
A supporting sac, the chorionic cavity
How is the yolk sac suspended in the supporting sac?
By a connecting stalk
What happens to the embryonic spaces as the embryo develops?
- Yolk sac disappears
- Amniotic sac enlarges a lot
What is the remnant of the yolk sac?
Vestigial structure in the umbilical cord
Why does the amniotic sac enlarge a lot?
Needs to meet the needs of the growing embryo and fetus
What is the chorionic sac occupied by?
The expanding amniotic sac
What happens to the amniotic and chorionic membrane?
It gets pushed together, and fuses
What is the clinical relavence of the membrane formed from the fusion of the amniotic and chorionic membrane?
It is the membrane that ruptures when the 'waters break'
What is true of implantation?
It is interstitial
What happens in implantation?
The uterine epithleium is breached, and the conceptus implants within the stroma
What is the result of the breaching of the uterine epithelium in implantation?
There is a very close relationship between the fetal and maternal tissue
What can breaching cause in implantation?
What is the relavence of bleeding caused by breaching in implantation?
It can lead to dating confusion, as it can be confused with a light menstrual period
What happens to the placental membrane as the needs of the fetus increase?
It gets thinner
What is shown by the thinning of the placental membrane as the needs of the fetus increase?
The placenta itself has a develomental programme required to keep up with the needs of the fetus
What is meant by the human placenta being haemomonochorial?
One layer of trophoblast ultimately seperates maternal blood from fetal capillary wall
What are the aims of implantation?
- Establish basic unit of exchange
- Anchor placenta within endometrium
- Establish maternal blood flow within the placenta
What is the basic unit of exchange in the placenta?
What are the stages in villi development?
When are primary villi formed?
What are primary villi?
Early finger-like projections of trophoblast
What do primary villi consist of?
Cytotrophoblast surrounded by syncytiotrophoblast
When are secondary villi formed?
What happens in the development of secondary villi?
Invasion and population of mesenchyme into core
When are tertiary villi formed?
What happens in tertiary villi formation?
Invasion of mesenchyme core by fetal vessels
What is true of tertiary villi?
Structure now capable of conducting exchange
How is the placenta anchored within the endometrium?
With the establishment of the outermost cytotrophoblast shell
How is the endometrium prepared for implantation?
- Pre-decidual cells begin to develop
- Elaboration of the spiral artery blood supply
What are pre-decidual cells?
Specialised endometrial cells that control implantation
What is the purpose of the deciudal reaction?
Provides balancing force for invasive force of trophoblast
Why is it required that there is a balancing force for the invasive force of the trophoblast?
Prevents going through myometrium, into the vascular structures of the pelvis. Implantation is a very tissue destructive process, so needs to be mediated
What is created by the elaboration of the spiral arterial blood supply?
Creation of a low resistance vascular bed
Why is the creation of a low resistance vascular bed critical?
Because it maintains the high flow required to meet the fetal demand, particularly in late destation
What happens in elaboration of the spiral arterial blood supply in normal implantation?
The trophoblast starts to invade the spiral arteries and the maternal endothelium is displaced, so fetal tissue lines the spiral arteries
What does fetal tissue lining the spiral arteries allow?
The system to be low resistance
Describe the fetal membranes in week 5
Chorion has villi all round
Describe the fetal membranes in week 12
Chorion loses villi away from decidua basalis, and final disc shape achieved
Describe the fetal membranes in week 22
Amniochorionic membrane fuses to decidua parietalis, becoming composite membranes
To what extent to monozygotic twins share a membrane?
Varies, can be;
- Entirely independant
- Two amnions, but shared chorion
- Amnion and chorion are both shared
What degree of membrane sharing is most risky for monozygotic twins?
Amnion and chorion both shared
Give an example of an condition that may occur when the amnion and chorion are both shared
Twin to twin transfusion syndrome
What is the maternal aspect of the placenta divided into?
Describe the 1st trimester placenta
- Placenta established
- Placental barrier still relatively thick
- Complete cytotrophoblast layer beneath syncytiotrophoblast
What is the importance of the cytotrophoblast layer of the 1st trimester placenta?
Acts as a stem cell layer, as the syncytiotrophoblast layer is lost, and so needs to be replaced
Describe a term placenta
- Surface area for exchange dramatically increased
- Placental barrier now thin
What is the placental barrier made up of in a term placenta?
Just syncytiotrophoblast and fetal capillary endothelium
How does the placental barrier thin to reach the stage its at in a term placenta?
The cytotrophoblast layer is lost, and the distance between the capillary endothelium and syncytiotrophoblast gradually reduces due to morphological developmental programme of the villi, where undifferentiated mesoderm reduces
What is the importance of the thinning of the placental barrier?
Short distance for nutrients to get into fetal circulation
What makes up the fetal circulation?
Two umbilical arteries, and one umbilical vein
What do the umbilical arteries carry?
Deoxygenated blood, from the fetus to the placenta
What does the umbilical vein carry?
Oxygenated blood, from placenta to fetus
What are the functions of the placenta?
- Placental synthesis
- Endocrine function
What does the placenta synthesise?
- Fatty acids
Why does the fetus require cholesterol?
Precursor for the key steroid hormones to support pregnancy, oestrogen and progesterone
What are the categories of hormones produced by the placenta?
What protein hormones are produced by the placenta?
- Human chorionic gonadotrophin (hCG)
- Human chorionic somatomammotrophin
- Human chorionic thyrotrophin
- Human chorionic corticotrophin
When is hCG produced?
During the first two months of pregnancy
What is the function of hCG?
Supports the secretory function of the corpus luteum, maintaining it until the placenta takes over the production of progesterone and oestrogen
What is the clinical importance of hCG?
Excreted in maternal urine, therefore used as a basis for pregnancy testing
What may cause an increase in hCG?
- Twin pregnancy
- Trophoblast disease
What does human chorionic somatomammotrophin and hPL do?
Increases glucose availability to fetus
What are steroid hormones responsible for in pregnancy?
Maintaining the pregnant state by shutting down the hCG axis
When does placental production of steroid hormones take over from the corpus luteum?
By the 11th week
What is the effect of progesterone in pregnancy?
Why is an increase in appetite needed in pregnancy?
To lay down fat stores early in pregnancy for use later
What forms of transport are used in the placenta?
- Simple diffusion
- Active transport
What is simple diffusion?
Molecules moving down a concentration gradient
What molecules move by simple diffusion across the placenta?
- Urea and uric acid
What limits gas diffusion in placenta?
Flow limited, not gas limited
What is the result of gas diffusion across the placenta being flow limited?
It is dependant on good flow through the utero-placental circulation
Why is maintenance of adequate flow to the placenta essential?
Fetal O2 stores are small, and so fetus can't tolerate interference with exchange for very long
What is transported by facilitated diffusion in the placenta?
How is active transport achieved in the placenta?
Specific transports are expressed by the synctiotrophoblast
What is transported by active transport in the placenta?
- Amino acids
What are amino acids, iron, and vitamins required for by the fetus?
Development and growth of new structures
Why is passive immunity required in the fetus?
Fetal immune system is immature
Why is passive immunity effective?
Because the newborn is likely to encounter the same type of infections as the mother, and so it can borrow her immune system to afford the baby a degree of protection until its own system can mount an immune response
How does passive immunity pass to a fetus?
Receptor mediated process, maturing as pregnancy progresses
Is passive immunity to the fetus immunoglobulin class specific?
Yes, IgG only
How do IgG levels in fetal circualation compare to those in the mothers?
They are higher
What is the purpose of IgG in breastmilk?
Tops it up