W5L3 Thues Placentation Flashcards
Oviparous (amniote) species
produce eggs which develop externally, may have internal/external fertilisation
Ø Amniote eggs are adapted to survive in terrestrial environments – hard semipermeable shell protects embryo from harsh environments + allows gas exchange
Ø All nutrient provided by yolk
Viviparous species
give birth to live young, must have internal fertilisation
Ø Viviparity increases chance of survival by protecting young within a parent’s body
Ø Ovoviviparity: eggs retained within mother’s body until hatching
Ø Placental viviparity: placenta provides nutrient + gas exchange
§ Allows for larger offspring – not constrained by egg size, increased nutritional support
Ø Convergent evolution: distantly related organisms independently evolve similar traits
§ Viviparity estimated to have evolved >150 times
definition of placenta
§ Historical definition: an apposition or fusion of the foetal membranes to the uterine mucosa for physiological exchange
§ Modern definition: temporary organ that provides nourishment to embryo whilst inside uterus
§ Finite lifespan: ~270 days humans, ~600 days African elephants, ~1.5 days stripe-faced dunnart
§ True placenta: placenta forms inside the uterus (cartilaginous fish, lizards, snakes, marsupials + eutherians)
§ Placenta-like structures: ‘placenta’ forming outside the uterus (cartilaginous fish, ray-finned fish, frogs)
List of extraembryonic membrane
-amnion: liquid filled membrane that surround the fetus
-Yolk sac:
-Allantois:
-Chorion: cell layer that directly interacts with maternal tissues (chorionic villous)
Amnion in depth
Ø Extraembryonic ectodermal cells lined w/ extraembryonic mesoderm (nonvascularised)
Ø Cavity expands during gestation to 1L ~33-34 weeks
Ø Role: protective buffer against mechanical injury
yolk sac in depth
-lined by extraembryonic ectoderm + outside is well-vascularised extraembryonic mesoderm
Ø Birds/reptiles: provides yolk for nutrition
Ø Mammals: primordial germ cells arise in extraembryonic mesoderm near base of allantois (3rd week) + becomes visible in lining of yolk sac before migrating to gonads; provides extraembryonic haematopoiesis
Allantonis in-depth
-endodermally lined ventral outpocketing of hindgut (mesoderm + ectoderm)
Ø Other vertebrates: major respiratory organ + repository for urinary wastes
Ø Humans (vestige): role in respiration – BVs differentiated from mesodermal wall of allantois forms the umbilical circulatory arc, arteries + veins that supply placenta
Chorion in depth
Ø Extraembryonic mesoderm: blood vessels, Hofbauer cells (foetal macrophage), placental fibroblasts
Ø Trophectoderm:
§ Syncytiotrophoblast: outermost layer, multinucleated cell in direct contact w/ maternal blood
§ Cytotrophoblast: progenitor cells, can differentiate into syncytiotrophoblasts or extravillous trophoblasts, cell column which anchors placental villous to uterine decidua
§ Extravillous trophoblast: invades into uterus decidua to interact with maternal cells or remodel BVs to create placenta blood supply
Mammalian Placentation
§ Placentas are optimised to facilitate exchange of gases + nutrients b/w mother + foetus
§ Trophoblast invasiveness varies with different species
Ø Oviparous: maternal endometrial epithelium + foetal trophoblasts separated
Ø Epitheliochorial (horse): maternal endometrial epithelium in contact with foetal chorion trophoblast
Ø Synepitheliochorial (sheep): uterine epithelium lost but trophoblast cells have not invaded into endometrium
Ø Endotheliochorial (cat): maternal endothelium (BVs) in contact with foetal trophoblast
Ø Haemochorial (human): foetal trophoblasts bathed in maternal blood
§ Greater area of placentation = greater distance for exchange
Ø Diffuse (epitheliochorial) > cotyledonary (synepitheliochorial) > zonary (endotheliochorial) > discoid (haemochorial)
Sheep placentation
§ Synepitheliochorial, cotyledonary + non-invasive placenta
§ Foetal cotyledon + maternal caruncle come together to form a placentome
Ø Cotyledon: in-growths of chorionic villi, receives 1-3 branches from foetal umbilical vessels + lies under the surface of the trophoblast
Ø Caruncle: maternal arteries supply each caruncle + split into capillaries b/w the termini villi of foetal cotyledon
§ Organisation of foetal + maternal vessels means that in capillary beds, blood can flow in opposite directions = maximises opportunities for metabolic exchange
Human Placentation
- Haemochorial: Villi and outer surface of chorionic plate bathed in maternal blood
- Fetus connected to placenta via umbilical cord
- Placenta invasive to uterine tissue
- Hematopoietic tissue
Haemochorial, discoid and invasive - human
-Stem villi connect the chorionic plate to the basal plate forming a labyrinthine series of maternal blood spaces.
-The maternal spiral artery at the decidual base of the placenta ejects its blood into these maternal blood spaces
-In the mature human placenta, tongues or villi of chorionic syncytiotrophoblast, containing cores of mesodermal tissue in which fetal blood vessels run, penetrate deeply into the maternal tissue to form an extensive network.
-Only a thin layer of chorionic syncytiotrophoblast separates the fetal blood vessels from maternal blood.
-At the tips of the terminal villi, the capillaries are dilated and form tortuous loops. Thus, fetal blood flow through the tips will be slow, allowing for direct exchange of metabolites with maternal blood.
Placental Vasculature – Maternal Spiral Arteries
In normal placental development, extravillous trophoblast cells invade the maternal spiral arteries.
During this process, the cytotrophoblast cells differentiate from an epithelial phenotype to an endothelial phenotype.
Ø Unremodelled decidual spiral artery: thick layer of vascular SMCs = pulsatile
Ø Remodelled decidual spiral artery: outer vascular smooth muscle layer removed by uNKs, T cells + macrophages; endothelial cells of spiral arteries completely replaced by trophoblast cells
§ Slows rate of flow through vessel = villous not subjected to blunt force injury
Placenta STRUCTURAL Function
§ Large surface area: microvilli covered by syncytiotrophoblast, continuous layer and multinucleus cell, bathed in maternal blood
§ Epithelial plates: 5-10% of placental SA, extremely thin barrier b/w foetal + maternal circulation for diffusion
Placenta transport fuction
electively permeable placental barrier, nutrient transfer (sugars, amino acids, lipids), gas exchange (O2, CO2), waste removal (urea)
