What are the four main foetal membranes?
2. Yolk Sac
From what embryonic cell layer is the chorionic membrane derived from?
Trophoblast & mesoderm. The two layers forming the chorion completely surround the entire conceptus.
The same two layers also form the membrane of the amniotic cavity.
What is the function of the chorion?
Fuses with maternal endometrium to form placenta.
Which embryonic (blastocystic) cells form the yolk sac?
Hypoblast cells next to the blastocoele.
From which blastocyst/embryonic cell layer is amnion formed?
It is formed from epiblast cells; specifically, the mesoderm layer.
Mesoderm develops between embryo & primitive ectoderm → surrounds yolk sac → amniotic wings fuse to form amniotic cavity
What is the function of the amnion?
It forms a protective sac around the embryo.
Fusion of amniotic wings forms double sac around embryo, chorion & amnion: internal-sac membrane made of trophectoderm & mesoderm
What is the allantois & how is it formed? What is its function?
The allantois is a diverticulum from the primitive gut. It fuses with the chorion, forming allantochorion.
Collects embryonic waste.
The chorion fuses with the maternal endometrium to form the placenta. What is the type of placenta formed when the yolk sac does not regress but rather fuses with the chorion and the maternal endometrium?
Choriovitelline placenta. It is usually transitory.
What is the name of the type of placenta formed by most domestic animals when the chorionic membranes (trophoblast & mesoderm) fuse with the maternal endometrium?
Chorioallantoic placenta. This includes the outermost-layer of trophoblast cells and mesoderm cells (forming the chorion) plus the allantoic membrane.
What are the only mammals to produce yolky eggs with shells?
Monotremes - eg., duck-billed platypus
Placentas can be classified by the distribution of the chorionic villi in the maternal endometrium. What are the different classifications?
What is a diffuse placenta?
Diffuse means that the embryo's chorionic villi are distributed all over the chorion to fuse with the endometrium of the mother.
What are examples of two species that have diffuse placentas? How do they differ?
Sow: chorionic villi form villous areolae, spread all over the chorion to penetrate the endometrium to form the foetal-maternal surface. Vessels from EACH villus merge to form large vessels that enter the umbilical cord.
Mare: instead of villi, the horse foetus has microcotyledons all over its chorionic membrane that penetrate the endometrium.
What is a zonary placenta?
Externally, it looks like a band of foetal & maternal tissue around the middle of the conceptus. It consists of three zones:
1. Transfer Zone - site of foetal-maternal exchange
2. Pigmented Zone - maternal necrotic tissue & haemorrhage
3. Allantochorion - contains waste
Dogs & cats have zonary placentas.
What is a discoid placenta?
Round patch of chorionic tissue filled with vasculature (arteries and veins) immersed in pools of blood. The nutrients and waste are exchanged between mother and baby in the pools of blood into the vessels, which merge to form the umbilical cord of the foetus.
Primates & rodents have discoid placentas.
What is a cotyledonary placenta? What species have cotyledonary placentas?
Round "buttons" formed by the fusion of chorionic tissue and maternal caruncles. The maternal caruncles rise into the chorionic tissue, which surrounds the caruncles - these form placentomes. These cotyledons are the sites of foetal-maternal exchange.
Ruminants have cotyledonary placentas.
Cows & giraffes have "hamburger" type placentomes, where the maternal caruncles cause the chorionic membranes to bulge outward - convex.
Sheep & goats have "doughnut" type placentomes, where the maternal caruncles are concave within the chorionic tissue.
What are the four types of placenta classified by the proximity of the foetal & maternal blood supplies, from "most distant" to most intimate?
This classification takes into account the number of maternal and foetal tissue layers between the foetal blood supply and the maternal blood supply.
1. Epitheliochorial - 6 layers
2. Endotheliochorial - 4 layers
3. Haemochorial - 3 layers
4. Haemoendothelial - no layers between foetal & maternal endothelium
Which species have the epitheliochorial placentas?
Most livestock animals:
Pigs, Horses, Ruminants
Which animals have the endotheliochorial placentas?
Dogs & cats.
Which speces have haemochorial placentas?
Primates, humans & rodents
What does ovaviviparous mean?
Eggs with shells are produced but they are retained in the uterus and the offspring "hatch" out while inside the womb. Many snakes and lizards are ovaviviparous.
What type of placentas can reptiles have?
Choriovitelline (fusion of yolk sac & chorion with endometrium - no mesoderm) - this is transitory usually
Chorioallantoic - normal
How is the placenta classified by "types of implantation"?
This classifies the depth of the embryo's implantation in the maternal endometrium.
1. Central & superficial - ruminants, ungulates
2. Eccentric - carnivores & rodents
3. Interstitial & invasive - humans & primates
What is decidua in terms of placental implantation?
In species with invasive interstitial placentation (humans, primates), the uterine endometrial stroma may undergo a decidual reaction during, or even before implantation.
The decidua is a thickened region of endometrium –
the stromal cells show altered secretory activity, and
increased vascular growth and permeability.
Very marked in humans - driven by steroids & relaxin
What are the different types of chorionic interdigitation in the placenta?
Folded - pigs
Lamellar - carnivores
Villous - equids, ruminants, humans, rodents
What is the chorionic girdle?
This describes a band in the horse placenta. By about Day 35, the trophoblast cells invade the maternal endometrium to form endometrial cups. These produce gonadotrophins (eCG) transiently.
These produce secondary ovulations and secondary CLs that secrete P4 until Day 100.