1. VAN 505 –Veterinary Anatomy-IV (Embryology and Teratology)
Week – Structure and types of mammalian placenta.
Books: Mc Geady, T.A., Quinn, P.J., FitzPatrick, E.S. and Ryan, M.T Veterinary Embryology, Blackwell Synergy, Chap- 2.

2. Tutorial Class
1. Describe placenta and what is its function? 2. List down the four types of placenta and describe each type. 3. Define: Cotyledon, Caruncle and Placentome. 4. What are the three classifications of placentation types found in animals Based on Layers Between Fetal and Maternal Blood? 5. There are how many placentomes present in sheep, goats and cows? 6. Implantation is the attachment of the placenta in the uterine wall, when does implantation occur in the Dogs and cats?

3. Assignment 3 Developmental stages of the avian embryo.
Illustration of the different developmental stages of chicken embryo from day one to day 21 of incubation.

4. The placentas of all eutherian (placental) mammals provide common structural and functional features, but there are striking differences among species in gross and microscopic structure of the placenta.
Two characteristics are particularly divergent and form bases for classification of placental types:
1. The gross shape of the placenta and the distribution of contact sites between fetal membranes and endometrium.
2. The number of layers of tissue between maternal and fetal vascular systems.
Differences in these two properties allow classification of placentas into several fundamental types.

5. The placenta (also known as afterbirth) is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply, fights against internal infection and produces hormones to support pregnancy.

6. Placentation 
refers to the formation, type and structure, or arrangement of placentas.
Placentation occurs inside the uterus.
The function of placentation is to transfer nutrients from maternal tissue to a growing embryo.

7. Placentation occurs after the implantation of the embryo into the uterine wall and involves the remodeling of blood vessels in order to supply the needed amount of blood.
The developing fetus is connected to it via an umbilical cord.
In humans, placentation takes place 7–8 days after fertilization.

8. Implantation in Ruminants
Implantation in ruminants is non-invasive and some authors prefer to use the term attachment.
There is close attachment between embryonic membranes and the endometrium overlying caruncles at 5 weeks in cattle and 3 weeks in sheep.
Shortly thereafter, the placenta is established.

9. Ruminants have a cotyledonary placenta.
= Instead of having a single large area of contact between maternal and fetal vascular systems, these animals have numerous smaller placentae.
The terminology used to describe ruminant placentation is:
Cotyledon: the fetal side of the placenta
Caruncle: the maternal side of the placenta
Placentome: a cotyledon and caruncle together

10. Caruncles
- are oval or round thickenings in the uterine mucosa resulting from proliferation of subepithelial connective tissue.
caruncles are readily visible in the non-pregnant uterus. They are the only site in the uterus to form attachments with fetal membranes.
Patches of chorioallantoic membrane become cotyledons by developing villi that extend into crypts in the caruncular epithelium.

11. The image below shows caruncles in an incised non-pregnant sheep uterus (left) and cross sections through placentomes from a midgestation sheep pregnancy (right). Bovine placentomes looks similar, but have a convex appearance rather than the concave shape seen in sheep.
Pregnant sheep, goats and cattle have between 75 and 125 placentomes. Deer also have a cotyledonary placenta, but only 4 to 6 placentomes which are correspondingly larger.

12. The image shows an incised uterus from a pregnant sheep, roughly 50 days of gestation. The numerous button-shaped structures are placentomes, and the surfaces in view are actually cotyledons - the fetal side of the placentome. The slightly milky- looking membrane covering and between placentomes is the chorioallantois. The fetus is clearly visible inside the amnion.

13. Implantation in Dogs and Cats
Dogs and cats are litter bearing species, and prior to fixation and implantation, the blastocysts become evenly spaced throughout the uterine horns.
In dogs, implantation occurs roughly 18 to 20 days after the preovulatory LH surge (about diestrus day 8 to 10).
In cats, implantation has been reported to occur 12 to 14 days after mating.
Implantation is centric and antimesometrial.

14. Gross Structure of the Placenta
The zonary placenta takes the form of a band that encircles the fetus. In dogs and cats, it is complete, while in species like ferrets and raccoons, it is incomplete (i.e. two half bands)

15. The images above show dissection of a near-term cat uterus removed surgically.

16. In 1604, Fabricius introduced a placental classification scheme based upon the macroscopic structure of the sites where attachment occurs between the embryo and the endometrium of the uterus.
He listed four main placental types. These are now referred to as the diffuse, cotyledonary, zonary, and discoidal placentas.

17. Classification Based on Placental Shape and Contact Points
Examination of placentae from different species reveals striking differences in their shape and the area of contact between fetal and maternal tissue:
1. Diffuse:
2. Cotyledonary:
3.Zonary:
4. Discoid

18. Classification Based on Placental Shape and Contact Points
1. In diffuse placentae, seen in horses, pigs, camels, lemurs, opossums, kangaroos, and whales, the chorionic sac meets the uterine endometrium over its entire surface.
The villi of the chorion are distributed evenly throughout the surface of the chorion, and they extend into processes in the uterine endometrium. (total coverage)
Almost the entire surface of the allantochorion is involved in formation of the placenta.

19. Classification Based on Placental Shape and Contact Points
2. Cotyledonary placentae,
 common to ungulates such as cows, deer, goat, and giraffe, have their villi clumped together into circular patches called cotyledons.
The fetal cotyledon meets with maternal regions called caruncles to form the placentome where maternal-fetal exchanges take place. ("buttons")

20. Classification Based on Placental Shape and Contact Points
2. Cotyledonary placentae,
Multiple, discrete areas of attachment (cotyledons) are formed by interaction of patches of allantochorion with endometrium.
The fetal portions of this type of placenta are called cotyledons, the maternal contact sites (caruncles), and the cotyledon-caruncle complex a placentome.

21. Classification Based on Placental Shape and Contact Points
3. The zonary placenta
is characteristic of carnivores, the chorionic villi have aggregated to form a broad band that circles about the center of the chorion.
The placenta takes the form of a complete or incomplete band of tissue surrounding the fetus.
Such zones may be complete circles (such as those in dogs and cats) or incomplete (such as those in bears and seals).

22. Classification Based on Placental Shape and Contact Points
3. The zonary placenta (circular or partially circular band)
It is thought that zonary placentae form from diffuse placentae in which the villi at the ends regress, leaving only those in the center to function.
At the edges of the zonary placenta is the hemophagous organ, which is green.
The color is due to the degradation of hemoglobin into bilivirdin. This green organ provides iron for the developing fetus.
Seen also in seals, bears, and elephants.

23. Classification Based on Placental Shape and Contact Points
4. The discoid placenta 
is seen in numerous groups -- humans, mice, insectivores, rabbits, rats, and monkeys.
In such placentae, part of the chorion remains smooth, while the other part interacts with the endometrium to form the placenta. The maternal blood cells are in direct contact with the fetal chorion. (disc shaped)

26. Classification Based on Layers Between Fetal and Maternal Blood
Just prior to formation of the placenta, there are a total of six layers of tissue separating maternal and fetal blood.
There are three layers of fetal extraembryonic membranes in the chorioallantoic placenta of all mammals, all of which are components of the mature placenta:
1. Endothelium lining allantoic capillaries
2. Connective tissue in the form of chorioallantoic mesoderm
3. Chorionic epithelium, the outermost layer of fetal membranes derived from trophoblast

27. Classification Based on Layers Between Fetal and Maternal Blood
There are also three layers on the maternal side, but the number of these layers which are retained - that is, not destroyed in the process of placentation - varies greatly among species.
The three potential maternal layers in a placenta are:
1. Endothelium lining endometrial blood vessels
2. Connective tissue of the endometrium
3. Endometrial epithelial cells

28. Classification Based on Layers Between Fetal and Maternal Blood
The placentation types found in animals are:
1. endotheliochorial placentation
In this type of placentation, the chorionic villi are in contact with the endothelium of maternal blood vessels. (e.g. in most carnivores like cats  and dogs)
2. epitheliochorial placentation
Chorionic villi, growing into the apertures of uterine glands ( epithelium). (e.g. in ruminants, horses, whales, lower primates)
3. hemochorial placentation (e.g. in higher order primates, including humans, and also in rabbits, guinea pigs, mice, and rats)