Transcript lecture 8 File

VAN 504 Lecture 09
Structure and types of mammalian
placenta
Placenta (plah-sen’tah) is a Latin word.
It means “a flat cake
• The placenta is an organ characteristic of
mammals that connects the developing fetus to
the uterine wall to allow nutrient uptake, waste
elimination, and gas exchange via the mother's
blood supply.
• "True" placentas are a defining characteristic
of eutherian or "placental" mammals
Placental Development
• The morphogenesis of the placenta during early
gestation is closely related to those extra embryonic or
fetal membranes that are differentiated into the yolk
sac, amnion, allantois, and chorion. The fetal
membranes participate in the formation of the
placenta, either separate or in certain combinations,
and give rise to three basic types of placentation which
differ in regard to the identity of the fetal membranes
involved: chorionic, chorioallantoic & yolk sac
placentation.
• Among these types the chorioallantoic placentation
derived from the fusion of the allantois with the
chorion is characteristic of the farm animals.
Placenta
PLACENTA:
•
•
is a fetomaternal organ.
It has two components:
– Fetal part – develops from the chorionic sac
– Maternal part – derived from the
endometrium
PLACENTAL SHAPE: defines the proportion of surface area
shared between fetal membranes and maternal uterine
tissue where exchange occurs
PLACENTAL TYPE: defines the structure of cell layers
separating fetal blood from maternal blood
•
The placenta and the umbilical cord are a
transport system for substances between the
mother and the fetus.
Functions of placenta
• A site of exchange of gases and metabolites
between the maternal and fetal circulation.
• A major endocrine organ producing steroid and
protein hormones.
• Protection.
• Nutrition.
• Respiration.
• Excretion.
• Hormone production,
(progesterone,estrogen,Gonadotrophins
Placental Shapes
• Determined by the
distribution of villi over
the chorionic surface.
• Diffuse – noninvasive
• Cotyledonary –
placentomes are the
point of high
throughput
maternal/fetal contact
• Zonary/Discoid –
invasive; most direct
contact between fetal
and maternal blood
Diffuse Placenta
Uterine
Endometrium
Fetal
Chorion
Cotyledonary Placenta
Cotyledonary Placenta
Umbilicus
Fetus
Placentome
Placentation by Species
SPECIES
SHAPE
Cow
Cotyledonary
Ewe,goat
Cotyledonary
Sow
Diffuse
Mare
Diffuse
Dog, Cat
Zonary
Primates, Human Discoid
Rodents
Discoid
TYPE
Synepitheliochorial
Synepitheliochorial
Epitheliochorial
Epitheliochorial
Endotheliochorial
Hemochorial
Hemochorial
Hormones Produced by the Placenta
• Equine Chorionic Gonadotropin: maintains primary CL, responsible
for formation and maintenance of accessory CL.
• Human Chorionic Gonadotropin: maintain CL
• Progesterone: in some species (ewe, mare, woman) the placenta
takes over progesterone production later in gestation.
”progesterone block” – inhibits myometrial contractions
• Estrogen: peak of E2 signals preparturient period in some species
• Placental Lactogen: stimulates growth of fetus and mammary
glands
• Relaxin: softens connective tissue in the cervix and relaxes pelvic
ligaments
Fetal Growth
Growth: period of development from embryo to fully developed
fetus, prior to parturition
Example of development relative to time for the bovine
Calcification of Bone Matrix
Extensive Bone Formation
Tooth Formation
Hair, eyes, muzzle
Hair over entire body
70 days
180 days
110 days
150 days
230 days
Types of Placentas
• Maternal Blood
• Maternal endothelium
• Maternal connective tissue
• Maternal epithelium
• Fetal epithelium
Type = degree of
invasiveness, based on
layers separating
maternal blood from the
fetal epithelium
Least Invasive
epitheliochorial
• Fetal connective tissue
synepitheliochorial
• Fetal endothelium
endotheliochorial
• Fetal Blood
Most Invasive
hemochorial
Histological classification of placentas is based on the degree
of removal of the maternal layers
KEY CHARACTERISTICS OF MAMMALIAN PLACENTA
Foetal capillary (from umbilical artery)
Endothelial layer
Connective tissue layer
(may be minimal)
Cellular layer
(may be trophoectoderm + maternal
epithelium or a syncytium of the two,
or solely trophoectoderm)
Connective tissue layer
(may be minimal)
Maternal capillary
(in haemochorial placenta of primates
the endothelium is degraded)
EPITHELIOCHORIAL
Maternal endometrial epithelium
intact (horse, pig), all layers
preserved.
SYNEPITHELIOCHORIAL
Syncytium of maternal epithelium
and Chorion (ruminants), all layers
preserved.
ENDOTHELIOCHORIAL
Removal of maternal endometrial
epithelium And connective tissue
(dogs, cats)
HAEMOCHORIAL
Removal of maternal endothelium
(human, some rodents)
• The primitive type of placenta is a condition where the
endothelial walls, blood vessels, and mucosa of both the
mother and the fetus remain intact. This is known as an
EPITHELIOCHORIAL PLACENTA.
• SYNDESMOCHORIAL - when maternal epithelium is broken
down (e.g. cows).
• ENDOTHELIOCHORIAL - when maternal epithelium and mucosa
are both broken down (e.g. dogs).
• HEMOCHORIAL - when all of the maternal tissues are broken
down to the point that the fetal epithelium is literally bathed
directly by the maternal blood cells (e.g. humans, other
primates, rodents)
• (Essentially, all this has to do with the degree of destruction of
the maternal part of the placenta.)
• The classification of the placenta by microscopic structure
is based upon the maternal and fetal tissues that are
actually in contact.
• The basic structure comprises on the maternal side blood
vessel, connective tissue and epithelium and on the fetal
side chorionic epithelium, connective tissue and blood
vessel.
• While all layers are preserved in the epitheliochorial (horse
and pig), and synepitheliochorial placentas (ruminants)
• some layers on the maternal side are lost as in
endotheliochorial (cat) and hemochorial (human).
• In ruminants fetal chorionic binucleate cells migrate to form
a syncytum at the junction of the maternal and fetal tissue.
Classification based on histological structure
1. Epitheliochorial type: this type is the most superficial placenta and
lacks significant invasion of the uterine lining.
Pockets of columnar trophoblasts are loosely applied to the
maternal endometrial epithelium. No destruction or invasion of the
maternal tissues occurs and no layers are removed.
• The foetal chorion Is In contact with eplthelium of the uterus hence
it is called epithello chorial placenta. In between foetal, maternal
parts six layers are present.
I. Endothelium of mother blood vessel.
ii. Maternal syndesmose connective tissue.
lii. Epitheliurn of mother
Iv. Chorion of foetus.
v. Foetus connective tissue (syndesmose
vi.Endothellum of foetal blood vessel
The epitheliochorial type is found in horses, pigs and ruminants.
Contd
2.Syndeumose chorial placenta
The chorion will come in contact with
syndesmose of mother’s uterus. Hence it’s
called syndesmose chorial.
• Ex: Sheep, Cow.
3. Endotheliochorial type: the maternal uterine
epithelium and connective tissue disappear after
implantation, and the trophoblasts come into
direct contact with the maternal endometrial.
• The endotheliochorial type occurs in orders from
all four major clades of eutherian mammals
(Euarchontoglires, Laurasiatheria, Xenarthra and
Afrotheria), including carnivores.
4. Hemochorial type: this type is the most invasive
placenta.
• All maternal tissue layers disappear through erosion,
leading to direct connection between the chorion and
maternal blood.
• There are hemomonochorial (primates), hemodichorial
(rabbits), and hemotrichorial (rats and mice) placentas,
with one, two and three trophoblast layers,
respectively
The haemochorial placenta shows the intimate juxtaposition
of foetal and maternal blood allowing efficient exchange
A
The haemochorial placenta
Foetal capillaries
B
Umbilical vein
Umbilical arteries
Chorio
nic villi
Maternal
Maternal
blood
arteriole
pool
Maternal
venule
Notice the expansions at
the’turn around’ to allow
slower blood flow and better
equilibration with maternal
blood
5. Hemo endothelial placenta
float In mother’s blood. Hence it called hemo
endothelial placenta Hence
Rat, Rabbit,
Histological Classification
Tissues
epithelialchorial
syndesmochorial
endothelialchorial
hemohemochorial endothelial
Maternal
endothelium
conn. tissue
epithelium
+
+
+
+
+
-
+
-
-
-
Fetal
epithelium
conn. tissue
endothelium
+
+
+
+
+
+
+
+
+
+
+
+
+
pig
horse
ruminant
*ruminant
dog
cat
human
rat
rabbit
Species
Gross anatomical classification of placentas is based on the pattern
of contact between chorion and endometrium & Shape
•
•
•
DIFFUSE
Uniform distribution of chorionic
villi
over contact surface (horse, pigs)
COTYLEDONARY
Villi restricted to defined area
(cotyledons) (ruminants)
Diffuse Placenta
Horse
Pig
Microcotyledons
• ZONARY
• Girdle of chorionic villi around
middle of chorionic sac
(dogs,cats)
DISCOIDAL
Disc-shaped area on
chorionic sac
(humans, rodents)
• Microscopic Structure of the Placenta
• Dogs and cats have an endotheliochorial type of placenta.
• In this type of placenta, the endometrial epithelium under the
placenta does not survive implantation, and fetal chorionic
epithelial cells come to be in contact with maternal endothelial
cells.
• During implantation, cytotrophoblast cells surrounding the central
third of the chorioallantois proliferate to form a syncytium called
syncytiotrophoblast.
• The syncytiotrophoblast erodes through the endometrial
epithelium and flows around maternal capillaries.
• Initially, the invading fetal cells are in the form of villi, but villi soon
coalesce to form a labyrinthine-type of placenta.
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
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
• The placenta classification on nature of contact:
• There are two types: nondeciduate and
Deciduate type.
• NONDECIDUATE TYPE PLACENTA:
• The chorianic villi are simple projections, they lie
in contact with uterus. They have a loose contact.
There is no fusion. At the time of birth of embryo
uterus is not damaged. This type is characteristic
of farm animals.
DECIDUATE TYPE PLACENTA:
• The allantochorianic villi penetrate into
uterine valli.
• They are intimately fused. Hence maternal
epithelium is lost along with fetal membrane
at parturition. Bleeding occurs.
• It is restricted to the hemochorial placenta
(human).
Glucose is the dominant energy yielding substrate for the
foetus with little use of fatty acids
ENERGY SUBSTRATES
Glucose oxidation accounts from 50% oxygen use
Most of the rest is due to amino acid and lactate oxidation
Rather little is from fatty acid oxidation
(Notice that maternal energy metabolism is almost the mirror of this
with a shift to fatty acid oxidation and a shift away from amino acids
and glucose)
Shape or Distribution of Chorionic Villi
•
•
•
•
Cotelydonary - cow, sheep
Diffuse - pig, horse
Zonary - dog, cat
Discoid - human
Cotelydonary
Placenta
Cow
Ewe
Placentome
Placental Attachment in Ruminant
Binucleate
Cell
Chorion
Migrate and
fuse with
uterine
epithelium
Uterine
Epithelium
Syncytium
Capillary
Stroma
Fusion of
Binucleate
cells and
uterine
epithelium
(multinucleate)
Day 23 Migration of Bicnucleate Cells
and Formation of Syncytium
Binucleate
Cell
Endometrial
Stroma
Multinucleated Cells in
Uterine Epithelium
Cotelydonary
Placenta
Cow
Convex
Cotelydon (Chorion
ConcaveEwe
Caruncle
70 - 120
Endometrium
90 - 100
Binuclear Giant Cells
• 20% of fetal placenta
• Invade endometrium
• Source
– Placental lactogen
– Pregnancy specific protein B
Placental Lactogens (Protein
Hormones
• Prolactin-like activity:Stimulates Milk Synthesis
• Not Present in Pig and Mare
• May regulate maternal metabolism to facilitate fetal growth
• High levels in the last 1/3 of gestation.
• High levels facilitate higher milk production.
• Dairy cows have higher blood concentrations than beef cows
Microcotelydons
Microcotelydon
(Fetal)
Epithelium
Microcotelydon
(Maternal)
Endometrial
Glands
Uterine
Arteries
Increase placental
surface area
Microcote
lydon
E
n
d
o
m
e
t
ri
u
Uterine
Veins
m
Zonary Placenta
(bitch, queen)