Placentation Histology

Ectoderm, mesoderm, endoderm

Three germ layers of embryo (from epiblast)

Functions of placenta

• Permits developing fetus to 1) acquire nutrients from another and 2) dispose of waste products via maternal vasculature

• Provides physical and biological barriers against environment and maternal immune system

• A source of hormones for maintenance of pregnancy (progesterone, gonadotropins)

Allantochorionic membrane

Fetal contribution to placenta

Yolk sac, amnion, chorion, allantois

Extra embryonic membranes

Amnion

Part of amniotic sac

Innermost membrane that covers and protects embryo

Allantois

Membrane that contributes to formation of umbilical cord

Hypoblast

Primitive endoderm

Trophoblast, hypoblast, extraembryonic mesoderm

Layers during early embryo development

Yolk sac

Forms from hypoblast (primitive endoderm) and extraembryonic mesoderm

Mesodermal layer: Contains blood vessels

Nutrient transport: Blood from yolk sac carries nutrients to embryo

Site of hematopoiesis

Fate: Drawn into abdominal cavity and digested

Choriovitelline

Yolk sac placenta

Mostly temporary

Yolk sac + chorion (trophoblasts)

Up to 21 days in dogs and cats

Up to 8 weeks in horses

Amnion

Formation: Folds of trophectoderm (inner) + extraembryonic mesoderm (outer)

Lift from the sides and ends of the embryo and fuse

• Outer layer becomes chorion

• Inner layer becomes ___

Avascular

Contains fluid

• Protects embryo

• Prevents adhesions

Ruminants, pigs, chickens

In which species does the connection between the chorion and amnion layers persist?

Chorion

Formation: Trophectoderm (outer) + extraembryonic mesoderm (inner)

Part of chorioallantoic placenta

Contacts endometrial blood vessels and begins exchange with maternal blood

Allantois

From hindgut

Forms the allantoic umbilical circulation

Part of chorioallantoic placenta

Collects urine as fetal kidney develops

Membrane ruptures in vagina at parturition in horses, dogs, and cats

Choriovitelline, chorioallantoic

Two types of placentas

Choriovitelline placenta

Yolk sac

Transient in most

Definite placenta in marsupials

Chorioallantoic placenta

Major type of placenta

Variation is species-dependent

Before

Does the yolk sac develop before or after the allantois?

When yolk sac wall combines with chorion and contacts the uterine wall

When does the choriovitelline placenta develop?

Yolk sac

Where are vessels derived from in the choriovitelline placenta?

Choriovitelline

Which type of placenta is transient in all domestic animal species?

Chorion, allantois

The chorioallantoic placenta is formed by fusion of which parts?

Allantochorion

Contacts endometrium in chorioallantoic placenta

Chorioallantoic placenta

Most common type of placenta in most mammals

Very well vascularized by the umbilical arteries and veins derived from the allantois

Caruncle (maternal) + cotyledon (fetal)

What makes up the placentome?

Chorion

Principal layer that contacts the endometrium

Chorion frondosum, chorion laeve

What is the chorion divided into?

Chorion frondosum

Rough chorion

Leaflike

Highly modified into folds and villi to increase surface area for exchange

Chorion laeve

Smooth chorion

Not modified for exchange with maternal endometrium

Deciduate formation and endometrium tissue lost at birth, shape of contact areas, number of maternal layers between maternal and fetal blood, chorioallantoic projections

What is the classification of chorioallantoic placenta based on?

Deciduate

Invasive

Uterine lining that forms the maternal part of placenta

Base of endometrium

Specialized decidual cells

• Large, hormone-producing (such as progesterone)

Endometrium shed at birth with hemorrhage likely

Carnivores, primates, and rodents (rabbits, rats)

Nondeciduate

Non-invasive

Minimal loss of endometrium

Low blood loss

Can peel away the placenta from endometrium

Hoofed animals

Diffuse, cotyledonary, zonary, discoid

Shapes of contact areas when classifying chorioallantoic placentas

Diffuse

Entire surface of uterus

Microplacentomes (horses)

Chorionic villi (pigs)

Cotyledonary

Localized tufts of frondosum

Uterine caruncles

Placentomes

Zonary

Frondosum band

Discoid

Flattened disc

True

True or false?:

Three fetal layers of the placenta are always intact.

Maternal layers

Which type of layer changes numbers between maternal or fetal blood when classifying different types of placenta?

(Does the number of maternal layers or fetal layers change in each type?)

3

Maternal layers in epitheliochordial placenta

2

Maternal layers in synepitheliochorial placenta

1

Maternal layers in endotheliochorial placenta

0

Maternal layers in hemochorial placenta

Endothelium of fetal (allantoic blood vessels), mesoderm CT (chorioallantoic), epithelium of chorion (trophoblast cells)

Three fetal layers

Epithelium of uterus (endometrial surface), CT of uterus (collagen, fibroblasts, decidual cells), endothelium of uterus (capillaries)

Three maternal layers

Epitheliochorial

All three maternal layers present

All six layers present

Horses, pigs, donkeys, whales

Synepitheliochorial

Trophoblast cells fuse with maternal epithelium

Fused cells form syncytial cells

Binucleate and multinucleate

Some endometrium can be eroded in some areas

Cows, goats, sheep, deer (most non-deciduate hoofed species)

Endotheliochorial

Uterine epithelium and CT are absent

Uterine capillaries contact chorioallantoic membrane and trophoblasts (cytotrophoblast + syncytiotrophoblasts)

Carnivores (dogs, cats), elephants

Hemochorial

Only three layers present

Uterine capillaries completely eroded

Maternal blood has direct contact with chorioallantoic trophoblasts (cytotrophoblast + syncytiotrophoblasts)

Humans (primates), rodents

Syncytiotrophoblasts

Multinucleated trophoblasts

Act as a defense barrier to prevent immune cell invasion

Folded, villous, lamellar, labyrinthine

Types of chorioallantoic projections

Folded

Macroscopic folds— rugae (wrinkles)

Microscopic pilca (fold)

Pigs

Villous

Chorionic villi into endometrial crypts

Horses, ruminants

Labyrinthine/lamellae

Complex network of channels

Envelops endothelium of maternal blood vessels

Forms lamellae folds of syncytiotrophoblasts

Carnivores (dogs, cats)

Pigs

Which species?:

• Nondeciduate: No loss of uterine tissue

• Diffuse

• Epitheliochorial: all 3 maternal layers

• Folded: macroscopic folds (rugae), microscopic (plica)

Horses

Which species?:

• Nondeciduate: No loss of uterine tissue

• Diffuse

• Epitheliochorial: all 3 maternal layers

• Villous (microcotyledons, microcaruncles, microplacentomes)

Microplacentomes

Fetal/uterine tissue interaction

Areolae spaces of glandular secretions

Endometrial cups

Chorionic epithelium breaks off at around day 30

Moves into endometrium

Large cells produce equine chorionic gonadotropin (eCG)

Stimulates CL-progesterone secretion

Human: syncytiotrophoblasts produce human chorionic gonadotropin (hCG)

Cow, ewe

Which species?:

• Nondeciduate to partially deciduate: no loss of uterine tissue

• Cotyledonary: focal areas

• Villous: placentome

• Synepitheliochorial: fusion of chorion and uterine epithelial cells

Convex placentome

Focal areas in cow

Concave placentome

Fusion of uterine caruncle and fetal cotyledon in ewe

Dogs, cats

Which species?:

• Deciduate: maternal tissue lost at birth

• Zonary: belt modified for exchange

• Endotheliochorial: fetal epithelium contacts maternal endothelium

• Labyrinthine

Trophoblast

Which layer is destined to become the placenta?

Inner cell mass

What becomes the embryo proper?

Allantoic calculi

Free-floating calcified bodies