Placenta: Structure, Function and Neurotransmitters

• Placenta is derived from both fetal and maternal tissues
• Placenta can produce neurotransmitters like serotonin and dopamine
• Need to read Rosenfeld CS. Placental serotonin signaling, pregnancy outcomes, and regulation of fetal brain development, BOR 2019

Placenta definition

• Latin for "a cake"
• Temporary organ
• Composed of both fetal and maternal tissue

General functions

• Facilitates exchange between fetus and mother but prevents mixing of fetal and maternal blood
• Produces endocrine/neurotransmitters like progesterone and gonadotropins
• Gonadotropins include PMSG or eCG
• IFN-τ is specific to ruminants, vital for maternal recognition of pregnancy, and prevents luteolysis to maintain pregnancy
• Can synthesize neurotransmitters like serotonin (5-HT) and dopamine that can induce paracrine effects, and affect fetal brain development
• The placentas of various species produces many unique proteins, whose function is unknown but may be useful for pregnancy detection such as pregnancy associated glycoproteins (PAG)

Placentation

• Formation and attachment of the placenta via blastocyst
• Blastocyst nourished by secretions from uterine glands also known as uterine milk or histiotrophe

Extraembryonic Membranes Formation

• Yolk Sac
• Splanchnopleure is the layer
• Endoderm is the layer
• Splanchnic mesoderm has blood vessels in this layer
• The yolk sac is vascular
• Amnion and Chorion
• Somatopleure is the layer (chorioamniotic fold)
• Ectoderm is the layer
• Somatic mesoderm does not have blood vessels in this layer
• The amnion and chorion are avascular
• Allantois
• Splanchnopleure is the layer
• Endoderm is the layer
• Splanchnic mesoderm has blood vessels in this layer
• Chorion fuses with allantois
• Allantochorion is the chorioallantoic membrane
• Vascular due to allantoic component
• Contacts endometrium (maternal blood vessels) to form the placenta
• Permits exchange with maternal blood forming hemotrophe

Placentae Classification

• Two major types in domestic animals: yolk sac and chorioallantoic placenta
• Yolk Sac Placenta
• Also known as choriovitelline or omphaloid placenta
• Yolk sac fuses with chorion
• Transient in most species
• Chorioallantoic Placenta
• Allantoic placenta
• Major type
• Carried through most of gestations
• Species dependent structural differences

Placenta Classification Based on Loss of Endometrium

• Basal classification
• Nondeciduate: no loss of endometrium at birth
• Deciduate: loss of endometrium at birth
• Nondeciduate: minimal to no loss of endometrium at birth and little blood loss at birth
• Typical of ungulates and generally occurs in species with non-invasive placentae where the chorioallantois can be peeled away from the endometrium
• Deciduate: part of endometrium is shed at birth, which might cause hemorrhage during parturition; decidua is the endometrium that is shed at delivery
• Typical of carnivores

Gross Anatomy Contact Areas

• Chorion laeve is smooth and not modefied for exchange
• Chorion frondosum has branching roots which increases the area for gas, nutrient and waste exchange
• Three types in domestic mammals.
• Diffuse: chorion frondosum is found entrily
• Cotyledonary: has velcro like 1:1 fetal cotyledon linked to uterine caruncle
• Zonary: forms band around belt

Diffuse Placenta

• Has chorion frondosum over entire surface
• Present in sow, camel and mare.
• Cotyledonary Placenta = Multiplex placenta
• Has localized areas of chorion frondosum or cotyledons attach to caruncles of endometrium to form placentomes
• "Momma can drive a car but baby cannot" to recall
• Present in ruminants
• Zonary Placenta
• Chorion frondosum forms a band
• Present in carnivores

Numbers of Layers Separating Fetal and Maternal Blood

• Six total possible layers separating fetal and maternal blood:
• Uterine endothelium
• Uterine connective tissue (lamina propria)
• Uterine epithelial lining
• Epithelium of chorioallantoic membrane = trophoblast (fetal) = cytotrophoblast cells
• Connective tissue of chorioallantoic membrane (fetal)
• Fetal endothelium of allantoic blood vessels
• The number of maternal layers varies, but the number of fetal layers stays the same for all species
• Based on this classification scheme, there are four main types: Epitheliochorial, Synepitheliochorial/syndesmochorial, and Endotheliochorial, and Hemochorial
• Epitheliochorial: all three maternal layers are intact, so there are 6 layers separating maternal and fetal blood; present in mare, sow, and cow
• Synepitheliochorial/syndesmochorial: uterine epithelial lining is absent in some areas, and/or uterine cells have fused with fetal cells; the lamina propia of endometrium contacts the chorioallantois in some areas; in these areas only five layers separate maternal and fetal blood; present in ewe and doe partially
• Endotheliochorial: uterine epithelial lining and connective tissue are absent, so only four layers separate maternal and fetal blood; uterine capillaries contact chorioallantois, and it is present in carnivores
• Hemochorial: maternal blood contacts chorioallantois, so only three layers separate maternal and fetal blood; present in primates and rodents

Chorioallantoic Projections

• Three types:
• Folded
• Villous
• Labyrinthine
• Folded
• Macroscopic folds = plicae
• Microscopic folds = rugae
• Present in the sow Villous
• Chorionic villi fit into maternal crypts
• Present in the mare and ruminants
• Labyrinthine
• Chorioallantois forms a complex maze-like network
• Present in carnivores

Porcine Placenta

• Least invasive so this can be pulled away
• Gestation lasts 114 days, with attachment beginning 12-13 days after ovulation
• 10% rule holds true
• Non-deciduate - no loss of uterine tissue at birth
• Diffuse placentation
• Folded
• Epitheliochorial with 6 layers separating maternal and fetal blood
• Contains areolae: spaces with uterine glandular secretions and maternal trophoblast cells in this region are tall columnar with microvilli
• Fetal connective tissue looks like mesenchyme, is mucus-like, and stains pale

Equine placenta

• Gestation 340 days
• Attachment 35-40 days after ovulation
• 10% rule holds true
• Non-deciduate
• Diffuse
• Villous
• Epitheliochorial
• Microcotyledons = microplacentomes = microcaruncles when fetal and uterine tissues interact at this areolae region

Endometrial Cups

• Fetal epithelium breaks off from the chorion about day 30-35 of gestation, establishing colonies in the endometrium that grows over time
• The cells produce equine chorionic gonadotropin (eCG = PMSG) Acts like FSH - stimulating follicle development
• eCG stimulates ovarian follicular development, the follicles producing high concentrations of estrogen and forming secondary corpora lutea to generate a lot of progesterone to maintain the pregnancy
• Maternal immune system rejects these partially foreign fetal cells
• Endometrial cups become non-functional at day 120 of gestation

Hippomanes

• Allantoic calculi, white structures floating in allantoic fluid
• Fetal connective tissue is very dense, unlike other species

Ruminant Placenta

• Gestation length - cow=280 days, ewe and doe 150 days
• Attachment of chorioallantoic membrane, 28 - 32 days post-ovulation
• Attachment of chorioallantoic membrane, ewe and doe, 14-16 days post ovulation
• 10% rules holds true
• Non-deciduate partially deciduate, cotyledonary, villous, and epitheliochorial
• Cow epitheliochorial, a ewe and doe-synepitheliochorial.
• Key cells, binucleate, trinucleate, trophoblastic, and Cryptdial
• Fetal connective tissues cells stain lightly.

Species Differences in Ruminents

• Cow-placentomes convex, ewe and doe placentomes are concave dark, esp. in black-faced sheep, because of melanocytes

Carnivore Placenta

• Gestation- dog and cat- 63 days
• Cat attachment begins around day 11 to 12 post ovulation
• Bitch attachment around day 14-17 post ovulation
• attachment begins very late
• the 10% rule no longer applies,
• Terminate pregnancy with estrogen but not advised because of adverse effects.
• Deciduate - maternal tissue loss at birth.
• Zonary- belt modified for exchange.
• Labyrinthine.
• Endotheliochorial, fetal epithelium contacts maternal

Layer Of Carnivore Placenta

• The chorioallantois stains lightly
• Labyrinth =maze.
• Choronic Villi form form lamella,
• Cat is very regular and very layered.
• Dog has irregular lamellae that is hard to stain
• core.
• Cytothrophoblasts next to CT
• synctiotrophoblasts= syncytiotrophoblasts = one large multinicleted mass of cytoplasm
  • Between cytotrophoblasts and maternal capillaries. Form from fused cytotrophoblasts equal to syncytia.
• Erodes Erode uterine epithelium + lamina propria
• → endotheliochorial.
• Deciduate-located base with a uterus
  • prominent in cats - richromatic nuclei pale stain
    • From bone marrow.
  • From endometrial fibroblasts. Prevent rejection .
• may inhibit T cell response

Zones

• Spongy layer (zone)- enlarged uterine glands
  • Separation here during partition, which means loss during birth (deciduate)
  • Supraglandular part
    • Is more prominent in dogs
    • Glandular
• More prominent in dogs.
• Marginal hematomas/blackspots.
  • Outside where the attachement layer of placenta turns black to green.
• Provide iron for the fetus
• More noticable in dogs but can be present in cat

Explore the placenta's dual fetal and maternal origins, its role in nutrient exchange, and its endocrine functions. Learn about the hormones and neurotransmitters it produces, such as serotonin and dopamine, which influence fetal development. IFN-τ in ruminants prevents luteolysis ensuring pregnancy maintenance.