BGDA Practical Placenta - Maternal Decidua

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Uterus

Human placenta classified as Haemochorial where the chorion comes in direct contact with maternal blood.

Menstrual Changes

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  Proliferative phase

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  Uterine gland proliferative phase

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  Secretory phase

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  Uterine gland secretory phase

• Endometrium - 3 layers in secretory phase of menstrual cycle: compact, spongy, basal
• Myometrium - muscular layer outside endometrium, contracts in parturition
• Perimetrium - tunica serosa of the uterus continuous with the peritoneal wall

Endometrial Layers

• Compact - implantation occurs in this layer, dense stromal cells, uterine gland necks, capillaries of spiral arteries.
• Spongy - swollen stromal cells, uterine gland bodies, spiral arteries.
• Basal - not lost during menstruation or childbirth, own blood supply.

Uterine glands

• still well-developed and highly active at 6 weeks (GA).
• gradually regress in length and epithelium height as the first trimester advances.

Decidual Reaction

• occurs initially at site of implantation and includes both cellular and matrix changes
• reaction spreads throughout entire uterus, not at cervix
• deposition of fibrinoid and glycogen and epithelial plaque formation (at anchoring villi)
• presence of decidual cells are indicative of pregnancy

Fibrinoid layer - (Nitabuch's layer) is thought to act to prevent excessively deep implantation.

Artery Dilatation - due to extravillous trophoblast cells invading uterine wall and maternal spiral arteries replacing both smooth muscle with fibrinoid material and part of vessel endothelium. There is also a proliferation of maternal blood vessels.

Cervix - at mouth of uterus, secretes mucus (CMP), forms a plug/barrier, mechanical and antibacterial.

Decidua

Historic drawing decidua parts

The endometrium becomes the decidua and forms 3 distinct anatomical regions (at approx 3 weeks)

• Decidua Basalis at implantation site
• Decidua Capsularis enclosing the conceptus
• Decidua Parietalis the remainder of uterus
  • Decidua Capsularis and Parietalis fuse eventually fuse and uterine cavity is lost by 12 weeks

Maternal Blood Vessels

Uterine and Placental Vasculature in Non-pregnant, Pregnant and immediate Post-partum State Diagrammatic representation of uterine and placental vasculature (red shading = arterial; blue shading = venous) in the non-pregnant, pregnant and immediate post-partum state. Normal pregnancy is characterized by the formation of large arterio-venous shunts that persist in the immediate post-partum period. Extravillous cytotrophoblast invasion in normal pregnancy (diamonds) extends beyond the decidua into the inner myometrium resulting in the formation of funnels at the discharging tips of the spiral arteries. Pregnancies complicated by severe preeclampsia are characterized by minimal arterio-venous shunts, and thus narrower uterine arteries. Contrast with severe preeclampsia.

Additional Information

Fibrinoid

Exist as 2 forms of extracellular matrix:

1. Fibrin-type fibrinoid is a maternal blood-clot product which replaces degenerative syncytiotrophoblast
2. Matrix-type fibrinoid is secreted by invasive extravillous trophoblast cells.

Fibrinoid layer (Nitabuch's layer) is thought to act to prevent excessively deep implantation.

Decidualization - process of endometrial stromal cells (fibroblast-like) change in morphology (polygonal cells) and protein expression and secretion (specific decidual proteins: prolactin, insulin-like growth factor binding protein-1, tissue factor, interleukin-15, and VEGF).

1. Estrogen and progesterone - receptive phase, luminal and glandular epithelial cells change in preparation for blastocyst adplantation.
2. Human Chorionic gonadotropin - luminal epithelium endoreplication leading to epithelial plaque formation.
3. Human Chorionic gonadotropin - trophoblast invasion and decidualization of human stromal fibroblasts.

Artery Dilatation - due to extravillous trophoblast cells invading uterine wall and maternal spiral arteries replacing both smooth muscle with fibrinoid material and part of vessel endothelium. There is also a proliferation of maternal blood vessels.

Other changes

• Endoreplication - rounds of nuclear DNA replication without intervening cell or nuclear division (mitosis).
• Cytokines - of maternal origin also act on placental development.
• Natural Killer (NK) cells - 30% of all the decidual cells towards the end of the first trimester of pregnancy. These lymphocytes are present in the maternal decidua in large numbers (70%, normal circulating blood lymphocytes 15%) close to the extravillous trophoblast cells. Have a cytolytic potential against virus-infected and tumor-transformed cells.