STRUCTURES AND FUNCTIONS OF THE MATURE PLACENTA, BEGINNING IN WEEK 9
- The human placenta is hemochorial; that is, it allows direct contact between maternal blood and fetal tissues. As we’ll see, chorionic villi provide the surfaces for exchange of materials, and, therefore, are the functional units of the placenta.
- Because fetal growth is dependent upon sufficient oxygen, nutrients, and other substances provided in the maternal blood, insufficient vascular remodeling has major clinical consequences, including fetal growth restriction, eclampsia, and even spontaneous abortion.
- Derived from both maternal and offspring tissues.
Week 9
Differential growth and regression of the chorionic villi initiates formation of the mature placenta.
Key Structures:
- Embryo is suspended within the amniotic cavity, which is bound by the amniotic membrane.
- Chorion plate:
- Chorion Frondosum grows thick with villi at the embryonic pole.
- Chorion Laeve is the thinner area, where the villi regress.
- Chorionic Cavity surrounds the amniotic cavity and embryo.
- Endometrium -> Decidua:
Its regions are named according to their location in reference to the embryo:- Decidua basalis, which comprises the decidua adjacent to the chorion frondosum.
- Decidua capsularis, which comprises the decidua encapsulating the embryo.
- Decidua parietalis, which comprises the decidua of the uterine wall opposite the embryo.
Week 20
Expansion of the embryo pushes these layers against each other, leading to fusion and the formation of the mature placenta; a mucous plug blocks the cervical opening.
Embryonic Pole:
- Placenta comprises fused chorion frondosum and decidua basalis.
Abembryonic Pole:
- Chorion laeve is greatly reduced, and fused with the amniotic membrane.
- Decidua capsularis has regressed (due to expansion of the embryo).
- Decidua parietalis fuses with the chorion laeve.
Placental anatomy
Growth of the fetus is driven by dynamic interactions between the mother and fetus. In healthy pregnancies, both fetal and maternal needs are met by physiological processes that operate for mutual benefit. However, when maternal health is poor, physiological compromises are made that put the mother and/or fetus at greater risk.
The placenta serves as an interface between maternal and fetal environments.
- Fetal Side
- Amniotic membrane gives the surface a smooth, shiny appearance.
- Umbilical cord extends from the placenta to the fetus; holds umbilical arteries and vein.
- Chorionic plate that gives rise to the chorionic villi (specifically, to the stem villi, which then give rise to branches). “Anchoring villi,” make contact with the decidua; “floating” villi do not.
- Fetal blood vessels travel through the umbilical cord and within the chorionic plate to reach the villi.
- Maternal Side
- Decidual, aka, basal plate forms septa, which demarcate cotyledons; cotyledons give the maternal surface of the placenta a bumpy appearance.
- Intervillous Space
-The space between the chorion and decidual plate.- Maternal vasculature passes through the decidua and opens to the intervillous space; spiral arteries, which have a cork-screw like appearance, provide the arterial blood supply to the intervillous space.
Recall that the intervillous spaces are derived from connections formed between the synctiotrophoblast and maternal sinusoids during early embryonic development.
- Transfer of materials occurs across the villous surface:
Materials leave the maternal blood, enter the intervillous space, cross the villous surface, and then enter the fetal vessels.
Wastes, etc. exit the fetal blood, cross the villous surface, enter the intervillous space, and enter the maternal veins.
Placental Physiologic Roles
The placenta is a temporary multi-functional organ; it performs many tasks that are taken over by various organ systems after birth.
Exchange of materials occurs in both directions across the placenta:
- From the maternal side, oxygen, nutrients, hormones, antibodies, and some drugs enter the fetal circulation.
- From the fetal side, carbon dioxide and other metabolic wastes, hormones, and red blood cell antigens enter the maternal circulation.
Another important role of the placenta is hormone production, including: - Human chorionic growth hormone, estrogen, and progesterone, all of which are necessary for maintenance of the decidua and, therefore, the pregnancy.
- Placental lactogen and growth hormone, which induce maternal insulin resistance later in pregnancy. By inducing insulin resistance, these hormones ensure that maternal nutrients are readily available for fetal growth.
Placental Blood Barrier
The placenta prohibits passage of many pathogens from the mother to the fetus.
- However, there are several important exceptions to this rule, which can be summed up with the TORCH rule:
Toxoplasmosis, Others, Rubella, Cytomegalovirus, and Herpes simplex can all traverse the placenta. - The Zika virus is an example of a TORCHeS exception: as you are probably aware, the Zika virus can be transmitted from an infected mother to a developing offspring, resulting in serious birth defects, including microcephaly. Current research is focused on preventing its transmission across the placenta, thereby avoiding fetal infection.
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