Placenta is an important organ for material exchange between the fetus and the mother. It is a mother-child tissue-binding organ that grows from the embryonic germ membrane and the mother's endometrium during pregnancy. The fetus develops in the womb, relying on the placenta to obtain nutrition from the mother, while the two parties maintain considerable independence. The placenta also synthesizes a variety of hormones, enzymes and cytokines to maintain a normal pregnancy.
1. Substance exchange function
Gas exchange is oxygen exchange, carbon dioxide exchange; nutrient supply, supply all nutrients needed for fetal development; exclude metabolites in the fetus.
2. Defense function
Although the barrier effect of the placenta is extremely limited, it has a certain barrier function against some bacteria, pathogens and drugs.
3. Synthesis function
Such as human chorionic gonadotropin, human placental lactogen, estrogen, progesterone, tocolytic enzyme, thermostable alkaline phosphatase, cytokines and growth factors.
4. Immune function
The specific mechanism for making the mother able to accept and not reject the fetus is currently unknown.
5. Other functions
(1) Storage function. Such as early pregnancy, placenta grows quickly. A large number of nutrients (protein, glycogen, calcium, iron, etc.) are stored in placental cells for fetal growth needs.
(2) Metabolic regulation function. The placenta has a function equivalent to that of the liver. It not only stores nutrients, but also has a regulating effect. In the later stages of development, the fetal liver gradually grows and completes, and the placenta's metabolic regulating function gradually decreases and even disappears. The placenta can also transform and synthesize a number of substances, and perform a variety of functions of the digestive tract, lungs, kidneys, liver and endocrine glands, and can regulate these functions to protect the fetus and the mother and make the pregnancy go smoothly.
Development of the placenta and fetus is a continuous process that begins at the time of fertilization. The first three days of development occur within the fallopian tube. Four days after fertilization, the morula (a solid mass of blastomere cells) enters the uterus. On the fifth day after fertilization, the morula becomes a blastocyst as fluid accumulates and polarization of the cells occurs. The blastocyst has an outer layer of cells (trophoblast) that will form the placenta and fetal membranes, an inner cell mass at one pole that will form the embryo, and a fluid filled cavity. The inner and outer cell masses multiply and the fluid cavity enlarges until the expanded blastocyst hatches out of the zona shell. Initially it is bathed in uterine secretions that provide oxygen and metabolic substrates; however, these secretions soon become inadequate for support of further development. Therefore, within 24 hours of hatching (approximately day 6 after fertilization), the blastocyst implants in the uterine lining, which provides access to substrates (glycogen filled stromal cells) necessary for continued growth. Implantation involves movement of the blastocyst to an optimal location (typically the mid to upper anterior or posterior wall of the uterus), adhesion, and invasion. As the trophoblast erodes deeper into the decidua, vacuoles form and become confluent to form lacunae by day 13 after fertilization. The lacunar space eventually becomes the intervillous space.The progenitor cytotrophoblast cell is the stem cell of the placenta. These cells proliferate throughout gestation, differentiating along two pathways to form either villous cytotrophoblast, which ultimately can become syncytiotrophoblasts (outer cellular layer), or extravillous cytotrophoblasts (inner cellular layer, extravillous trophoblast [EVT]). Syncytiotrophoblast is a specialized epithelium that has several functions, including transport of gases, nutrients, and waste products and synthesis of peptide and steroid hormones that regulate placental, fetal, and maternal systems. EVT has a proliferative component and an invasive component. There is also a migratory EVT, which is neither invasive nor proliferative. These cells populate the cell islands, septum, chorionic plate, and chorion laeve. At four to five weeks of gestation (menstrual dates), EVT erupts in columns with proliferative trophoblast at the base and invasive trophoblast at the distal portion of the column. Invasive EVT that invades decidua is called interstitial EVT, whereas EVT that invades and remodels the spiral arteries is called endovascular EVT. Endovascular invasion (intramural or intra-arterial) involves replacement or displacement of vascular smooth muscle and endothelial cells and transforms the narrow spiral arteries into wide uteroplacental arteries (algorithm 1). Anastomoses between the dilated spiral arteries and endometrial veins form maternal sinusoids, which eventually distribute blood into the low resistance vascular network of the lacunar system, thus establishing the uteroplacental circulation.
There are four levels of placental maturity: 0, Ⅰ, Ⅱ, and Ⅲ. Level 1 signifies that the placenta is basically mature; Level 2 signifies that the placenta is mature; Level 3 signifies that the placenta is aging. Due to calcification and cellulose deposition, the ability of the placenta to deliver oxygen and nutrients is reduced, and the fetus is always at risk. In the second trimester (13-28 weeks), the placenta is grade 0; in the third trimester (30-32 weeks), the placenta is grade Ⅰ; after 36 weeks, the placenta is grade Ⅱ (more mature). If placenta Ⅲ is found before 37 weeks, premature placenta should be considered, and vigilance of intrauterine growth retardation may occur. The placenta at 38 weeks entered grade III, indicating that the placenta was mature.