Exosomes, a type of extracellular vesicles (EVs), were first discovered in sheep reticulocytes in 1983, and after four years, the name ‘exosome’ was officially given to them. A variety of cells secrete exosomes under both normal and pathological conditions. Exosomes are mainly products of the multivesicular body formed by intracellular lysosomal invagination, and released into the extracellular space by fusion of the outer membrane of the multivesicular body with the cell membrane. Exosomes are present in body fluids including blood, saliva, urine, cerebrospinal fluid, and breast milk. Since their discovery, they have been found to play crucial roles in many physiological and pathological processes, and the last 10 years have witnessed the explosion of studies on exosomes and other EVs. They carry large amounts of materials, including proteins, lipids, and miRNA/mRNA/DNA, and are thus involved in intercellular communication[3-5]. For example, tumor-derived exosomes promote tumor growth by inhibiting anti-tumor immune responses and stimulating tumor proliferation and metastasis. Disease-associated EVs play a role in disease pathogenesis and exert a wide range of effects on the target cell, therefore, EVs can be regarded as biomarkers for early diseases, and they may also be used as drug carriers for targeted therapy.
The male reproductive system consists of testes, ducts, accessory glands, and external genital organs: the penis and scrotum. The role of the testes is to produce sperm and hormones; the epididymal ducts transport, store, and assist sperm maturation. Accessory glands secrete most of the liquid part of the semen, and the penis containing the urethra serves for ejaculation. The glandular tissue of single testis contains 200–300 lobules, with up to three seminiferous tubules for each lobule. In the interstitial space, specialized Leydig cells secrete androgens. Male sexual dysfunction is mainly divided into erectile dysfunction (ED), and hypogonadism. In addition, infertility may also occur even when the sexual function is normal, sometimes due to sperm abnormalities such as azoospermia and oligozoospermia. The pathogenesis of many male infertility-related diseases has been uncovered recently.
The female reproductive system consists of the internal and external reproductive organs and related tissues. Female genitalia include the vagina, uterus, fallopian tubes, and ovaries. Reproduction requires a complex sequence of comprehensive events in the female body, including ovulation, sperm transport, sperm capacitation, fertilization, embryo implantation, fetal support, and childbirth[10, 11].
Recently, it was found that EVs are very important for tumor development, the cardiovascular system, and the immune system[1, 12]. In addition, there are many studies about the effects of EVs on reproductive processes such as gametogenesis, hormone secretion, and embryo development. This review summarizes the effects of EVs derived from various tissues or cells on male and female reproduction systems.
After the summary of EVs in the male reproductive system, their significance in female genitalia also deserves to be mentioned. As described previously, the reproductive process that occurs in females is not limited to oogenesis but also includes transport and capacitation of sperm, fertilization, cleavage, embryonic development, and childbirth. In addition to these processes, many other factors are related to female reproduction, such as hormone levels (for example, estrogen and progestin), sexual behaviors, and libido. Any defects of these processes may influence the outcome of pregnancy. There are many studies regarding the role of EVs in female reproductive processes, and most focus on the influence of embryo-derived EVs on the mother and/or placenta. The relationship between reproduction and EVs from different maternal sources needs to be elucidated.
The main function of the ovary as the female gonad is to produce ova and secrete sex hormones, which promote and maintain the development of female sexual characteristics. The microenvironment of the ovary is maintained by follicular fluid, and miRNAs carried by EVs in the follicular fluid are associated with follicular growth and oocyte maturation, proliferation of granulosa cells, cumulus expansion, meiosis and mitosis of the early embryo[65-67]. Female patients who received intracytoplasmic sperm injection demonstrated increased CD63 and CD81-positive exosomes in their follicular fluid and upregulated miRNA such as miR-29a, miR-99a, miR-100, miR-132, miR-212, miR-214, miR-218, miR-508-3p, and miR-654-3p. Most of these upregulated exosomal miRNAs participate in WNT, MAPK, ERBB, and TGF-β signaling pathways, and it is speculated that these miRNAs play an important role in growth, maturation, and meiosis of oocytes and may be related to the development of fertilized eggs. It is notable that the characteristics of miRNAs carried by EVs in the follicular fluid vary with the age of women, suggesting that miRNAs carried by EVs can be used as biomarkers of age-related decline of oocyte quality. Subsequently, da Silveira et al. found that EVs in the ovaries are associated with age-related fertility decline in horses. They found that the presence of mir-23a carried by exosomes in the ovaries is involved in apoptosis of granulosa cells. The mir-23a can cause changes to TGF-β signaling in granulosa cells, which in turn affects the development of the follicle and ovulation. In addition, exosomes derived from ovaries/oocytes express Uroplakin (UP) protein, which can affect fertilization.
The fallopian tube connects the ovary and the uterus. Al-Dossary et al. identified EVs derived from the fallopian tube in 2013 and named these oviductal bodies. CD9+ exosomes were also detected in the fluid of the oviduct, and plasma membrane Ca2+-ATPase 4a (PMCA4a) was identified in it. PMCA4a plays central role in the Ca2+ efflux pump in the sperm of mice, and this is essential for capacitation. In addition, experiments in turtles suggest that EVs secreted by fallopian tube epithelial cells and glands can help the fusion of sperm with the egg. Although there are only a few known results of mammalian experiments, the roles of EVs derived from mammalian fallopian tubes are mainly clear. It has been reported that EVs secreted by the oviduct can prolong the survival time of embryos and improve embryo quality. Studies have proven that the EVs of the fallopian tube fluid can significantly increase the birth rate by inhibiting apoptosis and promoting differentiation, which may be considerably beneficial to the development of assisted reproductive technology. By analyzing the contents of EVs secreted by the oviduct, it has been identified that miRNA miR-34c and miR-449a are associated with defects of cilia in fallopian tubes and infertility. Proteins contained in fallopian tube-derived EVs are related to maternal estrus. In addition, some RNAs in these EVs are associated with the expression of cilia, development of the embryo, and transcripts encoding ribosomal proteins.
There are EVs in the microenvironment of the uterus. Researchers have discovered EVs with strongly positive CD9 and CD63 signals from endometrial epithelial cell cultures (ECC-1). Specific miRNAs carried by these include hsa-miR-200c, hsa-miR-17, and hsa-miR-106a, which are closely related to the implantation of the embryo. Proteins or miRNAs transported by EVs may be very valuable biomarkers for endometrial diseases in humans. EVs secreted by uterine epithelial cells into the uterine cavity are important for embryonic development, as confirmed by removal of uterine glands resulting in embryos often failing to develop or aborting. Research from the group of Javier showed that EVs released from human endometrial MSCs can partially recover the youthful abilities of aged oocytes and improve the total blastomere count.
Besides the reproductive system, many studies have isolated EVs directly from the circulatory system of the mother. EVs secreted from leukocytes, erythrocytes, and platelets can affect the physiological status of women during pregnancy through different mechanisms. Pregnant women with eclampsia have more EVs in their peripheral blood. Platelet-derived EVs may cause eclampsia, and they might be associated with blood coagulation. The beneficial effects of aspirin on preeclampsia-related risk factors may be at least partially explained by the influence of aspirin on the activity of platelets. Other EVs secreted by endothelial cells are also increased in patients with eclampsia. In addition, EVs secreted by maternal intestinal flora may cause the rupture of embryolemma, thus leading to premature labor and other consequences (Table 1).
Sources of EVs Carrier/correlation factor of EVs Functions Literatures Leukocytes Inflammatory cytokines (IL-1, IL-8)
and nuclear factor (NF-Kβ) and tissue factor
In women with preeclampsia, the concentration of EVs derived from leukocyte is elevated, which can help remove and regulate syncytiotrophoblast-derived vesicles and placental fragments released into the maternal circulation, and initiate inflammatory coagulation through its cargo. [81-84] Platelets P selectin, tissue factor, etc. Platelet-derived EVs in women with preeclampsia may be involved in increased thrombin generation and fibrin clot formation [85,86] Erythrocytes Increased concentration of EVs from erythrocytes in preeclampsia may be due to erythrocytes rupture and hemolysis, which may be related to extensive thrombosis.  Endothelial cells Soluble fms-like tyrosine kinase 1 (sFlt1), soluble endoglin (sEnd) and placental growth factor (PlGF),
certain procoagulant molecules
In patients with preeclampsia, circulating EVs from endothelium are increased, and sFlt1 competitively binds to PlGF and vascular endothelial growth factor, inhibiting endothelial protection. sEnd is an anti-angiogenic protein that prevents capillary formation and increases vascular permeability. PAI-1 in EVs helps elucidate the link between endothelial dysfunction and extensive coagulation [87,88] Plasma The signal to reduce prostaglandins Reducing the prostaglandin 2α (pgf2α) in the uterus, which is associated with uterine infection.  Plasma The miR-133, miR-30, miR-99,
miR-23, and so on
In diabetic pregnancy, these exosomes can be engulfed by embryonic cells through the placental barrier, affecting heart development, possibly via exosomal miRNAs.  Intestinal flora The EVs secreted by the intestinal flora of pregnant women are associated with premature rupture of membranes and neonatal infection. 
Table 1. Effects of EVs from pregnant mothers
After the fusion of sperm and egg, the embryonic development process occurs in the uterus. This process includes the development of the fertilized egg to a blastocyst, trophoblast cells coming in contact with the endometrium, and blastocyst implantation in the endometrium. Then, the inner cell mass (ICM) proliferates and differentiates into the ectoderm and endoderm, forming the amniotic cavity and yolk sac. Meanwhile, the blastoderm appears and differentiates into inner, outer, and middle germ layers gradually, finally forming the embryo. During these processes, the embryo also produces some substances such as protein, tissue factors and immunosuppressive factors to affect development. The placenta amnion can secrete EVs with abundant content to regulate embryonic development. The placental trophoblast can secrete many EVs as well, thus influencing the reconstruction of intrauterine spiral arteries and invasion of cytotrophoblast cells. Additionally, these EVs may be associated with some diseases in pregnant women, such as eclampsia. Moreover, EVs derived from trophoblasts also affect the immune system, preventing fetal rejection, and protecting the placenta from viral infections (Table 2).
Sources Of EVs Carrier material Functions Literatures Amnion mir-21 EVs loading mir-21 are helpful for the growth of the embryo.  Amnion Molecules involved in blood coagulation EVs express phosphatidylserine and tissue factor, they also can significantly shorten the plasma coagulation time and increase the production of factor Xa and thrombin. [93,94] Amnion Histone (H) 3, heat shock protein (HSP) 70, activated form of pro-senescence
Protein involved in stress response. The activated form of pro-senescence and term parturition associated marker p38 mitogen activated protein kinase (MAPK) (P-p38 MAPK) co-localized with exosome, so these exosomes are related with the outcome of pregnancy.  Trophoblast Synctin-1 Induce peripheral blood mononuclear cell (PBMC) activation by the production of cytokines and chemokines, so these EVs modulate immune cell activation and the responses of immune cells to subsequent lipopolysaccharide stimulation.  Trophoblast mir-141 and C19MC Promote invasion and proliferation of extravillous trophoblast cells (EVTs). Trophoblasts secrete EVs containing mir-141 and C19MC, which are associated with invasion. [97-99] Trophoblast EVs released from syncytiotrophoblast stimulated the production of inflammatory cytokines TNF-alpha, IL12p70, and IL-18, these EVs are potential contributors to altered systemic inflammatory responsiveness in pregnancy.  Trophoblast Tissue factor and soluble vascular endothelial growth factor receptor
Tissue factors activate the coagulation system; and sflt-1 have anti-angiogenic effect.  Trophoblast Immunosuppressive factors Prevent fetal rejection, inhibit maternal immune reactions, and activate T lymphocyte and natural killer cells to protect the fetus.  Trophoblast miRNA - C19MC Protect the placenta from viral infections and transfer antiviral infections to non-placental cells. [13,103,104] Embryo PIBF Inducing the increase of IL-10, and IL-10 contributes to the Th2 dominant immune responses during pregnancy. 
Table 2. Effect of embryo-derived EVs on reproduction