Approximately 7% of men are infertile [13]. The causes of male-factor infertility are diverse and complex. Chromosomal and genetic diseases (e.g., Klinefelter syndrome, Y chromosome deletions, trisomy 21), exposure to environmental chemicals, radiation, cancer, and chemotherapeutic agents can lead to infertility [14] due to their adverse effects on sperm function and quality in the testis. Despite the development of various organoids over the past decade, testicular organoids have received increasing attention. Testicular organoids are similar to the testes in structure and function and serve as highly useful model systems to study male infertility and the mechanism of germ cell niche, germ cell functions, and the interaction between germ cells for spermatogenesis [15,16]. In addition, testis organoids can be used as high-throughput drug and toxicity screening tools that can replace animal experiments owing to ethical issues [17].

The fallopian tubes play an important role in the female reproductive system as sites for gamete and embryo transport, fertilization, sperm reservoir, and embryonic development [18]. Improper functioning of fallopian tubes can cause infertility in females. Fallopian tube organoids were derived from the female reproductive system by isolating cells using enzymatic digestion and culturing the cells by embedding them in Matrigel [8]. These organoids were able to maintain the structure and functionality of fallopian tubes, as evident from their ability to respond to female hormones (estrogen and progesterone), the presence of cilia and secretions, and folding of the epithelium [19].

The human endometrium is a complex multicellular and dynamic tissue that undergoes the menstrual cycle in response to female steroid hormones [20]. During pregnancy, the blastocyst first implants at the functional layer of the endometrium, which is the outermost layer of the uterus [21]. Thus, the endometrium is essential for female reproduction in mammals, and defects in its function or cyclic remodeling can cause implantation failure, pregnancy disorders, endometriosis, or endometrial cancers [22]. Endometriosis is a chronic inflammatory disease that causes pelvic pain and excessive bleeding due to the proliferation of endometrial tissue outside the uterus [23]. Endometrial or uterine cancer is the 4th most common gynecological malignancy in women in the United States [24]. Although many studies have investigated endometrial cancer, little is known about the cellular and molecular mechanisms behind its pathology. The main limitation of these studies was the lack of an accurate research model system, as mouse models do not physiologically resemble human endometrial development and function in vivo [9]. For instance, the process of endometrial decidualization varies between mice and humans [25]. Therefore, results obtained from animal models cannot be directly translated to humans. To address these obstacles, human endometrial organoids were derived from primary endometrial cells to mimic the human endometrium. Endometrial organoids have been shown to recapitulate the morphology and function of the adult human endometrium in response to estrogen and progesterone [26]. Endometrial organoids can be used not only to elucidate the mechanism of pathological diseases, such as endometriosis and endometrial cancer, but also to contribute to the development of therapeutic agents.

The human brain is divided into three regions—the forebrain, midbrain, and hindbrain—which are primarily composed of neurons and glial cells [27]. However, the function, development, and pathology of many disorders of the human brain are not yet fully understood, and it is difficult to study their mechanisms in vivo, as the human brain is the central system that regulates the body and cannot be cultured outside the body. Therefore, an in vitro model to understand human brain development and disorders is needed. Efforts have been made toward 2D culturing of brain tissues or organs by differentiating neural stem cells and PSCs into neurons; however, the structure and function of the brain could not be recapitulated [28]. Watanabe et al. [29] developed a 3D culture method to generate different brain regions from mouse or human PSCs. Brain organoids were then generated in vitro, as tiny organs that could recapitulate many features of the brain. In 2013, 3D cerebral organoids were generated in vitro that could grow up to a few millimeters from various brain tissues such as the retina, dorsal cortex, ventral forebrain, midbrain-hindbrain boundary, choroid plexus, and hippocampus [30]. In addition, organoids from specific brain regions, such as the midbrain and hippocampus, were established through the organoid culture system [31]. Brain organoids provide an excellent research platform for understanding the development of various disorders of the human brain, such as schizophrenia and Alzheimer disease.

In 2009, Sato et al. [6] first established intestinal organoids derived from leucine-rich repeat-containing G protein-coupled receptor 5 (Lgr5)+ stem cells using a Matrigel 3D culture system. These organoids form crypt-villus structures with cellular heterogeneity to mimic the physiology and organization of the actual intestine. When transplanted into mice, intestinal organoids showed long-term survival, and their regenerative ability was also confirmed [32]. Hindgut spheroids were cultured from human PSCs in Matrigel to promote the maturation of intestinal organoids. Although intestinal organoids are derived from PSCs, gastric and lingual organoids—derived from parts of the digestive tract—are generated from adult pyloric stem cells and adult tongue epithelium, respectively [33]. Organoid technology can be used as a powerful tool to study gastrointestinal diseases, intestine-microbe interactions, and colorectal cancer.

The liver is a critical organ that performs various functions such as detoxification, protein synthesis, and the production of bile, which is necessary for digestion [34]. To conduct studies on liver function and regeneration, liver organoids resembling the adult liver in various species were generated from induced PSCs, hepatoblasts, and adult tissue-derived cells [35,36]. Liver organoids can be used as in vitro culture models to replace animal models for studying drug toxicity, metabolism, personalized medicine, and regenerative medicine.