The skin is the outermost layer covering the human body. It has an important function as a barrier and an organ in charge of immunity and is indispensable for maintaining life. The epidermal cells actively proliferate to keep the skin surface fresh, while the dermis, composed mainly of collagen and elastic fibers, helps maintain homeostasis. Skin changes observed with the naked eye always have corresponding microscopic and electron microscopic changes. Epidermal cell–cell junctions are mediated by structures called desmosomes, and epidermal–dermal adhesions are maintained by structures called hemidesmosomes. Acquired abnormalities in these structures can cause blistering within the epidermis or between the epidermis and dermis. Melanin pigment is synthesized by melanocytes. In vitiligo vulgaris, which is an acquired depigmentation disorder, a decrease or loss of melanocytes is observed, whereas in oculocutaneous albinism, which is an inherited depigmentation disorder, the number of melanocytes is normal but mature melanosomes are lost due to the dysfunction of melanin synthesis enzymes. Thus, the appearance of the skin is a manifestation of inner changes, and visualization of its structure and function is essential for the advancement of medicine and biology.

The skin, being the organ that covers the body surface and interfaces with the external environment, is constantly exposed to various external stimuli such as pathogens, chemicals, and ultraviolet radiation. In order to counteract these, the skin forms a robust barrier structure and houses complex mini-organs such as hair follicles and sweat glands. Additionally, various immune cells like T cells and dendritic cells migrate within the skin, orchestrating diverse immune responses. The ability to observe these processes three-dimensionally and over time, known as “in vivo imaging,” becomes a powerful tool in dermatological research. In this article, we outline fundamental research and insights obtained from observing mouse skin using two-photon excitation microscopy. Furthermore, in the latter part, we discuss the current status and prospects of in vivo imaging of human skin.

Intravital fluorescence imaging is a promising technique and has been applied for in vivo experiments in various research fields including cancer research. Recently, there has been a growing interest in applying this intravital fluorescent imaging technique to study different disease process and complex biology in vivo. Particularly, intravital fluorescence imaging using various functional fluorescent proteins and dyes, in conjunction with appropriate fluorescence microscopy, allows us visualization of cell behavior as well as cell function in vivo. For instance, we tried to apply the intravital fluorescence imaging technology to monitor cancer cell behavior and function such as cell cycle progression and signal transduction in vivo. We have developed Fucci (Fluorescent ubiquitination-based cell cycle indicator) to visualize cell cycle progression in vitro and in vivo¹⁾. We also developed two-photon excitation microscopy-based experimental platforms that enable the prospective analysis of cancer stem cell dynamics with sufficient spatiotemporal resolution and revealed that cell-matrix interface regulates dormancy in human colon cancer stem cells²⁾. In addition, we tried to show the interaction between cancer cells and tumor microenvironment such as blood vessels and lymphatic vessels by intravital fluorescence imaging technique in vivo. Moreover, we developed various fluorescence imaging equipment including two-photon microscope and light-sheet microscope. Intravital fluorescence imaging will be useful to investigate biology and test the effectiveness of therapeutic agents.

The skin is an organ located in the outermost layer of the body, and it is important to be able to measure human skin in vivo (as it is, non-labeling) in dermatology, cosmetics science, and percutaneous absorption research. Within skin, the epidermis is of particular importance as it considered to correlate to skin condition. Specifically, smooth epidermal differentiation is thought to result in the generation of a healthy stratum corneum, which maintains the skin’s protective barrier function. Many dermatological studies using immunostaining with fluorescent dyes or proteins have had limited success in revealing skin functions as these methods are known to affect the main components of the epidermis, including the water, lipids, and proteins. Spectroscopic techniques have high potential in the measurement of human skin components, as it can obtain the molecular vibrational signature of skin non-invasively in vivo/ex vivo. Near-infrared spectroscopy (NIR) can be used to visualize water, while spontaneous Raman can be used to analyze water, lipids, and free-amino acids in the skin. For application of Raman microscopy to the molecular imaging of human skin, non-linear Raman spectroscopy, such as coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS), which enables faster measurement than with spontaneous Raman, is an attractive technique. In my talk, I gave examples of visualizing water in 2D using NIR imaging. I also introduced the latest results of intracellular morphologies in the human epidermis during the epidermal differentiation process using SRS, water distribution in the human stratum corneum using CARS and SRS, and the potential of coherent Raman microscopy in the label-free bio-imaging of skin to provide a cellular-level analysis of the skin’s functions.

Alopecia symptoms are a disease that takes a long time to treat and is often intractable. It is often not itchy or painful, but it is a troublesome condition that significantly reduces quality of life, reduces the range of activities, causes emotional stress, affects school life, and imposes physical and mental restrictions on all aspects of human life, including love, friendships, and sports. Since it takes time for treatment to make the results, the main response in the meantime is to use wigs and cosmetics, which in many cases have significantly improved quality of life. This paper outlines various methods of coping with the condition, including wigs, cosmetics, hair growth products, and art makeup.