How many layers does a cat's skin have? Skin diseases are essentially diseases that manifest on the skin, so before treating a cat's skin disease, we need to understand the structure of the cat's skin.
I. Epidermis
The epidermis forms the outermost layer of the skin, acting as a barrier against harsh external environments and protecting the body from various chemical, physical, and biological factors. Although thin, the epidermis, aided by hair, keratinocytes, and glands, provides strong protection. The epidermis is tightly integrated with the dermis, exchanging cells and fluids to ensure adequate nutrient supply. The epidermis is composed of stratified squamous (flat) epithelium, only 0.2-0.5 mm thick. The thickest epidermis is found in the nose and foot pads (1.5 mm). The epidermis is composed of keratinocytes, pigment cells, Langer's cells, etc. Keratinocytes account for 85% of epidermal cells, with slightly different morphologies in different layers. In the basal layer, they appear as tightly connected columnar epithelium, connected to the basement membrane. As cells divide, daughter cells enter the spinous layer, exhibiting a polyhedral shape; in the granular layer, they appear as flattened cells. The stratum corneum appears as a flattened cell with missing nuclei. Keratinocytes are not only an important structural component of the skin but also crucial members of the skin's immune system. Their ability to engulf and process antigens is even greater than that of specialized immune cells in the epidermis—Langer cells. Under the influence of gamma interferon, keratinocytes have a stimulatory and buffering effect on T lymphocytes. After interacting with antigens, they produce interleukin-1, which further stimulates the production of a wider range of cytokines, initiating and suppressing immune responses. Interleukin-1 can also be released into the dermis, causing an inflammatory response. 
II. Dermal-epidermal junction - basement membrane
The basement membrane is the foundation of the epidermis. The epidermis is firmly fixed to the dermis through it, maintaining normal epidermal function and proliferation, maintaining tissue structure, helping wound healing, and also serving as a barrier between the dermis and epidermis, maintaining the exchange of cells and body fluid elements between the epidermis and dermis.
The basement membrane consists of four parts:
(1) Tonefilament-hemidemosome complex (attached to basal cells);
(2) The main components of the transparent plate are collagen XVII (i.e., 180kDa bullous pemphigoid antigen, BP180) and fixed fibers;
(3) The dense plate contains type IV collagen, laminin subtypes, nestin, and basement membrane proteoglycans, etc.
(4) Subbasal membrane: A dense, thin layer of fixed fibers and acid-resistant filaments extending into the superficial dermis. In very rare bullous pemphigoid, the primary target of autoantibodies is collagen XVII (BP180).
III. Dermis
The dermis is the main part of the skin, firm, resilient, and elastic. It provides physical, vascular, and nerve support to the epidermis and is the body's complete connective tissue. The dermis is mainly composed of dermal fibers and soluble polymers, with cells diffused throughout. Most of the skin's appendages are located within the dermis.
(1) Dermal fibers: collagen and elastin.
(2) Soluble polymers: proteoglycans and hyaluronic acid.
(3) Dermal cells: fibroblasts and dendritic cells.
(4) Accessory structures: follicular units, arrector pili muscles, blood vessels, lymphatic vessels, and nerves. Dermal lesions often occur within and around blood vessels. Lesions of other dermal components are very rare, such as: E.D. disease (also known as: generalized dysplasia of elastic fibers) caused by abnormal collagen structure, which is a hereditary disorder of collagen synthesis and distribution, mainly manifested as excessive stretching of the skin.
