The skin is the largest and one of the most important organs of the body. It forms a barrier to protect the body of the cat from infections, parasites, and the elements. It also maintains the body's internal environment, preventing loss of moisture and other body constituents. Because the skin is on the outside of the body, it is easily exposed to outside elements and susceptible to injury and disease. It is also very visible, so disorders are readily detected during an examination.
The skin is made up of layers of cells, lubricating (sebaceous) glands, blood vessels, nerve endings, and hair follicles which produce hairs. The skin cells form layers, namely the tough outer covering called the 'epidermis' and the deeper layer called the 'dermis.'
The epidermis is composed of older cells that form a tough, almost impervious, protective outer barrier. As the outer cells erode, other cells mature and move up to replace them. The epidermis varies in thickness. In the more exposed areas, such as the head and back, the epidermis is thicker than areas such as the armpits and belly.
The deeper layer (dermis) contains hair follicles, blood vessels, nerves and sebaceous (oil) glands. Hair follicles and sebaceous glands are more prevalent on the back than on the belly. Hair and nails are made of a hard substance called keratin.
Types of hair
Kittens are not born naked. Their skin is covered by a short, soft, and sometimes wooly-like hair. Sometimes the kitten hair, or fur, is a similar color to what is expected as an adult, other times it is not. Siamese kittens are born almost entirely white or cream-colored and develop their dark 'points' (ears, legs, tail) as they age.
Most kittens develop a coarser, longer, and occasionally darker coat by six to eight months of age. Breeds and individuals have different rate of coat development. Factors such as day length, hormones, average outdoor temperature, and nutrition may influence coat development as well.
Cats have four types of hair. There are short fluffy hairs called secondary hairs. Other names for secondary hairs include underfur and undercoat. The second type is the longer and stiffer outer hairs called primary hairs. The primary hairs are also referred to as guard hairs, outer hairs, or outer coat. Cats, unlike some other mammals, also have hairs called awn hairs which are thinner than the primary hairs. Finally, whiskers are actually specially developed hairs, called tactile hairs, which help the cat sense her surroundings.
The ratio of the number of primary to secondary hairs differs by age and breed. Newborn kittens lack primary hairs. That is why their coats are short and soft. Usually by six months of age most kittens have developed a good growth of primary hairs so their hair coats are longer and more coarse. Many variations exist amongst breeds as to the exact length, color, and texture of the hair coat. These coat differences are largely the result of the ratio of primary to secondary hairs and the texture of these individual hairs.
Each hair grows from a simple opening within the skin called a hair follicle. A kitten is born with all of the hair follicles it will ever possess. Any future differences or changes of the hair coat will be due to changes within the follicle. Each hair shaft produced by a hair follicle will eventually die and be removed (shed) and replaced by a new hair shaft produced by that hair follicle. Cats continually shed old dead hair from the follicle and replace it with a new live and growing hair. There is no such thing as a nonshedding breed. The extent or rapidity to which an individual sheds is, however, governed by such factors as age, amount of sunlight, outside temperature, breed, sex, hormones, allergies, nutrition, etc.
Breeds and individuals within every breed will shed and regrow hair at varying rates. If a cat sheds often it is more noticeable than if she sheds extensively but for a period of only several weeks. Indoor cats, because of artificial heat and more importantly light, tend to shed in a more or less continuous fashion. Strictly outdoor cats kept outside tend to shed for several weeks during major seasonal changes, most notably in spring and fall. Usually they grow more secondary hairs or underfur in the fall for warmth. In the spring they lose the underfur and replace much of it with the longer primary or guard hairs. The hair coat changes in appearance and texture but the absolute numbers of hair follicles and hair does not.
The hair of a cat does not grow continuously, but in cycles, similar to our eyebrows. Anagen is the first phase, in which the hair is produced. The new hair grows along side the old hair, which is subsequently lost. Catagen is an intermediate stage in the cycle, and telogen is the resting phase in which the follicle is basically dormant. The hair follicles are not all in the same phase at the same time, which is why we do not see a lot of bald cats!