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Skin & Muscles

Objective :

  • To learn the procedure for collection of skin and muscle.
  • To learn the structure of skin and the major muscles of rat.

Theory :

  • The skin consists of three main components namely, (1) the epidermis, a superficial lining of epithelial cells covering the body surface; (2) the dermis, a richly vascularised connective tissue situated beneath the epidermis; and (3) the cutaneous adnexa or appendages, a series of epithelial structured situated in the dermis that are connected to the epidermal surface.
  • The number, composition, and thickness of these three components are extremely variable, and it is impossible to describe a 'normal' cutaneous structural pattern. Not only does the skin differ from species to species, but within each species there are strain variations, as well as numerous variations determined by different topographic and physiologic requirements. Additional variations can be caused by age, sex, hormonal status, and genetic constitution.
  • The epidermis is a stratified epithelium that is separated from the dermis by a thin basal lamina. The most superficial cells are continuously replaced by proliferation and upward migration of the deeper layers. This is accomplished by cell division of the basal cells; cuboidal or columnar cells, with proliferative capacity, can also migrate and differentiate into suprabasal cells that constitute the spinous layer.
  • In general, rodents have a thin epidermis, only 2-3 cell layers thick, dogs and horses have a somewhat thicker epidermis and pigs have the thickest of all. A general rule of thumb is that the more sparsely-haired a species is, the thicker its epidermis will be, in particular the horny keratin layer. Marine mammals such as whales have epidermis that is exceedingly thick, up to ten cell layers or more, with an even thicker modified keratin covering.
  • The cutaneous adnexa consist of hair follicles, in addition to sweat and sebaceous glands. The hair follicles are downgrowths of the epidermis in which an ordered array of keratinized cells is gradually pushed upward in the form of hair shafts. These cells give rise to hair by a process of terminal differentiation, analogous to, but more complicated than, the process described for epidermal differentiation.
  • Muscle cells  are specialised contractile cells which generate motile forces by the interaction of proteins, actin and myosin. These are of 3 types namely, skeletal muscle, smooth muscle and cardiac muscle. Skeletal muscle is responsible for the movement of skeleton and organs. It is also called as the voluntary muscle.
  • The arrangement of the contractile proteins gives rise to the appearance of prominent cross-striations histologically and hence are often called as the striated muscle. In the smooth muscles, the striations are absent and forms the muscular component of visceral structures  such as blood vessels, GI tract, uterus, etc. The cardiac muscles have structural and functional characteristics intermediate between those of the skeletal and smooth muscles and they provide continuous, rhythmic contractility of the heart. 
  • Muscles function in antagonistic groups. For example, several muscles contract to bend the forearm. Muscles antagonistic to this motion contract to straighten the forearm. e.g. Flexion/Extension. Muscles that flex or extend the forearm are an example of an antagonistic group.
  • When a joint is flexed the distal bone moves toward the proximal bone so that the angle between them decreases. Extending a joint increases the angle between the two bones.  While one set of muscles is contracting and getting shorter the other set is relaxing and getting longer.

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