介绍:
Integumentary System
The integument,the largest organ of the body,is composed of skin and skin appendages--nails,hair,sweat glands,and sebaceous glands.The total weight and overall surface area of skin in the adult are 3-5kg and 1.5-2㎡,respectively.Skin thickness,between 0.5 and 3mm,varies regionally;skin is thickest on the back and thinnest on the eyelid.As well as serving as a protective barrier against injury(e.g.,abrasions,cuts,burns),infection pathogens,and ultraviolet radiation,skin assists in body temperature regulation,vitamin D synthesis,ion excretion,and sensory reception(touch and pain),and it has a remarkable regenerative capacity.It consists of the epidermis and the dermis.A loose layer of subcutaneous connective tissue,the hypodermis,attaches skin to underlying structures and permits movement over most body parts.
Epidermisd:
The epidermis consists primarily of cells called keratinocytes,which make up more than 90% of the cell population.Other epidermal cells are melanocytes.Merkel cells,and Langerhans cells.Four or five distinct layers constitute the epidermis.The stratum basale,or germinativum,is the deepest;it consists of a single layer of closely packed,basophilic cuboidal to columnar epithelial cells,resting on a basement membrane.These cells have oval nuclei that often show mitotic figures;;they continuously undergo cell division to replace cells that move outward through the epidermis.The next layer, the stratum spinosum,is several cells thick and has polyhedral cells that become progressively flatter toward the surface.Cell shrinkage caused by a fixation artifact accentuates the processes and creates spines or prickles--thus the name prickle cells.The next layer,the stratum granulosum,consists of three to five layersof flattened cells,their axes aligned parallel to the epidermal surface.They contain numerous basophilic granules,the keratohyalin granules,Superficial to this layer is a thin,translucent,lightly eosinophilic layer,known as the stratum lucidum.Absent in thin skin but present in thick skin,it consists of a few layers of tightly packed squamous cells that lack organelles and nuclei.The outermost layer,the stratum corneum,is made of dead,anucleate cornified cells;its thickness varies regionally.The protein keratin replaces cytoplasm in its cells.The most superficial cells are continuously shed in a process known as desquamation.
Dermis:
The dermis,a richly vascularized connective tissue,provides mechanical support,pliability,and tensile strength to skin.Blood vessels furnish nutrients and are involved in thermoregulation.The thick dermis is divided into two layers;a superficial palillary layer of loose connective tissue containing type 1 and 3 collagen fibers interspersed with elastic fibers,connective tissue cells,and rich network of capillaries;and a deeper reticular layer of dense irregular connective tissue consisting of coarse,interlacing bundles of collagen fibers,mostly type 1.Aside from fibroblasts,other connective tissue cells in the dermis include macrophages,mast cells,adipocytes,plasma cells,and lymphocytes.
A rich nerve supply throughout the dermis includes a complex network of sensory nerves and efferent sympathetic innervation to sweat glands,vascular smooth muscles, and arrector pili muscles.Myelinated nerve fibers supply nerve endings to the epidermis and encapsulated sensory receptors in the dermis including Meissner and Pacinian corpuscles.Located in dermal papillae,Meissner corpuscles are mechanoreceptors that mediate touch.Pacinian corpuscles are larger encapsulated receptors in deeper regions of dermis and subcutaneous tissue.
Skin appendages
Eccrine sweat glands are simple,coiled tubular glands consisting of secretory and narrower excretory duct portions.With cholinergic innervation,they mainly serve a thermoregulatory role and maintain body temperature by evaporative heat loss.They also aid ion excretion and may,under normal conditions,produce 500-750ml or more of sweat daily in response to thermal and emotional stimuli.
Apocrine sweat glands,also known as odoriferous sweat glands,are large,branched glands found in axillae,scrotum,prepuce,labia minora,nipples,and perianal regions.They are less coiled than sccrine sweat glands,and many coils anastomose to form an interwining tubular network.The sac-like lumen of the secretory tubules is lined by simple cuboidal epithelium and,compared with the eccrine glands,has a wider diameter and larger,more numerous myoepithelial cells that share a basement membrane with secretory epithelium.
Except for lips,palms,soles,and a few other sites,hairs cover most of the body surface.They develop from epidermis,cross dermis,and often extend into subcutaneous connective tissue.Each hair comprises a free shaft and a root,which is enclosed at its lower end by a tubular hair follicle,composed of epidermal(epithelial) and dermal(connective tissue)parts.The long axis of each follicle usually lies oblique to the plane of the epidermal surface.Hair are keratinized threads that vary in thickness and length depending on body region.
Sebaceous glands are usually associated with hair and are located between a hair follicle and its arrector pili muscle in the dermis.They are holocrine glands in which part of the secretory product,known as sebum,is made of lipid-rich decomposed cells.Most sebaceous glands empty secretions by a duct into the upper part of the hair follicle near the hair shaft.These simple or branched alveolar glands are pale staining and ovoid.A thin connective tissue typically open into a common duct that is lined by stratified squamous epithelium,which is continuous with the outer epithelial root sheath of the hair follicle.
Nails are modification of the stratum corneum of the epidermis on the dorsal aspect of terminal phalanges of fingers and toes.The slightly convex,semitransparent nail plate is composed of multiple layers of squamous-shaped keratinized cells contain hard keratin and so do not desquamate.The undersurface of both exposed and concealed parts of the nail plate is the nail bed,It consists of stratum germinativum of the epidermis and underlying dense dermis,which lacks subcutaneous tissue but is firmly attached to periosteum of terminal phalanged.
