nThe dermis, which contains the hair follicles (found only in thin skin) and sebaceous and sweat glands, consists of 2 layers of vascular connective tissue that blend at their common border.
nA. Papillary Layer: This layer of loose connective tissue, rich in elastic fibers, lies directly beneath the epidermal basement membrane. Its projections--dermal papillae-interdigitate with the epidermal ridges, increasing the area of contact. Special collagen fibers, anchoring fibrils, extend from this layer into the epidermal basal lamina to reinforce the dermal-epidermal junction. The papillary layer contains immunoprotective cells, a rich capillary net work, and abundant free nerve endings, some of which penetrate the epidermis. The tips of many dermal papillae contain encapsulated touch receptors called Meissner's corpuscles.
nB. Reticular Layer: Beneath the papillary layer is a thicker layer of dense irregular connective tissue. Also richly vascularized, this layer contains many arteriovenous anastomoses, or shunts, that control the amount of blood reaching the papillary capillaries and thus aid in regulating heat loss and blood pressure. The reticular layer also contains a rich supply of nerves in both free and encapsulated endings leg, Pacinian corpuscles).
Showing posts with label histology. Show all posts
Showing posts with label histology. Show all posts
GENERAL FEATURES OF THE SKIN
A. General Functions: The skin is the largest and heaviest organ. It protects against microorganisms, toxic substances, dehydration, ultraviolet radiation, impact, and friction. It also acts as a sensory receptor and has a role in excretion, vitamin D metabolism, and regulation of blood pressure and body temperature.
B. General Organization: Human skin (the integument) is of 2 types. Thick skin, restricted to the palms of the hands and soles of the feet, lacks hairs and has abundant sweat glands. Thin skin, which has hairs, covers the rest of the body. Thick or thin, the skin consists of 2 distinct but tightly attached layers, the epidermis and dermis, which are underlain by the hypodermis.
1. Epidermis. This outer (superficial) layer of skin, composed of keratinized stratified squamous epithelium, derives from embryonic surface ectoderm. It is avascular, receiving nourishment from vessels in the underlying dermis. Its only innervation is by unencapsulated (free) nerve endings. The epidermal layer is further divided into 5 stratea; these layers, in order from superficial to deep, are the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. The thickness of these layers differs in thick and thin skin.
2. Dermis, This inner (deeper) layer is a vascular connective tissue of mesodermal origin. It can be further divided into a superficial papillary layer and a deeper reticular layer. The papillary layer contains extensive capillary networks, which nourish the epidermis. The reticular layer contains many arteriovenous anastomoses that help regulate blood pressure and body temperature. It is richly supplied with free nerve endings, a variety of encapsulated sensory receptors, and autonomic fibers that control the vascular smooth muscle. Even in thick skin, the dermis is much thicker than the overlying epidermis.
3. Hypodermis, Although not a part of the skin, this layer of mesoderm-derived loose connective and adipose tissue underlying the dermis flexibly binds the skin to deeper structures. Its thickness varies, depending on nutritional status, level of activity, body region, and gender. The hypodermis is also called the subcutaneous fascia and, where thick enough, the panniculus adiposus
Structures Associated With the Skin: Glands (sebaceous and sweat), hairs, and nails arise from epidermal downgrowths into the dermis during embryonic development. These structures, which are mainly of epithelial origin, require epitheliomesenchymal interactions between the epidermis and dermis for their formation and maintenance
B. General Organization: Human skin (the integument) is of 2 types. Thick skin, restricted to the palms of the hands and soles of the feet, lacks hairs and has abundant sweat glands. Thin skin, which has hairs, covers the rest of the body. Thick or thin, the skin consists of 2 distinct but tightly attached layers, the epidermis and dermis, which are underlain by the hypodermis.
1. Epidermis. This outer (superficial) layer of skin, composed of keratinized stratified squamous epithelium, derives from embryonic surface ectoderm. It is avascular, receiving nourishment from vessels in the underlying dermis. Its only innervation is by unencapsulated (free) nerve endings. The epidermal layer is further divided into 5 stratea; these layers, in order from superficial to deep, are the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. The thickness of these layers differs in thick and thin skin.
2. Dermis, This inner (deeper) layer is a vascular connective tissue of mesodermal origin. It can be further divided into a superficial papillary layer and a deeper reticular layer. The papillary layer contains extensive capillary networks, which nourish the epidermis. The reticular layer contains many arteriovenous anastomoses that help regulate blood pressure and body temperature. It is richly supplied with free nerve endings, a variety of encapsulated sensory receptors, and autonomic fibers that control the vascular smooth muscle. Even in thick skin, the dermis is much thicker than the overlying epidermis.
3. Hypodermis, Although not a part of the skin, this layer of mesoderm-derived loose connective and adipose tissue underlying the dermis flexibly binds the skin to deeper structures. Its thickness varies, depending on nutritional status, level of activity, body region, and gender. The hypodermis is also called the subcutaneous fascia and, where thick enough, the panniculus adiposus
Structures Associated With the Skin: Glands (sebaceous and sweat), hairs, and nails arise from epidermal downgrowths into the dermis during embryonic development. These structures, which are mainly of epithelial origin, require epitheliomesenchymal interactions between the epidermis and dermis for their formation and maintenance
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