Lec 23 - Integument

• The skin is organized into three distinct layers

  • epidermis (uppermost)
  • dermis
  • hypodermis
• The dermis further divides into

  • papillary layer (LCT, has lymph vessels)
  • reticular layer (DICT, gives skin its stretchiness and resistance to force)
• The skin is the largest organ in the body - makes up 15 - 20% body mass
• The epidermis is the epithelial layer and is avascular
• The vast of the nervous tissue, vasculature, hair bulbs, ducts, and skin modifications are found in the dermis
• The hypodermis is made up of Loose connective tissue, find a lot of adipose tissue, and glandular tissue
• The epidermis is made up of stratified squamous keratinized epithelium
• Functions of the skin

  • barrier
  • water homeostasis
  • temperature homeostasis
  • peripheral nervous system receptors
  • vitamin D production (endocrine)
  • excretion
  • immunologic functions
  • endocrine
• The blood supply that feeds the skin is mostly coming in from the subcutaneous plexus which is in the hypodermis
• Blood supply in the skin

  • subcutaneous plexus - in the hypodermis
  • cutaneous plexus - in the dermis
  • subpapillary plexus - in the papillary layer of the dermis which allows nutrient provision to the epidermis
• Layers of the epidermis

  • stratum basale
  • stratum spinosum
  • stratum granulosum
  • stratum lucidum (only in thick skin)
  • stratum corneum
• The stratum basale is the layer of the epidermis that contains mitotically active cells, pigmented melanocytes, and sensory cells. This layer is one cell thick
• The stratum spinosum is a layer of the epidermis that contains many layers of keratinocytes and langerhans cells
• The stratum granulosum is a layer of the epidermis that contains darkly stained keratohyalin granules (secretory granules) - usually 1-2 cell layers thick
• The stratum lucidum is a layer of the epidermis that is only present in thick skin (palms and soles), it is a bluish layer, where apoptosis occurs, and lots of lipids are released for the function of water proofing the skin
• The stratum corneum is the outermost layer of the epidermis where cells are dead and keratinized. They contain lipids between the keratinocytes
• Thick skin does not contain hair follicles - therefore ther are no sebaceous glands
• Fingerprints are formed by epidermal ridges - formed by the interdigitation of the epidermis and dermis (interpapillary peg and dermal papilla)
• Keratinocytes are linked together by a lot of intermediate filaments and desmosomes and anchored to the basement membrane by hemidesmosomes and integrins
• The special cells of the epidermis are

  • langerhans cells (dendritic cells
  • merkel cells (mechanoreceptors)
  • melanocytes (pigmented cells)
• The special cells of the epidermis are not intrinsically native to skin, they migrate in as the skin is developing (most from mesenchyme)
• Langerhans cells (dendritic cells) are monocytes derived from lymphoid precursor cells - they are the macrophages of the skin and are located in the stratum spinosum
• Merkel cells are mechanoreceptor cells that detect fine touch and are located in the stratum basale
• Melanocytes are pigmented cells that produce melanin. They are located in the stratum basale but they contain projections that project up into the stratum spinosum
• Skin pigmentation

  • tyrosine converted into dopa which is the precursor of melanin
  • melanin accumulates in pre-melanosomes after synthesis from dopa
  • melanosomes (secretory vesicles) migrate upward to the tip of the melanocyte processes that invaginate between keratinocytes
  • keratinocytes phagocytose the tips of the processes - called melanosomes
  • melanosomes release the melanin into the cytoplasm, which then accumulates at the UV side of the nucleus
• Everyone has the same amount of melanocytes but different types of melanin and different people break down melanin more readily
• Langerhans cells phagocytose and present antigen on its surface then migrate out of the skin into the dermis and out into the lymphatic vessels, go to a lymph node and interacts with T-cells
• Merkel cells are associated with the peripheral nervous system and connect with free nerve endings. They secrete granules that are going to interact like a synapse with the free nerve ending. They respond to mechanical movements, really fine touch and pressure. They are found in the stratum basale
• Label the layers of the skin

  A) epidermis

  B) dermis

  C) papillary layer

  D) reticular layer

  E) hypodermis

  
• Label the layers of the epidermis

  A) stratum basale

  B) stratum spinosum

  C) stratum granulosum

  D) stratum lucidum

  E) stratum corneum

  
• The encircled cell is an example of a melanocyte - dark staining nucleus with clear halo cytoplasm