Welcome to Anatomy 101.

This is your instructor Sheila. Today, I am going to tell you about the integumentary system. What is this system? It is the most important system because it is comprised of hair, nails, skin, and various glands, muscles and nerves. It regulates body temperature, blood reservoir, protection from the external environment, excrection and absorption, synthesis of Vit D, and cutaneous sensations. The skin is composed of two layers: the epidermis and the dermis. There is also a subcutaneous layer (aka the hypodermis) that is comprised of areolar and adipose tissue. This serves as a storage for fat and contains large blood vessels that supply the skin.

The epidermis is a keritinized stratified squamous epithelium that can be classified in 4 catergories.
1) Keratinocytes: arranged in 4-5 layers. produces keratin and lamellar granules, which releases a waterproofing sealant.
2) Melanocytes: produces melanin which contributes to skin color and absorbs damaging UV light.
3) Langerhan cells: participate in the immune response mounted again microbes that invade the skin and are easily damaged byt UV light.
4) Merkel cells: located in the deepest layer of the epidermis; functions in sensation of touch.

There are several distinct layers in the keratinovytes in the various stages of development in the epidermis. From the deepest to the superficial:
1) Stratum Basale: composed of Langerhan cell, Melanocytes, and Merkel cells. composed of single cuboidal or columnar keratinocytes, some of hich are stm cecll that undergo cell division to cotinually produce new keratinocytes.
2) Stratum Spinosum: superficial to the stratum basale. 8-10 layers of polyhedral keratinocytes fit closely together, flattened. this arrangement provides strength and flexibility to the skin. Projections are composed of Langerhan cell and Melanocytes.
3) Stratum Granulosm: This marks the transition of the deep layer from the superficial layer. Here you will notice a pronounced staining of the granules of a protein called keratohyalin which converts tonofilaments to keratin. Is located at the middle of the epidermis, 3-5 layers of flattened keratinocytes that are undergoing apoptosis.
4) Stratum Lucidum (Note: this only is present in thick skin): 3-5 layers of clear, flat, dead keratinocytes that contain large amounts of keratin and thickened plasma membranes.
5) Stratum Corneum: 25-30 layers of dead keratinocytes. These cells are continuously shedding and are replaced by cells from deer strata.

What is keratinization? As cells move from one epidermal layer to another, they accumulate more keratin. So the whole process in which cells from the deep stratum basale moves to the superficial layer, the cells are becoming keratinized and die.

Dermis is the other layer that your skin is made of. That's where all the good stuff like support comes into play. This is techinically the second layer of your skin. It is composed mainly of connective tissue unlike it's other half the epidermis (which is composed of epithelium tissue). So collagen fibers and elastic fibers and few cells like fibroblasts, macrophages and adipocytes would be found in the dermis.

There are two regions that the dermis is sectioned into. There the papillary region and the reticular region.
1) Papillary Region is only made up of one-fifth of the dermis. It contains the dermal papillae, corpuscles of touch, and free nerve endings; not to mentioni areolar connective tissue containing fine elastic fibers. Of course, this is the superficial portion of the dermis.
2) Reticular Region is made up of dense irregular connective tissue containing bundles of collagen and some coarse elastic fibers. The spaces betwwn fibers are occupies by a few adipose cells, hair follicles, nerves, sebaceous (oil) glands, and sudorigerous (sweat) glands. In sum, the reticular region provides the skin with strength and extensibility.