Other Cells of Not of The Immune System

Red Blood Cell/Erythrocytes are the most common type of cell in the blood. They are packed full of hemoglobin and contain practically none of the usual cell organelles. Erythrocytes have an average life span of 120 days before being phagocytosed and digested by macrophages in the spleen. A lack of oxygen or shortage of erythrocytes stimulates cells in the kidney to synthesize and secret erythropoietin in the blood which stimiulates the production of more erythrocytes. Another colony stimulating factor called interleukin-3 (IL-3) promotes the survival and proliferation of erythroid progenitor cells.

Platelets are miniature cells without a nucleus. They circulate in the blood and help stimulate blood clotting at sites of tissue damage. Platelets secrete a mitogen called platelet derived growth factor (PDGF) which stimulates cell division. When blood clots, platelets incorporated in the clot are triggered to release PDGF. The serum of a cell which is prepared by allowing blood to clot and taking the cell free liquid that remains contains this PDGF. Compare the term "plasma" which is prepared by removing cells from blood without allowing clotting to occur.

Cells of the Skin

Keratinocytes: differentiated activity is the synthesis of intermediate filament proteins called keratins, which give the epidermis its toughness. As keratinocytes mature, they produce keratin hyaline granules which release their contents into the cytoplasm as the cell dies and lamellar bodies which are involved in the formation of the water barrier in skin.

Melanosomes: are located in the basal layer of the epidermis and are of neural crest original rather than ectoderm as with the epithelium. Melanosomes produce melanin which help protect developing keratinocytes from DNA damages due to UV light.

Langerhands Cells are phagocytic cells found in the epidermis and develop from bone marrow cells, as do most macrophages. They process antigens and present them to lymphocytes.

Merkel Cells also reside in the epidermis and function as mechanoreceptors. Nerve endings are closely associated with these cells to transmit the sensory signal.

Cells of the Blood Vessels

Endothelial cells: line the blood vessels and mediate rapid responses to neural signals for blood vessel dilation by releasing NO to make smooth muscle relax in the vessel wall.

Cells of the Lung

Type I alveolar cells cover most of the wall of alveoli. They are squamous (thin and flat) to allow gas exchange.

Type II alveolar cells are interspersed among type I alveolar cells and secret surfactant which is a phospholipid rich material that forms a film on water surfaces thereby reducing surface tension.

Cells of the Liver

Hepatocytes of the liver play a central part in carbohydrate and lipid metobolism of the body. They remain connected with the lumen of the gut via a system of small channels and larger ducts and secrete waste products of their metabolism and an emulsifying agent, bile, which helps in the absorption of fats.

Cells of the Connective Tissue/Muscle, cartilage, Bone

Skeletal muscle cells are often referred to as muscle fibers because of their elongated shape. Each one is a syncytium, containing many nuclei within a common cytoplasm. Myoblasts are the precursor cells to skeletal muscle fibers. They differentiate and fuse with one another to form multinucleate skeletal fibers. Humans do not usually generate new skeletal muscle fibers but small inactivate cells lying in close contact with the muscle cell called satellite cells can be activated to proliferate and fuse to repair damaged muscle.

Smooth muscle cells are non-striated spindle shaped cells that have one central nucleus.

Heart muscle cells like smooth muscle cells are single cells with one nucleus but they resemble skeletal muscle cells in that they appear striated.

Fibroblasts are dispersed in connective tissue through the body where they secrete a nonrigid extracellular matrix that is rich in type I and/or type III collagen.

Stromal cells are found in the bone marrow.

Osteoclasts demolish old bone matrix. Osteoclasts are capable of tunneling deep into bone, forming cavities which are then lined by a blood capillary. The walls of the tunnel become lined with a layer of osteoblasts.

Osteoblasts deposit new bone matrix.

Cells of the Nervous System

Nerve cells or neurons have an extended shape with a long axon and branching dendrites connecting it through synapses to other cells.

Glial cells create an enclosed protective environment in which neurons can function. Glial cells and neurons of the central nervous system in vertebrates derive from the part of the ectoderm that rolls up to form the neural tube, while those of the peripheral nervous system derive mainly from the neural crest.

Retinal ganglion cells are neurons that transmit signals from the eye to the brain.

Olfactory Sensory Neurons have a single axon extending form their basal end toward the brain and contain oderant receptor proteins.