Within the bone marrow, all blood cells originate from a single type of unspecialized cell called a stem cell. When a stem cell divides, it first becomes an immature red blood cell, white blood cell, or platelet-producing cell. The immature cell then divides, matures further, and ultimately becomes a mature red blood cell, white blood cell, or platelet. The rate of blood cell production is controlled by the body’s needs. Normal blood cells last for a limited time (ranging from a few hours to a few days for white blood cells, to about 10 days for platelets, to about 120 days for red blood cells) and must be replaced constantly. Merk Manual

Erythrocytes (Red Blood Cells): 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 stimulates the production of more erythrocytes.

RBCs fill a critical role by transporting oxygen and metabolic waste between the lungs and other cells and tissues. This role has been optimized by the unique features of RBCs, which lack nuclei, mitochondria, Golgi, the endoplasmic reticulum (ER), and most other major organelles so as to maximize oxygen-carrying capacity. Nonetheless, RBCs respond actively to changing tissue environments and dynamically alter their cell shapes on short timescales in order to thread narrow capillary networks and splenic tissues. (Marcotte, “The protein organization of a Red Blood cell” Cell Reports 40(3), 2022)

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.

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 granuleswhich release their contents into the cytoplasm as the cell dies and lamellar bodies which are involved in the formation of the water barrier in .

Melanosomes: are located in the basal layer of the  and are of  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 . 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.

Platelets are another name for thrombocytes and are miniature cells without a nucleus. They circulate in the blood and help stimulate blood clotting at sites of tissue damage. Plates can be identified by staining for CD61 and CD62P.

Platelets secrete a 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.

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.

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.

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.

–Satellite cells: The normal mechanisms involved in muscle tissue regeneration initially invovles the recruitment of satellite cells. Muscle Satellite cells are a distinct lineage of myogenic progenitors which are located between the basal lamina and sarcolemma of mature myofibers. During the regeneration cycle, satellite cells are activated and migrate from the myofibers to the site of regeneration to give myoblasts. Most of the poriferating myoblasts differentiate into myotubes. The myotubes mature and are incorporated into muscle fibres. The remamaining myoblasts return to the myfibers to renew the satelllite cell population and thus the capactiy to continue the regneration cycle. (WO2006/083183).

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.

–Cardiospheres are undifferentiated cardiac cells that grow on self-adherent clusters as described in WO 2005/012510 and Messina et al. “Isolation and expansion of adult cardiac stem cells from human and murine heart” Circulation Research, 95: 911-921 (2004). For example, CDCs can be generated by plating cardiospheres on a solid surface which is coated with a substance which encourages adherence of cells to a solid surface of a culture vessel, e.g., fibronectin, a hydrogel, a polymer, laminin, serum, collagen, gelatin or poly-D-lysine and expanding the same as an adherent monolayer culture. (Hemmati, US 2019/0099370). Kreke (US 2018/0169150) disclsoes methods for preparing cardiosphere dervied cells (CDCs) suitable for allogeneic cardiac stem cell therapy that includes receiving donar cardiac tissue from a subject, processing the piece of donor cardiac tissue into a plurality of tissue explants, enzymatically digesting the explants, culturing the explants area until cells migrate from teh explant, collecitng the cells that migrate from the explant, culturing the collected cells in order ot generate CDCs, harvesting teh CDCs, filtering the harvested CDCsto remove particles greater than about 50 um.

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

Fibrocytes: are a CD45-positive hematopietic stem cells. These cells can be detected in tissue sections by colocalization of reactivity for collagen and CD45.

Stromal cells are found in the bone marrow.

Cells of the Bone Tissue

Living bone tissue exhibits a dynamic equilibrium between formation of bone, which is called deposition, and break-down of bone, which is called resorption. Three types of cells found in bone, osteocytes, osteoblasts and osteoclasts, are involved in this equilibrium. Osteoblasts promote formation of bone tissue whereas osteoclasts are associated with resorption (dissolution of bone matrix and mineral). Resorption is a fast and efficient process compared to bone formation and can release large amounts of mineral from bone.

Osteoblasts: are devied form mesenchymal stem cells. They produce the bone matrix which consists of collagenous and non-collagenous proteins (sucha s type I collagen and osteocalsin, respectively) and control its subsequent mineralisation that is the deposition of hydroyapatite. Thus, the rate of bone formation will depend upon individual osteoblast activity, their lifespan and the number of precursor cells recruited. Parathormone (PTH) and PTH related protein (PTHrp), the only other known ligand of the PTH receptor, increase the lifespan of mature osteopblasts by reducing their rate of apptosis, which explains their anabolic effects on bone. Teh calcium-sensing receptor (CaR) which is located on the surface of the parathyrodoid gland senses extracellular levels of ionised calcium and controls calcium homeostasis by regulating the release of PTH.

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. Resorption is stimulated by the secretion of parathyroid hromone in response to decreasing concentration of calcium ion in extracellular fluids. Inhibition of resorption is a funciton of calcitonin. The metabolites vitamin D alter the responsiveness of bone to parathyroid hormone and calcitonin. Osteoprotegerin ligand (OPGL) is a member of the TNF family of cytokines which promotes formation of osteoclasts through bidning to the receptor activator NF-kappaB. Osteoprotegerin (OPG), on the other hand, inhibits the formation of osteoclasts by sequestering OPGL and preventing OPGL associaiton with ODAR (US2004/0033535).

Osteoclasts are issued form haemopoietic stem cells and are formed by the fusion of cells form the monocyte-macrophage cell line. Osteoclastogenesis requires the presence of rANKL (Receptor Activator of Nuclear facto kappabea ligand), a member of the TNF cytokines and M-CSF (macrophage-colony stimulating factor). Once RANKL is epxressed by the osteoblasts, it activates its receptor RANK on the cells of the osteoclast lineage leading to their proliferation, maturation, activation and survival, ultimately resulting into increased bone resorption.

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. Most neurons consist of three parts: a cell body, dendrites and an axon. The cell body contains the nucleus. Dendrites are thin, highly branched extensions that receive incoming stimulation and conduct electrical impulses to the cell body. The axon is a single extension of cytoplasm that conducts impulses away from the cell body. most axons and dendrites can be measured in millimeters, but some can be quite long. For example, the cell bodies of neurons that control the muscles in your feet lie in the spinal cord, therefore, their axons may extend over a meter.

Teh brain and spinal cord form the central nervous system (CNS) of vertebrates and sensory and motor neurons from teh peripheral nervous system (PNS). Sensory neurons of the peripheral nervous system carry information about the environment to the CNS. Interneurons in teh CNS provide links between sensory and motor neurons. Motor neurons of teh PNS system carry impurlsesto muslces and glands.

Glial cells create an enclosed protective environment in which neurons can function. Neuroglia or glial cells do not conduct implsses. They function to lend support, aid in nourishment and provide protection for neurons. They are also intrumental in the regulation of neurotranons. 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  to the brain.

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