Molecular Biology of the Cell - Volume 3 - Alberts B., Bray D., Lewis J., Raff M., Roberts K., Watson J. 1994

From Cells to Multicellular Organisms
Maintenance of Normal Tissue Organization
Soft Cells and Hard Matrix: Bone Growth, Renewal, and Repair

Bone is a highly dense, specialized Connective Tissue. Much like reinforced concrete, the bone matrix consists of two components: tough fibers (type II Collagen fibrils) that resist tension, and hard particles (calcium phosphate in the form of hydroxyapatite crystals) that withstand compression. The volume of space occupied by collagen is nearly equal to that occupied by calcium phosphate. In adult bones, the fibrils are arranged in orderly layers reminiscent of plywood Structure: in each layer, they lie parallel to one another, but at right angles to the fibrils in both adjacent layers.

Despite its hardness, bone is dynamic and constantly undergoes remodeling. Its dense Extracellular matrix is permeated by channels and cavities filled with living Cells, which account for about 15% of the weight of compact bone. These cells participate in the continuous process of bone remodeling. Cells of one type break down the old bone matrix, while cells of another type form new matrix. This mechanism ensures the renewal of the matrix within the bone.

Bone can grow only by apposition, i.e., the deposition of additional matrix along with cells onto the free surface of hard tissue. In the embryo, appositional bone growth must be coordinated with the growth of other Tissues so that the Organism can increase in size without significant distortion of proportions. The growth of most of the Skeleton, especially the long BONES OF THE limbs and trunk, is coordinated through a complex strategy. In the embryo, miniature "models" of future bones are first formed from Cartilage. Each such model grows, and as new cartilage is formed, the older cartilage is replaced by bone. The growth and destruction of cartilage and the deposition of bone during development are so finely coordinated that in the adult animal, even after reaching half a meter in length, the bone retains almost the exact same shape as the initial cartilaginous model, which was no more than a few millimeters long. Without delving deeply into the geometry of this process, we will focus on the cellular activity underlying the growth of bone and cartilage in the embryo and the renewal of the corresponding tissues in the adult organism; here, The Role of interactions between different Cell types is manifested particularly clearly.

17.8.1. Cartilage is capable of interstitial growth [39, 42]

The "cooperation" between bone and cartilage is based on their contrasting properties. Both tissues develop from mesenchymal cells that secrete large amounts of extracellular matrix containing collagen. However, the bone matrix is hard, whereas cartilage is deformable because it consists primarily of a high concentration of Proteoglycans (section 14.2.4) and type II collagen. Therefore, unlike bone, cartilage is capable of interstitial growth: it can increase in volume because cells already surrounded by the matrix continue to secrete it.

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Fig. 17-43. A. Schematic representation of a section through a cartilaginous rod during its formation, showing the surrounding fibrous perichondrium. Each chondrocyte fills a lacuna within the cartilage matrix. B. Cross section through such a rod in a chicken embryo at an early stage of development. As the tissue grows, The amount of cartilage matrix per chondrocyte will increase significantly, and the boundary between the cartilage and perichondrium will become more sharply defined. (Courtesy of Peter Gould.)

Cartilage cells, or chondrocytes, are separated from one another by the matrix. Each cell occupies a small cavity, or lacuna, within the matrix. Cartilage typically lacks Blood capillaries, and the viability of its cells is maintained through the diffusion of nutrients and gases via the matrix to and from relatively distant Blood Vessels. Most of cartilage is surrounded by the perichondrium, a dense layer of connective tissue containing collagen (Fig. 17-43). Cartilage grows from within as chondrocytes secrete new matrix, while the fibrous perichondrium acts as a corset that limits shape changes. New cells are also generated during cartilage growth: a chondrocyte residing in its lacuna divides to give rise to two cells, each of which subsequently secretes new matrix that soon isolates them from one another (Fig. 17-44). New cells may also enter the cartilage from the perichondrium. Perichondrial cells, resembling fibroblasts, are believed to divide and undergo a transition whereby they begin to form cartilage matrix around themselves and rapidly become true chondrocytes. This process occurring in vivo is presumably analogous to the in vitro transformation of connective tissue cells described above (section 17.7.2).

Fig. 17-44. Growth of cartilage. The tissue expands as chondrocytes divide and produce more matrix. The newly synthesized matrix immediately surrounding the cells is highlighted in a darker shade of gray. Cartilage can also grow through the recruitment of fibroblasts from the perichondrium and their differentiation into chondrocytes (see Fig. 17-43).

17.8.2. Osteoblasts secrete bone matrix, whereas osteoclasts destroy it [41, 43]

Bone is a more complex tissue than cartilage. The bone matrix is secreted by osteoblasts, which lie On the surface of pre-existing matrix and lay down new bone material. Some osteoblasts remain free on the surface, while others gradually become embedded within their own secretory product. This freshly produced material (consisting mainly of type I collagen) is called osteoid. It rapidly transforms into dense bone matrix As a result of calcium phosphate deposition. Once enclosed within the hard matrix, the original bone-forming cell, now termed an osteocyte, loses The ability to divide, although it continues to secrete matrix in small amounts. Like a chondrocyte, an osteocyte occupies a small cavity, or lacuna, in the matrix, but unlike chondrocytes, it is not isolated from its peers: very narrow canaliculi extend from each lacuna, containing processes of the residing osteocyte that allow it to form Gap Junctions with neighboring osteocytes (Fig. 17-45). Although these osteocyte networks neither secrete nor resorb matrix themselves, they likely play an important regulatory role in controlling The activity of cells that do.

While osteoblasts deposit bone matrix, osteoclasts resorb it (Fig. 17-46). Osteoclasts are large, multinucleated macrophage-like cells. Like other macrophages, they develop from monocytes produced in the hematopoietic tissue of the Bone Marrow. Osteoclast precursors—monocytes—enter the bloodstream and accumulate at sites of bone resorption; there they fuse with one another to form multinucleated osteoclasts, which invade the superficial layers of the bone matrix and gradually dissolve it.

Osteoclasts are capable of carving deep tunnels into the compact bone material, creating cavities that are subsequently invaded by other cells. Blood capillaries sprout along the axis of such a tunnel, and its walls become lined with a layer of osteoblasts (Fig. 17-47). Osteoblasts deposit new bone in concentric layers, which gradually fills the cavity, leaving only a narrow canal around the new blood vessel. Many osteoblasts become trapped within the bone matrix and form concentric rings of osteocytes there. While some tunnels are being filled with bone, others are newly excavated by osteoclasts within older concentric systems. The results of this continuous remodeling are clearly visible in histological sections of compact bone (Fig. 17-48).

Fig. 17-45. This diagram illustrates how osteoblasts lining the bone surface secrete organic bone matrix (osteoid) and differentiate into osteocytes as they become embedded within this matrix. The newly formed matrix soon calcifies. Osteoblasts themselves are thought to originate from osteogenic stem cells—close relatives of fibroblasts.

Fig. 17-46. An osteoclast, a giant cell that degrades the bone matrix. The "ruffled border" is the site of secretion of acids (to dissolve the bone mineral) and Hydrolases (to digest the organic Components of the matrix). The Cell is shown in cross section. Osteoclasts are variable in shape, motile, and frequently extend processes to resorb bone at multiple sites simultaneously. Osteoclasts develop from monocytes and can be regarded as specialized macrophages. (R. V. Krstic: ULTRASTRUCTURE OF THE Mammalian Cell: An Atlas. Berlin: Springer, 1979.)

Fig. 17-47. Diagram illustrating the remodeling process of compact bone. Osteoclasts, acting in small groups, tunnel through old bone, advancing by approximately 50 µm per day. They are followed by osteoblasts, which line the tunnel walls and begin forming new bone by depositing matrix at a rate of 1-2 µm per day. Simultaneously, capillaries sprout along the axis of the tunnel. Eventually, the tunnel becomes progressively filled with concentric layers of new bone, leaving only a narrow central canal open. Each such canal not only provides access for osteoblasts and osteoclasts but also contains one or more blood vessels that supply the nutrients required for the survival of bone cells. Typically, 5% to 10% of bone is replaced in this manner each year in a healthy adult mammal. (After Z. F. G. Jaworski, B. Duck, G. Sekaly, J. Anat., 133, 397-405, 1981.)

Fig. 17-48. Micrograph of a Cytology/practical/72.html">Cross section of compact bone from a long bone, showing the outlines of tunnels created by osteoclasts and subsequently filled in by osteoblasts. The section was prepared by the ground-section method. The dense matrix is preserved, but the cells are destroyed; however, the lacunae and canaliculi that were once occupied by osteocytes and their processes are clearly visible. The alternating light and dark concentric rings correspond to the changing orientation of collagen fibers in successive layers of bone matrix laid down by osteoblasts lining the walls at different periods in the individual's life. (This appearance is obtained by observing the specimen between two partially crossed polarizing filters.) Note that a portion of an older system of concentric bone lamellae (at the bottom right, with a narrow central canal) has been partially resorbed and replaced by a newer system in which the central canal remains wide—apparently because it is still in The process of being filled.

Many aspects of these processes remain poorly understood. Bones, for example, possess a remarkable capacity to remodel their structure to adapt to mechanical loads. This implies that matrix deposition and resorption are regulated in some way by local mechanical stresses. The specific mechanisms that determine whether matrix will be deposited on a given bone surface by osteoblasts or resorbed by osteoclasts remain unknown. Growth factors secreted by bone cells embedded deep within the matrix likely play an important role in this regulation (section 17.7.1). These factors could be released upon matrix degradation or in response to appropriate mechanical loading.

17.8.3. In the developing organism, osteoclasts destroy cartilage to pave the way for bone growth [44]

The replacement of cartilage by bone during organismal development is also believed to depend on osteoclast activity. As cartilage matures, cells in certain regions enlarge significantly at the expense of the surrounding matrix, and the matrix itself becomes mineralized like bone through the deposition of calcium phosphate crystals. At the same time, chondrocytes in these areas swell and die, leaving large empty spaces. Osteoclasts and blood vessels invade these spaces, destroying the remnants of the cartilage matrix, while osteoblasts following closely behind begin to deposit bone matrix. The only remnant of cartilage in the long bones of an adult animal is a thin layer forming a smooth covering in joint regions where one bone articulates with another (Fig. 17-49).

However, cells capable of forming new cartilage persist in the connective tissue surrounding the bone. In the event of a bone fracture, cells from the adjacent region will effect repairs by hastily reproducing the initial embryonic process: cartilage is first deposited to fill the gap, and is subsequently replaced by bone.

17.8.4. Body structure is stabilized by its connective-tissue framework, as well as by selective Cell Adhesion [45]

An individual bone, much like the organism as a whole, is a dynamic system that maintains its structure through a balance between the opposing functions of various specialized cells. Any dynamic system poses The problem of stability, and this leads to the general question of how body structure is maintained. We have seen how cells in various types of tissues maintain their differentiated state, how new cells are formed to replace lost ones, and how the extracellular matrix is remodeled and renewed. But why do cells of different types not gradually mix together, and why does chaos not ensue? Why does the overall structure not become distorted or change its proportions as old elements are replaced by new ones?

Fig. 17-49. Scheme of The Development of a long bone (such as the Femur or humerus) from a miniature cartilaginous model. Uncalcified cartilage is shown in light gray, calcified cartilage in dark gray, bone in black, and blood vessels in red. Cartilage does not turn into bone, but is gradually replaced by it through the activity of osteoclasts and osteoblasts, which invade the cartilage along with blood vessels. Osteoclasts destroy the cartilaginous and bone matrix, while osteoblasts secrete bone matrix. The process of ossification begins in the embryo and is completed only by the end of Puberty. The resulting bone consists of a thick-walled hollow cylinder of compact bone surrounding a central cavity filled with bone marrow. Note that not all bones develop in this way. For example, the flat bones of the Skull form directly as bony plates, without a preliminary cartilaginous model stage. (D. W. Fawcett; A Textbook of Histology, 11th ed. Philadelphia: Saunders, 1986, with modifications.)

Of course, over time the organism does become deformed to some extent: this is one of the manifestations of Aging. Yet what is striking is how small these changes are. Despite continuous remodeling, the skeleton provides a rigid structure whose dimensions remain nearly unchanged. This can be explained by the fact that various PARTS OF THE bone are renewed not all at once, but little by little, much like repairing a building by replacing bricks one by one. In addition to the conservative nature of this renewal process, active homeostatic mechanisms are also at work. For example, minor deviations of a bone from its normal shape alter the pattern of mechanical loads, and these loads regulate tissue remodeling in order to restore the bone to its normal shape (Fig. 17-50).

The growth and renewal of many soft parts of the body are homeostatically controlled so that each detail corresponds precisely to its designated place. The epidermis proliferates to cover the entire body surface, and once this goal is achieved, cell migration ceases as a result of contact inhibition (Section 11.6.8); connective tissue proliferates just enough to fill a wound defect, and so on. But something more than the Regulation of Cell number is required. The renewal of differentiated cells of various types must maintain not only the correct numerical ratios between them, but also their proper relative arrangement. Tissue renewal inevitably involves cell movements, and these movements must be restricted in some way.

Fig. 17-50. Diagram illustrating the remodeling process of a long leg bone following a malunited fracture. Deformations in the newly formed bone lead to unusual stresses. In areas of increased bone compression, The rate of Bone Formation increases relative to the rate of its destruction. Conversely, where compression is reduced, the rate of bone growth decreases. Thus, the bone gradually remodels, approaching its normal state.

Limiting factors can be of various kinds. For example, glands and other aggregations of specialized cells are often enclosed in dense connective-tissue capsules. Cells of certain types die if they find themselves outside their usual environment and are deprived of specific growth factors presumably required for their survival. Perhaps the most important mechanism holding various cells in their proper places is selective adhesion: Cells of the same type tend to "stick" to one another (Section 14.3.5), forming either dense masses (as, for example, in smooth Muscle) or epithelial layers (the lining of the intestine, etc.).

As discussed in Section 14.3.7, this mechanism allows dissociated epidermal cells, for instance, to spontaneously reaggregate into a properly structured epithelium. More generally, continuous sheets of epithelial cells demarcate separate Regions of the body, thereby maintaining cellular compartmentalization and restricting their distribution to the appropriate territories.

It is clear how complex and delicate the control and coordination mechanisms must be to preserve body structure and tissue Organization amidst constant remodeling and renewal. The critical role of these mechanisms is starkly and brutally revealed when they break down, as we will see in the final chapter of the book when discussing the problem of Cancer.

Conclusion

Cartilage and bone consist of cells embedded in a dense matrix. Cartilage, with its pliable matrix, is capable of interstitial growth, whereas rigid bone can grow only by the deposition of new material on its surface. Nevertheless, bone undergoes continuous remodeling through the concerted action of osteoclasts, which degrade the matrix, and osteoblasts, which synthesize it. Some osteoblasts become entombed within the matrix, transform into osteocytes, and participate in regulating bone matrix turnover. Most long bones develop from miniature cartilaginous "models" that serve as templates for the deposition of bone substance as they grow, driven by the combined activity of osteoblasts and osteoclasts. Bone fracture healing in the adult occurs in a similar manner: the break is first filled with cartilage, which is later replaced by bone. Although Bone tissue, like most other tissues, is continuously renewed, this dynamic process is regulated so that the overall macroscopic structure remains unchanged. Thus, through this and other mechanisms (such as selective intercellular adhesion), body organization is robustly maintained despite the constant replacement of nearly all its components.

Appendix. Census of Cell Types of the Adult Human Body

How many different cell types exist in the adult human body? A large textbook of histology typically mentions about 200 types deserving of a distinct name. These traditional names—unlike, for example, the names of spectral colors—do not refer to separate parts of some arbitrarily divided continuum; most of them correspond to discrete, clearly distinguishable categories. Within any given category, some variations are often observed: Skeletal Muscle fibers that move the Eyeball are much smaller than the fibers of major leg Muscles; auditory Hair cells in different regions of the cochlea may be tuned to different sound frequencies, and so on. However, there are no continuous transitions between such disparate cell types of the adult body as, for example, a muscle fiber and an auditory hair cell.

Traditional histological Classification is based on cell shape and structure as visible under the Microscope, and on its chemical nature, very crudely assessed by staining with various Dyes. Finer Methods make it possible to distinguish new subclasses within the framework of traditional classification. Thus, modern immunology has established that the former category of "lymphocytes" encompasses more than ten different cell types (see Chapter 18). Similarly, pharmacological and physiological studies have shown that There are many distinct varieties of smooth muscle cells; for example, in the uterine wall these cells are highly sensitive to estrogen, and in the late stages of Pregnancy to oxytocin, whereas analogous cells in the intestinal wall lack these properties. Important differences of another kind between cells are revealed in embryological experiments like those discussed in Chapter 16; these have shown that in many cases outwardly similar cells from different regions of the body are nonequivalent—in the sense that they possess internal differences in their potential for further development and in their ability to influence other cells. For example, connective-tissue cells from different regions of the dermis must be nonequivalent, since under their influence the overlying epidermal cells behave differently (Section 16.6.4). Within categories such as "fibroblast," there are likewise likely many subtypes whose chemical differences cannot yet be directly detected.

In view of the above, any cell classification will be somewhat arbitrary regarding the level of detail in subdivision. Our census includes only those cell types of the adult human body that are considered distinct in any major modern textbook of histology. They are grouped in approximate accordance with their function. We have made no attempt to subdivide the class of Central Nervous system Neurons. When a distinct cell type, such as a keratinizing epidermal cell (keratinocyte), successively acquires different names as it matures, we provide only two of them—one for the differentiating cell and one for the stem cell. Subject to these qualifications, the 210 cell names contained in the census constitute a more or less comprehensive list of the various ways The Genome is expressed as phenotypes in normal adult human cells.

Keratinizing epithelial cells

Epidermal keratinocyte (= differentiating epidermal cell)

Epidermal basal cell (stem) Nail keratinocyte Nail bed basal cell (stem) Hair shaft cells medulla cell cortex cell

cuticle cell Hair ROOT sheath cells

cuticular

Huxley's layer

Henle's layer

Outer hair matrix cell (stem cell)

Cells of wet stratified barrier epithelia

Superficial epithelial cell of stratified squamous epithelium of the cornea, Tongue, Oral Cavity, Esophagus, anal canal, distal Urethra, and Vagina

Basal cell of the same epithelial types (stem cell)

Epithelial cell of the Urinary Tract (lining the Urinary Bladder and urinary passages)

Epithelial cells with exocrine function

Salivary gland cell

Mucous cell (secretion rich in Polysaccharides)

Serous cell (secretion rich in glycoprotein Enzymes)

Von Ebner's gland cell in the tongue (secretion washes taste buds)

Mammary gland cell, milk-secreting

Lacrimal gland cell, tear-secreting

Ceruminous gland cell of the ear, cerumen-secreting

Eccrine sweat gland cell, glycoprotein-secreting (dark cell)

Eccrine sweat gland cell, small molecule-secreting (clear cell)

Apocrine sweat gland cell (secretes odoriferous substances, responsive to Sex Hormones)

Cell of the gland of Moll in the eyelid (specialized sweat gland)

Sebaceous gland cell, lipid-rich sebum-secreting

Bowman's gland cell in the Nose (secretes fluid to wash the olfactory epithelium)

Brunner's gland cell in the duodenum, secreting an alkaline solution of mucus and enzymes

Seminal Vesicle cell, secreting seminal fluid components, including fructose (as an energy source for sperm motility)

Prostate Gland cell, secreting Other components of seminal fluid; Bulbourethral gland cell, secreting mucus

Bartholin's gland cell, secreting lubricating fluid for the vagina

Littre's gland cell, mucus-secreting

Endometrial cell, secreting primarily CARBOHYDRATES

Isolated mucus-secreting goblet cell of the respiratory and digestive tracts

Mucosal lining cell

of The Stomach

Gastric chief cell (secretes pepsinogen)

Gastric parietal cell (secretes HCl)

Pancreatic acinar cell (secretes digestive enzymes and bicarbonate)

Paneth cell in the Small Intestine (secretes Lysozyme)

Type II alveolar cell in the lung (secretes surfactant)

Cell

Club cell in the lung (function unknown)

Hormone-secreting cells

Growth Hormone-secreting cells of the anterior pituitary

follicle-stimulating hormone luteinizing hormone prolactin

adrenocorticotropic hormone thyroid-stimulating hormone

Melanocyte-stimulating hormone-secreting cell of the intermediate pituitary

Cells of the posterior pituitary secreting

oxytocin

vasopressin

Gastrointestinal cells secreting

serotonin

endorphin

Somatostatin

gastrin

secretin

cholecystokinin

Insulin

Glucagon

bombesin

Thyroid cells secreting the thyroid hormone Calcitonin

Parathyroid gland cells secreting parathyroid hormone; oxyphil cells (function unknown)

Adrenal gland cells,

secreting

adrenaline

noradrenaline

Steroid Hormones

mineralocorticoids

glucocorticoids

Gonadal cells secreting

testosterone (Leydig cells in the Testes)

estrogen (theca interna cells of the ovarian follicle)

Cells of the juxtaglomerular apparatus of the Kidney: juxtaglomerular cells (secrete renin); macula densa cells, functionally probably similar; peripolar cells, possibly involved in Erythropoietin secretion; mesangial cells

Epithelial absorptive cells of the gastrointestinal tract, exocrine glands, and urogenital tract

Microvillar brush border cell (in the small intestine)

Striated duct cell of an exocrine gland

Gallbladder epithelial cell

Brush border cell of the proximal renal tubule

Distal renal tubule cell

Non-ciliated cell of the vas deferens

Epididymal cells: principal cell, basal cell

Cells responsible for metabolic processes and storage of reserve Materials

Hepatocyte (Liver cell)

Adipocytes: white fat cell, brown fat cell, hepatic lipocyte

Epithelial cells performing primarily a barrier function, lining the Lungs, intestine, exocrine glands, and urogenital tract

Type I pneumocyte (lining the air space of the lung)

Pancreatic duct cell (centroacinar cell)

Non-striated duct cells of Sweat Glands, Salivary Glands, Mammary Glands, etc.

Renal glomerulus parietal cell

Renal glomerulus podocyte

Cell of the thin segment of the Loop of Henle (in the kidney)

Collecting duct cell (in the kidney)

Duct cell of the seminal vesicle, prostate gland, etc.

Epithelial cells lining closed internal cavities

Endothelial cells of Blood and Lymphatic vessels

fenestrated

continuous

splenic

Synovial cells (lining joint cavities and secreting mainly hyaluronic acid)

Serous cells (lining the peritoneal, pleural, and pericardial cavities)

Squamous cells lining the perilymphatic space of the ear, squamous cell

columnar cell of the endolymphatic sac with microvilli without microvilli

"dark" cell

cell of the vestibular membrane

basal cell of the stria vascularis

marginal cell of the stria vascularis

Claudius cell

Boettcher cell

Choroid plexus cell (secretes CEREBROSPINAL FLUID)

Flattened cell of the pia mater and arachnoid mater Cells of the ciliary epithelium of the eye

pigmented

non-pigmented

Corneal "endothelial" cell

Ciliated cells with a propulsive function

Cells of the respiratory tract

Cells of the oviduct and endometrium (in females)

Cells of the rete Testis and vas deferens (in males)

Ependymal cells lining the Brain cavities

Extracellular matrix-secreting cells

Epithelial

Ameloblast (secretes tooth enamel)

Planum semilunatum cell of the vestibular apparatus (secretes proteoglycan)

Interdental cell of The Organ of Corti (secretes the substance of the tectorial membrane overlying the hair cells of this organ)

Non-epithelial (connective tissue)

Fibroblasts (of loose connective tissue, cornea, tendons, reticular tissue of bone marrow, etc.)

Pericyte of a blood capillary

Cell of The Nucleus pulposus of the intervertebral disc

Cementoblast/cementocyte (secretes tooth root cementum, similar to bone substance)

Odontoblast/odontocyte (secretes tooth dentin)

Chondrocytes of hyaline cartilage fibrocartilage elastic cartilage Osteoblast/osteocyte

Primary osteogenic cell (osteoblast stem cell)

Hyalocyte of the vitreous body of the eye

Stellate cell of the perilymphatic space of the ear

Contractile cells

Skeletal muscle cells: red (slow), white (fast), intermediate

muscle spindle with nuclear bag

muscle spindle with nuclear chain

satellite cell (stem cell)

Cardiac muscle cells: conventional

nodal (pacemaker)

Purkinje fibers

Smooth muscle cells (various)

Myoepithelial cells

of the iris

of exocrine glands

Blood and immune system cells

Erythrocyte

Megakaryocyte

Macrophages and related cells

monocyte

connective tissue macrophages (various)

Langerhans cell (in the epidermis)

osteoclast (in bone)

dendritic cell (in lymphoid tissues)

microglial cell (in the central nervous system)

Neutrophil, Eosinophil, Basophil, Mast cell, T lymphocytes

T helper cell

T-suppressor

T-killer

B lymphocytes

producing

IgM

IgG

IgA

IgE

Killer cell

Stem cells and committed progenitors for blood and immune system (various)

Sensory transducers

Photoreceptors

rods cones

blue-sensitive

green-sensitive

red-sensitive

Auditory receptor cells

inner hair cells of the organ of Corti

outer hair cells of the organ of Corti

Acceleration and gravity receptors

hair cells of the vestibular apparatus

type I

type II

Taste receptor cells taste bud cell

type II

Olfactory receptor cells (olfactory neurons)

basal cell of the olfactory epithelium (stem cell for olfactory neurons)

Blood pH receptors (carotid body cells)

type I

type II

Tactile receptor cells

Merkel cell in the epidermis

primary tactile neurons (various)

Thermoreceptor cells

primary thermoreceptive neurons

cold-sensitive

warmth-sensitive

Pain receptors

primary pain-sensing neurons (various)

Position and tension receptors in The Musculoskeletal System (primary proprioceptive neurons, various)

Autonomic neurons

Cholinergic (various)

Adrenergic (various)

Peptidergic (various)

Supporting cells of Sense Organs and peripheral neurons

Supporting cells of the organ of Corti

inner pillar cell

outer pillar cell

inner phalangeal cell

outer phalangeal cell

border cell

Hensen cell

Supporting cell of vestibular system

Supporting cell of taste bud (taste bud cell, type I)

Supporting cell of olfactory epithelium

Schwann cell

Satellite cell (encapsulating peripheral neurons)

Enteric glial cell

Neurons and glial cells of the central nervous system

Neurons (vast variety of types, still poorly classified)

Glial cells

astrocytes (various)

oligodendrocyte

Lens cells

Anterior lens epithelial cell

Lens fiber cell (crystallin-containing cell)

Pigment cells

Melanocyte

Retinal pigment epithelial cell

Germ Cells

Oogonium/oocyte

Spermatogonium (stem cell for spermatocyte)

Spermatocyte

Nurse cells

Ovarian follicle cell

Sertoli cell (in the testis)

Thymic epithelial cell

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