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55-468: The dermis is composed of three major types of cells : fibroblasts , macrophages , and mast cells . Apart from these cells, the dermis is also composed of matrix components such as collagen (which provides strength ), elastin (which provides elasticity ), and extrafibrillar matrix , an extracellular gel-like substance primarily composed of glycosaminoglycans (most notably hyaluronan ), proteoglycans , and glycoproteins . The papillary dermis

110-510: A Honey-comb , but that the pores of it were not regular". To further support his theory, Matthias Schleiden and Theodor Schwann both also studied cells of both animal and plants. What they discovered were significant differences between the two types of cells. This put forth the idea that cells were not only fundamental to plants, but animals as well. Langer%27s lines Langer's lines , Langer lines of skin tension , or sometimes called cleavage lines , are topological lines drawn on

165-420: A membrane that envelops the cell, regulates what moves in and out (selectively permeable), and maintains the electric potential of the cell . Inside the membrane, the cytoplasm takes up most of the cell's volume. Except red blood cells , which lack a cell nucleus and most organelles to accommodate maximum space for hemoglobin , all cells possess DNA , the hereditary material of genes , and RNA , containing

220-641: A nucleus , and prokaryotic cells , which lack a nucleus but have a nucleoid region. Prokaryotes are single-celled organisms such as bacteria , whereas eukaryotes can be either single-celled, such as amoebae , or multicellular , such as some algae , plants , animals , and fungi . Eukaryotic cells contain organelles including mitochondria , which provide energy for cell functions; chloroplasts , which create sugars by photosynthesis , in plants; and ribosomes , which synthesise proteins. Cells were discovered by Robert Hooke in 1665, who named them after their resemblance to cells inhabited by Christian monks in

275-516: A nucleus , and other membrane-bound organelles . The DNA of a prokaryotic cell consists of a single circular chromosome that is in direct contact with the cytoplasm . The nuclear region in the cytoplasm is called the nucleoid . Most prokaryotes are the smallest of all organisms, ranging from 0.5 to 2.0 μm in diameter. A prokaryotic cell has three regions: Plants , animals , fungi , slime moulds , protozoa , and algae are all eukaryotic . These cells are about fifteen times wider than

330-454: A cell. Some (such as the nucleus and Golgi apparatus ) are typically solitary, while others (such as mitochondria , chloroplasts , peroxisomes and lysosomes ) can be numerous (hundreds to thousands). The cytosol is the gelatinous fluid that fills the cell and surrounds the organelles. Many cells also have structures which exist wholly or partially outside the cell membrane. These structures are notable because they are not protected from

385-434: A discrete nucleus, usually with additional genetic material in some organelles like mitochondria and chloroplasts (see endosymbiotic theory ). A human cell has genetic material contained in the cell nucleus (the nuclear genome ) and in the mitochondria (the mitochondrial genome ). In humans, the nuclear genome is divided into 46 linear DNA molecules called chromosomes , including 22 homologous chromosome pairs and

440-407: A diverse range of single-celled organisms. The plants were created around 1.6 billion years ago with a second episode of symbiogenesis that added chloroplasts , derived from cyanobacteria . In 1665, Robert Hooke examined a thin slice of cork under his microscope , and saw a structure of small enclosures. He wrote "I could exceeding plainly perceive it to be all perforated and porous, much like

495-414: A fluid mosaic membrane. Embedded within this membrane is a macromolecular structure called the porosome the universal secretory portal in cells and a variety of protein molecules that act as channels and pumps that move different molecules into and out of the cell. The membrane is semi-permeable, and selectively permeable, in that it can either let a substance ( molecule or ion ) pass through freely, to

550-405: A functional three-dimensional protein molecule. Unicellular organisms can move in order to find food or escape predators. Common mechanisms of motion include flagella and cilia . In multicellular organisms, cells can move during processes such as wound healing, the immune response and cancer metastasis . For example, in wound healing in animals, white blood cells move to the wound site to kill

605-427: A limited extent or not at all. Cell surface membranes also contain receptor proteins that allow cells to detect external signaling molecules such as hormones . The cytoskeleton acts to organize and maintain the cell's shape; anchors organelles in place; helps during endocytosis , the uptake of external materials by a cell, and cytokinesis , the separation of daughter cells after cell division ; and moves parts of

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660-410: A map of the human body. They are parallel to the natural orientation of collagen fibers in the dermis , and generally parallel to the underlying muscle fibers. Langer's lines have relevance to forensic science and the development of surgical techniques. The lines were first discovered in 1861 by Austrian anatomist Karl Langer (1819–1887), though he cited the surgeon Baron Dupuytren as being

715-427: A monastery. Cell theory , developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann , states that all organisms are composed of one or more cells, that cells are the fundamental unit of structure and function in all living organisms, and that all cells come from pre-existing cells. Cells are broadly categorized into two types: eukaryotic cells , which possess a nucleus , and prokaryotic cells , which lack

770-435: A nucleus but have a nucleoid region. Prokaryotes are single-celled organisms , whereas eukaryotes can be either single-celled or multicellular . Prokaryotes include bacteria and archaea , two of the three domains of life . Prokaryotic cells were the first form of life on Earth, characterized by having vital biological processes including cell signaling . They are simpler and smaller than eukaryotic cells, and lack

825-436: A pair of sex chromosomes . The mitochondrial genome is a circular DNA molecule distinct from nuclear DNA. Although the mitochondrial DNA is very small compared to nuclear chromosomes, it codes for 13 proteins involved in mitochondrial energy production and specific tRNAs. Foreign genetic material (most commonly DNA) can also be artificially introduced into the cell by a process called transfection . This can be transient, if

880-461: A process called eukaryogenesis . This is widely agreed to have involved symbiogenesis , in which archaea and bacteria came together to create the first eukaryotic common ancestor. This cell had a new level of complexity and capability, with a nucleus and facultatively aerobic mitochondria . It evolved some 2 billion years ago into a population of single-celled organisms that included the last eukaryotic common ancestor, gaining capabilities along

935-443: A process of nuclear division, called mitosis , followed by division of the cell, called cytokinesis . A diploid cell may also undergo meiosis to produce haploid cells, usually four. Haploid cells serve as gametes in multicellular organisms, fusing to form new diploid cells. DNA replication , or the process of duplicating a cell's genome, always happens when a cell divides through mitosis or binary fission. This occurs during

990-399: A tendency to pucker and remain obvious, although sometimes this is unavoidable. The orientation of stab wounds relative to Langer's lines can have a considerable impact upon the presentation of the wound. Langer's lines include breast static tension lines, which mark a guide for breast surgery incisions. Keloids are more common when incision is given across Langer's lines. Sometimes

1045-411: A thicker and more complex epidermis, increased sensory abilities, and the absence of hair and sebaceous glands. The ridges increase friction for improved grasping. Dermal papillae also play a pivotal role in hair formation, growth and cycling. In mucous membranes , the equivalent structures to dermal papillae are generally termed "connective tissue papillae", which interdigitate with the rete pegs of

1100-458: A typical prokaryote and can be as much as a thousand times greater in volume. The main distinguishing feature of eukaryotes as compared to prokaryotes is compartmentalization : the presence of membrane-bound organelles (compartments) in which specific activities take place. Most important among these is a cell nucleus , an organelle that houses the cell's DNA . This nucleus gives the eukaryote its name, which means "true kernel (nucleus)". Some of

1155-460: Is encoded in its DNA sequence. RNA is used for information transport (e.g., mRNA ) and enzymatic functions (e.g., ribosomal RNA). Transfer RNA (tRNA) molecules are used to add amino acids during protein translation . Prokaryotic genetic material is organized in a simple circular bacterial chromosome in the nucleoid region of the cytoplasm. Eukaryotic genetic material is divided into different, linear molecules called chromosomes inside

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1210-697: Is detected. Diverse repair processes have evolved in organisms ranging from bacteria to humans. The widespread prevalence of these repair processes indicates the importance of maintaining cellular DNA in an undamaged state in order to avoid cell death or errors of replication due to damage that could lead to mutation . E. coli bacteria are a well-studied example of a cellular organism with diverse well-defined DNA repair processes. These include: nucleotide excision repair , DNA mismatch repair , non-homologous end joining of double-strand breaks, recombinational repair and light-dependent repair ( photoreactivation ). Between successive cell divisions, cells grow through

1265-576: Is from cyanobacteria -like organisms that lived between 3 and 3.5 billion years ago. Other early fossils of multicellular organisms include the contested Grypania spiralis and the fossils of the black shales of the Palaeoproterozoic Francevillian Group Fossil B Formation in Gabon . The evolution of multicellularity from unicellular ancestors has been replicated in the laboratory, in evolution experiments using predation as

1320-781: Is less well-studied but is involved in the maintenance of cell shape, polarity and cytokinesis. The subunit protein of microfilaments is a small, monomeric protein called actin . The subunit of microtubules is a dimeric molecule called tubulin . Intermediate filaments are heteropolymers whose subunits vary among the cell types in different tissues. Some of the subunit proteins of intermediate filaments include vimentin , desmin , lamin (lamins A, B and C), keratin (multiple acidic and basic keratins), and neurofilament proteins ( NF–L , NF–M ). Two different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Cells use DNA for their long-term information storage. The biological information contained in an organism

1375-510: Is present in some bacteria outside the cell membrane and cell wall. The capsule may be polysaccharide as in pneumococci , meningococci or polypeptide as Bacillus anthracis or hyaluronic acid as in streptococci . Capsules are not marked by normal staining protocols and can be detected by India ink or methyl blue , which allows for higher contrast between the cells for observation. Flagella are organelles for cellular mobility. The bacterial flagellum stretches from cytoplasm through

1430-577: Is the basic structural and functional unit of all forms of life . Every cell consists of cytoplasm enclosed within a membrane ; many cells contain organelles , each with a specific function. The term comes from the Latin word cellula meaning 'small room'. Most cells are only visible under a microscope . Cells emerged on Earth about 4 billion years ago. All cells are capable of replication , protein synthesis , and motility . Cells are broadly categorized into two types: eukaryotic cells , which possess

1485-599: Is the uppermost layer of the dermis. It intertwines with the rete ridges of the epidermis and is composed of fine and loosely arranged collagen fibers. The papillary region is composed of loose areolar connective tissue . It is named for its fingerlike projections called papillae or dermal papillae specifically, that extend toward the epidermis and contain either terminal networks of blood capillaries or tactile Meissner's corpuscles . The dermal papillae (DP) (singular papilla , diminutive of Latin papula , 'pimple') are small, nipple-like extensions (or interdigitations) of

1540-401: The selective pressure . The origin of cells has to do with the origin of life , which began the history of life on Earth. Small molecules needed for life may have been carried to Earth on meteorites, created at deep-sea vents , or synthesized by lightning in a reducing atmosphere . There is little experimental data defining what the first self-replicating forms were. RNA may have been

1595-611: The DNA is not inserted into the cell's genome , or stable, if it is. Certain viruses also insert their genetic material into the genome. Organelles are parts of the cell that are adapted and/or specialized for carrying out one or more vital functions, analogous to the organs of the human body (such as the heart, lung, and kidney, with each organ performing a different function). Both eukaryotic and prokaryotic cells have organelles, but prokaryotic organelles are generally simpler and are not membrane-bound. There are several types of organelles in

1650-467: The S phase of the cell cycle . In meiosis, the DNA is replicated only once, while the cell divides twice. DNA replication only occurs before meiosis I . DNA replication does not occur when the cells divide the second time, in meiosis II . Replication, like all cellular activities, requires specialized proteins for carrying out the job. Cells of all organisms contain enzyme systems that scan their DNA for damage and carry out repair processes when it

1705-488: The attachment of bacteria to specific receptors on human cells ( cell adhesion ). There are special types of pili involved in bacterial conjugation . Cell division involves a single cell (called a mother cell ) dividing into two daughter cells. This leads to growth in multicellular organisms (the growth of tissue ) and to procreation ( vegetative reproduction ) in unicellular organisms . Prokaryotic cells divide by binary fission , while eukaryotic cells usually undergo

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1760-416: The cell in processes of growth and mobility. The eukaryotic cytoskeleton is composed of microtubules , intermediate filaments and microfilaments . In the cytoskeleton of a neuron the intermediate filaments are known as neurofilaments . There are a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments. The prokaryotic cytoskeleton

1815-444: The cell membrane(s) and extrudes through the cell wall. They are long and thick thread-like appendages, protein in nature. A different type of flagellum is found in archaea and a different type is found in eukaryotes. A fimbria (plural fimbriae also known as a pilus , plural pili) is a short, thin, hair-like filament found on the surface of bacteria. Fimbriae are formed of a protein called pilin ( antigenic ) and are responsible for

1870-449: The cell, glucose is broken down to make adenosine triphosphate ( ATP ), a molecule that possesses readily available energy, through two different pathways. In plant cells, chloroplasts create sugars by photosynthesis , using the energy of light to join molecules of water and carbon dioxide . Cells are capable of synthesizing new proteins, which are essential for the modulation and maintenance of cellular activities. This process involves

1925-436: The cell. mRNA molecules bind to protein-RNA complexes called ribosomes located in the cytosol , where they are translated into polypeptide sequences. The ribosome mediates the formation of a polypeptide sequence based on the mRNA sequence. The mRNA sequence directly relates to the polypeptide sequence by binding to transfer RNA (tRNA) adapter molecules in binding pockets within the ribosome. The new polypeptide then folds into

1980-911: The cytoskeleton. In August 2020, scientists described one way cells—in particular cells of a slime mold and mouse pancreatic cancer-derived cells—are able to navigate efficiently through a body and identify the best routes through complex mazes: generating gradients after breaking down diffused chemoattractants which enable them to sense upcoming maze junctions before reaching them, including around corners. Multicellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms . In complex multicellular organisms, cells specialize into different cell types that are adapted to particular functions. In mammals, major cell types include skin cells , muscle cells , neurons , blood cells , fibroblasts , stem cells , and others. Cell types differ both in appearance and function, yet are genetically identical. Cells are able to be of

2035-548: The dense concentration of collagenous , elastic , and reticular fibers that weave throughout it. These protein fibers give the dermis its properties of strength , extensibility , and elasticity. Within the reticular region are the roots of the hair , sebaceous glands , sweat glands , receptors , nails , and blood vessels. The orientation of collagen fibers within the reticular dermis creates lines of tension called Langer's lines , which are of some relevance in surgery and wound healing. Cell (biology) The cell

2090-532: The dermis into the epidermis . At the surface of the skin in hands and feet, they appear as epidermal, papillary or friction ridges (colloquially known as fingerprints ). Blood vessels in the dermal papillae nourish all hair follicles and bring nutrients and oxygen to the lower layers of epidermal cells. The pattern of ridges produced in hands and feet are partly genetically determined features that are developed before birth. They remain substantially unaltered (except in size) throughout life, and therefore determine

2145-452: The direction of Langer's lines within a specific area of the skin is important for surgical operations, particularly cosmetic surgery . If a surgeon has a choice about where and in what direction to place an incision, they may choose to cut in the direction of Langer's lines. Incisions made parallel to Langer's lines may heal better and produce less scarring than those that cut across. Conversely, incisions perpendicular to Langer's lines have

2200-493: The earliest self-replicating molecule , as it can both store genetic information and catalyze chemical reactions. Cells emerged around 4 billion years ago. The first cells were most likely heterotrophs . The early cell membranes were probably simpler and more permeable than modern ones, with only a single fatty acid chain per lipid. Lipids spontaneously form bilayered vesicles in water, and could have preceded RNA. Eukaryotic cells were created some 2.2 billion years ago in

2255-464: The exact direction of the collagen fibers are unknown, because in some regions of the body there are differences between different individuals. Also, the lines described by Kraissl differ in some ways from Langer's lines, particularly on the face. Other authors have created topological skin maps. Kraissl's lines differ from Langer's lines in that while Langer's lines were defined in cadavers, Kraissl's lines have been defined in living individuals. Also,

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2310-633: The external environment by the cell membrane. In order to assemble these structures, their components must be carried across the cell membrane by export processes. Many types of prokaryotic and eukaryotic cells have a cell wall . The cell wall acts to protect the cell mechanically and chemically from its environment, and is an additional layer of protection to the cell membrane. Different types of cell have cell walls made up of different materials; plant cell walls are primarily made up of cellulose , fungi cell walls are made up of chitin and bacteria cell walls are made up of peptidoglycan . A gelatinous capsule

2365-430: The first to recognise the phenomenon. Langer punctured numerous holes at short distances from each other into the skin of a cadaver with a tool that had a circular-shaped tip, similar to an ice pick . He noticed that the resultant punctures in the skin had ellipsoidal shapes. From this testing he observed patterns and was able to determine "line directions" by the longer axes of the ellipsoidal holes and lines. Knowing

2420-465: The formation of new protein molecules from amino acid building blocks based on information encoded in DNA/RNA. Protein synthesis generally consists of two major steps: transcription and translation . Transcription is the process where genetic information in DNA is used to produce a complementary RNA strand. This RNA strand is then processed to give messenger RNA (mRNA), which is free to migrate through

2475-523: The functioning of cellular metabolism. Cell metabolism is the process by which individual cells process nutrient molecules. Metabolism has two distinct divisions: catabolism , in which the cell breaks down complex molecules to produce energy and reducing power , and anabolism , in which the cell uses energy and reducing power to construct complex molecules and perform other biological functions. Complex sugars can be broken down into simpler sugar molecules called monosaccharides such as glucose . Once inside

2530-412: The increase in the surface area prevents the dermal and epidermal layers from separating by strengthening the junction between them. With age, the papillae tend to flatten and sometimes increase in number. The skin of the hands and fingers and the feet and toes is known by forensic scientists as friction ridge skin. It is known by anatomists as thick skin, volar skin or hairless skin. It has raised ridges,

2585-399: The information necessary to build various proteins such as enzymes , the cell's primary machinery. There are also other kinds of biomolecules in cells. This article lists these primary cellular components , then briefly describes their function. The cell membrane , or plasma membrane, is a selectively permeable biological membrane that surrounds the cytoplasm of a cell. In animals,

2640-432: The microorganisms that cause infection. Cell motility involves many receptors, crosslinking, bundling, binding, adhesion, motor and other proteins. The process is divided into three steps: protrusion of the leading edge of the cell, adhesion of the leading edge and de-adhesion at the cell body and rear, and cytoskeletal contraction to pull the cell forward. Each step is driven by physical forces generated by unique segments of

2695-455: The other differences are: Many groups of eukaryotes are single-celled. Among the many-celled groups are animals and plants. The number of cells in these groups vary with species; it has been estimated that the human body contains around 37 trillion (3.72×10 ) cells, and more recent studies put this number at around 30 trillion (~36 trillion cells in the male, ~28 trillion in the female). All cells, whether prokaryotic or eukaryotic , have

2750-467: The patterns of fingerprints, making them useful in certain functions of personal identification. The dermal papillae are part of the uppermost layer of the dermis, the papillary dermis, and the ridges they form greatly increase the surface area between the dermis and epidermis. Because the main function of the dermis is to support the epidermis, this greatly increases the exchange of oxygen, nutrients, and waste products between these two layers. Additionally,

2805-409: The plasma membrane is the outer boundary of the cell, while in plants and prokaryotes it is usually covered by a cell wall . This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of phospholipids , which are amphiphilic (partly hydrophobic and partly hydrophilic ). Hence, the layer is called a phospholipid bilayer , or sometimes

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2860-443: The same genotype but of different cell type due to the differential expression of the genes they contain. Most distinct cell types arise from a single totipotent cell, called a zygote , that differentiates into hundreds of different cell types during the course of development . Differentiation of cells is driven by different environmental cues (such as cell–cell interaction) and intrinsic differences (such as those caused by

2915-429: The superficial epithelium. Dermal papillae are less pronounced in thin skin areas. The reticular dermis is the lower layer of the dermis, found under the papillary dermis, composed of dense irregular connective tissue featuring densely-packed collagen fibers. It is the primary location of dermal elastic fibers. The reticular region is usually much thicker than the overlying papillary dermis. It receives its name from

2970-748: The uneven distribution of molecules during division ). Multicellularity has evolved independently at least 25 times, including in some prokaryotes, like cyanobacteria , myxobacteria , actinomycetes , or Methanosarcina . However, complex multicellular organisms evolved only in six eukaryotic groups: animals, fungi, brown algae, red algae, green algae, and plants. It evolved repeatedly for plants ( Chloroplastida ), once or twice for animals , once for brown algae , and perhaps several times for fungi , slime molds , and red algae . Multicellularity may have evolved from colonies of interdependent organisms, from cellularization , or from organisms in symbiotic relationships . The first evidence of multicellularity

3025-434: The way, though the sequence of the steps involved has been disputed, and may not have started with symbiogenesis. It featured at least one centriole and cilium , sex ( meiosis and syngamy ), peroxisomes , and a dormant cyst with a cell wall of chitin and/or cellulose . In turn, the last eukaryotic common ancestor gave rise to the eukaryotes' crown group , containing the ancestors of animals , fungi , plants , and

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