Fertilisation or fertilization known as generative fertilisation, pollination, fecundation and impregnation, is the fusion of gametes to initiate the development of a new individual organism or offspring. This cycle of fertilisation and development of new individuals is called sexual reproduction. During double fertilisation in angiosperms the haploid male gamete combines with two haploid polar nuclei to form a triploid primary endosperm nucleus by the process of vegetative fertilisation. In Antiquity, Aristotle conceived the formation of new individuals through fusion of male and female fluids, with form and function emerging in a mode called by him as epigenetic. In 1784, Spallanzani established the need of interaction between the female's ovum and male's sperm to form a zygote in frogs. In 1827, von Baer observed a therian mammalian egg for the first time. Oscar Hertwig, in Germany, described the fusion of ova from sea urchin; the evolution of fertilisation is related to the origin of meiosis, as both are part of sexual reproduction, originated in eukaryotes.
There are two conflicting theories on how the couple meiosis–fertilisation arose. One is; the other is. The gametes that participate in fertilisation of plants are the pollen, the egg cell. Various families of plants have differing methods. In Bryophyte land plants, fertilisation takes place within the archegonium. In flowering plants a second fertilisation event involves another sperm cell and the central cell, a second female gamete. In flowering plants there are two sperm from each pollen grain. In seed plants, after pollination, a pollen grain germinates, a pollen tube grows and penetrates the ovule through a tiny pore called a micropyle; the sperm are transferred from the pollen through the pollen tube to the ovule. Pollen tube growth Unlike animal sperm, motile, plant sperm is immotile and relies on the pollen tube to carry it to the ovule where the sperm is released; the pollen tube penetrates the stigma and elongates through the extracellular matrix of the style before reaching the ovary.
Near the receptacle, it breaks through the ovule through the micropyle and the pollen tube "bursts" into the embryo sac, releasing sperm. The growth of the pollen tube has been believed to depend on chemical cues from the pistil, however these mechanisms were poorly understood until 1995. Work done on tobacco plants revealed a family of glycoproteins called TTS proteins that enhanced growth of pollen tubes. Pollen tubes in a sugar free pollen germination medium and a medium with purified TTS proteins both grew. However, in the TTS medium, the tubes grew at a rate 3x that of the sugar-free medium. TTS proteins were placed on various locations of semi in vevo pollinated pistils, pollen tubes were observed to extend toward the proteins. Transgenic plants lacking the ability to produce TTS proteins exhibited slower pollen tube growth and reduced fertility. Rupture of pollen tube The rupture of the pollen tube to release sperm in Arabidopsis has been shown to depend on a signal from the female gametophyte.
Specific proteins called FER protein kinases present in the ovule control the production of reactive derivatives of oxygen called reactive oxygen species. ROS levels have been shown via GFP to be at their highest during floral stages when the ovule is the most receptive to pollen tubes, lowest during times of development and following fertilization. High amounts of ROS activate Calcium ion channels in the pollen tube, causing these channels to take up Calcium ions in large amounts; this increased uptake of calcium causes the pollen tube to rupture, release its sperm into the ovule. Pistil feeding assays in which plants were fed diphenyl iodonium chloride suppressed ROS concentrations in Arabidopsis, which in turn prevented pollen tube rupture. Bryophyte is a traditional name used to refer to all embryophytes that do not have true vascular tissue and are therefore called "non-vascular plants"; some bryophytes do have specialised tissues for the transport of water. A fern is a member of a group of 12,000 species of vascular plants that reproduce via spores and have neither seeds nor flowers.
They differ from mosses by being vascular. They leaves, like other vascular plants. Most ferns have what are called fiddleheads that expand into fronds, which are each delicately divided; the gymnosperms are a group of seed producing plants that includes conifers, Cycads and Gnetales. The term "gymnosperm" comes from the Greek composite word γυμνόσπερμος, meaning "naked seeds", after the unenclosed condition of their seeds, their naked condition stands in contrast to the seeds and ovules of flowering plants, which are enclosed within an ovary. Gymnosperm seeds develop either on the surface of scales or leaves modified to form cones, or at the end of short stalks as in Ginkgo. After being fertilised, the ovary starts to develop into the fruit. With multi-seeded fruits, multiple grains of pollen are necessary for syngamy with each ovule; the growth of the pollen tube is controlled by the vegetative cytoplasm. Hydrolytic enzymes are secreted by the pollen tube that digest the female tissue as the tube grows down the stigma and style
The fecal–oral route describes a particular route of transmission of a disease wherein pathogens in fecal particles pass from one person to the mouth of another person. Main causes of fecal–oral disease transmission include lack of adequate sanitation, poor hygiene practices. If soil or water bodies are polluted with fecal material, humans can be infected with waterborne diseases or soil-transmitted diseases. Fecal contamination of food is another form of fecal-oral transmission. Washing hands properly after changing a baby's diaper or after performing anal hygiene can prevent foodborne illness from spreading; the common factors in the fecal-oral route can be summarized as five Fs: fingers, fields and food. Analingus, the sexual practice of licking or inserting the tongue into the anus of a partner, is another route. Diseases caused by fecal-oral transmission include diarrhea, cholera and hepatitis; the foundations for the "F-diagram" being used today were laid down in a publication by WHO in 1958.
This publication explained transmission routes and barriers to the transmission of diseases from the focal point of human feces. Modifications have been made over the course of history to derive modern-looking F-diagrams; these diagrams are used in many sanitation publications. They are set up in a way that fecal–oral transmission pathways are shown to take place via water, hands and soil. To make it easier to remember, words starting with the letter "F" are used for each of these pathways, namely fluids, flies, fields, fomites. Rather than only concentrating on human feces, animal feces should be included in the F-diagram; the sanitation and hygiene barriers when placed prevent the transmission of an infection through hands and food. The F-diagram can be used to show how proper sanitation can act as an effective barrier to stop transmission of diseases via fecal–oral pathways; the process of transmission may involve multiple steps. Some examples of routes of fecal–oral transmission include: water that has come in contact with feces and is not treated properly before drinking.
Eating feces, in children, or in a mental disorder called coprophagia eating soil One approach to changing people's behaviors and stopping open defecation is the community-led total sanitation approach. In this process "live demonstrations" of flies moving from food to fresh human feces and back are used; this can "trigger" villagers into action. The list below shows the main diseases, they are grouped by the type of pathogen involved in disease transmission. Vibrio cholerae Clostridium difficile Shigella Salmonella typhii Vibrio parahaemolyticus Escherichia coli Campylobacter Hepatitis A Hepatitis E Enteroviruses Norovirus acute gastroenteritis Poliovirus Rotavirus – Most of these pathogens cause gastroenteritis. Entameba histolytica Giardia Cryptosporidium Toxoplasma gondii Tape worms Ascariasis and other soil transmitted helminthiasis Waterborne diseases are diseases caused by pathogenic microorganisms that most are transmitted in contaminated fresh water; this is one particular type of fecal-oral transmission.
Neglected tropical diseases contains many diseases transmitted via the fecal-oral route. Toilet Vector control
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians have a larval phase of their life cycle; the larva's appearance is very different from the adult form including different unique structures and organs that do not occur in the adult form. Their diet may be different. Larvae are adapted to environments separate from adults. For example, some larvae such as tadpoles live exclusively in aquatic environments, but can live outside water as adult frogs. By living in a distinct environment, larvae may be given shelter from predators and reduce competition for resources with the adult population. Animals in the larval stage will consume food to fuel their transition into the adult form. In some species like barnacles, adults are immobile but their larvae are mobile, use their mobile larval form to distribute themselves; some larvae are dependent on adults to feed them. In many eusocial Hymenoptera species, the larvae are fed by female workers.
In Ropalidia marginata the males are capable of feeding larvae but they are much less efficient, spending more time and getting less food to the larvae. The larvae of some species do not develop further into the adult form; this is a type of neoteny. It is a misunderstanding; this could be the case, but the larval stage has evolved secondarily, as in insects. In these cases the larval form may differ more than the adult form from the group's common origin. Within Insects, only Endopterygotes show different types of larvae. Several classifications have been suggested by many entomologists, following classification is based on Antonio Berlese classification in 1913. There are four main types of endopterygote larvae types: Apodous larvae – no legs at all and are poorly sclerotized. Based on sclerotization, three apodous forms are recognized. Eucephalous – with well sclerotized head capsule. Found in Nematocera and Cerambycidae families. Hemicephalus – with a reduced head capsule, retractable in to the thorax.
Found in Tipulidae and Brachycera families. Acephalus – without head capsule. Found in Cyclorrhapha Protopod larvae – larva have many different forms and unlike a normal insect form, they hatch from eggs which contains little yolk. Ex. first instar larvae of parasitic hymenoptera. Polypod larvae – known as eruciform larvae, these larva have abdominal prolegs, in addition to usual thoracic legs, they poorly sclerotized and inactive. They live in close contact with the food. Best example is caterpillars of lepidopterans. Oligopod larvae – have well developed head capsule and mouthparts are similar to the adult, but without compound eyes, they have six legs. No abdominal prolegs. Two types can be seen: Campodeiform – well sclerotized, dorso-ventrally flattened body. Long legged predators with prognathous mouthparts.. Scarabeiform – poorly sclerotized, flat thorax and abdomen. Short legged and inactive burrowing forms.. Crustacean larvae Ichthyoplankton Spawn Non-larval animal juvenile stages and other life cycle stages: In Porifera: olynthus, gemmule In Cnidaria: ephyra, strobila, hydranth, medusa In Mollusca: paralarva, young cephalopods In Platyhelminthes: hydatid cyst In Bryozoa: avicularium In Acanthocephala: cystacanth In Insecta: Nymphs and naiads, immature forms in hemimetabolous insects Subimago, a juvenile that resembles the adult in Ephemeroptera Instar, intermediate between each ecdysis Pupa and chrysalis, intermediate stages between larva and imago Protozoan life cycle stages Apicomplexan life cycle Algal life cycle stages: Codiolum-phase Conchocelis-phase Marine larval ecology Media related to Larvae at Wikimedia Commons The dictionary definition of larva at Wiktionary Arenas-Mena, C.
Indirect development, transdifferentiation and the macroregulatory evolution of metazoans. Philosophical Transactions of the Royal Society B: Biological Sciences. Feb 27, 2010 Vol.365 no.1540 653-669 Brusca, R. C. & Brusca, G. J.. Invertebrates. Sunderland, Mass.: Sinauer Associates. Hall, B. K. & Wake, M. H. eds.. The Origin and Evolution of Larval Forms. San Diego: Academic Press. Leis, J. M. & Carson-Ewart, B. M. eds.. The Larvae of Indo-Pacific Coastal Fishes. An Identification Guide to Marine Fish Larvae. Fauna Malesiana handbooks, vol. 2. Brill, Leiden. Minelli, A.. The larva. In: Perspectives in Animal Phylogeny and Evolution. Oxford University Press. P. 160-170. Link. Shanks, A. L.. An Identification Guide to the Larval Marine Invertebrates of the Pacific Northwest. Oregon State University Press, Corvallis. 256 pp. Smith, D. & Johnson, K. B.. A Guide to Marine Coastal Plankton and Marine Invertebrate Larvae. Kendall/Hunt Plublishing Company. Stanwell-Smith, D. Hood, A. & Peck, L. S.. A field guide to the pelagic invertebrates larvae of the maritime Antarctic.
British Antarctic Survey, Cambridge. Thyssen, P. J.. Keys for Identification of Immature Insects. In: Amendt, J. et al.. Current Concepts in Forensic Entomology, chapter 2, pp. 25–42. Springer: Dordrecht
The nematodes or roundworms constitute the phylum Nematoda. They are a diverse animal phylum inhabiting a broad range of environments. Taxonomically, they are classified along with insects and other moulting animals in the clade Ecdysozoa, unlike flatworms, have tubular digestive systems with openings at both ends. Nematode species can be difficult to distinguish from one another. Estimates of the number of nematode species described to date vary by author and may change over time. A 2013 survey of animal biodiversity published in the mega journal Zootaxa puts this figure at over 25,000. Estimates of the total number of extant species are subject to greater variation. A referenced article published in 1993 estimated there may be over 1 million species of nematode, a claim which has since been repeated in numerous publications, without additional investigation, in an attempt to accentuate the importance and ubiquity of nematodes in the global ecosystem. Many other publications have since vigorously refuted this claim on the grounds that it is unsupported by fact, is the result of speculation and sensationalism.
More recent, fact-based estimates have placed the true figure closer to 40,000 species worldwide. Nematodes have adapted to nearly every ecosystem: from marine to fresh water, from the polar regions to the tropics, as well as the highest to the lowest of elevations, they are ubiquitous in freshwater and terrestrial environments, where they outnumber other animals in both individual and species counts, are found in locations as diverse as mountains and oceanic trenches. They are found in every part of the earth's lithosphere at great depths, 0.9–3.6 km below the surface of the Earth in gold mines in South Africa. They represent 90% of all animals on the ocean floor, their numerical dominance exceeding a million individuals per square meter and accounting for about 80% of all individual animals on earth, their diversity of lifecycles, their presence at various trophic levels point to an important role in many ecosystems. They have been shown to play crucial roles in polar ecosystem; the 2,271 genera are placed in 256 families.
The many parasitic forms include pathogens in animals. A third of the genera occur as parasites of vertebrates. Nathan Cobb, a nematologist, described the ubiquity of nematodes on Earth as thus:In short, if all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, if, as disembodied spirits, we could investigate it, we should find its mountains, vales, rivers and oceans represented by a film of nematodes; the location of towns would be decipherable, since for every massing of human beings, there would be a corresponding massing of certain nematodes. Trees would still stand in ghostly rows representing our highways; the location of the various plants and animals would still be decipherable, had we sufficient knowledge, in many cases their species could be determined by an examination of their erstwhile nematode parasites. Modern Latin compound of nemat- "thread" + -odes "like, of the nature of". In 1758, Linnaeus described some nematode genera included in the Vermes.
The name of the group Nematoda, informally called "nematodes", came from Nematoidea defined by Karl Rudolphi, from Ancient Greek νῆμα and -eiδἠς. It was treated as family Nematodes by Burmeister. At its origin, the "Nematoidea" erroneously included Nematodes and Nematomorpha, attributed by von Siebold. Along with Acanthocephala and Cestoidea, it formed the obsolete group Entozoa, created by Rudolphi, they were classed along with Acanthocephala in the obsolete phylum Nemathelminthes by Gegenbaur. In 1861, K. M. Diesing treated the group as order Nematoda. In 1877, the taxon Nematoidea, including the family Gordiidae, was promoted to the rank of phylum by Ray Lankester; the first clear distinction between the nemas and gordiids was realized by Vejdovsky when he named a group to contain the horsehair worms the order Nematomorpha. In 1919, Nathan Cobb proposed, he argued they should be called "nema" in English rather than "nematodes" and defined the taxon Nemates, listing Nematoidea sensu restricto as a synonym.
However, in 1910, Grobben proposed the phylum Aschelminthes and the nematodes were included in as class Nematoda along with class Rotifera, class Gastrotricha, class Kinorhyncha, class Priapulida, class Nematomorpha. In 1932, Potts elevated the class Nematoda to the level of phylum. Despite Potts' classification being equivalent to Cobbs', both names have been used and Nematode became a popular term in zoological science. Since Cobb was the first to include nematodes in a particular phylum separated from Nematomorpha, some researchers consider the valid taxon name to be Nemates or Nemata, rather than Nematoda, because of the zoological rule that gives priority to the first used term in case of synonyms; the phylogenetic relationships of the nematodes and their close relatives among the protostomian Metazoa are unresolved. Traditionally, they were held to b
In biology and biochemistry, a lipid is a biomolecule, soluble in nonpolar solvents. Non-polar solvents are hydrocarbons used to dissolve other occurring hydrocarbon lipid molecules that do not dissolve in water, including fatty acids, sterols, fat-soluble vitamins, diglycerides and phospholipids; the functions of lipids include storing energy and acting as structural components of cell membranes. Lipids have applications in the food industries as well as in nanotechnology. Scientists sometimes broadly define lipids as amphiphilic small molecules. Biological lipids originate or in part from two distinct types of biochemical subunits or "building-blocks": ketoacyl and isoprene groups. Using this approach, lipids may be divided into eight categories: fatty acids, glycerophospholipids, sphingolipids and polyketides. Although the term "lipid" is sometimes used as a synonym for fats, fats are a subgroup of lipids called triglycerides. Lipids encompass molecules such as fatty acids and their derivatives, as well as other sterol-containing metabolites such as cholesterol.
Although humans and other mammals use various biosynthetic pathways both to break down and to synthesize lipids, some essential lipids can't be made this way and must be obtained from the diet. In 1815, Henry Braconnot classified lipids in two categories and huiles. In 1823, Michel Eugène Chevreul developed a more detailed classification, including oils, tallow, resins and volatile oils. In 1827, William Prout recognized fat, along with protein and carbohydrate, as an important nutrient for humans and animals. For a century, chemists regarded "fats" as only simple lipids made of fatty acids and glycerol, but new forms were described later. Theodore Gobley discovered phospholipids in mammalian brain and hen egg, called by him as "lecithins". Thudichum discovered in human brain some phospholipids and sphingolipids; the terms lipoid, lipin and lipid have been used with varied meanings from author to author. In 1912, Rosenbloom and Gies proposed the substitution of "lipoid" by "lipin". In 1920, Bloor introduced a new classification for "lipoids": simple lipoids, compound lipoids, the derived lipoids.
The word "lipid", which stems etymologically from the Greek lipos, was introduced in 1923 by Gabriel Bertrand. Bertrands included in the concept not only the traditional fats, but the "lipoids", with a complex constitution. In 1947, T. P. Hilditch divided lipids into "simple lipids", with greases and waxes, "complex lipids", with phospholipids and glycolipids. Fatty acids, or fatty acid residues when they are part of a lipid, are a diverse group of molecules synthesized by chain-elongation of an acetyl-CoA primer with malonyl-CoA or methylmalonyl-CoA groups in a process called fatty acid synthesis, they are made of a hydrocarbon chain. The fatty acid structure is one of the most fundamental categories of biological lipids, is used as a building-block of more structurally complex lipids; the carbon chain between four and 24 carbons long, may be saturated or unsaturated, may be attached to functional groups containing oxygen, halogens and sulfur. If a fatty acid contains a double bond, there is the possibility of either a cis or trans geometric isomerism, which affects the molecule's configuration.
Cis-double bonds cause the fatty acid chain to bend, an effect, compounded with more double bonds in the chain. Three double bonds in 18-carbon linolenic acid, the most abundant fatty-acyl chains of plant thylakoid membranes, render these membranes fluid despite environmental low-temperatures, makes linolenic acid give dominating sharp peaks in high resolution 13-C NMR spectra of chloroplasts; this in turn plays an important role in the function of cell membranes. Most occurring fatty acids are of the cis configuration, although the trans form does exist in some natural and hydrogenated fats and oils. Examples of biologically important fatty acids include the eicosanoids, derived from arachidonic acid and eicosapentaenoic acid, that include prostaglandins and thromboxanes. Docosahexaenoic acid is important in biological systems with respect to sight. Other major lipid classes in the fatty acid category are the fatty esters and fatty amides. Fatty esters include important biochemical intermediates such as wax esters, fatty acid thioester coenzyme A derivatives, fatty acid thioester ACP derivatives and fatty acid carnitines.
The fatty amides include N-acyl ethanolamines, such as the cannabinoid neurotransmitter anandamide. Glycerolipids are composed of mono-, di-, tri-substituted glycerols, the best-known being the fatty acid triesters of glycerol, called triglycerides; the word "triacylgl
Rubbing alcohol refers to either isopropyl alcohol or ethanol based liquids, or the comparable British Pharmacopoeia defined surgical spirit, with isopropyl alcohol products being the most available. Rubbing alcohol is undrinkable if it is ethanol based, due to the bitterants added, they are liquids used as a topical antiseptic. They have many industrial and household uses; the term "rubbing alcohol" has become a general non-specific term for either isopropyl alcohol or ethyl alcohol rubbing-alcohol products. The United States Pharmacopeia defines'isopropyl rubbing alcohol USP' as containing 70 percent by volume of pure isopropyl alcohol and defines'rubbing alcohol USP' as containing 70 percent by volume of denatured alcohol. In Ireland and the UK, the comparable preparation is surgical spirit B. P. which the British Pharmacopoeia defines as 95% methylated spirit, 2.5% castor oil, 2% diethyl phthalate, 0.5% methyl salicylate. Under its alternative name of "wintergreen oil", methyl salicylate is a common additive to North American rubbing alcohol products.
Individual manufacturers are permitted to use their own formulation standards in which the ethanol content for retail bottles of rubbing alcohol is labeled as and ranges from 70-99% v/v. All rubbing alcohols are unsafe for human consumption: isopropyl rubbing alcohols do not contain the ethyl alcohol of alcoholic beverages; the term "rubbing alcohol" came into prominence in North America in the mid-1920s. The original rubbing alcohol was used as a liniment for massage; this original rubbing alcohol was rather different from today's formulated surgical spirit. The name "rubbing" emphasized that the alcohol was not intended for consumption, a significant distinction in Prohibition-era America. All rubbing alcohols are flammable, they have an bitter taste from additives. The specific gravity of Formula 23-H is between 0.8691 and 0.8771 at 15.56 °C. Isopropyl rubbing alcohols contain from 50% to 99% by volume of isopropyl alcohol, the remainder consisting of water. Boiling points vary with the proportion of isopropyl alcohol from 80 °C to 83 °C.
Colorless, products may contain color additives. They may contain medically-inactive additives for fragrance, such as wintergreen oil, or for other purposes. To prevent against alcohol abuse in the United States, all preparations classified as Rubbing Alcohols must have poisonous additives to limit human consumption in accordance with the requirements of the US Treasury Department, Bureau of Alcohol and Firearms, using Formula 23-H, it contains 87.5–91% by volume of absolute ethyl alcohol. The rest consists of water and the denaturants, with or without color additives, perfume oils. Rubbing alcohol contains in each 100 ml more than 355 mg of sucrose octaacetate or more than 1.40 mg of denatonium benzoate. The preparation may be colored with one or more color additives. A suitable stabilizer may be added. Product labels for rubbing alcohol include a number of warnings about the chemical, including the flammability hazards and its intended use only as a topical antiseptic and not for internal wounds or consumption.
It should be used in a well-ventilated area due to inhalation hazards. Poisoning can occur from ingestion, absorption, or consumption of rubbing alcohol. Why Is Drinking Rubbing Alcohol Bad
A pulmonary alveolus is a hollow cavity found in the lung parenchyma, is the basic unit of ventilation. Lung alveoli are the ends of the respiratory tree, branching from either alveolar sacs or alveolar ducts, which like alveoli are both sites of gas exchange with the blood as well. Alveoli are particular to mammalian lungs. Different structures are involved in gas exchange in other vertebrates; the alveolar membrane is the gas exchange surface. Carbon dioxide rich blood is pumped from the rest of the body into the capillaries that surround the alveoli where, through diffusion, carbon dioxide is released and oxygen is absorbed; the alveoli are located in the respiratory zone of the lungs, at the ends of the alveolar ducts and alveolar sac, representing the smallest units in the respiratory tract. They provide total surface area of about 75m2. A typical pair of human lungs contain about 480 million alveoli; each alveolus is wrapped in a fine mesh of capillaries covering about 70% of its area. An adult alveolus has an average diameter of 200 µm, with an increase in diameter during inhalation.
The alveoli consist of an epithelial layer and an extracellular matrix surrounded by small blood vessels called capillaries. In some alveolar walls there are pores between alveoli called Pores of Kohn; the alveoli contain elastic fibers. The elastic fibres allow the alveoli to stretch, they spring back during exhalation in order to expel the carbon dioxide-rich air. There are three major types of cell in the alveolar wall: two types of alveolar cell and a large phagocyte known as an alveolar macrophage. Type I cells form the structure of the alveoli. Type I alveolar cells are squamous and cover 90–95% of the alveolar surface. Type I cells are involved in the process of gas exchange between blood; these cells are thin – the electron microscope was needed to prove that all alveoli are covered with an epithelial lining. These cells need to be so thin to be permeable for enabling an easy gas exchange between the alveoli and the blood. Organelles of type I alveolar cells such as the endoplasmic reticulum, Golgi apparatus and mitochondria are clustered around the nucleus.
The nuclei occupy large areas of free cytoplasm. This reduces the thickness of the cell; the cytoplasm in the thin portion contains pinocytotic vesicles which may play a role in the removal of small particulate contaminants from the outer surface. In addition to desmosomes, all type I alveolar cells have occluding junctions that prevent the leakage of tissue fluid into the alveolar air space. Type I pneumocytes are susceptible to toxic insults. In the event of damage, type II cells can proliferate and differentiate into type I cells to compensate. Type II cells secrete pulmonary surfactant to lower the surface tension of water and allows the membrane to separate, therefore increasing its capability to exchange gases; the surfactant is continuously released by exocytosis. It forms an underlying aqueous protein-containing hypophase and an overlying phospholipid film composed of dipalmitoyl phosphatidylcholine. Type II alveolar cells cover a small fraction of the alveolar surface area. Type II cells are capable of cellular division, giving rise to more type I and II alveolar cells when the lung tissue is damaged.
These cells are granular and cuboidal. Type II alveolar cells are found at the blood-air barrier. Although they only make up <5% of the alveolar surface, they are numerous. The alveolar macrophages called dust cells, destroy foreign materials and microbes such as bacteria. Type I cells are flat cells lining the alveolar walls; each alveolus is surrounded by numerous capillaries, is the site of gas exchange, which occurs by diffusion. The low solubility of oxygen necessitates the large internal surface area and thin walls of the alveoli. Weaving between the capillaries and helping to support them is an extracellular matrix, a meshlike fabric of elastic and collagenous fibres; the collagen fibres, being more rigid, give the wall firmness, while the elastic fibres permit expansion and contraction of the walls during breathing. Type II cells in the alveolar wall contain secretory granular organelles known as lamellar bodies that fuse with the cell membranes and secrete pulmonary surfactant; this surfactant is a film of fatty substances, a group of phospholipids that reduce alveolar surface tension.
The phospholipid are stored in the lamellar bodies. Without this coating, the alveoli would collapse and large forces would be required to re-expand them. Type II cells start to develop at about 26 weeks of gestation, secreting small amounts of surfactant. However, adequate amounts of surfactant are not secreted until about 35 weeks of gestation - this is the main reason for increased rates of infant respiratory distress syndrome, which drastically reduces at ages above 35 weeks gestation. Type II pneumocytes will replicate to replace damaged type I cells. MUC1, a human gene associated with type II pneumocytes, has been identified as a marker in lung cancer. Another type of cell, known as an alveolar macrophage, resides on the internal surfaces of the air cavities of the alveoli, the alveolar ducts, the bronchioles, they are mobile scavengers that serve to engulf