21st March is the World Down Syndrome Day. This syndrome is a chromosomal combination that has always been part of the human condition. It exists in all regions of the world, and usually it has variable effects on learning styles, physical characteristics or health. It affects 1 in 700 children, making it the most common chromosomal abnormality and the first cause of mental disability. With this article I want to introduce a little more this syndrome.
WHY IS IT CALLED THAT?
Its name comes from the English doctor John Langdon Down who described a group of patients with intellectual disabilities and similar physical characteristics, in 1866. These patients had Down syndrome.
However, already existed artworks with people with Down syndrome (Figure 1), but Langdon Down was the first one to group them in a subcategory within individual with cognitive impairment.
Figure 1. “The Adoration of the Christ Child” (1515). This oil painting, made by a follower of Jan Joest van Kalkar, shows two people with Down syndrome (Source: Arte y síndrome de Down)
It is called syndrome because the affected people express a known set of symptoms or signs that they may appear together, although its origin is unknown. Even though physical features are common, each person with Down syndrome is a unique individual and can present the characteristics in different degrees or not.
WHAT ARE THE FEATURES THAT CHARACTERIZE THEM?
Diminished muscle tone
Small ears
Slanting eyes
Short nose
Flat back of head
Single crease in the palm of the hand: simian crease: complete fusion between heart line and headline (Figure 2)
When they are children present retardation in reaching capabilities as sitting independently, wandering, first words…
WHICH IS THE ROLE OF GENETICS?
In 1959, Jérôme Lejeune, a French doctor, saw that people with Down syndrome had 47 chromosomes in each cell instead of 46. This extra chromosome was 21 (Figure 3). The article Why I look similar to my parents? reminds us what a chromosome is.
Figure 3. Male karyotype, person with Down syndrome (Source: Mireia Ramos, Cerba Internacional SAE)
So, Down syndrome or trisomy 21, is a result of an extra chromosome. But having and extra copy of chromosome 21 can be given by three phenomena.
NONDISJUNCTION
It is the major cause and represents 95% of cases. It is produced by an error in the process of cell division. It means that when parent’s cell divides there is an error, and the son inherits two copies of chromosome 21 instead of one.
Then the son has 3 chromosomes 21: 1 comes from one parent and 2 come from the other parent, which are transmitted together.
TRANSLOCATION
During the process of cell division of one parent, a chromosome 21 joins with other chromosome, usually a chromosome 14.
Then the son has 3 chromosomes 21: one comes from one parent and two come from the other parent.
It represents 4% of cases, and it is important to identify it to avoid passing the translocation to another child, if the couple wants another child.
MOSAICISM
It is the least common cause because it represents 1% of cases. After fertilization nondisjunction occurs, but not in all cells. This causes cells with 46 chromosomes and cells with 47, forming a mosaic.
Cells with 47 chromosomes have an extra chromosome 21.
HAVING A CHILD WITH DOWN SYNDROME
It has been found that the age of the mother is related to have a child with Down syndrome, i.e., the risk of having a baby with Down syndrome is greater among mother age 35 and older.
Trisomy 21 is the most trisomy accepted by nature, so in pregnancy test doctors always study it. If they detect a foetus has Down syndrome, the couple can choose to go ahead or to stop pregnancy.
People with Down syndrome are increasingly integrated into our society. Their IQ is 45-48, when the standard range is around 100, but with a special school integration support is highly beneficial and IQ can go up to 70.
Nowadays, more and more companies offering workplaces for them and this should not surprise us, because after all they only have an extra chromosome (Figure 4).
Figure 4. Keep calm it’s only and extra chromosome (Source: Pinterest)
El día 21 de marzo se celebra el Día Mundial del Síndrome de Down. Este síndrome es una combinación cromosómica natural que siempre ha formado parte de la condición humana. Existe en todas las regiones del mundo y, normalmente, tiene efectos variables en los estilos de aprendizaje, las características físicas o la salud. Afecta a 1 de cada 700 niños, siendo la anomalía cromosómica más frecuente y la primera causa de discapacidad intelectual. Con este artículo quiero dar a conocer un poco más este síndrome.
¿POR QUÉ SE LLAMA ASÍ?
Su nombre se debe al médico inglés John Langdon Down que, en 1866, describió a un grupo de pacientes con discapacidad intelectual que mostraban características físicas muy parecidas. Estos pacientes sufrían síndrome de Down.
Sin embargo, ya existían obras de arte donde aparecían personas con síndrome de Down (Figura 1), pero fue Langdon Down el primero en agruparlos en una subcategoría dentro de los individuos con deterioro cognitivo.
Figura 1. “La adoración del niño Jesús” (1515). Esta pintura al óleo, hecha por un discípulo de Jan Joest van Kalkar, muestra a dos personas con síndrome de Down (Fuente: Arte y síndrome de Down)
Se llama síndrome porque las personas afectadas expresan un conjunto de síntomas o signos conocidos, que pueden aparecer juntos, aunque su origen es desconocido. Pese a que los rasgos físicos sean comunes cada persona con síndrome de Down es un individuo único y puede presentar las características en diferentes grados o no.
¿CUÁLES SON LOS RASGOS QUE LOS CARACTERIZAN?
Hipotonía: tono muscular bajo, falta de fuerza en los músculos
Orejas pequeñas
Fisuras palpebrales oblicuas: forma característica de los ojos, inclinados hacia arriba
Nariz pequeña
Braquicefalia: parte de atrás de la cabeza plana, sin curvatura
Manos con pliegue simiano: las líneas que forman una M en la palma de la mano están fusionadas (Figura 2)
Cuando son pequeños presentan retraso en alcanzar capacidades como sentarse con independencia, deambular, primeras palabras…
¿QUÉ PAPEL JUEGA LA GENÉTICA?
En 1959, el médico francés, Jérôme Lejeune vio que las personas con síndrome de Down en vez de tener 46 cromosomas en cada célula tenían 47. Este cromosoma de más era el 21 (Figura 3). El artículo ¿Por qué me parezco a mis padres? nos recuerda qué es un cromosoma.
Figura 3. Cariotipo masculino de síndrome de Down (Fuente: Mireia Ramos, Cerba Internacional SAE)
El síndrome de Down o trisomía 21, que es como se le empezó a llamar entonces, es consecuencia de un cromosoma 21 extra. Pero tener un cromosoma 21 de más se puede dar por tres fenómenos.
NO DISYUNCIÓN
Es la causa mayoritaria, representando un 95% de los casos. Se debe a un error en el proceso de división celular. Es decir, que en el momento en que la célula del progenitor se tiene que dividir porque el hijo herede sólo un cromosoma 21, hay un error y el hijo hereda los dos cromosomas 21 de este progenitor. Como consecuencia el hijo tendrá 3 cromosomas 21: uno por parte de un progenitor y dos, que no se han podido separar, por parte del otro progenitor.
TRANSLOCACIÓN
En el momento de la división celular, de uno de los progenitores, un cromosoma 21 se une a otro cromosoma, mayoritariamente a un cromosoma 14. Entonces, el hijo tendrá 3 cromosomas 21: uno por parte de un progenitor y dos, el que tiene que recibir y el que está unido a un cromosoma 14, por parte del otro progenitor.
Representa el 4% de los casos y es importante identificarla parar evitar transmitir la translocación si la pareja quiere tener otro hijo.
MOSAICISMO
Es la causa menos frecuente ya que representa un 1% de los casos. Se produce una no disyunción después de la fecundación, pero no en todas las células. Esto provoca que haya células con 46 cromosomas y células con 47, formando un mosaico.
Las células con 47 cromosomas tendrán un cromosoma 21 extra.
TENER UN HIJO CON SÍNDROME DE DOWN
Se ha visto que la edad de la madre está relacionada con tener un hijo con síndrome de Down, es decir, cuando mayor sea la madre, la probabilidad de tener un hijo con síndrome de Down aumenta. Hay un aumento progresivo a los 30-35 años y a partir de los 38 la incidencia es exponencial.
Como es la trisomía más aceptada por la naturaleza, en las pruebas de embarazo se mira de forma rutinaria. Si se detecta que el feto tiene síndrome de Down la pareja puede escoger si seguir adelante o parar el embarazo.
Las personas con síndrome de Down cada vez están más integradas en nuestra sociedad. De acuerdo que su coeficiente intelectual es de 45-48, cuando el rango estándar ronda los 100, pero la integración escolar con un soporte especial es altamente beneficiosa y pueden llegar hasta un coeficiente intelectual de 70.
Cada vez hay más empresas que ofrecen lugares de trabajo para ellos y esto no nos tendría que extrañar, porque al fin y al cabo sólo tienen un cromosoma de más (Figura 4).
Figura 4. “Calma, sólo es un cromosoma extra” (Fuente: Pinterest)
El dia 21 de març es celebra el Dia Mundial de la Síndrome de Down. Aquesta síndrome és una combinació cromosòmica natural que sempre ha format part de la condició humana. Existeix en totes les regions del món i, habitualment, té efectes variables en els estils d’aprenentatge, les característiques físiques o la salut. Afecta a 1 de cada 700 nens, fent que sigui l’anomalia cromosòmica més freqüent i la primera causa de discapacitat intel·lectual. Amb aquest article pretenc donar a conèixer una mica més aquesta síndrome.
PER QUÈ S’ANOMENA AIXÍ?
El seu nom es deu al metge anglès John Langdon Down que, el 1866, va descriure un grup de pacients amb discapacitat intel·lectual que mostraven característiques físiques molt semblants. Aquests pacients patien la síndrome de Down.
No obstant, ja existien obres d’art on es mostraven persones amb síndrome de Down (Figura 1), però és Langdon Down el primer en agrupar-los en una subcategoria dins dels individus amb deterioració cognitiva.
Figura 1. “L’adoració del nen Jesús” (1515). Aquesta pintura a l’oli, feta per un deixeble de Jan Joest van Kalkar, mostra a dues persones amb síndrome de Down (Font: Arte y síndrome de Down)
S’anomena síndrome perquè les persones afectades expressen un conjunt de símptomes o signes coneguts, que poden aparèixer junts, tot i que el seu origen és desconegut. Encara que els trets físics siguin comuns, cada persona amb síndrome de Down és un individu únic i pot presentar les característiques en diferents graus o no.
PER QUINS TRETS ES CARACTERITZEN?
Hipotonia: baix to muscular, falta de força als músculs
Orelles petites
Fissures palpebrals obliqües: forma característica dels ulls, inclinats cap amunt
Nas petit
Braquicefàlia: part del darrere del cap plana, sense curvatura
Mans amb plec simià: les línies que formen una M a la palma de la mà estan fusionades (Figura 2)
Quan són petits presenten retard en assolir capacitats com seure amb independència, deambular, primeres paraules…
QUIN PAPER JUGA LA GENÈTICA?
El 1959, el metge francès, Jérôme Lejeune va veure que les persones amb síndrome de Down en comptes de tenir 46 cromosomes a cada cèl·lula en tenien 47. Aquest cromosoma de més era el 21 (Figura 3). L’article Per què m’assemblo als meus pares? ens recorda què és un cromosoma.
Figura 3. Cariotip masculí de síndrome de Down (Font: Mireia Ramos, Cerba Internacional SAE)
Aleshores, la síndrome de Down o trisomia 21, que és com se la va començar a anomenar llavors, és conseqüència d’un cromosoma 21 extra. Però tenir un cromosoma 21 de més pot ser degut a tres fenòmens.
NO DISJUNCIÓ
És la causa majoritària, representant un 95% dels casos. Es deu a un error en el procés de divisió cel·lular. És a dir, en el moment en què la cèl·lula del progenitor s’ha de dividir perquè el fill hereti només un cromosoma 21, hi ha un error i el fill hereta els dos cromosomes 21 d’aquest progenitor. Com a conseqüència tindrà 3 cromosomes 21: un per part d’un progenitor i dos, que no s’han pogut separar, per part de l’altre progenitor.
TRANSLOCACIÓ
En el moment de la divisió cel·lular, d’un dels progenitors, un cromosoma 21 s’uneix a un altre cromosoma, majoritàriament a un cromosoma 14. Aleshores, el fill tindrà 3 cromosomes 21: un per part d’un progenitor i 2, el que ha de rebre i el que està enganxat al cromosoma 14, per part de l’altre progenitor.
Representa el 4% dels casos i és important identificar-la per evitar que la parella, si vol tenir un altre fill, transmeti la translocació.
MOSAICISME
És la causa menys freqüent ja que representa un 1% dels casos. Es produeix una no disjunció després de la fecundació, però no en totes les cèl·lules. Això provoca que hi hagi cèl·lules amb 46 cromosomes i cèl·lules amb 47, formant un mosaic.
Les cèl·lules amb 47 cromosomes tindran un cromosoma 21 extra.
TENIR UN FILL AMB SÍNDROME DE DOWN
S’ha vist que l’edat de la mare està relacionada amb tenir un fill amb síndrome de Down, és a dir, com més gran sigui la mare augmenta la probabilitat de tenir un fill amb síndrome de Down. Hi ha un augment progressiu als 30-35 anys i a partir dels 38 la incidència és exponencial.
Com que és la trisomia més acceptada per la naturalesa, en les proves d’embaràs es mira de forma rutinària. Si es detecta que el fetus té síndrome de Down la parella pot triar si seguir endavant o parar l’embaràs.
Les persones amb síndrome de Down cada cop estan més integrades a la nostra societat. D’acord que el seu coeficient intel·lectual és de 45-48, quan el rang estàndard volta els 100, però la integració escolar amb un suport especial és altament beneficiosa i poden arribar fins a un coeficient intel·lectual de 70.
Cada cop hi ha més empreses que ofereixen llocs de treball per a ells i això no ens hauria d’estranyar, perquè al cap i a la fi només tenen un cromosoma de més (Figura 4).
Figura 4. “Calma, només és un cromosoma de més” (Font: Pinterest)
The presence of feathers is one the main characteristics of modern birds. Currently many dinosaur fossils show us that feathers appeared long before birds. Yet the feathers that those Mesozoic animals had weren’t exactly the same as the ones current birds have. The evolution of feathers was a long and gradual process, and in this entry we’ll review the most important evolutionary stages that brought those dinosaurs to develop anatomically modern feathers.
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Today’s feathers
Feathers are fundamental structures for the life of modern birds. Feathers help them insulate from cold and hot weather, make them waterproof, camouflage, allow them to fly and in many species, feathers are very important in the mating rituals. In many birds, plumage allows us to differentiate between different species, telling a male and a female apart, and even allows us to know the age of an individual.
Male golden pheasant (Chrysolophus pictus) photographed at Kuala Lumpur’s Bird Park, showing us different types of feathers. Photo by Bjørn Christian Tørrissen.
Feathers are the most complex integumentary structures found in vertebrates. These are formed in the epidermis, in little follicles which produce keratin. The β-keratin of the bird’s feathers, claws and beak is much more folded than the α-keratin found in mammalian’s hair, hooves or horns, making the first a much stronger structure. Feathers are resistant and light structures, but in many birds they correspond to a third of their body weight.
Modern feathers have a central shaft divided into two parts: the proximal part which inserts to the body called the calamus, and the rachis, the distal part from which the laminar part of the feather appears. This is called the vane and is disposed on both sides of the rachis. The laminar part is made of parallel ramifications called barbs, which have ramifications called barbules which also have ramifications in the shape of small hooks called barbicels that make barbules cross-attach to each other. The superior end of the vane (pennaceous part) barbules are perfectly arranged by the barbicels, while in the inferior end (plumulaceus part) barbules lack barbicels and so they float free from each other.
According to its structure, in current birds we can find two main types of feathers:
Contour feathers: These are the feathers that make up the shape of the bird. These are long, flat feathers with a well-developed rachis and well-arranged barbs. These can be further classified into generic contour feathers, which cover the head, neck, trunk and limbs of the animal, and the flight feathers, called rectrices the ones in the tail (symmetric) and remiges the ones in the wings (asymmetric).
Down feathers: These are found forming a second layer under the contour feathers. These are feathers with a short rachis and with disordered barbs floating freely. Its main function is to thermally insulate the bird. Natal down feathers covering most bird hatchlings in some time of their lives are called “neossoptilus”.
Barn owl hatchling (Tyto alba) covered in down feathers. Photo by Maxgreen.
Apart from these two types, there are other kinds of feathers in birds, such as the semiplumes (with an intermediate structure between contour and down feathers) and the bristles and filoplumes (with few barbs and mainly with a sensory function).
Different types of feathers we can find on modern birds, drawings by Osado. From left to right: Rectrix (tail), remex (wing), generic contour feather, semiplume, down feather, bristle and filoplume.
Origin and evolution of feathers
Probably dinosaurs develped the first feathers as a system to avoid the loss of body heat. Having a covering feathers, a layer of warm air becomes trapped around the animal, making its body temperature more stable. That’s why some scientists think that many dinosaur species had an almost endothermic metabolism (mesothermy), with high and constant body temperature. Nevertheless, primitive feathers or “protofeathers” were very different from modern feathers.
Reconstruction of Deinonynchus by Stephen Czerkas, at the Natural History Museum of Vienna. Photo by Domser.
As we will now see, protofeathers went through different evolutionary stages before becoming modern feathers. Even if here we present you these stages linearly, it doesn’t mean that when a new kind of protofeather appeared the previous one disappeared. Just like modern birds sport different kinds of feathers, many dinosaurs presented different combinations of protofeathers, which only represented different levels of specialization.
Stage 1: A single filament
Drawing about the origin and formation of the first protofeathers. Extracted from Prum & Brush (2002).
The first known protofeathers were nothing more than a cylindrical hollow spine-like filament, which formed on a follicle’s collar. Even though feathers and protofeathers are typically exclusive characteristics of theropods, this first protofeathers have also been found in two groups of non-theropod dinosaurs. These are the Heterodontosauridae and Psittacosauridae families, many species of which had spines homologous to stage 1 protofeathers which probably also served to retain body heat.
Reconstruction of a heterodontosaurid named Fruitadens. Drawing by Smokeybjb.
In theropods, feathers appeared in a group named Coelurosauria, which includes animals like the tyrannosaur, the velociraptor and modern birds. The oldest feathered coelurosaur known is Sciurumimus, which literally means “squirrel mimic”. This fossil got its name for its fully feathered tail, covered in filamentous protofeathers similar to a squirrel’s.
Reconstruction of a juvenile Sciurumimus based on the skeleton found in Bavaria. Drawing by Franz Joseph.
Stage 2: A plumulaceous protofeather
Second stage in the evolution of feathers, in which a division in the follicle produces various barbs with a single origin. Extracted from Prum & Brush (2002).
The next step on the evolution of feathers was the division of the cellular collar of the follicle, which brought the branching of the filament. The result is a plumulaceous protofeather with unbranched barbs originating in a calamus. Stage 2 protofeather are similar to down feathers of current birds and have been found in a wide variety of theropod fossils.
These protofeathers provided a better insulation, helping the animal to keep its body heat. It is also believed that it’s likely that the smallest dinosaurs were more fully covered in protofeathers, since smaller animals loose heat faster than bigger animals and so, they need more mechanisms to retain body heat. Bigger coelurosaur species like Tyrannosaurus may have lost their protofeathers much like modern elephants have lost almost all their body hair. Yet, it is possible that some species presented protofeathers after birth and during the first stages of life, and after growing up they would either loose them or only present them on some body parts.
Reconstruction of a juvenile Juravenator in which we can appreciate how it was covered both with protofeathers and scales. Drawing by Tom Parker.
Yet in a Chinese paleontological site, the two biggest feathered dinosaurs known were discovered. The first to be discovered was Beipiaosaurus, a strange looking coelurosaur of about 3 metres long with long claws, which presented protofeathers both filamentous (stage 1) and plumulaceous (stage 2). This species shared its habitat with Yutyrannus, a 9 metre-long animal up to 1400 kilos of weight, which had almost all its body covered in plumulaceous protofeathers. These two animals probably lived in a humid and cold environment, and their coat of protofeathers helped them to keep their warmth when temperatures would fall.
Reconstruction of four Yutyrannus and a pair of Beipiaosaurus on their habitat. Drawing by Brian Choo.
Stage 3: Fusion and branching
Drawing of the evolution of feathers from stage 1 to 3. Extracted from Sues (2001).
The third stage in the evolution of feathers gave rise to a protofeather with a central rachis made from the fusion of some barbs (3a) and a protofeather with barbules branching from the main barbs (3b). The combination of these two characters produced a pennaceous, vaned protofeather similar to the ones found in modern birds but less firm, as it lacked the hooked barbicels of modern feathers.
Fossils of stage 3a protofeathers where we can see a central rachis from which various barbs extend. Extracted from Perrichot (2008).
Stage 4: Hooks to maintain order
Modified drawing from Prum & Brush (2002) of the apparition of hooks on the barbules of the stage 4.
It is in this stage where we can start talking about present day feathers. The stage 3 structure with a rachis, barbs and barbules, developed small hooks on the barbules which made them cross-attach and keep the vane together. These feathers were like the ones found in modern birds, the contour feathers, which present a central shaft and a symmetric vane.
Reconstruction of the troodontid called Anchiornis, where the wide cover of feathers it presented can be seen. Drawing by Matt Martyniuk.
These feathers are found in many different dinosaurs, many of which had begun to develop adaptations for flight, or at least gliding. Despite this, we can also find them in typically running dinosaurs like Velociraptor, a terrestrial predator about the size of a turkey, with a long mouth and a sickle-shaped claw on its hind legs. This claw is thought to be used mainly to kill their prey, but some scientists think that they used their claw to climb trees and ambush their prey from above. Maybe their feathers served them to make more controlled leaps when they fell on their victims.
Drawing of a velociraptor attacking an oviraptotosaur. Drawing by Durbed.
These feathers are also found in the oviraptorosaurs, a coelurosaurian group with beak and few or no teeth. Even if they couldn’t fly, they probably used their arm feathers to incubate their eggs (like the Avimimus genre) and the ones on the tail for display and communication with other members of their species (like the Caudipteryx, and Nomingia genera).
Reconstruction of the oviraptorosaurian Nomingia, in which we can see the fan of feathers on its tail. Drawing by Smokeybjb.
Other dinosaurs like Scansoriopteryx had an arboreal lifestyle, and the feathers on its arms allowed it to glide from one tree to the other both to hunt and to escape predators. A relative of this animal called Epidexipteryx, even though not having feathers on its arms (as far as is known) presented long vaned feathers on the tail, probably to send visual signals to other members of its species.
Reconstruction of Epidexipteryx in which the long vaned tail feathers can be appreciated. Reconstruction by Nobu Tamura.
Stage 5: Asymmetry brings flight
Drawings and fossils of all the different stages of the evolution of feathers. Extracted from McKellar et al (2011).
Finally the last stage in the evolution of feathers is the appearance of asymmetric feathers, with a displaced rachis making one half of the vane wider than the other. These feathers are the remiges found on the wings of birds, which not only increase drag during gliding, but also allow the animal to leave the ground and fly.
Photo of a Willow tit (Poecile montanus) taking flight, where we can perfectly see the asymmetric remiges on its wings. Photo by David Mark.
Even if it’s generally assumed that apart from birds no other dinosaur group accomplished powered flight, there’s one group which got really close. The microraptorians were a group of small feathered dinosaurs characterized by presenting flight feathers, not only on their front limbs but also on their hind limbs. The most famous of them, Microraptor, had asymmetrical flight feathers on its arms, legs and, unlike modern birds, on its tail.
Drawing of the silhouette of Microraptor gliding. Extracted from Xu et al.
Even if it’s usually considered a glider, some authors argue that possibly Microraptor was capacitated to fly. Some skeletal characteristics indicate that some microraptorians may have been better suited for flight than Archaeopteryx, the ancestor of modern birds. For example, Microraptor presented a fused more developed sternum than Archaeopteryx, which would have made for a major anchor point for the flight muscles.
Nevertheless, Archaeopteryx is still considered the nearest to the ancestor of modern birds that, even if it wasn’t a great flier, it already had the different kinds of feathers found on current birds. Probably many more dinosaurs were covered with feathers or protofeathers, but in this entry we have only seen the species which show irrefutable evidence of having them. As we have seen, the road to reach modern feathers was long and gave rise to a wide diversity of dinosaur species, but after a meteorite practically extinguished life on Earth 65 million years ago, only one group of feathered dinosaurs survived and thrived.
Fossil of Archaeopteryx lithographica from the late Jurassic found at southern Germany. Photo by James L. Amos.
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References
The next sources have been consulted during the elaboration of this entry:
All you need is Biology 