Shubin mentions 4 arches that compose different strictures found in creating a human on page 88. These arches represent gill like formations. In your own words, describe what each arch develops into and what the significance of the fetus taking on a 'gill' structure while developing.
Nikhil Pereira (nikhil.pereira3@gmail.com)
The gill arches are still seen in many cartilaginous fish as stiff loops that support and separate the gills themselves. (Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia) However, through continuity and change, the gill arches formed the structures we now see in the human body today. All of the arches formed different and vital nerve, bone, cartilagenous, and muscular structures. However, all the structures formed stayed more or less in the same order as the previous arches. (arch 1 above arch 2. etc) (Shubin 88) When we examine the new "gill" structures, we can define exactly what these structures have become. The first gill arch forms part of the external carotid and maxillary arteries and muscles we use to chew, the tensor tympani muscle, which is the muscle responsible for muffling sounds in our ears, the mandible, which is the lower jaw, the incus and malleus, two of the ear bones, and the trigeminal nerve, which is the fifth cranial nerve and responsible for sensory information from our face. The second gill arch creates the facial muscles and nerves, the and muscles in the cheeks. Part of the arch also migrates upwards and forms the stapes (the third ear bone) and hyoid, or lingual, bone which aids in swallowing and anchors muscles to move the tongue. The third arch also creates the hyoid bone as well as the thymus and the glossopharyngeal nerves, which are twelve pairs of nerves that receive special sensory, visceral sensory, and parasympathetic fibers from the pharynx, tonge, and middle ear. The fourth and smallest arch creates the thyroid and epiglottis cartilage, the aortic arch, and the muscles in the soft palate.
ReplyDelete(http://www.nlm.nih.gov/medlineplus/ency/imagepages/9350.htm) (http://musom.marshall.edu/anatomy/grosshom/z_devbranc.html) (http://home.comcast.net/~wnor/cranialnerves.htm)
(http://home.comcast.net/~wnor/lesson11.htm)
The gill structures can be seen in the fetus till about the third week of gestation. After this point, the gill arches begin to change into what they have morphed into in the human body. This shows in important concept of continuity and change through embryology. Humans as fetuses have gill structures first and hyoid processes later. Therefore, through embryology, we can see that many of the structures that were present in our ancestors later morph in the structures that we see in ourselves today. This has helped us understand how different creatures have evolved from their inner fish. (http://onlinelibrary.wiley.com/doi/10.1111/j.1469-7580.2005.00472.x/full)
The four arches, which appear in the developing embryo, form bone tissue, muscle, and blood vessels. These arches are called the branchial arches or the pharyngeal arches. The arches organize themselves so that each of their cells can divide to make up the parts of the head/neck that they are destined to become. According to Shubin, the complex mix of cells in each of the arches are made up of cells that “divided right there” and ones that “migrated a long way to enter the arch itself” (Shubin 87).
ReplyDeleteThe skeletal parts that the first arch tissues form are the malleus and incus of the middle ear; the anterior ligament of the malleus; the maxilla (upper jaw), the mandible (lower jaw), and connecting bones of the jaws; and the palatine bone that creates the roof of the mouth. The muscular parts that the first arch tissues form are the muscles of mastication (chewing), the tensor tympani muscle in the ear, and a number of muscles underneath the jaw. Also, the trigeminal nerve, the maxillary artery, and the external carotid artery are formed from the first arch.
The second pharyngeal arch forms the stapes of the ear and the hyoid bone (lesser horn) of the throat as skeletal derivatives. Muscularly, the second arch creates the muscles of facial expression, the buccinator muscles of the cheeks, the platysma muscle of the neck, and the tiny stapedius muscle to stabilize the stapes. The facial nerve and the stapedial artery also come from it.
The third pharyngeal arch also helps create the hyoid bone (greater horn), and it forms the thymus, whose main function is “to develop immature T-cells into immunocompetent T-cells” after being created in the bone marrow(http://www.acm.uiuc.edu/sigbio/project/updated-lymphatic/lymph5.html). The thymus is located behind the sternum and in front of the heart. The only muscular contribution from the third arch is the stylopharyngeus muscle, located on the side of the head. The glossopharyngeal nerve and the common/internal carotid arteries are also created.
The fourth pharyngeal arch builds the thyroid cartilage, the largest of nine cartilages that make up laryngeal skeleton of the larynx. The thyroid also interestingly “extends anteriorly, forming the laryngeal prominence or ‘Adam’s Apple’”(http://www.getbodysmart.com/ap/respiratorysystem/larynx/thyroid_cricoid/tutorial.html). The male larynx grows larger than the female larynx, which is why the Adam’s Apple is more easily seen in men than in women and why men have deeper voices. The epiglottis is also created by the fourth arch, which is attached to the entrance of the larynx and is covered with a mucous membrane. This membrane can trap pathogens so that ciliated cells push the mucus upwards(to avoid disease in the lungs). The cricothyroid muscle and all the intrinsic muscles of the soft palate are derived from the fourth arch. The cricothyroid muscle is involved in producing tension and elongation of the vocal cords. The nerves made by the fourth arch are the vagus nerve and the superior laryngeal nerve, and the right arch creates the subclavian artery while the left arch creates the aortic arch, which carries oxygenated blood away from the heart to all the tissues of the body.
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ReplyDeleteThe human embryo oddly builds out of a gill structure, even though humans do not have gills after birth. As Aparna stated earlier, this portrays the theme of continuity and change through embryology. To add to her point, this continuity and change is brought upon by evolution. Because humans are terrestrial animals, they have no use for actual gill slits because the lungs are used for gas exchange between the body and the outside environment. Embryology shows us the connection between fish and humans, and proves the evolutionary truth that we are related to fish, and they have passed down the embryonic trait of gill structures to us.
Sources:
Your Inner Fish by Neil Shubin
http://www.eneurosurgery.com/pharyngealarches.html
http://www.acm.uiuc.edu/sigbio/project/updated-lymphatic/lymph5.html
http://www.getbodysmart.com/ap/respiratorysystem/larynx/thyroid_cricoid/tutorial.html
Austin Lee (austinklee7@gmail.com)
Just as Aparna and Austin had previously explained, four gill arches that are commonly found in fish have been found in human embryonic gills that then develop into crucial structures that are required by our advanced body plan. Shubin states, “every head on every animal from a shark to a human shares those four arches in development” (Shubin, 90).
ReplyDeleteTo explain these arches further, similarities are found between the gills of sharks and humans. The first arch in sharks and humans supports and develops into the jaw (Shubin, 91). Austin had stated that the first arch is dedicated to forming or supporting the different structures of the jaw including the maxilla or upper bone and mandilla or lower bone. Contrary to this point, Shubin had stated, “the second rod breaks up to form two bones that support the jaws: a lower one that compares with our hyoid and an upper one that supports the upper jaw” (Shubin, 91). This may bring confusion to some, but both arches contribute to building the structure of the jaw in different aspects, and the bones Shubin was referring to that the second rod become a part of are different from the maxilla and mandilla that Austin described in his response. For the first arch, the difference, as Shubin explains, is the first arch in human embryo adds to the development of the ear bones that does not occur in sharks (Shubin, 91). This is because sharks and other fish do not have bone but rather cartilaginous skeletons. Furthermore, sharks do not have external ears like humans do and therefore do not need the development of these bones (“Ichthyology”, Florida Museum of Natural History). The second arch develops into the upper and lower part of the jaw similar to the hyoid as explained earlier as well as three bones of the middle human ear (Shubin, 91). Therefore, as Shubin bluntly put it, the second arch provides for our ability to swallow and hear. In sharks, the upper bone structure of the jaw from the second arch, as compared to the middle ear bone like stapes, causes it to become retractable. The third arch further develops the hyoid bone and the thymus. As we had learned in the last unit, the thymus is dedicated to the development of T-cells that play a crucial role in cell-mediated immunity in the body. The fourth arch forms the thyroid cartilage and the epiglottis cartilage. The thyroid cartilage makes up the lower part of the larynx and is actually the “Adam’s apple” that we see more pronounced in male necks (“Thyroid Cartilage”, InnerBody). The epiglottic cartilage is the cartilage that folds over the flap that prevents food from entering the trachea in the back of the mouth (Campbell, 885).
Sonia Doshi (soniadoshi7@gmail.com)
(con't)
ReplyDeleteThe reason that the fetus takes on this gill structure is a result of the concept of evolution. While Aparna and Austin make good points in regards to continuity and change as it relates to the gill arches in the embryo, evolution and selective advantage are the key purposes of these fetal structures. As Shubin had stated earlier, every animal begins with these arches in their early developmental stages. These arches provide a stepping stone to jumpstart the development of each of these creatures. As an embryo, humans are so small that their “controlling” structures must be accommodated to their size. The arches provide this sort of advantage. While they remain as gills in fish and sharks, these arches give way to greater structures in species such as humans. Because fish are underwater creatures and are bone-less, the gills provide a way to circulate water over their gills in order to provide oxygen to their body and breathe, and the arches will develop into their main structures such as their jaws or their cartilage but will keep the general gills intact (Campbell, 916). As animals evolved into more advanced species over time such as humans, a greater number of more complex structures needed to be developed within the bodies of these species. Because the gill arches of fish and early animals had already provided this first step of development, their was no selective advantage for advanced species to have evolved to have an entirely new way to develop their own physique. Instead, the function of these arches simply expanded while the functioning time-span decreased. These arches only remain during the fetal stage in mammals such as humans because our greatest development occurs then, and we have no use for gills being land mammals, so there would be no selective advantage for this to remain within our bodies. We’ve basically adapted as a species, over time, to have “gill arches 2.0”.
Sonia Doshi (soniadoshi7@gmail.com)