Recall the Reproduction and Development unit.
How does our body develop from a zygote to our full-grown body?
How are organs, such as the brain, developed from our germ layers?
Compare and contrast the embryonic development of a chicken, frog, and a human.
In what ways is our development similar? Different?
Be sure to include terms such as ectoderm, blastopore, primitive streak, etc. in your response.
(Keigo Tanaka; tanakarus3@hotmail.com)
In humans, a zygote is formed as the sperm fertilizes an egg. From there, the zygote travels down the fallopian tubes and into the uterus. While it is traveling, it starts dividing by mitosis, but remains one mass. It goes through cleavage, which means that it becomes many cells all in a ball. This ball is called a blastocyst. In the blastocyst, there is a blastocoel, a space, at one end and an inner cell mass at another end. As the cell attaches to the endometrial epithelium, the inner cell mass becomes the epiblast and the hypoblast. The trophoblast, the side of the blastocyst with the blastocoel, is mainly used for support. As implantation is completed, gastrulation begins. The cells move further into uterine lining through a primitive streak and make the mesoderm and endoderm. During this, the trophoblast keeps going deeper into the endometrium. By the end of gastrulation, three distinct germ layers have formed: the endoderm, mesoderm, and the ectoderm (Campbell 1034).
ReplyDeleteAfter gastrulation, the embryo starts to form. There are four distinct extraembryonic membranes that form during development. The chorion completely surrounds the embryo and whose function is gas exchange. The amnion (the reason reptiles and mammals are called amniotes) cushions the embryo and prevents dehydration and mechanical shock. The allantois disposes of wastes and also contributes in gas exchange (Campbell 1033). The yolk sac contains yolk, which provides nutrients for the embryo until birth. However, what nutrients the yolk sac does not contain, the albumen around the embryo does. The albumen is actually what you see when you crack open a chicken egg and see white, goopy stuff. It’s the “egg white” (Jones and Watt 2).
The germ layers develop into organs based roughly on their organization in vertebrates. There are three germ layers in vertebrates. The ectoderm is roughly on the outside part of the embryo and therefore develops into the epidermis, cornea and lens of the eye, nervous system, tooth enamel, and the adrenal medulla, among others. The mesoderm is located about between the three layers, and therefore develops into the skeletal system, muscular system, excretory system, and many other systems. The endoderm is named because it’s roughly on the inside of the embryo and it develops into the liver, pancreas, epithelial lining of the digestive and respiratory tracts, and many other inner parts (Campbell 1032).
benitorosenberg12@comcast.net
http://www.ncbi.nlm.nih.gov/books/NBK10037/
When an egg and sperm meet and eventually fertilize, a zygote is produced. The human zygote lacks yolk and is holoblastic. The zygote starts to divide; the first division done approximately 36 hours after fertilization, the second division about 60 hours, and the third division about 72 hours later. These blastomeres are equal in size and during the eight-cell stage, the blastomeres are tightly adhered to each other and the outer surface of the embryo takes on a smoother appearance. After cleavage is complete, the embryo has 100+ cells around a central cavity and goes down the oviduct to the uterus - this embryonic stage being called the blastocyst, and the inner cell mass clustered at one end of the cavity. The trophoblast, which is the outer epithelium of the blastocyst, initiates implantation in the uterus by secreting enzymes that break down molecules of the endometrium. After implantation is completed, gastrulation starts and cells move in from the epiblast through a primitive streak, forming mesoderm and endoderm. The trophoblast (which continues expanding into the endometrium), the mesodermal cells from the epiblast, and the endometrial tissue all form the placenta, which is the organ that mediates exchange of nutrients, gases, and wastes between the embryo and the mother. At the end of gastrulation, the developing human has grown from a zygote to a 3 layered embryo with extraembryonic mesoderm and membranes. The development of a child in its mothers womb takes about 9 months to complete before being ready to be born. After being born, babies continue to develop throughout their life, going through several physical stages and milestones, from crawling and walking to the menstrual cycle and the beginning of growth of body and pubic hairs. Going through all of these stages, a human grows from a tiny single celled zygote to a multi-cellular, fully developed human being.
ReplyDeleteOrgans, such as the brain, develop through the process of organogenesis, which involves the three germ layers (endoderm, ectoderm, and mesoderm) in localized shape changes in tissues and individual cells. The brain, specifically, is developed from the neural tube, which is developed from the neural plate, which is developed from the ectoderm. The anterior end of the neural tube, which is called the telencephalon, expands rapidly by cell proliferation and eventually develops into the brain. Eventually, some of these cells stop dividing and are differentiated into neurons and glial cells. The neurons migrate to different parts of the brain and organize into different brain structures.
In the chicken, the type of zygote cleavage is meroblastic and there is lots of yolk in the zygote. Gastrulation occurs via primitive streak. In the frog, the type of zygote cleavage is holoblastic and there is a moderate amount of yolk in the zygote. Gastrulation occurs via invagination, then followed by involution. In humans, the type of zygote cleavage is holoblastic and there is very little amount of yolk in the zygote. Gastrulation occurs via primitive streak, like the chicken. Although human zygotes lack yolk, the early development and organogenesis follow a general pattern that is similar between birds and other reptiles.
Sujin Ko (sujinko93@gmail.com)
Sources: Campbell
http://en.wikipedia.org/wiki/Human_development_(biology)
http://en.wikipedia.org/wiki/Organogenesis
http://en.wikipedia.org/wiki/Neural_development
Shubin explains how teeth development is progressed by a relationship between a inner and outer layer, where the outer layer sets out molecular precursors of enamel and the inner layer makes the dentine and pulp inside. (78) The interaction between these two layers allows the folding of tissues to happen and allows them to secrete organ molecules. The upbringing of teeth by layer interactions has also led to the similar processes of hair, feathers, and breast development, where layers come forth and secrete proteins. Shubin is able to describe layer formation processes with analogies to assembly processes in factories, where different structures form by similar technique. The folding of these specific layers is similar to the layer formation of germ layers in a gastrula from embryogenesis. Animals with two layers (dipoblastic) have the outer ectoderm and inner endoderm. In animals with three layers (tripoblastic) three layers are formed (ectoderm, endoderm, and in addition a layer between called mesoderm). The formation of tooth enamel is part of the ectoderm. (Campbell)
ReplyDeleteWhen sperm is present in oviduct during the period for ovulation, fertilization takes place as the sperm meets the egg. A diploid (2n chromosome number) zygote is formed by fertilization. A cascade of events is triggered for specific mitotic divisions to occur for zygote division. The division of cells is called cleavage. Cleavage can be holoblastic (total/entire) or meroblastic (partial). In early development, mitotic divisions form into a morula (solid ball of cells). The morula’s continuous division forms the blastula (hollow ball of cells). A blastocoel, main cavity, is formed by pressure of cells on enother and is filled with fluids secreted. A thickening along the anterior (front-posterior (back) axis is formed caused by piling up of cells, called the primitive streak. More rapid divisions in the later stages form a two-layer gastrula, and two germ layers (ectoderm and endoderm) become present. In a three-layer gastrula, in vertebrates, a mesoderm layer develops. These three tissues give rise to adult tissues. Cleavage is followed by gastrulation, which is then followed by organogenesis. Inductive processes govern the organogenesis . As the notochord develops from dorsal surfaces, it induces the ectodermal tissues to form a neural plate. This forms the neural tube, to differentiate more into the brain.(http://en.wikipedia.org/wiki/Gastrulation)
Organs, such as the brain, are developed from the ectoderm layer. The ectoderm forms into the neural tube, which houses the spine and upper portions form the brain. The neural tube also differentiates into the spinal cord as well as the brain.
Frogs go through holoblastic cleavage (entire) and the yolk is distributed unequally in zygote. Gastrulation occurs by involution, where the outer cells of the blastula roll over the edge of the dorsal lip. Humans too, go through holoblastic cleavage. Humans require little yolk because the embryo gets nutrients delivered by the placenta from the mother. Humans also form a primitive streak, and cells of the epiblast move midline to enter the blastocoel. Chickens go through meroblastic cleavage (partial), and have a large amount of yolk. The yolk is distributed unequally in the zygote like the frog. Like humans, gastrulation forms a primitive streak.
Human development begins as a male’s sperm cell fertilizes a female’s egg cell (ovum), as it starts in many other sexual organisms, to form a single zygote. This usually occurs in the female’s uterine tube (fallopian tube). Around 24 hours following conception, the zygote begins to divide through cleavage. Three days after fertilization, the zygote becomes a 16-celled morula. Further division turns the morula into the blastula (called a blastocyst in mammals), in which there is a blastocoel. The blastocoel is a fluid-filled cavity within the blastula, making the blastula a hollow ball of cells. At this stage, the organism is unrecognizable, because many species start development in this similar way. Differentiation begins when gastrulation occurs, which is when the blastopore appears on the dorsal side of the late blastula. A crease is formed through invagination, when cells push inward through the blastopore into the surface of the embryo. Gastrulation creates a gastrula, which has three germ layers in humans and many other organisms (two in less complex animals). These germ layers are the endoderm, the mesoderm, and the ectoderm. Each of these layers have specialized cells that eventually form different tissues, which help our bodies function with different systems, simultaneously working to complete our daily processes.
ReplyDeleteMany animals in evolutionary history share the same beginnings of development because different species are derived from past organisms. This concept is shown in the phylogenic trees of animal history. For example, an evolutionary tree based on morphological comparison categorizes phylum Annelida nearby phylum Arthropoda, meaning that the species from the two phyla are derived from a similar ancestor.
Organs are developed through further growth of the embryo, by division of the cells in each of the germ layers. Generally, the ectoderm forms the outermost organs of the body, the endoderm forms the innermost organs (respiratory and digestive systems), and the mesoderm forms many of the parts in between. More specifically, the ectoderm develops into the epidermis of the skin, the epithelial lining of the mouth and anus, the cornea and lens of the eye, the sensory receptors in the epidermis, the adrenal medulla, tooth enamel, and the epithelium of the pineal and pituitary glands. The mesoderm creates the notochord, the skeletal system, the muscular system, the excretory system, the circulatory and lymphatic systems, the reproductive system, the dermis of the skin, the lining of the body cavity, and the adrenal cortex. The endoderm forms the epithelial lining of the digestive tract and respiratory system, the reproductive system, the liver, the pancreas, the thymus (where T-Cells mature), and the thyroid and parathyroid glands.
Sujin Ko and Kyle Kim both explained that organogenesis was the development of organs from the germ layers, but they did not specifically mention what guides the cells to create these tissues and organs. The Organizer region is DNA which directs cells in each of the germ layers to build the body. Also, the Organizer not only aids humans in bodybuilding, but it is present in all organisms with bodies. It was found by Hans Spemann and Hilde Mangold in 1924, after they conducted an experiment testing the effect of a transplant of organizer tissue from one organism to another. They proved that this one small tissue had the ability to guide other cells, shown through producing twins after transplanting this one tissue. They also showed that embryos go through a “’Regulative mode’ of development: at least some cells develop according to their position relative to other cells, rather than entirely according to their lineage history” (http://www.bioinfo.org.cn/book/Great%20Experments/great30.htm). The Organizer region regulates the building of an animal’s body by sending signals to other cells, ordering them to form different tissues and organs.
... (From last post)
ReplyDeleteEmbryologic differences have come up as evolution has created more selective advantages for embryo development. Fish and ambhibians, including frogs, perform external fertilization and external development because they live in moist environments, allowing the male’s sperm to swim to eggs for fertilization. On the other hand, chickens, humans, and other terrestrial animals must undergo internal fertilization for more of an “aim and fire” approach for fertilization. Through internal fertilization, sperm cells do not have to swim as far, increasing the chance for successful copulation. However, birds go through external development by laying eggs. This differs from humans, who must develop internally. Internal development provides a selective advantage in survival because the embryo is protected within the mother, and it can receive nutrients directly from her. In contrast, chicken eggs contain a large amount of yolk, which acts as the source of nutrients for the developing chick. Aquatic eggs of frogs also do not need much yolk because nutrients can be obtained directly from the surrounding water. Chickens have their yolk concentrated on one side of the egg, the vegetal pole, so they must undergo meroblastic cleavage. Frogs and humans, however, perform holoblastic cleavage because the little amount of yolk in them is more equally divided. In holoblastic cleavage, the cleavage furrow passes all the way through the cells to divide the embryo completely; in meroblastic cleavage, the cleavage furrow can’t pass through the yolk, dividing the egg incompletely.
Sources:
Your Inner Fish by Neil Shubin
Campbell Textbook
http://www.embryo.chronolab.com/cleavage.htm
http://www.bioinfo.org.cn/book/Great%20Experments/great30.htm
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/F/FrogEmbryology.html
Austin Lee (austinklee7@gmail.com)