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n order to better understand the universe in which we live, MedTab introduces the "chicken" from a far plant called "Earth." During a recent visit to Earth, Doctor Treslegs, a member of the Alien Abduction Society (AAS), was able to sit in on a "developmental biology" conference, undetected. According to the information he gathered it appears as though us Puppeteers develop within the womb in an orderly fashion through the joining of certain elements.

This article, specifically, has to do with a process the humans call, induction. Induction can be thought of as a signaling between cells in which one cell (or a group of cells) sends a message to responder cells in order to stimulate (or inhibit) certain qualities. These qualities include, be are not limited to: stimulation or inhibition of mitosis within the responding cells; stimulation of the responder cells to secrete signaling molecules; and stimulation of the responding cells to restrict their potency and become more specified (toward their final phenotype). As you will see, humans have found, for example, that lenses within the eye are derived from the ectoderm. Through a series of inductive cues, the ectoderm invaginates and becomes the lens. In this case, the tissue which sends the message to the ectoderm (and thus inducing it) is the optic vesicle of the eye.

Can all tissue types respond to all inducing signals?

It turns out that the answer to this question is NO. Using the example of the lens from above, it turns out that if the optic vesicle is transplanted to other regions of the body containing ectoderm, the lens will fail to develop because the ectoderm is not "competent" to respond to the inducing signal of the optic vesicle. Thus, both the inducing tissue and the responder tissue, and their spatial relationship to each other, are incredibly important for normal development. We invite you to explore our website and learn about how induction aids in the formation of many body parts.

Below we begin by briefly touching on the topics of fertilization, cleavage, gastrulation, neurulation, and organogenesis.

Fertilization

Fertilization is the combination of two haploid gametes to form a single diploid cell. In sexually reproducing species a male sperm combines with a female ovum (egg) to create a zygote. Once the diploid zygote cell is created it can divide and specialize to give rise to the individual organism.

The reaction between a sperm and an egg is a complicated and regulated mechanism. For starters, the sperm has to have the ability to enter the egg and transfer its genetic information. To enter the egg sperm have an enzymatic layer, the acrosome. The enzymes released by the acrosome digest the zona pellucida of the egg (Tosney 2005). Once through the zona pellucida the sperm can fuse with the egg membrane to create one membrane (Fox 1998).

At this point regulation is very important; the egg cannot allow two sperm fertilize it. If two sperm fertilize an egg a condition, called polyspermy, occurs and the cell dies. To prevent polyspermy the fertilization pathway has two key regulatory mechanisms. The first way to prevent polyspermy is a mechanism called the fast block (Tosney 2005). In the fast block mechanism the fertilized egg changes its electrical potential. The depolarization change in electrical potential prevents additional sperm from binding. However, this is not a permanent solution to preventing polyspermy (Fox 1998). So, the egg also uses a method called the slow block.

Upon sperm entering the egg, a rush of calcium ions trigger the cortical granules release enzymes that initiate the slow block response (Fox 1998). The cortical granules fuse with the plasma membrane of the egg and enzymatic activity dissolve protein posts that hold the vitelline and plasma membrane together. This reaction causes water to rush into the egg which expands the vitelline envelope creating a fertilization membrane. The fertilization member prevents additional sperm from fertilizing the egg (Tosney 2005).

For information on Puppeteer fertilization, please consult the “Sexes” page.

Cleavage

Cleavage immediately follows fertilization, and is a series of extremely rapid mitotic divisions in which the enormous volume of zygote cytoplasm is divided into numerous smaller cells, eventually forming a sphere-like structure known as the blastula (Gilbert, 2003).

The mammalian cleavage process has been noted to contain many unique properties. This process is among the slowest in the animal kingdom, as it generally takes about 12-24 hours. The orientation of mammalian blastomeres is another unique aspect, as it undergoes the cleavage pattern known as rotational cleavage (Gilbert, 2003).

Mammalian cleavage is also unique in that the blastomeres do not all divide at the same time, and thus mammalian embryos do not increase exponentially, but often contain an odd numbers of cells. Unlike other non-mammalian development, the switch from mother to zygotic genome becomes activated during early cleavage, allowing for the zygote to produce the necessary proteins for continued cleavage and development to occur (Gilbert, 2003).

However, the most crucial difference to mammalian cleavage is the phenomenon of compaction. Compaction involves the increased expression of cell adhesion proteins, such as E-cadherin, inducing the blastomeres to become adherent and spread on each other to form a compact ball of cells (Tosney, 2005). This tightly packed arrangement is stabilized by tight junctions that form between the outside cells, while leaving gap junctions that allow for small molecules and ions to pass between them (Gilbert, 2003).

Image of mammalian zygote undergoing compaction

The 8-cell embryo continues to divide to produce a 16-cell morula, which subsequently divide to become the trophoblast cells, which produce the extra-embryonic structures, while the inner cell mass will give rise to the embryo and its associated yolk sac, allantois, and amnion (Gilbert, 2003). The process of cleavage appears to be similar in the Pierson’s Puppeteer.

Image of trophoblast and inner cell mass formation

(Click on Image to view Original Source)

Gastrulation

After cleavage has ceased, the blastomeres undergo dramatic movement where they change their positions relative to one another. This series of extensive cell rearrangement is called gastrulation and is very critical for the developing embryo. The end of gastrulation occurs by the formation of a gastrula and the three germ layers, ectoderm, mesoderm and endoderm. These layers specify all other parts of the developing embryo.

In the bird and the mammal development the primitive streak forms, which defines the axes of the embryo and starts in the posterior of the animal and moves towards the anterior. Cells ingress along the primitive streak, forming the three major layers of the embryo mentioned above. At the head fold, the node develops and moves from the anterior to the posterior, laying down the notochord. This notochord induces pattern and neural tissue creating a gradient of maturity leaving the anterior structures more developed than the posterior structures.

This is a SEM of the primitive streak in a chick embryo.

Used with permission from © K. Tosney

Once this process of gastrualation occurs, the anterior/posterior axis is specified by HOX gene expression. In the alien development gastrulation occurs in much of the same way; however there is rapid convergence with the different expression of HOX genes. Most importantly, in the case of the alien gastrulation begins in the posterior end of the embryo and since the HOX gene code is reversed, the posterior end is in the top of the neck for the developing alien.

(Click on Image to view Original Source)

Neurulation

In mammals there are two mechanisms by which the neural tissue of the body is formed. The anterior undergoes primary neurulation while the posterior undergoes secondary neurulation. In primary neurulation the epithelium thickens and many processes occur (i.e. convergent extension, spread of the ectoderm, anchorage of the underlying cells, etc.) to cause the epithelium to elevate and fold. there is then cell-cell recognition, differential adhesion, and reorganization, which allow closure to occur, giving rise to a neural tube and an overlying ectoderm.

In secondary neurulation, which occurs in the trunk of the presumptive spinal cord, a process known as cavitation occurs, forming a hollow neural tube. Differences with this mechanism from primary neurulation results from differential gene expression.

Organogenesis

What is Organogenesis?

Organogenesis is the development of the organs and tissues of an embryo. This process begins after gastrulation when the three embryonic tissue layers (endoderm, mesoderm, and ectoderm) develop. The organogenesis of a mouse and of a Puppeteer are fairly similar, differing mostly with respect to brain organogenesis.

Formation of the Heart

The heart is one of the first structures to develop during organogenesis. The heart is formed from splanchnic mesoderm (the most ventral mesoderm) (Tosney). The heart originally forms outside the body as a thickened tube. It later develops its chambers and ingresses into the body cavity (Tosney). The endoderm regresses to form a place for the heart, and no endoderm is in the heart (Tosney). A Puppeteer’s heart is one of the first organs to form, as in the mouse. Their hearts usually are larger, with the left ventricle significantly larger than the mouse’s because the heart must pump blood up two necks instead of one. Otherwise, a Puppeteer’s heart forms the same way as the mouse heart does.

Formation of the Brain

The brain also begins development early, although it continues to develop through gestation and even after birth (Tosney). The brain’s neurons are “born” at different times and migrate to the outermost layer of the brain. Each layer migrates past the old layers to the outermost portion of the brain; therefore the brain has an “inside-out” organization (Tosney). There are three general types of neurons: sensory, motor, and commissural. Sensory neurons have axons in the brain, spinal cord, muscles, and skin (Tosney). Motor neurons have axons in muscles (Tosney). Commissural neurons have axons that stay in the brain and spinal cord (Tosney). Some of the axons in the body are myelinated (surrounded by cells) to send signals more quickly. In the central nervous system (brain and spinal cord), oligodendrocytes ensheath the axon. In the peripheral nervous system (nerves not in the brain or spinal cord), Schwann cells ensheath the axon (Gilbert). A Puppeteer’s brain is more specialized than a mouse brain. In the mouse, the brain is right in the head, close to the eyes and mouth. In a Puppeteer, the brain is located under the main between the two necks’ connection to the rest of the body. A Puppeteer’s brain, therefore, forms in the inside-out organization, but the two brain hemispheres grow away from each other, instead of next to each other as in the mouse, in order to accommodate the optic and other nerves that go to the eye, head, muscles, and mouth.

Formation of the Limbs

The limbs form from outgrowths of the ectoderm called limb buds and their muscles come from somites (developmental units that form during neurulation) (Tosney). Mesenchyme from somites migrates to the inside of the limbs and will form bones and muscles. The Puppeteer embryo has three limb buds, two for the from legs and one for the back leg. The necks and heads do not form from limb buds, but rather from a splitting of the region of the embryo anterior to the brain.

Formation of the Urinary System

The urinary system begins with the formation of the kidney. Nephrogenic mesenchyme (of mesodermal origin) near the posterior portion of the embryo develops into the pronephros and the nephric duct (the primordial kidney) (Tosney). The pronephros extends posteriorly down the nephric duct, finally degenerating as its replacement, the mesonephros, forms (Tosney). The mesonephros will be replaced with the final kidney, the metanephros (Tosney). The dorsal aorta interacts with the mesonephros to form the glomerulus, which is the part of the kidney that has contact with many blood vessels (Tosney). Waste leaves the blood and enters the kidney. The metanephros will induce the metanephric mesenchyme to form the ureters, which lead to the bladder where urine is stored (Tosney). As the embryo develops, the kidneys move anteriorly to their proper location (Tosney). Puppeteers have urinary systems that are fairly similar to a mouse urinary system.

Formation of the Digestive and Respiratory Systems

The organs of the digestive system and respiratory system all branch off from the embryonic gut. The lungs branch off the from the anterior portion of the gut; while the liver, spleen, gall bladder, and pancreas branch off from the middle part of the gut (Gilbert). The intestines are just specialized portions of the gut tube, as are the esophagus and stomach (Liem et al.). The formation of mouse and Puppeteer digestive and respiratory systems are the same, with the notable exception that the Puppeteer has two esophagi that both empty into the same stomach.

Sources:

Fox, Richard. Lecture Notes Bio 112. Lander University. <http://www.lander.edu/rsfox /112devel.html>. 1998.

Gilbert, Scott F. Developmental Biology. 7^th ed. Sunderland: Sinauer Associates, Inc., 2003. p. 364-368

Liem, Karel F., et al. Functional Anatomy of the Vertebrates: An Evolutionary Perspective. 3^rd ed. Brooks/Cole: Belmont, 2001.

Tosney, Kathryn W. “Heart and circulatory system.” Biology 208: Embryology. Ann Arbor. 17 Nov. 2005.

Tosney, Kathryn W. “Limb Development.” Biology 208: Embryology. Ann Arbor. 29 Nov. 2005.

Tosney, Kathryn W. “Neural Development.” Biology 208: Embryology. Ann Arbor. 27 Oct. 2005.

Tosney, Kathryn W. “Urinary System.” Biology 208: Embryology. Ann Arbor. 15 Nov. 2005.

Tosney, K. "Fertilization." Biology 208: Embryology. Ann Arbor. 2005.

Tosney, Kathryn W. “Early Vertebrate Development.” Biology 208: Embryology. Ann Arbor. September 29, 2005.