The Best Laid (Body) Plans --- Discussion of Chapter 6 "Your Inner Fish"

Indeed all tetrapods have one head and four limbs arranged in two pairs, a tail and a variety of other gross morphological features. We, human beings, have the same basic body plan, too! (except the tail) But why? And where did this basic body plan come from? Chapter 6 of Your Inner Fish would give us the answer.

Here are some key terms in chapter 6. Let's discuss them one by one. :) 

I. Embryology:  The study of the development of an embryo from the fertilization of the ovum to the fetus stage. The study focuses on comparing different species at their early stage. By comparing, scientists are able to find the common structures of various species.

At the very early stage --- all look similar, almost the same. 

II. Germ layers:  Karl Ernst Von Baer, a Russian biologist and a founding father of embryology,  discovered that there are three layers in embryos (also known as germ layers). 

The endoderm is one of the germ layers formed during animal embryogenesis --- the process by which the embryo forms and develops. 

Cells migrating inward along the archenteron form the inner layer of the gastrula, which develops into the endoderm. Endoderm forms many of the inner structures of our bodies, including digestive tract and numerous glands that are associated with it. 

The mesoderm germ layer forms in the embryos of triploblastic animals. During gastrulation, some of the cells migrating inward contribute to the mesoderm, an additional layer between the endoderm and the ectoderm. The formation of a mesoderm led to the development of a coelom. Organs formed inside a coelom can freely move, grow, and develop independently of the body wall while fluid cushions and protects them from shocks. 

The mesoderm has several components which develop into tissues: intermediate mesoderm, paraxial mesoderm, lateral plate mesoderm, and chorda-mesoderm. The chorda-mesoderm develops into the notochord. The intermediate mesoderm develops into kidneys and gonads. The paraxial mesoderm develops into cartilage, skeletal muscle, and dermis. The lateral plate mesoderm develops into the circulatory system (including the heart and spleen), the wall of the gut, and wall of the human body. Also, it forms tissue in between skeleton and muscles. 

The ectoderm is the outer layer of the embryo, and it forms from the embryo's epiblast. The ectoderm develops into the surface ectoderm, neural crest, and the neural tube.

The surface ectoderm develops into: epidermis, hair, nails, lens of the eye, sebaceous glands, cornea, tooth enamel, the epithelium of the mouth and nose.
The neural crest of the ectoderm develops into: peripheral nervous system, adrenal medulla, melanocytes, facial cartilage, dentin of teeth.
The neural tube of the ectoderm develops into: brain, spinal cord, posterior pituitary, motor neurons, retina.

III. Organizer: 

In 1903, Hans Spemann discovered that some cells have the capacity to form a whole new individual on their own at the early stage of embryo. 

In 1920, Mangold discovered Organizer, which can direct other cells to form an entire body plan. Organizer could control the activity of the embryo. The importance of the organizer experiment is the discovery that a part of the mesoderm influences the ectoderm as the ectoderm differentiates into central nervous system tissue. 

All of the cells within a complex multicellular organism such as a human being contain the same DNA; however, the body of such anorganism is clearly composed of many different types of cells. What, then, makes a liver cell different from a skin or muscle cell? That is because the particular combination of genes that are turned on (expressed) or turned off (repressed) dictates cellular morphology (shape) and function. This process of gene expression is regulated by cues from both within and outside cells, and the interplay between these cues and the genome affects essentially all processes that occur during embryonic development and adult life. DNA interactions control and show how genes work.