Embryogenesis
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Contents |
Introduction
Beginning immediately at fertilization, the single cell divides again and again in controlled and organized steps until a distinct multicellular organism is formed. But what are the stages of this process and how are they controlled? The study of these developmental processes is called embryology.
The stages of development in embryology can be broken down into fertilization, cleavage, gastrulation, and neurolation.
Fertilization
Fertilization begins with the fusion of two gametes: the egg (ovum) and the sperm. The result of this fusion is the fertilized egg or zygote. During fertilization, the genetic material of the sperm and egg are combined. Each gamete is haploid, that is, it contains one-half of the normal number of chromosomes for the species. At fertilization, the gametes combine to produce a zygote with a full number of chromosomes- fertilization restores the diploid number. Because of the distinct differences between egg and sperm, the sperm only deliveries its genetic material and as a result, all organelles and most cellular content inside the zygote are from the egg. As soon as the sperm enters the egg, the cell membrane of the egg undergoes changes that prevent the entrance of additional sperm, this ensures that exactly the right number of chromosomes are combined (one set from each parent).
Cleavage
Within a few hours of the chromosomes coming together, the first cell division begins. The first division splits the cell into two daughter cells called blastomeres. These two blastomeres then divide into four blastomeres, the four blastomeres divide into eight, and so on. During cleavage, the total number of cells increases, but the size of each cell decreases. The reason for this strange situation is that cell division occurs so rapidly that there is not enough time for the individual cells to grow bigger. This constant doubling of cells results in a multicellular embryo of roughly the same size as the original single celled zygote.
Blastulation
The solid ball of blastomeres formed through repeated cleavage is called a morula. Soon after cleavage is complete however, the morula rearranges itself into a single layer of cells with a fluid-filled center; the embryo at this stage is called a blastula and it has just undergone blastulation.
Gastrulation
Through a process of invagination, in which a region of cells in the blastula fold in on themselves, the gastrula is formed. The result is a double-layer cup of cells. The opening of the inner folded region is called the blastopore, the outer layer of cells is termed the ectoderm and the inner layer of cells is termed the endoderm. The inner endodermal layer surrounds a new cavity, a begining of a primitive gut known as the archenteron. After formation of the two cell layer gastrula, a small portion of cells breaks off to produce a third layer known as the mesoderm.
The ectoderm, mesoderm and endoderm are the three germ layers from which all cells, tissues and organs develop.
Cells of the ectoderm differentiate into the epidermis, hair, nails, claws, sweat glands, tooth enamel, brain, and spinal cord. Mesoderm differentiates into muscles, blood, blood vessels, heart, spleen, reproductive organs, and kidneys. Endoderm differentiates into the cells lining the digestive and respiratory systems, the liver, gallbladder, and pancreas.
Neurulation
As differentiation continues, the three layers continue to divide and become the different regions of the body. One specific event that occurs is the formation of the nervous system. The formation of the notochord (by some mesoderm cells) triggers overlying ectoderm cells to be pulled inwards through a cellular inductive effect. These ectoderm cells continue to expand into a neural fold and eventually close in on themselves forming a neural tube. This tube then expands to give rise to the brain and spinal cord. Cells from the neural fold but on the outside of the neural tube expand radially throughout the body to become the components of the peripheral nervous system (including the sensory ganglia, Schwann cells, and adrenal medulla).
Test Yourself!
See the Embryogenesis Section Test to ensure you understand this material and how it might be asked on the MCAT.