... and that's how I became interested in embryology. Well, sort of. As soon as I saw this diagram in this Dictionary of biology (Penguin, 2004) straight away started the process of deep study of basic terminology when it comes to development of an embryo:
blastula - name of the stage of development
blastocoele - cavity in the middle of blastocyst
blastoderm - first (outer) row of cell in a blastocyst
blastomere - name of one cell in a blastocyst
blastopore - transitory opening for injecting ES cells
ES cells stands for embryonic stem cells, which are the ones that are still not differenciated into specific type. First type of cells produced in the brainstem are unspecified, or "sort of" universal, and they grow to produce a cluster of cells called morula. This is a stage prior to blastula, in which the basic cleavage (or division) of those cells occur.
One cell becomes two, four, eight, sixteen, and so on, and that's how morula is created. Then the outer layer and inner cavity is formed, and this is already second stage called blastula. Diagram shows roughly this stage, when an artificial injection of ES cells can occur.
After that blastula follows another stage called gastrula - the stage where first differentiation happen. Firstly, cells are divided into three main categories - germ layers:
- ectoderm - outermost cells
- mesoderm - middle layer inbetween
- endoderm - innermost cells
The ectoderm leads to development of skin, nervous system (neurons and glia), and gland cells. Mesoderm leads to bones, muscles, kidneys and gonads. Endoderm developes into yolk cells and alimentary canal (mainly epithelials in liver, lungs, stomach, pharynx, and intestine).
Mesoderm further divides into dorsal and ventral, which in layterms means the one at the back and the one in front respectively. Ectoderm further divides into three types - neural plate (neurons and glia) on one side, neural crest (cartilage) in the middle, and epidermis (skin, gland cells) on the other side. All these processes of differentiation are collectivelly named gastrulation movements, and once they are finished, the new stage called neurula will start.
Neurula, is called this way, because it signified the presence of neural plate at the begining of this stage. Neural plate is rising from ectodermal tissue, and further develop into so-called neural tube, which is the begining of the formation of spine.
Neurula, is called this way, because it signified the presence of neural plate at the begining of this stage. Neural plate is rising from ectodermal tissue, and further develop into so-called neural tube, which is the begining of the formation of spine.
So that would be about the embryo. In human, the embryo becomes a foetus at about seven weeks from beginning, which is marked by the appearance of first bone being formed. If you notice the blue color underlined sentence, you will probably realize that there are probably not sufficiently defined limits, where exactly the human life starts. Some would place it exactly inbetween an embryo and feotus, some at the conception, still some would go even further.
Many people would place it even before that - the intention of conception. So this topic and the debate remains still open, as I read here and there. What do you think?
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