CHAPTER 6 



Embedding 



In the early days of histology and cytology, sections were cut by 

 hand. Skilled workers could cut ordinary plant tissue admirably, 

 for each cell was held in place by a firm cell-wall. Zoologists were 

 at a disadvantage, which they sought to overcome by using fixa- 

 tives that hardened the tissues. Indeed, their attention was focused 

 on this, rather than on the primary object of fixation ; and accord- 

 ingly they called their fixatives hardening agents. When micro- 

 tomes came into general use, it was found convenient to embed 

 tissues in suitable media which would hold the cells and intercellu- 

 lar matter in place while the razor slid past them. Animal tissues 

 are seldom sectioned today without having been embedded, and 

 it is therefore unnecessary that fixatives should have a hardening 

 eff"ect. 



Embedding media must necessarily be substances that are 

 capable of easy conversion from liquid to solid form. The liquid 

 penetrates the tissues to a greater or lesser extent and is then con- 

 verted into a solid. We shall see that this conversion may involve 

 hydrogen bonding, covalent linkage, crystallization, or polymer- 

 ization. 



It is sometimes suggested that embedding media may be divided 

 into two groups: those that penetrate the cells, and those that 

 merely surround them. Actually, it is doubtful whether any sharp 

 distinction can be drawn. Some embedding media certainly enter 

 cells, and indeed may penetrate their nuclei (p. 75) ; but whether 

 any particular medium penetrates any particular kind of cell or 

 merely surrounds it may depend on the fixative used. Those 

 fixatives that coagulate proteins in the form of a coarse sponge- 

 work leave the tissues in a state that favours the entry of embed- 



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