L I M B M O R P H O G E N E S I S I N V E R T E B R A T E S
197
survey, which is mostly concerned with the main morphogenetic process
involved in limb development, i.e. the formation of the skeletal segments. Two histochemical methods, McManus' technique for polysaccharides and v. Kossa's technique for alkaline phosphatase, have
proved very useful for the study of metabolic changes occurring during
chondrification. Each precartilaginous blastema undergoes a series of
typical histochemical changes before it reaches the cartilaginous stage.
Although some of these changes may differ from species to species, they
are always the same in all skeletal elements of a particular species and
may therefore be taken as chronological landmarks in the formation of
cartilage. Regarded in this manner, the study of chondrification of the
limb mesoderm may be of some help in retrospectively understanding the
chronology of the inductive processes that have occurred at earlier
stages of the limb development.
The first three changes simultaneously taking place in the transformation of mesoderm into precartilage are cell condensation, intracytoplasmic synthesis of glycogen and loss of alkaline phosphatase.
This latter histochemical peculiarity of course concerns only those
species in which the enzyme is present in the undifferentiated cells,
i.e. mole and chick. The presence of acid phosphatase was recently
demonstrated in young precartilaginous cells of the mole embryo limb
bud; however, the variations of the acid phosphatase activity during
the whole period of chondrification has not yet been studied. The next
significant event occurring in the precartilaginous cells is a notable
increase of cytoplasmic glycogen which may reach such a high concentration that the cell nuclei become completely indistinct. A zymoresistant mucopolysaccharide, probably containing chondroitin sulphuric acid, then appears outside the cell membranes and forms an
extracellular network enclosing each precartilaginous cell in a small
discrete chamber. While the amount of P.A.S. positive cartilaginous
substance is gradually increasing in the extracellular spaces, there is a
parallel decrease of glycogen in the cells themselves: they soon become
smaller and lie freely in their cartilaginous chambers. The amount of
RNA in the precartilaginous differentiating cells, though higher than
in the loose mesenchyme, does not show any significant variation during
the process of chondrification. Right from the beginning of this process,
alkaline phosphatase is synthesized in the mesodermal cells surrounding
the precartilages. These differentiating perichondrial cells flatten and
arrange themselves into numerous layers close to the inner skeletal
element. They have little RNA and no glycogen.
The process of chondrification was studied histochemically at all
stages of the mole embryo limb buds. Except for some minor variations,
the main developmental steps in the formation of the cartilaginous
197
survey, which is mostly concerned with the main morphogenetic process
involved in limb development, i.e. the formation of the skeletal segments. Two histochemical methods, McManus' technique for polysaccharides and v. Kossa's technique for alkaline phosphatase, have
proved very useful for the study of metabolic changes occurring during
chondrification. Each precartilaginous blastema undergoes a series of
typical histochemical changes before it reaches the cartilaginous stage.
Although some of these changes may differ from species to species, they
are always the same in all skeletal elements of a particular species and
may therefore be taken as chronological landmarks in the formation of
cartilage. Regarded in this manner, the study of chondrification of the
limb mesoderm may be of some help in retrospectively understanding the
chronology of the inductive processes that have occurred at earlier
stages of the limb development.
The first three changes simultaneously taking place in the transformation of mesoderm into precartilage are cell condensation, intracytoplasmic synthesis of glycogen and loss of alkaline phosphatase.
This latter histochemical peculiarity of course concerns only those
species in which the enzyme is present in the undifferentiated cells,
i.e. mole and chick. The presence of acid phosphatase was recently
demonstrated in young precartilaginous cells of the mole embryo limb
bud; however, the variations of the acid phosphatase activity during
the whole period of chondrification has not yet been studied. The next
significant event occurring in the precartilaginous cells is a notable
increase of cytoplasmic glycogen which may reach such a high concentration that the cell nuclei become completely indistinct. A zymoresistant mucopolysaccharide, probably containing chondroitin sulphuric acid, then appears outside the cell membranes and forms an
extracellular network enclosing each precartilaginous cell in a small
discrete chamber. While the amount of P.A.S. positive cartilaginous
substance is gradually increasing in the extracellular spaces, there is a
parallel decrease of glycogen in the cells themselves: they soon become
smaller and lie freely in their cartilaginous chambers. The amount of
RNA in the precartilaginous differentiating cells, though higher than
in the loose mesenchyme, does not show any significant variation during
the process of chondrification. Right from the beginning of this process,
alkaline phosphatase is synthesized in the mesodermal cells surrounding
the precartilages. These differentiating perichondrial cells flatten and
arrange themselves into numerous layers close to the inner skeletal
element. They have little RNA and no glycogen.
The process of chondrification was studied histochemically at all
stages of the mole embryo limb buds. Except for some minor variations,
the main developmental steps in the formation of the cartilaginous