Memoirs: Observations on the Shape of the Nucleus and its Determination

Abstract

Variations in the shape of the nucleus have been described in different animal cells. In addition, the following factors have been shown to be responsible for nuclear shape: (1) Surface tension : when this is equal over the surface of the nuclear membrane, the nucleus tends towards the spherical condition. When surface tension varies over the interface between nucleus and cytoplasm, nuclear polymorphism may result. (2) Mechanical deformation of the nucleus is common and may be due to various causes, chief amongst which are: (a) Pressure from cytoplasmic inclusions, e.g. fat, lecithin, and yolk; (b) Tonofibrillae; (c) in striated muscle, the influence of the Membranes of Krause which constrict the nucleus along its length--and limit its ends--by their prolongation from the myofibrillae into the sarcoplasm. (3) The centrosome, which has been shown (in the resting cell) often to repel that part of the nuclear membrane which is nearest to it. (4) The relation between cell shape and nuclear shape has been briefly discussed. It has been noted that the nucleus never comes into contact with the cell membrane, except in the rarest instances due to the intervention of mechanical factors. Evidence has been brought forward in favour of our view that there is a mutual repulsion between cell membrane and nuclear membrane. (5) Canaliculi and incisions in the nuclear membrane have been described in various cells. (6) The unfolding of such incisions during development and differentiation of some such cells has been described. (7) Intranuclear rodlets and their importance in the maintenance or the modifying of nuclear shape have been discussed. (8) Mitotic division and a certain degree of nuclear differentiation have been shown often to be incompatible--thereby accounting for amitosis in certain highly specialized nuclei. (9) The need for care in distinguishing between nuclear incisions and genuine amitotic division of nuclei has been emphasized.