Studies in the Dynamics of Histogenesis

Abstract

1. The region of most active mitosis per mm. of cross-section in the intestine is the entodermal epithelial tube. The mitotic figures primarily follow the path of a right-handed helix. In one of the twenty embryos the mitotic figures describe the path of a right-handed helix. 2. The region of least active or relatively passive growth per mm. of cross-section is the mesenchyme, derived from the splanchnic mesoderm surrounding the epithelial tube. 3. The rapid expansion, due to epithelial growth in a rotating spiral manner, of the intestinal lumen is greater than the activity manifest in the surrounding mesenchyme. This causes a pressure in the latter resulting in a flattening and elongation of the mesenchymal cells. The successive changes in shape of these cells through the spherical, ellipsoidal, and spindle cellular phases are seen. The mesenchymal wall decreases in thickness, due to tension caused by epithelial tubular dilation. 4. The rotating spiral growth of the epithelial cells causes the formation of a series of mesenchymal cellular and fibrillar concentric rings due to the centripetal force of the former. 5. The circular, smooth muscle cells are differentiated in the outer, more condensed margins of the ring. At these points the developing tensional stresses are greater than within the ring. 6. The inner circular smooth muscle coat is the first one differentiated and is incident to the rapid growth of the epithelial tube in diameter. The former soon tends to restrict the growth of the epithelial tube in diameter. The tube, pursuing the lines of least resistance, grows in length. During the period of rapid growth in length the outer longitudinal muscle coat is in the process of formation. 7. The tensional stresses to which the elongated strained mesenchymal cells are subjected appear to be a dynamic stimulus to smooth muscle differentiation. 8. From this study of a closely graded and progressive series of sections of intestinal development, the conclusion is drawn that muscle tissue is not self-differentiating, in the strict sense of the term, but that the tension of differential growth acts as the stimulus to smooth muscle differentiation.

Ultrastructural and morphometric analysis of the separation of two thigh muscles in the chick

“…Muscle separation, therefore, is subject to complex spatial-temporal control that is separate from general maturation processes. In addition, motor nerves (Paterson, 1888;Shellswell and Wolpert, 1977;Shellswell, 1977;Harrison, 1905), tensile forces imposed by skeletal growth (Carey, 1920;Shellswell and Wolpert, 1977), a prepatterning by vasculature (Caplan and Koutroupas, 1973;Wilson, 19861, and a prespecification of myogenic precursors to become parts of particular muscles (Chevallier and Kieny, 1982;Lance-Jones, 1988a,b) have all been shown to be unlikely to cause muscle cleavage. The separation of muscles is currently thought to be a n autonomous process carried out by those cells within or near the muscle masses, and the patterning is generally considered to make use of a postulated three-dimensional system of positional information (see Shellswell and Wolpert, 1977).…”

Genes, forces, and forms: Mechanical aspects of prenatal craniofacial development

“…It was as early as in 1874 when Wilhelm His sen. (His, 1874) explained our body form as a consequence of mechanical forces exerted by tissues growing in a confined space. Eben J. Carey was also convinced that forces act during morphogenesis when he explained the formation of the annular muscles of the colon and esophagus (Carey, 1920a(Carey, ,b, 1935.…”

“…Muscle fibers arise wherever mesenchymal tissue is available that can be dilated by adjacent expanding tissue (Carey, 1920a(Carey, ,b, 1921(Carey, , 1936.…”

“…At first glance, it is difficult to give credence to the very unusual mechanical interdependencies between cell growth, spatial impediment and subsequent folds and bulges (His, 1874), and the concept of tissue differentiation in fields of specific mechanical stress (Carey, 1920a(Carey, ,b, 1921Blechschmidt, 2004). It seems too mechanistic and too simplistic.…”

“…The key idea, however, is that the cells do not follow an intrinsic program of differentiation during morphogenesis but are able to react to the different situations into which they have been maneuvered by their own growth and that of their neighboring cells (Carey, 1920a(Carey, ,b, 1921Blechschmidt, 2004). The genetic code provides the basis for condensed mesenchymal cells to react by forming cartilage.…”