PROBLEMS IN CORNEAL MORPHOGENESIS
85
two corneal layers fuse at metamorphosis (Harms, 1923; Popov, 1938)
under the control of thyroxin (Kaltenbach, 1953). In some fish the two
corneae remain separated by a space throughout life. Until metamorphosis in the anuran (Lewis, 1905; Dürken, 1913; Fischel, 1917; Groll,
1923; Beliaeva, 1950), and throughout life in those fish which retain an
external cornea (Harms, 1914), the external cornea forms skin structures
when the underlying eye is removed.
III. Anterior Epithelium
Following its induction by the lens and eye cup, the anterior epithelium increases in area by mitosis in the basal layer. This mitosis also
gives rise to additional layers of cells (the intermediate cells) between
the basal cuboidal layer and the superficial squamous layer. Mitotic
activity in the basal layers continues throughout life to replace cells
which are lost at the surface. The rate of mitosis follows a diurnal
rhythm, and is maximal at night and minimal during the day (e.g. cat:
Fortuyn-van Leyden, 1917, 1926; frog: Meyer, 1954; mouse: Vasama
and Vasama, 1958). In Amhystoma punctatum larvae, hypophysectomy
abolishes this cycle and reduces the overall mitotic rate (Scheving and
Chiakulas, 1961, 1962). The nature of the endocrine mechanism which
is operating in this instance is suggested by the experiment of Sigelman
et al. (1954) which demonstrated an increase in cell division in the cornea] epithelium of the rat following parenteral administration of cortisone. It is not yet known when diurnal cycling makes its appearance during
embryonic development.
The cytogenesis of the anterior epithelium has been studied intensively. The squamous layers at the free surface of the anterior epithelium have the following characteristics in the differentiated state:
strongly periodic acid-Schiff positive (PAS + ) orthochromatic substances of two types (a diastase resistant component which is spread
evenly throughout the cytoplasm, and diastase soluble granules);
sulfhydryl and disulfide groups; sudanophilic substances (possibly
phospholipids) ; and a blue-white fluorescence (Wislocki, 1952). In the
chick embryo both the diffuse PAS positivity and the glycogen granules
are present from very early stages in development (Yoneyama, 1932;
Coulombre, 1961; Vasilyeva, 1961, 1962). Sheldon (1956) followed the
cytogenesis of the squamous cells in the post-partum mouse. The cell
membrane, which is about 80 Â thick, is composed of a central electron
lucent layer bounded by two electron dense layers, and possesses
scattered microvilli. The nuclear membrane is double contoured. The
cytoplasm contains Golgi substance, small rod-shaped mitochondria,
numerous filaments 50 Â in width, and granular endoplasmic reticulum.
85
two corneal layers fuse at metamorphosis (Harms, 1923; Popov, 1938)
under the control of thyroxin (Kaltenbach, 1953). In some fish the two
corneae remain separated by a space throughout life. Until metamorphosis in the anuran (Lewis, 1905; Dürken, 1913; Fischel, 1917; Groll,
1923; Beliaeva, 1950), and throughout life in those fish which retain an
external cornea (Harms, 1914), the external cornea forms skin structures
when the underlying eye is removed.
III. Anterior Epithelium
Following its induction by the lens and eye cup, the anterior epithelium increases in area by mitosis in the basal layer. This mitosis also
gives rise to additional layers of cells (the intermediate cells) between
the basal cuboidal layer and the superficial squamous layer. Mitotic
activity in the basal layers continues throughout life to replace cells
which are lost at the surface. The rate of mitosis follows a diurnal
rhythm, and is maximal at night and minimal during the day (e.g. cat:
Fortuyn-van Leyden, 1917, 1926; frog: Meyer, 1954; mouse: Vasama
and Vasama, 1958). In Amhystoma punctatum larvae, hypophysectomy
abolishes this cycle and reduces the overall mitotic rate (Scheving and
Chiakulas, 1961, 1962). The nature of the endocrine mechanism which
is operating in this instance is suggested by the experiment of Sigelman
et al. (1954) which demonstrated an increase in cell division in the cornea] epithelium of the rat following parenteral administration of cortisone. It is not yet known when diurnal cycling makes its appearance during
embryonic development.
The cytogenesis of the anterior epithelium has been studied intensively. The squamous layers at the free surface of the anterior epithelium have the following characteristics in the differentiated state:
strongly periodic acid-Schiff positive (PAS + ) orthochromatic substances of two types (a diastase resistant component which is spread
evenly throughout the cytoplasm, and diastase soluble granules);
sulfhydryl and disulfide groups; sudanophilic substances (possibly
phospholipids) ; and a blue-white fluorescence (Wislocki, 1952). In the
chick embryo both the diffuse PAS positivity and the glycogen granules
are present from very early stages in development (Yoneyama, 1932;
Coulombre, 1961; Vasilyeva, 1961, 1962). Sheldon (1956) followed the
cytogenesis of the squamous cells in the post-partum mouse. The cell
membrane, which is about 80 Â thick, is composed of a central electron
lucent layer bounded by two electron dense layers, and possesses
scattered microvilli. The nuclear membrane is double contoured. The
cytoplasm contains Golgi substance, small rod-shaped mitochondria,
numerous filaments 50 Â in width, and granular endoplasmic reticulum.