Differentiation Of Pigment Cells Research Articles

Behavior and differentiation process of pigment cells in a tropical sea urchin Echinometra mathaei.

The behavior and differentiation processes of pigment cells were studied in embryos of a tropical sea urchin Echinometra mathaei, whose egg volume was one half of those of well-known sea urchin species. Owing to earlier accumulation of pigments, pigment cells could be detected in the vegetal plate even before the onset of gastrulation, distributed dorsally in a hemi-circle near the center of the vegetal plate. Although some pigment cells left the archenteron during gastrulation, most of them remained at the archenteron tip. At the end of gastrulation, pigment cells left the archenteron and migrated into the blastocoele. Unlike pigment cells in typical sea urchins, however, they did not enter the ectoderm, and stayed in the blastocoele even at the pluteus stage. It is of interest that the majority of pigment cells were distributed in the vicinity of the larval skeleton. Aphidicolin treatment revealed that eight blastomeres were specific to pigment cell lineage after the eighth cleavage, one cell cycle earlier than that in well-known sea urchins. The pigment founder cells divided twice, and the number of pigment cells was around 32 at the pluteus stage. It was also found that the differentiation of pigment cells was blocked with Ni2+, whereas the treatment was effective only during the first division cycle of the founder cells.

MITF: a stream flowing for pigment cells.

Microphthalmia-associated transcription factor (MITF) is a transcription factor with a basic-helix-loop-helix-leucine zipper (bHLHZip) structure. Mutations of the MITF gene cause a variety of phenotypes, most notably in pigmented cells, in several species. In humans, haploinsufficiency of MITF causes Waardenburg syndrome type 2, while a dominant-negative mutation causes Tietz syndrome. Four isoforms have been cloned so far: MITF-M is the most abundant and is expressed in neural-crest-derived melanocytes; MITF-A is expressed in various cultured cells including retinal pigment epithelium (RPE) and enriched in RPE of embryonal and developing eyes; MITF-H are expressed in many types of cultured cells and in the heart tissue; MITF-C is expressed in many types of cultured cells, but not in melanocytes. Many growth factor signaling pathways have been implicated for regulation of MITF at both protein and promoter levels. Most notably, Steel factor/c-Kit signaling pathway was linked to phosphorylation of MITF at Ser73 and Ser409 through activation of MAP kinase and RSK-1, respectively. Phosphorylation of MITF is also conducted at Ser298 through GSK3beta, although the signaling pathway for this event still remains to be elucidated. IGF-1 and HGF/SF pathways may merge with the c-Kit signaling pathway. WNT and MSH signaling pathways regulate MITF positively at the promoter level. Endothelins may regulate MITF at the protein and promoter levels. MITF is involved in the differentiation, growth and survival of pigment cells, employing a number of signaling pathways.

Surviving Drosophila eye development

During eye development, cell death interplays dynamically with events of differentiation to achieve the remarkably patterned structure of the fly compound eye. Mutations in genes that affect the normal developmental process can lead to excessive death of progenitor cells, or, alternatively, to the differentiation of supernumerary neurons, pigment and cone cells due to survival of cells that would normally be eliminated. These data reveal that eye development contains cell selection processes: only certain cells are selected to undergo differentiation, and supernumerary cells are actively eliminated by cell death pathways to achieve the highly ordered lattice of the eye. The final number of cells that comprise the eye is controlled through a balance of cell proliferation with proper cell differentiation and removal by cell death.

Role of integrins in differentiation of chick retinal pigmented epithelial cells in vitro.

When retinal pigmented epithelial cells (PEC) of chick embryos are cultured under appropriate conditions, the phenotype changes to that of lens cells through a process known as transdifferentiation. The first half of the process, characterized by dedifferentiation of PEC, is accompanied by a marked decrease in adhesiveness of PEC to collagen type I- or type IV-coated dishes. To understand the underlying mechanisms of this change, we analyzed the expression of integrins, which are major receptors for extracellular matrix components. Northern blot analysis with cDNA probes for chicken α3, α6, α8, αv, β1 and β5 integrin mRNA showed that the genes for all these integrins are transcribed at similar levels in PEC and dedifferentiated PEC (dePEC). Further analysis of β1 integrin, which is a major component of integrin heterodimers, showed that although the protein amount of β1 integrin was not changed, its localization at focal contacts seen in PEC was lost in dePEC. When anti-β1 integrin antibody was added to the PEC culture medium, a decrease of cell-substrate adhesiveness occurred, followed by a gradual change in both morphology and gene expression patterns to ones similar to those of dePEC. These findings suggest that an appropriate distribution of β1 integrin plays an essential role in maintaining the differentiated state of PEC through cell-substrate adhesion.

Mechanism of Pseudoalbinism in Flatfish: An Association Between Pigment Cell and Skin Differentiation

AbstractTwo groups of flounder P. olivaceus larvae were reared under different conditions to provide either normally pigmented or pseudoalbinic metamorphosed juveniles. The process of the differentiation of skin and pigment cells during postembryonic development was analyzed by means of histological, histochemical and immunofluorescent assay methods. In parallel with these assays, the differentiation of pigment cells was examined with the use of organ and cell culture in vitro. The results obtained strongly suggested that pseudoalbinism was evoked as a result of disruption of the mechanisms that controlled the establishment of asymmetric skin structures during metamorphosis.

From the Crest to the Periphery: Control of Pigment Cell Migration and Lineage Segregation

Pigment cells are one of many cell types derived from the neural crest. This review focuses on the mechanisms that control the timing and pathways of migration of pigment cells into the epidermis and determinants that control the differentiation of pigment cells. Several factors may control the timing and pattern of pigment cell migration in the dorsolateral space including the loss of inhibitory molecules in the pathway, the appearance of chemotactic molecules emanating from the dispersing dermatome, and the differentiation of pigment cells, which may be the only neural crest derivative capable of utilizing the substratum found in the dorsolateral path. Control of pigment cell differentiation remains controversial. A working model presented in this review suggests that multipotent neural crest cells that disperse ventrally upon separation from the neural tube preserve neurogenic ability and lose melanogenic ability, whereas those cells that are arrested at the entrance to the dorsolateral path lose neurogenic ability so that the population becomes primarily melanogenic. During the time that the latter population is arrested in migration it is speculated that the neural crest cells are exposed to an environment comprised of specific extracellular matrix molecules and/or growth factors that enhance pigment cell differentiation.

Inhibition of neural crest cell differentiation by embryo ectodermal extract.

The white mutation in Mexican axolotls has long been thought to be a defect associated with the embryonic extracellular environment, but not with embryonic neural crest cells. Thus it was believed that pigment cells in white axolotls disappear from the skin during early development, not because they are intrinsically defective but because they have no choice but to move into an unfavorable environment. We present evidence to suggest that: (1) white neural crest cells are in fact intrinsically different from dark (wild-type) cells, and (2) an inhibitor is produced in white embryonic ectoderm that actively suppresses the migration, differentiation, and survival of pigment cells in this animal. How these observations fit into the existing body of literature on the white mutant and a model for how the white phenotype might develop are discussed.

Pattern Formation: The Differentiation of Pigment Cells from the Embryonic Neural Crest

This chapter illustrates the usefulness of the amphibian neural crest and pigment cells for studying vertebrate pattern formation. The fact that perturbation of pigment cell metabolism, substances intrinsic to the skin, lectins, and disruption of the extracellular matrix (ECM) can all influence pigment patterns (i.e., pigment cell migration and/or differentiation) provides researchers with ample opportunities for experimentation. From the point of origin atop the neural tube, neural crest cells migrate to seemingly precise positions within an embryo and then differentiate to form a wide variety of cell types and structures, among which are included sympathetic and parasympathetic neurons, craniofacial structures, endocrine and paracrine cells, and pigment cells. The accessibility of the neural crest and the relative ease of manipulating these cells without causing lethal damage to the embryo contribute to the popularity of amphibian embryos as a model experimental system for studying both migration and differentiation of crest cells.

Pigment cell pattern formation in Taricha torosa: The role of the extracellular matrix in controlling pigment cell migration and differentiation

The neural crest is a population of highly migratory mesenchymal cells that ultimately localize in specific sites and differentiate into a variety of cell types. This report describes studies on the factors governing the migratory pathways, differentiation, and ultimate localization of the neural crest-derived pigment cells (black melanophores and yellow xanthophores) in the California newt, Taricha torosa. Melanophores first appear scattered in the dorsal portion of the lateral neural crest migratory pathway (between the somites and the ectoderm). These cells are eventually found in two stripes: a dorsal stripe that runs along the apex of the somites, and a midbody stripe near the somite-lateral plate mesoderm border. Melanophores are not seen in the dorsal fin of prehatching embryos. Xanthophores can be identified with the light microscope using NH 4OH-induced autofluorescence of pteridines and in the transmission electron microscope (TEM) by the presence of pterinosomes. Xanthophores first appear scattered among the melanophores over the surface of the somites; these cells eventually are found between the two melanophore stripes and in the dorsal fin. We were interested in determining the roles of the extracellular matrix (ECM) in controlling the formation of pigment cell patterns in T. torosa. Immunocytochemistry, Alcian blue staining of paraffin sections and ruthenium red staining of thin sections (accompanied by Streptomyces hyaluronidase and chondroitinase ABC digestion) were used to identify the composition and distribution of the ECM surrounding the pigment cells at various stages during development. The adhesive glycoprotein fibronectin is found in the dorsal portion of the lateral neural crest migratory pathway as well as in the dorsal fin matrix. Glycosaminoglycans (GAG) are found primarily in the dorsal fin and in the ECM surrounding the notochord. The dorsal fin ECM contains hyaluronate (HA), which was identified in the TEM as Streptomyces hyaluronidase-sensitive 3–5 nm microfibrils, as well as sulfated proteoglycan aggregates. We then confronted T. torosa neural crest cells in vitro with known ECM molecules. When neural folds are explanted onto tissue culture plastic in half-strength L-15 medium containing 10% fetal calf serum (FCS), cells migrate from the explant and differentiate into melanophores after 6 to 9 days. Xanthophores appear in the cultures 2 to 4 days after the appearance of melanophores. When cultured on three-dimensional collagen gels, xanthophores migrate significantly farther ( P < 0.01) onto and into the collagen than melanophores (336 ± 183 vs 196 ± 160 μm from the edge of the explant). When 2.5 mg/ml chondroitin sulfate (CS) is present in the collagen gel, the distance that both pigment cell types migrate from the explant is reduced, with the result being that only xanthophores invade the GAG-rich matrix. When 1 mg/ml HA is present in the collagen gel, the differentiation of pigment cells is inhibited. Melanophores appear 48 hr later than in control gels without HA, and the number of melanophores in the explant after 10 days is significantly reduced ( P < 0.01; 26.6 vs 1.1 melanophores/explant). When 1 mg/ml of HA is added to the FCS-enriched medium over neural crest cells spreading on tissue culture plastic, there is a similar delay and inhibition of pigment cell differentiation. With 2 mg/ml of CS there is no effect on pigment cell differentiation in vitro. Melanophores eventually appear in the dorsal fin of T. torosa several weeks after hatching. When fragments of dorsal fin that contain no apparent melanophores are transferred onto tissue culture plastic, melanophores appear in the explants after a few days in culture. These results suggest the following model of ECM-cell interactions during pigment cell pattern formation in T. torosa: Pigment cells differentiate in regions of the embryo that contain relatively little GAG. Xanthophores are able to invade the GAG-rich dorsal fin, but melanophores can not. The melanophores that eventually appear in the dorsal fin are derived from the neural crest cells that invaded the fin during early development, and were delayed in differentiating by the presence of HA.

The pigmentary system of developing axolotls

ABSTRACT The axanthic mutant in the Mexican axolotl (Ambystoma mexicanum) was analysed with respect to the differentiation of pigment cells. Transmission electron micrographs revealed the presence of melanophores and cells that are described as unpigmented xanthophores in axanthic skin. Iridophores apparently failed to differentiate in axanthic axolotls (a pattern similar to that observed in melanoid axolotls). Chromatographic analyses of skin extracts confirmed that there are no pteridines (xanthophore pigments) in axanthic skin, suggesting that the axanthic gene may affect pteridine biosynthesis at some point early in the biosynthetic pathway. Why iridophores fail to differentiate in these animals is not known, but this, too, may be related to an inability to synthesize pigments properly. Xanthophore and iridophore pigments both presumably derive from purine precursors. Finally, all axanthic animals were found to be infected by a virus. Electron microscopic results demonstrated the presence of numerous macrophages in the dermis of the skin, occupying positions typical of pigment cells. The virus was localized primarily in macrophages, but was also observed in pigment cells. The virus is, as yet, uncharacterized but is thought to contribute to the low survivability of axanthic adults.

The pigmentary system of developing axolotls: III. An analysis of the albino phenotype

ABSTRACT The albino mutant in the Mexican axolotl (Ambystoma mexicanum) is analysed with respect to the differentiation of pigment cells. Pigment cells were observed with the transmission electron microscope in order to determine any unusual structural characteristics and to determine what happens to each of the cell types as development proceeds. Chemical analyses of pteridine pigments were also carried out, and the pattern of pteridines in albino animals was found to be more complex than, and quantitatively enhanced (at all developmental stages examined) over, the pattern observed in comparable wild-type axolotls. The golden colour of albino axolotls is due primarily to sepiapterin (a yellow pteridine) and secondarily to riboflavin (and other flavins). Coincident with enhanced levels of yellow pigments, xanthophore pigment organelles (pterinosomes) in albino skin reach a mature state earlier than they do in wild-type axolotl skin. This morphology is conserved throughout development in albino animals whereas it is gradually lost in the wild type. Unpigmented melanophores from albino axolotls are illustrated for the first time, and in larval albino axolotls the morphology of these cells is shown to be very similar to xanthophore morphology. In older albino animals xanthophores are easily distinguished from unpigmented melanophores. Iridophores seem to appear in albino skin at an earlier stage than they have been observed in wild-type skin. Morphologically, wild-type and albino iridophores are identical.

Studies on the development and growth of mammalian nervous system by aggregation chimeras: [formula omitted], [formula omitted], [formula omitted]. [formula omitted], [formula omitted], [formula omitted], [formula omitted]. [formula omitted], [formula omitted], [formula omitted] & [formula omitted]. Dept. Physiol. Keio University, Tokyo, Central Inst. Exp. Animals, Kawasaki, Dept. Anatomy, Hikkaido University, Sapporo

The neural crest is a population of highly migratory mesenchymal cells that ultimately localize in specific sites and differentiate into a variety of cell types. This report describes studies on the factors governing the migratory pathways, differentiation, and ultimate localization of the neural crest-derived pigment cells (black melanophores and yellow xanthophores) in the California newt, Taricha torosa. Melanophores first appear scattered in the dorsal portion of the lateral neural crest migratory pathway (between the somites and the ectoderm). These cells are eventually found in two stripes: a dorsal stripe that runs along the apex of the somites, and a midbody stripe near the somite-lateral plate mesoderm border. Melanophores are not seen in the dorsal fin of prehatching embryos. Xanthophores can be identified with the light microscope using NH4OH-induced autofluorescence of pteridines and in the transmission electron microscope (TEM) by the presence of pterinosomes. Xanthophores first appear scattered among the melanophores over the surface of the somites; these cells eventually are found between the two melanophore stripes and in the dorsal fin. We were interested in determining the roles of the extracellular matrix (ECM) in controlling the formation of pigment cell patterns in T. torosa. Immunocytochemistry, Alcian blue staining of paraffin sections and ruthenium red staining of thin sections (accompanied by Streptomyces hyaluronidase and chondroitinase ABC digestion) were used to identify the composition and distribution of the ECM surrounding the pigment cells at various stages during development. The adhesive glycoprotein fibronectin is found in the dorsal portion of the lateral neural crest migratory pathway as well as in the dorsal fin matrix. Glycosaminoglycans (GAG) are found primarily in the dorsal fin and in the ECM surrounding the notochord. The dorsal fin ECM contains hyaluronate (HA), which was identified in the TEM as Streptomyces hyaluronidase-sensitive 3–5 nm microfibrils, as well as sulfated proteoglycan aggregates. We then confronted T. torosa neural crest cells in vitro with known ECM molecules. When neural folds are explanted onto tissue culture plastic in half-strength L-15 medium containing 10% fetal calf serum (FCS), cells migrate from the explant and differentiate into melanophores after 6 to 9 days. Xanthophores appear in the cultures 2 to 4 days after the appearance of melanophores. When cultured on three-dimensional collagen gels, xanthophores migrate significantly farther (P < 0.01) onto and into the collagen than melanophores (336 ± 183 vs 196 ± 160 μm from the edge of the explant). When 2.5 mg/ml chondroitin sulfate (CS) is present in the collagen gel, the distance that both pigment cell types migrate from the explant is reduced, with the result being that only xanthophores invade the GAG-rich matrix. When 1 mg/ml HA is present in the collagen gel, the differentiation of pigment cells is inhibited. Melanophores appear 48 hr later than in control gels without HA, and the number of melanophores in the explant after 10 days is significantly reduced (P < 0.01; 26.6 vs 1.1 melanophores/explant). When 1 mg/ml of HA is added to the FCS-enriched medium over neural crest cells spreading on tissue culture plastic, there is a similar delay and inhibition of pigment cell differentiation. With 2 mg/ml of CS there is no effect on pigment cell differentiation in vitro. Melanophores eventually appear in the dorsal fin of T. torosa several weeks after hatching. When fragments of dorsal fin that contain no apparent melanophores are transferred onto tissue culture plastic, melanophores appear in the explants after a few days in culture. These results suggest the following model of ECM-cell interactions during pigment cell pattern formation in T. torosa: Pigment cells differentiate in regions of the embryo that contain relatively little GAG. Xanthophores are able to invade the GAG-rich dorsal fin, but melanophores can not. The melanophores that eventually appear in the dorsal fin are derived from the neural crest cells that invaded the fin during early development, and were delayed in differentiating by the presence of HA.

The pigmentary system of developing axolotls

ABSTRACT The melanoid mutant in the Mexican axolotl (Ambystoma mexicanum) is analysed with respect to the differentiation of pigment cells. Pigment cells were observed with the transmission electron microscope in order to determine any unusual structural characteristics and to determine what happens to each of the cell types as development proceeds. Chemical analysis of pteridine pigments was also carried out, and changes in pteridine biosynthesis were found to correlate well with changes in xanthophore morphology and number. In melanoid axolotls, as development proceeds, melanophore numbers increase, xanthophores decrease, and iridophores fail to differentiate at all. This is considered to result from: (a) conversion of xanthophores (that are present in young larvae) to melanophores; (b) the gradual programming of the majority of chromatoblasts to become, exclusively, melanophores, and (c) the failure of some chromatoblasts (possibly iridoblasts) to differentiate altogether. The ultrastructural and chemical evidence presented in this study is compared to similar data for wild-type axolotls, and a mechanism regarding how the melanoid gene might act is suggested.

Pigment cell differentiation in the fire-bellied toad, Bombina orientalis. I. Structural, chemical, and physical aspects of the adult pigment pattern.

Wild-collected adults of Bombina orientalis are bright green dorsally and red to red-orange ventrally. As a prelude to an analysis of the differentiation of pigment cells in developing B. orientalis, we describe structural and chemical aspects of the fully differentiated pigment pattern of the "normal" adult. Structurally, differences between dorsal green and ventral red skin are summarized as follows: (1) Dorsal green skin contains a "typical" dermal chromatophore unit comprised of melanophores, iridophores, and xanthophores. Red skin contains predominantly carotenoid-containing xanthophores (erythrophores), and skin from black spot areas contains only melanophores. (2) In ventral red skin, there is also a thin layer of deep-lying iridophores that presumably are not involved in the observed color pattern. (3) Xanthophores of red and green skin are morphologically distinguishable from each other. Dorsal skin xanthophores contain both pterinosomes and carotenoid vesicles; ventral skin xanthophores contain only carotenoid vesicles. Carotenoid vesicles in dorsal xanthophores are much larger but less electron dense than comparable structures in ventral xanthophores. The presence of carotenes in ventral skin accounts for the bright red-orange color of the belly of this frog. Similar pigments are also present in green skin, but in smaller quantities and in conjunction with both colored (yellow) and colorless pteridines. From spectral data obtained for xanthophore pigments and structural data obtained from the size and arrangement of reflecting platelets in the iridophore layer, we attempt to explain the phenomenon of observed green color in B. orientalis.

Differentiation of Lens and Pigment Cells in Cultures of Brain Cells of Chick Embryos

Dissociated cells of brains (tel- and diencephalons) of 3.5-day-old chick embryos were cultivated in vitro under the cell culture conditions which are known to be permissive for neural retinal cells (NR cells) to transdifferentiate into lens and/or pigmented epithelial cells (PE cells). The differentiation of lentoid bodies (LBs) with lens-specific delta-crystallin and PE cells with melanin granules was observed in such brain cultures. LBs appeared in two different phases, i.e., 2-3 days and 16-30 days of cultivation, and after 40 days of culture these structures were formed in all 60 culture dishes. Sometimes, LBs were observed in foci of PE cells formed during earlier stages of brain cultures. When similar brain cultures were prepared with older embryos of 5-, 8.5-, 14-, and 16-days of incubation, no differentiation of lens and PE cells was observed.

Neoplastic Pigment Cells Induced by N-Methyl-N-nitrosourea (MNU) in Xiphophorus and Genetic Control of Their Terminal Differentiation

Neoplastic pigment cells can be induced by N-methyl-N-nitrosourea (MNU) in certain genotypes of the fish Xiphophorus that never develop such cells spontaneously. In some, the neoplastic cells retain their incompletely differentiated stage and proliferate, apparently due to unrestrained division, into malignant pigment cell neoplasms (‘melanomata’). In other fish most of the neoplastic pigment cells become terminally differentiated and form harmless ‘hyperplastic foci’. Terminal differentiation of MNU-induced neoplastic pigment cells appears to be predominantly controlled by a gene complex designated ‘differentiation’ (Diff).

Age-dependent changes in the capacity of transdifferentiation of retinal pigment cells as revealed in clonal cell culture

Changes in the capacity of transdifferentiation of retinal pigment cells into lens cells with developmental age were examined by culturing cells obtained from chicks at various developmental stages ranging from 5-day-old embryos to 1-year-old adults. Secondary clonal cultures were prepared from pigment cells harvested from primary cultures of the cells dissociated from the material at different ages. The percentage of colony formation to the total number of inoculated cells, of colonies with lens differentiation, and of colonies with the differentiation of pigment cells were obtained in cultures of the material derived from the donors at different ages. The total clonal efficiency did not alter with age. Colonies with lens cells appeared only in cultures started from embryos of less than 15 days. The percentage in the formation of pigmented colonies decreased with the age of the donor. Cells of post-hatching chicks produced only unidentifiable colonies without lens and pigment cells. The results indicate that determination of retinal cells alters with age.

Changes in surface morphology of pigmented retinal cells during differentiation in clonal culture

The changes in surface morphology during the reexpression of differentiation of chick retinal pigmented epithelial cells (RPE) in clonal culture have been studied using the scanning electron microscope (SEM) and transmission electron microscope (TEM) and compared with those described in vivo. Three-week-old colonies demonstrated a gradual change in apical surface morphology along any colony radius. At the outer edge, the cell surfaces were either smooth with a few small filamentous protrusions or showed a varying number of large blebs. Toward the centre of the colony the surfaces demonstrated a gradual increase in filamentous protrusions. The apical surfaces of the most densely pigmented cells at the centre of the colony consisted mainly of small rounded protrusions. The changes in surface morphology of cells in the centre of younger colonies during redifferentiation were similar to those found along the radius of a 3-week-old colony. The results show that older colonies have all of the morphological stages of the redifferentiation process (and possibly the biochemical ones as well) arranged along any radius.The basal surfaces of all the colonies were covered by a thin acellular membrane that stained positively with periodic acid–Schiff (PAS) and which may contain fibronectins and appears to be involved in cellular attachment.

The Focal differentiation of pigment cells

A study has been made of the normal development and of the regeneration after excision of the groups of large pigment cells which form the spotted skin pattern of the gecko Eublepharis macularius, together with the effects of neonatal graft transplantation on this pattern. The results all indicate strongly that such groups of specialized pigment cells are not clones but the product of an induction process. This is then compared with the neural reflex mechanism by which the skin pattern of Chamaeoleo dilepis is formed.

AGE-DEPENDENT CHANGE IN THE TRANSDIFFERENTIATION ABILITY OF CHICK NEURAL RETINA IN CELL CULTURE.

We examined how the transdifferentiation ability of neural retinal cells into lens and/or pigment cells in call culture is changed with the development of the donor. Cells dissociated from neural retinas of chick embryos ranging from 3-day-old to the stage immediately before hatching and of 3-day-old chicks were cultured for about 60 days. The results clearly indicated that the transdifferentiation ability decreased with age. The latest developmental stage at which the differentiation of lens cells took place was in 18-day-old embryos. A gradual decrease in this ability was shown by the comparison of crystallin content in cultures prepared from embryos at different stages. The differentiation of pigment cells was recognized in cultures of neural retinas earlier than in 15-day-old embryos. Such loss of the ability of neural retinal cells to transdifferentiate into pigment cells earlier than into lens cells can be partially attributed to inhibitory factors accumulated in medium conditioned with many neuronal cells present in cultures.