Genetic Regulation of Melanocyte Differentiation

Melanocytes originate from the neural crest in vertebrates and migrate to the body surface where they differentiate into functional cells. Genes involved in melanocyte differentiation can be classified into two groups. One of them consists of the functional genes that control proteins specific to the function of the melanocyte. As the representative gene of this category, albino (c) locus in the mouse is considered to control tyrosinase, the key enzyme in melanogenesis. cDNA for mouse tyrosinase has been cloned and sequenced. The cDNA can be used to detect tyrosinase mRNA synthesized during melanocyte differentiation. On the other hand, genes such as brown (b) or pink-eyed dilution (p) have been assumed to control melanosome proteins. The other category consists of genes that regulate the expression of these functional genes directly or indirectly. In the mouse, so-called white-spotting genes and genes of the agouti series are considered to fall into this category. Based on the fact that mutations at the white-spotting loci result in the absence of melanocytes in a particular area of skin, it is assumed that some of these loci control the factors that promote either differentiation or migration of melanoblasts and are candidates for the classic regulator genes Genes at the agouti (a) locus in the mouse determine the type of melanin synthesized in hair follicle melanocytes, that is eumelanin or pheomelanin. An interesting feature of this locus is that the site of gene action is not within the melanocytes but in the cells surrounding them. The results of our study indicate that the gene product of the a-locus interacts with α-MSH at the α-MSH receptor site, regulates the cellular cAMP level via a signal transduction system and, in turn, determines the type of melanin synthesized in the cells.     

The Effect of Hydrocortisone on the Epidermal Proliferation in an Organ Culture of Chick Embryonic Skin

Hydrocortisone is regarded as an initiator of keratinization in embryonic skin. The present investigation dealt with the effect of hydrocortisone on the proliferation of epidermal cells during early development: Cell kinetic analyses using ³H-thymidine autoradiography were applied to a skin organ culture prepared from a 13-day chick embryo.
Hydrocortisone at a concentration between 0.01 and 1.0 μg/ml was effective in initiating a morphological change leading to the epidermal keratinization in vitro and caused a marked decrease in the mitotic and labeling indices of epidermal basal cells, the decrease being maximum at 2 days of culture previous to the morphological change.
During continuous labeling with ³H-thymidine, the number of labeled basal cells reached 100% within 2 days in the control and 4 days in the culture treated with hydrocortisone. This confirmed that the growth fraction of epidermal basal cells was 1.0 even after the administration of hydrocortisone.
The duration of each cell cycle phase at 2 days of culture was determined by percent labeled mitoses and double-labeling analyses. It was concluded that hydrocortisone extended the generation time of epidermal basal cells at this time point about three fold over the control. This extension was mainly due to the elongation of the G ₁ phase.     

Macrophage Migration Inhibition by Serum from Desensitized Animals Previously Sensitized with Tubercle Bacilli

MIGRATION of peritoneal exudate cells removed from guinea-pigs or mice exhibiting delayed hypersensitivity is inhibited by specific antigen^1–3. This in vitro macrophage migration inhibition has been regarded as a useful immunological test for delayed skin hypersensitivity^4,5. Studies of the mechanism of this phenomenon revealed that, in contact with specific antigen, lymphocytes from sensitized animals released into the medium a specific substance (migration inhibitory factor; MIF) capable of inhibiting the migration of normal macrophages^6,7. When injected intradermally into normal guinea-pigs, MIF elicits inflammatory reactions characterized by induration, erythema and mononuclear cell infiltration⁸.     

Growth and curvature in seedlings of Pisum sativum and an ageotropic mutant

In an attempt to explain the influence of gravity on the behaviour of ageotropic plant organs, a pea mutant (Pisum sativum ageotropum) and normal pea (Pisum sativum cv. Sabel) were examined. The mutant has a significantly lower germination rate (large seeds: 25%, small seeds: 10%) than normal pea seeds (55%). Removal of testa increased germination dramatically, the values obtained were 63 and 89%, respectively. Immediately after imbibition the mutant from which the testa had been removed, developed more slowly than normal pea seeds; after 28 h the difference in elongation rate between the two types was reversed. When continuously stimulated geotropically in the horizontal position the elongation in the mutant is larger than in the normal pea roots kept in the same position. During a 24 h period starting 48 h after imbibition the mutant root elongated 45.0 mm while the value for the normal pea root was 11.5 mm. The course of the geotropic curvature in roots of the two types has been followed during a period of 24 h. Normal pea roots develop an asymmetry in the extreme root tip region after 30 min of horizontal stimulation. After prolonged stimulation (exceeding 2 h) the asymmetry has disappeared and the curvature distributed over the entire growth region. When roots of normal pea are stimulated continuously at various angles, the optimum angle of geotropic response is 90° with decreasing responses in the order 135° (i.e. the root tip is pointing obliquely upward) and 45°. The presumed ageotropic behaviour of the mutant has only to a certain extent been confirmed in the present study. When stimulated at 135° a slight positive curvature developed; stimulation at 90° and 45° gave a slight negative curvature.     

EFFECTS OF HYPOPHYSECTOMY ON IMPLANTED TESTES OF THE NEUTER MEDAKA, ORYZIAS LATIPES

The effects of hypophysectomy on spermatogenesis in testes implanted to neuter individuals of the medaka, oryzias latipes , have been histologically studied.
The neuter fish produced by prolonged androgen treatment during larval life maintained the implanted testes for more than 8 months, irrespective of the genetic sex of the host. Histological examination revealed that all the implanted testes were filled with numerous cysts containing cells at all spermatogenetic stages, as in the normal testes. Each cyst contained only cells at the same stage. Furthermore, the implanted testes had fertile spermatozoa. In the experiment of regeneration of amputated anal fins, the implanted testes were found to be able to induce the formation of papillar processes, although the number of processes was one-half of that induced by normal testes.
Hypophysectomy of the fish was successfully achieved by orbital approach. The survival rate of the operated fish was 95% a week after the operation and still 50% 14 months later. Hypophysectomy inhibited the transformation of spermatogonia to spermatocytes in spermatogenesis.
The presence of the capacity to carry out normal spermatogenesis and induce papillar processes in the testis-implanted neuter fish suggests that the hypothalamus of the neuter fish was not seriously affected by prolonged androgen treatment.