Gene expression in sheep skin and wool (hair).

We sequenced 2939 ESTs from fetal and adult sheep skin. Stages of gestation were picked to coincide with the major events in skin appendage (wool follicle) formation. Clustering analysis generated a nonredundant set of ESTs 2435 strong (83% nonredundant). Approximately 24% of these gave no hit to NCBI build 29 of the human genome, while 35% were tentatively classified by putative function based on BLASTX hits with a p(N) of <10(-4). In addition to bioinformatics analysis of our ESTs and gene mapping, we have generated a large EST spatial expression data set using in situ hybridization. One thousand one hundred forty-two ESTs have been used for in situ localization; about 31% are from adult sheep skin, 39% from late gestation fetal sheep skin, and 30% from midgestation fetal sheep skin. These probes have been used in over 3000 hybridization experiments. In this report, we summarize the results of in situs on adult sheep skin.

Characterization and distribution of epidermal growth factor receptors in the skin and wool follicles of the sheep fetus during development.

We have determined the binding affinity and capacity and relative distribution of epidermal growth factor (EGF) receptors in the skin of the Merino sheep fetus before and during the development of the wool follicle population. Autoradiography of tissue sections incubated with [125I]EGF revealed that label was confined predominantly to the epidermis and dermoepidermal junction before follicle formation, at 30 and 55 d of gestation. During follicle initiation (Days 60 to 65), receptor activity was distributed over the epidermis, including the epidermal aggregations of primordia at the dermoepidermal junction. However, receptor concentrations, as revealed by grain counts of autoradiographs, were reduced in these regions when compared with 55-d skin. The receptor distribution over the epidermis and its derivatives did not alter during subsequent follicle development, although the intensity of labeling increased as the follicles matured. Specific receptor binding was not observed above background levels in the dermis and dermal papillae during all stages of follicle development. At follicle maturation, EGF receptors were widely distributed over the cells of the epidermis and the epidermal derivatives of the cutaneous appendages but were particularly localized in the sebaceous glands and outer root sheath (see also Wynn et al. 1989). EGF immunoreactive material has also been found at these sites (du Cros et al. 1992), suggesting an autocrine role for EGF in the regulation of cell function. It is likely that the differentiation-promoting activities of EGF may predominate over those of growth, because the receptor-bearing cells were not members of rapidly proliferating populations.

Growth factor expression in skin during wool follicle development.

A variety of growth factors are likely to be involved in initiation and morphogenesis of wool follicles. To enable direct comparisons of the expression of different growth factors, reverse transcriptase-polymerase chain reactions (RT-PCR) were developed for ovine and murine TGF alpha, TGF beta 1, TGF beta 2, TGF beta 3, IGF1, IGF2, and FGF-2, which could all be carried out on a single cDNA sample. These RT-PCR were used with 16 sheep RNA samples from different foetal stages, neonatal sheep and mouse skin. The mRNAs for these growth factors were detected throughout gestation in sheep skin, except for TGF beta 1 mRNA which was not expressed in 51-day-old skin, but was expressed in 54-day and older samples. Since the first microscopically visible changes of follicle initiation occur around 62 days gestation, these results suggest that TGF beta 1 expression may be a signal for follicle initiation.

Retarded and excessive development of skin appendages in fetal lambs in response to thyroidectomy before wool follicle appearance.

The impact on wool follicles of development in an athyroid environment was studied in a series of twin fetal lambs by surgically thyroidectomizing one of each pair before the appearance of follicle buds and comparing development of epidermal appendages in it with their development in the normal co-twin. Thyroidectomy was undertaken at 51 to 54 days' gestation, i.e. after approximately one-third of the gestation period. Each treated fetus was then replaced in the uterus, allowing pregnancy to continue. Eight pairs of twins were removed at intervals from 67 to 122 days' gestation and skin samples from the thyroidectomized and the intact twins were compared. Micromorphometric examination of the samples was used to assess quantitatively the effects of thyroid deprivation on wool follicle development. In thyroidectomized fetuses there was a failure of keratinization in primary wool follicles, an absence of secondary follicles, a tendency to excessive follicular branching and sweat gland development, and a paucity of sebaceous gland formation. The density of wool follicles was substantially increased, but the mean cross-sectional area of these follicles was reduced. The effects of very early thyroidectomy imply that the thyroid plays a role in the stimulation and regulation of wool follicle differentiation. To test the reversibility of the effects observed in the skin of thyroidectomized fetuses, grafts from these animals were transplanted to normal, young fetal lambs. Subsequent examination of grafted skin revealed that complete keratinization had occurred but that none of the other abnormal features had been reversed.

[Histologic and histochemical studies of the prenatal development of the wool follicle in Egyptian fat-tailed sheep]. / Histologische und histochemische Untersuchungen der pränatalen Entwicklung der Wollfollikel bei ägyptischen Fettschwanzschafen.

Thickness of epidermis, diameter and density of follicles as well as the ratio of secondary to primary follicles went up with significance (P less than 0.01) along with development. Medium concentrations of neutral and acid mucopolysaccharides were histochemically recorded from the corium, while high levels were present in basal stratum and hair pivots. High concentrations of alkaline phosphatase were recorded from basal stratum, hair pivots, and papillae and moderate concentrations from sweat and sebaceous glands, with the latter containing also glycogen and lipids.

The role of a reaction-diffusion system in the initiation of primary hair follicles.

A mechanism based on a reaction-diffusion system is proposed for the initiation of hair follicles in the epidermis during fetal development. It is demonstrated that initiation of primary follicles in a series of waves, within the proposed mechanism, is a consequence of the size and shape dependent properties of the reaction-diffusion system without the need for the propagation of signals through the skin. The observed trio grouping of follicles and variation of primary follicle density per unit skin area during development are also correctly predicted. An explanation, based on the reaction-diffusion system and the variation of its characteristic spatial wavelength with time during development, is suggested for the termination of both primary and secondary follicle initiation as well as follicle neogenesis. The proposed initiation mechanism is basically the same as that used to explain various spatial patterns observed in hair fibre formation (Nagorcka & Mooney, 1982).

Effects of maternal nutrition on the initiation of secondary wool follicles in foetal sheep.

The initiation of secondary (S) wool follicles usually takes place between about 95 and 135 days of gestation. Severe underfeeding during the first half of this period did not significantly inhibit the initiation of S follicles, but severe underfeeding during the latter half of this period resulted in a significantly lower number of S follicles and this number was not increased by refeeding ewes to a high level between 132 days and term. A similar reduction in S follicle initiation was effected when ewes were moderately underfed throughout most of pregnancy, but when the ewes were refed between 120 days of gestation and term the numbers of S follicles initiated were not significantly different from those of a group of well-fed animals. It is concluded that S follicle initiation is most affected by maternal undernutrition between about 115 and 135 days of gestation.

Epidermal growth factor: a critical factor in fetal maturation?

Epidermal growth factor (EGF) infused over 3-14 days into fetal sheep of 110-125 days gestation resulted in a number of morphological and endocrine changes. Striking hypertrophy of the skin, wool follicles and their accessory structures was seen, together with a reduction in the ratio of secondary to primary follicles and degenerative changes in wool fibres associated with shedding of fibres. Adrenal, thyroid, liver and kidney weights were increased while thymus weight was decreased. The increase in adrenal size resulted from cortical hypertrophy and was associated with increased cortisol secretion. Thyroid hypertrophy was accompanied by an increase in colloid stores, decreased plasma thyroxine and reverse triiodothyronine (T3) concentrations, unchanged plasma T3 and thyroid-binding globulin and raised thyrotropin (TSH) levels. Thyrotropin receptor affinity and content per gram of tissue were unchanged. Fetal and maternal plasma prolactin and growth hormone levels, and fetal plasma placental lactogen levels, were unchanged, although there was a significant rise in maternal plasma placental lactogen concentrations with high doses of EGF. Other maturational parameters such as switching from fetal to adult haemoglobin and liver glycogen content were unaffected.