Simple, reliable detection of latex microspheres in high quality tissue sections

Penetration Profile of Microspheres in Follicular Targeting of Terminal Hair Follicles

The transfollicular administration of pharmacologically active molecules is of current therapeutic interest, mainly with regard to delivery to specific sites of the hair follicle (HF) and the reduction of hepatic metabolism and systemic toxicity. HF are privileged pathways for specific molecules depending on formulations, which enter faster into these shunts than through the stratum corneum. The aim was to optimize the delivery of fluorescent microspheres into the HF, thereby, developing a standardized protocol for follicular targeting with microspheres. The number of HF showing penetration, as well as the depth of penetration, was determined. Freshly excised skin samples with terminal HF were divided into groups, with or without prior treatment with cyanoacrylate skin surface stripping-technique (CSSS). Thereafter microspheres at a size of 0.75-6.0 microm were applied according to the developed standardized protocol. Skin biopsies were obtained, shock-frozen, and sectioned in 5 microm slices. We demonstrated a selective penetration route of the microspheres into the HF. Optimal microsphere size proved to be approximately 1.5 microm, with a 55% rate of all HF, and with a maximum penetration depth of >2300 microm. Without previous CSSS treatment of the skin, the transfollicular microsphere penetration was below 27% with a maximum penetration depth of 1000 microm. Thus, the basis for follicular targeting of essential structures containing stem cells for keratinocytes, melanocytes, and mast cells has been laid.

Histological methods to determine blood flow distribution with fluorescent microspheres

We evaluated several histological methods and determined their advantages and disadvantages for histological studies of tissues and organs perfused with fluorescent microspheres. Microspheres retained their fluorescence in 7-10 microm serial sections with a change in the antimedium from toluene when samples were fixed in formalin and embedded in paraffin. Several antimedia allowed both wax infiltration of tissue and preservation of microsphere fluorescence. Histoclear II was the best substitute for toluene. When samples were fixed in formalin and embedded in glycol methacrylate, thinner (3-5 microm) sections provided greater histological detail but had fewer microspheres per section. Air dried lung tissue followed by Vibratome sectioning provided thick sections (100 microm) that facilitated rapid survey of large volumes of tissue for microspheres but limited histological detail, and the air drying procedure was restricted to lung tissue. Samples fixed in formalin followed by Vibratome sectioning of unembedded tissue provided better histological detail of lung tissue and was also useful for other organs. These sections were more difficult to handle and to mount on slides compared to air dried tissue, whereas fixed tissue embedded in gelatin provided better tissue support for Vibratome sectioning. Rapid freezing followed by cryo-microtome sectioning resulted in frozen sections that were relatively difficult to handle compared to embedded or unembedded tissue; they also deteriorated relatively rapidly with time. Paraffin sections were stained with hematoxylin and eosin or with aqueous methyl green, although tissue autofluorescence by itself was usually sufficient to identify histological features. Methacrylate sections quenched tissue autofluorescence, and Lee's stain or Richardson's stain were used for staining sections. Toluene based mountants such as Cytoseal quenched fluorescence, particularly the red fluorescent microspheres. Aqueous based mountants such as Aquamount, Crystal/Mount, Fluoromount-G were substituted, although such preparations were not as permanent as Cytoseal mounted coverglasses and tended to cause fading of stained sections.