Primary cell cultures from farm animals
When searching through the literature, it is quite easy to find cell culture systems for any desired tissue. However, not all published protocols are accepted as standard procedures, and there is a large variety of culture conditions. In general, care should be taken when comparing methods and extrapolating a system from one species to the other. There are only a few publications dealing with primary cultures derived from veterinary relevant species (as compared to those from laboratory animals). It was the use of laboratory animals, which has resulted in the development of standard procedures - but these must be adapted for each species. It can be expected that species-specific cell culture systems will become important to complement results obtained by genome and proteome analysis, both in human and in veterinary research.

Nevertheless, primary cell cultures of several tissues have found their way into basic and clinical veterinary research. The choice of animal and organ is often made on practical grounds, like availability of tissue or slaughterhouse procedures. For example, isolation of porcine pneumocytes from slaughterhouse material will be difficult, as many lungs are affected by the scalding procedure. On the other hand, kilograms of bovine adrenals can be collected in the slaughterhouse, from which pure adrenal cortex cells are easily prepared that are viable for some weeks and maintain differentiated functions. Fig. 4 shows bovine adrenal cortex cells cultured for six days in serum-free medium and treated with ACTH, cAMP, dexamethasone, cycloheximide, and tunicamycin.

   Endocrine tissue
For these technical reasons it is not surprising to find high-ranking publications using bovine adrenal cortex cultures [5,10]. The protocols leading to a standardised procedure have been developed in the late 70ies [14, 18]. More information concerning optimal culture conditions of these cells can be found in the literature [9, 24],. Other endocrine tissue has been successfully cultured, like bovine and porcine thyroid cells [2]. A detailed description of cell cultures from porcine pancreas has been published [20]. Primary cell cultures from the porcine reproductive system have also been described, like endometrial cells [6], cells from the ovary [11], and Leydig cells [3, 13]. Bovine mammary epithelial cell culture was employed to study the synthesis and secretion of IGF binding proteins [12]., and bovine pituitary cells for the analysis of signaling pathways [22].

   Liver
The isolation of cells from the liver has been a challenge for nearly two decades. The main problem is the rapid onset of cell death post mortem. However, the importance of this tissue led to large efforts in method development to maintain viable and differentiated cultures from large animals. Bovine, canine and sheep primary hepatocytes were studied for their hormonal responsiveness under serum free conditions [7, 8, 16]. Adult chicken hepatocytes were analysed for their differentiation state under serum free conditions [23]. The optimisation of differentiation was also the focus of studies using porcine hepatocytes [15, 19]. These refinements are of great importance, as improvement of the quality of primary cultures makes them a viable alternative to in vivo experiments, particularly for pre-screening. Thus liver cell culture systems were employed to study aspects of inflammation in bovine hepatocytes [1, 17].

A methological option that needs more attention is the preparation of cell cultures from animals that had died in veterinary clinics. In a recent publication the development of equine adipocyte cultures from expired animals has been described [21].

   Skeletal muscle
Skeletal muscle is also easy to obtain and to process into viable cultures. It can be obtained from a variety of animals, and used even many hours post mortem. A recent review [4] summarises isolation and culture protocols from animals of veterinary interest. In the case of skeletal muscle culture it is important to realize that these do not result from outgrowth of existing fibers. It is the dormant stem cells, satellite cells, which are isolated and which fuse in vitro to form new myotubes.

It has been the aim of this mini-review to briefly summarize the principles of primary cell culture and to show that protocols are available for various species of veterinary relevance. The above list of tissues as a source of primary cell culture is by no means exhaustive; cartilage, bone, skin, kidney, spleen, blood derived cells, lung and intestinal tissues have also successfully been processed for primary cell cultures.


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