**2.2. Organotypical culture or organ culture**

Organ culture or organotypical culture consists of culturing an organ's fragments or explants. Regarding the thyroid, this began with 2 to 3 h incubations (the term usually used to refer to cultures lasting less than 24h) of sheep thyroid fragments in the presence of radioactive iodide thereby demonstrating *in vitro* the ion's incorpoproportion into diiodinethyronine (DIT) and T4 [40]. When the transmission electron microscope was developed in the 1970s, this led to an ultra-structural description of thyrocytes *in vitro*; the first descriptions of thyrocytes' morphological changes in different culture conditions were made. It was shown that organ culture thyrocytes had reduced RER and GC in the absence of TSH [41,42]. Thyrocyte follicular architecture and ultra-structure were rapidly lost in some of the models which were described. Approaching the 1990s attempts were made to use very small fragments (less than 1mm3) in organ cultures (called mini organ cultures) which lasted 2 to 3 days without necrosis, exhibited iodide, sulphate and phosphate transport, synthesised a 19S Tg (normally glycosylated and iodised) [43] and were maintained for up to 7 days without cell death when coated with collagen [44]
