After years of controversy it is now clear that these cells develop from the self fusion of macrophages. It was in the beginning of 1980 that the monocyte phagocytic system was recognized as precursor of osteoclasts. Osteoclast formation requires the presence of RANKL (receptor activator of nuclear factor κβ ligand) and M-CSF (Macrophage colony-stimulating factor). These membrane-bound proteins are produced by neighbouring stromal cells and osteoblasts, thus requiring direct contact between these cells and osteoclast precursors. M-CSF acts through its receptor on the osteoclast, c-fms (colony-stimulating factor 1 receptor), a transmembrane tyrosine kinase-receptor, leading to secondary messenger activation of tyrosine kinase Src. Both of these molecules are necessary for osteoclastogenesis and are widely involved in the differentiation of monocyte/macrophage derived cells. RANKL is a member of the tumour necrosis family (TNF), and is essential in osteoclastogenesis. RANKL knockout mice exhibit a phenotype of osteopetrosis and defects of tooth eruption, along with an absence or deficiency of osteoclasts. RANKL activates NF-κβ (nuclear factor-κβ) and NFATc1 (nuclear factor of activated t cells, cytoplasmic, calcineurin-dependent 1) through RANK. NF-κβ activation is stimulated almost immediately after RANKL-RANK interaction occurs and is not upregulated. NFATc1 stimulation, however, begins ~24–48 hours after binding occurs and its expression has been shown to be RANKL dependent. Osteoclast differentiation is inhibited by osteoprotegerin (OPG), which is produced by osteoblasts and binds to RANKL thereby preventing interaction with RANK. It may be important to note that while osteoclasts are derived from the hematopoietic lineage, osteoblasts are derived from mesenchymal stem cells.
This kind of insecurity leads to overcompensation: compulsive formality, strict social observances and exaggerated displays of assurance.In 1950, Kurt Schneider described the "fanatic psychopaths" and divided them into two categories: the combative type that is very insistent about his false notions and actively quarrelsome, and the eccentric type that is passive, secretive, vulnerable to esoteric sects but nonetheless suspicious about others.The descriptions of Leonhard and Sheperd from the sixties describe paranoid people as overvaluing their abilities and attributing their failure to the ill-will of others; they also mention that their interpersonal relations are disturbed and they are in constant conflict with others.In 1975, Polatin described the paranoid personality as rigid, suspicious, watchful, self-centered and selfish, inwardly hypersensitive but emotionally undemonstrative. However, when there is a difference of opinion, the underlying mistrust, authoritarianism and rage burst through.In the 1980s, paranoid personality disorder received little attention, and when it did receive it, the focus was on its 
potential relationship to paranoid schizophrenia. The most significant contribution of this decade comes from Theodore Millon who divided the features of paranoid personality disorder to four categories:1) behavioral characteristics of vigilance, abrasive irritability and counterattack,
2) complaints indicating oversensitivity, social isolation and mistrust,
3) the dynamics of denying personal insecurities, attributing these to others and self-inflation through grandiose fantasies
4) coping style of detesting dependence and hostile distancing of oneself from others. Controversy
Due to repeated concerns of the validity of PPD and poor empirical evidence, it has been suggested that PPD be removed from the DSM.