Mechanism of action
For its use in cancer, its mechanism of action is unknown, but on a cellular level, laboratory tests show that tretinoin forces APL cells to differentiate and stops them from proliferating; in people there is evidence that it forces the primary cancerous promyelocytes to differentiate into their final form, allowing normal cells to take over the bone marrow. Recent study shows that ATRA inhibits and degrades active PIN1.For its use in acne, tretinoin (along with other retinoids) are vitamin A derivatives that act by binding to two nuclear receptor families within keratinocytes: the retinoic acid receptors (RAR) and the retinoid X receptors (RXR). These events contribute to the normalization of follicular keratinization and decreased cohesiveness of keratinocytes, resulting in reduced follicular occlusion and microcomedone formation. The retinoid-receptor complex competes for coactivator proteins of AP-1, a key transcription factor involved in inflammation. Retinoids also down-regulate expression of toll-like receptor (TLR)-2, which has been implicated in the inflammatory response in acne. Moreover, tretinoin and retinoids may enhance the penetration of other topical acne medications.The combination of the 10% benzoyl peroxide and light results in more than 50% degradation of tretinoin in about 2 hours and 95% in 24 hours. This lack of stability in the presence of light and oxidizing agents has led to the development of novel formulations of the drug. When microencapsulated tretinoin is exposed to benzoyl peroxide and light only 1% degradation takes place in about 4 hours and only 13% after 24 hours. Biosynthesis
Tretinoin is synthesized from beta-carotene.
Ossification of the posterior longitudinal ligament (OPLL) is a process of fibrosis, calcification, and ossification of the posterior longitudinal ligament of the spine, that may involve the spinal dura. Once considered a disorder unique to people of Asian heritage, it is now recognized as an uncommon disorder in a variety of patients with myelopathy. Causes
The causes of OPLL are unknown. However, genetic and environmental factors appear to play a role in pathogenesis. OPLL may also be associated with diffuse idiopathic skeletal hyperostosis

Diagnosis
Myeolography, including post-myelographic CT is likely the most effective imaging study an accurate diagnosis. Treatment
Surgical management options include extensive cervical laminectomy with or without an additional posterior arthrodesis, anterior decompression and arthrodesis, and posterior cervical laminoplasty. Treatment decisions can be made based on a grading systems devised by Hirabayashi et al., supplemented by the Nurick myelopathy classification system. Prognosis
Most patients suffer from only mild symptoms. Symptoms typically last approximately 13 months. Of patients without myelopathy at initial presentation, only 29% of them will develop myelopathy within 30 years. Epidemiology
The age range of patients with OPLL is from 32 to 81 years (mean = 53), with a male predominance. Prevalence is higher in those of Japanese or Asian ancestry (2-3.5%)  and rarer in other racial groups (0.16%). Schizophrenia patients in Japan may have as high as 20% incidence. References


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