The cause is genetic mosaicism; that is, some of, or most of, the germ cells of one parent have a dominant form of OI, but not enough of their somatic cells do to cause symptoms or obvious disability in the parent—the parents different cells have two (or more) sets of slightly different DNA. : 1513  It has been clinically observed that ≈5–10% of cases of OI types II and III are attributable to genetic mosaicism. : 532

Pathophysiology
People with OI are either born with defective connective tissue, born without the ability to make it in sufficient quantities, or, in the rarest genetic types, born with deficiencies in other aspects of bone formation such as chaperone proteins, the Wnt signaling pathway, the BRIL protein, et cetera. In type I the collagens structure itself is normal, it is just its quantity that is low. : 1516  Types II, III and IV are usually, but not always, related to a deficiency of type I collagen. One possible deficiency arises from an amino acid substitution of glycine to a bulkier amino acid, such as alanine, in the collagen proteins triple helix structure. The larger amino acid side-chains lead to steric effects that creates a bulge in the collagen complex, which in turn influences both the molecular nanomechanics and the interaction between molecules, which are both compromised. Depending on both the location of the substitution and the amino acid being used instead, different effects are seen which account for the type diversity in OI despite the same two collagen genes being responsible for most cases.
A placental site nodule (PSN) is benign remnant from a previous pregnancy. Presentation
They are typically asymptomatic and found incidentally. Pathology
PSNs are intermediate trophoblastic remnants. Diagnosis
PSN are diagnosed by examining the tissue under a microscope, usually obtained with a dilation and curettage. Typically, they consist of pink (hyaline) material using the standard stain and contain few cells. Bizarre multinucleated cells may be present; however, there is no mitotic activity. The differential diagnosis includes (cervical) squamous cell carcinoma, gestational trophoblastic disease, and exaggerated placental site. Prognosis
PSN are benign. Once removed, they do not require any treatment and do not recur. See also
Gestational trophoblastic disease
Placental site trophoblastic tumour

References


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