Parathyroid Grafts Research Articles

Analysis of the efficacy of different amounts of parathyroid grafts in the treatment of secondary hyperparathyroidism.

This study compares the efficacy of two different ranges of parathyroid transplantation weights with the aim of determining a preferable range for transplantation weight. From May 2018 to June 2023, 79 patients underwent total parathyroidectomy with autotransplantation. Demographic data, symptoms, and pre- or postoperative biochemical indicators were compared between two different ranges of parathyroid transplantation weights. All 79 surgeries were successful, with a total of 316 parathyroid glands reported among the patients. The patients were diagnosed with parathyroid hyperplasia. Postoperatively, itching, bone pain, and muscle weakness disappeared, while serum parathyroid hormone and phosphate levels significantly decreased. With an average follow-up of 12 months, no transplant-dependent recurrence was observed. Parathyroid transplantation with a weight of 30-50mg is a feasible, safe, and effective surgical approach.

OR23-06 Long Term Outcomes Of Parathyroid Autografts In Primary Hyperparathyroidism

Abstract Disclosure: E. Chuki: None. A. Graf: None. A. Ninan: None. R. Tora: None. T. Abijo: None. N.M. Center: None. N. Nilubol: None. L.S. Weinstein: None. S.K. Agarwal: None. W.F. Simonds: None. S. Jha: None. Background: Autologous implantation of parathyroid tissue is frequently utilized in patients with heritable forms of primary hyperparathyroidism (PHPT). There is sparse data on long-term functional outcome of these grafts and post-graft recurrence of PHPT. Objective: We aimed to investigate the long-term outcome of parathyroid autografts and assess the frequency of post-graft PHPT recurrence. Methods: Retrospective study of patients with PHPT who underwent parathyroid autografts from 1991 to 2020. Results: We identified 115 patients with PHPT who underwent 135 parathyroid autografts. A significant majority (106 of 115 patients, 92%) had a heritable form of PHPT, with MEN1 being the most common (93 of 115 patients, 81%). Median follow-up duration since graft was 10 [4-20] years. 119 (88%) were to brachioradialis, 11 (8%) to sternocleidomastoid and 5 (4%) to deltoid. There were 86 immediate (64%) and 49 delayed (36%) grafts. Of the 135 grafts, 54 (40%) were fully functional at last follow-up, 13 (10%) partially functional, 44 (32%) non-functional while functional status of 24 (18%) grafts could not be ascertained. Age at graft, thymectomy prior to autograft, graft type (delayed/immediate) or duration of cryopreservation did not predict functional outcome. There were 45 (83%) post-graft recurrences of PHPT among 54 fully functional grafts at a median duration of 8 [4-15] years after grafting. Parathyroidectomy was performed in 42/45 but surgical cure was attained in 18/42 (43%) only. 12 of 18 (67%) recurrences were graft-related while the remaining 6 (33%) had a neck or mediastinal source. Median time to recurrence was 16 [11-25] years in neck or mediastinal source versus 7 [2-13] years in graft-related recurrences. Median PTH gradient was significantly higher at 23 [20-27] in graft-related recurrence versus 1.3 [1.2-2.5] in neck or mediastinal source (P = 0.03). Nineteen patients had multiple parathyroid grafts. Median pre-repeat-graft PTH of 7.5 [6.5-18.5] pg/mL with ionized calcium of 1.15 ± 0.145 (reference:1.09-1.30) mmol/L served as an indicator of the need for a repeat graft. Discussion: Post-graft recurrence occurs frequently within the first decade after grafting. Source of excess PTH in post-graft recurrence of PHPT can be challenging to localize. Time to recurrence after graft is significantly shorter and PTH gradient higher for graft-related recurrence. Presentation: Saturday, June 17, 2023

Co-transplantation with adipose-derived cells to improve parathyroid transplantation in a mice model

BackgroundAccidentally removed parathyroid glands are still challenging in neck surgery, leading to hypoparathyroidism characterized with abnormally low levels of parathyroid hormone. Parathyroid auto-transplantation is usually applied in compensation. To improve the efficiency of parathyroid transplantation, we introduced a method by co-transplanting with adipose-derived cells, including stromal vascular fractions (SVFs) and adipose-derived stem cells (ADSCs), and investigated the underlying molecular mechanisms involved in parathyroid transplantation survival.MethodsRat and human parathyroid tissues were transplanted into nude mice as parathyroid transplantation model to examine the effects of SVFs and ADSCs on grafts angiogenesis and survival rates, including blood vessel assembly and parathyroid hormone levels. Several angiogenic factors, such as vascular endothelial growth factor (VEGF)-A and fibroblast growth factor (FGF) 2, were assessed in parathyroid grafts. The effects of hypoxia were investigated on ADSCs. The modulatory roles of the eyes absent homolog 1 (EYA1), which is vital in parathyroid development, was also investigated on angiogenic factor production and secretion by ADSCs. All experimental data were statistically processed. Student’s t test was used to assess significant differences between 2 groups. For multiple comparisons with additional interventions, two-way ANOVA followed by Tukey’s post hoc test was performed. P < 0.05 was considered as significant.ResultsSVFs improve rat parathyroid transplantation survival and blood vessel assembly, as well as FGF2 and VEGF-A expression levels in parathyroid transplantation mice. Functional human parathyroid grafts have higher microvessel density and increased VEGF-A expression. The supernatant of ADSCs induced tubule formation and migration of human endothelial cells in vitro. Hypoxia had no effect on proliferation and apoptosis of human ADSCs but induced higher angiogenic factor levels of VEGF-A and FGF2, modulated by EYA1, which was confirmed by parathyroid glands transplantation in mice.ConclusionsAdipose-derived cells, including ADSCs and SVFs, improve parathyroid transplantation survival via promoting angiogenesis through EYA1-regulating angiogenetic factors in vitro and in vivo. Our studies proved an effective method to improve the parathyroid autotransplantation, which is promising for clinical patients with hypoparathyroidism when parathyroid glands were accidentally injured, removed, or devascularized.

Autotransplantation of parathyroid grafts into the tibialis anterior muscle after parathyroidectomy: a novel autotransplantation site.

BackgroundSurgical management of renal secondary hyperparathyroidism (sHPT) is varying. Total parathyroidectomy with heterotopic autotransplantation (TPTX + AT) is one of the standard surgical procedures in sHPT, but there is no consensus about the optimal site for graft insertion. At the surgical department of the University Hospital of Heidelberg we prefer the autotransplantation into the tibialis anterior muscle. The aim of this study was to assess the long-term function of the auto-transplanted parathyroid tissue in this type of surgical procedure.MethodsThe function of the autograft of 42 patients was assessed 8.2 ± 2.5 years after surgery, using a modified Casanova-test of the leg bearing the parathyroid tissue. Ischemic blockage was induced by tourniquet and the levels of parathyroid hormone (PTH) were assessed during the test.ResultsAt the point of assessment, the ischemic blockage led to a significant reduction in the concentration of PTH (≥50 % of the baseline value) in 19 patients (45 %) indicating well-functioning autografts. In 11 patients (26 %), ischemic blockage did not cause any change in the concentration of PTH (≤20 % of the baseline value), indicating functioning residual parathyroid tissue from another site. The source of PTH production was classified as unidentifiable in five patients (12 %). Two patients had developed graft-dependent recurrent HPT (5 %) without therapeutic consequences and three patients suffered from persistent symptomatic hypoparathyroidism (7 %).ConclusionsThese results indicate that TPTX + AT into the tibialis anterior muscle is a successful surgical treatment for renal HPT and that the modified Casanova-test is a suitable diagnostic tool for autografts function.

Induction of tolerance to parental parathyroid grafts using allogeneic thymus tissue in patients with DiGeorge anomaly

DiGeorge anomaly can affect both thymic and parathyroid function. Although athymia is corrected by allogeneic thymus transplantation, treatment options for hypoparathyroidism have been unsatisfactory. Parathyroid transplantation offers the potential for definitive cure but remains challenging because of graft rejection. Some allogeneic parathyroid grafts have functioned in adult recipients in the context of immunosuppression for renal transplantation. Other efforts have attempted to reduce the allogenicity of the parathyroid grafts through manipulation of the parathyroid tissues before transplantation (by using encapsulation or special culture techniques). Recently, we demonstrated the efficacy of parental parathyroid transplantation when combined with allogeneic thymus transplantation in an infant with complete DiGeorge anomaly. The recipient developed tolerance toward the parathyroid donor. The parathyroid graft has functioned for 5 years after transplantation without the need for continued immunosuppression or calcium supplementation. We observed that matching of the allogeneic thymus graft to the parathyroid donor HLA class II alleles that are unshared with the recipient appears to be associated with the induction of tolerance toward the parathyroid graft. Further work is needed to determine the optimal means for using combined allogeneic thymus and parental parathyroid transplantation to correct hypoparathyroidism in patients with both complete and partial DiGeorge anomaly.

Mechanisms of tolerance to parental parathyroid tissue when combined with human allogeneic thymus transplantation

The induction of tolerance toward third-party solid organ grafts with allogeneic thymus tissue transplantation has not been previously demonstrated in human subjects. Infants with complete DiGeorge anomaly (having neither thymus nor parathyroid function) were studied for conditions and mechanisms required for the development of tolerance to third-party solid organ tissues. Four infants who met the criteria received parental parathyroid with allogeneic thymus transplantation and were studied. Two of 3 survivors showed function of both grafts but subsequently lost parathyroid function. They demonstrated alloreactivity against the parathyroid donor in mixed lymphocyte cultures. For these 2 recipients, parathyroid donor HLA class II alleles were mismatched with the recipient and thymus. MHC class II tetramers confirmed the presence of recipient CD4(+) T cells with specificity toward a mismatched parathyroid donor class II allele. The third survivor has persistent graft function and lacks alloreactivity toward the parathyroid donor. All parathyroid donor class II alleles were shared with either the recipient or the thymus graft, with minor differences between the parathyroid (HLA-DRB1∗1104) and thymus (HLA-DRB1∗1101). Tetramer analyses detected recipient T cells specific for the parathyroid HLA-DRB1∗1104 allele. Alloreactivity toward the parathyroid donor was restored with low doses of IL-2. Tolerance toward parathyroid grafts in combined parental parathyroid and allogeneic thymus transplantation requires matching of thymus tissue to parathyroid HLA class II alleles to promote negative selection and suppression of recipient T cells that have alloreactivity toward the parathyroid grafts. This matching strategy may be applied toward tolerance induction in future combined thymus and solid organ transplantation efforts.

Persisted/recurrent hyperparathyroidism associated with development of multi-drug resistance phenotype and proliferation of parathyroid transplants.

The surgical treatment of secondary hyperparathyroidism (HPTH) requires sub-total excision of parathyroid glands or total excision with their autotransplantation. Although this approach has been considered as a safe method of treatment, in this report we describe persisted/recurrent HPTH after parathyroid transplantation. Due to parathormone (PTH) hypersecretion and uncontrolled proliferation, the parathyroid grafts were removed and used for generation of cell cultures, which further have been subjected to in vitro studies. As a control we used parathyroid tissue, obtained during multiorgan harvesting. We found increased proliferation and up-regulated PTH production by the graft-derived, but not control in vitro cultured cells. Moreover, due to decrease of in vivo radiotracer uptake by parathyroid grafts, the expression of multi-drug resistance-involved factors, including P-glycoprotein (P-gp/mdr1), multi-drug resistance-associated protein (mrp) and bcl-2 have been investigated using RT-PCR. The analysis revealed increased expression of both, mdr1 and mrp in graft-derived cells, in contrast to control cells, which did not express P-gp/mdr1 or mrp. However, we did not observe any difference in expression of bcl-2 between analyzed cells. The up-regulated expression of P-gp/mdr1 on graft-derived cells was further confirmed by immunofluorescence studies. The described case indicates potential risk associated with transplantation of parathyroid tissue. Our results confirm a role of MDR phenomenon in occurrence of false negative results in parathyroid tissue scintigraphy studies. Moreover, they indicate that standard histological examination of transplanted material could not be sensitive enough to exclude any potential danger of abnormal graft progression. Thus, they could support the concept to use encapsulated parathyroid transplants.

Immunogenicity of parathyroid allografts in the rat: immunosuppressive dosages effective in passenger leukocyte-rich small bowel transplants are not effective in parathyroid gland transplants with few passenger leukocytes.

The passenger leukocytes harboured within an allograft induce a massive allo-immune response that leads to allograft rejection if not countered by immunosuppression. We compared the response to short-term immunosuppression of parathyroid gland transplants possessing few passenger leukocytes with that of passenger leukocyte-rich small bowel transplants. Heterotopic parathyroid and orthotopic small bowel transplantation was performed in a Wistar Furth-to-Lewis rat strain combination. Immunosuppression with cyclosporine A (CsA) was administered in different dosages for 14 days. Dysfunctional allografts were examined immunohistologically. CsA more effectively suppressed the immune response provoked by immunogenic small bowel grafts than that induced by less-immunogenic parathyroid grafts. Immunosuppression with 20 mg/kg per day induced long-term survival in the small bowel (165+/-21 days) but not in the parathyroid (28+/-3 days). All rejected grafts featured massive cellular infiltration by activated T cells as a sign of immune rejection. Immunosuppressive dosages effective in passenger leukocyte-rich small bowel transplants were not as effective in parathyroid gland transplants harbouring few passenger leukocytes. In spite of the paucity of passenger leukocytes in parathyroid grafts it is more difficult to control by immunosuppression the immune response to them than that to the passenger leukocyte-rich small bowel.

Human parathyroid grafts survived and functioned in mice treated with Anti-CD4 monoclonal antibody

Human parathyroid grafts survived and functioned in mice treated with Anti-CD4 monoclonal antibody

Experimental parathyroid transplantation: human parathyroid grafts survived and functioned in mice treated with anti-CD4 monoclonal antibody

Permanent hypoparathyroidism is one of the most difficult of all endocrine disorders to treat medically. To examine the possibility that xenotransplantation can be used to treat hypoparathyroidism, human parathyroid tissues were transplanted into mice. Human parathyroid tissue was taken from specimens excised from patients with hyperparathyroidism. Fresh human parathyroid tissue was implanted under kidney capsule of CBA (H2k) mice. Some mice were treated intraperitoneally with depleting anti-CD4 monoclonal antibody (mAb, YTA 3.1, 100 microg/dose, days -1. 0. 1, 2, 3, and 5). Mice were killed 30 days after transplantation. Survival of parathyroid grafts was examined microscopically and human parathyroid hormone in serum was measured by ELISA. All parathyroid grafts survived under kidney capsule and human parathyroid hormone was strongly detected in serum (621 +/- 576 pg/mL) when recipients were treated with short-course treatment of anti-CD4 mAb. Conversely, no parathyroid tissue was seen microscopically in any recipient mice without anti-CD4 mAb treatment. Human parathyroid hormone was undetectable by ELISA in naive mice and mice transplanted with human parathyroid tissue without short-course treatment of anti-CD4 mAb. Xenogeneic human parathyroid tissue survived and functioned in mice treated with short-course treatment of anti-CD4 mAb.

Long-term results of subcutaneous parathyroid grafts in uremic patients.

Parathyroid glands are normally surrounded (entirely or partially) by fatty tissue. Subcutaneous parathyroid grafts are thus located in a normal environment. Therefore, we postulated that the late results of subcutaneous implantation of parathyroid tissue in uremic patients should be at least as good as those reported for intramuscular grafting. We also challenged the idea that the recurrence rate of renal hyperparathyroidism after surgery depended solely on the type of hyperplasia (diffuse vs nodular) observed in the implanted tissue. A retrospective study of a series of patients without loss to follow-up. A university hospital and 9 affiliated dialysis units. Fifty-nine patients (33 women and 26 men) operated on for renal hyperparathyroidism underwent the resection of at least 4 parathyroid glands followed by presternal subcutaneous implantation of parathyroid tissue. They were followed up for 12 to 130 months (median, 38 months). Failure of treatment, recurrence of disease, and hypoparathyroidism. During the study period, 9 patients had to undergo another operation: 2 (3%) for persistent hyperparathyroidism due to a fifth ectopic gland and 7 (12%) for recurrence of hyperparathyroidism resulting from hypertrophy of the subcutaneous grafts. Four patients received a kidney transplant. The prevalence of hypoparathyroidism (intact parathyroid hormone serum level <1.6 pmol/L with a normal or low serum calcium concentration) was 14% (8 of 59 patients), and the curve representing the distribution of intact parathyroid hormone serum concentrations among operated on patients was shifted to the left when compared with the curve of patients who underwent hemodialysis and who had no indication for parathyroid surgery. In this latter group, the peak of the curve was situated between 1 and 2 times the upper normal limit, while it was in the normal range 12 to 130 months after total parathyroidectomy and subcutaneous parathyroid autotransplantation. No relation was observed between the recurrence rate of the disease and the histological characteristics of the parathyroid grafts. Also, their function was not influenced by the presence or absence of aluminum deposits in bone biopsy specimens that were obtained at the time of cervical exploration. The late results of total parathyroidectomy and presternal subcutaneous grafting compare favorably with the published data on other surgical techniques proposed for the treatment of renal hyperparathyroidism. The ease with which the hypertrophied grafts are removed when the disease recurs warrants further use of this procedure.

Survival of human parathyroid tissue xenotransplanted in nude mice after 9 to 55 months' cryopreservation.

Survival of human parathyroid tissue xenotransplanted after cryopreservation was studied. Peroperative biopsies from 26 patients were cryopreserved and xenotransplanted into nude mice after 9 to 55 months. At 8 to 12 weeks after transplantation, the morphology of the transplanted tissue was compared to that of the original tissue after thawing and before transplantation. Morphologically viable tissue was observed in 20 out of 26 nude mice (77%). Based on the morphological appearance, the parathyroid transplants were arranged into four "quality" groups. No correlation existed between the quality of the transplants and duration of storage, or between the age and sex of the patients. There was no correlation between initial clinical diagnosis or histopathological patterns (primary, secondary and tertiary hyperplasia [n=16], adenoma [n=9], one case undetermined) and transplant survival. After thawing and transplantation, all parathyroid grafts, except one, were morphologically either of the same or somewhat lower quality.

Total parathyroidectomy and forearm autotransplantation for chronic renal failure: a useful tool for the general surgeon in private practice.

We reviewed 12 cases of chronic renal failure in patients with secondary hyperparathyroidism who had total parathyroidectomy and forearm autotransplantation during the years 1978 to 1984. All had elevated parathyroid hormone and calcium levels before the procedure. Using a technique previously described, with relatively large parathyroid grafts in the forearm, calcium levels were effectively reduced and maintained below the upper limit of the normal range (10.5 mg/dl). Only one patient had symptomatic hypocalcemia postoperatively. Phosphorous levels were also significantly reduced to more physiologic values. When necessary, the forearm autografts allowed simplified titration of calcium levels on an outpatient basis.

Functional capacity of solid tissue transplants in the brain: evidence for immunological privilege.

The capacity of the mammalian brain to support the physiological function of allografts was assessed in parathyroidectomized Fischer strain rats bearing either isografts or immunogenic DA allografts of parathyroid glands implanted in their cerebral cortices. Established isografts and allografts survived indefinitely in the brain, maintaining normal serum calcium levels, with equal numbers of spontaneous failures (18-21%) in each group. Similarly, both MHC-compatible and incompatible skin allografts survived and were 'functional' at 40-50 days postgrafting as assessed by: continued formation of keratin; the presence of differentiated hair follicles and sebaceous glands; and frequent mitotic figures. No serum alloantibodies were induced by either MHC-incompatible parathyroid glands or skin in this site. However, both types of allografts were promptly rejected or failed to become established in the brains of specifically presensitized hosts. Furthermore, when Fischer hosts with long-established intracerebral DA parathyroid grafts received orthotopic DA skin grafts, their parathyroid grafts were rejected along with first-set rejection of the skin grafts. The tempo of this cellular immune response and the primary alloantibody response that accompanied it indicate that although the intracerebral grafts failed to induce detectable host sensitization or suppression, they remained susceptible to immune effectors. Thus, by using strongly immunogenic, adult tissues, we have established that the rat cerebral cortex is an immunologically privileged site, and the privilege is not dependent on lack of graft immunogenicity or alterations in host responsiveness. Furthermore, Ia+ (possible antigen-presenting) cells were rare in the cortical parenchyma sites used for transplantation though numerous in the choroid plexus of the ventricles and in certain areas of white matter. Therefore, privilege probably reflects deficient graft antigen presentation related to the paucity of Ia+ cells as well as to the brain's poor lymphatic drainage.

Factors that influence the assessment of parathyroid graft function.

Autogenous parathyroid grafts are used in the treatment of primary and secondary parathyroid hyperplasia and for salvaging normal parathyroid glands removed during thyroid surgery. Placement of the autogenous grafts in the forearm may allow assessment of graft function by comparing the patient's background level of immunoreactive parathyroid hormone (iPTH) with the iPTH value in the antecubital vein above the parathyroid graft. Among patients who on clinical grounds seem to have functioning parathyroid tissue, significant iPTH gradients can be demonstrated in only approximately 80%. Several technical and clinical factors can prevent demonstration of an iPTH gradient in patients who in fact do have functioning parathyroid grafts. Hypercalcemia may suppress iPTH secretion. PTH secretion may be intermittent. High background levels of iPTH due to renal failure may transform a significant numerical gradient for iPTH into an insignificant percentage change in iPTH. It may be technically difficult to obtain blood from the particular vein bearing effluent from the parathyroid graft. The regional specificity of the iPTH assay employed may have an important influence on the magnitude of the apparent iPTH gradient. Knowledge of these factors should maximize the chance of documenting parathyroid graft function.

Immune privilege in the testis. I. Basic parameters of allograft survival.

The fate of solid tissue allografts--skin or parathyroid glands--implanted in the interstitial tissue of the testis was investigated using inbred rats. The results affirm that the testis is an immunologically privileged site despite its efficient lymphatic drainage. Skin allografts survived at least several days longer than orthotopic grafts of similar size, whether major histocompatibility complex (MHC)-compatible or MHC-incompatible, and failed to induce an alloantibody response in most recipients or to prime for secondary antibody responses on rechallenge. Further assessment employed parathyroid grafts that allowed appraisal of their function and its duration, by monitoring serum calcium levels. Most intratesticular MHC-incompatible parathyroids survived for at least twice as long as control grafts in nonprivileged sites, with a median survival time (MST) of 41 days--and one-third of the grafts functioned at 100 days. MHC-compatible grafts fared even better (MST of 60 days), some surviving more than 400 days. Most F1 hybrid grafts survived virtually indefinitely. Splenectomy 5-23 days prior to implantation had a beneficial rather than a detrimental effect on the privilege afforded intratesticular parathyroid allografts. Allograft rejection was accompanied by antibody production in only one-half the animals. Grafts that had undergone functional rejection at the time of recovery usually had an intense mononuclear cell infiltrate, and long-term surviving grafts displayed varying degrees of cellular infiltration among cords of healthy, functional chief cells. Accepted parathyroids were destroyed by active immunization of the host but were unaffected by passively administered alloantibodies. The possible mechanisms controlling graft rejection in this unique privileged site are discussed.

Transplantation of the Parathyroid Glands: Clinical and Experimental Results

Parathyroid transplantation offers the surgeon versatility in managing the patient with hyperparathyroidism. Indications, clinical experience, and results of parathyroid grafts in a series of 75 patients are presented.

THE UTERUS AS A RECIPIENT SITE FOR PARATHYROID HOMOTRANSPLANTATION.

Syngeneic and allogeneic parathyroid gland transplants to the uterus of pseudopregnant and non-pseudopregnant, parathyroid-ectomized rats were performed. The functional survival of these grafts was assessed by determination of subsequent serum calcium levels. Histological evidence revealed that implantation took place in such a way that the uterine epithelium covered the ectopic parathyroid. Implantation directly upon a site where decidual proliferation had been produced did not occur. Syngeneic parathyroid grafts to the uterus functioned indefinitely throughout many estrus cycles. Allogeneic transplants of parathyroid glands to pseudopregnant and non-pseudopregnant uteri were almost uniformly rejected by 20 days following transplantation. Histological evidence was compatible with typical homograft rejection.

Parathyroid transplants in rats; a comparison of their survival time with that of skin grafts.

1. A reliable method of parathyroid grafting in rats, and of assessing the functional state of such grafts in terms of blood calcium has been devised. 2. Parathyroid homografts between rats of proven genetic diversity may be rejected as promptly as skin; but the majority survive for an indefinite period, there being no intermediate class. 3. Surviving parathyroid grafts in this genetically diverse combination invariably succumb in the presence of the immunity induced by a later skin graft from the same donor. 4. Parathyroid grafts between rats of less genetic diversity have usually responded to skin graft-induced immunity in the same manner as those between more dissimilar rats. The less divergent group, however, revealed an occasional persistent functioning of parathyroid grafts even in the face of the sensitivity incidental to an accelerated reaction to skin. 5. The hypothesis is presented that the parathyroid tissue of the rats studied was deficient in effective transplantation antigens as compared with those of their skin. This weakness in apparent antigenicity may be a reflection of a deficiency in production, availability, or content of antigens in the tissue transferred.