Energy Deposition leads to Bone Loss

Reference 

Experiment Description 

Result 

Overgaard, 1988 

In vivo. Patients receiving post-mastectomy photon radiation (8 MV) had the number of rib fractures evaluated with chest radiograms. 

The frequency of fractures increased dose-dependently between 40 and 50 Gy (12 fractions) and between 50 and 55 Gy (22 fractions), resulting in a maximum of 48% of patients with rib fractures at 50 Gy. 

Holm et al., 1996 

In vivo. Rectal carcinoma patients received preoperative radiotherapy with photons at 25 Gy (500 irradiated, 527 control). The source of photons was either ⁶⁰Co or a 6-21 MV linear accelerator. The incidence of hospitalizations for femoral neck or pelvic fracture was determined at a 5-year follow-up. 

Patients irradiated with 25 Gy had an incidence of pelvic fracture of 5.3%, while significantly fewer non-irradiated patients were admitted for fracture (2.4%). 

Dickie et al., 2009 

In vivo. Lower extremity soft tissue sarcoma patients receiving radiotherapy were divided into patients with lower extremity fractures (n=21) and patients without fractures (n=53). The average dose received was compared between the two groups. 

Radiotherapy patients that had a bone fracture received an average dose of 45 Gy. Patients without a fracture had a lower average dose of 37 Gy. In addition, the maximum dose received by patients with a fracture was 64 Gy, while the maximum dose received by non-fractured patients was 59 Gy. 

Nishiyama et al., 1992 

In vivo. Patients with uterine cervix carcinoma from 1989 to 1990 with or without 4 MV photon irradiation to lumbar vertebrae had bone mineral content (measured in mg CaCO₃ eq/cm3) determined. Radiation was given in 1.8 Gy fractions over 5 weeks for a total dose of either 22.5 or 45 Gy to the vertebrae (radiation plan dependent). 

The control group did not show a change in bone mineral content. Both 22.5 and 45 Gy reduced bone mineral content by about 55 mg. 

Stavnichuk et al., 2020 

In vivo. A meta-analysis that extracted the percent change in bone density in 148 astronauts from articles from 1971 to 2019. The longer the spaceflight, the higher dose of IR the astronauts received, although IR was not the only stressor that the astronauts would have received. 

In missions less than 30 days, bone density was reduced up to 10%. In missions from 30 to 250 days, the estimated reduction in bone density was 0.1% per month in upper limbs and 0.8% per month in lower limbs. 

Bandstra et al., 2008 

In vivo. 58-day-old, female, juvenile, C57BL/6J mice were exposed to whole-body irradiation with 0.5, 1, and 2 Gy of 250 MeV protons at a rate of 0.7 Gy/min. Microarchitecture measurements, including trabecular BV/TV, Tb.Sp, and vBMD, were measured in the proximal tibiae. Three-point bending tests on the left femora were performed to assess mechanical parameters. 

Following exposure to 2 Gy of proton radiation, mice showed significant changes in bone structure compared to the non-irradiated controls, including a 20% loss of trabecular BV/TV, an 11% increase in Tb.Sp, and a 19% decrease in trabecular vBMD. BV/TV also decreased by 13% at 1 Gy. 0.5 Gy irradiation did not result in significant changes to trabecular bone structure. BV/TV and vBMD followed a decreasing trend at 1 and 2 Gy, and Tb.Sp similarly showed a linear, dose-dependent increase. No significant changes to mechanical strength were observed at any dose. 

Hamilton et al., 2006 

In vivo. 9-week-old, juvenile, female, C57BL/6 mice were exposed to 2 Gy whole-body irradiation from different sources, including LET=0.23 keV/µm ⁶⁰Co gamma rays, LET=0.4 keV/µm protons, LET=13 keV/µm ¹²C, and LET=148 keV/µm ⁵⁶Fe. 4 months post-exposure, microarchitectural parameters, including trabecular BV/TV, Tb.Sp, Tb.Th, Tb.N, cortical porosity (Ct.Po), cortical volume (Ct.V), and Conn.D (integrity), were measured in the proximal tibiae. 

Compared to non-irradiated controls, mice from all radiation groups experienced significant decreases in trabecular BV/TV following exposure to 2 Gy of IR, including decreases of 29% for gamma rays, 35% for protons, 39% for ¹²C, and 34% for ⁵⁶Fe. Tb.Th showed a LET-dependent difference in IR-induced bone loss, with high LET sources (¹²C and ⁵⁶Fe) showing significant decreases of 10% and 11%, respectively, while changes caused by low LET sources (gamma rays and protons) were non-significant. Only proton-irradiated mice experienced significant changes in Tb.N, and Tb.Sp, with a 20% decrease in Tb.N and a 22% increase in Tb.Sp. Trabecular Conn.D declined significantly in all radiation groups following exposure, with decreases of 54% for gamma rays, 64% for protons, 54% for ¹²C, and 46% for ⁵⁶Fe. Ct.Po and Ct.V did not change significantly compared to the control after exposure to gamma, proton, ¹²C, or ⁵⁶Fe radiation. 

Willey et al., 2010 

In vivo. 20-week-old, adult, female, C57BL/6 mice were exposed to whole body irradiation with 2 Gy of 140 kVp X-rays at a rate of 1.36 Gy/min. Microarchitectural parameters, including BV/TV, Conn.D, Tb.N, Tb.Th, Tb.Sp, Ct.V, Ct.Po, polar moment of inertia (pMOI), the percent eroded surface at the endocortical surface (Ec.ES/Ec.BS), vBMD, and marrow volume (Ma.V) were measured in the tibiae. 

The irradiated group experienced a 30% decrease in BV/TV and a 53% decrease in Conn.D in the proximal tibia after 3 weeks. Similar changes occurred in the distal femur and the fifth lumbar vertebrae. Decreases in vBMD and Tb.N and increases in Tb.Sp were observed from 1-3 weeks in the proximal tibia, distal femur, and the fifth lumbar vertebrae. vBMD decreased a maximum of 44%, Tb.N decreased a maximum of 13%, and Tb.Sp increased a maximum of 15%. There was no significant change in Tb.Th. Neither endocortical or periosteal Ct.V, Ct.Po, Ma.V, or pMOI changed significantly after exposure to X-rays. Ec.ES/Ec.BS increased by 68% at week 3. 

Ghosh et al., 2016 

In vivo. 16-week-old, adult, male C57BL/6 mice were exposed to whole body irradiation with 1 Gy of LET=150 MeV/µm ⁵⁶Fe heavy ion radiation at a rate of 0.1 Gy/min. Microarchitectural measurements, including BV/TV, Tb.Th, Tb.Sp, and Tb.N, were measured in the cancellous bone of the tibia. 

Compared to non-irradiated controls, mice that underwent total body irradiation experienced a 14% decrease in BV/TV, an 11% increase in Tb.Sp, and a 14% decrease in Tb.N. The resulting change in Tb.Th after irradiation was not significant. 

Alwood et al., 2010 

In vivo. 4-month-old, adult, male, C57BL/6 mice were exposed to irradiation with 0.5 Gy (low dose) and 2 Gy (high dose) of 1 GeV/nucleon ⁵⁶Fe heavy ions at a rate of 0.45 Gy/min and 0.9 Gy/min, respectively. 1-month post-irradiation, microarchitectural parameters, including BV/TV, Tb.Sp, Tb.N, cortical thickness (Ct.Th), cortical bone area (Ct.BA), and Conn.D, were measured in the mineralized cancellous bone tissue of the fourth lumbar vertebra. Stress transfer was assessed within the fourth lumbar vertebra. The elastic modulus of the cancellous centrum compartment and whole-vertebral body were determined with an axial compression test. 

Compared to non-irradiated controls, mice that were exposed to 2 Gy of heavy ions showed a 14% decrease in cancellous BV/TV, a 9% decrease in Tb.N, and an 18% decrease in Conn.D, as well as a 12% increase in Tb.Sp. The average cancellous tissue stress increased by 27% within the centrum following 2 Gy. The centrum elastic modulus (30%) and whole-vertebral body elastic modulus (10%) were decreased at 2 Gy. Mice that received a 0.5 Gy dose did not exhibit a significant degradation in bone structure or mechanical properties. Ct.Th and Ct.BA were not significantly affected. 

Green et al., 2012 

In vivo. 8- and 16-week-old (young and mature adult) C57BL/6J mice were irradiated with 5 Gy of ¹³⁷Cs gamma rays at a rate of 0.6 Gy/min. 8 weeks post-irradiation, microarchitectural parameters, including BV/TV, Tb.N, Tb.Sp, and Conn.D, were measured in the proximal tibial bones of the mice. 

Compared to non-irradiated controls, mice showed decreases of 45% and 51% for BV/TV, 34% and 21% for Tb.N, and 81% and 85% for Conn.D, as well as a 56% and 28% increase in Tb.Sp, in young and mature adults, respectively. 

Bandstra et al., 2009 

In vivo. 16-week-old, adult, male, C57BL/6 mice were irradiated with 0.47 Gy of LET=151.4 keV/µm ⁵⁶Fe heavy ions at a rate of 4 Gy/min. Nine weeks after irradiation, microarchitectural parameters, including BV/TV, Conn. D, Tb.Sp, Tb.Th, Tb.N, Ct.V (excluding marrow volume), cortical total volume (Ct.TV, including marrow volume), Ct.Po, pMOI and vBMD, were measured in the trabecular bone of the proximal humerus. 

Compared to non-irradiated controls, mice saw a 17% decrease in BV/TV and a 4% decrease in Tb.Th in the trabecular bone of their proximal humerus. While the changes to BV/TV and Tb.Th were statistically significant, the changes to the other microarchitecture parameters were not significant. After exposure to 0.47 Gy radiation, the proximal humerus experienced a significant decrease in BV (4%), TV (3%), and pMOI (6%), as well as a significant increase in Ct.Po (6%), compared to the control. After exposure to 0.18 Gy radiation, the proximal tibia experienced non-significant changes to all endpoints. 

Yumoto et al., 2010 

In vivo. 16-week-old, adult, male, C57BL/6 mice were exposed to whole-body irradiation with 0.1, 0.5, and 2 Gy of LET=150 keV/µm ⁵⁶Fe heavy ions at a rate of 0.2-1 Gy/min. 3 days after irradiation, BV/TV, Tb.Th, Tb.N, and Conn. D were measured in the proximal tibiae of the mice. 

Compared to non-irradiated controls, 0.1 and 0.5 Gy irradiation resulted in significant 16% and 18% decreases in BV/TV, respectively. 2 Gy radiation did not have a significant effect on trabecular BV/TV. 0.1 and 0.5 Gy irradiation similarly decreased Tb.N by 7% and 5%, respectively, while changes following 2 Gy irradiation were non-significant. Following 0.1 and 0.5 Gy irradiation, Conn. D decreased by 21% and 24%, respectively. Tb.Th was not affected by IR at any of the measured doses. 

Alwood et al., 2017 

In vivo. 16-week-old, adult, male, C57BL/6J mice were irradiated with 0.05, 0.1, 0.5, or 2 Gy of either LET=0.52 keV/µm protons or LET=175 keV/µm ⁵⁶Fe heavy ions. At 5 weeks and 1 year after exposure, microarchitectural parameters, including BV/TV, Tb.N, Tb.Th, Tb.Sp, Ct.BV, and Ct.Th, were measured in the proximal tibial metaphysis of the mice. 

At 5 weeks post-exposure, IR affected BV/TV and Tb.N in an identical manner. High doses of ⁵⁶Fe radiation (0.5 and 2 Gy) resulted in a 16% and 31% decrease, respectively, in both parameters compared to non-irradiated controls, while 2 Gy of protons similarly caused a 22% reduction in both. 0.5 Gy of protons caused non-significant decreases in BV/TV and Tb.N (11 and 13%, respectively). 2 Gy of proton irradiation also resulted in an increase in Tb.Sp, but it did not affect Tb.Th. Low doses (0.05 and 0.1 Gy) did not have an effect on bone loss after exposure to either protons or ⁵⁶Fe heavy ions. Ct.BV and Ct.Th were not significantly affected in the femur midshaft. 

Lloyd et al., 2012 

In vivo. 16-week-old, adult, female, C57BL/6 mice were exposed to whole body irradiation with 1 Gy of low LET protons at a rate of ~0.6 Gy/min. Microarchitectural parameters, including BV/TV, Conn. D, Tb.N, Tb.Sp, Ct.BV, Ct.TV, Ct.Po, and pMOI were measured in the proximal tibia and distal femur of the mice. Three-point bending tests on the left femur were performed to assess mechanical parameters. 

Compared to non-irradiated controls, BV/TV, Conn. D, and Tb.N in the proximal tibiae of the mice decreased significantly by 16%, 28%, and 7.7%, respectively, while Tb.Sp increased significantly by 9%. Microarchitectural parameters of the distal femur were not as affected, with BV/TV and Conn. D decreasing significantly by 22% and 37%, respectively, while Tb.N and Tb.Sp were unchanged. Ct.BV, Ct.TV, Ct.Po, and pMOI were not significantly affected by radiotherapy in the femur or tibiae. Mechanical strength was not significantly changed by radiation. 

Chandra et al., 2017 

In vivo. The distal metaphyseal region of right femurs of 8- to 10-week-old male mice were irradiated with 8 Gy of focal SARRP (small animal radiation research platform) X-ray radiation at a rate of 1.65 Gy/min. vBMD, BV/TV, Tb.N, and Tb.Sp were measured from the femurs of the mice. Linear elastic analysis was performed to assess stiffness. 

Compared to non-irradiated controls, irradiated mice experienced a 30% decrease in vBMD, a 31% decrease in BV/TV, a 13% decrease in Tb.N, and a 19% increase in Tb.Sp. Trabecular bone stiffness decreased 56%. 

Chandra et al., 2014 

In vivo. Three-month-old female Sprague-Dawley rats were irradiated at the proximal metaphyseal region of the right tibiae with 16 Gy of SARRP X-rays, fractionated into two 8 Gy doses at a rate of 1.65 Gy/min. Stiffness, BMD, BV/TV, Tb.N, and Tb.Sp were measured from the tibiae of the rats. 

Compared to non-irradiated controls, IR exposure resulted in a 14.3% decrease in BMD, a 17.7% decrease in BV/TV, a 17.7% decrease Tb.N, and a ~25% increase in Tb.Sp at 28 days post-exposure. Trabecular stiffness was decreased 51%. 

Hui et al., 2014 

In vivo. 16-week-old adult female BALB/c mice were exposed to a single 16 Gy dose of 250 kVp X-rays. The BV/TV and Ct.Th of the distal femurs of irradiated mice were measured. 

Compared to non-irradiated controls, irradiation resulted in the mice experiencing a ~55% decrease in trabecular BV/TV at 30 days post-exposure. Ct.Th increased significantly by ~12% at day 8 post-exposure. 

Wright et al., 2015 

In vivo. The hindlimbs of 20-week-old adult male mice were irradiated with 2 Gy of 320 kV X-rays at a rate of 1.6 Gy/min to the right hindlimb. 7 days post-irradiation, microarchitectural measurements, including BV/TV, Conn. D, Tb.N, Tb.Th, and Tb.Sp, were measured in the tibia and femur of the affected hindlimb. 

Compared to baseline levels, 2 Gy of IR resulted in a 22% and 14% (significant only against controls) decrease in BV/TV, a 50% and 45% (significant only against baseline) decrease in Conn. D, a 16% (significant only against baseline) and 13% decrease in Tb.N, and a 20% (significant only against baseline) and 16% increase in Tb.Sp in the proximal tibia and distal femur, respectively. 

Oest et al., 2018 

In vivo. An experiment was done on 6-week-old female BALB/cj mice exposed to 5 Gy X-ray radiation (225 kV beam at 17 mA) to the femur. Changes in BV/TV, Conn.D, Tb.Th, Tb.N, Ct.BA and Ct.Th were measured up to 26 weeks after exposure. Three-point bending tests were used to assess the mechanical properties of the whole bone and of cortical bone at the mid-diaphysis of the femur. 

In metaphyseal trabecular bone at 12 weeks, BV/TV was decreased by 69%, Tb.N by 79%, and Conn.D by 93% compared to the sham group. Tb.Th was increased compared to controls until 8 weeks. In the epiphyseal compartment, similar trends were seen. BV/TV decreased by 21%, Tb.N decreased by 30%, connectivity density decreased by 51%, and Tb.Th increased by 12%. Ct.Th decreased 8.1% and Ct.BA decreased 8.3% in the mid-diaphysis after 12 weeks compared to controls. In the metaphyseal region, cortical parameters increased. By 12 weeks, bending strength was reduced by 14.1% and bending stiffness was reduced by 13.3%. For cortical bone at 12 weeks, flexural strength decreased 5.7% and the flexural modulus decreased 4.9%. 

Zou et al., 2016 

In vivo. Male Sprague-Dawley rats were exposed to 20 Gy radiation (0.8 Gy/min) using ¹³⁷Cs gamma ray irradiation chamber for tibia and distal femur. Non-irradiation body parts were shielded, and contralateral sides of the femur and tibia were also harvested. BMD, BV/TV, Ct.Po, Tb.Th, and Tb.N of the irradiated femur were determined 12 weeks after exposure. Three-point bending tests were performed on the femur to assess mechanical parameters. 

Trabecular BMD of the irradiated femur was reduced by 21.2% in comparison with the control group. Trabecular BV/TV was reduced by 30.8% at the irradiated femur. Compared to the control group, BS/BV was increased by 32.9% at the irradiated femur. Both Tb.Th and Tb.N decreased after irradiation 17.5% and 18.1%, respectively. Tb.Sp increased after irradiation by 39% in the irradiated femur. Ct.Po was increased by 13.8% and 17.9%. Regarding tibia, BMD decreased 8.5%, and trabecular bone volume did not change significantly at 2 weeks post irradiation but decreased significantly in both irradiated and contralateral tibia at 12 weeks. The maximum loading of the femur was decreased 32.6% after 12 weeks. 

Zhang et al., 2019 

In vivo. An experiment was done on 4-week-old male C57BL/6J mice exposed to 2 Gy X-ray radiation at the mid-shaft of the left femur. Changes in BMD, BV/TV, Tb.Th, Tb.N were measured 7 and 28 days after exposure. 

7 days after irradiation, substantial degeneration of trabecular microarchitecture, with losses of 19% for BMD, 17% for BV/TV, 16% for Tb.Th, and an increase of 31% for Tb.Sp. Irradiated femurs showed further degeneration after 28 days. BMD decreased 15%, BV/TV decreased 42%, Tb.Th decreased 17%, Tb.N decreased 30%, and Tb.Sp increased 62%. 

Reference 

Experiment Description 

Result 

Holm et al., 1996 

In vivo. Rectal carcinoma patients received preoperative radiotherapy with photons at 25 Gy (500 irradiated, 527 control). The source of photons was either ⁶⁰Co or a 6-21 MV linear accelerator. The incidence of hospitalizations for femoral neck or pelvic fracture was determined at a 5-year follow-up. 

By 5 years post-radiotherapy, 5.3% of irradiated patients were admitted for a fracture, while 2.4% of non-irradiated patients were admitted for a fracture. 

Nishiyama et al., 1992 

In vivo. Patients with uterine cervix carcinoma from 1989 to 1990 with or without 4 MV photon irradiation to lumbar vertebrae had bone mineral content (measured in mg CaCO₃ eq/cm3) determined. Radiation was given in 1.8 Gy fractions over 5 weeks for a total dose of either 22.5 or 45 Gy to the vertebrae. 

The control group did not show a change in bone mineral content over time. Bone mineral content was 140 mg in the pre-treatment for the irradiated group. Bone mineral content was 95 mg after irradiation (5 weeks), 84 mg after 3 months, 74 mg after 6 months, and 71 mg after 12 months. 

Hui et al., 2014  

In vivo. 16-week-old adult female BALB/c mice were exposed to a single 16 Gy dose of 250 kVp X-ray radiation. The BV/TV of the distal femurs of irradiated mice were measured. 

Trabecular BV/TV initially increased relative to the non-irradiated control on day 3, but gradually declined to day 8 until it was ~55% lower relative to controls on day 30. Ct.Th increased significantly by ~12% at day 8 post-exposure. 

 Zou et al., 2016 

In vivo. Male Sprague-Dawley rats were exposed to 20 Gy radiation (0.8 Gy/min) using ¹³⁷Cs gamma ray irradiation chamber for tibia and distal femur. Non-irradiation body parts were shielded, and contralateral sides of the femur and tibia were also harvested. BMD, BV/TV, Ct.Po, Tb.Th, and Tb.N of the irradiated femur were determined 12 weeks after exposure. Three-point bending tests were performed on the femur to assess mechanical parameters. 

Trabecular BMD of the irradiated femur was reduced by 21.2% after 12 weeks. Trabecular BV/TV was reduced by 30.8% after 12 weeks. Compared to the control group, BS/BV was increased by 32.9% after 12 weeks. Both Tb.Th and Tb.N decreased after 12 weeks 17.5% and 18.1%, respectively. Tb.Sp increased after 12 weeks by 39% in the irradiated femur. Ct.Po was increased by 13.8% and 17.9% after 12 weeks. Regarding tibia, BMD decreased 8.5% after 12 weeks, and trabecular bone volume did not change significantly at 2 weeks post irradiation but decreased significantly in both irradiated and contralateral tibia at 12 weeks. The maximum loading of the femur was decreased 32.6% after 12 weeks. 

 Oest et al., 2018 

In vivo. An experiment was done on 6-weeks old female BALB/Cj mice exposed to 5 Gy X-ray radiation to the femur. Changes in BV/TV, Conn.D, Tb.Th, Tb.N, Ct.BA and Ct.Th were measured up to 26 weeks after exposure. Three-point bending tests were used to assess the mechanical properties of the whole bone and of cortical bone at the mid-diaphysis of the femur. 

 In metaphyseal trabecular bone BV/TV, Tb.N, and Conn.D increased slightly during the radiation period but declined almost linearly between 1 and 26 weeks, reaching 69%, 79%, and 93% below the initial values, respectively, by 12 weeks. Tb.Th was increased. In the epiphyseal compartment, similar trends can be seen. By 12 weeks, BV/TV, Tb.N, and Conn.D declined linearly after exposure reaching 21%, 30%, and 51% below the control group, respectively. Tb.Th was increased. All mechanical parameters increased over time up to 26 weeks, but the parameters of irradiated mice were lower than those for control mice. Both cortical parameters were decreased about 8% in the mid-diaphysis by 12 weeks. By 12 weeks, bending strength was reduced by 14.1% and bending stiffness was reduced by 13.3%. For cortical bone at 12 weeks, flexural strength decreased 5.7% and the flexural modulus decreased 4.9%. 

Alwood et al., 2017 

In vivo. 16-week-old, male, C57BL6/J mice were subjected to low LET protons or high LET ⁵⁶Fe ions using either low (5 or 10 cGy) or high (50 or 200 cGy) doses. Trabecular microarchitectural parameters such as BV/TV, and Tb.N were measured in the in the proximal tibial metaphysis.  

In the proximal tibia, 50 and 200 cGy ⁵⁶Fe induced a reduction in BV/TV (16 percent and 31%, respectively) and Tb.N (16 percent, and 31%, respectively) at 5 weeks after irradiation, compared to the control group. For protons, 200 cGy resulted in a 22% reduction in BV/TV and Tb.N, while 50 cGy resulted in a trend toward lower BV/TV and Tb.N. After 1 year, no changes in any endpoints were observed other than a 25% decrease in both BV/TV and Tb.N at 200 cGy (non-significant). 

Zhang et al., 2019 

In vivo. An experiment was done on 4-week-old male C57BL/6J mice exposed to 2 Gy X-ray radiation at the mid-shaft of the left femur. Changes in BMD, BV/TV, Tb.Th, Tb.N were measured 7 and 28 days after exposure. 

7 days after irradiation, substantial degeneration of trabecular microarchitecture occurred, with losses of 19% for BMD, 17% for BV/TV, 16% for Tb.Th, and an increase of 31% for Tb.Sp. Irradiated femurs showed further degeneration after 28 days. BMD decreased 15%, BV/TV decreased 42%, Tb.Th decreased 17%, Tb.N decreased 30%, and Tb.Sp increased 62%. 

 Green et al., 2012 

In vivo. Eight-week-old and 16-week-old mice were irradiated with 5 Gy of ¹³⁷Cs gamma rays. BV/TV, Conn.D, Tb.Sp, and Tb.N were measured 2 days, 10 days, and 8 weeks post radiation in the proximal tibia. 

None of the microarchitecture parameters indicated significant bone loss at 2 days post-irradiation. BV/TV, Tb.N, Tb.Sp, and Conn.D all demonstrated significant bone loss at 10 days and 8 weeks post-irradiation. By 8 weeks, mice showed decreases of 45% and 51% for BV/TV, 34% and 21% for Tb.N, and 81% and 85% for Conn. D, as well as a 56% and 28% increase in Tb.Sp, in young and mature mice, respectively.