The present prospective, randomized study, compares the spinal fusion obtained after instrumentation and the use of a biologic product (patient's mesenchymal cells obtained from his/her own bone marrow which will be fixed in human bone tissue form a donor), with the current procedure that consists in instrumented spinal fusion and the use of each patient's bone obtained from his/her iliac crest.

The working hypothesis proposes that the tissue engineering is a valid and useful technique to achieve bone regeneration, avoiding the need for obtaining patient's iliac crest and its associated morbidity.

Prospective, open-label, randomized, parallel, single-dose phase I-II clinical trial in which 62 patients affected with L4-L5 degenerative spondylolisthesis grade I-II according to Meyerding and/or with L4-L5 degenerative discopathy needing spinal fusion will enter the trial with the primary objective of assessing the feasibility and safety of "ex-vivo" expanded autologous mesenchymal stem cells fixed in allogenic bone tissue in spinal fusion. Secondary objectives are to assess the efficacy of the implantation by imaging (computerized helicoidal tomography and X-ray) and clinical questionnaires (pain by visual analogue scale, quality of life by SF-36 and Oswestry Disability Index).

Patients will be randomized to one of the two treatment arms (instrumented spinal fusion and the tissue engineering product composed by "ex-vivo" expanded autologous mesenchymal stem cells fixed in allogenic bone tissue in spinal fusion or the standard treatment of instrumented spinal fusion and patient's bone iliac crest). Thereafter, patients will be followed for 12 months.

Imaging assessment will be done by an independent blinded radiologist.

Inclusion Criteria:

• L4-L5 degenerative spondylolisthesis grade I-II Meyerding isolated or associated to more than one level, and/or L4-L5 degenerative discopathy isolated or associated to more than one level.
• 18 to 85 years of age (male and female)
• Informed Consent Form signed
• The patient is able to understand the nature of the study

Exclusion Criteria:

• Previous spine surgery
• L4 isthmic spondylolisthesis
• Smoker (more than 10 cigarettes a day)
• Systemic or local infection
• Patients treated with steroids (oral or systemic) within 3 months of entering the study, or treated with biphosphonates for more than 10 years
• Positive serology for HIV-1 or HIV-2, Hepatitis B (HBsAg), Hepatitis C (Anti-HCV-Ab) or Syphilis.
• Pregnant woman or intended to become pregnant, or breath feeding
• Neoplasia within the previous 5 years, or without remission
• Immunosuppressive states (except diabetes mellitus. Treatment with therapies is allowed)
• Significant abnormal laboratory tests that contraindicates the surgery.
• Participation in another clinical trial or treated with an investigational medicinal product the previous 3 months
• Other pathologic conditions or circumstances that difficult participation in the study according to medical criteria
• The patient does not accept to be followed-up for a period that could exceed the clinical trial length
• The patient is legally dependent