<h3>Study Objective</h3> To analyze the histological and immunohistochemical features of ovarian tissue and endometrium in women with diminished ovarian reserve and to investigate the correlation between these parameters and the effectiveness of infertility treatment <h3>Design</h3> Prospective study, Level II. <h3>Setting</h3> National Medical Research Center. <h3>Patients or Participants</h3> 70 patients with diminished ovarian reserve who underwent drug-free in vitro activation using a modified surgical technique. Two bilateral ovarian cortical tissue samples and one endometrial sample were obtained from each patient during the surgery. 40 patients out of 70 underwent ovarian stimulation after at least a one-month postoperative period. <h3>Interventions</h3> Bilateral ovarian cortex biopsy via laparoscopy, ovarian tissue fragmentation in aliquots of 1 mm<sup>2</sup>, and hysteroscopy with endometrial biopsy were performed in all patients, followed by histological and immunohistochemical analysis (expression of proteins of the Hippo signaling pathway (YAP, MST-1, LATS-1) and PTEN (inhibitor of PI3K signaling pathway). The Human Protein Atlas datasets were used as a comparison group. <h3>Measurements and Main Results</h3> We revealed an impaired functioning of intracellular signaling pathways (primarily YAP- and PTEN-dependent) both in ovarian tissue and endometrial samples. Moreover, the expression of the proteins targeted in the study was significantly reduced in patients who responded to stimulation (n=31). Histological analysis of tissue samples of patients with diminished ovarian reserve indicated profound changes in various structures of the ovaries: surface epithelium, vascular component, stroma of ovarian cortex. In 25% of patients, chronic endometritis was also determined. <h3>Conclusion</h3> Specific histological and immunohistochemical features of ovarian tissue and endometrium associated with diminished ovarian reserve and the outcome of ovarian stimulation were shown.