title: Revisiting the human seminiferous epithelium cycle.
abstract: Can all types of testicular germ cells be accurately identified by microscopy techniques and unambiguously distributed in stages of the human seminiferous epithelium cycle (SEC)?
  
  
  
  By using a high-resolution light microscopy (HRLM) method, which enables an improved visualization of germ cell morphological features, we identified all testicular germ cells in the seminiferous epithelium and precisely grouped them in six well-delimitated SEC stages, thus providing a reliable reference source for staging in man.
  
  
  
  Morphological characterization of germ cells in human has been done decades ago with the use of conventional histological methods (formaldehyde-based fixative -Zenker-formal- and paraffin embedding). These early studies proposed a classification of the SEC in six stages. However, the use of stages as baseline for morphofunctional evaluations of testicular parenchyma has been difficult because of incomplete morphological identification of germ cells and their random distribution in the human SEC.
  
  
  
  Testicular tissue from adult and elderly donors with normal spermatogenesis according to Levin's, Johnsen's and Bergmann's scores were used to evaluate germ cell morphology and validate their distribution and frequency in stages throughout human spermatogenesis.
  
  
  
  Testicular tissue from patients diagnosed with congenital bilateral agenesis of vas deferens (n = 3 adults) or prostate cancer (n = 3 elderly) were fixed in glutaraldehyde and embedded in araldite epoxy resin. Morphological analyses were performed by both light and transmission electron microscopy.
  
  
  
  HRLM method enabled a reliable morphological identification of all germ cells (spermatogonia, spermatocytes and spermatids) based on high-resolution aspects of euchromatin, heterochromatin and nucleolus. Moreover, acrosomal development of spermatids was clearly revealed. Altogether, our data redefined the limits of each stage leading to a more reliable determination of the SEC in man.
  
  
  
  Occasionally, germ cells can be absent in some tubular sections. In this situation, it has to be taken into account the germ cell association proposed in the present study to classify the stages.
  
  
  
  Our findings bring a new focus on the morphology and development of germ cells during the SEC in human. Application of HRLM may be a valuable tool for research studies and clinical andrology helping to understand some testicular diseases and infertility conditions which remain unsolved.
  
  
  
  Experiments were partially supported by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Fundação de Amparo à Pesquisa de Minas Gerais (FAPEMIG) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). The authors declare that there are no conflicts of interest.
  
  
  
  Not applicable.
authors.affiliation: Departamento de Morfologia, Instituto de Ciências Biológicas, Universidade Federal de Minas Gerais (UFMG), CEP 31.270-901 Belo Horizonte, Brazil.
authors.forename: F
authors.initials: F
authors.lastname: Nihi
authors.affiliation: Departamento de Morfologia, Instituto de Ciências Biológicas, Universidade Federal de Minas Gerais (UFMG), CEP 31.270-901 Belo Horizonte, Brazil.|Departamento de Biologia Estrutural, Instituto de Ciências Biológicas e Naturais, Universidade Federal do Triângulo Mineiro (UFTM), CEP 38.025-015 Uberaba, Brazil.
authors.forename: M L M
authors.initials: MLM
authors.lastname: Gomes
authors.affiliation: Departamento de Morfologia, Instituto de Ciências Biológicas, Universidade Federal de Minas Gerais (UFMG), CEP 31.270-901 Belo Horizonte, Brazil.
authors.forename: F A R
authors.initials: FAR
authors.lastname: Carvalho
authors.affiliation: Departamento de Cirurgia, Faculdade de Medicina, Universidade Federal de Minas Gerais (UFMG), CEP 30.130-100 Belo Horizonte, Brazil.|Departamento de Serviço de Nefrologia e Urologia do Hospital das Clínicas da Universidade Federal de Minas Gerais (UFMG), CEP 31.270-901 Belo Horizonte, Brazil.
authors.forename: A B
authors.initials: AB
authors.lastname: Reis
authors.affiliation: Departamento de Serviço de Nefrologia e Urologia do Hospital das Clínicas daUniversidade Federal de Minas Gerais (UFMG), CEP 31.270-901 Belo Horizonte, Brazil.
authors.forename: R
authors.initials: R
authors.lastname: Martello
authors.affiliation: Departamento de Biologia, Instituto de Ciências Biológicas, Universidade Federal de Juiz de Fora (UFJF), CEP 36.036-900 Juiz de Fora, Brazil.
authors.forename: R C N
authors.initials: RCN
authors.lastname: Melo
authors.affiliation: Departamento de Morfologia, Instituto de Ciências Biológicas, Universidade Federal de Minas Gerais (UFMG), CEP 31.270-901 Belo Horizonte, Brazil.
authors.forename: F R C L
authors.initials: FRCL
authors.lastname: Almeida
authors.affiliation: Departamento de Morfologia, Instituto de Ciências Biológicas, Universidade Federal de Minas Gerais (UFMG), CEP 31.270-901 Belo Horizonte, Brazil.
authors.forename: H
authors.initials: H
authors.lastname: Chiarini-Garcia
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000375:Aging; D001706:Biopsy; D006059:Gonadal Dysgenesis / Q000473:pathology; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008297:Male; D008853:Microscopy; D046529:Microscopy, Electron, Transmission; D008954:Models, Biological; D009919:Orchiectomy; D000072356:Parenchymal Tissue / Q000166:cytology / Q000254:growth & development / Q000473:pathology / Q000648:ultrastructure; D011471:Prostatic Neoplasms / Q000473:pathology / Q000601:surgery; D012670:Seminiferous Epithelium / Q000166:cytology / Q000254:growth & development / Q000473:pathology / Q000648:ultrastructure*; D013091:Spermatogenesis; D013094:Spermatozoa / Q000166:cytology / Q000254:growth & development / Q000473:pathology / Q000648:ultrastructure*; D013737:Testis / Q000002:abnormalities; D014649:Vas Deferens / Q000002:abnormalities
keywords: acrosomic system; germ cells; high-resolution light microscopy; seminiferous epithelium cycle; testis