title: Cellular and molecular maturation in fetal and adult ovine calcaneal tendons.
abstract: Processes of development during fetal life profoundly transform tendons from a plastic tissue into a highly differentiated structure, characterised by a very low ability to regenerate after injury in adulthood. Sheep tendon is frequently used as a translational model to investigate cell-based regenerative approaches. However, in contrast to other species, analytical and comparative baseline studies on the normal developmental maturation of sheep tendons from fetal through to adult life are not currently available. Thus, a detailed morphological and biochemical study was designed to characterise tissue maturation during mid- (2 months of pregnancy: 14 cm of length) and late fetal (4 months: 40 cm of length) life, through to adulthood. The results confirm that ovine tendon morphology undergoes profound transformations during this period. Endotenon was more developed in fetal tendons than in adult tissues, and its cell phenotype changed through tendon maturation. Indeed, groups of large rounded cells laying on smaller and more compacted ones expressing osteocalcin, vascular endothelial growth factor (VEGF) and nerve growth factor (NGF) were identified exclusively in fetal mid-stage tissues, and not in late fetal or adult tendons. VEGF, NGF as well as blood vessels and nerve fibers showed decreased expression during tendon development. Moreover, the endotenon of mid- and late fetuses contained identifiable cells that expressed several pluripotent stem cell markers [Telomerase Reverse Transcriptase (TERT), SRY Determining Region Y Box-2 (SOX2), Nanog Homeobox (NANOG) and Octamer Binding Transcription Factor-4A (OCT-4A)]. These cells were not identifiable in adult specimens. Ovine tendon development was also accompanied by morphological modifications to cell nuclei, and a progressive decrease in cellularity, proliferation index and expression of connexins 43 and 32. Tendon maturation was similarly characterised by modulation of several other gene expression profiles, including Collagen type I, Collagen type III, Scleraxis B, Tenomodulin, Trombospondin 4 and Osteocalcin. These gene profiles underwent a dramatic reduction in adult tissues. Transforming growth factor-β~1 expression (involved in collagen synthesis) underwent a similar decrease. In conclusion, these morphological studies carried out on sheep tendons at different stages of development and aging offer normal structural and molecular baseline data to allow accurate evaluation of data from subsequent interventional studies investigating tendon healing and regeneration in ovine experimental models.
authors.affiliation: Faculty of Veterinary Medicine, Teramo, Italy.
authors.forename: Valentina
authors.initials: V
authors.lastname: Russo
authors.forename: Annunziata
authors.initials: A
authors.lastname: Mauro
authors.forename: Alessandra
authors.initials: A
authors.lastname: Martelli
authors.forename: Oriana
authors.initials: O
authors.lastname: Di Giacinto
authors.forename: Lisa
authors.initials: L
authors.lastname: Di Marcantonio
authors.forename: Delia
authors.initials: D
authors.lastname: Nardinocchi
authors.forename: Paolo
authors.initials: P
authors.lastname: Berardinelli
authors.forename: Barbara
authors.initials: B
authors.lastname: Barboni
mesh_terms: D000125:Achilles Tendon / Q000033:anatomy & histology / Q000166:cytology / Q000196:embryology / Q000254:growth & development / Q000378:metabolism; D000818:Animals; D015415:Biomarkers / Q000378:metabolism; D015153:Blotting, Western / Q000662:veterinary; D017630:Connexins / Q000378:metabolism; D005109:Extracellular Matrix / Q000378:metabolism; D020869:Gene Expression Profiling / Q000662:veterinary; D007150:Immunohistochemistry / Q000662:veterinary; D020932:Nerve Growth Factor / Q000378:metabolism; D015675:Osteocalcin / Q000378:metabolism; D039904:Pluripotent Stem Cells / Q000378:metabolism; D012756:Sheep / Q000196:embryology / Q000254:growth & development; D042461:Vascular Endothelial Growth Factor A / Q000378:metabolism
chemical_list: D015415:Biomarkers; D017630:Connexins; D042461:Vascular Endothelial Growth Factor A; D015675:Osteocalcin; D020932:Nerve Growth Factor
keywords: blood vessels; calcaneal tendon; connexins; extracellular matrix molecules; fetus; growth factors; nerve fibers; pluripotency stem cell markers