Fetal development and growth of the human erector spinae with special reference to attachments on the surface aponeurosis.

PURPOSE: The longissimus (LO) and iliocostalis (IC) of adults consist of myofibers extending from the superolateral to the inferomedial side of the back and, because of the same course, they are fused in the thoracolumbar region. The LO also has a medial attachment to the long myofibers of the transversospinalis (TS) showing a course from the superomedial to the inferolateral side. However, there is apparently no information regarding when and how these similar longitudinal muscles differentiate from a cluster of dorsomedial myotome cells. METHODS: We examined sagittal and horizontal sections of the trunks of 39 human embryos and fetuses (18-330 mm crown-rump length). RESULTS: At 6-7 weeks gestational age (GA), the surface aponeurosis appeared prior to and independent of the thoracolumbar fascia. At 6-9 weeks GA, the LO myofibers had a postero-inferior course, from the transverse process to the initial aponeurosis, whereas the TS myofibers had a postero-superior course, from a lateral extension of the intertransverse ligament to the aponeurosis. However, the IC consisted of supracostal longitudinal myofibers and was distant from the LO until 12 weeks GA. Because of the lack of ligamentous attachments and ribs, myofibers of the TS, LO, and IC took a similar inferior course in the lumbar region. When the early TS was represented by the transverso-aponeurotic muscle, consequently, the LO corresponded to the aponeuro-transversal muscle and was independent from the IC. CONCLUSION: The classical model of TS and LO development does not recognize the essential role of the aponeurosis identified here.

An alternative didactic, functional and topographic systematization of the spinal muscles.

Back muscles are commonly described in a topographically-oriented manner without necessarily following morphological criteria. In this manner, non-standard terms may be employed which convey incorrect morphological concepts and demanding more time from both faculty and students to transmit knowledge. We propose a classification system for spinal muscles incorporating morphological concepts with the goal of facilitating knowledge transfer and suggest the term "spinal muscles". Those muscles were systematically divided and classified in seven strata from anterior to posterior: vertebro-appendicular (VA), transversarium (Tr), deep post-transversarium (DPT), middle post-transversarium (MPT), superficial post-transversarium (SPT), deep spino-appendicular (DSA) and superficial spino-appendicular (SSA). Besides topography and function, this system incorporates innervation and embryological origins of each muscle. The extrinsic (VA, DSA, SSA) or intrinsic (Tr, DPT, MPT, SPT) nature of these muscles in relation to the spine and also the topographic relationship to the transverse process is represented in this system. Specific areas of functional, nervous and developmental transition exist on Tr and DPT strata due to being adjacent to extrinsic strata. We believe this system represents a more modern and concise teaching strategy for back muscles which may be employed partially or fully within any program. We envision its full version may be particularly useful in postgraduate medical training for specialties dealing with the spinal column such as neurosurgery, orthopedic surgery and physical medicine and rehabilitation.

Development of the epaxial muscles in the human embryo.

Although the intrinsic muscles of the back are defined by their embryological origin and innervation pattern, no detailed study on their development is available. Human embryos (5-10 weeks development) were studied, using Amira3D® reconstruction and Cinema4D® remodeling software for visualization. At Carnegie Stage (CS)15, the epaxial portions of the myotomes became identifiable laterally to the developing vertebrae. At CS16, these portions fused starting cranially to form a longitudinal muscle column, which became innervated by the dorsal branches of the spinal nerves. At CS17, the longitudinal muscle mass segregated into medial and lateral columns (completed at CS18). At CS18, the medial column segregated again into intermediate and medial columns (completed at CS20). The lateral and intermediate columns did not separate in the lower lumbar and sacral regions. Between CS20 and CS23, the cervical portions of the three columns segregated again from lateral to medial resulting ventrolaterally in rod-like continuations of the caudal portions of the columns and dorsomedially in spade-like portions. The observed topography identifies the iliocostalis and splenius as belonging to the lateral column, the longissimus to the intermediate column, and the (semi-)spinalis to the medial column. The medial (multifidus) group acquired its transversospinal course during closure of the vertebral arches in the early fetal period. Hence, the anatomical ontology of the epaxial muscles is determined by craniocaudal and lateromedial gradients in development. Three longitudinal muscle columns, commonly referred to as the erector spinae, form the basic architectural design of the intrinsic muscles of the back. Clin. Anat. 29:1031-1045, 2016. © 2016 Wiley Periodicals, Inc.