Heads and tails: The notochord develops differently in the cranium and caudal fin of Atlantic Salmon (Salmo salar, L.)

Abstract

While it is well known that the notochord of bony fishes changes over developmental time, less is known about how it varies across different body regions. In the development of the Atlantic salmon, Salmo salar L., cranial and caudal ends of the notochord are overlaid by the formation of the bony elements of the neurocranium and caudal fin, respectively. To investigate, we describe how the notochord of the cranium and caudal fin changes from embryo to spawning adult, using light microscopy, SEM, TEM, dissection, and CT scanning. The differences are dramatic. In contrast to the abdominal and caudal regions, at the ends of the notochord vertebrae never develop. While the cranial notochord builds a tapering, unsegmented cone of chordal bone, the urostylic notochordal sheath never ossifies: adjacent, irregular bony elements form from the endoskeleton of the caudal fin. As development progresses, two previously undescribed processes occur. First, the bony cone of the cranial notochord, and its internal chordocytes, are degraded by chordoclasts, an undescribed function of the clastic cell type. Second, the sheath of the urostylic notochord creates transverse septae that partly traverse the lumen in an irregular pattern. By the adult stage, the cranial notochord is gone. In contrast, the urostylic notochord in adults is robust, reinforced with septae, covered by irregularly shaped pieces of cellular bone, and capped with an opistural cartilage that develops from the sheath of the urostylic notochord. A previously undescribed muscle, with its origin on the opistural cartilage, inserts on the lepidotrich ventral to it.

Hox genes control homocercal caudal fin development and evolution

“…S7A). In addition, b13a −/− mutants either lacked or had a significantly reduced opisthural cartilage, a specialized structure derived from the notochordal sheath ( 38 ) that covers the posterior end of the notochord (yellow arrowheads, fig. S7A) ( 7 ).…”

“…The evolution of the homocercal tail involved reduction and internalization of this notochordal extension, resulting in an enclosed notochord ending in an opisthural cartilage over the last series of hypurals in stem teleost lineages (Fig. 1, B and D) ( 4 , 38 , 40 , 73 , 74 ). Many teleost lineages with derived morphologies, such as mormyrids, toadfishes, and atherinomorphs ( 75 – 77 ), further specialized by ossifying the notochord’s posterior end and losing the opisthural cartilage ( 1 – 3 ).…”

Evolutionary Insights into Muscle Fiber Distribution in the Twin Tails of Ornamental Goldfish

“…5 and 12A), it is reasonable to infer that this distribution pattern of slow muscle fibers within the deep muscles of the caudal region has been evolutionarily and developmentally conserved across these closely related teleost species. From the evidence that slow muscle fibers were found in a deep muscle in the caudal region of several teleost species from different lineages, it is expected that there may be a developmental mechanism specific to teleost fish that differentiates slow muscle fibers in the deep muscles at the caudal level (Flammang, 2014; Flammang and Lauder, 2008, 2009; Kryvi et al, 2021; Nag, 1972). Further comparison of immunohistochemical features of slow and fast muscle fibers in more distantly related teleost species will reveal whether this is correct.…”

“…It also very well could be that the function of the notochord and its attached tissues at the most posterior level differs from that of equivalent tissues (notochord and neural tube) at the mid-trunk region. Indeed, it has been shown in salmon species ( Salmo salar ) that the posterior- and anterior regions of the notochord could be very different (Kryvi et al, 2021). Thus, it is reasonable to consider that applying the same molecular mechanisms directly to explain the differentiation processes of both trunk and tail muscle fibers could be challenging.…”

“…Presumably due to the complexity of the muscle arrangement and structure in the caudal region, there has been limited immunohistochemical research conducted on muscle fibers in this area. In fact, while early research showed that the caudal muscle of teleost species contains both slow and fast muscles, their distribution patterns in muscle tissues and their relative location with the other skeletal tissues have not been well investigated yet (Flammang and Lauder, 2008, 2009; Kryvi et al, 2021; Nag, 1972). Thus, in this study, we conducted a detailed immunohistochemical analysis of the musculoskeletal system in the twin-tail goldfish, especially, focusing on the caudal region.…”