Development of the ciliary body: morphological changes in the distal portion of the optic cup in the human.

This study seeks to determine the main events that occur in the development of the ciliary body (CB) in the 5-14th week of development. The CB develops from the distal portion of the optic cup (OC) and the neighboring mesenchyme. During the 5th week of development, 4 zones were observed in the distal portion of the OC: in zone 1, the epithelia of the outer and inner layers of the OC came into contact. This contact coincided with the appearance of mainly apical granule pigments. This zone corresponded to the anlage of the epithelial layers of the CB. In zone 2, the cells surrounded the marginal sinus and contained scarce pigment granules and nuclei in the basal position. This zone corresponded to the anlage of the iris. Zone 3 was triangular in shape and its vertex ran towards the marginal sinus and corresponded to common cell progenitors. Zone 4 corresponded to the retinal pigment epithelium anlage and the neural retina anlage. We determined the onset of the stroma and the ciliary muscle anlage at the end of the 7th week. In the 13-14th week, we observed the anlage of the orbicularis ciliaris (pars plana of the CB) and corona ciliaris (pars plicata of the CB), in addition to the anlage of the ciliary muscle. Our study, therefore, establishes a precise timetable of the development of the CB.

Development of the platysma muscle and the superficial musculoaponeurotic system (human specimens at 8-17 weeks of development).

There is controversy regarding the description of the different regions of the face of the superficial musculoaponeurotic system (SMAS) and its relationship with the superficial mimetic muscles. The purpose of this study is to analyze the development of the platysma muscle and the SMAS in human specimens at 8-17 weeks of development using an optical microscope. Furthermore, we propose to study the relationship of the anlage of the SMAS and the neighbouring superficial mimetic muscles. The facial musculature derives from the mesenchyme of the second arch and migrates towards the different regions of the face while forming premuscular laminae. During the 8th week of development, the cervical, infraorbital, mandibular, and temporal laminae are observed to be on the same plane. The platysma muscle derives from the cervical lamina and its mandibular extension enclosing the lower part of the parotid region and the cheek, while the SMAS derives from the upper region. During the period of development analyzed in this study, we have observed no continuity between the anlage of the SMAS and that of the superficial layer of the temporal fascia and the zygomaticus major muscle. Nor have we observed any structure similar to the SMAS in the labial region.

Development of the juxta-oral organ in rat embryo.

The aim of this work is to clarify the development and morphology of the juxta-oral organ (JOO) in rat embryos from Day (E)14 to 19. Furthermore, in the region of the JOO, an analysis was made of the expression of the monoclonal antibody HNK-1, which recognizes cranial neural-crest cells. In this study, we report that JOO develops from an epithelial condensation at the end of the transverse groove of the primitive mouth at E14. During E15, it invaginates and is disconnected from the oral epithelium. At E16, the JOO forms an solid epithelial cord with three parts (anterior, middle, and posterior) and is related to the masseter, temporal, medial pterygoid, and tensor veli palatini muscles. During E17-19, no significant changes were detected in their position. Both the mesenchyme caudal to the anlage of the JOO at E14, as well as the mesenchyme that surrounds the bud of the JOO at E15, expressed positivity for HNK-1. Our results suggest that the mesenchyme surrounding the JOO at E15 could emit some inductive signal for the JOO to reach its position at E16. This work shows for the first time that the cranial neural-crest-derived mesenchyme participates in the development of the JOO.

Histological study of the extratympanic portion of the discomallear ligament in adult humans: a functional hypothesis.

This study was carried out on histological aspects of the extratympanic portion of the discomallear ligament (DL) in adult humans. The temporomandibular joint (TMJ) was dissected bilaterally in 20 cadavers; in 15 cases the articular disc (AD) and the retroarticular tissue were extirpated. The extratympanic portion of the DL had the shape of a base-down triangle, in relation to the AD, and an upper vertex, in relation to the petrotympanic fissure. In five cases, the base, measured bilaterally, had an average length of 6.4 mm, while the distance from the base to the upper vertex averaged 9.3 mm in length. The extratypanic portion of the DL is an intrinsic ligament of the TMJ, composed of collagen fibres and abundant elastic fibres. We propose that this ligament could act as a tensor of the synovial membrane in movements of the TMJ.

Incidence and development of the human supracochlear cartilage.

The supracochlear cartilage is known as an accessory cartilage of the chondrocranium situated between the otic capsule and the trigeminal ganglion. Although claimed to appear regularly during human development, its incidence and development have been reported only scarcely in the literature. The aim of this study was to describe the position and relationships of the supracochlear cartilage during its development. This study was made in 96 human specimens of 7-17 weeks of development, belonging to a collection of the Embryology Institute of Complutense University of Madrid. In addition, three-dimensional reconstruction of the supracochlear cartilage was made from 1 specimen. This cartilage, spherical in shape, appeared bilaterally in 23 specimens and unilaterally (left side) in 5. In our results, the supracochlear cartilage was found in 26.5% of the cases and was related to the trigeminal ganglion, the dura mater of the trigeminal cavity and the otic capsule. In 4 specimens, bilaterally, the supracochlear cartilage was continuous with the otic capsule. This work suggests that, based on the structures to which the supracochlear cartilage is related, it could be derived from the cranial neural crest.

Anatomical relationships of the cleidoatlanticus muscle. Interpretation about its origin.

An unusual muscular variation, the cleidoatlanticus muscle, was observed on the right-hand side of the lateral cervical region. The upper third of the muscle was concealed by the sternocleidomastoid muscle. There was a loop of nerves surrounding the muscle, formed by an anastomosis between the transverse cervical nerve and the greater auricular nerve. A fine vascular-nervous pedicle (formed by a small branch from the transverse cervical artery and by a branch from the medial supraclavicular nerve) entered the deep surface of the muscle at the junction of its middle and lower thirds. Taking into account the relationships that presented with the superficial branches of the cervical plexus, we consider that the cleidoatlanticus muscle is derived from the sternocleidomastoid muscle.

Development of the mandibular condylar cartilage in human specimens of 10-15 weeks' gestation.

This study analyses some morphological and histological aspects that could have a role in the development of the condylar cartilage (CC). The specimens used were serial sections from 49 human fetuses aged 10-15 weeks. In addition, 3D reconstructions of the mandibular ramus and the CC were made from four specimens. During weeks 10-11 of development, the vascular canals (VC) appear in the CC and the intramembranous ossification process begins. At the same time, in the medial region of the CC, chondroclasts appear adjacent to the vascular invasion and to the cartilage destruction. During weeks 12-13 of development, the deepest portion of the posterolateral vascular canal is completely surrounded by the hypertrophic chondrocytes. The latter emerge with an irregular layout. During week 15 of development, the endochondral ossification of the CC begins. Our results suggest that the situation of the chondroclasts, the posterolateral vascular canal and the irregular arrangement of the hypertrophic chondrocytes may play a notable role in the development of the CC.

Morphogenesis of the human excretory lacrimal system.

The aim of this study was to determine the principal developmental stages in the formation of the excretory lacrimal system in humans and to establish its morphogenetic period. The study was performed using light microscopy on serial sections of 51 human specimens: 33 embryos and 18 fetuses ranging from 8 to 137 mm crown-rump length (CR; 5-16 weeks of development). Three stages were identified in the morphogenesis of the excretory lacrimal system: (1) the formative stage of the lacrimal lamina (Carnegie stages 16-18); (2) the formative stage of the lacrimal cord (Carnegie stages 19-23); and (3) the maturative stage of the excretory lacrimal system, from the 9th week of development onward. A three-dimensional reconstruction of the excretory lacrimal system was performed from serial sections of an embryo at the end of the embryonic period (27 mm CR).

Origin of the styloglossus muscle in the human fetus.

The origin of the styloglossus muscle was histologically studied bilaterally in nine human fetuses (18 sides). In all cases, the muscle originated in Reichert's cartilage, which gives rise to the temporal styloid process. We identified three types of variation: type A, an accessory muscle fascicle originating from the mandibular angle, found in 7 cases (12 sides); type B, where the styloglossus muscle was attached to the mandibular angle by fibrous tracts, found in three cases (4 sides); and type C, where an accessory muscle fascicle arose from the fibrous tract connecting Reichert's cartilage to the mandibular angle; found in one case. In all cases (2 sides), the styloglossus muscle was innervated by the hypoglossal nerve. Relationships between the styloglossus muscle and vasculonervous elements of the prestyloid and retrostyloid spaces were analysed.

Morphogenesis of the juxtaoral organ in humans.

The juxtaoral organ was studied using light microscopy in 55 human embryos and 90 fetuses at different stages of development. The juxtaoral organ arises from the epithelium at the bottom of the transverse opening of the primitive mouth during O'Rahilly stage 16 and becomes detached from the epithelium after O'Rahilly stage 18. The juxtaoral organ is innervated by the buccal nerve from O'Rahilly stage 20 onward, and its connective tissue capsule is clearly visible after week 11 of development. This study enabled us to describe three main periods of juxtaoral organ development: (1) the period of epithelial condensation and invagination, at O'Rahilly stages 16-17; (2) the period during which the juxtaoral organ becomes detached from the oral epithelium and is innervated, at O'Rahilly stages 18-23; and (3) the period during which the connective tissue capsule is formed, after week 11 of development. We also analysed the juxtaoral organ of five additional fetuses by immunohistochemistry with anti-NF-200 to verify their innervation. The results show that the juxtaoral organ may have a function in the mechanical activity of the region.

Morphogenesis of the human lacrimal gland.

The aim of this study was to determine the main stages of the lacrimal gland's developmental process in humans and to establish its precise morphogenetic timetable. Its onset is generally assumed to take place at O'Rahilly's stage 21, arising from an epithelial thickening of the superior extreme of the temporary conjunctival fornix. However, the present study points to a prior stage in the process: the presence of epithelial-mesenchymal changes in embryos at O'Rahilly's stage 19. The study was performed using light microscopy on serial sections of 37 human specimens: 23 embryos and 14 fetuses ranging from 15 to 137 mm crown-rump length (7-116 weeks of development). Three stages in lacrimal gland morphogenesis were identified: (1) the presumptive glandular stage, O'Rahilly's stages 19-20, characterized by a thickening of the superior fornix epithelium together with surrounding mesenchymal condensation; (2) the bud stage, generally assumed to be the first manifestation of glandular origin, characterized initially by the appearance of nodular formations in the region of the superior conjunctival fornix and concluding with the appearance of lumina within the epithelial buds; and (3) the glandular maturity stage, weeks 9-16, the period in which the gland begins to take on the morphology of adulthood.