Evaluación de la operación de la infraestructura

The most extensive studies into the development of the craniofacial region in mammals have centred on the human and the mouse, whilst rat development has not been studied as extensively. However a developmental staging system has been proposed by Witschi which I have summarized in Tables A1 and A2. Below is a description of the morphogenetic events that occur in the craniofacial region of the human embryo. Of course, no study dealing with development of the head and face can completely ignore the essential contribution provided by the neural crest cells, but as this has been covered extensively elsewhere (for examples see Le Douarin et al. ,

1993; Thorogood, 1994) it will not be described here in any significant detail. The many detailed accounts of mammalian craniofacial development have concentrated on the mouse and human, whilst the rat seems to have been largely neglected. Many of the morphological events that occur during the development of the rat head and face can be extrapolated from those events which contribute to the human craniofacial development as described below, which has been adapted from Sadler, 1990 and is summarized in Fig. A l.

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The most extensive studies into the development of the craniofacial region in mammals have centred on the human and the mouse, whilst rat development has not been studied as extensively. However a developmental staging system has been proposed by Witschi which I have summarized in the Appendix as Tables Al and A2. Below is a description of the morphogenetic events that occur in the craniofacial region of the human embryo. Of course, no study dealing with development of the head and face can completely ignore the essential contribution provided by the neural crest cells, but as this has been covered extensively elsewhere (for examples see Le Douarin et al., 1993; Thorogood, 1994) it will not be described here in any significant detail.

The many detailed accounts of mammalian craniofacial development have concentrated on the mouse and human, whilst the rat seems to have been largely neglected. Many of the morphological events that occur during the development of the rat head and face can be extrapolated from those events which contribute to the human craniofacial development as described below, which has been adapted from Sadler, 1990 and is summarized in Fig. 1.3.

The Human

The initial shape of the embryonic head and face is determined by several factors. The cranium forms in relation to the developing brain; the craniofacial region is formed by the chondrocranium; and the mandible forms from the first branchial arch and its derivatives.

The branchial arches consist of a core of mesenchymal tissue, covered on their external aspect by surface ectoderm, and on the inside by epithelium derived from the

endoderm. In addition to mesenchyme derived from the mesoderm of the paraxial and lateral neural plate, the core of each arch is infiltrated by neural crest cells, which migrate into the arches to contribute to the skeletal components of the face. The craniofacial musculature is derived from the original branchial arch mesoderm and each arch is thus characterized by its own muscular components, which in turn carry their own nerve. Wherever the muscle cells might migrate, they carry their cranial nerve component with them. In addition each arch has its own arterial component. First Branchial Arch

The first branchial arch comprises a dorsal portion known as the maxillary process, which extends forward inferior to the eye region, and a ventral portion, the mandibular process which gives rise to Meckel’s cartilage. As the embryo develops, Meckel’s cartilage regresses and disappears, with the exception of its dorsal end which persists and give rise to the incus and malleus. Through the process of membranous ossification, the mesenchymal component of the maxillary process subsequently forms the premaxilla, maxilla, zygomatic bone and part of the temporal bone. The mandible is similarly formed by membranous ossification of the mesenchymal tissue surrounding Meckel’s cartilage.

The first branchial arch musculature is comprised of the muscles of mastication (temporal, masseter, and pterygoids), the anterior belly of the digastric, the mylohyoid, the tensor tympani, and tensor palatini.

The muscles of the various arches do not always attach to the bony or cartilaginous components of their own arch, sometimes migrating into the surrounding regions. The nerve supply to the muscles of the first arch is provided only by the mandibular branch of the trigeminal nerve. As the first arch mesenchyme also

contributes to the dermis of the face, the sensory supply of the facial skin is provided by the ophthalmic, maxillary, and mandibular branches of the trigeminal nerve.

Second Branchial Arch

The cartilage of the second or hyoid arch (Reichert’s cartilage) gives rise to the stapes, the styloid process of the temporal bone, the stylohyoid ligament, and, ventrally to the lesser horn and the upper part of the body of the hyoid bone. The muscular component of the hyoid arch comprises the stapedius, the stylohyoid, the posterior belly of the digastric, the auricular, and the muscles of facial expression, which are all innervated by the facial nerve, the nerve of the second arch.

Third Branchial Arch

The cartilage of this arch gives rise to various components of the hyoid bone. The musculature is limited to the stylopharyngeal muscle, which is innervated by the glossopharyngeal nerve, the nerve of the third arch.

Fourth and Sixth Branchial Arches

The cartilaginous components of these arches fuse to form the thyroid, cricoid, arytenoid, comiculate, and cuneiform cartilages of the larynx. The muscles of the fourth arch (the cricothyroid, the levator palatini, and the constrictors of the pharynx) are innervated by the superior laryngeal branch of the vagus, the nerve of the fourth arch.

The Face

The facial prominences consist primarily of neural crest-derived mesenchyme, and they are formed predominantly by the first branchial arch. The maxillary prominences, can be distinguished lateral and the mandibular prominences caudal to

the stomodeum. The frontonasal prominence, formed by proliferation of mesenchyme ventral to the brain vesicles, constitutes the upper border of the stomodeum. On either side of the frontonasal prominence are local thickenings of the surface ectoderm, the nasal (olfactory) placodes. At a later stage in development, the nasal placodes invaginate to form the nasal pits, creating a ridge of tissue that surrounds each pit, forming the nasal prominences. The prominences on the outer edge of the pits are the lateral nasal prominences, whilst those on the inner edge are the medial nasal prominences.

As the embryo gets older, the maxillary prominences continue to grow medially, compressing the medial nasal prominences toward the midline, and eventually they fuse. Therefore, the upper lip is formed by the two medial nasal prominences and the two maxillary prominences. The lower lip and jaw are formed from the mandibular prominences, which merge across the midline.

Initially, the maxillary and nasal prominences are separated by a deep furrow, the nasolacrimal groove. The ectoderm in the floor of this groove forms a solid epithelial cord, which detaches from the rest of the overlying ectoderm, ultimately forming the nasolacrimal duct. Following detachment of the cord, the maxillary and lateral nasal prominences merge with each other. The nasolacrimal duct runs from the medial comer of the eye to the inferior meatus of the nasal cavity. The maxillary prominences then enlarge to form the cheeks and maxillae.

The nose is formed from five facial prominences: the frontal prominence gives rise to the bridge; the merged nasal prominences provide the crest and tip; and the lateral nasal prominences form the sides (alae).

Intermaxillary Segment

As a result of the medial growth of the maxillary prominences, the two medial nasal prominences merge not only at the surface, but also at a deeper level. The structures formed by the two merged prominences are together known as the intermaxillary segment. It is composed of a labial component, which forms the philtrum of the upper lip; an upper jaw component, which carries the incisor teeth; and a palatal component which forms the triangular primary palate. Cranially, the intermaxillary segment is continuous with the rostral part of the nasal septum, which is formed by the frontal prominence.

Secondary Palate

The definitive palate is predominantly formed by two shelf-like processes which grow out from the maxillary prominences. These processes, the palatal shelves, initially grow obliquely downwards on either side of the tongue. Later, these shelves ascend enabling them to reach a horizontal position above the tongue and fuse with each other, thus forming the secondary palate.

Anteriorly, the shelves fuse with the triangular primary palate and at the same time as this fusion, the nasal septum grows down and merges with the rostral aspect of the newly formed secondary palate.

Tongue

Three swellings derived from the first branchial arch, two lateral lingual swellings and one medial swelling, the tuberculum impar, mark the initial appearance of the tongue. A second medial swelling, the copula or hypobranchial eminence, is formed by the mesoderm of the second, third and part of the fourth arch. Finally, a third medial swelling, formed by the posterior part of the fourth branch, marks the

development of the epiglottis. Immediately behind this swelling is the laryngeal orifice, which is flanked by the arytenoid swellings.

The lateral lingual swellings overgrow the tuberculum impar and merge with each other, thereby forming the anterior two-thirds of the body of the tongue. The mucosa covering the body of the tongue also originates from the first branchial arch, and is innervated by the mandibular branch of the trigeminal nerve.

Age Size

TABLE Al

RAT DEVELOPMENT

In document Evaluación ex post a nivel de culminación del proyecto de riego tecnificado San José, Provincia Azángaro – Puno (página 63-74)