Anterior-posterior

Observation of the patient may reveal use of accessory muscles of respiration (manifested as paradoxical movements of the chest and abdomen, in a "see-saw"-type motion), pursed-lips breathing, and hyperinflation of the chest with increased anterior-posterior diameter ("barrel chest"). 

What Carroll s team discovered is that the genes and the entire regulatory pathway that integrates them and which control anterior/posterior wing development in the Drosophila (or its common ancestor with butterflies) have been recruited and modified to develop the eyespot focus. This discovery of the facility with which new developmental functions can evolve. .. within extant structures (Keys et al., 1999, p. 534) would have been impossible without the successful why-necessary answer to the proximate question of developmental biology. 

Anterior Posterior Anterior Posterior Anterior Posterior... 

As a functional analysis of animal development, an accurate fate map was carried out using the silkworm, Bombyx mod [128]. Among the beam sizes tested (60- to 250-pm c[)), 250-pm c[) was adopted for the fate mapping of egg at the cellular blastderm stage. When eggs were irradiated near the anterior, posterior, or dorsal periphery, no defective larvae were observed. A close correlation was observed between the site of irradiation and the site of the defect induced. Head defects were induced only by irradiation at the anterior half of the egg, whereas defects in thoracic and abdominal segments were induced by irradiation at the middle and the posterior part of the egg, respectively. Based on the correlations, a fate map of the B. mod egg was established [128]. 

The anterior-posterior axis in Drosophila is defined at least in part by the products of the bicoid and nanos genes. The bicoid gene product is a major anterior morphogen, and the nanos gene product is a major posterior morphogen. The mRNA from the bicoid gene is synthesized by nurse cells and deposited in the unfertilized egg near its anterior pole. 

FIGURE 28-37 Regulatory circuits of the anterior-posterior axis in a Drosophila egg. The bicoid and nanos mRNAs are localized near the anterior and posterior poles, respectively. The caudal, hunchback, and pumilio mRNAs are distributed throughout the egg cytoplasm. The gradients of Bicoid (Bed) and Nanos proteins lead to accumulation of Hunchback protein in the anterior and Caudal protein in the posterior of the egg. Because Pumilio protein requires Nanos protein for its activity as a translational repressor of hunchback, it functions only at the posterior end. 

Development of a multicellular organism presents the most complex regulatory challenge. The fate of cells in the early embryo is determined by establishment of anterior-posterior and dorsal-ventral gradients... 

Fig. 4. (continued) (c) Projection of the five compartmental boundaries onto the fate map26 of the third instar wing disc. The dotted line 4 is the postulated compartmental line in the ventral thorax needed to complete the anterior-posterior, dorsal-ventral (twofold) symmetry. 

The striking feature of the compartmental lines on the wing disc is their clear twofold symmetry. Anterior, posterior, ventral, and dorsal quan-drants look fundamentally similar. Good evidence suggests that these compartmental lines arise in situ in the growing disc, and do not reflect movement of cells relative to one another. Therefore, the question arises of how such symmetrical lines might arise. 

The cerebellum lies posterior to the brainstem and is separated from it by the fourth ventricle. Anatomically it is divided into two hemispheres, each consisting of three lobes (anterior, posterior, and flocculonodular). The function of the cerebellum is to help plan and coordinate motor activity and to assume responsibility for comparing the actual movement with the intended motor pattern. The cerebellum interprets various sensory input and helps modulate motor output so that the actual movement closely resembles the intended motor program. The cerebellum is also concerned with the vestibular mechanisms responsible for maintaining balance and posture. Therapeutic medications are not usually targeted directly for the cerebellum, but incoordination and other movement disorders may result if a drug exerts a toxic side effect on the cerebellum. 

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