Книги по МРТ КТ на английском языке / The Embryonic Human Brain An Atlas of Developmental Stages. Third Edition. 2006. By Ronan O'Rahilly

A

a

b

c

B

Figure 17–4. Graphic reconstruction prepared from transverse sections to show a median view of the brain. The asterisk indicates the junction with the spinal cord. (A) The sulcus limitans (a), the hypothalamic sulcus (b), and the marginal ridge (c). The sulcus limitans separates the alar and basal laminae of the rhombencephalon and mesencephalon. However, it does not continue into the diencephalon. The hypothalamic sulcus of the diencephalon separates the thalamus sensu lato from the hypothalamus sensu lato (Table 18–1). (B) The medial surface of each cerebral hemisphere has greatly increased and a hemispheric stalk can be identified. This is the junctional area between the telencephalon and the diencephalon.

Shown here are the boundaries between the different parts of the brain, the newly developed recesses (arrows), the commissures, the sulci, and the entrance into the optic ventricle (black dot in the diencephalon). Four small arrows indicate the isthmic, supramamillary, inframamillary, and infundibular recesses. The boundaries are summarized in Table 15–2.

The posterior commissure has suddenly appeared and consists of two subcommissures: the posterior commissure sensu stricto with fibers of the medial longitudinal bundle, and the commissure of the superior colliculi. Other commissures are the supraoptic in the caudal part of the chiasmatic plate, the supramamillary (dagger), and the commissure of the oculomotor nerves. The ventral commissure of the rhombencephalon, which consists of fibers that cross throughout the length of the floor-plate, has not been indicated here. The commissural plate itself does not yet contain crossing fibers.

In the inset, features of the medial aspect of the lateral wall of the left cerebral hemisphere have been superimposed on the median reconstruction. The interventricular foramen is surrounded by the hippocampal area of the telencephalon and by the medial ventricular eminence of the diencephalon, as well as by the ventral thalamus. The medial eminence is invading telencephalic territory and is penetrated by blood vessels. The lateral ventricular eminence (Fig. 19–6) is visible in the left hemispheric wall. The internal cerebellar swelling lies between the sulcus limitans and the rhombic lip. Reconstructions from a different embryo were published by Hines (1922).

Sulcus limitans

A

B

Tel.

Velum transversum

Figure 17–5. Median section. All mesenchymal components of the head have increased greatly. Many skeletal elements can already be reconstructed: the stapes, the styloid process, the cartilage (Meckel’s) of pharyngeal arch 1, the thyroid laminae, and the cricoid cartilage. The primordia of the so-called hypophysial cartilages (trabeculae; polar cartilages) are present in the area of the future hypophysial fossa. The brain sits on a firm basal condensation of mesenchyme, which, in its rostral part, represents the future nasal septum. This restrains the growth of the cerebral hemispheres in a basal direction. Bar: 1 mm.

The mesenchyme is beginning to form the future dural limiting layer, especially in basal areas, and, beginning at the rostral tip of the notochord, it continues to spread between the diencephalon and the rhombencephalon, i.e., at the site of the mesencephalic flexure. The condensed mesenchyme at the flexure is the primordium of the tentorium cerebelli, which is compressed between the basilar artery and the hypothalamus (Fig. 16–5). The dural limiting layer already contains openings (pori durales) for cranial nerves 3, 4, 5, and 12.

The presence of the dural limiting layer between the pia mater and the skeletogenous sheath initiates the formation of the subarachnoid space. The space clearly begins its development before the presence of choroid plexuses. The roof of the fourth ventricle is very thin. Two areas are beginning to become the areae membranaceae rostralis et caudalis. They are formed by flattened ependymal cells, and their position is indicated by two arrows. Such areas (in the adult mouse) are thought to permit the passage of cerebrospinal fluid containing macromolecules, by way of interependymal cellular clefts. The “ependymal fluid” within the ventricles is believed to be produced by the lining cells that will develop later into ependymal and choroid plexus cells.

Already in the previous stage, the vertebral arteries join the basilar, and an anastomosis is present between the basilar and the internal carotid arteries (Fig. 16–13D). An anterior choroid artery is now developing and was reconstructed by Padget (1948, Fig. 5). The main cerebral arteries are now well established.

The caudal part of the spinal cord, which forms by secondary neurulation, reaches the caudal tip of the body. The definitive number of somites (usually 39 pairs: O’Rahilly and Muller,¨ 2003) seems to be present at this and the next stage. The normal regression of the caudalmost part of the spinal cord may begin at approximately this time.

The key shows the position of the sulcus limitans and the alar and basal laminae (cf Fig. 21–8). The neurohypophysis (characterized by folded walls) and the infundibular recess are present in all embryos of this stage.

Cartilage appears in occipital sclerotomes 1–4, and the hypoglossal foramen has formed in sclerotome 4. The sclerotomic mass is shown in the key.

Med. forebrain bundle

funiculus

Figure 17–10. The cranial nerves and tracts. All cranial nerves have been present since the previous stage. The last of the regular cranial nerves to appear are the abducent and the cochlear, at stage 16. The olfactory fibers, however, become clear in the present stage. Two bundles of nerve fibers, a lateral and a medial, can be distinguished, and at the site of their entrance into the hemispheres, the olfactory bulb will develop in the next stage. The hypoglossal nerve emerges from the occipital sclerotomic mass as either one (as seen here) or two trunks. The ventral ramus of the first cervical nerve joins the hypoglossal. Cervical nerves 1 and 2 unite to form the inferior root of the ansa cervicalis, and the loop is completed by the superior root, which descends from the hypoglossal nerve. Ganglia are indicated by small circles.

Many tracts have appeared during the previous week (from stage 14 onwards). The order of their appearance as determined from graphic reconstructions is: lateral longitudinal fasciculus, common afferent tract, medial longitudinal fasciculus, dorsal funiculus, preopticohypothalamotegmental tract, mesencephalic tract of the trigeminal nerve, medial prosencephalic fasciculus, medial tectobulbar tract, and mamillotegmental and habenulo-interpeduncular tracts. The medial prosencephalic fasciculus (medial forebrain bundle) contains mainly ascending fibers that terminate in the amygdaloid and preoptic regions. The habenulo-interpeduncular tract (fasciculus retroflexus) delineates the synencephalon rostrally. Some of the nuclei and areas of loose intermediate layer are indicated by stippling. The term “nucleus” refers here to zones of lower cellular density and slightly larger cellular size. The amygdaloid nuclei form the most histologically advanced area of the cerebral hemispheres. The amygdaloid region is related to the medial ventricular eminence (Fig. 19–6). The subthalamic nucleus, red nucleus, interstitial nucleus, nucleus of the posterior commissure, and nucleus of the mesencephalic tract of the trigeminal nerve, and the locus caeruleus (asterisk) are recognizable. The cells of the locus caeruleus occupy the isthmus and rhombomere 1. Its tall cells, twice the size of ordinary neurons, lie at the innermost aspect of the intermediate layer, ventral to the sulcus limitans. The interstitial nucleus is indicated by a dagger. The gustatory fibers begin to separate from the common afferent tract as the tractus solitarius.

The tectal area of the mesencephalon, shown in stippling, has developed an intermediate layer.

In the cerebellum, cells of the rhombic lip participate in the formation of the dentate and isthmic nuclei, and later (stage 23) in the development of the external germinal layer.

Figure 17–11. Photomicrograph showing the periphery of the amygdaloid area (the cortex of the future temporal pole), which contains multiple tall cells with long fibers; rostral is to the left-hand side of the page. The neurons for the amygdaloid complex belong to the earliest postmitotic neurons. An initial burst of cell formation for the septal nuclei occurs at this time. Bar: 0.1 mm.

Figure 17–12. Relationship of the olfactory bulb to other parts of the developing limbic

system at approximately 61 –7 weeks.

/2

(A) Three-dimensional right lateral view of the amygdaloid complex, hippocampal primordium, septal area, and olfactory bulb of right and left sides. The amygdaloid complex is at the future temporal pole of the cerebral hemisphere. (B) Schematic representation of the right amygdaloid complex, seen from the direction of the arrow in (A), with four main nuclei. Two chief fiber connections exist: (1) between olfactory bulb via olfactory tubercle to the medial nucleus and (2) between olfactory bulb and the basolateral and anterior nuclei. The arrows at the right indicate a lateral to medial ( ) and a dorsal to ventral (†) “rotation” during the fetal period. (C) A posterosuperior view of the adult amygdaloid nuclei showing that the definitive arrangement differs from the embryonic (After Neuwenhuys et al., cited in Benninghoff’s Anatomie).

From Muller¨ and O’Rahilly (2006) by permission of the Journal of Anatomy.