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

A

1

2

3

4

5

6

7

B

C

Syn.

Caudal

par.

Rostral

par.

D1

Figure 14–7. The neuromeres at stage 14 as shown in graphic reconstructions. The distinction that can now be made between the rostral and caudal parts of the parencephalon completes the series of 16 neuromeres. The initial appearance of the neuromeres is summarized in Table 10–1. The rostral parencephalon contains the opening of the optic ventricle on each side and is associated with the infundibular region and the ventral thalamus, whereas the caudal parencephalon presents the dorsal thalamus and the mamillary region. (A) Longitudinal section of the brain stem at the level indicated in the key. The mesencephalon is joined to the rhombomeres by the isthmus. Bar: 0.25 mm. (B) Section through the forebrain along the line B. The nasal discs and their neural crest are visible on each side. In the key drawing the asterisks denote the sulcus medius, which separates the dorsal (d) from the ventral (v) thalamus. The hypothalamic cell cord (stippled) indicates longitudinal differentiation in the diencephalon even before the hypothalamic sulcus appears at the next stage. (C) Section through the forebrain along the line C. The four compartments separated by horizontal lines are synencephalon, caudal parencephalon, rostral parencephalon, and D1 (immediately above the chiasmatic plate). The sulcus medius is marked by an arrowhead. Portions of the optic stalk and optic cup are visible on the right-hand side of the photomicrograph. Bar: 0.25 mm.

Figure 15–2. Graphic reconstruction prepared from transverse sections to show a median view of the brain. The asterisk indicates the junction with the spinal cord. The cerebral hemisphere is limited from the diencephalon by an internal crest, the torus hemisphericus (Fig. 15–4), which begins at the velum transversum and ends as a basal thickening in the medial ventricular eminence (Table 15–2). In the development of the basal nuclei (Fig. 19–6), the lateral ventricular eminence, which is telencephalic, has appeared (Fig. 15–2, inset). The medial eminence, however, is entirely diencephalic, although it has been misinterpreted by others as the lateral eminence. The chiasmatic plate is well defined and is limited by preoptic and postoptic recesses. The infundibular recess is appearing adjacent to the adenohypophysial pouch. The caudal part of the diencephalon (formerly D2) is the synencephalon, which will give rise to the prerubrum and the pretectum. At this level, the epiphysis cerebri begins as a thickening, but a recess is not yet present.

A characteristic feature of this stage is the appearance of longitudinal zones in the diencephalon (Fig. 15–4 and Table 18–1). For example, the hypothalamic sulcus delineates the hypothalamus from the thalamus. In addition, the hypothalamus comprises subthalamus and hypothalamus sensu stricto (separated by a faint sulcus); the thalamus consists of epithalamus, dorsal thalamus, and ventral thalamus (Table 18–1). The dorsal and ventral thalami are incompletely separated in more advanced embryos by the beginning formation of the zona limitans intrathalamica, which is marked on the interior relief by the marginal ridge. Blood vessels begin to penetrate the wall of the diencephalon.

The midbrain consists of two neuromeres: M1 and M2. The sulcus limitans ends at the rostral limit of M1. M2 is joined to the hindbrain by the isthmus. The supramamillary commissure is marked by a dagger. In the hindbrain, the cerebellar plate is distinct and extends over two rhombomeres: the isthmus and Rh. 1. The roof of the isthmus forms the superior medullary velum. The locus caeruleus is indicated (Fig. 15–10). The notochord is partly surrounded by sclerotomic material that represents the future basioccipital component of the skull.

The inset drawing below left shows the contiguity of the torus hemisphericus and the medial ventricular eminence. The lateral eminence is visible in the lateral wall of the (left) cerebral hemisphere.

The boundaries between the major divisions of the brain are listed in Table 15–2.

TABLE 15–1. The Sequence of Appearance of Key Features of the Brain, Including the Main Divisions and Subdivisions

Myelencephalon

TABLE 15–2. Boundaries Between the Main Subdivisions of the Brain