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

Developing

atrium

Figure 25–1. Approximately 230 mm (about 24–26 weeks). This horizontal section is taken near the end of trimester 2. It resembles very closely a corresponding section through the adult brain, except for the almost total absence of sulci and gyri. Moreover, the insula is not yet buried by its opercula. The level of the section is that of the interthalamic adhesion and the epiphysis cerebri. (The interthalamic adhesion appears at about the junction of trimesters 1 and 2, and disappears at approximately 40–60 years of age, as shown in a graph in Lemire et al., 1975, p. 179). The third ventricle is sectioned twice, as is also the caudate nucleus. The anterior and posterior horns of each lateral ventricle are evident. Also shown are the pulvinar (a), the habenula (b), and the suprapineal recess (c). The section is based on a photomicrograph in Feess-Higgins and Larroche (1987). The right lateral view of the ventricular system (after Kier, 1977) illustrated in an inset shows the triangular part of the lateral ventricle, where the inferior and posterior horns diverge. This part is frequently referred to in radiology as the atrium, and its floor is the collateral trigone.

TABLE 25–1. Relationships of Internal Capsule in the Fetus and in the Adult

abbreviation CA can still be used (as proposed here) for Cellular Areas 1 to 4. In a different system, five hippocampal fields are designated H1 to H5.

The hippocampal formation is the “earliest group of cortical areas in humans” and “hippocampal pathways develop prior to isocortical pathways” (Hevner and Kinney, 1996).

The dentate gyrus possesses a pool of progenitor cells that continues to produce new granule cells throughout life.

The embryonic development of the hippocampus and dentate gyrus has already been illustrated in Figures 15–4, 16–6B, C, 16–11, 18–2, 20–13, 21–17, and 23–24.

THE HIPPOCAMPAL

FORMATION

The dentate gyrus, the hippocampus, the subiculum, and the parahippocampal gyrus (entorhinal area) are conveniently grouped as the hippocampal formation.

The hippocampus was probably first observed by Achillini of Bologna (1463–1512). It was named by Aranzi (Arantius) of Bologna (1530–1589), who described it as “a raised, partially attached, whitish substance” showing “an uneven, bent form that resembles the appearance of a hippocampus, that is, a sea-horse.” The name is an example of Witz der Namengebung (Burdach). The alternative name Cornu Ammonis, now avoided as are other eponyms, was used by Winslow (1669–1760) because of a supposed resemblance to the curling horn of a ram, which was the crest of the Egyptian deity Ammon.

The name Cornu Ammonis gave rise to the abbreviation CA and the structure was divided by Lorente de No´ into CA1 to CA4; CA4 is now frequently included in CA3. Although the preferred term is hippocampus, the

NEUROTERATOLOGY: AGENESIS OF THE CORPUS CALLOSUM

Agenesis of the corpus callosum, a complete or partial lack of callosal fibers between the cerebral hemispheres, is discovered usually as an accidental finding. The anterior commissure may be enlarged, providing supplementary neocortical interconnections. It is assumed that the condition arises in the commissural plate, the callosal component of which may fail to develop or may not grow caudally. At least some instances may represent a failure of commissuration rather than an agenesis. Defects of the corpus callosum and the adjacent region are considered by some to be types of cerebral dysraphia.

More than 50 syndromes involve callosal agenesis, which may be autosomal dominant, autosomal recessive, or X-linked. Moreover, some 40 genes are relevant.

Figure 25–3. Development of the hippocampal formation. (A) and (B) During trimester 1. The embryonic development is characterized mainly by cellular production and migration (Table 25-2). The rostral, middle, and occipital regions are alike before the appearance of the corpus callosum. Here the occipital (retrocommissural) part is represented. (A) A horizontal section at 49 mm (9 21 weeks), at the level of the insula. The rectangle indicates the portion (at 44 mm) shown in A’. (A′ ) A structure resembling cortex represents the future pyramidal layer of the hippocampus. The marginal layer of the dentate area is very broad. (B) A horizontal section at 85 mm (12 weeks). The pyramidal layer (hippocampal plate) is continuous with the dentate primordium and with the cortical plate. The rectangle indicates the portion (at 56 mm) shown in B′ . (B′ ) A stream of migrating cells leads from the ventricular layer to the dentate primordium. The hippocampal sulcus is evident. The curved, Cellular Areas, CA 1–3, are marked. The fimbria has appeared and is well-delineated near the dentate area.

A and B are modified from drawings, and A′ and B′ are based on photomicrographs, in Humphrey (1965).

ADENOHYPOPH.

Pars tuberalis intermedia distalis

Hypophysial fossa

Sphenoid

ANTERIOR POSTERIOR

C H A P T E R

CH

335 mm

GL

B

26

TRIMESTER 3 AND THE NEWBORN

Approximately 250–335 mm in Greatest Length; Approximately 26 Postfertilizational Weeks to Birth

The sulci and gyri increase rapidly in number during trimester 3, although they are not quite as numerous as in the adult. Their sequential appearance is

listed in Table 26–1 (p. 306).

The duration of prenatal life is generally about 38 postfertilizational weeks, the mean being 264 days. The range 35 to 40 postfertilizational weeks is considered to indicate an infant at “term.” Commonly used figures at birth are about 335 mm for the greatest length (exclusive of the lower limbs) and about 500 mm for the crown–heel length. The biparietal diameter is approximately 95 mm and the head circumference is of the order of 350 mm. The body weight at birth varies from 2500 to 4000 g or more, with an average of about 3350 g. The brain weight, which also varies considerably, ranges from about 300 to 400 g at birth and continues to increase rapidly up to the fourth year, more slowly up to the twelfth year.

A useful general summary of the central nervous system in the newborn has been provided by Robinson and Tizard (1965). An important account of the brain stem of two newborns was published by Sabin (1900, cf. Fig. 26–5B). It contains precise drawings of two series of sections: 22 coronal and 27 transverse. See also Sabin (1901, 1902). Photomicrographs and drawings of the motor cortex of a newborn have been published by Mar´ınPadilla and Mar´ın-Padilla (1982) and by Mar´ın-Padilla (1992).

Probably the most valuable treatise on the cerebral hemispheres during the fetal period is that by Kahle (1969), because, unlike more recent accounts, it is based on a series of careful graphic reconstructions. Studies of the fetal cerebral cortex, in continuity with those of Conel on the newborn, and on the postnatal cortex of 1 to 6 years, have been published by Rabinowicz (1986, for references). Useful photomicrographs, drawings, and measurements of neocortical areas during the fetal period have been published by Mar´ın-Padilla (1970, 1988a, and 1992).

The subplate of the primary visual cortex disappears during the last prenatal weeks, whereas that of the prefrontal cortex persists as long as 6 months after birth (Kostovic´ and Rakic, 1990). The changes include relocation of fibers in the cortical plate, incorporation of subplate neurons in the gyral white matter, and cell death. Postnatally, cortico-cortical and association fibers continue to grow, and overproduction of dendritic spines as well as excessive synaptogenesis have been recorded.

Precision

It is incorrect to refer to the trunk of the corpus callosum as its body, because the entire structure is a body, Latin corpus. Moreover, the central part of the lateral ventricle is not a body because all parts of the ventricle are cavities.

Sexual Differentiation of the

Brain

Although the importance of estrogen is acknowledged, uncertainty persists regarding “the onset of estrogen synthesis in the [human] brain, the sensitivity to estrogen of distinct neuronal populations, and the appearance of sexual dimorphisms” (Beyer, 1999).

It has been proposed that autism, under the influence of prenatal androgens, may represent an “extreme male brain:” the white matter is greater, the internal capsule and corpus callosum are proportionally reduced, and the amygdaloid body is abnormally large (Baron-Cohen et al., 2005).