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TRIMESTER 2

301

 

 

TABLE 25–2. Summary of Development of Hippocampal Formation

 

A. Embryonic Period

 

 

 

Figure

Hippocampus

Dentate Area

Fibers

 

 

 

 

15–4

Hippocampal thickening on both sides

 

 

 

of lamina terminalis

 

 

16–6 B,C

Hippocampal thickening extends from

 

 

 

future frontal to future temporal pole.

 

 

16–11

Thickening is accompanied by a cell-

 

 

 

free marginal layer

 

 

18–2

Hippocampal primordium has grown

Dentate area is a small rim intercalated

 

 

and reaches forebrain septum

between hippocampal thickening

 

 

 

 

and area epithelialis.

 

18–5

Rostral and temporal parts are alike

 

 

20–13

 

 

 

Hippocamposeptal and

 

 

 

 

precommissural fibers

21–17

Thin sheath of marginal cells appears

Migrating cells form primordium

 

 

opposite hippocampal thickening

of dentate area

 

23–24

About 5 rows of scattered marginal cells

 

 

B. Fetal Period

 

 

 

GL (mm)

Weeks

Hippocampus

Dentate Gyrus

Fibers

 

 

 

 

 

32

8

 

Migrating cells

Fimbria

44

9

Distinct pyramidal layer.

Patches of cells (Fig. 25–3A′ )

Columns of fornix

 

 

CA 1–3 distinguishable

 

 

49

9 1

H1, H2, and H3 can be distinguished

 

 

 

2

 

 

 

50–83

9 1 –12

Marginal cells in marginal layer

 

 

 

2

 

 

 

56

10

 

Spherical cell mass (Fig. 25–3B′ ).

Hippocampal commissure

 

 

 

Fimbriodentate sulcus

 

61

10

 

Polymorphic and granular layers

Alveus forming

65

10 1

Marginal layer with Cajal–Retzius cells

 

 

2

 

 

 

79–85

12

 

Granular layer more distinct

Alveus complete

 

 

 

and clearly separated from H3

 

120

14

Involution complete; hippocampal

 

 

 

 

matrix wider than subicular

 

 

150

17

Fig. 25–3C

 

Formation of synapses

 

 

 

 

mainly in marginal layer

200

21

Fig. 25–3D; ventricular layer

 

Connections in subdivisions

 

 

seems to be exhausted

 

of hippocampus

≥300

32–45 Hippocampus has grown in volume;

 

by 20 weeks

 

 

walls of hippocampal sulcus

 

 

 

 

are fused; molecular layer

 

 

 

 

still has some Cajal–Retzius cells

 

 

Newborn

 

Hippocampal surface only one-third

 

 

 

 

of adult area

 

 

 

 

 

 

 

Sources: A: based on authors’ research. B: based on data from various authors, including Humphrey (1965), Rakic and Yakovlev (1968), Kahle (1969), Bogolepova (1995), Hevner and Kinney (1996), and Bogolepova (1997).

302

C h a p t e r 2 5 : TRIMESTER 2

A

B

A¢

B¢

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).

TRIMESTER 2

303

Figure 25–3. Continued (C) and (D)

Development of the hippocampal formation during trimester 2. Cellular production and migration continue. As the corpus callosum develops, the more rostral parts of the hippocampal primordium regress. The formation, which became curved and multilayered in trimester 1, becomes S-shaped and then rolled during trimester 2. (C) A coronal section at 150 mm (17 weeks). The width of the pyramidal layer in CA1 narrows progressively from the subiculum towards CA2 to CA4. Its cells are small and immature. Three layers can be identified already in the dentate gyrus. The dentate area presents a wide marginal layer, the subiculum is double-layered, and the entorhinal cortex is bipartite. (D) At 203 mm (21 weeks). The pyramidal layer is still populated with mostly immature neurons, as is also the dentate gyrus. Fibers connecting the entorhinal cortex, hippocampus, and subiculum are present by about 19 weeks, and connections between other subdivisions of the hippocampal formation may be established about one week later. Connections with the neocortex, however, are still only beginning at 22 weeks (Hevner and Kinney, 1996). (E) Summary of the change in shape from 9 weeks to curved and multilayered at 10 weeks, S-shaped at 17 weeks, and infolded at 21 weeks.

(F) The hippocampal formation of the adult. C and D are drawn from photomicrographs in Arnold and Trojanowski (1996).

With the completion of the hippocampal arch, it reaches from the olfactory bulb to the region of the uncus. Its transformation into induseum griseum begins rostrally during trimester 2 (Kier et al., 1995, 1997).

Stippling, fimbria. Black, dentate gyrus. Horizontal hatching, hippocampus. Orange-brown, pia mater. The subiculum and the parahippocampal gyrus are left white.

ADENOHYPOPH.

Pars tuberalis intermedia distalis

 

 

Figure 25–4. The hypophysis

 

 

cerebri. (A) Sagittal section through

3rd ventricle

 

the fetal organ. The cleft represents

 

 

the cavity of the initial

Stalk

NH

adenohypophysial pouch. (B) The

 

 

greater part of both the

 

 

adenohypophysis and the

 

 

neurohypophysis lies in the

 

Cleft

hypophysial fossa of the sphenoid

 

bone (still cartilaginous here).

 

 

Hypophysial fossa

Sphenoid

ANTERIOR POSTERIOR

fibers

A

 

B

 

C

D

Figure 25–5. A comparison of basal views of the brain at (A) 31 mm (stage 23, 8 weeks), (B) 68 mm (11 weeks), (C) ca 130 mm (ca. 15 weeks),

(D) ca. 210 mm (22 weeks). The inferior views illustrate the development of some of the olfactory structures (olfactory bulb and striae in white in A to C). Already at 8 weeks (A) the lateral olfactory fibers (striae laterales or gyrus olfactorius lateralis), which had formed by stage 17, reach the area of the amygdaloid nuclei (although mitral cells are only beginning to be present in the olfactory bulb). Medial olfactory fibers (striae mediales or gyrus olfactorius medialis) are shown in B and C. (Some fibers also are already present at stage 17.) The striae are sometimes termed gyri because they are covered by a small amount of gray matter. The lateral olfactory tract reaches the temporal lobe and is continued by the gyrus ambiens (C). The changing relationship of the olfactory structures can be studied further in views by Macchi (1951). The optic nerves, chiasma, and tracts in A to C are shown in black, as is also the trigeminal nerve.

View A is based on Muller¨ and O’Rahilly (1990b), B is after Hochstetter (1919), C is after Kollmann’s Handatlas der Entwicklungsgeschichte des Menschen and Corning’s Lehrbuch der Entwicklungsgeschichte des Menschen, D is by courtesy of Marvin D. Nelson, M.D., Children’s Hospital, Los Angeles.

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.

The Embryonic Human Brain: An Atlas of Developmental Stages, Third Edition. By O’Rahilly and Muller¨ Copyright C 2006 John Wiley & Sons, Inc.

305

306

 

C h a p t e r 2 6 : TRIMESTER 3 AND THE NEWBORN

 

 

 

 

 

 

 

TABLE 26–1. The Usual Sequence of Appearance of Cerebral Sulci and Gyri Arranged in Groups

 

 

 

Sulci

 

Gyri

 

 

 

 

 

 

 

Group

Weeks

Lateral

Medial

Lateral

Medial

 

 

 

 

 

 

A

10–15

Lateral

Callosal

 

 

 

 

 

 

 

 

B

16–19

Parieto-occipital

Cingulate

 

Gyrus rectus

 

 

 

calcarine

 

cingulate

 

 

 

 

 

Cuneus

 

 

 

 

 

 

C

20–23

Central

Collateral

Superior temporal

Parahippocampal

 

 

Superior temporal (T1)

 

 

 

 

 

 

 

 

 

D

24–27

Precentral

Occipitotemporal

Precentral

 

 

 

Inferior temporal (T2)

 

Middle temporal

 

 

 

Postcentral

 

Inferior temporal

 

 

 

Superior frontal (F1)

 

Medial & lateral

 

 

 

Intraparietal

 

occipitotemporal

 

 

 

 

 

Postcentral

 

 

 

 

 

Superior frontal

 

 

 

 

 

Superior & inferior

 

 

 

 

 

parietal lobules

 

 

 

 

 

 

 

E

28–31

Inferior frontal (F3)

 

Middle frontal

 

 

 

Middle frontal (F2)

 

Inferior frontal

 

 

 

 

 

Pars triangularis

 

 

 

 

 

 

 

F

32–35

 

 

 

Paracentral lobule

 

 

 

 

 

 

Source: Based largely on data supplied by Marvin D. Nelson, M.D., Los Angeles.

 

Tectum

A

Tegmentum

 

 

 

 

 

Cerebral

 

Substantia nigra

peduncle

 

 

 

Crus cerebri

 

B

 

 

 

Figure 26–1. A likely migration (arrow) of alar (A) into basal (B) territory has long been suspected in the midbrain. Moreover, the origin of the different cells of the substantia nigra is highly complex (Verney et al., 2001a,b). This section at the junction of trimesters 2 and 3 shows that the adult form and features have clearly been attained.

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