Файл: The Embryonic Human Brain An Atlas of Developmental Stages. Third Edition. 2006. By Ronan O\'Rahilly-1.pdf

The orbitomeatal plane

Figure 6–1. The main planes of reference and the chief terms of position shown in an embryo of stage 23 (8 weeks). (A) The body as a whole. (B) The head, including the brain.

Telencephalon

Prosencephalon

Diencephalon

Mesencephalon

Isthmus

Rhomb-

encephalon Metencephalon

Myelencephalon

Figure 6–2. The location of the three divisions and six subdivisions of the brain as seen in the newborn.

Spinal

cord

Cerebral hemisphere

Midbrain

Pons Brain

stem

Medulla

Because the first morphological indication of the nervous system appears at stage 8 (approximately 3 postfertilizational weeks), the following brief

statement concerning stages 1 to 7 is provided. Full details of these early stages have been published in a monograph by O’Rahilly and Muller¨ (1987a).

Some evidence exists in mammals (and probably including the human) that a labile axis of bilateral symmetry is determined by the site of penetration of a spermatozoon (reviewed by Denker, 2004).

Stage 1 is the unicellular embryo that is formed at fertilization, normally in the lateral end (ampulla) of the uterine tube. A new, genetically distinct human organism is thereby formed, although the embryonic genome does not become activated until the next stage.

Stage 2 (2–3 days) is the cleaving embryo that proceeds along the uterine tube.

Stage 3 (4–5 days) is characterized by the appearance of a cavity within the cellular mass, at which time the embryo is termed a blastocyst and lies in the uterine cavity. The embryonic disc now presents dorsal and ventral surfaces. The inner cell mass, mainly the epiblast, has been used for the production of embryonic stem cells that retain the ability to develop into all types of cells, including germ cells.

Stage 4 (6 days) is the attachment of the blastocyst to the uterine lining (endometrium), an event that heralds the beginning of implantation within the uterine mucosa.

Stage 5 (7–12 days), the continuation of implantation, is complicated and is subdivided into three substages,

according to whether (5a) the trophoblast is solid, (5b) the trophoblast contains lacunae, or (5c) the lacunae form a vascular circle. The amniotic cavity and umbilical vesicle (so-called yolk sac) appear during stage 5. At stage 5c a thickening of the hypoblast at one end of the embryonic disc may possibly indicate the rostral end of the body even before the appearance of the primitive streak (Luckett, discussed by O’Rahilly, 1973).

Stage 6 (about 17 days) is marked by the appearance of chorionic villi. During this stage, at approximately 2 12 postfertilizational weeks, axial features (particularly a cellular proliferation termed the primitive streak) develop, and hence the embryo acquires right and left sides, as well as rostral and caudal ends. The embryonic disc is now about 0.2 mm in length. Twins (other than certain conjoined examples) arise before the appearance of axial features, i.e., during approximately the first fortnight. Retinoic acid is believed to be implicated in the patterning of the rostrocaudal axis and the induction of the expression of Hox genes.

Stage 7 (Fig. 7–1, about 19 days). An axial structure known as the notochordal process is characteristic of this stage, when the embryonic disc measures approximately 0.4 mm. Although no morphological sign of the nervous system is yet present, the site of the future neural plate is dorsal and lateral to the notochordal process.

Stage 8 (about 23 days) is the subject of the following chapter. The epiblast is now being transformed into the neural plate in the region rostral to the neurenteric canal. Canalization of the notochordal process occurs (i.e., the

B

Amniotic cavity

Important

The embryo shows dorsal and ventral surfaces at stage 3 (about 1 12 postfertilizational weeks), and right and left sides, as well as rostral and caudal ends, at stage 6 (about 2 12 weeks.)

Prechordal plate

Notochordal process

&plate Primitive streak

&node

Site of future cloacal membrane

Allantoic diverticulum

Ectoderm

Neural

ectoderm

Neurosomatic junction

folds Neural tube

Neural

crest

The embryo is an ovoid (frequently pear-shaped) disc with a longitudinal axis and distinct cephalic and caudal ends. In one-quarter of examples, referred to as

stage 8b, the neural groove can be detected in the neural plate as a very shallow sulcus bounded by faint neural folds. This is the first visible sign of the future nervous system, and most of the folds represent the brain. The neural plate is closely related to two median features: the prechordal plate and the notochordal process.

Stage 8 has been generally listed as 18 days, but is probably more usually about 23 days.

Although few morphological features are visible at the time of initial development of the brain, stage 8 and its

organization are of crucial importance for an understanding of the developing nervous system. It is of interest that the primordium of the brain appears before the heart or any other organs become visible.

Studies of early neurogenesis indicate that differences concerning neural induction among vertebrate classes are present from the beginning of neurulation, and that mammalian neurulation is extremely complex.

Important

The first indication of the neural groove and folds is found during this stage.