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Structural organization of the human cerebral cortex prior to the appearance of the cortical plate
Insights
Human cerebral cortex development begins earlier than previously thought, around the sixth week of gestation. Early corticipetal fibers and Cajal-Retzius neurons form a primitive structure before the cortical plate emerges.
Area of Science:
- Developmental Neuroscience
- Neurobiology
- Human Embryology
Background:
- Previous assumptions placed human cerebral cortex development initiation around the eighth week of gestation.
- Early embryonic stages of the human brain, specifically cerebral vesicle formation, are critical for understanding cortical organization.
Purpose of the Study:
- To investigate the early developmental timeline and structural organization of the human cerebral cortex before the cortical plate stage.
- To identify the precise timing of initial cortical development and associated neuronal structures.
Main Methods:
- Study of two human embryos at Carnegie stages 18 (43 days) and 20 (50 days).
- Histological examination focusing on the cerebral vesicle and associated fiber tracts using silver staining methods.
Main Results:
- Human cerebral cortex development commences around the sixth week (stage 18), earlier than the eighth week.
- Primitive corticipetal fibers from the mid-brain reach the developing cerebral vesicle by stage 18, forming a primordial plexiform lamina.
- Embryonic Cajal-Retzius neurons differentiate by stage 18, contributing to the functionally active primordial plexiform lamina by stage 20.
Conclusions:
- The human cerebral cortex begins its ontogeny around the sixth week of gestation.
- A primitive "premammalian" cortical organization, the primordial plexiform lamina, is established by 50 days of gestation.
- This early structure, composed of corticipetal fibers and Cajal-Retzius neurons, precedes the formation of the mammalian neocortical plate.
Abstract:
The early development and the structural organization of the human cerebral cortex, prior to the appearance of the cortical plate (Carnegie stage 22, ca. 54 days), was studied in two embryos: 43 (stage 18) and 50 day old (stage 20), respectively. It has been shown that the human cerebral cortex begins its ontogenetic development around the sixth rather than around the eighth week of gestation as it has been previously assumed. The human cerebral cortex starts to develop soon after the cerebral vesicles have been formed (stage 15) and a primitive internal capsule has been established (stage 17, ca. 41 days). By stage 18 of human development fibres from this primitive internal capsule have reached and probably have penetrated into the developing cerebral vesicle, through its more superficial zone. Fibres from this primitive internal capsule have been traced backward through the ventral thalamus to the mesencephalic tegmentum. The possible existence of primitive ascending fibres from the mid-brain which terminate in the superficial zone of the developing cerebral cortex (tegmento-thalamostriato-cortical tract) is suggested. The arrival of these primitive corticipetal fibres establishes in the outer zone of the cerebral cortex a primordial plexiform lamina or an external white matter. Horizontal-bipolar cells (embryonic Cajal-Retzius neurons) begin to differentiate by stage 18 of human development (43 days in our case). By stage 20 (50 days in our case), the primordial plexiform lamina is well established, extends throughout the entire surface of the developing cerebral cortex, and is considered to be functionally active. It is, by this age, a superficial, 40 micrometers thick, complex fibrillar neuronal organization composed of numerous horizontal corticipetal fibres (demonstrable with silver methods), horizontal-bipolar Cajal-Retzius neurons and a few other, less defined, cellular elements. This primordial plexiform lamina is considered to represent a primitive "premammalian" cortical organization. The next event in cortical ontogenesis is the appearance of the cortical plate or the mammalian neocortical grey at stage 22 (ca. 54 days). Migrating neuroblasts attracted toward the preexisting primordial plexiform lamina and guided by glial fibres start to accumulate within it.(ABSTRACT TRUNCATED AT 400 WORDS)