The development of Pacinian corpuscles

SummaryThe fine structural development of Pacinian corpuscles on the interosseous membrane of the rat was investigated from day 18 of gestation until 2 months after birth.At the initial stage of development on day 19–20 of gestation, Pacinian corpuscles consist of a cylindrical sensory terminal surrounded by one layer of cells which are apposed to the terminal and send off short lamellar processes towards the axolemma. These presumptive inner core cells accumulate around the terminal in continuation of the Schwann cell sheath, which indicates their Schwann cell origin. Both the inner core cells and their lamellae rapidly increase in number. At birth, the sensory terminal is already enclosed in a rudimentary inner core comprising several layers of loosely arranged lamellae, with cell nuclei accumulated at the outer circumference of the inner core. The sensory terminal sends off axonal processes from random sites on its circumference, and ends as a bulb which projects numerous axonal processes in all directions. The organelle content of the terminal consists of many mitochondria oriented lengthwise among microtubules and neurofilaments; clear and dense core vesicles are found in groups beneath the axolemma and at the bases of axonal processes and branches, whereas the processes themselves contain a microfilamentous network. Numerous coated invaginations and vesicles are found at the axolemma and in the lamellae enclosing the axon, indicating uptake of macromolecules on both sides of the periaxonal cleft. The outer capsule begins to form around the inner core shortly before birth; in neonatal rats, it consists of approximately five attenuated lamellae.During the first postnatal week, the inner core lamellae increase in number, become tightly packed together and concentrically arranged. The bilateral symmetry of the inner core is established 5–12 days after birth, when 2 opposite radial clefts are formed, bisecting the inner core cylinder into 2 corresponding sectors. From the cellular layer of the inner core, cytoplasmic arms penetrate through the radial cleft, giving rise to hemilamellae of one or both sectors. The axonal processes eventually become aligned 2–3 weeks after birth so that they project into the radial clefts, except for the bulbous ultraterminal part from which they project in all directions.During the 2 postnatal months studied, the inner core grows continuously in length, but its diameter remains practically unchanged. The outer capsule grows considerably during this time, as both the number of capsular layers and the spacing between them continue to increase. Pacinian corpuscles are on the average 475.8 ± 10.7 μm in length and 220.8 ± 6.3μm in diameter by day 60 postnatal, their size having increased about four fold from birth.