Granule cells appearance

The majority of the inputs to the granule cells are excitatory, each of which provides a small depolarizing current to the membrane of the cell body. The point of contact between the axonal projection from the neuron and an adjacent cell is termed the s)mapse which under the electron microscope appears as a swelling at the end of the axon. Most synapses are excitatory and are usually located along the dendritic branches of the neuron. The contributions of the individual excitatory s)mapses are additive and, as a result, when an excitatory stimulus occurs a wave of depolarizing current travels down the axon to stimulate the adjacent cell body. However, some synapses are inhibitory, usually fewer in number and strategically located near the cell body. These synapses, when activated, inhibit the effects of any excitatory currents which may travel down the dendritic processes and thereby block their actions on the neuron (Figure 2.1). 

Doucette et al. 2000). At lower doses neonates show delayed effects on eye opening and olfactory conditioned place preference (Doucette et al. 2003). The latter supports the high sensitivity of the mitral-granule cells to domoic acid as described earlier. Evidence indicates that the sensitivity of the neonates results from insufficient renal clearance of toxin, allowing increased bioavailability in the blood (Xi et al. 1997). Domoic acid has been detected in the blood of neonates, transmitted via milk from lactating mother rats previously exposed to domoic acid but at levels appearing to be well below symptomatic doses (Maucher and Ramsdell 2005). 

The epidermal cells appear to be involved in calcifying both the exocuticle and the endocuticle. Early in postmoult, strings of granules containing inorganic calcium were observed to be extruded from the epidermis into the endocuticle. At the same time, the calcification of the exocuticle occurred as a wave of hardening moved from its outer edge inwards (Travis, 1960). The... 

Initial studies with mast cell cultures or granules demonstrated that mast cells appear to have several important effects on connective tissue elements. Subba Rao et al. (1983) showed that cultured rat embryonic skin fibroblasts phagocytose rat mast cell granules added to the culture medium or released by co-culmred mast cells, and that this is followed by secretion of collagenase and p-hexosaminidase. Similarly Yoffe et al. (1984) demonstrated that granules derived from mast cells purified from dog mastocytomas induced a 10- to 50-fold stimulation of collagenase production by human... 

Flgure 12.1 Electron micrograph of an apoptotic human neutrophil showing the characteristic chromatin aggregation, prominent nucleolus and dilated cytoplasmic vacuoles. Note that the cell membrane is intact and the granule structure appears normal. (EM taken by Jan Henson), (x 11 000). 

The waste products are partially deposited on the Bruch s membrane (Young, 1987) in the form of drusen. The accumulation of lipofuscin in RPE cells appears detrimental to its function (Flood et al., 1984) and causes photoreceptor death (Dorey et ak, 1989). With age, the number of RPE cells decreases in the central retina, and they become pleomorphic (Dorey et ak, 1989). Other changes are also frequent. They include atrophy, depigmentation, hyperplasia, hypertrophy, and cell migration. The melanin concentration in the RPE cells decreases with age, especially in Caucasians, but also in blacks (Feeney-Burns et al., 1984). The melanin granules are slowly (over decades) digested by lysosomes (Bums and Feeney-Burns 1980). 

In the cerebellar cortex, the most frequently noted pathology is a decreased number of Purkinje cells (Kemper and Bauman, 1998 Palmen et al., 2004 Bauman and Kemper, 2005 Whitney et al., 2008). This pathology is most marked in the posterior lateral part of the cerebellar hemispheres and the adjacent archicerebellar cortex and occurs without evidence of loss of neurons in the inferior olive in the brain stem (Kemper and Bauman, 1998 Bauman and Kemper, 2005). The Purkinje cells have an intimate relationship with the axons of the inferior olivary neurons in the brain stem, such that loss of Purkinje cells at any time after birth leads to loss of neurons in the inferior olive (Holmes and Stewart, 1908 Norman, 1940 Sakai et al., 1994). Since this intimate relationship between the Purkinje cell and the inferior olive is established in the human brain sometime after 29-30 weeks of gestation (Rakic and Sidman, 1970), it is likely that the decrease in number of Purkinje cells occurred before this time. In those brains with a marked decrease in the number of Purkinje cells, there appears to be a concomitant decrease in the number of granule cells (Bauman and Kemper, 2005). The relationship between the number of granule cells and the number of Purkinje cells noted in the autistic brain has been elucidated in rat studies. With prenatal loss of Purkinje cells the number of granule cells is adjusted such that the ratio of Purkinje cells to the number of granule cells is maintained (Chen and Hilman, 1989). 

---