Robert Hadek

The Structure of the Mammalian Egg

Publisher Summary This chapter reviews the structure of the mammalian egg, and discusses the recent concepts of some of its functions that have come mostly through the application of submicroscopic techniques. The oocytes in most eutherian mammals are in the meiotic prophase at the time of birth, having thus completed the mitotic phase of their development. Following the diplotene stage (of meiotic prophase) most mammalian oocytes pass through a dictyate period-with the exception of the human gamete, which appears to remain in a prolonged diplotene stage. The nuclear membrane of the oocyte seems to pass through a number of morphological variations, some of them possibly indicative of nucleocytoplasmic exchanges (annuli, nuclear pores, and annulate lamellae), and it also seems to contribute to the formation of the meiotic spindle. The old question of granulosa cell and oocyte connections has been clarified by submicroscopic techniques. It is now certain that the two cells are separated by a double set of membranes. These membranes undergo a number of morphological changes (apposition, folding, invagination) during oocyte growth and development, ultimately acquiring a large number of villous processes. Of the cell organelles in all the eutherian eggs studied, mitochondria alone have been observed to undergo morphological changes, possibly also converting into “cytoplasmic bodies”. Of vitellus, or “vitelline material,” with the exception of the aforementioned “cytoplasmic bodies,” only free lying lipid granules and crystalloid inclusions have been observed in the egg of the eutherian mammal. And also the multivesicular bodies seem to be regular cytoplasmic components in the egg of eutherian mammals.

Submicroscopic changes in the penetrating spermatozoon of the rabbit

Submicroscopic studies on the spermatozoon of the rabbit have shown that ( a ) neither incubation with oviduct washings nor with freshly shed ova has any effect on the morphology of mature spermatozoa which could be compared with penetration changes; ( b ) the penetrating spermatozoon while passing through the zona pellucida apparently loses its cell membrane and gradually becomes denuded of its head cap; ( c ) the spermatozoa which reach the vicinity of the vitelline membrane have no head cap, cell membrane covering, or cytoplasmic layer and the “apical body” is elongated.

Study on the fine structure of rabbit sperm head

Histochemical and submicroscopic studies were conducted on the head of the spermatid and of the ejaculated spermatozoon of the rabbit. The histochemical studies have established the fact that acrosome and head cap development in the rabbit spermatid are almost identical with the development of the same structures in the rat and guinea pig. Submicroscopic observations on the spermatid head have shown a changing relation between the acrosome cap and head cap, on the one hand, and the nucleus and cell membrane on the other. Immediately after its appearance the acrosome vesicle was found in the vicinity of the nucleus. However, in the maturation phase a cytoplasmic layer appeared between the nucleus and acrosome and head cap, respectively. Ejaculated spermatozoa were found to be composed of a nucleus surrounded by a cytoplasmic layer containing an apical body, the acrosome, and the head cap. The whole head is ensheathed within a cytoplasmic membrane. After an hour of incubation with a freshly shed ovum in a suspension of physiological saline, the spermia show a knob-like thickening of the acrosome and an elongation of the cone-like apical body. Spermatozoa in the vicinity of the vitelline membrane appeared to be devoid of a cell membrane, acrosome, and head cap and the cone was elongated within a thin cytoplasmic coat.

Electron microscope study on primary liquor folliculi secretion in the mouse ovary

This paper reports a study by light and electron microscopy on changes observed in the granulosa cells of the primary ovarian follicle of the mouse. These changes are, apparently, concomitant with primary follicular fluid secretion. The staining of thick sections of OSO4-fixed, Epon-embedded specimens with Azure B and periodic acid-Schiff reagent has revealed the presence of follicular fluid between granulosa cells and in the follicular spaces at an early phase of follicular development. Electron microscope observations have shown within the granulosa cells a sequence of changes which appear to be associated with cellular secretory functions. These changes begin with the enlargement of some of the Golgi canaliculi and are followed by an apparent discontinuity of their course. In the next phase smooth-walled cisternae appeared and were concentrated in the cortical area of the cells. The ultimate sequence was the discharge of the cisternal contents into the intercellular spaces. The follicular fluid appeared electron opaque within the intercellular spaces where it appeared to fill the spaces between thin, somewhat more electron dense, fibrillar micelles. Call-Exner bodies were observed as cytoplasmic droplets.

Submicroscopic study on the cortical granules in the rabbit ovum

Round to oval cortical granules 0.08 to 0.2 μ in diameter were observed in the ovum of the rabbit immediately before ovulation. If fertilization did not occur, the number of these granules appeared to increase. Although occasional granules were observed within the vitellus of the ovum, they appeared in larger numbers in the cortex, where they formed a row beneath the vitelline membrane.

The localization of calcium in skeletal muscle: Its distribution in muscles in which the caffeine-induced contracture was arrested

The intracellular precipitation of calcium was effected by fixation of muscles in 1% OsO 4 containing 2% potassium pyroantimonate (K 2 H 2 Sb 2 O 7 · 4H 2 O). In these preparations granules of electron dense pyroantimonate salts were observed in the terminal cisternae of the sarcoplasmic reticulum, closely applied to the luminal aspect of the cisternal membrane at its junction with the T system. Along the myofibrils the precipitate was most dense in the I band adjacent to the A–I junction. Further, the reproducible pattern of I band precipitate and its relation to sarcomere length suggest that this store of calcium may represent the intracellular site of calcium activation during contraction. The distribution of precipitate in muscles exposed to Ringers solution containing caffeine indicate that this alkaloid acts at some site other than the sarcoplasmic reticulum, such as the T system or the junction between the T system and the sarcoplasmic reticulum.

Transmission electron microscopy and stereo ultrastructure of the T system in frog skeletal muscle

Transmission electron microscopy revealed the presence of T tubules near the sarcolemma in the extensor longus digiti IV muscle of the frog. Due to the complicated termination of the T tubules, their continuity with the extracellular space was never clearly evident. With scanning electron microscopy, however, highly organized parallel rows of openings or apertures corresponding to the level of the T system were seen on the muscle surface of air-dried specimens. The apertures varied from 1000 to 2000 A in diameter, and were always found at the level of the I band in the vicinity of the Z disk. The significance of the apertures, and their relation to previous studies is discussed.

A study of nonmyelinated nerves in the rat and rabbit heart

Submicroscopic studies of the papillary muscle and ventricular myocardium of the rat and rabbit have revealed: (1) a dearth of nervous elements—an observation which contradicts the comparatively rich innervation suggested by investigators studying silver-stained preparations by light microscopic techniques; (2) the presence of nonmyelinated nerve fibers only; and (3) the apparent termination of nonmyelinated fibers at a significant distance from the myocardial cells.

SUBMICROSCOPIC STUDY ON THE SPERM-INDUCED CORTICAL REACTION IN THE RABBIT OVUM.

Cortical reactions were studied on OsO 4 -fixed rabbit ova obtained 12 and 60 hours after matings to a vasectomized male and 12 hours after matings to fertile males. In the eggs obtained 12 hours after sterile matings single, comparatively thin villous cytoplasmic processes projecting into the perivitelline area, and a row of cortical granules within the vitellus, below the cell membrane, were observed. In specimens obtained 12 hours after fertile matings, the presence of spermatozoa in the vicinity of the vitelline membrane appeared to cause a change in the shape and number of these processes. An increase in their number and a change in their shape (width, branching) was followed by a localized bulging of the cortex of the ovum, which took such proportions that one could speak of a cortical “cytoplasmic wave,” or possibly of “fertilization cone.” This localized cortical reaction apparently served to surround and to engulf the spermatozoon. Although multiple spermatozoa were observed aligned on the cortex, the cortical “cytoplasmic wave,” or “fertilization cone,” appeared to be restricted to one spot on the surface of the ovum and only one spermatozoon was enclosed by it. Ova obtained 60 hours after sterile matings have shown comparatively small cytoplasmic processes projecting into the perivitelline area at almost regular intervals and a row of cortical granules associated with the cell membrane.

Ultrastructure of sinus venosus innervation in Petromyzon marinus

Sections of the sinus venosus from the young adult Petromyzon marinus revealed the presence of numerous axon profiles. These nonmyelinated axons are bound singly or in multiples within Schwann cell sheaths. At points of neuromuscular association, the axon protrudes from an incomplete Schwann cell investment and approaches within 40–50 nm of adjacent myocardial sarcolemmae with only basal laminae intervening between the two. Points of neuromuscular association are characterized by the presence of abundant electron lucent vesicles (40–50 nm in diameter) within the axon crowded against the axon membrane nearest the myocardial cell. No axons were observed within the atria or ventricles. Innervation of chromaffin cells containing 120 nm vesicles with electron dense cores was not observed.