Don W. Fawcett

Separation of mouse spermatogenic cells by sedimentation velocity: A morphological characterization☆

Abstract A method utilizing sequential enzymatic incubation in collagenase (1 mg/ml) and trypsin (2.5 mg/ml) has been developed for the dissociation of the seminiferous epithelium. A significant advantage of this method is that, following collagenase incubation and washings in an enriched Krebs-Ringer bicarbonate buffer solution, isolated seminiferous tubules are obtained which are free of interstitial cells. The “purified” seminiferous epithelium is then dissociated with trypsin. A further advantage of this dissociation technique has been a reduction in the number of symplasts (multinucleate cells) which form by the opening up of the intercellular bridges that occur between synchronously differentiating clusters of germ cells. Both the elimination of the interstitial cells and the reduction in the number of symplasts have made possible the recovery of more highly enriched germ cell fractions. The homogeneity of the cell fractions was determined by light and electron microscopy. Integrity of the isolated cells was verified by Trypan blue exclusion and measurement of oxygen consumption.

Morphogenetic factors influencing the shape of the sperm head

Abstract There is abundant evidence based upon study of both protozoa and metazoa that microtubules are involved in the establishment and maintenance of cell shape. It has also been suggested that microtubules may play a role in the development of the specific shapes of sperm heads. The present investigation has explored spermatid differentiation in mammals, birds, insects, and annelids. It is concluded from these comparative observations that the form of the sperm head is probably not a consequence of external modeling by pressures applied to the condensing spermatid nucleus by microtubules in the surrounding cytoplasm or by filaments in the ectoplasm of the supporting cell. It is suggested instead that the shape may be largely determined from within by a specific genetically controlled pattern of aggregation of DNA and protein during condensation of the chromatin.

The Sertoli cell occluding junctions and gap junctions in mature and developing mammalian testis

Abstract Special occluding junctions between Sertoli cells near the base of the seminiferous epithelium are the structural basis of the blood-testis permeability barrier. In micrographs of thin sections, multiple punctate pentalaminar contacts between apposed membranes are observed in the junctional regions. In freeze-fractured mature testis, the junctional membranes exhibit up to 40 parallel circumferentially oriented rows of intramembrane particles preferentially associated with the B-fracture face, but with complementary shallow grooves on the A-face. Short rows of particles may remain with the A-face resulting in discontinuities in the B-face particle rows. In addition, elongate aggregations of particles of uniform size (∼70 A) arranged in one or more closely packed rows are occasionally found adjacent to the linear depressions on the A-face of the Sertoli junction. These are interpreted as atypical gap junctions. In immature testis, occluding junctions are absent but typical gap junctions are common. These gradually disappear. In the second postnatal week, linear arrays of particles appear on the B-face. Initially meandering and highly variable in direction, these gradually adopt a consistent orientation parallel to the cell base. The establishment of the blood-testis barrier appears to be correlated with this reorganization of the intramembrane particle rows. Sertoli junctions were shown to be resistant to hypertonic solutions that rapidly dissociate junctions of other epithelia. Sertoli junctions thus differ from other occluding junctions in their (1) basal location, (2) large number of parallel particle rows, (3) absence of anastomosis between rows, (4) preferential association of the particles with the B-face, (5) intercalation of atypical gap junctions, (6) unusual resistance to dissociation by hypertonic solutions.

Physiologically Significant Specializations of the Cell Surface

This paper presents a brief review of the ultrastructure of some of the structural elaborations of the plasma membrane that may be regarded as adaptations for specific cell functions. Among the relatively stable specializations of the free surface, it considers the striated border of the intestinal mucosa, the brush border in the nephron, and filamentous coatings on the membranes of certain cells of the gastric mucosa. It then turns to those transient configurations of cell surfaces involved in the dynamic processes of pinocytosis, phagocytosis, and liberation of secretory products and considers the turnover of membranes associated with these phenomena. Discussion of specializations of the contact surfaces includes the desmosomes and terminal bars and the present status of intercellular cement and interfacial canals. A section on specializations of the cell base considers the basement membrane and basal infoldings of the plasma membrane in cells engaged in active transport. And finally, there is a description of the terminal web and the marginal band of nucleated erythrocytes—specializations of the superficial cytoplasm concerned with maintenance of cell shape.

The Structure of the Mammalian Spermatozoon1

Publisher Summary The head of the human spermatozoon consists of the nucleus and a head cap. The postnuclear cap and cytoplasmic sheath often described in the literature are not found in electron micrographs and apparently do not exist. Contrary to common belief, the nucleus extends throughout the length of the head and is definitely not confined to the posterior half. During differentiation of the spermatogenic cells a sequence of changes is observed in the state of aggregation of the nuclear chromatin, that leads to the formation of a homogeneous, dense sperm nucleus in which no ordered fine structure is detectable with the electron microscope. The so-called head vacuoles commonly found in human spermatozoa are cavities in the condensed chromatin of the nucleus. They are variable in size and number, irregular in outline, and lack a limiting membrane. They seem to arise as defects in the condensation of the chromatin and probably have no important functional significance. A distinct apical acrosome is lacking in the human sperm. The nucleus is covered only by a thin head cap and by the continuous membrane that invests the entire cell. The cap is composed of a thin layer of amorphous material enveloped by a continuous membrane. The membrane is the persisting limiting membrane of the acrosomal vesicle that develops in the early spermatid, and the layer of amorphous substance which it contains seems to be derived from the acrosomal granule. The acrosome and head cap in mammalian spermatozoa are components of a single structure that might be better described by the term acrosomal cap.

Changes in the acrosome of guinea pig spermatozoa during passage through the epididymis

SummaryElectron microscopic observations have been made on the structure of guinea-pig spermatozoa in successive segments of the epididymal duct. Marked changes are observed in the shape and internal structure of the acrosome as the sperm move through the epididymis. The morphological basis for this continuing differentiation of the sperm is traced back to certain unusual features of acrosome formation in the early spermatids of this species. The possible relation of the progressive development of the acrosome to the known increase in fertilizing capacity of the spermatozoa during their passage through the epididymis is discussed.

The anatomy of the mammalian spermatozoon with particular reference to the guinea pig

SummaryA description of the guinea pig spermatozoon has been presented using descriptive terms that are applicable to mammalian spermatozoa in general. The spermatozoon consists of two principal parts, the head and the tail. The tail is further subdivided into four regions, the neck, middle-piece, principal-piece and end-piece. The principal components of the head are the acrosomal cap and the nucleus. On its posterior surface, the nucleus bears a shallow implantation fossa lined by the nuclear envelope which is locally specialized to form the basal plate. The neck contains a connecting-piece bearing an articular surface that attaches to the basal plate lining the implantation fossa. The connecting-piece is formed by the convergence and fusion of nine longitudinal segmented columns which merge caudally with nine outer dense fibers that surround the axial filament complex throughout the major part of the length of the flagellum. In the middle-piece, the core of longitudinal fibers is surrounded by a mitochondrial sheath. The end of the middle-piece is marked by the annulus, a thin dense ring to which the flagellar membrane is firmly adherent. In the principal-piece, the core of longitudinal fibers is enclosed in a fibrous sheath composed of closely spaced circumferential ribs that attach to two longitudinal columns running along opposite sides of the sheath for its entire length.It is suggested that accurate communication would be facilitated by abandonment of the terms “galea capitis”, “head cap”, “post-nuclear cap”, “implantation plates”, “thin laminated fibers”, “protoplasmic bead”, “middle-piece bead”, “ring-centriole”, “Jensens ring”, “spiral sheath”, “tail helix”, and “tail sheath”.

OBSERVATIONS ON THE ULTRASTRUCTURE OF NUCLEATED ERYTHROCYTES AND THROMBOCYTES, WITH PARTICULAR REFERENCE TO THE STRUCTURAL BASIS OF THEIR DISCOIDAL SHAPE.

SummaryThe marginal band of nucleated erythrocytes in the toadfish is found, in electron micrographs, to be composed of about twenty-five microtubules approximately 200 Å in diameter. These form a bundle that encircles the erythrocyte just beneath the plasma membrane. These observations support the interpretation of Meves 1904, that this relatively stiff equatorial band may contribute to the maintenance of the discoid shape of nucleated erythrocytes in fish, amphibians, reptiles and birds.Similar microtubules form an annular bundle encircling the nucleus in fish thrombocytes. The number of tubular elements involved here is in excess of one hundred and they are located deep to the ectoplasmic layer instead of immediately beneath the plasmalemma. The term endoplasmic ring is therefore proposed for this structure.Comparative observations on nucleated erythrocytes of various species are presented showing that the density and fine structure of the material occupying the interchromosomal areas of the nucleus, always matches the cytoplasm and is related to the hemoglobin concentration of the species. These ultrastructural observations are consistent with the optical absorption and biochemical findings of other investigators indicating the presence of intranuclear hemoglobin in nucleated erythrocytes. Crystalline order is occasionally found in electron micrographs of the hemoglobin rich areas of the nucleus in toadfish erythrocytes but is not found in the cytoplasm.

The Sarcoplasmic Reticulum of Skeletal and Cardiac Muscle

This paper traces the development of our present concept of the structural organization of the sarcoplasmic reticulum in striated muscle and reviews the physiologic evidence for its participation in intracellular impulse conduction. Comparative observations are presented showing that this system of membrane-limited tubules is particularly well developed in exceptionally fast-acting skeletal muscles. These findings are interpreted as evidence supporting the hypothesis that the reticulum is involved in the coupling of excitation to contraction, but it is considered likely that it also has other important functions in muscle metabolism. The sarcoplasmic reticulum of cardiac muscle is found to be much less extensive and less precisely arranged in relation to the cross-banded pattern of the myofibrils, than it is in skeletal muscle. It is believed, nevertheless, that it may prove to have a significant role in the physiology of the myocardium.

In vitro synthesis of cholesterol and testosterone from acetate by rat epididymis and vas deferens.

Summary The epididymis and vas deferens from the rat are capable of synthesizing cholesterol and testosterone from acetate. The significance of androgen synthesis by the epididymis is discussed.