Three-dimensional imaging of human stem cells using soft X-ray tomography

Three-dimensional imaging of human stem cells using transmission soft X-ray tomography (SXT) is presented for the first time. Major organelle types--nuclei, nucleoli, mitochondria, lysosomes and vesicles--were discriminated at approximately 50 nm spatial resolution without the use of contrast agents, on the basis of measured linear X-ray absorption coefficients and comparison of the size and shape of structures to transmission electron microscopy (TEM) images. In addition, SXT was used to visualize the distribution of a cell surface protein using gold-labelled antibody staining. We present the strengths of SXT, which include excellent spatial resolution (intermediate between that of TEM and light microscopy), the lack of the requirement for fixative or contrast agent that might perturb cellular morphology or produce imaging artefacts, and the ability to produce three-dimensional images of cells without microtome sectioning. Possible applications to studying the differentiation of human stem cells are discussed.

Rediscovering Boveri's centrosome in Ascaris (1888): its impact on human fertility and development

We rediscover and review the brilliant work of Theodore Boveri, over a 100 years ago, on the centrosome of the round worm Ascaris and show how it impacts on our understanding of human fertilization and embryogenesis. Boveri was able to make fundamental predictions on the mechanics of fertilization and the dominant role of the sperm centrosome (Boveri's rule), which is now applicable to most animals. Using advanced digital imaging by light and electron microscopy, we explore centrosomal dynamics during Ascaris fertilization and the first cell cycle during cleavage. Twenty figures are presented in this visual publication. Humans follow Boveri's rule, as do most mammals excluding some rodents, and there is a remarkable similarity of the events of fertilization and cleavage in Ascaris and humans, the latter of which has been documented since 1991. The role of the sperm centrosome (centriole) in egg activation, polarity, embryogenesis, infertility and cancer is discussed. An attempt is made to portray the images Boveri may have visualized, in his painstaking drawings presented in his thesis in 1888. We now know the origins of the centrosome in human somatic cells--predominantly from the sperm cell. The impact of Boveri's work on human development is highlighted in this age of assisted reproductive technology.

Human embryonic stem cells and their spontaneous differentiation

Human embryonic stem cells (hESCs) usually grow in saucer-shaped colonies with thickened rims and can form spherical human embryoid bodies (hEBs) under non-adherent conditions. A problem associated with ES cell culture is the spontaneous differentiation of cells into a variety of cell types representing all three germ layers, which is evident in both hESC colonies and hEBs. This presentation deals with the precise origins of hESCs and their spontaneous differentiation in vitro. We have used advanced digital microscopy, including transmission electron microscopy (TEM) to define the fine structure of these cells. We present images of undifferentiated hESCs and their spontaneous differentiation into basic embryonic cell types such as nerve, muscle, connective tissue, epithelium, and digestive tract progenitors, representing all three primary germ layers: embryonic ectoderm, mesoderm and endoderm. It appears that hESCs work in concert and interact with one another, as in tissue formation of the embryo. Our fine structural observations agree mostly with those of the Thomson group. Digital microscopy of plastic sections and TEM are invaluable tools in the precise characterization of cells forming these tissues and a combined study with immunofluorescent markers is most desirable.

Nine-day-old human embryo cultured in vitro: a clue to the origins of embryonic stem cells

The aims of this study were to investigate whether the human embryo could sustain development beyond the blastocyst stage in vitro and to identify the precise origins of embryonic stem cells (ES cells) from the embryoblast. A frozen-thawed 4-cell embryo was cultured to the post-blastocyst stage. This 9-day-old embryo presented a solid mass of inner cells (resembling a tumour) surrounded by surface trophoblast cells. Clumps of multinucleated syncytiotrophoblast cells were evident at one pole. Most cells resembled those of blastocysts. However, there were groups of comparatively undifferentiated cells within the inner cell mass somewhat resembling ES cells documented previously, that might give a clue as to their origins. The embryo attempted to form an amnion with a cavity, but did not present a bilaminar, discoidal structure as expected in week 2 of development, and hence was abnormal.

Characterization of human gamete centrosomes for assisted reproduction

Centrosomes of human gametes need to be characterised in more detail, since it was recently shown in 1991 that the human embryo inherits the dominant paternal centrosome at fertilization. Thus there has been a resurgence of centrosomal research in the last decade. The sperm centrosome, containing a single centriole, is preserved and dominant while the egg centrosome is reduced and inactivated during oogenesis, since there should be only one functional centrosome to ensure normal development. This presentation reviews the ultrastructure of gamete centrosomes and attempts to define their respective roles in assisted fertilization. Human testicular and ovarian tissues, sperm, eggs and zygotes were examined by routine TEM. Developing sperm cells and oogonia have two functional centrioles (diplosomes) in their centrosomes, showing the typical "9+0" organisation of microtubule triplets--common to somatic cells. The diplosomes are surrounded by pericentriolar material (PCM), which nucleate microtubules (MT) that organize the cytoskeleton and mitotic spindles. During spermiogenesis, when the spermatids transform into mature sperm, there is a partial reduction of the male centrosome, during which the proximal centriole (PC) is retained intact in the sperm neck, proximal to the nucleus, while the distal centriole (DC) which gave rise to the sperm flagellum, is partially reduced and merges distally with the sperm axoneme in the midpiece and tail. The proximal functional centriole is located in a "black box" in the neck, composed of the capitulum beneath the basal plate and flanked laterally by 9 segmented columns and shows the typical "9+0" organisation of triplets of MT. The PC contains dense material both within and outside the triplets, and the central doublet of axonemal MT terminates in a clump of dense material below its lower vault. The DC is disorganised proximally showing loss of triplets and cannot function as a typical centriole, since the central doublets of axonemal MT traverse through it. Oogonia present a pair of well-defined centrioles, which are involved in cell division. These are lost during oogenesis and the mature oocyte is devoid of centrioles, as in most mammals. Neither does the human oocyte have granular centrosomal material at meiotic spindle poles, in contrast to mouse oocytes which have a dominant maternal centrosome. Thus the oocyte centrosome is greatly reduced and inactivated. Functional centrosomal structure is, however, restored after fertilization in the zygote with some maternal input around the sperm centriole, which duplicates at the pronuclear stage, forms a sperm aster and proceeds to form the first mitotic spindle. This is the ancestor of centrosomes in embryonic, foetal and adult somatic cells. In 1991 (Sathananthan, 1991), we postulated that sperm centrosomal dysfunction could lead to aberrant embryonic development based on centriolar defects in sperm with impaired motility. This hypothesis has now gained acceptance and further evidence to support this theory of infertility is presented.

Egg-sperm interactions in humans: ultrastructural aspects

A large cumulus mass usually covers the human ovulated oocyte, and voluminous clusters of cumulus cells are still seen after fertilization around the egg. Cumulus cells surround oocytes and fertilized eggs also during in vitro fertilization (IVF) procedures. This study describes, by transmission and scanning electron microscopy, the morphology and the microtopography of the cells forming the human cumulus mass surrounding IVF samples (insemined but not fertilized oocytes and fertilized eggs). Particularly emphasized is their morphodynamic role in sperm-egg interactions. A comparison with the behavior in vivo of cumulus-enclosed oocyte/fertilized eggs has been also performed. All patients have given their informed consent to participate in this protocol. An inner layer (corona radiata cells) and an outer layer (proper cumulus cells) can be microtopographically recognized in the cumulus mass. Numerous cumulus-corona cells, particularly after fertilization, showed ultrastructural characteristics typical for steroid synthetic cells, thus undergoing a sort of "luteinization" parallel to that occurring in the sister granulosa cells of the postovulatory follicle. This steroid synthetic activity, particularly enhanced in vitro but present also in vivo, may be finalized to the release of small amount of steroids (estrogens and progesterone) in the oocyte/fertilized egg milieu. Various proteins, secreted by other cell subpopulations--as revealed in other studies by our research group--, may even enrich this milieu. Lymphocytes and macrophages were often found in the cumulus mass. They may modulate the steroid secretion of the neighboring cumulus cells by production of cytokines, mimicking what occurs in the ovarian follicle and, later, in the corpus luteum. Spermatozoa, both normal (acrosome-intact or--reacted) and abnormal, were frequently seen in the cumulus mass, free in the intercellular spaces or close to the cumulus cells, that can induce sperm capacitation and acrosome reaction. Leukocytes and cumuluscorona cells appeared both capable of actively phagocytizing supernumerary and/or abnormal sperms. Such spermiophagic response is present in a lesser extent around oocytes and eggs fertilized in vivo. In vitro, instead, cumulus spermiophagy leads to the elimination of a large part of the excess spermatozoa that have reached the oocyte, thus restoring in an extracorporeal medium the spermiophagic activity physiologically exerted by leukocytes and epithelial cells in the female and male genital tracts. In conclusion, the cumulus mass surrounding oocytes and fertilized eggs appears as a highly dynamic system, in which various subpopulations of cells cooperate in order to provide a suitable and healthy microenvironment for fertilization and early embryo development.

Mitochondrial morphology during preimplantational human embryogenesis

The structure, distribution, and function of mitochondria during human oogenesis and early development is reported. Oogonia show a sparse and even distribution of mitochondria, which are oval or elongated. Except around nuclei, growing oocytes from small antral follicles have more dense rounded or oval mitochondria, associated with the rough endoplastic reticulum. Mitochondria in fully grown, germinal vesicle (GV) oocytes present an inert appearance, with a dense matrix and a few arch-like or transverse cristae. At this stage mitochondria are usually absent from the cortical part of the cytoplasm. Mitochondria in metaphase I and II oocytes, including fertilized oocytes, present a similar structure, but they are numerous and evenly spread in the ooplasm, associating closely with vesicles or aggregates of tubular smooth endoplasmic reticulum. The most substantial change in distribution occurs at the pronuclear stage, when there is a central conglomeration of mitochondria around the pronuclei in both monospermic and dispermic embryos, which persists up to syngamy. In structure and distribution, mitochondria in blastomeres of 2-16-cell embryos remain virtually unchanged and resemble those of mature oocytes, though perinuclear aggregation can be evident. Mitochondria are usually excluded from meiotic and mitotic spindles but locate peripherally, apparently providing energy for centrosomal, cytoskeletal, and chromosomal activity during cell division. Morphogenetic changes in mitochondrial structure occur in the 8-cell cleaving embryo, the morula and the blastocyst (apparently accompanying the onset of nuclear and mitochondrial transcription), when they become progressively less electron dense and often develop clear areas in their matrices. Elongating mitochondria with inner mitochondrial membranes arranged into transverse cristae appear in expanding blastocysts, in the trophoblast, embryoblast, and endodermal cells. These mitochondria seem to play a role in blastocyst differentiation, expansion, and hatching, with their morphological changes reflecting increased cellular activity.

Fine structure of human oogonia in the foetal ovary

Foetal ovarian tissue is now being cultured or frozen, to generate oocytes for assisted reproduction, an emerging technology. This study examines the ultrastructure of oogonia at 13-15 weeks of gestation, which could be used as a control for culture and freezing of foetal ovaries. Oogonia are largely located in the ovarian cortex, whilst primordial germ cells (PGC) and somatic follicle cells compose the surface epithelium. Oogonia and PGC have large vesicular nuclei with clear cytoplasm, compared to dense follicle cells, which have polymorphic nuclei. Follicle cells intermingle with oogonia and establish close contacts - beginning of folliculogenesis. Nuclei of oogonia contain one to three highly reticulated nucleoli, reflecting high levels of RNA synthesis at the onset of growth. Rough endoplasmic reticulum (RER) form stacks of cisternae associated with numerous ribosomes. Prominent organelles in the ooplasm are elongated mitochondria with dense matrices and tubular cristate presenting a multilocular appearance. Typical Golgi complexes, dense bodies and clear vacuoles are present and microfilaments are located beneath the plasma membrane. The most remarkable feature of oogonia is that they have typical juxtanuclear centrioles (diplosomes) with dense pericentriolar material, which nucleate microtubules, characteristic of functional centrosomes organizing the cytoskeleton. The mature oocyte has no centrioles, since the maternal centrosome is inactivated or reduced, while the paternal is dominant. Centrioles are most likely involved in mitosis of oogonia.

Development of the human dispermic embryo

In a recent CD-ROM, we portrayed the microstructure of the pre-implantation human embryo (Sathananthan et al., 1999), which was a multimedia production with computer colour-enhanced electron micrographs of mainly monospermic embryos. This disk portrays light and electron micrographs of over 250 tripronuclear (3PN), dispermic, human embryos during pre-implantation development, viewed in thick and thin Araldite sections, as well as appearances of whole embryos flat embedded in Araldite blocks visualized with the light microscope. The 100 figures were computerized (IBM TIFF format), edited and labelled using Adobe Photoshop 5. Some of the figures were coloured on computer. The early development of 3PN embryos overtly resembles that of normal embryos but there are important differences in their microstructure which are portrayed in this presentation. This is a multicentric study involving researchers from four IVF centres.

Is the acrosome reaction a prerequisite for sperm incorporation after intra-cytoplasmic sperm injection (ICSI)?

There is debate as to whether the acrosome reaction is necessary for sperm incorporation after intra-cytoplasmic sperm injection (ICSI). Ultrastructural evidence is presented to show that the acrosome reaction could occur in the ooplasm before sperm incorporation in mature human oocytes or the acrosome could be discarded intact before sperm incorporation in immature oocytes, matured in vitro. Both germinal vesicle and growing follicular oocytes showed sperm chromatin decondensation, with discarded acrosomes close to the sites of incorporation, and were able to form male pronuclei. This is probably the first report of microfertilization of a growing oocyte with a reticulate nucleolus by ICSI. The acrosome reaction, when it occurs, is preceded by acrosome swelling and is followed by vesiculation of surface membranes exposing the inner acrosome membrane, as observed on the surface of the zona during IVF or in the perivitelline space after subzonal sperm injection. These sperm were probably capacitated at the time of ICSI. There was subtle evidence of leaching of the acrosomal matrix from intact discarded acrosomes and from partially depleted acrosomes attached to decondensing spermheads. These sperm were probably not fully capacitated at the time of ICSI. It is concluded that both the acrosome reaction and acrosome deletion are possible prerequisites to sperm incorporation after ICSI.

Paternal centrosomal dynamics in early human development and infertility

PURPOSE

Our purpose was to demonstrate the dynamics of the human sperm centrosome during fertilization and cleavage.

METHODS

Human gametes, fertilized oocytes, and preimplantation embryos were examined by transmission electron microscopy.

RESULTS

The functional sperm centrosome containing a typical centriole (proximal) is inherited at fertilization and forms a sperm monoaster. It then replicates and is perpetuated during cleavage. It organizes the mitotic apparatus at each stage of cleavage up to the hatching blastocyst stage. Bipolar spindles are formed in all monospermic and most dispermic embryos. Occasionally, two sperm asters and tripolar spindles are formed in dispermic embryos. Centrioles are associated with pronuclei and nuclei at interphases when they duplicate and occupy pivotal positions at spindle poles during mitoses. The maternal centrosome is not functional.

CONCLUSIONS

The human embryo shows paternal centrosome inheritance and perpetuation like most other animals. Inheritance of defective centrosomes may lead to abnormal cleavage and contribute to infertility.

Mitosis in the human embryo: the vital role of the sperm centrosome (centriole)

The pattern of sperm centrosomal (centriolar) inheritance, centrosomal replication and perpetuation during mitosis of the human embryo is reviewed with a series of electron micrographs. Embryonic cleavage involves repeated mitoses, a convenient sequence to study centriolar behaviour during cell division. After the paternal inheritance of centrioles in the human was reported (Sathananthan et al., 1991a), there has been an upsurge of centrosomal research in mammals, which largely follow the human pattern. The human egg has an inactive non-functional centrosome. The paternal centrosome contains a prominent centriole (proximal) associated with pericentriolar material which is transmitted to the embryo at fertilization and persists during sperm incorporation. Centriolar duplication occurs at the pronuclear stage (interphase) and the centrosome initially organizes a sperm aster when male and female pronuclei breakdown (prometaphase). The astral centrosome containing diplosomes (two typical centrioles) splits and relocates at opposite poles of a bipolar spindle to establish bipolarization, a prerequisite to normal cell division. Single or double centrioles occupy pivotal positions on spindle poles and paternal and maternal chromosomes organize on the equator of a metaphase spindle, at syngamy. Bipolarization occurs in all monospermic and in most dispermic ova. Dispermic embryos occasionally form two sperm asters initially and produce tripolar spindles (tripolarization). Anaphase and telophase follows producing two or three cells respectively, completing the first cell cycle. Descendants of the sperm centriole were found at every stage of perimplantation embryo development and were traced from fertilization through cleavage (first four cell cycles) to the morula and hatching blastocyst stage. Centrioles were associated with nuclei at interphase, when they were often replicating and occupied pivotal positions on spindle poles during mitosis. Sperm remnants were associated with centrioles and were found at most stages of cleavage. Centrioles were found in trophoblast, embryoblast and endoderm cells in hatching blastocysts. Pericentriolar, centrosomal material nucleated astral and spindle microtubules. Abnormal nuclear configurations observed in embryos reflect mitotic aberrations. The bovine embryo closely resembles the human embryo in centriolar behaviour during mitosis. It is concluded that the sperm centrosome is the functional active centrosome in humans and is likely the ancestor of centrioles within centrosomes in foetal and adult somatic cells. The role of the sperm centrosome in embryogenesis and male infertility is discussed, since it is of clinical importance in assisted reproduction.

Ultrastructure of the human egg

This report on the fine structure of human oocyte is based on 20 years research where over 2000 eggs were examined by TEM in conjunction with our research on various methods of assisted reproduction. The eggs were routinely fixed in glutaraldehyde/osmium tetroxide, flat embedded in araldite, serially sectioned and examined by TEM. The oocytes were usually recovered after gonadotrophin stimulation. The general organisation of the mature human oocyte conforms to that of other mammals but has some unique features. The oocyte has the basic cell organelles such as mitochondria, lysosomes, two types of smooth endoplasmic reticulum (SER)-vesicular and tubular aggregates, multivesicular residual bodies, lipofuschin, microfilaments and microtubules. Golgi, RER and ribosomes are very rare and the egg has no yolk. It's surface has few microvilli, pinocytolic caveolae and 1-3 layers of cortical granules. The zona pellucida is composed of fine fibrils and granules embedded in an amorphous matrix and encloses a perivitelline space containing polar bodies. Remnants of corona cell junctions may be found at the oolemma. The metaphase II spindle is often oriented perpendicular to the surface and is barrel-shaped, anastral and lacks centrioles. Osmiophilic centrosomes are not demonstrable in human eggs since the maternal centrosome is inactive. The sperm centrosome organizes mitotic spindles of the embryo after fertilization, whereas in mice the maternal centrosome is active and dominant during cleavage. The stages of peri-ovulatory maturation and differences in oocyte structure during maturation are also presented. Oocytes ageing in culture show progressive swelling of vesicular SER culminating in vacuolation, denser mitochondria (clouding together or associated with vacuoles) peripheral conglomerations or centripetal migration of cortical granules and increased lysosomal activity. Prolonged culture also causes displacement and disorganisation of metaphase II spindles, loss of microtubules and consequent displacement of chromosomes. Evidently the cytoskeleton becomes disorganized. Some observations on oocyte maturation in vitro and spontaneous activation of oocytes are included.

inheritance of sperm centrioles and centrosomes in bovine embryos

Immature cumulus oocyte complexes (COCs) were aspirated from ovarian follicles of slaughtered cow and matured for 24 h in TCM 199 medium with hormones. Eighty-five percent of oocytes matured with subsequent abstriction of the polar body. Matured COCs were then inseminated with frozen-thawed semen (2 x 10(6)/mL final concentration). Eighteen hours after insemination, fertilized COCs were vortexed, washed, and cultured to the pronuclear stage and syngamy (24-36 h postinsemination) and fixed for TEM. Unfixed embryos achieved a cleavage rate of 54%, with 29% developing to blastocysts. Fertilization was confirmed by TEM. Examination of fertilized bipronuclear ova revealed the presence of a sperm aster associated with sperm midpieces, tails, and male pronuclei in several embryos. Further examination of embryos at syngamy showed centrioles at one pole of the first mitotic bipolar spindle in two embryos. Since the mature oocyte at metaphase II has no centrioles at spindly poles, this centriole was most likely derived from the sperm, which has a single proximal centriole associated with pericentriolar material in its neck region, like most mammalian sperm. Tripronuclear ova produced disorganized bipolar spindles or, rarely, tripolar spindles. Bovine embryos, too, follow Boveri's rule of paternal inheritance as in man and most animals. It is possible that both paternal centrosomes (centrioles) and maternal centrosomes are involved in the organization of bipolar spindles in these embryos, quite unlike the mouse embryo where maternal centrosomes seem to organize the first mitotic spindle. The bovine embryo appears to be an appropriate model to study centriolar inheritance.

Centrifugation of bovine oocytes for nuclear micromanipulation and sperm microinjection

The reproductive biotechnologies of intracytoplasmic sperm microinjection, nuclear transfer and DNA microinjection require the visualization of cytoplasmic components and nuclei of oocytes and early embryonic cells. Bovine oocytes were matured and fertilized in vitro, and then centrifuged at the germinal vesicle, metaphase II and pronuclear stages and at syngamy. These (n = 536) were examined using light and transmission electron microscopy. The organelles stratified in five distinct zones in a consistent pattern in both oocytes and zygotes, though relative fractions changed in organelle composition after fertilization. These comprised a centripetal lipid zone, below which was a vesicular zone, then a supra-equatorial band of smooth endoplasmic reticulum (SER), a clear zone and a centrifugal mitochondrial zone. Cortical granules were located peripherally, single or clumped together, in the clear and mitochondrial zones. The nuclei were usually found associated with the SER or Golgi membranes, and chromatin was clumped at one pole within the nucleus. The maturation spindles were often located beneath the oolemma in all zones, while the first mitotic spindle was usually located in the clear zone. Some of the oocytes were activated by centrifugation and completed maturation.

The sperm centriole: its inheritance, replication and perpetuation in early human embryos

The inheritance, replication and perpetuation of the sperm centriole in the early human embryo are reported. Both normal monospermic and abnormal dispermic embryos (n = 127) were examined by transmission electron microscopy. Centrioles were traced from fertilization to the hatching blastocyst stage. The sperm proximal centriole is introduced into the oocyte at fertilization and remains attached to the expanding spermhead during sperm nuclear decondensation, as it forms the male pronucleus. A sperm aster is initially formed after the centriole duplicates at the pronuclear stage. At syngamy, centrioles occupy a pivotal position on opposite spindle poles, when the first mitotic figure is formed. Bipolar spindles were found in the majority of embryos, while tripolar spindles were seen in four dispermic embryos at syngamy. Two single centrioles were detected at two poles of two tripolar spindles, while two additional centrioles were located on the sides of a bipolar spindle of a dispermic embryo. Sperm tails were detected near spindle poles at syngamy and in later embryos. Typical centrioles showing the characteristic pin-wheel organization of nine triplets of microtubules were evident. During centriolar replication, the daughter centriole grows laterally from the parent and gradually acquires pericentriolar material (PCM). The two centrioles are surrounded by a halo of electron-dense PCM, which nucleates microtubules, thus making it a typical centrosome. The usual alignment of diplosomes at right angles to each other was maintained. Centrioles were detected at all stages of embryonic cleavage from the 1-cell through 8-cell stages, right up to the hatching blastocyst stage. They were closely associated with nuclei at interphase, when they were often replicating, and were prominently located at spindle poles during the first four cell cycles. In blastocysts, they were detected in trophoblast, embryoblast and endoderm cells respectively. It is evident that the sperm centrosome is the functional active centrosome in human, while the female is inactive but may contribute some centrosomal material to the zygote centrosome. It is very likely that the paternal centriole is the ancestor of the centrioles in fetal and adult somatic cells.

Functional competence of abnormal spermatozoa

The processes of abnormal sperm penetration and incorporation into human oocytes during IVF and after sperm microinjection, assessed by TEM, are reviewed. A spectrum of morphologically abnormal sperm with head, neck and midpiece defects penetrate the egg vestments of oocytes (1-3 h after insemination with sperm from normal donors) in both unfertilized and normally fertilized oocytes. Sperm with aberrant head shapes, acrosomal and nuclear defects penetrate the outer zona pellucida but are rarely encountered in the inner zona and perivitelline space, showing that the zona prevents abnormal sperm penetration. Grossly abnormal sperm are, however, incorporated into the ooplasm of zona-denuded oocytes. Microinjection of poor-quality sperm from male-factor patients into the perivitelline space or directly into the ooplasm of oocytes also reveals a variety of structural defects conforming to those observed in washed sperm pellets, highlighting the difficulties of selection of 'normal' sperm for microinjection. Abnormal sperm have been seen to interact and fuse with the oocyte in the perivitelline space, and to be incorporated into the ooplasm. Those with nuclear and neck (centriolar) defects are of particular significance, as they might contribute to aberrant development. The functional competence of immotile, round-headed and epididymal sperm is also briefly discussed.

Ultrastructural changes during meiotic maturation in mammalian oocytes: unique aspects of the human oocyte

This paper reviews the process of peri-ovulatory oocyte maturation and the ultrastructural organization of the human egg and compares it with that of the mouse. The main thrust of the paper is on the human, since there are several reviews on the mouse. Both preovulatory and postovulatory events at fertilization, as well as some of the aberrant features of maturation are covered. Some changes induced by oocyte culture and cooling in the human are also included. The report attempts to focus on unique features of the human oocyte and shows a variety of ultrastructural differences between human and murine oocytes, which may well reflect differences in their physiology and biochemistry. Based on these differences and further observations on the process of fertilization of both species, particularly with respect to the inheritance of paternal centrioles, it is concluded that the mouse may not be a suitable model for the development and refinement of current procedures in human assisted reproductive technology.

Early sperm-egg interaction after sperm microinjection

The early events of sperm-egg interaction occurring 1-3 h after multiple sperm injection into the perivitelline space (PVS) and after direct injection into the ooplasm of human oocytes are reported. The sperm acrosome reaction occurred in the PVS but was not detected within the ooplasm. Sperm in the PVS were incorporated into the ooplasm in the usual manner described in vitro, after completion of the acrosome reaction, and a block to polyspermy was evident at the oolemma. However, sperm incorporation into the ooplasm was not clearly defined and requires further investigation. Sperm were also incorporated into oolemma-bound vacuoles within the ooplasm and breaches in the ooplasm were seen after intracytoplasmic injection. Both normal and abnormal sperm were found in the PVS and ooplasm, even though sperm from donors with normal semen parameters were used. The effect of freezing in liquid nitrogen on demembranation of sperm for microinjection is also reported.

Understanding the fundamentals of embryology in assisted reproductive technology

This paper emphasises the need to understand the fundamentals of embryology in relation to in vitro fertilisation (IVF) and associated assisted reproductive technologies (ART). It introduces the reproductive technologist and others involved in such programmes to the events that occur during early embryogenesis, which have led to recent developments in IVF/ART. It also covers some of the contributions made by IVF/ART in understanding early events of development, particularly during the first week of human life. The reader is referred to some of the widely used embryology texts and audio-visual aids and also to selected reviews and papers published recently that would help toward a better understanding of early development and ART.

The effects of cooling mouse oocytes

The effects of cooling and warming on meiotic spindles of mouse oocytes have been assessed by transmission electron microscopy. Intact cumulus-oocyte complexes were immediately cooled from 37 to 15, 4, 0, and -7 degrees C (seeding temperature) for 15 min in a programmed biological freezer and fixed at these temperatures. Other complexes, cooled to these temperatures, were rapidly warmed to 37 degrees C and incubated for 2 hr before fixation at 37 degrees C. Of 334 oocytes assessed at various temperatures, at least 100 were examined for metaphase II spindles. Spindle microtubules completely disappear at 0 and -7 degrees C, while complete or partial depolymerization of microtubules was observed at 4 degrees C. Cooling to 15 degrees C did not cause major disruptions of spindle structure in most oocytes. Chromosomes tended to rotate or clump at lower temperatures but chromosome scatter outside the spindle zone was rarely observed. Centrosomal material was fragmented at 4 degrees C and occasionally at 15 degrees C and was not evident at the spindle poles at 0 and -7 degrees C. Kinetochores were seen at all temperatures. Spindle structure was evidently restored in the majority of oocytes on rewarming at 37 degrees C. Changes in the ooplasm induced by cooling were elongation and disruption of vesicular smooth endoplasmic reticulum, especially between lipid globules and disappearance of fibrillar inclusions. Cortical granule exocytosis was not observed on cooling, while microfilaments were intact. Swelling of membranous organelles was also observed in cumulus cells. Most of the cytoplasmic changes were also reversed on rewarming. The response of mouse oocytes to cooling is compared to that of human oocytes, reported previously.

Centrioles in the beginning of human development

We demonstrate the presence of centrioles in fertilized human oocytes at syngamy. Single or double centrioles within centrosomes were detected by transmission electron microscopy at one pole of the first cleavage spindle in normal and dispermic embryos (25-26 hr after insemination). Sperm centrioles were also closely associated with the male pronucleus (16-20 hr after insemination) in pronuclear stage embryos. A tripolar spindle derived from a tripronuclear embryo is also demonstrated with two centrioles at one pole. The data provide evidence that human centrioles, as those in most other animals, and unlike the mouse, are paternally derived, thus supporting Boveri's classical theory. Furthermore, this study provides insights to the proposed mechanisms of aberrant cleavage patterns of dispermic human embryos.

Fine structure of the human corpus cavernosum

This paper demonstrates the ultrastructure of the human corpus cavernosum from eight male transexuals (aged 20 to 30 years) undergoing penectomy. The presence of collagen, smooth muscle, endothelial cells lining cavernous spaces, mast cells, and different types of nerve terminals, including those of a nonadrenergic and noncholinergic type, are illustrated.

Subzonal transfer of multiple sperm (MIST) into early human embryos

Microinsemination sperm transfer (MIST) is a technique whereby sperm are transferred into the perivitelline space (PVS) with the aid of a micromanipulator. MIST is now used to investigate whether blastomere membranes of early human embryos are capable of fusing with the sperm as in the metaphase II oocyte. Between 10 and 30 sperm were transferred into 11 donated human embryos between pronuclear and 16 cell stage. After culture for 6-24 hr in vitro, the embryos were fixed for transmission electron microscopy (TEM). Both acrosome-intact and acrosome-reacted sperm were located in the PVS and between blastomeres. Sperm in the PVS were sometimes penetrating the inner regions of the zona. Sperm-blastomere membrane fusion was not observed, but sperm tail incorporation by phagocytosis was occasionally evident. Sperm heads incorporated into blastomeres were often located in membrane-bound vesicles. Both acrosome-intact and acrosome-reacted sperm heads were found in vacuoles. Acrosome-reacted sperm heads were lying passively in vacuoles or were undergoing degenerative changes at their surfaces. Sperm chromatin decondensation was not observed in any of the sperm heads that were detected in the blastomeres. The evidence presented clearly shows that sperm heads are incapable of expanding their chromatin to form typical male pronuclei following MIST into early human embryos.

Assessment of the human sperm acrosome reaction using concanavalin A lectin

A method for assessment of the human sperm acrosome reaction is reported using fluorescein isothiocyanate (FITC)-conjugated Concanavalin A (ConA). The technique involved labelling prefixed spermatozoa, where only those spermatozoa that showed a complete loss of the acrosome bound FITC-ConA to the acrosomal region. Competitive sugar binding studies demonstrated that binding of ConA lectin to the acrosomal area of human spermatozoa was inhibited in the presence of 0.2 M D-mannose. Staining with the supravital stain Hoechst 33258 (H258) concomitantly with FITC-ConA allowed determination of only those spermatozoa that had undergone a true and not degenerative acrosomal loss. Incubation of human spermatozoa with 0, 1, 5, and 25 microM calcium ionophore, A23187, for 60 min demonstrated that changes in acrosomal status due to the different treatment protocols may be determined by the dual-staining method. Electron microscopy studies revealed that gold-conjugated ConA bound specifically to the surface of the inner acrosomal membrane of acrosome-reacted spermatozoa. A significant correlation (r = +.97) between transmission electron microscopy (TEM) and FITC-ConA labelling methods of acrosomal status assessment was achieved. The simple ConA labelling procedure reported here therefore provides a reliable method for quantitation of the physiological acrosome reaction of a population of human spermatozoa.

Human micro-insemination by injection of single or multiple sperm: ultrastructure

The process of micro-insemination by single or multiple sperm transfer into the perivitelline space (PVS) or by direct sperm injection into oocytes was examined by transmission electron microscopy. Spermatozoa from normal and oligozoospermic men were injected into oocytes, obtained from consenting IVF patients, mostly by zona-puncture using micromanipulators. Spermatozoa were washed by the Percoll or Ficoll methods and capacitated using Whittingham's T6 or modified Tyrode's medium or incubated in strontium medium before injection. The women were stimulated by three IVF methods and oocytes were recovered by laparoscopy or ultrasonography. Sixty-one oocytes were cultured in T6 or Ham's F-10 media (3-24 h) and were subjected to micromanipulation. Four oocytes were also studied after zona-drilling. Normal 2-pronuclear ova were developed after single-sperm transfer satisfying all morphological criteria of fertilization. Both monospermic and polyspermic fertilization resulted after multiple sperm transfer, indicating that a vitelline block to polyspermy may exist in humans. The majority of oocytes examined were unfertilized. Spermatozoa with intact or reacted acrosomes and those undergoing the acrosome reaction were found in the PVS and in the ooplasm. Abnormal spermatozoa were also seen in these locations. Quantitation of acrosomal status in 16 oocytes after multiple-sperm transfer, revealed that 24% of spermatozoa were acrosome-reacted or reacting in the PVS following Ficoll entrapment, while 76% of spermatozoa were intact (33% of these abnormal). Sperm transfer seemed to be the least invasive, while direct sperm injection was comparatively destructive to oocytes. Drilling with acid made larger breaches in the zona when compared with mechanical perforation and spermatozoa occasionally escaped through breaches. Three 2-pronuclear ova obtained after multiple sperm transfer have resulted in two pregnancies, in cases of severe oligozoospermia, during the course of this study.

Human fertilization by micro-injection of immotile spermatozoa

Microfertilization of human oocytes with spermatozoa from a man with immotile cilia syndrome is reported, confirming a preliminary investigation where a zona-free donor oocyte was fertilized with spermatozoa from the same patient. Oocytes from his spouse were obtained by laparoscopy after routine stimulation with clomiphene citrate, human menopausal and chorionic gonadotrophins, and were cultured for 4-6 h in Whittingham's T6 medium, supplemented with 10% of her serum. The spermatozoa were washed and processed in the same medium and capacitated for 6-8 h before micromanipulation. Three of five mature oocytes were fertilized by micro-injection of a single immotile spermatozoon into the perivitelline space. One oocyte produced a two-pronuclear ovum assessed 19 h after injection, while the other two produced 2-cell embryos with blastomeres of equal size, 22 h after injection. These embryos cleaved to 3-8-cell stages in culture before embryo replacement. No pregnancy resulted from embryo transfer. The results conclusively demonstrate that human oocytes can be fertilized successfully with immotile spermatozoa by micro-injection and the work has profound implications in the treatment of severe male infertility.

The effects of ultrarapid freezing on meiotic and mitotic spindles of mouse oocytes and embryos

Preovulatory mouse oocytes and 2-cell embryos were frozen with dimethyl sulfoxide and propanediol by an ultrarapid method. The survival of frozen oocytes was low (33-34%) compared to that of 2-cell embryos (78-79%) with either cryoprotectant. Development to blastocysts after postthaw culture was about 7-15% for oocytes and 79-80% for the embryos. Ultrarapid freezing preserves cell structure quite well as revealed by electron microscopy, but meiotic oocytes and late 2-cell embryos undergoing mitosis showed evidence of spindle disorganization involving loss or clumping of microtubules resulting in some scattering of chromosomes. Embryos developed from frozen eggs showed clear evidence of micronuclear formation and incomplete incorporation of chromosomal material into main nuclei. These experiments confirm our observations on freezing of human oocytes and show that spindle microtubules are sensitive to freeze-thawing and that cryopreservation could cause chromosomal aberrations during early development. A cautious approach to the introduction of oocyte freezing in human in vitro fertilization (IVF) programs is advocated.

The effects of cooling human oocytes

Preovulatory human oocytes were cooled to 0 degrees C at 1 degree C/min, with or without the cryoprotectant dimethyl sulphoxide (DMSO), to assess the effects of cooling on the meiotic spindles and on oocyte structure. Batches of oocytes, cultured for 3-9 h, were held at 0 degrees C for 20 or 60 min and then fixed for transmission electron microscopy (TEM) either at 0 or 8 degrees C. Control oocytes were not cooled and were fixed at 22 or 37 degrees C for comparison. TEM revealed that 80% of the oocytes were at metaphase II, while 20% were at metaphase I and most had resumed meiosis recently. Control oocytes had more or less barrel-shaped meiotic spindles composed of microtubules (MT), some associated with chromosomes at kinetochores. Both metaphase I and II spindles were disassembled when cooled and fixed at 0 degrees C, with or without DMSO, due to extensive depolymerization of MT. The few MT that survived were found at the poles or were bundled together or were associated with chromosomes. Kinetochores were not prominent. Some oocytes cooled with DMSO and fixed at 0 or 8 degrees C showed evidence of MT, but the spindles were still disorganized and were abnormal in structure. Chromosomes tended to clump together or were dislocated in the cortical ooplasm in cooled oocytes, but widespread scattering was not observed. This was particularly evident in the absence of DMSO. Elements of the endoplasmic reticulum, Golgi, mitochondria and the cytosol were also adversely affected in some of the cooled oocytes and their surrounding cumulus cells. The results show that meiotic spindles are very sensitive to simple cooling and that DMSO does not provide substantial stabilization of the meiotic spindle even at 0 degrees C. The findings are discussed with reference to recent work on frozen human and mouse oocytes.

Fine structure of early human embryos frozen with 1,2 propanediol

Previous studies by a French group (Fertil Steril 44:645-651, 1985) have shown that two- to eight-cell human embryos can survive slow freeze-thawing with propanediol in a biological freezer. These embryos were assessed for morphological appearance by phase-contrast microscopy. We assessed the structure of 25 frozen-thawed one- to 12-cell embryos, obtained from our in vitro fertilization (IVF) and GIFT programmes, by phase-contrast and electron microscopy, using the same method of cryopreservation. One-fourth of the embryos examined had all cells intact, and more than one-half the embryos had over 50% of their cells well preserved. Some of these embryos had unequal blastomeres and cytoplasmic fragments. Ultrastructural assessment revealed good preservation of fine structure in the intact blastomeres of all embryos and maintenance of cell-to-cell contacts. Most cytoplasmic organelles, cell membranes, and nuclei were well preserved compared to nonfrozen controls. The cells that were cryoinjured showed varying degrees of disorganization of the cell membrane, cytosol, and cellular membranes, including swelling and disruption of the nuclear envelope. Disruption of the zona was somewhat rare. Small cytoplasmic fragments were less prone to cryoinjury than blastomeres. The use of propanediol for embryo cryopreservation seems to be feasible; frozen embryos with more than 50% cells intact have produced 10 pregnancies after embryo transfer (Fertil Steril 46:268-272, 1986). Replacement of 17 frozen embryos in seven patients has resulted in a twin pregnancy in Singapore. However, the effects of freezing on the mitotic spindles of embryonic cells need to be investigated further.

Morphology and fertilizability of frozen human oocytes

Human oocytes were frozen and thawed by four methods previously used for cryopreservation of human embryos. Most of these oocytes were inseminated after thawing to assess their capacity to fertilize and form pronuclear ova. Their morphology was assessed by phase-contrast microscopy used in routine IVF. Twenty-three oocytes were examined by electron microscopy to critically evaluate the effects of cooling and cryopreservation and to confirm fertilization. Morphological survival was observed in more than 60% of the oocytes examined after freeze-thawing. The main features of cryoinjury were cracks in the zona pellucida, disruption of the plasma membrane and extensive disorganization of the ooplasm. Subtle changes in the cytosol of cumulus cells was also observed. Cooling to 0 degrees C or -6 degrees C had little effect on cytoplasmic structure. Spindles were damaged in two frozen oocytes. Cumulus cell activity, sperm binding to the zona, sperm penetration of the zona seem to be largely unaffected by freeze-thawing. Fertilization was observed in eight oocytes after postthaw insemination and three embryos (8-cell to morula stages) were developed from pronuclear ova on further culture. Both monospermic and polyspermic fertilization were confirmed by electron microscopy and micronuclei were detected in three pronuclear ova. The genetic implications of these nuclear aberrations are discussed. These preliminary studies indicate that oocyte freezing needs to be integrated cautiously with clinical IVF by further assessment of embryos developed from frozen oocytes.

Early penetration of human sperm through the vestments of human eggs in vitro

Penetration of human sperm through the vestments of human oocytes during the first 3 h after insemination was investigated to determine the time taken for sperm capacitation, which precedes the acrosome reaction and fertilization. Twelve oocytes from women who became pregnant by IVF, recovered at laparoscopy after appropriate stimulation, were examined by electron microscopy. Follicular development was controlled by administration of clomiphene citrate, hMG, and hCG. Oocytes were cultured in Whittingham's T6 medium for 7-14 h before insemination and were fixed 1-3 h after insemination with preincubated sperm obtained from fertile men. All oocytes had matured and eight were normally fertilized 2-3 h after insemination. The acrosome reaction had already begun 1 h after insemination when 10-30% of sperm had reacted. Sperm-oocyte membrane fusion occurred 2 h after insemination, and sperm decondensation and pronuclear formation were in progress 3 h after insemination. Variable numbers of sperm (30-60%) had reacted acrosomes after 2-3 h of insemination. Many sperm penetrating the cumulus were intact or had partially reacted acrosomes. Intact, partially and fully reacted sperm were found at the surface of the zona. Intact sperm were bound to the zona by their plasma membranes. Sperm penetrating the zona had reacting or reacted acrosomes exposing their inner acrosome membranes. Those approaching the perivitelline space had a persistent equatorial vestige of the acrosome with intact plasma membrane. The acrosome reaction involved multiple fusions of the sperm plasma and outer acrosome membranes, resulting in vesiculation. This study shows that human sperm could complete capacitation and initiate the acrosome reaction within 1 h of insemination in vitro.

The origin and distribution of cortical granules in human oocytes with reference to Golgi, nucleolar, and microfilament activity

The origin and distribution of cortical granules were investigated in human preovulatory oocytes at various phases of maturation. Twenty-five oocytes obtained from unstimulated small antral follicles and from stimulated large antral and mature follicles were examined by transmission electron microscopy. Ovarian stimulation in women was accomplished by administering Clomid followed by hMG or hCG or both. Small antral follicle oocytes were dissected from ovarian biopsies, while the other oocytes were recovered by laparoscopy. Some oocytes were allowed to mature in Ham's F-10 or Whittingham's T-6 media before routine fixation in glutaraldehyde/osmium. Cortical granules originate from typical, hypertrophic Golgi complexes during early maturation and continue till its completion. Evidently there are two waves of cortical granule synthesis, the first more prolific than the second. The first occurred in small antral follicle oocytes, when there was a peak in Golgi activity, and the second was observed at the germinal vesicle stage, particularly at the onset of resumption of meiosis. Golgi complexes became progressively scarce as oocytes completed first maturation. Golgi membranes were also involved in the formation of lysosomes. A well-defined band of microfilaments was detected in small antral follicle oocytes which seemed to prevent the cortical granules, organized in a single layer, from migrating to the periphery. This band gradually became disorganized at the germinal vesicle stage as oocytes resumed meiosis, when cortical granules were apparently migrating to the surface. Metaphase I and mature oocytes had one to three discontinuous layers of cortical granules beneath the oolemma. The general organization of oocytes was also investigated and the roles of the nucleolus and endoplasmic reticulum in relation to Golgi activity and cell secretion were discussed.

The application of electron microscopy in the evaluation of two- to four-cell human embryos cultured in vitro for embryo transfer

Eighteen two- to four-cell embryos, cultured in vitro for 32-65 hr after insemination, were examined by transmission electron microscopy to assess their normality and developmental potential. These stages are now being widely used for embryo transfer in in vitro fertilization clinics. They were obtained by inseminating preovulatory oocytes aspirated at laparoscopy, with or without ovarian stimulation, by methods which have yielded normal pregnancies. The organization of seven embryos was apparently normal and their blastomeres had cellular organelles usually present in fertilized ova. Details of their ultrastructure including subtle changes observed on prolonged culture are described. Other embryos showed some normal and obvious abnormal features, such as partial fragmentation and multinucleated blastomeres, or evidence of degeneration.

The ultrastructure of the preovulatory human egg fertilized in vitro

Preovulatory eggs in cumulus were inseminated in vitro with washed spermatozoa which had been preincubated for 1.5 hours. After 3 hours, three eggs were processed for electron microscopy and each was sectioned serially from pole to pole. In the two eggs which had been fertilized, the expanded chromatin of the fertilizing sperm head and the chromatin of the ovum were almost completely surrounded by a developing pronuclear envelope. In one of the penetrated eggs the developing male pronucleus and associated midpiece and sperm tail were located within an incorporation cone. The surface of the cone was free of microvilli but contained a zone of microfilaments immediately beneath the plasma membrane. A similar zone of microfilaments was present beneath the advancing surface of the extruding second polar body (PB2) which was connected to the ovum by an interbody and microtubules of the meiotic spindle. Cortical granules were completely absent from the fertilized eggs but were present in the unfertilized egg. PB2 contained a nucleus at a stage of development similar to that of the early pronuclei.

Degeneration of possible serotonergic nerves in the myenteric plexus of Mytilus induced by 5,7-dihydroxytryptamine (5,7-DHT)

Initial observations on the effects of 5,7-DHT on the myenteric plexus of Mytilus reveal extensive lesioning of monoamine nerves containing dense-cored vesicles. At least, two types of monoaminergic nerves could be recognized among the degenerating axons. The most affected were profiles with a predominance of large granular vesicles (56-200 nm) which resemble possible indoleamine nerves in the lamprey gut. The other type had small granular vesicles (40-56 nm) resembling catecholamine nerves of vertebrates. These two types of nerves possibly represent serotonergic and dopaminergic nerves, in accordance with pharmacological, biochemical and physiological evidence available in molluscs. There were, however, nerves with a population of medium-sized granular vesicles (56-130 nm) which were also damaged. Two other types of nerve profiles were unaffected by the drug. One conforms to cholinergic nerves while the other resembles "purinergic" nerves in the vertebrate gut. These results confirm previous studies with dihydroxytryptamines on molluscan smooth muscle.

Degeneration of monoamine nerves in anterior byssus retractor muscle of Mytilus induced by 5,6-dihydroxytryptamine

Preliminary ultrastructural studies on the effects of 5,6-Dihydroxytryptamine (5,6-DHT) on the anterior byssus retractor muscle (ABRM) of Mytilus show degeneration of 2 types of monoaminergic nerves after 10 days of drug treatment. One type contained large granular vesicles (560-1,680 A) possibly represent serotonergic and dopaminergic nerves, thought to innervate this muscle. Two other types of profiles seemed to be unaffected by the drug. One conforms to cholinergic nerves while the other has a predominance of large opaque vesicles (1,200-2,500 A). The significance of these findings is discussed in the light of recent observations on the neurotoxic effects of 5,6-DHT on vertebrate and molluscan nerves.

On two new nematodes of the genus Spironoura (Oxyuroidea: Kathlaniidae) from a Ceylon tortoise

S. fernandoi and S. pillaii are described as new species and are shown to differ specifically from each other and from S. japonensis, the only other species that has been reported as having spines on the spicules.

Cytological and cytochemical studies of centrifuged eggs of the slug Avion ater rufus L

The stratification of various cell organelles and of important chemical substances have been studied in the eggs of the slug, after moderate centrifugation.

As in other molluscs the egg contents stratify typically into three well-defined zones - lipid, L hyaline and yolky zones - but a distinct equatorial band of inclusions consisting mainly of F phagosomes and associated lysosomes was detected in the most centrifugal region of the hyaline zone.

The sub-stratification of various cell inclusions in their respective zones was determined in some detail.

The role of the cell membrane and egg cortex in the redistribution of these inclusions and the nature of the ergastoplasm are discussed in the light of electron-microscope studies of eggs of this slug and of the sea urchin.