Surface mapping of binding of oviductin to the plasma membrane of golden hamster spermatozoa during in vitro capacitation and acrosome reaction

The role of oviduct-specific glycoprotein (OVGP1) in modulating biological functions of gametes and embryos

Diverse lines of evidence indicate that the mammalian oviduct makes important contributions to the complex process of reproduction other than being simply a conduit for the transport of gametes and embryos. The cumulative synthesis and transport of proteins secreted by oviductal secretory cells into the oviductal lumen create a microenvironment supporting important reproductive events, including sperm capacitation, fertilization, and early embryo development. Among the components that have been identified in the oviductal fluid is a family of glycosylated proteins known collectively as oviduct-specific glycoprotein (OVGP1) or oviductin. OVGP1 has been identified in several mammalian species, including humans. The present review summarizes the work carried out, in various mammalian species, by many research groups revealing the synthesis and secretion of OVGP1, its fate in the female reproductive tract upon secretion by the oviductal epithelium, and its role in modulating biological functions of gametes and embryos. The production and functions of recombinant human OVGP1 and recombinant OVGP1 of other mammalian species are also discussed. Some of the findings obtained with immunocytochemistry will be highlighted in the present review. It is hoped that the findings obtained from recent studies carried out with recombinant OVGP1 from various species will rekindle researchers’ interest in pursuing further the role of the oviductal microenvironment, of which OVGP1 is a major component, in contributing to the successful occurrence of early reproductive events, and the potential use of OVGP1 in improving the current assisted reproductive technology in alleviating infertility.

Addition of exogenous proteins detected in oviductal secretions to in vitro culture medium does not improve the efficiency of in vitro fertilization in pigs

This work was designed to study whether HSP70-1A, HSP90α, ezrin or PDI4, proteins previously identified in porcine oviductal secretions, have a role in zona pellucida (ZP) resistance to enzymatic digestion, in vitro fertilization (IVF) and sperm viability. In vitro matured porcine cumulus oocyte complexes were denuded and i) incubated for 1 h in TALP medium supplemented or not with each exogenous oviductal protein and in presence or absence of heparin to assess ZP digestion time by pronase; and ii) inseminated with fresh ejaculated boar spermatozoa in medium supplemented or not with each exogenous oviductal protein to assess their effect on fertilization results. Finally, spermatozoa were incubated in Tyrode's medium (0, 1 and 20 h) supplemented or not with HSP-701A, HSP-90α or ezrin, to assess simultaneously sperm viability and acrosome status by means of flow cytometry. Although all proteins increased the ZP digestion time, this increase was lower than 1 min, being ezrin the protein with a stronger effect. Presence of heparin in the medium reinforced the ZP hardening effect of ezrin and HSP-701A up to one more min, but not HSP-90α nor PDI4. Sperm penetration, but not IVF efficiency, increased when gametes were cocultured in medium containing PDIA4 whereas sperm penetration and polyspermy rates decreased in presence of ezrin and HSP proteins. This reduction was not the result of a detrimental effect of proteins on sperm viability or acrosome reaction. In conclusion, addition of exogenous proteins detected in oviductal secretions to artificial media does not reproduce the effect of adding such secretions nor improve the final efficiency of the porcine IVF system.

Sperm Proteome after Interaction with Reproductive Fluids in Porcine: From the Ejaculation to the Fertilization Site

Ejaculated sperm are exposed to different environments before encountering the oocyte. However, how the sperm proteome changes during this transit remains unsolved. This study aimed to identify proteomic changes in boar sperm after incubation with male (seminal plasma, SP) and/or female (uterine fluid, UF; and oviductal fluid, OF) reproductive fluids. The following experimental groups were analyzed: (1) SP: sperm + 20% SP; (2) UF: sperm + 20% UF; (3) OF: sperm + 20% OF; (4) SP + UF: sperm + 20% SP + 20% UF; and (5) SP+OF: sperm + 20% SP + 20% OF. The proteome analysis, performed by HPLC-MS/MS, allowed the identification of 265 proteins. A total of 69 proteins were detected in the UF, SP, and SP + UF groups, and 102 proteins in the OF, SP, and SP + OF groups. Our results showed a higher number of proteins when sperm were incubated with only one fluid than when they were co-incubated with two fluids. Additionally, the number of sperm-interacting proteins from the UF group was lower than the OF group. In conclusion, the interaction of sperm with reproductive fluids alters its proteome. The description of sperm-interacting proteins in porcine species after co-incubation with male and/or female reproductive fluids may be useful to understand sperm transport, selection, capacitation, or fertilization phenomena.

Role of Protein Glycosylation in Host-Pathogen Interaction

Host-pathogen interactions are fundamental to our understanding of infectious diseases. Protein glycosylation is one kind of common post-translational modification, forming glycoproteins and modulating numerous important biological processes. It also occurs in host-pathogen interaction, affecting host resistance or pathogen virulence often because glycans regulate protein conformation, activity, and stability, etc. This review summarizes various roles of different glycoproteins during the interaction, which include: host glycoproteins prevent pathogens as barriers; pathogen glycoproteins promote pathogens to attack host proteins as weapons; pathogens glycosylate proteins of the host to enhance virulence; and hosts sense pathogen glycoproteins to induce resistance. In addition, this review also intends to summarize the roles of lectin (a class of protein entangled with glycoprotein) in host-pathogen interactions, including bacterial adhesins, viral lectins or host lectins. Although these studies show the importance of protein glycosylation in host-pathogen interaction, much remains to be discovered about the interaction mechanism.

Structural and functional characterization of buffalo oviduct-specific glycoprotein (OVGP1) expressed during estrous cycle

Oviduct-specific glycoprotein (OVGP1) is a high molecular weight chitinase-like protein belonging to GH18 family. It is secreted by non-ciliated epithelial cells of oviduct during estrous cycle providing an essential milieu for fertilization and embryo development. The present study reports the characterization of buffalo OVGP1 through structural modeling, carbohydrate-binding properties and evolutionary analysis. Structural model displayed the typical fold of GH18 family members till the boundary of chitinase-like domain further consisting of a large (β/α)8 TIM barrel sub-domain and a small (α+β) sub-domain. Two critical catalytic residues were found substituted in the catalytic centre (Asp to Phe118, Glu to Leu120) compared with the active chitinase. The carbohydrate-binding groove in TIM barrel was lined with various conserved aromatic residues. Molecular docking with different sugars revealed the involvement of various residues in hydrogen-bonding and non-bonded contacts. Most of the substrate-binding residues were conserved except for a few replacements (Ser13, Lys48, Asp49, Pro50, Asp167, Glu199, Gln272 and Phe275) in comparison with other GH18 members. The residues Trp10, Trp79, Asn80, Gln272, Phe275 and Trp334 were involved in recognition of all six ligands. The α+β sub-domain participated in sugar-binding through Thr270, Gln272, Tyr242 and Phe275. The binding assays revealed significant sugar-binding with purified native and recombinant OVGP1. Phylogenetic analysis revealed that OVGP1 was closely related to AMCases followed by other CLPs and evolution of OVGP1 occurred through several gene duplications. This is the first study describing the structural characteristics of OVGP1 that will further help to understand its interaction with gametes to perform crucial reproductive functions.

S100 A9 is expressed and secreted by the oviduct epithelium, interacts with gametes and affects parameters of human sperm capacitation in vitro

Our previous findings demonstrate that some oviductal secretion proteins bind to gametes and affect sperm physiology and gamete interaction. One of these proteins possesses an estimated molecular weight of 14 kDa. The objective of this study was to isolate and identify this 14 kDa protein, to localize it in the human oviduct, to detect gamete binding sites for the protein, and to evaluate its effects on sperm capacitation parameters and gamete interaction. Explants from the human oviductal tissues of premenopausal women were cultured in the presence of [³⁵ S]-Methionine-proteins ([35S]-Met-proteins). De novo synthesized secreted [³⁵ S]-Met-proteins were isolated from the culture media by affinity chromatography using their sperm membrane binding ability and analysed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Using liquid chromatography-tandem mass spectrometry peptide sequencing, human S100 A9 was identified as one of the isolated proteins from the 14 kDa protein band. S100 A9 was detected in oviduct epithelium and oviduct secretion using immunohistochemistry and a Western blot. S100 A9 binding to human oocytes and spermatozoa was assessed by indirect immunofluorescence. The acrosome reaction (AR) affected S100 A9 ability to bind sperm cells. The presence of S100 A9 significantly increased both the induced AR and the sperm protein tyrosine phosphorylation, with respect to controls. However, the protein did not affect sperm-zona pellucida interaction. Results indicate that S100 A9 is present in the human oviduct and that it modulates parameters of sperm capacitation in vitro. Hence, the protein might contribute to the regulation of the reproductive process in the oviductal microenvironment.

Profiling of proteins secreted in the bovine oviduct reveals diverse functions of this luminal microenvironment

The oviductal microenvironment is a site for key events that involve gamete maturation, fertilization and early embryo development. Secretions into the oviductal lumen by either the lining epithelium or by transudation of plasma constituents are known to contain elements conducive for reproductive success. Although previous studies have identified some of these factors involved in reproduction, knowledge of secreted proteins in the oviductal fluid remains rudimentary with limited definition of function even in extensively studied species like cattle. In this study, we used a shotgun proteomics approach followed by bioinformatics sequence prediction to identify secreted proteins present in the bovine oviductal fluid (ex vivo) and secretions from the bovine oviductal epithelial cells (in vitro). From a total of 2087 proteins identified, 266 proteins could be classified as secreted, 109 (41%) of which were common for both in vivo and in vitro conditions. Pathway analysis indicated different classes of proteins that included growth factors, metabolic regulators, immune modulators, enzymes, and extracellular matrix components. Functional analysis revealed mechanisms in the oviductal lumen linked to immune homeostasis, gamete maturation, fertilization and early embryo development. These results point to several novel components that work together with known elements mediating functional homeostasis, and highlight the diversity of machinery associated with oviductal physiology and early events in cattle fertility.

Effect of recombinant and native buffalo OVGP1 on sperm functions and in vitro embryo development: a comparative study

Background

An oviduct- specific glycoprotein, OVGP1, is synthesized and secreted by non-ciliated epithelial cells of the mammalian oviduct which provides an essential milieu for reproductive functions. The present study reports the effects of recombinant buffalo OVGP1 that lacks post-translational modifications, and native Buffalo OVGP1 isolated from oviductal tissue, on frozen- thawed sperm functions and in vitro embryo development.

Results

The proportion of viable sperms was greater (P < 0.05) in the recombinant OVGP1-treated group compared to the native OVGP1-treated group at 2 h, 3 h, and 4 h of incubation. The proportion of motile sperms at 3 h and 4 h of incubation; and membrane- intact sperms at 4 h was greater (P < 0.05) in the native OVGP1-treated group compared to the control and recombinant OVGP1-treated groups. The proportion of capacitated and acrosome- reacted sperms was greater (P < 0.05) in the native OVGP1-treated group compared to the recombinant OVGP1 group at 4 h. The rates of cleavage of embryos and their development to the blastocyst stage were greater (P < 0.05) in the presence of either native or recombinant OVGP1 in comparison to control at 10 μg/mL concentration as compared to 5 or 20 μg/mL.

Conclusions

The study suggests that both native and recombinant OVGP1 impart a positive effect on various sperm features and in vitro embryo development. However, native OVGP1 was found to have a more pronounced effect in comparison to recombinant non-glycosylated OVGP1 on various sperm functions except viability. Hence, our current findings infer that glycosylation of OVGP1 might be essential in sustaining the sperm functions but not the in vitro embryo development.

Electronic supplementary material

The online version of this article (doi:10.1186/s40104-017-0201-5) contains supplementary material, which is available to authorized users.

Recombinant human oviductin regulates protein tyrosine phosphorylation and acrosome reaction

The mammalian oviduct synthesizes and secretes a major glycoprotein known as oviductin (OVGP1), which has been shown to interact with gametes and early embryos. Here we report the use of recombinant DNA technology to produce, for the first time, the secretory form of human OVGP1 in HEK293 cells. HEK293 colonies stably expressing recombinant human OVGP1 (rHuOVGP1) were established by transfecting cells with an expression vector pCMV6-Entry constructed with OVGP1 cDNA. Large quantities of rHuOVGP1 were obtained from the stably transfected cells using the CELLSPIN cell cultivation system. A two-step purification system was carried out to yield rHuOVGP1 with a purity of >95%. Upon gel electrophoresis, purified rHuOVGP1 showed a single band corresponding to the 120–150 kDa size range of human OVGP1. Mass spectrometric analysis of the purified rHuOVGP1 revealed its identity as human oviductin. Immunofluorescence showed the binding of rHuOVGP1 to different regions of human sperm cell surfaces in various degrees of intensity. Prior treatment of sperm with 1% Triton X-100 altered the immunostaining pattern of rHuOVGP1 with an intense immunostaining over the equatorial segment and post-acrosomal region as well as along the length of the tail. Addition of rHuOVGP1 in the capacitating medium further enhanced tyrosine phosphorylation of sperm proteins in a time-dependent manner. After 4-h incubation in the presence of rHuOVGP1, the number of acrosome-reacted sperm induced by calcium ionophore significantly increased. The successful production of rHuOVGP1 can now facilitate the study of the role of human OVGP1 in fertilization and early embryo development.

Recombinant hamster oviductin is biologically active and exerts positive effects on sperm functions and sperm-oocyte binding

Studies carried out in several mammalian species suggest that oviductin, also known as oviduct-specific glycoprotein or OVGP1, plays a key role in sperm capacitation, fertilization, and development of early embryos. In the present study, we used recombinant DNA technology to produce, for the first time, recombinant hamster OVGP1 (rHamOVGP1) in human embryonic kidney 293 (HEK293) cells. rHamOVGP1 secreted in the culture medium was purified by affinity chromatography. The resulting protein migrated as a poly-dispersed band of 160-350 kDa on SDS-PAGE corresponding to the molecular mass of the native HamOVGP1. Subsequent mass spectrometric analysis of the purified rHamOVGP1 confirmed its identity as HamOVGP1. Immunocytochemistry demonstrated binding of rHamOVGP1 to the mid-piece and head of hamster sperm and to the zona pellucida (ZP) of ovarian oocytes. In vitro functional experiments showed that addition of rHamOVGP1 in the capacitation medium further enhanced tyrosine phosphorylation of two sperm proteins of approximately 75 kDa and 83 kDa in a time-dependent manner. After 3 hours of incubation in the presence of rHamOVGP1, a significant increase in acrosome reaction was measured. Pretreatment of either sperm or oocyte with 20 μg/ml of rHamOVGP1 prior to sperm-egg binding assay significantly increased the number of sperm bound to the ZP. Addition of rHamOVGP1 in the medium during sperm-egg binding with either oocyte or sperm pretreated with rHamOVGP1 also saw an increase in the number of sperm bound to ZP. In all experimental conditions, the effect of rHamOVGP1 on sperm-oocyte binding was negated by the addition of monoclonal anti-HamOVGP1 antibody. The successful production and purification of a biologically active rHamOVGP1 will allow further exploration of the function of this glycoprotein in reproductive function.

Oviductal secretion and gamete interaction

Experimental evidence from the last 30 years supports the fact that the oviduct is involved in the modulation of the reproductive process in eutherian mammals. Oviductal secretion contains molecules that contribute to regulation of gamete function, gamete interaction, and the early stages of embryo development. The oviductal environment would act as a sperm reservoir, maintaining sperm viability, and modulating the subpopulation of spermatozoa that initiates the capacitation process. It could also contribute to prevent the premature acrosome reaction and to reduce polyspermy. Many studies have reported the beneficial effects of the oviductal environment on fertilization and on the first stages of embryo development. Some oviductal factors have been identified in different mammalian species. The effects of oviductal secretion on the reproductive process could be thought to result from the dynamic combined action (inhibitory or stimulatory) of multiple factors present in the oviductal lumen at different stages of the ovulatory cycle and in the presence of gametes or embryos. It could be hypothesized that the absence of a given molecule would not affect fertility as its action could be compensated by another factor with similar functions. However, any alteration in this balance could affect certain events of the reproductive process and could perhaps impair fertility. Thus, the complexity of the reproductive process warrants a continuous research effort to unveil the mechanisms and factors behind its regulation in the oviductal microenvironment.

Sequence analysis of feline oviductin and its expression during the estrous cycle in the domestic cat (Felis catus)

Oviductins belong to a family of oviduct-specific glycoproteins believed to play an important role in fertilization and/or early embryonic development. Oviductin cDNA between species is highly conserved and shares 58% to 98% similarity in the deduced amino acid sequences. Our objective in this study was to sequence the full open reading frame of the feline oviductin and to examine its expression during the estrous cycle on both mRNA and protein level. The obtained cDNA containing the full open reading frame was determined to be 1677 nucleotides coding for a deduced protein of 558 amino acids. Identities between species range from 74% (mouse) to 80% (human, baboon, and rhesus) within the N-terminal protein region. Major differences were localized in the carboxy terminal region, which corresponds to exon 11 of the gene. Feline oviductin contained one putative N-linked glycosylation site, six O-linked glycosylation sites, a potential heparin binding site, and two cholesterol recognition and/or interaction amino acid consensus (CRAC) domains. Oviductin expression was analyzed by real time quantitative polymerase chain reaction (RT-qPCR) and immunohistochemistry. Both approaches revealed an estrous cycle-dependent expression in the ampulla and isthmus. Quantitative PCR showed highest oviductin mRNA copy numbers in the early and late follicular stage and reduced mRNA expression during all other stages. With the exception of the early follicular stage, feline oviductin mRNA abundance was not significantly different in the oviductal segments ampulla and isthmus. A prominent immunolabeling was seen in the early and late follicular stage which disappeared after ovulation, indicating a function of the protein during sperm storage and fertilization.

Effect of oviductal proteins on structural and functional characteristics of cryopreserved sperm in Murrah buffaloes

The present study was undertaken to elucidate the effect of non-luteal oviductal proteins on sperm characteristics in Murrah buffaloes. Oviducts from healthy buffaloes were collected immediately after slaughter and the oestrous cycle phase was determined as either luteal or non-luteal based on ovarian morphology. Non-luteal oviducts (n = 80) were flushed from the isthmic end of the oviduct with PBS, fluid was centrifuged at 10,000 g at 4 degrees C for 20 min and then dialysed and clarified. The supernatant obtained was lyophilized to concentrate the protein and stored at -20 degrees C till use. Sixteen good quality ejaculates from four Murrah buffalo bulls were collected using an artificial vagina. After fresh semen analysis, each ejaculate was split into two parts and extended in Tris-citrate-egg yolk glycerol dilutor. Part I of the split ejaculate was treated with non-luteal oviductal proteins at the dose rate of 1 mg/ml of diluted semen, while part II remained as control. The extended semen was equilibrated for 4 h at 5 degrees C, filled in 0.5 ml French straws, exposed to LN(2) vapour, plunged into LN(2) and then stored at -196 degrees C. The equilibrated and frozen-thawed semen was evaluated for sperm motility, viability, acrosomal integrity, cervical mucus penetration test and hypo-osmotic sperm swelling test (HOST). In frozen-thawed semen, the percentage of sperm motility, viability and acrosomal integrity was significantly (p < 0.05) higher in the treatment group compared to the control group. The incorporation of non-luteal oviductal proteins in the extender increased the ability of sperm to penetrate cervical mucus both after equilibration and the freeze-thaw process. Similarly, the proportion of sperm with intact plasma membrane, as revealed by HOST values, was also significantly (p < 0.05) higher in the treatment group (32.6%) than the control group (27%) in frozen-thawed semen. It was inferred that incorporation of non-luteal whole oviductal fluid proteins improved the sperm quality in frozen-thawed semen in Murrah buffaloes.

Oviduct-specific glycoprotein and heparin modulate sperm-zona pellucida interaction during fertilization and contribute to the control of polyspermy

Polyspermy is an important anomaly of fertilization in placental mammals, causing premature death of the embryo. It is especially frequent under in vitro conditions, complicating the successful generation of viable embryos. A block to polyspermy develops as a result of changes after sperm entry (i.e., cortical granule exocytosis). However, additional factors may play an important role in regulating polyspermy by acting on gametes before sperm-oocyte interaction. Most studies have used rodents as models, but ungulates may differ in mechanisms preventing polyspermy. We hypothesize that zona pellucida (ZP) changes during transit of the oocyte along the oviductal ampulla modulate the interaction with spermatozoa, contributing to the regulation of polyspermy. We report here that periovulatory oviductal fluid (OF) from sows and heifers increases (both, con- and heterospecifically) ZP resistance to digestion with pronase (a parameter commonly used to measure the block to polyspermy), changing from digestion times of approximately 1 min (pig) or 2 min (cattle) to 45 min (pig) or several hours (cattle). Exposure of oocytes to OF increases monospermy after in vitro fertilization in both species, and in pigs, sperm-ZP binding decreases. The resistance of OF-exposed oocytes to pronase was abolished by exposure to heparin-depleted medium; in a medium with heparin it was not altered. Proteomic analysis of the content released in the heparin-depleted medium after removal of OF-exposed oocytes allowed the isolation and identification of oviduct-specific glycoprotein. Thus, an oviduct-specific glycoprotein-heparin protein complex seems to be responsible for ZP changes in the oviduct before fertilization, affecting sperm binding and contributing to the regulation of polyspermy.

Murine SPAM1 is secreted by the estrous uterus and oviduct in a form that can bind to sperm during capacitation: acquisition enhances hyaluronic acid-binding ability and cumulus dispersal efficiency

Sperm uptake of epididymal sperm adhesion molecule 1 (SPAM1) in vitro has recently been shown to be a marker of sperm maturation, since acquisition of this surface hyaluronidase increases cumulus dispersal efficiency. Here, we demonstrate that this glycosyl phosphatidylinositol-linked sperm antigen, previously shown to be expressed during estrous in the female reproductive tract, is secreted in the uterine and oviductal fluids (ULF and OF respectively) in a 67 kDa form, which can bind to sperm. We show that it can be acquired by caudal sperm from Spam1 null, Spam1-deficient mutant, and wild-type (WT) mice in vitro during incubation in ULF or OF at 37 degrees C, as detected by immunocytochemistry and flow cytometry. SPAM1 binding after ULF incubation was localized predominantly to the acrosome and the mid-piece of the flagella of Spam1 null sperm in a pattern identical to that of WT sperm. After ULF incubation, WT sperm demonstrated a significantly (P<0.001) enhanced hyaluronic acid-binding ability, and the involvement of SPAM1 in this activity was shown by a significant (P<0.001) decrease in binding when sperm were exposed to SPAM1 antiserum-inhibited ULF. Importantly, when Spam1 null sperm were exposed to ULF with SPAM1 accessible (in the presence of pre-immune serum) or inaccessible (in the presence of SPAM1 antiserum) for uptake, there was a significant difference in cumulus dispersal efficiency. Taken together, these results suggest that in the sperm surface remodeling that occurs prior to and during capacitation, the fertilizing competence of sperm is increased via acquisition of SPAM1, and likely other hyaluronidases, from the female tract.

Sperm binding glycoprotein (SBG) produces calcium and bicarbonate dependent alteration of acrosome morphology and protein tyrosine phosphorylation on boar sperm

The oviduct is a dynamic organ which modulates gamete physiology. Two subpopulations of sperm have been described in the oviduct of sows, a majority with normal appearance in the deep furrows and a minority, centrally located, and showing damaged membranes. Sperm-oviduct interaction provides the formation of a sperm storage and allows the selection of sperm with certain qualities. Pig (Sus scrofa) oviductal sperm binding glycoprotein (SBG) binds to sperm and exposes Gal beta1-3GalNAc. This disaccharide may be recognized by boar spermadhesin AQN1, which seems to be involved in sperm interaction with the oviduct. SBG is present at the apical surface of the epithelial cells that surround the lumen of the oviduct rather than at the bottom of the crypts. These characteristics imply it could be involved in sperm interaction with this organ. In this study, we evaluate the effect of SBG over boar sperm. We show that the presence of SBG produces alterations of the acrosome morphology of sperm only when they are incubated in capacitating conditions. SBG binds to the periacrosomal region of sperm undergoing capacitation. Its presence induces an increase on the tyrosine-phosphorylation of a polypeptide of apparent molecular mass 97 kDa, as occurs with a 95 kDa protein in other mammalian sperm upon acrosomic reaction. Altogether, these results suggest that SBG might be involved in sperm selection by alteration of the acrosome of sperm that have already begun the capacitation process when they arrive to the oviduct.

Equine embryology: An inventory of unanswered questions

Carl Hartman's title of 47 years ago is invoked in tribute to his first recovery of a bovine embryo 30 years before that, and his legacy of an emphasis on the value of descriptive and comparative studies in reproductive biology. The horse's qualification as a farm animal has waned since those times but, in a conference understandably dominated by research in ruminants and pigs, there are lessons to be learned from some peculiarities of equine embryonic development. Morphological and physiological features of the conceptus and its interaction with its environment during the first month of pregnancy are described and discussed, with emphasis on conceptus expansion, experimental study of the capsule and its associated proteins, and steroid production and metabolism by the various tissues within the conceptus. It is also suggested that easy access to entire conceptuses at advanced stages of development in horses offers valuable opportunities for comparative investigation of early organogenesis and fetal membrane differentiation and, possibly, how they are affected by embryo manipulation in vitro.