Embryology 1 and 2 - Introduction and Gametogenesis PDF

Periods of the human body: Medical point of view —> first, second and third trimester Embryological point of view —>period of the egg (from fertilisation to implantation in the uterus mucosa. The egg is called “preimplantation embryo” and it goes from zygote, to morula, to blastocyst. First...

Periods of the human body: Medical point of view —> first, second and third trimester Embryological point of view —>period of the egg (from fertilisation to implantation in the uterus mucosa. The egg is called “preimplantation embryo” and it goes from zygote, to morula, to blastocyst. First week of gestation), period of the embryo (from implantation to the 8th week of development) and period of the fetus (from the 8th week on) However, development is FAR FROM COMPLETED at birth. Maturation is a long process that takes place after birth (es: the branching system of the lungs only fully develops at the 7th year of age). Development continues for the whole life and it includes aging and senescence. Dating of pregnancy (when do we determine when the fetus is due) —> 1. Fertilisation age: we count the weeks to delivery starting from fertilisation (difficult to know the exact moment of fertilisation + eggs may survive 2 days in the ovarian tube and spermatozoa 4 days), in this case pregnancy should last 38 weeks 2. Onset of the last menstrual period the pregnancy should last 40 weeks twoweeksdifference becauseauctiontakes afterindays peace themenstrual of period It is hard to assume the Estimated Due Date (EDD) based on the last menstrual period (LMP) because it is based on the assumption of a regular cycle of 28 days. How do we actually determine the EDD? Crown Rump Length (CRL) —> by measuring this length in the first trimester through an ultrasound we can determine the gestational age of the fetus Teratogen —> chemical, physical or biological agents that alter fetal morphology or function during a specific stage of development Resistant period (weeks 1 or 2) —> either the embryo dies or it survives unaffected Maximum susceptibility period (weeks 3-8, embryonic period) —> organogenetic period, all organs are undergoing morphogenesis (making blueprints of the organs) so it is really dangerous Lower susceptibility period (week 9-38, fetal period) —> the basic plan of organs is formed so a morphological alteration can’t happen. However, functional alterations can still happen (es: mental retardation) Causes of birth defects: Genetics —> chromosomal abnormalities or genes mutations Environmental factors —> drugs, alcohol, viruses, radiations and chemicals Multifactorial inheritance —> genetic and environmental factors together (genetic alterations activated by environmental factors) For 50%-60% of birth defects the cause is unknown A mutation of the gene LMNA (which codes of lamins —> main component of the nucleus membrane) can lead to the development of Hutchinson-Gilford progeria syndrome (accelerated aging, children die at 13) Movie suggestion —> JACK and story of Sam Berns (links in presentation) PHASES OF HUMAN EMBRYOLOGY: Gametogenesis Fertilisation Cleavage Gastrulation (bases for the formation of all the farther tissues) Mbootysitcure Formation of the body plan (morphogenesis —> folding, formation of a tube-within-a tube, formation of organ rudiments) Organogenesis position anatomical The embryo is a very curved object that changes shape in time see e the towards tone exine soiree ysectioni gems  OVERVIEW OF THE MALE GENITAL TRACT a seminal vesicles prostaticunetane prostate a a ream bueborethnegland epididymis.is gang ofcowper notinth scrotum abdomens reavertemperature t isneeneagon he g maturation spermatocytes INSIDE OF THE TESTIS 2 the reach they tract abdominal spermatozoa are madeinits wall WOMEN GENITAL TRACT Fanopiantubes I overiest endometrium www.o vaginaegomix regina GAMETOGENESIS —> process by which gametes are produced. It can be divided in four phases —> 1. Primordial germ cells (PGC) and their migration 2. Increase in the number of primordial germ cells 3. Reduction of chromosomal number by meiosis 4. Structural and functional maturation of egg and sperm GAMETOGENESIS PHASE 1 - formation and migration of PGC The embryonic disc in the 2nd week of gestation is made of two layers. The cells of one of the two layers (epiblast, intraembryonal) become primordial germ cells. Below the disc there is a yolk sack with all the nutrients needed. The PGC then migrate in the wall of the yolk sack (extraembryonal) (3rd week) and then they reenter the embryo and reach the point where the gonads will develop (5th to 6th week). To do this they pass through the dorsal mesentery (which sustains the intestine) and arrive in the posterior abdominal wall (where the gonads will be) Migration is a complex mechanism that requires dynamic rearrangement of the cytoskeleton and changes in the adhesive properties of cells. The cell moves by extending the lamellipodium (actin filaments) which adheres to the extracellular matrix, then the nucleus is moved and the adhesion is detached. Something can go wrong in the process of maturation and migration of PGCs —> tumours Teratomas are a type of germ cell tumours that include components/tissues derived by the 3 embryonic layers. They can form in the gonads or in the extragonadal site. Cells start to differentiate in the wrong places (there may be teeth, hair, bones…). It takes a lot for these tumour to grow (it can take years for them to be detectable).bones this emir eyes gartin G tissues They can be mature, immature or a mix depending on how normal aiggerent the cells look under a microscope. These tumours usually occur in women’s ovaries, men’s testicles and in the tailbone of children (sacrococcygeal teratomas). They may also occur in central nervous system, chest and abdomen. They can be both benign and malignant  theycanthenstay silent yearsona foreverinlife cop gene GAMETOGENESIS PAHSE 2 - increase in the number of PGCs by mitosis This process has a different pacing for males and females: In FEMALES —> the PGCs reach the ovary and then start to proliferate (they become 7 millions by the 5th month). When PGCs are in the ovary they are enveloped by support cells (follicle) and called oogonia. Oogonia then start to decrease over development —> at birth a female has 2 millions oogonia, at puberty 40.000 and then only 400 will be ovulated. By the 5th month all the oogonia enter meiosis I and then stop there (called primary oocytes, surrounded by primordial follicle). They will complete meiosis I only when ovulation will take place and meiosis II when fertilised. In MALES —> PGC in the gonads are called spermatogonia. They start to proliferate and go on till the 6th week, when they become dormant. They will only be reactivated during puberty (testosterone is capable of initiating meiosis I, then there is an intensive proliferation throughout all life —> males are fertile even at old age). GAMETOGENESIS PHASE 3 - reduction in chromosomal number by meiosis Reduction in chromosomes and reassortment of paternal and maternal genetic information (crossing over), formation of viable gametes (that a can be used for fertilisation) (each spermatogonia gives life to 4 spermatozoa, 1 oogonia gives life to one oocyte) Different pacing for males and females MEIOSIS in FEMALES —> synchronous (all in the 5th month), slow pace (1 meiotic division completes at ovulation and may take 50 years, II meiotic division only happens if the oocyte is fertilised). To sustain the egg materials like rRNA and mRNA are stored in the cytoplasm and form cortical granules (will be used if the egg is fertilised to support the first stages of cleavage). Second meiotic division happens with fertilisation. When oogonia start meiosis they become oocytes. Meiosis is accompanied by the development of a follicular complex. MEIOSIS in MALES —> asynchronous (not all spermatogonia start and finish meiosis at the same time). The first meiosis lasts 24 days and the second one takes place immediately after and lasts 8 hours. Spermiogenesis —> change of shape (to allow movement) Spermatogenesis starts with the proliferation of spermatogonia (16 days) and lasts 24 days —> 64 days total (proliferation of spermatogonia + meiosis I + meiosis II + spermatogenesis) A surge of testosterone at puberty leads to maturation of somatic cells (Sertoli cells, responsible for the formation of the seminiferous tubules), proliferation of PGCs and formation of spermatogonia and secondary sex characteristics (voice, hair…) GAMETOGENESIS PHASE 4 - structural and functional maturation of egg and sperm Spermatogenesis —> from spermatogonia to spermatids —> takes place in the wall of the seminiferous tubules (which communicate with the rete testis and the epididymis) of the Testis (where support cells/somatic cells (called Sertoli cells), and spermatogonia can be found) The seminiferous tubules are divided into two compartments —>basal compartment —> spermatogonia still undergoing mitosis and spermatocytes in the initial phases of meiosis —> adluminal compartment —> spermatocytes already undergoing meiosis I and meiosis II as These two compartments are Me separated by the projections of Sertoli cells (support cells) a When spermatogonia enter meiosis I (primary spermatocytes) they become antigenically different to their father and so they need to migrate to the adluminal compartment —> there’s the need for an immunosuppressive environment (that’s why Sertoli cells have to separate them) The maturation of spermatozoa is at different stages in different segments of the seminiferous tubules—> this allows a continuous production of spermatozoa The tubules are connected by interstitial tissue (stroma) in which unicellular endocrine glands can be found (Leydig cells) —> they produce testosterone overview on spend seems BASAL emowe tochange inantigens AD LUMINAL notmotileyet theywillbecomenotice is intheepi again The testis is an immune-privileged organ that protects itself from auto-antigens and the associated detrimental immune responses by forming a blood-testis barrier. Infection and inflammation of the male genital tract can break the immune tolerance and represent a significant cause of male infertility CLINICAL DROP: Disruption of the blood testis barrier due to a trauma exposes blood to antigens mounting the immune response —> sterility. It can also be caused by a leaky barrier Blood-testis barrier —> physical separation between the basal compartment and the adluminal compartment of the seminiferous tubules. It is made of junctional complexes of Sertoli cells. SPERMATOGENESIS recap Type A spermatogonia start mitosis —> some of the resultant spermatogonia continue to reproduce while others become Type B spermatogonia —> type B spermatogonia give rise to primary spermatocytes (still in primary compartment) —> primary spermatocytes start meiosis and migrate towards the adluminal compartment. Then mRNA is synthesised (24 days). Spermatocytes remain interconnected by cytoplasmic bridges —> the end of meiosis I leads to the formation of secondary spermatocytes, which then start meiosis II (8h) —> the end of meiosis II leads to the formation of Spermatids (still connected by cytoplasmic bridges (Sertoli cells), they share a lot of information). cytoskeletonundergoes a hugechange SPERMIOGENESIS From spermatids to spermatozoa —> changes in the morphology of spermatids that allow them to turn into fertilising cells. This process lasts around 24 days Nuclear events of spermiogenesis —> reduction in size and change in the shape of the nucleus (condenses, genetic material becomes packed) and condensation of chromosomal removed serveon material (histones are replaced by protamines) Cytoplasmic events of spermiogenesis —> elimination of cytoplasm (to become lighter) and formation of a head (changes in shape, condensation of the Golgi apparatus at the apical end of the nucleus —> the acrosome, originates at the other side from the centriole. It is a sort of helmet on the head of the spermatids which contains enzymes utile for fertilisation). Formation of a tail in spermiogenesis —> the flagellum originates form the centrioles (microtubules, allow motility). Mitochondria are arranged in the proximal portion of the flagellum (initial part) to provide energy needed for movement Aee thishappensinthewallsoftheseminiferoustubules og The membrane of the head is divided into different antigenic domains (happens during maturation in the male and in the female genital tract) Axoneme —> internal part of the flagellum, made of 9 pairs of microtubules enclosing another pair. to asskeleton The length of a spermatozoa is around 60 μm LOOK AT THE LINK FOR MOTOR PROTEINS Matins Theyprovided movement Defects in the axonal structure causes defects in sperm motility and often leads to male infertility Problems in the motility of cilia cause sperm dysfunction and thus infertility ofciliainthebody distribution Sertoli cells: Maintenance of the blood testis barrier (isolation of the antigenically different haploid germ cells from the male adult immune system) Secretion of the seminiferous tubular fluid (STF, they provide the right nutrients and hormones for spermatogenesis) (10-20 microlit/gram of testis) Secretion of androgen-binding protein (binds to testosterone and helps maintaining a high concentration of it in the testis) Secretion of other proteins —> inhibin for feed-back loop to hypothalamus (avoids a come back from a distant pituitary gland to the hypothalamus), mullerian-inhibiting factor (to prevent female primary sexual characteristic from developing), retinol-binding protein (transport of vitamin A from liver to other tissues) Maintenance and coordination of spermatogenesis Phagocytosis of residual bodies of sperm cells Adjoining segments of the seminiferous tubules present different stages of maturation of spermatozoa —> asynchronous meiosis t Cells linked by bridges are in the same stage. The reappearance of that same association in the same segment is a spermatogenic cycle (64 days) The distance along the tubule between the same stage is called a spermatogenic wave At the end of spermiogenesis and spermiation (release of spermatids in the lumen) mature spermatids/spermatozoa are not yet motile. Spermatozoa are propelled in the epididymis (where they acquire motility) by fluid pressure generated in the seminiferous tubules (STF produced by Sertoli cells and located in the lumen), ciliary movements and contractile cells. For spermatozoa to be functionally mature for fertilisation they must have 3 characteristics Motile Able to recognise the oocyte Able to pass across the membranes of the oocyte When spermatozoa reach the epididymis they stay there for 12 days and then they are stored in the lower part of the epididymis Maturation —> spermatids acquire motility thanks to the stimulation of the Factor Motility protein (FMP). FMP induces tubulin phosphorylation, an increase in Ca++ and an increase in cAMP (cyclic adenosine monophosphate) and this leads to the regulation of flagellar activity. Biochemical maturation —> the plasma membrane of the head of the spermatozoa is coated by glycoproteins The process of maturation doesn’t end in the male genital tract but in the FEMALE genital tract —> reaction of capacitation —> removal of the plasma membrane’s glycoproteins and reorganisation of its lipids and proteins. Capacitation allows the acrosomal reaction (digestion of the zona pellucida, fusion of the sperm membrane with the egg membrane and sinkage of the contents of the sperm’s head into the egg) which enables the spermatozoa to penetrate the zona pellucida of the oocyte rabbit Hormonal regulation of Spermatogenesis ⬇ ️ Hypophysis produces FSH (Follicle Stimulating Hormone) and LH (Luteinizing Hormone). Sertoli cells respond to FSH producing Androgen-Binding Proteins (ABP) and Anti-Mullerian Hormone (AMH) while Leydig cells respond to LH producing testosterone. The androgen-  binding protein binds testosterone and is carried into the fluid compartment of the seminiferous tubules where it exert a strong effect on spermatogenesis. The concentration of testosterone is higher in the adluminal part. Sertoli cells transform some testosterone into estrogens (some estrogens are then carried back in a paracrine fashion to Leydig cells together with a stimulating factor) Inhibin produced by Sertoli cells inhibits FSH secretion by reaching the pituitary gland via the blood stream Causes of male infertility: Less than 10 millions spermatozoa per ml of sperm Motility dysfunction Abnormal spermatozoa Altered genome Medication and drugs Endocrine disorders Environmental disorders Cigarettes Obstruction of genital duct system Male infertility is detectable in 30-50% of involuntary childless couples OOGENESIS externaepertwithpuicees fataiggerentstagesegmannation It includes the maturation of the oocyte u and that of the follicle (which together form a morpho-functional integrated unit). w Follicle = follicle + oocyte Three types of ovarian follicles —> 1. Primordial follicles 2. Growing follicles (primary and secondary) Limpert 3. Mature follicles or Graafian follicles mussels nopeeice.es puimoraiaegoeeiceecmeios.si support PRIMORDIAL FOLLICLES - oogonia + support cells All the oogonia of primordial follicles enter meiosis I give by the 5th month. After they begin meiosis I we call them oocytes. aiometreassoum Follicular cells are somatic cells. They have a layer of squamous granulosa cells (which becomes simple cuboidal granulosa in primary follicles and stratified granulosa cells in growing follicles) The follicular cells communicate with the oocyte through the zona pellucida (gap junctions and microvilli). This is essential to maintain the diplotene stage (meiosis I) Interaction between oocytes and follicles is essential —> The oocyte’s cytoplasm and the follicle’s one have different concentrations of cyclicAMP, a second messenger which inhibits MPF (maturation promoting factor), and cyclicGMP (cyclic guanosine monophosphate), which inhibits a Phosphodiesterase enzyme preventing cAMP from being transformed into 5’AMP (important trigger for meiosis). Follicular cells transport cAMP and cGMP to the oocyte’s cytoplasm thanks to gap junctions and microvilli. Primaryfollicle Each month 5-12 (up to 50) primordial follicles begin folliculogenesis and thanks to this some primordial follicles develop into primary follicles apical PRIMARY FOLLICLES —> The follicle becomes isoprismatic, there is the formation of the zona pellucida with glycoproteins (Zonula Proteins —> ZP 1-4) and glycosaminoglycans (used for basal recognition of spermatozoa) and follicular cells become cuboidal. The zona pellucida is a layer between the apical surface of follicular cells and the plasma membrane of the oocyte. It doesn’t obstruct communication between the two. pongeeiweeraees qgapjunctions proaceabstereogies g oocytematuration inhibitor f monessen environment aeroneyger emerspennana  Some of the primary follicles develop into growing follicles (secondary follicles) GROWING FOLLICLES (secondary follicles) The follicular cells develop into more than one layer and they form a structure called granulosa. The granulosa cells start to berriergromstrome express receptors for FSH (follicle stimulating hormone). The armariesnotbeaking immune system granulosa is supported by a basal lamina called membrana granulosa (barrier against capillaries). Outside of the granulosa the stroma is arranged in the theca folliculi. The theca folliculi is divided into two parts —> 1. Theca interna: highly vascularised and with a glandular appearance (produces estrogens precursor and has LH receptors) 2. Theca externa: protective and supportive envelope of cash oemme fibroblast-like cells. peaigmenema There can be only one follicle that survives —> some growing follicles degenerate and some enlarge by taking up fluids Secondary follicles become pre ANTRAL follicles and ANTRAL follicles Pre ANTRAL (or pre Graafian) follicles —> formation of small intracellular spaces (called Cell-Exner bodies) filled with fluid among granulosa cells ANTRAL (or Graafian) follicles —> the small intracellular spaces get together to form a single large cavity called antrum folliculi. The liquid contained in the antrum is called liquor folliculi and it is rich in hyaluronic acid and proteins. Thanks to hyaluronic acid more fluid is attracted (antrum grows larger and larger). 200μm In the meanwhile FSH receptors on granulosa cells trigger the production of estrogens while LH receptors in the theca interna trigger the production of testosterone. With the formation of the antrum the cells surrounding the oocyte form the cumulus  oophorus while the ones adjacent to the theca interna are the proper granulosa cells. There’s a progressive increase of FSH and LH receptors and hormone synthesis. The granulosa will also develop LH receptors. Testosterone synthesised in the theca interna is transported to the granulosa cells which then, thanks to the aromatase enzyme, transform it into estrogens. Maturation of follicles is divided into a gonadotropin-independent phase, a gonadotropin sensitive/responsive phase (they respond to gonadotropin) and a gonadotropin- dependent phase (if they don’t respond to gonadotropin they die) Of the 50 initial follicles only 3 reach the size of 8mm and eventually one of them becomes FSH independent (MATURE GRAAFIAN FOLLICLE) and starts releasing inhibin, which reduces the secretion of FSH thus causing the death of the other follicles. In the Graafian follicle the cumulus oophorus is multilayered and, at ovulation, it will become the corona radiata matureGroofionfollicle 10-12 hours before ovulation meiosis I resumes following a LH surge (and a smaller FSH surge). The cumulus oophorus cells respond to the LH surge by shutting the gap junctions used to communicate with the oocyte. cGMP decreases in the oocyte so the phosphodiesterase enzyme is activated and cAMP is turned into 5’AMP (which initiates meiosis) and MPF is activated. Meiosis I is completed while Meiosis II begins (and then gets arrested in metaphase). Now the primary oocyte turns into a SECONDARY OOCYTE. The completion of meiosis I leads to the formation of the first polar body in the perivitellin space (between the oolemma and the zona pellucida) The secondary oocyte and its granulosa cells form the TERTIARY FOLLICLE (2cm, super big, it protrudes on the surface of the ovary). Hyaluronic acid and water are formed in the antrum thus creating an increase in pressure. In the meanwhile follicular cells produce estrogens and estradiol to prepare the female genital tract for gametes transport (implantation of a fertilised egg in the uterus). Stigma —> region where, just before ovulation, the surface of the ovary will be digested and the blood flow will be stopped. This will allow the Graafian follicle to release the ovum at ovulation Follicular fluid in the antrum contains proteins similar to the serum but in low concentration, 20 enzymes, dissolved hormones (LSH, FSH, steroids) and negatively charged proteoglycans (attract water thus increasing the size of the antrum) OVULATION (day 14) It takes place around 38h after the LH and FSH surge. It is an inflammatory reaction of the follicle cells and of the theca cells and it leads to the rupture of the outer follicular wall and the ovarian surface. This leads to the loosening of cumulus oophorus cells and the sliding of the ovulated complex out of the ovary. Ovulated complex consists of: Ovum (secondary oocyte) Zona pellucida Corona radiata (part of the cumulus oophorus attached to the oocyte) Sticky matrix (fluid of the antrum cavity containing hyaluronic acid)  lowered Lgr5+ positive stem cells renovate the ovarian germinative epithelium after each ovulation. Lgr5+ —> Leucin-rich-containing G-protein-coupled receptor 5 —> marker of stem cells in many organs —> associated with very malignant ovarian cancer (uncontrolled proliferation) Mittleschmerz (mittle, mid + schmerz, pain) —> abdominal and pelvic pain that accompanies ovulation in some women. This is caused by the enlargement of the follicles before ovulation or by a slight bleeding in the abdominal/peritoneal cavity (after the rupture of the ovarian surface) —> inflammation. What remains in the ovary is the Corpus Luteum —> not part of the ovulation process but still important. Called corpus luteum due to its yellowish color. Corpus luteum —> the Theca and what remains of the granulosa start to collapse after ovulation (forming the corpus haemorrhagicum). The corpus haemorrhagicum then undergoes Luteinization and becomes whitish, increases in size, starts producing hormones (that’s why the color changes) and it becomes rich in vascular network (to secrete hormones). The hormonal production increases after ovulation (progesterone and estrogens) to trigger the development of the endometrium (inner lining of the uterus) for a possible implantation of an embryo. Endometrium —> portion that undergoes changes in the menstrual cycle and is responsible for the bleeding. The corpus luteum is functional for approximately 2 weeks after ovulation. If the oocyte is not fertilised it becomes the menstrual corpus luteum (ceases hormonal production after 10 days and reduces size) and then it turns into corpus albicans (scar tissue). If the oocyte is fertilised it becomes gravidic corpus luteum (remains functional for 5-6 months, after that placenta will produce hormones instead of it). Human chorionic gonadotropin (HCG) is produced by syncytiotrophoblast of the blastocysts and it maintains the luteum functional. graviaic corpusaesicans corpuswamy  Egg capture —> the ovulated complex is captured by the fimbriae of the uterine tube. Coming out of the infundibulum of the Fallopian tube there are protrusions called fimbriae. Fimbriae change their distance from the ovaries depending on the phase of the menstrual cycle. When we are ovulating the fimbriae are close to the ovaries and the ovulated complex slips out and gets captured by them Once the ovulated complex is captured and falls in the infundibulum the contraction of the smooth muscle cells of the uterine walls moves the ovulated complex towards the uterus. The transport happens in two phases —> slow transport: through the ampullary region (72h, gently propelled). Rapid phase from the isthmus to the uterus (8h). The cilia and the smooth muscle cells in the uterine tube aid the movement. section cross smoothmusceesceeesanacilia oftheuterinetube During slow transport oocytes move for a long time in the uterine tube to raise the chances of fertilisation (if a spermatozoa is in the tubes) 8 A blockage of the uterine tube is a major cause of infertility in women. This may be caused by an infection (es STDs), an inflammation or an endometriosis (anomalous presence of endometrium) Sperm transport and maturation Ejaculation —> rapid transit from the epididymis through the ductus deferens and urethra (common tract with the urinary tract) to the vagina. The spermatozoa are contained in a seminal fluid (around 2-6ml with a pH of 7.2-7.8). The seminal fluid contains prostatic secretions (acidic) and seminal vesicle secretions (basic) which aid the process of fertilisation (vesicles contain fructose for spermatozoa energy). Every ejaculation contains around 300 millions spermatozoa and each of them may retain their function (survive) in the female reproductive tract for around 80h. Not all spermatozoa are able to survive —> the environment of the vagina is protective —> 1° barrier: the pH of the vagina is very low (acidic, while spermatozoa’s pH is neutral/basic). The seminal fluid has buffering capacities (it buffers for a short time the environment of the vagina, just few minutes to allow the spermatozoa to reach the cervix of the uterus where pH is optimal for swimming (6-6.5)). The cervix itself has a protection —> 2° barrier: mucous. During ovulation the mucous becomes more fluid to allow the passage of some of the spermatozoa. If there is a fertilised egg already the mucous will be thicker The passage within the cervix is divided in two phases —> 1. Fast phase of passage: some sperm can reach the tube within 5-20 minutes but they are less likely to fertilise the egg because the egg hasn’t been moved in the genital tract yet + the spermatozoa hasn’t had the time to fully mature. The movement in this phase is due to the contraction of the female reproductive tract 2. Slow phase of passage: depends on the swimming of the spermatozoa through the cervical mucous (2-3 mm per hour), some of the spermatozoa might be stored in the cervical crypts so the passage in the cervical canal can happen even 2-4 days later (these spermatozoa are more likely to fertilise the egg because they are completely mature) Only several hundreds reach the uterine tube, mostly on the side of ovulation thanks to some chemical attractions. When they reach the uterine tube they must bind to the initial part (isthmus) for 24h for the capacitation reaction to happen (removal of glycoproteins added in the epididymis and removal of cholesterol that inhibits capacitation). The biochemical changes associated with capacitation cause changes in the plasma membrane which gets hyper polarised (they become much more motile and can swim faster). The membrane becomes more fluid and permeable. Capacitation also includes changes in protein phosphorylation and protein kinase activity, it increases bicarbonate concentration and intracellular pH, Ca++ and cAMP levels. Only capacitated spermatozoa are able to bind to the zona pellucida of the oocyte. The ampulla is the meeting place between the oocyte and the spermatozoa (oocyte us moving slowly) To break free from the Isthmus they have to become hyperactive —> a small bunch of spermatozoa is released at times. Thanks to a combination of smooth muscle contraction and swimming movement (flagellum) the spermatozoa can reach the ampullary portion (where fertilisation happens) Sperm chemotaxis —> taxonomically widespread (spread among all species) phenomenon involving the attraction of sperm toward eggs by egg-derived chemicals. Spermatozoa respond to cumulus-derived progesterone and chemoattractans (released by the oocyte) and to temperature gradients present in the tube (only capacitated spermatozoa can respond to this gradient) isthmus Benespermatozoa capacitated green notcapacitated facie