L1 RDA Intro to the RDA Block Module and Germ cells

Overview

The lecture opens the Reproduction, Development and Ageing (RDA) module by mapping its lectures, lab practicals and integrated cases, then covers germ cells in three parts: sperm (testicular anatomy, sperm structure, the three phases of sperm development, the spermatogenic cycle and wave, spermiation, and factors affecting sperm output), oocytes (origin of germ cells, ovarian reserve and its lifelong depletion, folliculogenesis, oogenesis and oocyte maturation), and fertilisation (sperm transport through the female tract, capacitation, and the events in the ampulla). The unifying contrast is that male gamete production is continuous and unlimited while female gamete production halts at birth from a fixed pool.

Module structure

Two streams run through the module, Reproduction/Development and Ageing, with lectures matched to lab practicals and integrated clinical cases.

Reproduction and development lectures: germ cells (Michael Pankhurst); child development (Paul Trani); growth (Ben Wheeler); anatomy of male and female reproductive systems (Latika Samalia); testicular endocrinology (Mike Pankhurst); ovarian and menstrual endocrinology (Mike Pankhurst); puberty (Ben Wheeler); fertilisation and implantation (Kanchana Subasinghe); contraception (Brenda Stebbings); assessment and treatment of fertility (Natalie Burger); placenta (Kanchana Subasinghe); abortion (Sarah Bush/Charlene Rapsey/Karen Layton/Helen Paterson); prenatal development and teratology (Megan Wilson); heart development (Megan Wilson); respiratory and GI development (Megan Wilson); urogenital development (Megan Wilson); adaptations of pregnancy (Greg Anderson); transition at birth (Jason Wister); physiology of parturition (Pauline Dawson); lactation (Greg Anderson).

Ageing lectures: ageing and problems of mobility (Lara Vlietstra); menopause (Brenda Stebbings, Charlene Rapsey); stroke recovery (Leigh Hale); prostate disease clinical (John Nacey, recorded); incontinence (Mark Weatherall); combined systems failure (Marie McLaughlin, Noelyn Hung); iatrogenic disease (Xaviour Walker).

Lab practicals: male reproductive anatomy; female reproductive anatomy; male reproductive histology; menstrual cycle histology; placenta; early development.

Integrated cases: Case 20 faltering growth; breast cancer; Case 23 infertility and pregnancy; sexuality in medicine; Case 24 sexual health and cancer of the cervix; cervical neoplasia; prostate cancer; Case 25 confusion.

Relevant vertical module lectures: Tamariki Hauora/child health (Esther Willing); childhood abuse (Paul Trani); Tangihanga, death and end of life care (Esther Willing); ethical issues of pregnancy and birth (Josephine Johnston); sex hormones (John Ashton); assisted reproduction (Josephine Johnston); sexually transmitted infections (Patricia Priest); sexuality through the lifespan (Charlene Rapsey); sexual response and sexual difficulties (Charlene Rapsey); sexually transmitted infections (Bruce Russell); sexuality and illness (Charlene Rapsey); mental health and ageing (Yoram Barak).

Module conveners are Dr Michael Pankhurst and Dr Charlene Rapsey.

Learning objectives of the germ cell lecture

• Describe the structure of spermatozoa.
• Describe the stages of sperm development.
• Understand the temporo-spatial spermatogenic cycle.
• Understand the loss of germ cells throughout female life.
• Outline the stages of follicle and oocyte development.
• Explain how oocyte maturation contributes to embryo development.
• Explain how fertilisation occurs in the salpinx.

Testicular versus ovarian reproductive function

Feature	Testicular	Ovarian
Release of gametes	Millions/day, continuous	~1/month, episodic
Mitotic/meiotic replication	Continuous	Halts at birth
Gamete production	Unlimited	Fixed number
Hormone production	Continuous	Episodic
Reproductive tract role	Transport gametes	Transport gametes; support conceptus

Testis anatomy and sperm structure

• Sperm develop in the seminiferous tubule. The earliest stages sit against the basement membrane and later stages sit in the adluminal compartment.
• Hormone production comes from Leydig cells (interstitial, outside the tubule) and Sertoli cells (lining the tubule).
• Gross arrangement: seminiferous tubules coiled within lobules separated by septa, draining via the rete testis and efferent ductules into the epididymis, then the ductus deferens.
• In cross-section the germ cell layers run from basement membrane inwards: spermatogonium, primary spermatocyte, spermatid, spermatozoa in the lumen. Leydig cells lie in the interstitial space between tubules.
• Sertoli cells are hard to see in a standard section. Ligating the efferent ducts empties the lumen of sperm, and the radiating Sertoli cell cytoplasmic processes spanning from the tubule wall to the centre become visible.

Sperm structure:

• Head: condensed nucleus, plus the acrosome, which contains glycoproteins and enzymes for penetration of the ovum.
• Flagellum: midpiece (contains mitochondria) and tail. A centriole sits between head and midpiece.

Blood-testis barrier

• Occludin and claudin junctions form the blood-testis barrier (BTB), which separates the basal from the adluminal compartment of the Sertoli cell.
• Basal compartment (on the basement membrane, adjacent to the tunica propria/peritubular myoid cells and Leydig cells) contains spermatogonia and preleptotene spermatocytes.
• The BTB lies between the preleptotene and pachytene spermatocyte. The adluminal compartment contains pachytene spermatocytes, round spermatids, elongating and elongated spermatids progressing toward the lumen.
• The slide poses two questions for the audience rather than answering them: why spermatocytes need to be cordoned off from blood, and what event happens after spermatocytes cross the BTB. From the rest of the lecture, meiosis is what occurs after crossing into the adluminal compartment. [slide does not elaborate on the immunological rationale]

The three phases of sperm development

Overall pathway: spermatogonia type A -> (mitosis) -> spermatogonia type B -> (differentiation) -> primary spermatocytes -> (migrate to adluminal compartment) -> first meiotic division -> secondary spermatocytes -> second meiotic division -> spermatids -> (spermiogenesis) -> spermatozoa.

1. Spermatocytogenesis (mitotic proliferation of germ cells, basal compartment)

• Begins with spermatogonial stem cells: Adark converting to Apale.
• Mitotic division of spermatogonia is very limited before puberty.
• Anticancer drugs can damage the DNA of proliferative spermatogonial stem cells.
• Several mitotic divisions occur, the number varying by species, and all products are identical (clonally related). The lineage runs A1 (n=1) -> A2 (n=2) -> A3 (n=4) -> A4 (n=8) -> intermediate spermatogonia (n=16) -> B spermatogonia (n=32) -> primary spermatocytes (n=64).
• Cell division is incomplete, so germ cells remain connected by cytoplasmic bridges and separate only when released into the lumen. In some cases complete cell division allows retention of some cells as stem cells.
• The primary spermatocyte replicates its DNA in preparation for meiosis and migrates to the adluminal compartment.

2. Meiotic division (adluminal compartment)

• Two nuclear divisions. Meiosis I creates genetic variation and produces secondary spermatocytes. Meiosis II produces four haploid spermatids.
• Function: generates genetic diversity and halves the chromosome number.

3. Spermiogenesis (remodelling and packaging for delivery to the oocyte, adluminal compartment)

• Testosterone is required for spermiogenesis.
• Major features: formation of the acrosome, development of the flagellum, chromatin condensation and nuclear remodelling, loss of cytoplasm (shed as residual bodies).
• Detail of the seven-step sequence: nuclear condensation compacts the nucleus; the Golgi apparatus forms a cap-like acrosome resembling a lysosome, containing enzymes to break down the zona pellucida; the flagellum is formed by a centriole, its axoneme containing nine outer doublet microtubules plus two single central ones, with the midpiece housing the mitochondria; cytoplasmic reduction sheds components no longer needed.

Spermatogenic cycle and wave

• Spermatogonial stem cells enter the spermatogenic cycle every 16 days. The regulation of this timing is unclear.
• The spermatogenic cycle takes 64 to 74 days, that is 4 to 4.6 cycles depending on when a spermatogonium is said to be differentiated and committed to spermatogenesis.
• Successive generations overlap, each beginning about 16 days after the previous one, so one region contains several generations at once running through cellular association stages I to VI (primary spermatocyte substages preleptotene, leptotene, zygotene, pachytene, diplotene; spermatid substages Sa, Sb1, Sb2, Sc, Sd1, Sd2).
• In humans, multiple different stages (cellular associations) are seen in a single seminiferous tubule cross-section, arranged as an irregular mosaic of patches (stages I to VI) along the tubule.
• In the rat, 14 sequential stages (I to XIV) are arranged as a smooth spiral wave along the tubule length, so a cross-section contains a single uniform stage.
• The spermatogenic wave ensures there is always sperm being released at some locations across the tubule.

Spermiation and maturation

• Spermiation is the release of sperm from the Sertoli cell into the lumen of the seminiferous tubule.
• It leaves behind the majority of the cytoplasm as a residual body.
• Sperm released from the testis are immature and incapable of movement or of binding and fertilising an oocyte.
• They require several days to mature in the epididymis.

Factors affecting sperm production

• For optimal spermatogenesis, testicular temperature must be 33 to 35 degrees C.
• Ambient temperature data (10 802 men, China): both total sperm number and progressively motile sperm count follow an inverted-U against air temperature exposure, peaking at around 13 degrees C (0% change) and falling increasingly at both the cold end (~5 degrees C) and the hot end (~30 degrees C), p-nonlinear < 0.001 for both.
• Average sperm concentration has declined between 1973 and 2011, shown by a downward-sloping trend across 185 studies of 42 935 individuals from North America, Europe, Australia, New Zealand, South America, Asia and Africa. North America, Europe, Australia and New Zealand were mostly responsible for the decline.
• Methodological limitations of that analysis: combining old data collected for other purposes, and variation in counting and sampling methods between labs.

Warning

The temperature slide places photographs of loose boxer shorts and tight briefs beside the graphs without any explanatory text linking them to the data, so the intended relationship is inferred rather than stated. A slide on the sperm corkscrew swimming motion (Gadelha et al. 2020, with a linked retraction notice) contains only an embedded video that renders as a black frame, so its content is not available.

Origin of germ cells

Primordial germ cells (PGCs) reach the gonad by migration in the embryo, shown in four sequential stages in the mouse:

1. Migration into the endoderm.
2. Migration along the endoderm (future hindgut).
3. Migration into the dorsal mesentery.
4. Migration into the genital ridges.

Ovarian reserve and germ cell loss

• Ovarian reserve is the number of primordial follicles remaining in the ovary. A primordial follicle consists of an oocyte with a surrounding follicular cell.
• Germ cell numbers peak at about 7 million around 5 months post conception, during oogonial proliferation, meiotic prophase and formation of primordial follicles.
• A steep perinatal decline through folliculogenesis and atresia leaves only 400 000 oocytes present in follicles at birth.
• From puberty to menopause only about 400 oocytes will be ovulated, so roughly 0.1% of follicles ovulate an egg; the rest are lost to atresia.
• Histology of ovarian cortex at 5, 19, 33 and 40 years shows the band of primordial follicles at the cortical edge narrowing progressively with age.
• Depletion of primordial follicles causes menopause.

Folliculogenesis

• Primordial follicles are dormant and the supply is finite, but they are always leaving dormancy, so the process is continuous.
• Sequence: ovigerous cords -> primordial follicle -> primary follicle (granulosa cells, theca, oocyte) -> secondary/preantral follicle -> antral follicle (antrum forms) -> preovulatory follicle (cumulus, mural granulosa, theca) -> ovulation -> corpus luteum (luteinised granulosa and theca cells).
• Hormone dependence rises with stage: primary, secondary and small antral follicles can survive without hormones; medium to large antral follicles require FSH to survive; large preovulatory follicles require LH to survive.
• Steroid production: theca makes androgen, granulosa cells convert that androgen to estrogen.
• FSH and LH are given to patients in assisted reproduction to promote maturation of additional follicles, enabling multiple oocytes to be obtained.

Oogenesis and oocyte maturation

• Meiotic sequence: oogonium -> (mitosis) -> primary oocyte -> (meiosis I, then arrest) -> secondary oocyte -> (meiosis II) -> fertilisation (ovum) -> zygote, with polar bodies extruded after meiosis I and after meiosis II/fertilisation.
• This meiotic sequence runs in parallel with follicular development, so each oocyte meiotic stage maps onto a follicle stage, ending with the cumulus-oocyte complex that the sperm meets at ovulation.
• Oocyte maturation comprises: growth, energy storage, mitochondrial expansion, zona pellucida formation, sperm-processing machinery, pronucleus-formation machinery, embryonic mRNA storage.
• The zona pellucida forms during folliculogenesis.
• Nuclear landmarks: the germinal vesicle (GV) is the oocyte nuclear membrane and is present before ovulation; germinal vesicle breakdown (GVBD) is its dissolution; the oocyte then reaches metaphase of meiosis II (MII), with a visible polar body, and meiosis halts there until fertilised by sperm.

Oocytes store material for embryos

• Early embryonic protein synthesis is from maternal mRNA.
• The embryo slowly turns on its own genome, so maternal transcripts decline while zygotic transcripts rise and the two curves cross over as development proceeds.
• Development runs across days 0 to 5: mature oocyte, fertilised oocyte (2PN), 2 cell embryo, 4 cell embryo, 8 cell embryo, morula, blastocyst.

Ovulation, corpus luteum and ovarian structures

• Ovulation and fertilisation occur in the uterine tube, also called the salpinx or oviduct.
• At ovulation the oocyte is expelled along with cumulus mass cells (corona radiata).
• Completion of the second meiotic division is triggered by sperm cell penetration.
• The remaining granulosa cells divide rapidly and enlarge to form the corpus luteum.
• The corpus albicans is a degenerated corpus luteum.
• Sequence seen in micrographs at ovulation and fertilisation: ovulation of a cumulus-oocyte complex (COC) -> cumulus expansion -> fertilisation (many sperm surrounding the oocyte) -> zygote with 2 pronuclei.
• Ovarian cross-section structures to recognise: ovarian epithelium, tunica albuginea, cortex and medulla, blood vessels; primordial follicles, primary follicles (oocyte, granulosa cells, zona pellucida), secondary follicle (vesicles, zona pellucida, theca interna, theca externa), mature (graafian) follicle (zona pellucida, oocyte, corona radiata, cumulus mass, antrum, theca interna, theca externa), degenerating follicle, corpus luteum and corpus albicans.

Warning

Several histology micrographs in this lecture carry single-letter labels that the slides never define: T, G and B on the corpus luteum section, and F, M, L, O and CL on the monkey and human ovary sections.

Social egg freezing

The lecture presents two news items as a discussion prompt and asks what issues about social egg freezing would concern you:

• Apple and Facebook offering to freeze eggs for female employees (2014), with Facebook paying up to $20 000 and Apple providing the perk from January, in an effort to attract more women.
• Freezing ovarian tissue may delay onset of menopause (2019), a procedure that could help women combat menopausal complications such as osteoporosis and cardiovascular disease.

Sperm transport in the female reproductive tract

1. Vagina/cervix: semen deposition.
2. Cervical canal, sperm swimming. Progress is impeded by cervical mucus, folds in the mucosal epithelium and an acidic environment, so progress is slow.
3. Uterine cavity, fluid movement. Muscular contractions of the uterus give rapid progress.
4. Uterine tube, sperm swimming. Sperm capacitation occurs here; mucosal folds thin out the sperm; cilia of the uterine tubes move fluid. It takes 6 to 12 hours for sperm to reach the uterine tube, plus a further 6 to 12 hours to reach the oocyte in the ampulla.
5. Uterine tube, ampulla: the typical fertilisation site, where oocyte chemotaxis signals attract sperm.

Capacitation and fertilisation in the ampulla

• Capacitation is biochemical activation of sperm that increases swimming speed, modifies surface proteins and starts the acrosome reaction.
• It occurs when sperm reach the uterine tube, where the biochemical environment induces intracellular changes in sperm Ca2+ and cyclic AMP.
• Flagellar movement becomes pronounced (hyperactivation), the trajectory changing from a looping pre-capacitation trace to a vigorous whip-like one; raised intracellular Ca2+ and cAMP are required.
• Just prior to fertilisation, the ampulla contains one ovulated oocyte (kakano), surrounded by cumulus cells and still at metaphase 2, and a few thousand sperm (tatea) out of the roughly 300 million in the ejaculate.
• Structures at the point of sperm-oocyte contact: metaphase spindle and chromosomes, oocyte plasma membrane, polar body 1, cortical granules, perivitelline space, zona, cumulus cells, and acrosome-reacted sperm at the zona.

Further reading

• Neal JM. Pituitary and Hypothalamus, in How the Endocrine System Works, 2nd ed.
• Ganong’s Review of Medical Physiology, 26e, Section III: Endocrine and Reproductive Physiology.
• Johnson and Everitt, Essential Reproduction, 8th ed., Wiley-Blackwell, 2018.
• Young et al., Wheater’s Functional Histology: A Text and Colour Atlas, 6th ed., 2006.
• Scientific review article: https://academic.oup.com/biolreprod/article/106/4/644/6548414

Self-test

1. Distinguish testicular from ovarian reproductive function across gamete release, mitotic/meiotic replication, gamete number and hormone production.
2. Describe the structure of a spermatozoon and state what the acrosome contains and does.
3. Explain where in the seminiferous tubule the earliest and latest germ cell stages sit, and name the two hormone-producing cell types of the testis.
4. Explain why Sertoli cells are difficult to see in a normal tubule section and describe the experimental manipulation that reveals them.
5. Describe the blood-testis barrier: what forms it, what it separates, and which germ cell stages lie on each side.
6. List the three phases of sperm development, giving the compartment and the achievement of each.
7. Describe the cell sequence from type A spermatogonium through to spermatozoon, naming the process at each transition.
8. Explain what incomplete cell division produces during spermatocytogenesis, when that connection is broken, and how stem cells are nonetheless retained.
9. Predict which germ cell population is most vulnerable to DNA-damaging anticancer drugs, and explain why prepubertal testis differs.
10. State the hormone required for spermiogenesis and list the four major structural changes of spermiogenesis.
11. Describe the axoneme structure of the sperm flagellum and say where the mitochondria sit.
12. State how often spermatogonial stem cells enter the spermatogenic cycle and how long the cycle takes.
13. Distinguish the arrangement of spermatogenic stages in the human tubule from that in the rat, and explain the functional consequence of the human pattern.
14. Define spermiation and explain why sperm leaving the testis cannot yet fertilise an oocyte.
15. What is the optimal testicular temperature for spermatogenesis, and what does the ambient temperature study show about sperm number and motility?
16. Describe the trend in average sperm concentration since 1973 and give two methodological limitations of the evidence.
17. Describe the four stages of primordial germ cell migration in the embryo.
18. Define ovarian reserve and give the germ cell number at its peak, the oocyte number at birth, and the number ovulated across reproductive life.
19. Explain what causes menopause in terms of the follicle pool, and describe the histological change seen between ages 5 and 40.
20. Describe the hormone dependence of each follicle stage, and explain why FSH and LH are given in assisted reproduction.
21. Distinguish the steroidogenic roles of theca and granulosa cells.
22. Describe oogenesis from oogonium to zygote, stating where meiosis arrests and what triggers completion of the second division.
23. List the components of oocyte maturation.
24. Distinguish GV, GVBD and MII stages of the oocyte.
25. Explain the maternal to zygotic transition and why the oocyte stores mRNA.
26. Describe what happens to the follicle after ovulation, and name the degenerated end stage.
27. Describe the path of sperm from deposition to the fertilisation site, giving the obstacle or driver at each level and the timings.
28. Define capacitation, state where it occurs, and name the two intracellular second messengers involved.
29. State what is present in the ampulla just prior to fertilisation, with numbers.
30. Integrative: explain how events during oocyte maturation in the ovary determine what the embryo can do in its first days after fertilisation.

Answers

Reveal answers

1. Testicular: millions of gametes released per day, continuously; mitotic/meiotic replication continuous; unlimited gamete production; continuous hormone production; tract transports gametes. Ovarian: about one gamete per month, episodic; replication halts at birth; fixed gamete number; episodic hormone production; tract transports gametes and supports the conceptus.
2. Head containing a condensed nucleus and an acrosome, plus a flagellum with a midpiece and tail; a centriole lies at the head-midpiece junction and mitochondria are in the midpiece. The acrosome contains glycoproteins and enzymes for penetration of the ovum, including enzymes that break down the zona pellucida.
3. Earliest stages lie against the basement membrane and later stages in the adluminal compartment, running spermatogonium, primary spermatocyte, spermatid, spermatozoa in the lumen. Hormones come from Leydig cells (interstitial) and Sertoli cells.
4. In a normal section the lumen is full of sperm and the Sertoli cell processes are obscured; ligation of the efferent ducts dilates and empties the lumen, so the radiating Sertoli cytoplasmic processes spanning the tubule wall to the centre become visible.
5. Occludin and claudin junctions form the BTB, separating basal from adluminal compartments. Basal side: spermatogonia and preleptotene spermatocytes. Adluminal side: pachytene spermatocytes, round, elongating and elongated spermatids. Meiosis follows crossing of the BTB.
6. Spermatocytogenesis, mitotic proliferation of germ cells in the basal compartment. Meiotic division in the adluminal compartment, generating genetic diversity and halving chromosome number. Spermiogenesis in the adluminal compartment, remodelling and packaging the germ cell for delivery to the oocyte.
7. Spermatogonia type A, by mitosis to spermatogonia type B, by differentiation to primary spermatocytes, which migrate to the adluminal compartment, then first meiotic division to secondary spermatocytes, second meiotic division to spermatids, then spermiogenesis to spermatozoa.
8. Incomplete cell division leaves the clonally related germ cells connected by cytoplasmic bridges; they separate when released into the lumen. In some cases complete cell division occurs, allowing retention of some cells as stem cells.
9. Proliferative spermatogonial stem cells, whose DNA anticancer drugs can damage. Before puberty, mitotic division of spermatogonia is very limited.
10. Testosterone is required. The four features are acrosome formation, flagellum development, chromatin condensation with nuclear remodelling, and loss of cytoplasm (shed as residual bodies).
11. The flagellum is formed by a centriole and its axoneme contains nine outer doublet microtubules plus two single central microtubules. Mitochondria sit in the midpiece.
12. Stem cells enter the cycle every 16 days (timing regulation unclear) and the spermatogenic cycle takes 64 to 74 days, that is 4 to 4.6 cycles.
13. Human tubules are a mosaic: several cellular associations (stages I to VI) occur in patches, so one cross-section shows multiple stages. Rat tubules run 14 sequential stages as a spiral wave with a single stage per cross-section. The human wave arrangement ensures sperm is always being released somewhere along the tubule.
14. Spermiation is release of sperm from the Sertoli cell into the tubule lumen, leaving most of the cytoplasm behind as a residual body. Released sperm are immature, incapable of movement or of binding and fertilising an oocyte, and need several days of maturation in the epididymis.
15. Testicular temperature must be 33 to 35 degrees C. Total sperm number and progressively motile sperm count both peak at about 13 degrees C ambient exposure and fall at both colder (~5 degrees C) and hotter (~30 degrees C) extremes, p-nonlinear < 0.001.
16. Average sperm concentration declined from 1973 to 2011 across 185 studies of 42 935 men, with North America, Europe, Australia and New Zealand mostly responsible. Limitations: old data combined that had been collected for other purposes, and variation in counting and sampling methods between labs.
17. Migration into the endoderm; migration along the endoderm (future hindgut); migration into the dorsal mesentery; migration into the genital ridges.
18. Ovarian reserve is the number of primordial follicles remaining in the ovary. Germ cells peak at about 7 million around 5 months post conception, only 400 000 oocytes are present in follicles at birth, and only about 400 are ovulated between puberty and menopause, about 0.1% of follicles.
19. Menopause is caused by depletion of primordial follicles. Cortical sections at 5, 19, 33 and 40 years show the band of primordial follicles at the cortical edge progressively narrowing with density falling.
20. Primary, secondary and small antral follicles survive without hormones; medium to large antral follicles need FSH; large preovulatory follicles need LH. FSH and LH are given in assisted reproduction to promote maturation of additional follicles so multiple oocytes can be obtained.
21. Theca cells make androgen; granulosa cells convert that androgen to estrogen. After ovulation both luteinise to form the corpus luteum.
22. Oogonium divides by mitosis to a primary oocyte, which begins meiosis I and arrests; on resumption it produces a secondary oocyte plus a polar body; meiosis II proceeds to metaphase and arrests until fertilisation, which triggers completion of the second meiotic division and extrusion of the second polar body, giving the ovum and then the zygote.
23. Growth, energy storage, mitochondrial expansion, zona pellucida formation, sperm-processing machinery, pronucleus-formation machinery, and embryonic mRNA storage.
24. GV is the germinal vesicle, the intact oocyte nuclear membrane, present before ovulation. GVBD is germinal vesicle breakdown, dissolution of that membrane. MII is metaphase of meiosis II, with a polar body visible, where meiosis halts until fertilisation.
25. Early embryonic protein synthesis uses maternal mRNA stored in the oocyte; the embryo then slowly turns on its own genome, so maternal transcripts decline as zygotic transcripts rise and the curves cross over during development from mature oocyte through 2, 4 and 8 cell stages, morula and blastocyst over days 0 to 5.
26. The oocyte is expelled with the cumulus mass cells (corona radiata); the remaining granulosa cells divide rapidly and enlarge to form the corpus luteum, which on degeneration becomes the corpus albicans.
27. Semen deposition in the vagina/cervix; through the cervical canal by swimming, slowed by cervical mucus, mucosal folds and an acidic environment; through the uterine cavity rapidly by uterine muscular contractions and fluid movement; into the uterine tube by swimming, with capacitation, thinning of sperm numbers by mucosal folds and ciliary fluid movement, taking 6 to 12 hours to reach the tube; then a further 6 to 12 hours to the oocyte in the ampulla, the typical fertilisation site, aided by oocyte chemotaxis signals.
28. Capacitation is biochemical activation of sperm that increases swimming speed, modifies surface proteins and starts the acrosome reaction. It occurs when sperm reach the uterine tube, and involves increases in intracellular Ca2+ and cyclic AMP, producing hyperactivated flagellar movement.
29. One ovulated oocyte surrounded by cumulus cells and still at metaphase 2, and a few thousand sperm out of the roughly 300 million in the ejaculate.
30. During folliculogenesis the oocyte grows, stores energy substrates and mRNA, expands its mitochondrial population, builds the zona pellucida, and assembles sperm-processing and pronucleus-formation machinery. That stored material is what allows fertilisation to be processed, pronuclei to form, and the first days of protein synthesis to run on maternal transcripts before the embryonic genome is switched on and zygotic transcripts take over.