REPRODUCTIVE BIOLOGY REVIEW
A) FOLLICULOGENESIS
Oogonia multiply in the fetal testis and then enter meiosis, becoming oocytes. Each oocyte is surrounded by a single layer of inactive epithelial cells, forming a primordial follicle. Primordial follicles are quiescent, remaining inactive until puberty at which point a given number of primordial follicles will be recruited to begin development with each follicular wave or estrous cycle.
As primordial follicles begin development, the epithelial cells increase in number by mitosis and take on characteristics of granulosa cells. Gap junctions appear between the granulosa cells and oocyte, and the zona pellucida forms.
The developing follicle is termed a secondary follicle when a layer of thecal cells forms outside the layer of granulosa cells. As pools of fluid form between the granulosa cells and coalesce to form a fluid-filled antrum, the follicle is termed a tertiary follicle. Tertiary follicles also are called antral, Graafian, or pre-ovulatory follicles. In the tertiary follicle, the oocyte within the zona pellicuda surrounded by several layers of granulosa cells (the cumulus cells) protrudes into the antrum, which is lined by antral granulosa cells with outer layers of mural granulosa cells, internal thecal cells and external thecal cells.
B) SPERMATOGENESIS
The testes consist of seminiferous tubules, which contain spermatogonia and Sertoli cells, intervening interstitial (Leydig) cells, and a central rete testis, through which spermatozoa move from the seminiferous tubules to the epididymis. Testosterone, produced by the Leydig cells, drives spermatogenesis. In male animals, testosterone is secreted as pulses and this periodicity is important for continuing spermatogenesis.
Unlike oogonia, which are a fixed population of cells that remain quiescent in meiosis until stimulated to develop, spermatogonia are a continually renewing population of cells. Diploid spermatogonia undergo mitosis to form spermatocytes, which than undergo meoisos to form haploid spermatids. Spermatids than undergo spermiogenesis; during this phase no more cell division occurs but instead the spherical spermatids undergo flattening and elongation of the head, condensation of the nucleus, and formation of the acrosome. Spermatozoa are continuously produced and moved into the epididymis, where physical and chemical changes occur that permit sperm to become motile. Mature spermatozoa are stored in the epididymis. If ejaculation does not occur, aged spermatozoa are resorbed by the epididymal epithelium or expelled into the urinary bladder.
C) ESTROUS CYCLE
1) MONOESTRUS VERSUS POLYESTRUS
Monoestrus = one cycle yearly followed by prolonged anestrus, regardless of pregnancy status
Polyestrus = repeated cycling until interrupted by pregnancy, illness, or season
2) STAGES OF THE ESTROUS CYCLE
- Proestrus - Stage of follicular growth prior to estrus
- Estrus - Stage where female will accept mating by the male ("standing heat")
- Diestrus / Metestrus - Luteal phase of the cycle. Metestrus refers most specifically to the time of development of the corpora lutea (CLs).
- Anestrus - Anestrus is lack of cyclicity. In dogs, a prolonged anestrus is a normal part of the cycle. In cats, anestrus occurs seasonally. Cats also may show anestrus while lactating. Dog and cats do not go through senile anestrus (menopause) although estrous cycles may become more erratic with increasing age. Pathologic causes of anestrus include karyotype or intersex abnormalities, Cushing's disease (hyperadrenocorticism), Addison's disease (hypoadrenocorticism), hypothyroidism, tumors or infections of the reproductive tract, systemic disease, and low body mass index / excessive athleticism.
D) OVULATION, INSEMINATION, AND FERTILIZATION
1) OVULATION
A surge of the gonadotropin luteinizing hormone (LH) induces ovulation in dogs and cats. Dogs are spontaneous ovulators; cats are induced or reflex ovulators. The surge of LH causes a change in hormone production in the follicle from estrogen to progesterone and begins development of the luteal cells. The follicular wall becomes hyperemic as inflammatory mediators move into the tissue. Inflammation and specific enzyme activity cause weakening of the follicular wall such that rupture and expulsion of the oocyte and surrounding cumulus cells can take place without a significant change in intrafollicular pressure.
After ovulation, the follicular cavity fills with blood and the granulosa and thecal cells form luteal cells and begin progesterone production in earnest. This new structure is called the corpus luteum (CL) and one forms at each ovulation sight. Hormones known to support luteal function are LH and prolactin. The hormone most commonly associated with lysis of the CL and cessation of progesterone production is prostaglandin F2alpha.
2) INSEMINATION
Insemination is introduction of semen into the female's reproductive tract. Semen may be deposited into the vagina, at the cervix, or into the uterus. The two types of penis in domestic animals are the musculocavernous type and the fibroelastic type. The former contains honeycombed erectile tissue that fills with blood to effect erection, and is the type found in dogs and cats. The canine penis also contains a bone, the os penis, which permits intromission of the penis before erection is complete.
Ejaculation of spermatozoa permits deposition of semen further cranially in the vagina than the penis can reach and most often occurs while the male is vigorously thrusting. Spermatozoa must traverse the cervix and associated cervical mucous, the uterus, and the utero-tubal junction to reach the uterine tube, which is the sight of fertilization. Sperm transport is aided by contractions of the female's reproductive tract, some of which are induced by presence of the male's penis and hormones present in the semen, and by the mass effect of the motile spermatozoa ejaculated as a group. Spermatozoa bind to the epithelium of the oviduct until an undefined signal causes their release and movement toward the eggs available for fertilization.
3) FERTILIZATION
Oocytes cannot be fertilized until they have extruded two polar bodies (secondary oocytes). Most species ovulate secondary oocytes. Bitches ovulate primary oocytes, which cannot be fertilized until they undergo maturation to secondary oocytes. This takes about 48 hours.
Spermatozoa undergo capacitation in the female's reproductive tract. Capacitation consists of acquision of hypermotility and completion of the acrosome reaction, which is activation of the sac of enzymes on the head of the spermatozoon. Multiple spermatozoa burrow through the cumulus cells and zona pellucida surrounding the oocyte. When the first spermatozoon reaches and binds to the inner vitelline membrane of the oocyte, the "zona reaction" is set off in which the zona hardens, preventing further sperm penetration. The sperm membrane of the successful spermatozoon binds to the vitelline membrane of the oocyte and the sperm and its nucleus are engulfed.
E) MATERNAL RECOGNITION OF PREGNANCY
Species that cycle continuously secrete specific compounds during pregnancy that stop follicular maturation and subsequent estrous cycling. For example, in ruminants this factor is interferon-tau, and in pigs it is estrogen, most likely estrone sulfate. No such product has been identified in bitches or queens.
STUDY AND REVIEW GUIDE
1. What are other names for a tertiary follicle? If a tertiary follicle is present, what hormones is the animal producing?
2. Does frequent collection of semen or use for natural breeding enhance spermatogenesis in dogs? Why or why not?
3. What hormone causes ovulation in dogs? Some women claim they can feel it when they ovulate; why might that be?
4. Regarding movement of spermatozoa, what actions might one choose to take when performing artificial insemination to mimic as well as possible what occurs during natural breeding?
5. What is capacitation? Do you think capacitated spermatozoa have a lengthened or shortened lifespan compared to uncapacitated sperm? Explain your answer.