REPRODUCTION
Asexual reproduction is common in lower animals, but SEXUAL reproduction is the major or only method for higher animals.
Some animals which reproduce sexually are MONOECIOUS (both male and female organs in one individual); however, most higher animals are DIOECIOUS (separate sexes).
Animal reproduction is primarily under hormonal control.
In evolutionary terms, reproductive behavior and physiology are highly plastic. Reproductive responses can be important species isolating mechanisms, and even closely related species may show marked differences.
[see Figure] The differences between males and females are largely due to QUANTITATIVE (not qualitative) differences in the reproductive hormones.
[Fig. 17-1] The gonads play a dual role. They produce the GAMETES (sex cells), but they also are endocrine glands, producing SEX HORMONES, which influence sexual behavior and the development of sexual characteristics.
The gonads, in turn, are largely controlled by anterior pituitary GONADOTROPINS, which are, in turn, controlled by GnRH from the hypothalamus.
MALE REPRODUCTIVE SYSTEM
The paired TESTES are the major organs of the male system. In humans (and most other mammals) the testes descend into an external sac (the SCROTUM). The lower temperatures outside the abdominal cavity are necessary for normal sperm development. In about 1 to 3% of male births, one or more testis fails to descend (= CRYPTORCHIDISM).
It is interesting to note that a significant number of mammalian species (>37%) are naturally ASCROTAL.
The testes produce the male haploid GAMETES (sex cells) known as SPERMATOZOA (aka sperm cells or sperm).
Sperm are produced in enormous numbers. About 300 million are released in a single EJACULATION.
SEMEN (the fluid ejaculated from the PENIS) contains the spermatozoa, but most of the volume is contributed by the secretions of the ACCESSORY GLANDS (e.g. seminal vesicles, prostate, etc.). These secretions serve to provide lubrication, to dilute the sperm and to provide a suitable chemical environment.
[see Figure] The onset of reproductive maturity (PUBERTY) is triggered in both sexes by alterations in brain function leading to increased secretions of GnRH.
[Fig. 17-7] The GERMINAL EPITHELIUM of the SEMINIFEROUS TUBULES within the testes initiates the process of SPERMATOGENESIS. The development of SPERMATOZOA is regulated by SERTOLI CELLS and is influenced by hormones (e.g. TESTOSTERONE and INHIBIN) and by ANDROGEN-BINDING HORMONE produced by the Sertoli cells.
[Fig. 17-6]
[Fig. 17-5] The seminiferous tubules produce the sperm and have a combined length of about 250 m.
[Fig. 17-4] The LEYDIG CELLS produce TESTOSTERONE. The hormone INHIBIN is produced by the SERTOLI CELLS within the tubules. The tight junctions between adjacent Sertoli cells form a BLOOD - TESTIS BARRIER (aka SERTOLI - CELL BARRIER). This helps to maintain the proper chemical conditions in the seminiferous tubules by restricting movement from the blood into the lumen.
[FIG. 17 - 9] Spermatogenesis occurs between adjacent Sertoli cells. The entire process takes on average about 64 days in humans.
[Fig. 17-11] SPERMATOGENESIS is a more or less continuous process throughout the reproductive life (though declining with age). In addition to its role in spermatogenesis, testosterone influences both primary and secondary sexual characteristics and sexual behavior.
Penile ERECTION, during male arousal, is a hydrostatic vascular event. Normally, the three cylindrical vascular compartments of the penis receive little blood (due to vasoconstriction of the small arteries) and the penis is flaccid. During sexual arousal, the sympathetic input contributing to the constriction is reduced and parasympathetic neurons, releasing NITRIC OXIDE as neurotransmitter, dilate the arteries and the penis engorges with blood.
EJACULATION (associated with male ORGASM) is a spinal reflex involving stimulation of the sympathetic nerves innervating the smooth muscles of the duct system.
[Fig. 17-10]
FEMALE REPRODUCTIVE SYSTEM
Female reproductive physiology is CYCLIC, and considerably more complex than the steady state system of the male.
The OVARIES are the major reproductive organs and, like the testes, they produce both gametes and reproductive hormones.
Female reproductive life begins at PUBERTY (i.e. MENARCHE) and ends at MENOPAUSE.
[Fig. 17-15] OOGENESIS (the development of OVA) is identical in many respects to spermatogenesis; however, only a few large gametes are produced. A few million PRIMARY OOCYTES are present (as PRIMORDIAL FOLLICLES) at birth, but only about 400 SECONDARY OOCYTES (in MATURE FOLLICLES) will be produced. The remaining follicles degenerate (ATRESIA).
[Fig. 17-13]
[see Figure] The lining of the UTERUS (ENDOMETRIUM) is hormonally prepared each cycle to receive and support a developing embryo (i.e. BLASTOCYST).
[see Figure] Primordial follicles (consisting of a primary oocyte surrounded by a layer of GRANULOSA CELLS) develop into PRIMARY FOLLICLES and, about once per month, a DOMINANT FOLLICLE is selected and continues to develop into a mature, fluid-filled follicle. During OVULATION, this follicle ruptures and the SECONDARY OOCYTE is released. The remaining cells form the CORPUS LUTEUM, which persists for a time and then regresses.
[Fig. 17-16] The reproductive hormones are produced by the follicle (i.e. THECA and GRANULOSA) cells. NOTE: The second meiotic division (to form an ovum) is not completed until after FERTILIZATION.
[Fig. 17-19] This slide depicts hormonal control during the early and mid follicular phases of the female reproductive cycle.
FSH stimulates the GRANULOSA CELLS and LH the THECA CELLS. ANDROGENS synthesized by the theca cells are converted to ESTROGEN by the granulosa cells.
[Fig. 17-20] Negative feedback control predominates during the early to mid follicular phase. NOTE that Estrogen secretion markedly increases during this period even though FSH is decreasing (This is due, in part, to an up-regulation of FSH receptors on the granulosa cells.
[Fig. 17-21] At HIGH plasma concentrations, estrogen has a POSITIVE feedback effect, leading to an LH surge, which triggers OVULATION.
The high concentrations of PROGESTERONE formed by the CORPUS LUTEUM after ovulation inhibit GnRH secretion and thus FSH and LH secretion during the luteal phase.
[Fig. 17-17] The above slide depicts OVARIAN events during the MENSTRUAL CYCLE. Day one of the cycle is determined by the onset of uterine bleeding (MENSTRUATION), even though this is not an ovarian event.
[Fig. 197-22] Estrogen promotes a thickening and increased vascularization of the ENDOMETRIUM of the UTERUS. This process is enhanced by progesterone. If CONCEPTION does not occur, the hormone levels drop and MENSTRUATION occurs.
[Fig. 17-18]
[see Figure]
[Fig. 17-23]
[see Figure] If COPULATION occurs on the day of ovulation (or within the five preceding days), FERTILIZATION (the fusion of egg and sperm) may occur. This occurs in the OVIDUCT (Fallopian tube), and only a few hundred sperm complete the trip up the female reproductive tract. Sperm must have resided in the tract for several hours before they are capable of fertilization (the process of CAPACITATION).
[see Figure] The fertilized egg (ZYGOTE) completes the second meiotic division and cell division begins, eventually forming a BLASTOCYST, which slowly makes its way to the UTERUS and digests its way into the ENDOMETRIUM (the process of IMPLANTATION) about 7 days after fertilization. CHORIONIC VILLI develop and become surrounded by pools of maternal blood. A structure of part maternal and part fetal origin (the PLACENTA) develops and persists throughout the pregnancy.
[Fig. 17-28] CHORIONIC GONADOTROPIN (CG) maintains the CORPUS LUTEUM during the critical first trimester (i.e. first 3 months) of pregnancy.
Under CG influence, the corpus luteum greatly enlarges and continues secretion of estrogen and progesterone.
[Fig. 17-26]
[see Figure] The PLACENTA secretes a variety of hormones including ESTROGEN and PROGESTERONE.
It supplies oxygen and nutrients to the FETUS and removes carbon dioxide and metabolic wastes. It receives as much as 10% of the total maternal cardiac output.
[Fig. 17-30] PARTURITION (the birth process) begins with LABOR (rhythmic contractions of the MYOMETRIUM). We do not yet fully understand what initiates spontaneous parturition in humans; however, once started, a number of POSITIVE FEEDBACK mechanisms are operative.
[see Animation]
The uterine contractions during labor rupture the extraembryonic membranes releasing AMNIOTIC FLUID and move the infant towards the mouth (CERVIX) of the uterus. The softened cervix is stretched, which triggers a wave of contractions forcing the infant down the birth canal and out into the world.
After DELIVERY, the contractions cease for a while, then resume to expel the fetal membranes and placenta as the AFTERBIRTH.
The first breath of the infant is triggered by increased blood carbon dioxide and/or by thermal and mechanical stimuli.
[see Figure] Even though PROLACTIN increases during pregnancy and the breasts are fully developed and enlarged, no MILK is secreted. LACTATION is inhibited by the high plasma levels of estrogen and progesterone. With the expulsion of the placenta, estrogen and progesterone levels markedly decline and milk production begins within one or two days after delivery (A watery fluid called COLOSTRUM is earlier produced).
[Fig. 17-31] This figure illustrates the progressive changes in breast anatomy that occur during pregnancy, lactation, and post-weaning.
[see Figure] Positive feedback effects (from the suckling infant) on the hypothalamus (and hence on both the anterior and the posterior pituitary) maintain high levels of PROLACTIN (for the production of milk) and OXYTOCIN (for the "let-down" of milk – i.e. its movement into the ducts from which it can be sucked out by the infant).
SEX DETERMINATION AND DIFFERENTIATION
[see Figure] Sex differentiation in the normal genetic male.
[see Figure] The female is the neutral (i.e. default) sex. In the absence of a functional SRY gene, female genitalia will develop (regardless of the genetic sex).
[see Figure]
[see Figure] The external genitalia will develop in response to the presence or absence of MIS and TESTOSTERONE.
PATTERNS OF SEXUAL REPRODUCTION
The human reproductive pattern is NOT representative of all mammals.
MENSTRUATION occurs in humans and Old World monkeys. Other mammals have cycles of ESTRUS. Females of other species are receptive to males only while they are "in heat" (i.e. Estrus condition). Ovulation usually occurs late in estrus.
Some mammals are MONESTRUS (i.e. have one (or two widely-spaced) breeding seasons per year and alternate between ANESTRUS (non-breeding) and estrus conditions). Examples = dog, seal, rhinocerous.
Other mammals are POLYESTRUS, with multiple breeding times per year, and with short periods of quiescence (= DIESTRUS) alternating with estrus. Examples = rat, horse, elephant.
Also, some mammals (e.g. humans, rats, dogs) are SPONTANEOUS OVULATORS. These species produce enough LH for ovulation without external stimulation. Other mammals (e.g. rabbit, mink) are INDUCED OVULATORS. These species require stimulation (e.g. copulation or the sight or smell of a male) before they release enough LH for ovulation.
Some mammals adjust the timing of the birth of their offspring by practicing either:
DELAYED FERTILIZATION – e. g. bats
Copulation occurs without ovulation in the Fall. The sperm are retained in the vagina. Fertilization occurs in the Spring.
Or
DELAYED IMPLANTATION – e.g. mink, badgers
Ovulation and fertilization occur following copulation; however, there is not enough PROLACTIN (which is a gonadotropin in some mammals) for implantation, so the embryos are retained in the uterus until a later time (e.g. early Spring).
Body size has an important impact on animal reproduction.
Animal …. Litter Size …. #/year …. Gestation Period (days)
Hamster… 1-12 … 3 … 16
Rabbit … 2-7 … 2-3 … 30
Cat … 4 … 1 … 63
Goat … 1-5 … 1 … 148
Human … 1 … 1 … 267
Cow … 1 … 1 … 281
Whale … 1 … 1 … 360
Elephant … 1 … 0.5 … 630
Non-mammalian animals are likewise highly variable in terms of reproductive physiology and behavior.
Sex determination in many species is environmental (not genetic). Some of the best studied examples occur in reptiles, many (but not all) of which exhibit TEMPERATURE - DEPENDENT SEX DETERMINATION (TSD). There are three patterns: