Reproduction in Mammals

Transcription

1 Reproduction in Mammals A. Introduction 1. The organs of the male and female reproductive systems ensure the continuation of the species. 2. They do this by producing gametes and by providing a method by which the gametes of the male (i.e., spermatozoa) can be united with a gamete (i.e., ovum) of the female. 3. The organs that produce the gametes are referred to as the primary sex organs. a. These are the gonads, the testicles in the male and the ovaries in the female. b. The primary sex organs also produce hormones that influence the development of male and female secondary sex characteristics and regulate the reproductive cycle. 1) In males, specialized cells in the testicles called Leydig cells produce hormones called androgens (e.g., testosterone). 2) In females, the ovaries produce estrogens and progesterone. B. Male reproduction 1. Introduction. a. The primary organs of the male reproductive system are the testicles. b. The other structures are considered the secondary organs. 2. Testicles. a. The testicles begin development in the abdominal cavity of the male. 1) In some mammals the testicles descend into the scrotum seasonally or permanently. 2) In other mammals, the testicles do not descend into a scrotum but are retained in the abdominal cavity. b. Each testicle is shaped like a flattened, elongated egg covered by a CT layer called the tunica albuginea. c. Each testicle contains seminiferous tubules where sperm are produced. 1) A cross-section through a tubule will show it is packed with cells undergoing spermatogenesis (just prior to and during the breeding season). 2) Mature sperm cells are derived from undifferentiated germ cells called spermatogonia (singular, spermatogonium), that lie just inside the outer wall and divide mitotically, producing new spermatogonia. 3) Some of the spermatogonia migrate away from the outer wall, increase in size, and become primary spermatocytes. 4) Primary spermatocytes are diploid; they become secondary spermatocytes when they meiotically divide into haploid cells. 5) Secondary spermatocytes divide to form spermatids. 6) Spermatids then differentiate into spermatozoa (mature sperm cells). d. The combined length of the seminiferous tubules in a mammal the size of a human is about 250 m.

2 2 e. Leydig cells, located in the interstitial spaces between the ST, synthesize and secrete testosterone. 3. Sperm a. Consists of 4 parts: 1) A head that contains an acrosome, which contains enzymes apparently to assist the sperm in entering the ovum. 2) A middle piece that contains mitochondria, which provide the energy needed by the tail. 3) A tail that propels the sperm. The tail is not activated until just prior to ejaculation. b. In humans, there may be in excess of 400 million sperm in 2 ml of semen expelled during ejaculation. 4. Semen. a. Semen consists of sperm plus secretions from the: 1) Epididymus. 2) Seminal vesicles. 3) Prostate gland. 4) Bulbourethral gland. c. A clinical examination of human semen is shown in the table below. Characteristic Reference Value Volume of ejaculate ml Sperm count million/ml Sperm motility (% of motile forms) 1 hour after ejaculation 70% or more 3 hours after ejaculation 60% or more Leukocyte count 0-2,000/ml ph Fructose concentration mg/100 ml 5. Accessory ducts. a. Tubulus rectus. b. Rete testis. c. Efferent ducts. d. Epididymus. e. Ductus deferentia. f. Ejaculatory ducts. 6. Accessory glands. a. Seminal vesicles. 1. Lie at the base of the urinary bladder. 2. Each has a duct that joins with a ductus deferens called the ejaculatory duct.

3 3 3. Secrete a viscid fluid that contributes to the formation of semen. 4. The secretions from the SV s contribute about 60% of the bulk of the semen in humans. b. Prostate gland. 1) Single (in humans), donut-shaped gland that surrounds the upper portion of the urethra just below the bladder. 2) Secretes a thin, alkaline, milky fluid that contributes to semen. 3) In older men, the prostate may enlarge and squeeze the prostatic portion of the urethra, making urination painful and difficult. c. Bulbourethral glands. 1) These 2 pea-sized organs (in humans) lie posterior to the prostate gland on either side of the urethra. 2) Fluid contains fructose and prostaglandins. 7. Penis. a. The penis is the copulatory organ of the male. b. It transports urine during urination and semen during ejaculation. c. It contains the urethra. d. A bacula occurs in the penis in many mammals. 8. Hormonal regulation: The table below summarizes the principle male reproductive hormones and their actions. Endocrine Gland and Hormone(s) Principle Site of Action Principle Action Anterior Pituitary FSH Testicle Stimulates development of seminiferous tubules; may stimulate spermatogenesis. LH (ICSH) Testicle Stimulates interstitial cells to secrete testosterone Testicles Testosterone General Before Birth stimulates development of 1 o sex organs. At puberty responsible for growth spurt; development of reproductive structures and 2 o sex characteristics. In adult responsible for maintaining 2 o sex characteristics; may stimulate spermatogenesis.

4 4 C. Female reproduction. 1. Introduction. a. The female reproductive system is designed to produce ova, to receive sperm, and provide protection and nourishment for the developing embryo. b. The reproductive organs also are important in the birth of the young and in the nourishing the offspring after birth. c. The primary reproductive structure of the female is the ovary. 2. The ovary and the ovum. a. An ovary is about the size and shape of large almonds (in humans); they are held against the lower walls of the pelvic cavity by broad ligaments. b. The ova are contained within the ovary in hollow structures called follicles. c. As a result of FSH, a primary follicle may grow and develop a cavity, called an antrum, within surrounding granulosa cells and become a secondary follicle. d. The ovum also undergoes changes during this time. 1) The primary oocyte within a primary follicle progresses through to metaphase II of meiosis within a secondary follicle. 2) In this process, a large secondary follicle is formed, together with a small polar body. 3) The secondary oocyte contained within a fully mature, or Graffian follicle, is expelled from the ovary in the process of ovulation. 4) The secondary oocyte can then enter the uterine or fallopian tube. 3. Uterine tubes. a. The paired uterine tubes receive mature ova from the ovary and serve as the site for fertilization. b. The inner surface of the oviduct is lined with a ciliated mucous membrane. The cilia beat, creating a current that draws the released ovum, which cannot move by itself, into the oviduct. 4. Uterus. a. A hollow, pear-shaped organ held in place by ligaments. b. The neck of the uterus, where it narrows and joins the vagina, is called the cervix. c. If an embryo becomes implanted in the uterine lining, the membranes of the uterus remain thick and enlarged, and the uterus serves as the womb, a protective, nourishing place for the developing baby. d. There are 6 types of mammalian uteri. 1) Monotreme type. 2) Marsupial type. 3) 4 types of eutherian uteri. a) Duplex. i. 2 uteri, 2 cervixes. ii. Found in the lagomorphs and rodents. b) Bipartite.

5 5 i. 2 uteri, 1 cervix. ii. Found in the cetaceans and carnivores. c) Bicornuate. i. Uterine horns Y-shaped, being separated medially but fused distally, where they form a common chamber, the body, which opens into the vagina through a single cervix. ii. Found in insectivores, some bats, perissodactyls, and artiodactyls. d) Simplex. i. All separation between the uterine horns is lacking, and the single uterus opens into the vagina through 1 cervix. ii. The simplex uterus is found in primates, some bats, and edentates. 5. The placenta. a. The placenta, a complex of embryonic and maternal tissues, performs several essential functions during pregnancy. b. The choriovitelline placenta. 1) Although eutherians are often referred to as placental mammals, marsupials also have a placenta, although not one as efficient as that found in the eutherians. 2) Most marsupials have a choriovitelline placenta. 3) Here, the yolk sac is greatly enlarged, serving the nutritional needs of the developing embryo. 4) Because the connection between the blastocyst and uterine wall is relatively weak, the system of nourishment to the embryo is inefficient. c. The chorioallantoic placenta. 1) In contrast to marsupials, eutherian mammals (and the marsupial bandicoots) have a chorioallantoic placenta. 2) In this case, the blastocyst adheres to the endometrium and then sinks very deeply into it, forming a strong adhesion. 3) Because the connection between the blastocyst and uterine wall is strong, the system of nourishment to the embryo is very efficient. 6. Vagina. a. An elastic canal that leads from the outside of the body to the cervix of the uterus. b. The vagina receives the penis during copulation. c. Additional functions are: 1) A site where sperm cells is deposited. 2) The birth canal for the young. 7. The estrus cycle. a. Introduction. 1) All female mammals except higher primates restrict copulation to a specific time of the year.

6 2) The reproductive season is controlled by hormones and the nervous system, and regulated by environmental and social cues. 3) In nonprimates, a period of brief receptivity shortly before and after ovulation is called estrus, or heat. 4) In the majority of mammals, ovulation is spontaneous, that is, it occurs without copulation. 5) In contrast, if the egg is shed from the ovary within a few hours following copulation, this situation is referred to as induced ovulation. Induced ovulators include rabbits, many carnivores, and some ground squirrels. b. The estrous cycle. 1) During the breeding season, the time span from 1 period of estrus to the next is called the estrous cycle. 2) A species that has 1 estrous cycle per year is termed monstrous. This condition can be found in some carnivores. 3) A species that has several cycles in a year is termed polyestrous. This condition can be found in rodents, rabbits, hares, and pikas. D. Reproductive variations. 1. Introduction. a. For most mammals, fertilization occurs within several hours of insemination. b. The fertilized egg becomes implanted in the uterus, and development continues until birth. c. Thus, we define the gestation period as the interval between fertilization and parturition. d. Mammals may exhibit modifications that lengthen the fertilization or gestation period, including delayed fertilization, delayed development, delayed implantation, and embryonic diapause. 2. Delayed fertilization. a. Occurs only in some bats. b. In bats that display delayed fertilization, copulation takes place in September or October before hibernation commences. c. Although follicular growth has taken place, ovulation does not happen at this time. d. The sperm are immotile and stored in either the uterus or the upper vagina, and then both sexes enter hibernation. e. When the female emerges from hibernation in the spring, the eggs are ovulated, spermatozoa become motile, and fertilization takes place. f. Young are born in early summer, when insects are abundant. 3. Delayed development. a. Occurs only in some bats. b. Delayed development differs from delayed fertilization in that the blastocyst implants shortly after fertilization, but development is very slow. For example, a 6

7 blastocyst may implant in the uterus in summer but may have a 7-month gestation period. 4. Delayed implantation. a. Occurs in a diverse group of mammals, including insectivores, rodents, bears, mustelids, seals, armadillos, certain bats, and 2 species of roe deer. b. In delayed implantation, ovulation, copulation, fertilization, and early cleavage of the zygote up to the blastocyst stage occurs normally. c. Development of the blastocyst is arrested, however, and each blastocyst floats freely in suspended animation in the reproductive tract until environmental conditions become favorable for implantation. d. Eventually, implantation occurs and development proceeds normally. 5. Embryonic diapause. a. Red kangaroos are the best example of embryonic diapause. b. See page 159 in Feldhamer et al. (2004). 7