Reproduction

Male Reproductive Physiology

Chromosomal sex: XY = Male

Regulation of Male Embryonic Sexual Development

Presence of sex-determining region of Y chromosome (SRY) and testis determining factor (tdf) directs differentiation of the "indifferent" gonads into testes.

Wolffian ducts, under the influence of testosterone (T), give rise to the internal male reproductive organs (e.g., epididymis, vas deferens, seminal vesicles).

Other embryonic structures give rise to other reproductive organs (penis, scrotum, prostate) and are mediated by dihydrotestosterone (DHT), which results from the conversion of testosterone in the target cell by the enzyme 5-alpha reductase.

What is the effect of a 5-alpha reductase deficiency?  ____________________________________________________________

Male brain differentiation requires conversion, within the brain, of T E₂ by the enzyme aromatase.

• Alpha-fetoprotein helps protect against masculinization of the female brain.

Testis - From Latin meaning "witness."

1. Seminiferous Tubules (ST)

• House germ cells and Sertoli cells.

• Site of spermatogenesis.

Spermatogonia Primary (1⁰)Spermatocytes Secondary (2⁰)Spermatocytes Spermatids Spermatozoa

          Sertoli Cells

• Support and nourish developing sperm

• Remove damaged germ cells by phagocytosis

• Provide blood-testis barrier

• Secrete mullerian inhibition factor (MIF)

• Secrete inhibin

• Secrete

activin

• Secrete androgen-binding protein (ABP)

         2. Cells of Leydig = interstitial cells

• Produce and secrete androgens (male sex hormones) which is referred to as steroidogenesis

• Found between seminiferous tubules

Functions of Androgens: To affect the development, growth, and maintenance of primary and secondary sex characteristics.

Primary Sex Characteristics = male sex and accessory organs

1. Testes - Produce sperm and testosterone.

2. Epididymis - Location of sperm maturation for motility and fertility, concentration, and storage between ejaculations.

3. Vas Deferens - Stores and transports sperm.

4. Seminal Vesicles - Produce seminal fluid; secrete fructose to supply energy to sperm.

5. Prostate - Contributes alkaline secretions to seminal fluid which neutralize acidic vaginal secretions.

6. Bulbourethral glands - Secrete mucus-like substance for lubrication of urethra = pre-ejaculatory fluid.

7. Penis - Copulation and urination.  See erection, emission, and ejaculation on pg. 693.

8. Scrotum - Houses testes and provides lower temperature for sperm maturation.

          1.  Distribution and pattern of body hair - Mustache, beard, underarm, pubic hair, body hair, recession of hairline

          2.  Growth of larynx and thickening of vocal cords which causes a deeper voice.

          3.  Behavior - Aggression, libido (sex drive)

          4.  Musculoskeletal development

Some Androgen Target Organs:  (However, some effects typically attributed to androgens may actually be due to estrogen.)

1. Bone - Anabolic actions of androgens trigger both the "growth spurt" and then the closure of epiphyseal plate to limit growth.

2. Skeletal and cardiac muscle - Larger in males

3. Red blood cells (RBC) - Stimulate hemoglobin synthesis and erythropoiesis (RBC production) - Greater number in males

4. Central nervous system

• Sexually dimorphic nucleus (SDN) of hypothalamus - Larger in males

• Erection center in spinal cord - Larger in males

Endocrine Regulation of Testicular Function:  Hypothalamic-pituitary-gonadal (testicular) axis

1. Hypothalamus		Anterior Pituitary		Testes
GnRH		LH, FSH		Testosterone

Castration causes an increase in LH and FSH which demonstrates that the testes exert negative feedback control on the reproductive axis.

GnRH is released in a pulsatile manner. Therefore, LH and FSH are also released in this manner.

LH receptors found on Leydig cells;  FSH receptors found on Sertoli cells.

• Both T and FSH appear necessary for spermatogenesis although there is significant debate on the relative roles of each.

2. Inhibin - Testicular protein that selectively inhibits FSH at the anterior pituitary gland.

3. Activin - Testicular protein that selectively stimulates FSH at the anterior pituitary gland.

4. Prolactin - May up regulate number of LH receptors on Leydig cells.

    Hyperprolactinemia (increased levels of PRL in blood), however, can cause testicular dysfunction (e.g., decreased testosterone, impotence).

5. Melatonin - Inhibits GnRH, gonadotropins, and testosterone secretion.

Female Reproductive Physiology

• Female reproductive physiology is more complex than male reproductive physiology due to cyclical nature of the reproductive system.

          Chromosomal sex:  XX = Female

• Female Embryonic Sexual Development

• Mullerian ducts develop into internal organs (uterus, oviducts).

• Other embryonic structures develop into the vagina, labia majora, labia minora, and clitoris.

Reproductive organs (primary sex characteristics)

• Ovaries: Produce ova (eggs)

• Produce E₂ and progesterone (in follicular cells); + reproductive development of primary and secondary sex characteristics; sex drive; prepares uterus for and maintains it during pregnancy; prepares mammary gland for milk production by stimulating duct formation.

• Oviducts = Uterine tubes = Fallopian tubes

• Receive secondary oocyte upon ovulation

• Site of fertilization and transport of fertilized ovum to uterus

• Uterus - Site of implantation of fertilized egg and houses developing embryo/fetus

• Vagina - Receives penis during copulation and serves as birth canal

• Secondary sex characteristics

• Development of mammary glands, broadening of the hips, fat deposition (breasts, hips, buttocks, thighs), distribution of body hair.

• Oogenesis - Formation of ova in the ovary. 

• At birth, the ovaries contain all the oocytes they will ever have.

Oogonia 1⁰ oocytes 2⁰ oocytes

If 2⁰ oocyte is fertilized by a sperm it will continue its development. If not, it will degenerate after ovulation.

• Considerable difference in time required for spermatogenesis vs. oogenesis - About 9-10 weeks vs. 10-50 years, respectively.

Endocrine Regulation of Ovarian Function: Hypothalamic-pituitary-gonadal (ovarian) axis

 

1. Hypothalamus		Anterior Pituitary		Ovary
GnRH		LH, FSH		Estradiol

Menstrual Cycle

• Cycle that repeats at approximately one-month intervals.

• Menstruation - Periodic shedding of the inner layer of the uterus (endometrium) which is accompanied by bleeding.

• Human females (and some other primates) that have menstrual cycles may permit copulation at any time of the cycle.

• Animals that exhibit estrous cycles are sexually receptive (in heat or estrus) only at a particular time in their cycles, which is around ovulation.

• Average menstrual cycle length - About 28 days; range 25 - 30 days in the majority of women. However, the cycle length can be influenced by stress, body composition, pregnancy, nursing, etc.

• Phases of menstrual cycle: Ovaries

• Follicular - Day 1 to ovulation (~ day 13)

• Luteal - Post-ovulation (day 15) to day 28

• Phases of the menstrual cycle: Endometrium

• Menstrual - Approximately days 1-4

• Proliferative - Days 5 to day 13

• Secretory - Ovulation (~ day 15) to day 28

• Follicular phase (days 1-4)

• Menstruation occurs due to decreased steroids which leads to shedding of the outer ~ 2/3 of endometrium.

• FSH, LH, estradiol, and progesterone are low.

• Ovarian follicles begin to grow.

• Follicular phase (days 5-13)

• FSH stimulates further growth of follicles.

• Estradiol levels increase due to FSH stimulation of follicles.

• Follicles continue to grow; graafian follicle (dominant follicle) develops.

• Endometrium increases in thickness.

• Increase in LH secretion in late follicular phase as a result of positive feedback effect of estradiol on LH that ultimately leads to the LH surge.

• Formation of spiral arteries in endometrium.

• Ovulatory phase (day 14)

• Massive LH surge triggers ovulation - Release of 2⁰ oocyte from graafian follicle into oviduct.

• Little change in endometrial lining during ovulation.

• Luteal phase (days 15-28)

• Development of corpus luteum from ruptured follicle.

• Estradiol and progesterone secretion increase, then fall at end of phase.

• LH and FSH decrease due to negative feedback effects of estradiol and progesterone. Serves to retard development of new follicles.

• Breakdown of corpus luteum (luteolysis) toward the end of luteal phase, if pregnancy does not occur.

• Development of mucous glands in endometrium due to progesterone. Contributes to the thickness of the endometrium during this phase.

• Slight increase in body temperature due to progesterone.