102 Reproductive System

Mature gonads perform dual function:-

• Gametogenesis (production gametes/reproductive cells)
  • Spermatozoa (sperm) - males
  • Ova (eggs) - females
• Secretion of sex hormones (steroid hormones)
  • Testosterone - males
  • Oestrogen & progesterone - females

These hormones are not exclusive to gender, each produces a little of the others hormones.

• Accessory sex glands:-
  • Empty supportive secretions into reproductive tract
  • In females, breasts can be classed as accessory glands

• Gonadotropin-releasing hormone (GnRH) secreted by the hypothalamus stimulates the anterior pituitary gland
• Anterior pituitary produces & secretes 2 gonadotropic hormones:-
  • Follicle stimulating hormone (FSH)
  • Luteinising hormone (LH)
    • in males sometimes known as interstitial cell stimulating hormone (ICSH)

• These hormones have 3 primary effects on the gonads:-
  • Maturation of sperm or ova
  • Stimulation of sex (gonadal) hormone secretion
  • Maintenance of structure of the gonads

Hypothalamus => GnRH => Anterior Pituitary Gland => FSH & LH => Gonads => Sex hormones

• GnRH is released in pulses, the anterioir pituitary gland only responds to pulses. 
  • Permanent elevation has no effect
• Once FSH and LH is released, the gonads release the primary sex hormones for that gender
  • These hormones promote gametogenesis
  • They have an effect on the rest of the reproductive tract & organs

Negative Feedback

• Except in the case of females where oestrogen stimulates FSH and LH in a positive feedback mechanism during the middle of the menstrual cycle

• External genitalia
  • Penis & scrotum
• Gonads
  • Testes
• Accessory sex organs
  • Other organs involved in aiding production and *********** of semen
• Develop of human embryo
  • in presence of testosterone

The testes are encased in the scrotum sac

• Spermatogenesis occurs at an optimum temperature roughly 2 degrees lower than the internal body temperature

They consist of 2 compartments

• Seminiferous tubules
  • 90% testis weight
  • Where spermatogenesis occurs
  • Length of over 2.5 football pitches!
• Interstitial tissue
  • Contains testosterone-secreting Leydig cells
  • Rich blood & lymphatic capillary network for hormone transport

Leydig Cells

• Lie in small connective tissue spaces between tubules
• Synthesis and release testosterone
• Stimulated to release testosterone by LH
• Testosterone:
  • stimulates male accessory organs
  • promotes development of male secondary sex characteristics
  • needed for spermatogenesis

Setoli Cells aka 'NURSE CELLS'

• Stimulated by FSH to promote spermatogenesis - respond to stimulation by testosterone & FSH to secrete paracrine agents that stimulate sperm proliferation & differentiation
• Form an unbroken ring around outer circumference of seminiferous tubule - provide a cell barrier to chemicals in plasma
• Nourish developing sperm
• Secrete paracrine agents that influence function of Leydig cells
• Phagocytose defective sperm

Spermatozoa are relatively simple cells

Head:

• Encloses a compact nucleus
• Capped by vesicle known as 'acrosome'
  • Contains enzymes that aid in penetration of protective layers surrounding the egg

Body & tail:

• Provide a propulsive mechanism

• Produced in the seminiferous tubules
• Seminiferous tubules converge to form network of interconnected tubes
• From this then enters into a single duct in the epididymis
  • Connects the seminiferous tubules to the vas (ductus) deferens
  • Where sperm are stored & mature
• Vas deferenscarries sperm from epididymis out of the scrotum sac into pelvic cavity
• The seminal vesicle, prostate gland and buldourethral glands all add secretions
  • Semen
  • Muscles within the abdominal cavity push onto the glands forcing the fluid into the seminal ducts
• The muscle within the penis becomes activated ==> ***********

• Semen complex - mixture of seminiferous fluid & sperm
  • Sperm contributes only a small percentage of the total volume =< 5%
• Van deferens from each testis joins with ducts from seminal vesicles (pair glands)
  • Produce fructose-rich fluid - 60% semen volume
• Van deferens continues as ejaculatory duct & enters prostate gland
  • Contributes up to 30% of bulk of semen
  • Slightly alkaline: helps neutralise the acidic environment of the vaginal tract
• Urethra carries semen out of the body through tip of the penis
  • Pair pea sized bulbourethral glands add secretions to make up last 10% of semen
  • Secretions prepare the urethra (lubricate) or spermatozoa and neutralise any acidic urine present

• Development of male reproductive structures requires the stimulation of testosterone production 'in utero' (in the uterus)
• LH stimulates the Leydig cells to produce testosterone
• FSH stimulates the Sertoli cells to produce spermatozoa
• Many biological actions of testosterone first require the conversion of 5-dihydrotestosterone in the cells of the target tissues
• Semen is a viscous fluid with components produced by the testes, the prostate gland and the seminal vesicles
• Testosterone production peaks in the early adult years and declines gradually after the age of 50 years

External genitalia

• ******** and labia

Gonads

• 2 Ovaries

Accessory sex organs

• Fallopian tubes, uterus, vagina

Develop in human embryo

• In absence of testosterone

Ovaries have several functions

• Oogenesis & maturation of oocyte
• Ovulation
• Secretion of femal sex hormones

At birth, ovaries contain 1-2 million ovarian follicles, each containing:

• Primary Oocyte (potential egg)
• Granulosa cells

At puberty granulosa cells secrete oestrogen

• Triggering menstruation
• Also stimulates breast development & pubic hair production

Oestrogen and progesterone are also involved in maintenance of accessory sex organs

Primary oocytes have begun meiosis but are arrested in prophase of 1st meiotic division

Some oocytes containing follicles are stimulated to develop during each menstrual cycle

Differs to males where spermatogenesis is continuous

• The immature femal reproductive system forms 'in utero' and matures during puberty
• The mature female reproductive system undergoes extensive cyclic changes focused on monthly ovulation and menstruation
• Oestrogen dominates the first half of the menstrual cycle, as the ovary prepares for ovulation and the uterus recovers from menstruation
  • Ovulation, induced by an LH surge, marks the midpoint of the menstrual cycle
• Progesterone dominates the second half of the menstrual cycle, preparing the uterus to receive a fertilised ovum
  • If implantation does not occur, progesterone production ceases, the uterine endothelium is lost during menstruation
• The monthly menstrual cycle continues until menopause, when the ovaries are no longer able to produce sufficient oestrogen to support the cycle