. Biology of Reproduction, lecture on Puberty
University of South
Dakota......Department of Biology

Biology of Reproduction

Fall 1998

click here for text only
text: Human Reproductive Biology 2nd Edition - R.E. Jones: Read pages 136-158 for this lecture


XII. Puberty = Biological Transition, Child to Reproductive Adult			back to X. Development of Sexual Behavior

	A. Pubescence = in the process of puberty

		1. not  development of  1o Sex Characteristics

			a. Gonads
						  			 }all structures necessary
			b. External Genitalia            for mating

			c. present at birth

		2. not development of 2o Sex Accessory Structures

			a. Vas deferens

			b. Seminal Vessicles, Prostate Gland

			c. uterus

			d. also present at birth

		3. Development of 2o Sex Characteristics

			a. pubic hair, larynx growth,
			    breast development, facial hair

			b. are not  developed until puberty

				i. both males and females

	B. Maturation of Gonads, Germ Cells, &
	    internal reproductive organs

		1. Menarche = time of first menstruation

			a. maturation of oviducts, uterus, and vagina

			b. not first ovulation necessarily

				i. characteristically anovulatory cycles

				ii. oligomenorrhea = menstrual cycles variable
				     in length

			c. average age = 12.3 years

				i. declining recently

		2. First Ovulation or fertile ejaculation

			a. Ovary: 500,000 follicles in a newborn
			    -> 83,000 just before puberty

				i. all lost to atresia since none are ovulated

	C. Adolescence

		1. time from puberty to Social adulthood

	D. Maturation of Hormone Regulation

		1. Controls the maturation process for maturation
		    of all other reproductive systems

		2. Rapid increase in production of gonadotropins
		    and sex steroids

		3. Maturation of adrenal androgen production (adrenarche)

			a. DHEA  & DHEAS in males and females at 6 or 7

				i. increased libido

		4. Feedback matures Endocrine responsiveness

			a. Child has very low FSH & LH
			    and therefore low steroids

			b. Low steroids in an adult would result high FSH and LH
			    because of low feedback

			c. Set-point for feedback in the child is much lower

				i. lower steroid levels shut off GnRH

					(1) set-point = gonadostat - may be controlled
					    by steroid receptor levels

					(2) set-point is 16x more sensitive in an child
					    than in an adult

				ii. Set-point slowly changes at puberty

					(1) gradual decrease "down regulation"
					    of steroid receptors?

					(2)  ~9 years to mature levels

				male						female

			¯ sensitivity to				¯ sensitivity to

			negative feedback				negative feedback

									Maturation of  +feedback

					(3) High levels of E2 in younger girls
					    does not result in an LH surge

			d. maturation of GnRH pulse generator
			    in the medial basal hypothalamus

				i. ­ amplitude and frequency of GnRH pulses

					(1) leptin from fat ­ GnRH

					(2) ­ NPY and Glu during puberty

						(a) NPY works with Gal
						    to stimulate GnRH release

							(i) Gal colocalized with GnRH
							    in ARC neurons

			e. GnRH receptors in the pituitary increase
			    during puberty

				i. GnRH pulses maintain (build?) GnRH-R

			f. LH and FSH receptors in the gonad increase
			   during puberty

				i. maturation of LH & FSH pulsatility

			g. GnRH, LH and FSH pulsatility are necessary
			    for normal gonadal axis function in adults

	E. Variation in the Timing of Puberty

		1. 15% of variation in age of puberty
		    attributable to heredity

			a. Identical Twins may be up to 2 months different
			    in the timing of menarche

			b. Fraternal Twins - 8 months

		2. Age of menarche has been declining
		   in developed countries

			a. 3-4 months / decade

			b. 1900 = 14.2 yrs. (avg.)   1980's = 12.3 yrs.

		3. Obese females tend to reach puberty earlier

			a. females with very low body fat have delayed puberty

				i. Primary Amenorrhea = delay of puberty

		4. Critical Body Fat Hypothesis

			a. % body fat is correlated with the age of puberty

			b. evidence that critical body weight 45kg (104 lb)
			    must be obtained before puberty can begin

				i. 17% of that must be fat

					(1) enough fat to carry through a pregnancy

			c. Fat cells have aromatase, convert androgen to E2

				i. E2 from fat necessary to start first GnRH/LH surge

			d. ­fatcritical also ®+ ­ leptin ®+ ­ GnRH, LH, FSH
				i. via ob/obb-R

				ii. anorexia ® ¯ fat ® loss of cyclicity:
                                     leptin restores it

		5. Stress ®+ CRH ®+ ACTH ®+ F

			a. CRH inhibits secretion of GnRH, and LH

			b. ACTH also directly inhibits GnRH and LH,
			    but also FSH and E2 or T

			c. Cortisol also directly inhibits
			    GnRH, LH, FSH E2 and T

				i. inhibits cells from taking up fat,
				   amino acids, and sugar

			d. Adrenal Axis hormones (CRH, ACTH, F) also inhibit
			   reproductive behavior

			e. Chronic Stress may delay puberty

				i. stress at a very early age may
				   advance the age of puberty

XIII. Reproductive Cycling

University of South
Dakota......Department of Biology