Biology Chapter 14: The Reproductive System

Mesoderm

All muscle, bone, and connective tissue, entire cardiovascular and lymphatic system, including blood. Urogenital organs (kidney, uteres, gonads, reproductive ducts), dermis of skin.

Development of Reproductive Systems

Early embryos have undifferentiated gonads, possessing both Wolffian ducts (develop into male internal genitalia) and Mullerian ducts (female internal genitalia). When Y is absent, mullerian form by default. If Y is present, the testes is formed, releasing testosterone and Mullerian inhibiting factor (MIF). Testosterone also enters the systemic circulation and is converted to dihydrotestosterone which helps develop the male genitalia.

Ectoderm

Entire nervous system, pituitary gland , adrenal medulla, cornea and lens, epidermis of skin and derivatives (such as hair), nasal, oral, anal epithelium.

Endoderm

GI tract epithelium (except mouth and anus), GI glands (liver, pancreas, etc.), respiratory epithelium, epithelial lining of urogenital organs and ducts, urinary bladder.

Hormones in females

GnRH from the hypothalamus stimulates the release of FSH and LH from the anterior pituitary. Under the influence of FSH, the granulosa and thecal cells develop during the follicular phase and secrete estrogen. Secretion of GnRH, FSH, and LH is initially inhibited by estrogen; however, estrogen, which increases throughout the follicular stage, reaches a threshold near the end of this phase and has a positive effect on LH secretion. This sudden surge in LH causes ovulation. After ovulation, LH induces the follicle to become the corpus luteum and to secrete estrogen and progesterone (this marks the beginning of the secretory phase). If pregnancy does not occur, the combined high levels of estrogen and progesterone feedback to strongly inhibit secretion of GnRH, FSH, and LH. When LH secretion drops, the corpus luteum regresses, no longer secretes estrogen or progesterone, and menstruation occurs.

Pregnancy

Human chorionic gonadotropin (hCG) takes the place of LH in maintaining the corpus luteum (which secretes estrogen, inhibin and progesterone- maintains endometrium lining). Now the corpus luteum does not degrade and menstruation does not occur.

LH and FSH in men and women

In men, LH acts on interstitial cells to release testosterone, and FSH stimulates sustenacular cells. In women, FSH stimulates granulose cells to secrete estrogen, and LH stimulates the formation of the corpus luteum and progesterone secretion.

Hormonal Control of Spermatogenesis

Testosterone (interstitial cells release this, also called Leydig cells) plays the essential role of stimulating division of spermatogonia. Luteinizing hormone (LH) stimulates the interstitial cells to secrete testosterone. Follicle Stimulating hormone (FSH) stimulates the sustenacular cells (protect and nurture the developing sperm both physically and chemically). The hormone inhibin is secreted by sustenacular cells; its role is to inhibit FSH release. Both LH and FSH are released by the anterior pituitary due to Gonadotropin releasing hormone (GnRH) released by the hypothalamus.

Oxytocin

secreted by the posterior pituitary gland; stimulates contraction of the uterus during childbirth. Also is necessary for milk let-down (release). It is released when suckling occurs so that breasts do not leak when infant is not nursing.

Prolactin

stimulates milk production