Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 46.4 The Interplay of Tropic and Sex Hormones Regulates Mammalian Reproduction
Mammalian reproduction is controlled by a hormonal cascade involving the hypothalamus, pituitary, and gonads. These hormones regulate gamete production, sex hormone secretion, and the female reproductive cycle, ensuring precise coordination for reproductive success.
1. Hormonal Regulation Overview
- Hypothalamus secretes GnRH (gonadotropin-releasing hormone)
- GnRH stimulates the anterior pituitary to release FSH and LH, which act on the gonads
- Gonads produce sex hormones: androgens (testosterone), estrogens (estradiol), and progesterone
- These regulate development, gamete production, sexual behavior, and secondary sex traits
2. Hormonal Control in Males
- FSH stimulates Sertoli cells to support spermatogenesis
- LH stimulates Leydig cells to produce testosterone
- Testosterone promotes sperm production and male secondary sex characteristics
- Negative feedback: testosterone inhibits GnRH, FSH, and LH
- Inhibin from Sertoli cells inhibits FSH specifically
- Leydig cells also produce local regulators like prostaglandins and renin
3. Hormonal Control in Females
- Female cycles include the ovarian and uterine (menstrual) cycles
- FSH and LH regulate follicle growth and ovulation
- Follicles release estradiol to stimulate uterine lining growth
- Mid-cycle LH surge triggers ovulation (around day 14)
- Corpus luteum forms post-ovulation and secretes estradiol and progesterone
- If no fertilization: corpus luteum degenerates → hormone levels drop → menstruation begins
- Menstrual phases:
- Follicular (days 0–14): follicle develops, estradiol rises
- Luteal (days 15–28): corpus luteum active
- Menstrual flow, proliferative, secretory phases in uterus mirror ovarian cycle
4. Menstrual vs. Estrous Cycles
- Menstrual cycles (humans, primates): endometrium is shed
- Estrous cycles (most mammals): endometrium reabsorbed; sexual receptivity only during estrus
- Humans can engage in sex throughout the cycle; other mammals cannot
5. Development and Secondary Sex Characteristics
- During embryogenesis, androgens drive male reproductive tract development
- At puberty, sex hormones cause:
- Males: voice deepens, facial/pubic hair grows, muscle mass increases
- Females: breasts develop, fat redistributes, menstrual cycles begin
6. Biological Sex and Gender
- SRY gene on Y chromosome directs testis formation in XY embryos
- XX embryos develop ovaries by default
- Intersex: individuals with mixed traits (e.g. XXY)
- Gender identity is distinct from biological sex (cisgender vs. transgender)
- Sexual orientation: includes heterosexual, homosexual, bisexual, asexual, etc.
7. Human Sexual Response
- Male and female reproductive structures share embryonic origins
- Four phases: excitement, plateau, orgasm, resolution
- Involves vasocongestion (blood flow) and myotonia (muscle tension)
- Orgasm: rhythmic contractions of reproductive organs
- Refractory period: present in males but not in females
In a Nutshell
Mammalian reproduction is governed by a complex hormonal interplay involving GnRH, gonadotropins, and sex hormones. These signals coordinate gamete production, menstrual and estrous cycles, and sexual behavior. In humans, hormonal regulation also influences gender development, secondary traits, and sexual response, reflecting a deep integration of endocrine and reproductive systems.