Embryonic Development of Oogonia

Factors Affecting Puberty Onset

• Genetics, environment, and psychological stress affect the onset of puberty
• Better nutrition has led to a decrease in the age of menarche in girls in the US
• There is a link between puberty onset and stored fat, especially in girls but also present in both sexes
• Leptin hormone plays a role in determining menarche
• Delay in puberty onset can occur in lean and highly active girls, such as gymnasts

Male Reproductive System

Hormonal Control

• Hypothalamus secretes Gonadotropin-releasing hormone (GnRH)
• Anterior pituitary secretes FSH and LH
• FSH stimulates Sertoli cells to secrete Androgen Binding Protein (ABP) and inhibin
• LH causes interstitial cells to secrete testosterone
• ABP and testosterone stimulate spermatogenesis
• Negative feedback control occurs through increased testosterone and inhibin

Anatomy

• Gonads, ducts, sex glands, and supporting structures
• Functions of various parts:
  • Penis
  • Testis
  • Epididymis
  • Scrotum
  • Prostate
  • Seminal vesicle
  • Urethra

Spermatogenesis

• Sperm forming cells go through two meiotic divisions
• Each of four spermatids develop into a sperm
• Second meiosis division gives four spermatids, each with 23 single-stranded chromosomes
• First meiosis division gives two secondary spermatocytes, each with 23 chromosomes that become double-stranded
• Testosterone release leads to spermatogenesis

Effects of Hormone Release

• Penis and scrotum grow
• Facial hair grows
• Larynx elongates, leading to a lowering of the voice
• Shoulders broaden
• Body, armpit, and pubic hair grow
• Musculature increases body-wide

Female Reproductive System

Hormone Release

• Estrogen release leads to:
  • Breast development and maturation
  • Hips broadening
  • Pubic hair growth
  • Folliculogenesis (process of growing and developing ovarian follicles in the ovary)

Human Reproductive System

• Essential features of human reproduction:
  • Liberation of an ovum at a specific time
  • Internal fertilization of the ovum by spermatozoa
  • Transport of the fertilized ovum to the uterus
  • Implantation of the blastocyst in the uterus
  • Formation of a placenta and maintenance during gestation
  • Birth of the child and maternal organ return

Unfertilized Egg

• Corona radiata: an outer layer made up of follicular (granulosa) cells that form around a developing oocyte in the ovary and remain with it upon ovulation
• Zona pellucida: a specialized extracellular matrix surrounding the developing oocyte

Fetal Reproductive System Development

• Development of reproductive systems begins soon after fertilization of the egg, around 3 weeks of development
• Germ cells migrate from the yolk sac to the genital ridge
• Females are the "fundamental" sex in development, without much chemical prompting; all fertilized eggs would develop into females

Sexual Organs Development

• Bipotential tissue: same group of cells has the potential to develop into either male or female gonads
• SRY protein actively recruits other genes to develop testes and suppresses genes important in female development
• Without SRY, different genes are expressed, oogonia form, and primordial follicles develop in the primitive ovary

Male Reproductive System Development

• Scrotum formation at 9 weeks
• Testosterone induces development of the mesonephric (Wolfian) duct to form the epididymis, vas deferens, and seminal vesicles
• Testis moves from the genital ridge across the pelvis to lie at the internal inguinal ring
• Leydig cells' testosterone secretion influences tissues to become male reproductive structures

Oogenesis

• Creation of an egg in the female fetus
• Oogenesis starts in the fetus at around 7 weeks' gestation, when primordial germ cells (PGC) colonize the newly formed ovary
• Oogonia undergo mitosis
• Number of oogonia increases rapidly from about 43,000 at 7 weeks' gestation to about 7 million at 14-15 weeks' gestation
• After the seventh month of embryonic development, the number of germ cells drops precipitously

Reproductive System Development

• Minimal changes in the reproductive system between infancy and puberty

Gonadal Changes during Puberty

• Puberty-related gonadal changes occur between 9-12 years of age
• Blood levels of LH and FSH increase
• High levels of LH and FSH initiate gonadal development
• Nocturnal GnRH pulsatility (LH secretion) precedes phenotypic changes by several years
• First phenotypic changes: breast development and testicular enlargement

Folliculogenesis

• Process of ovarian follicle growth and development

Changes during Puberty (Tanner Stages)

• Puberty consists of five stages:
  1. At age 8, the hypothalamus increases its production of GnRH
  2. GnRH triggers the anterior pituitary to release LH and FSH
  3. LH and FSH trigger testosterone production in the testes and estrogen production in the ovaries