Reproductive System Overview and Gametogenesis

Reproductive System Overview

• The reproductive system is a fundamental characteristic of life, responsible for creating new individuals.
• Sex differentiates biological characteristics, defined by genotype or phenotype.
• Gender differentiates masculinity and femininity.
• Gametes are reproductive cells (sperm and egg).

Gametogenesis

• Gametogenesis is the production of gametes.
• Females produce ova (takes 300 days).
• Males produce sperm (takes 74 days).
• Gametogenesis begins around the fifth week of embryonic development.
• Primordial germ cells (PGCs) migrate to genital ridges.
• PGCs divide mitotically during migration and differentiate into gametes.

Spermatogenesis

• Spermatogenesis occurs from puberty onward.
• Stem cells undergo mitosis to maintain stem cells and type B spermatogonia.
• Type B spermatogonia undergo further mitosis to double germ cells entering the next step.
• There are meiotic divisions followed by organelle and cytoplasm modifications which leads to the formation of spermatozoa.
• The spermatogenic cycle approximately takes 74 days.
• The entire testis produces millions of spermatogonia.

Spermatozoon Anatomy

• Spermatozoa are the only self-propelled swimming cells in the human body.
• They consist of a head, mid-piece, and tail.
• The head includes the nucleus and acrosome.
• The mid-piece contains mitochondria for energy.
• The tail contains microtubules for movement.

Oogenesis

• Oogenesis begins during embryogenesis and continues at puberty.
• Oogenesis produces an ovum.
• It starts with oogonia during embryonic development resulting in a population of about ~7 million cells.
• Most of the oogonia undergo atresia (destruction).
• About 600,000 remain at puberty and only about 400 will be ovulated throughout a woman's reproductive life.
• The oocyte undergoes two meiotic divisions with incomplete cytokinesis.
• Secondary meiotic division occurs with a male gamete present for fertilization.
• First meiotic division(MI) creates a primary oocyte, and the second meiotic division (MII) creates the secondary oocyte. These processes produce haploid nuclei with very little cytoplasm.

Male Reproductive Tract

• The male reproductive tract includes the testes, accessory glands, and ducts.
• The testes are paired ovoid gonads housed in the scrotum. They divide into 200-300 distinct tubules.
• The tunica albuginea surrounds the testes.
• Seminiferous tubules are formed by interconnected Sertoli cells.
• Sertoli cells direct germ cell development, scavenge cell debris, and produce seminiferous fluid.
• Sertoli cells also produce inhibin, FSH-responsive, and androgen-binding protein (ABP).
• Developing germ cells travel from spermatogonia through different stages (primary and secondary spermatocytes, spermatids) to reach spermatozoa in the lumen.
• Epididymis is a coiled tubule where sperm maturation occurs.
• Ductus deferens is a continuation of the epididymis.
• Ejaculatory duct joins ducts deferens and urethra
• Urethra is a tube that serves both urinary and reproductive functions.
• Accessory glands (seminal vesicles, prostate, and bulbourethral glands) produce seminal fluid which mixes with sperm.

Male Accessory Glands

• Seminal vesicles produce secretions rich in nutrients that provide energy and motility for the sperm.
• The prostate gland secretes a fluid that aids in sperm motility and viability.
• The bulbourethral glands produce a pre-ejaculate fluid that lubricates the urethra and neutralizes any residual urine.

Male External Genitalia

• The penis is the male external genitalia responsible for sexual intercourse.
• The penis is composed of the root, body, and glans. The body contains corpora cavernosa and corpora spongiosum containing penile urethra.
• The glans is the cone-shaped end of the penis
• The scrotum is a pouch of skin that encloses the testes. It helps regulate testicular temperature.

Female Reproductive Tract

• Ovaries are paired organs that produce ova and sex hormones. Ovaries are anchored to the uterus by the mesovarium.
• Female reproductive tract consists of the ovaries, uterine tubes, uterus, and vagina, external genitalia.
• Ovarian follicles develop oocytes.
• A developing follicle forms the primary, secondary, and tertiary follicle, and ovulation occurs once the egg is released in a mature follicle.
• Corpus luteum (formed from granulosa and thecal cells) is formed after ovulation.
• If no pregnancy occurs, the corpus luteum regresses to form the corpus albicans.
• Uterine tubes (fallopian tubes) are the tubes that connect the ovaries to the uterus where fertilization occurs. The uterine tubes have an infundibulum, ampulla, isthmus and intramural segment.

Female External Genitalia

• The vulva is the collective term for female external genitalia.
• It includes the mons pubis, labia majora, labia minora, vestibule, vaginal orifice and clitoris.
• The clitoris is an erectile body homologous to the male penis, containing corpus cavernosa.
• The vestibule is the region between the labia minora containing the openings of the urethra and vagina.

Hormonal Regulation

• Gametogenesis is regulated by the hypothalamic-pituitary-gonadal (HPG) axis.
• GnRH from the hypothalamus stimulates the anterior pituitary to release LH and FSH, which stimulate gamete and steroid production in the gonads.
• Steroid hormones (estrogen, progesterone, testosterone) regulate the menstrual cycle and other reproductive functions.

Menstrual Cycle

• The menstrual cycle is a monthly series of changes in the female reproductive system.
• It involves two intertwined cycles: the ovarian cycle, concerned with follicle development, and the uterine cycle, concerned with changes to the endometrium.
• The ovarian cycle has a follicular phase (follicle growth), ovulation (release of the egg), and a luteal phase (corpus luteum formation and regrowth of uterine lining).
• The uterine cycle has 3 phases: menstrual, proliferative, and secretory. These reflect cyclic changes to the lining of the uterus.

Puberty

• Puberty is the period of sexual maturation that marks the transition from childhood to adulthood.
• During puberty, there's an increase in the production of sex hormones (testosterone in males and estrogen and progesterone in females). This results in the development of secondary sexual characteristics and the initiation of gametogenesis.

Reproductive System Aging

• Reproductive abilities decline with age.
• Climacteric in females is associated with a drop in estrogen leading to menopause.
• Menopause is characterized by the loss of ovarian follicles and the cessation of menstruation.
• Similar effects occur in males as testosterone production decreases which leads to andropause. Various changes also occur in the male reproductive system such as decreased sperm count, reduced libido and erectile dysfunction.