Complete Reproductive System - BIOL 221

Main purpose of the reproductive system

Creates the next generation rather than maintaining homeostasis.

Specialized reproductive cells (gametes)

Sperm in males and oocytes (eggs) in females.

Gonads

Testes in males and ovaries in females.

How many chromosomes do body cells contain?

46 chromosomes.

How many chromosomes do gametes contain?

23 chromosomes.

What does fertilization restore?

The normal chromosome number (46).

Meiosis

Specialized cell division producing gametes and reducing chromosome number from 46 to 23.

How many sperm are produced from one starting cell?

4 sperm.

How many oocytes are produced from one starting cell?

1 oocyte.

Why does the egg keep most of its cytoplasm?

The future embryo depends on materials stored in the egg.

Why do sperm lose most of their cytoplasm?

To improve mobility and deliver DNA.

Male reproductive strategy

Continuous mass production of sperm.

Paired testes (male gonads)

Produce sperm.

Scrotum

Pouch of skin that holds the testes outside the body.

Penis

Delivers semen into female tract, contains urethra, and functions in urination.

Seminiferous tubules

Site of sperm production.

Spermatogenic cells

Divide and mature into sperm.

Sertoli (supporting) cells

Nourish and support developing sperm.

Leydig (interstitial) cells

Produce testosterone.

Step 1 of spermatogenesis

Spermatogonia divide by mitosis to maintain a continuous supply.

Step 2 of spermatogenesis

Some cells enter meiosis, producing 4 cells with 23 chromosomes.

Step 3 of spermatogenesis

Developing sperm lose cytoplasm, develop a flagellum, and concentrate mitochondria in midpiece.

Step 4 of spermatogenesis

Sperm release into tubule lumen and mature in the epididymis.

What does the sperm head contain?

Nucleus with 23 chromosomes and enzymes to penetrate the egg.

Function of the sperm midpiece

Contains many mitochondria that produce ATP for swimming.

Function of the sperm tail (flagellum)

Propels the sperm.

Why do mature sperm contain very little cytoplasm?

They sacrifice size and stored nutrients for mobility.

Accessory ducts

Epididymis, vas deferens, ejaculatory duct, and urethra.

Accessory glands

Seminal vesicles, prostate gland, and bulbourethral glands.

Pathway of sperm during ejaculation

Epididymis → Vas deferens → Ejaculatory duct → Urethra.

Seminal vesicles

Add nutrient-rich alkaline fluid with fructose for sperm energy.

Prostate gland

Adds milky alkaline fluid that helps activate sperm movement.

Bulbourethral glands

Add alkaline mucus that neutralizes acidic urine in the urethra.

Ejaculation

Rhythmic muscle contractions propel semen through the urethra.

Hormone pathway controlling the testes

Hypothalamus → GnRH → Pituitary → FSH & LH → Testes.

Luteinizing hormone (LH)

Stimulates Leydig cells to produce testosterone.

Follicle-stimulating hormone (FSH)

Works with testosterone to stimulate spermatogenesis.

Functions of testosterone

Sperm production, reproductive organ maturation, and secondary sex characteristics.

When does male puberty usually begin?

Between ages 9-14.

What initiates male puberty?

The hypothalamus begins releasing GnRH.

Male secondary sex characteristics

Deeper voice, facial/body hair, increased muscle mass, broader shoulders, heavier bones.

When does spermatogenesis occur?

Begins at puberty and continues throughout life.

Female reproductive structures

Ovaries, uterine tubes, uterus, vagina, and external genitalia (vulva).

Female reproductive tract

Uterine tubes, uterus, and vagina.

Ovaries

Female gonads that contain ovarian follicles.

Ovarian follicles

Contain a developing oocyte and surrounding granulosa cells.

Oogenesis

Production of oocytes (eggs).

What are oogonia?

Stem cells that give rise to oocytes.

When do most oogonia become primary oocytes?

Before birth.

Primary oocyte

Cell with 46 chromosomes that begins meiosis I before birth.

Primordial ovarian follicle

Primary oocyte surrounded by a single layer of follicular epithelial cells.

When does female reproductive development begin?

Before birth.

What happens to primary oocytes before birth?

They begin meiosis I and pause until puberty.

How many primary oocytes is a female born with?

Approximately 1-2 million.

Can females make new primary oocytes after birth?

No. The supply only decreases.

What happens at puberty?

Hormones stimulate ovarian follicles to begin developing each month.

How many follicles usually fully mature each month?

One.

What happens just before ovulation?

The primary oocyte completes meiosis I, forming a secondary oocyte.

Ovulation

Release of the secondary oocyte from the ovary.

What happens immediately after ovulation?

The secondary oocyte begins meiosis II and pauses again.

When is meiosis II completed?

Only if fertilization occurs.

Corpus luteum

Structure formed from the ruptured follicle after ovulation.

Corpus albicans

Fibrous scar formed when the corpus luteum degenerates.

How long does the secondary oocyte remain viable?

About 12-24 hours.

What happens if the oocyte is not fertilized?

It is shed with the uterine lining during menstruation.

What are the three phases of the ovarian cycle?

Follicular phase, ovulation, and luteal phase.

Follicular phase

Follicle develops.

Ovulation (ovarian cycle)

Oocyte leaves the ovary.

Luteal phase

Corpus luteum forms and later degenerates.

What are the phases of the uterine cycle?

Proliferative, secretory, and menstrual phases.

Proliferative phase

Uterine lining grows.

Secretory phase

Uterine lining secretes mucus and nutrients to support a developing embryo.

Menstrual phase

Uterine lining sheds and resets.

When does female puberty usually begin?

Between ages 8-13.

What initiates female puberty?

The hypothalamus begins releasing GnRH.

Female secondary sex characteristics

Breast development, fat redistribution, pubic and axillary hair growth, and widening of the pelvis.

How long does oogenesis continue?

From puberty until menopause.

Hormone pathway controlling the ovarian cycle

Hypothalamus → GnRH → Pituitary → FSH & LH → Ovaries.

Average menstrual cycle length

About 28 days (varies between people).

What happens during the follicular phase (days 1-14)?

FSH stimulates follicle development, granulosa cells secrete estrogen, and estrogen rebuilds the uterine lining.

What triggers ovulation?

An LH surge caused by very high estrogen levels.

What happens during ovulation?

The follicle ruptures, releasing the secondary oocyte, and the fimbriae sweep it into the uterine tube.

What happens during the luteal phase (days 15-28)?

The corpus luteum forms, secretes progesterone, and maintains the uterine lining.

What causes menstruation?

Falling hormone levels cause the uterine lining to shed.

What is considered day 1 of the menstrual cycle?

The first day of menstruation.

What hormones normally regulate the ovarian cycle?

Estrogen and progesterone.

Where is estrogen produced?

Developing ovarian follicles.

Where is progesterone produced?

Corpus luteum.

How does hormonal birth control work?

Synthetic estrogen and/or progesterone create negative feedback that suppresses GnRH, FSH, and LH.

What is the result of hormonal birth control?

Follicles do not fully mature, the LH surge is suppressed, and ovulation usually does not occur.

What causes menopause?

The ovarian follicle pool becomes depleted.

What happens during menopause?

Ovulation largely stops, progesterone drops dramatically, and estrogen also falls substantially.