Part 3

The Reproductive System

Gamete Formation

Introduction

Chromosomes carry genetic information

In humans, cells contain 46 chromosomes

Gametes carry only 23 chromosomes

Meiosis

Special type of cell division in the reproductive tract

Two cell divisions

Results in 4 daughter cells

Genetically unique

Spermatogenesis or oogenesis

Copyright © 2010 Pearson Education, Inc. Figure 27.5 (1 of 2)

Mother cell

(before chromosome replication)

Chromosome

replication

Chromosome

replication

2n = 4

MITOSIS

Replicated

chromosome Prophase

Chromosomes

align at the

metaphase plate

Sister chromatids

separate during

anaphase

2n 2n

Metaphase

Daughter

cells of

mitosis

Tetrad formed by

synapsis of replicated

homologous

chromosomes

Tetrads align at the

metaphase plate

Homologous chromosomes

separate but sister

chromatids remain together

during anaphase I

No further chromosomal

replication; sister chromatids

separate during

anaphase II

Daughter cells of meiosis II

(usually gametes)

n n n n

Prophase I

Metaphase I

Daughter cells

of meiosis I

Meiosis II

MEIOSIS

Meiosis

Non-disjunction

Chromosomes fail to separate properly

Trisomy

Trisomy 13 and 18 = usually fatal

Trisomy 21= Down syndrome

Monosomy

Always fatal

Spermatogenesis

Begins within testes at puberty

Spermatogonia

Stem cells divide mitotically

Some undergo a growth phase

Become primary spermatocytes

Undergo 2 divisions

1. Produces 2 secondary spermatocytes (haploid)

2. Produces 4 spermatids → mature into a spermatozoa

Spermatogenesis spermatogonia

spermatogonia

primary spermatocytes

secondary spermatocyte secondary spermatocyte

spermatid spermatid spermatid spermatid

spermatozoa

growth phase

Division 1

Division 2

Figure 27.7c

Basal lamina

Spermatogonium

(stem cell)

Cytoplasm of adjacent

sustentacular cells Sustentacular

cell nucleus

Tight junction between

sustentacular cells

Lumen of

seminifer- ous tubule

Late spermatids

Early

spermatids

Secondary

spermatocytes

Cytoplasmic

bridge

Primary

spermatocyte

Spermatozoa

Type B daughter cell

Type A daughter cell

remains at basal lamina

as a stem cell

(c) A portion of the seminiferous tublule wall, showing the spermato-

genic cells surrounded by sustentacular cells (colored gold)

Spermatogenesis

Spermatic cells give rise to sperm

Mitosis

Spermatogonia form spermatocytes

Meiosis

Spermatocytes form spermatids

Spermiogenesis

Spermatids become sperm

Figure 27.7b

Basal lamina

Spermatogonium

(stem cell)

Mitosis

Growth

Late spermatids

Early

spermatids

Secondary

spermatocytes

Primary

spermatocyte

Spermatozoa

Type B daughter cell

Enters meiosis I

and moves to

adluminal

compartment Meiosis I

completed

Meiosis II

Type A daughter cell

remains at basal lamina

as a stem cell

(b) Events of spermatogenesis,

showing the relative position

of various spermatogenic cells

Spermatogenesis Spermatids lose excess cytoplasm and form a tail, becoming

spermatozoa (sperm)

Major regions

1. Head Genetic region

Nucleus

Acrosome with hydrolytic enzymes

2. Midpiece Metabolic region

Mitochondria

3. Tail Locomotor region

Flagellum

Figure 27.8a, b

Centrioles Spermatid

nucleus

Golgi

apparatus Acrosomal

vesicle Mitochondria

Approximately 24 days

Excess

cytoplasm

Nucleus

Acrosome

Microtubules

Flagellum

Tail

Midpiece Head (a)

(b)

1 2

3

4

5

6 7

Spermatogenesis

Sperm are dependent on sugar in testes and semen

Do not survive long outside of the body

Gain motility in epididymis

Oogenesis

Production of female gametes

Begins in fetus

Oogonia multiply by mitosis

Develop into primary oocytes within follicles

Eventually produce estrogen

Primary oocytes begin meiosis but stall in prophase I

About 400,000 present at birth

Copyright © 2010 Pearson Education, Inc. Figure 27.11a

Medulla

Tunica

albuginea

Germinal

epithelium

Cortex

Oocyte Granulosa cells

Late secondary follicle

Antrum

Primary

follicles

Oocyte

Zona

pellucida Theca

folliculi

Ovulated

oocyte

Mesovarium and

blood vessels

Vesicular

(Graafian)

follicle

Corona

radiata

Developing

corpus luteum

Corpus luteum

Ovarian

ligament

Degenerating corpus

luteum (corpus

albicans)

(a) Diagrammatic view of an ovary sectioned to reveal the follicles in its interior

Oogenesis

Each month after puberty, a few primary oocytes are activated

One is selected each month to resume meiosis I

Result is two haploid cells

Secondary oocyte majority of cytoplasm + chromosomes

First polar body chromosomes

Figure 27.17

Meiotic events Follicle development

in ovary Before birth

Infancy and

childhood

(ovary inactive)

Primary oocyte

Primary oocyte (still

arrested in prophase I)

Vesicular (Graafian)

follicle

Primary follicle

Primordial follicle

Primordial follicle

Oocyte

Ovulated secondary

oocyte

In absence of

fertilization, ruptured

follicle becomes a

corpus luteum and

ultimately degenerates. Degenating

corpus luteum

Secondary follicle

Primary oocyte

(arrested in prophase I;

present at birth)

Oogonium (stem cell)

Each month from

puberty to

menopause

Meiosis I (completed

by one primary oocyte

each month in response

to LH surge)

First polar body

Mitosis

Growth

Meiosis II of polar

body (may or may

not occur)

Polar bodies

(all polar bodies

degenerate)

Ovum Second

polar body

Meiosis II

completed (only if sperm penetration occurs)

Sperm

Ovulation

Secondary oocyte

(arrested in

metaphase II)

Follicle cells

Spindle

Oogenesis

Secondary oocyte is ovulated

Sperm penetration of second oocyte completes meiosis II

Produces

Ovum (functional gamete)

Second polar body

The Ovarian Cycle

Ova prepared and released ≈ 28 days

Three consecutive phases

1. Follicular phase

Period of follicle growth (days 1–14)

2. Ovulation

Midcycle

3. Luteal phase

Period of corpus luteum activity (days 14–28)

The Ovarian Cycle

Follicular phase

Begins with slight increases in FSH

Stimulates growth of follicle

Slightly enhances estrogen production

Copyright © 2010 Pearson Education, Inc.

Hypothalamus

Late follicular and

luteal phases

Slightly

elevated

estrogen

and rising

inhibin

levels.

Positive feedback exerted by large in estrogen output.

Mature follicle Corpus luteum

Ovulated secondary oocyte

Ruptured

follicle

LH surge

Progesterone Estrogen Inhibin

Hypothalamus

Early and midfollicular phases

Travels via

portal blood

Granulosa cells

Inhibin

Androgens

Convert androgens to estrogens

Thecal cells

Anterior pituitary

GnRH

FSH LH

Figure 27.19 Feedback interactions in the regulation of ovarian function. Slide 9

1

1

2 2

2

5

5

4

8

6

8

7

3

Figure 27.20b

(b) Ovarian cycle: Structural changes in the ovarian

follicles during the ovarian cycle are correlated with

(d) changes in the endometrium of the uterus during

the uterine cycle.

Primary

follicle Secondary

follicle

Vesicular

follicle Ovulation

Corpus

luteum Degenerating

corpus luteum

Follicular

phase Ovulation

(Day 14)

Luteal

phase

The Ovarian Cycle

Hormonal Interactions

High estrogen levels induce surge of LH

Effects of LH surge Triggers ovulation

Transforms ruptured follicle into corpus luteum (CL)

Luteal phase

Copyright © 2010 Pearson Education, Inc.

Figure 27.20a Correlation of anterior pituitary and ovarian hormones with structural changes of the ovary and uterus.

(a) Fluctuation of gonadotropin levels: Fluctuating

levels of pituitary gonadotropins (follicle-stimulating

hormone and luteinizing hormone) in the blood

regulate the events of the ovarian cycle.

FSH

LH

The Ovarian Cycle

Luteal Phase

Corpus luteum remains functional only if pregnancy occurs

Functions

Produces estrogen and progesterone

Inhibit pituitary release of LH and FSH

Maintain uterine lining

The Ovarian Cycle

Luteal Phase

No fertilization corpus luteum degenerates ovarian

hormone levels drop sharply

Birth control pills

Mimic hormones produced by corpus luteum

Prevent ovulation

The Menstrual Cycle Definition

Cyclic changes in endometrium in response to ovarian hormones

Three phases

1. Days 1–5: Menstrual phase

2. Days 6–14: Proliferative (preovulatory) phase

3. Days 15–28: Secretory (postovulatory) phase

Figure 27.20d

(d) The three phases of the uterine cycle:

• Menstrual: Shedding of the functional layer of the

endometrium.

• Proliferative: Rebuilding of the functional layer of

the endometrium.

• Secretory: Begins immediately after ovulation.

Enrichment of the blood supply and glandular secretion of

nutrients prepare the endometrium to receive an embryo.

Both the menstrual and proliferative phases occur before ovulation, and

together they correspond to the follicular phase of the ovarian cycle. The

secretory phase corresponds in time to the luteal phase of the ovarian cycle.

Menstrual

phase

Menstrual

flow

Endometrial

glands

Blood vessels

Functional layer

Basal layer

Proliferative

phase

Secretory

phase

Days

The Menstrual Cycle

Menstrual phase (days 1-5)

Follicles growing within ovary during this time

Functional layer of endometrium sloughs

Menstrual flow occurs

Endometrial tissue, fluid, and mucus pass through vagina

The Menstrual Cycle

Proliferative phase (days 6-14) Estrogen from follicle stimulates growth of endometrium

Preparation for pregnancy

Ovulation usually occurs at the end of this cycle (day 14)

The Menstrual Cycle

Secretory phase

Corresponds with luteal phase of ovarian cycle

Progesterone and estrogen produced after ovulation

Further development of endometrium

The Menstrual Cycle

Secretory phase

In the absence of fertilization

Estrogen and progesterone levels fall

Endometrium deteriorates

CL degenerates

Another menstrual cycle begins

If fertilization occurs Human chorionic gonadotropin (HCG) produced

Maintains CL

Home pregnancy tests

Figure 27.20c

(c) Fluctuation of ovarian hormone levels:

Fluctuating levels of ovarian hormones (estrogens

and progesterone) cause the endometrial changes

of the uterine cycle. The high estrogen levels are

also responsible for the LH/FSH surge in (a).

Progesterone

Estrogens

Figure 27.20d

(d) The three phases of the uterine cycle:

• Menstrual: Shedding of the functional layer of the

endometrium.

• Proliferative: Rebuilding of the functional layer of

the endometrium.

• Secretory: Begins immediately after ovulation.

Enrichment of the blood supply and glandular secretion of

nutrients prepare the endometrium to receive an embryo.

Both the menstrual and proliferative phases occur before ovulation, and

together they correspond to the follicular phase of the ovarian cycle. The

secretory phase corresponds in time to the luteal phase of the ovarian cycle.

Menstrual

phase

Menstrual

flow

Endometrial

glands

Blood vessels

Functional layer

Basal layer

Proliferative

phase

Secretory

phase

Days

Menopause

Cessation of reproductive cycles

By age 45-50 ovarian follicles cease to respond to FSH and LH

Follicles stop producing estrogen

No inhibition of pituitary

No LH surge

No ovulation, no corpus luteum, no progesterone

Menopause

Symptoms associated with high FSH and LH levels

a) Vaginal dryness

b) Irritability/depression

c) Vasodilation of skin blood vessels hot flashes & night sweats

d) Thinning of skin, breast atrophy

Clinical findings

Increased cholesterol

Loss of bone mass (osteoporosis)