BIOL 1040 Chapter 27

Asexual Reproduction

the creation of genetically identical offspring by a single parent. 

Tpes of Asexual Reproduction Include . . . 

Fission

Cell Division

Budding

Fragmentation & Regeneration

Parthenogenesis

Binary Fission

Fission

as in Prokaryotes & Anemones. 

Cell Division

(binary fission) – as in unicellular protists. 

Budding

as in yeasts & hydras

Fragmentation & Regeneration

as in Starfish & Sponges. 

Parthenogenesis  

seen in fish, arthropods, amphibians, and lizards.  An egg develops without being fertilized

Binary Fission

the separation of a parent organism/cell into two (or more) individuals of about equal size

Advantages of Asexual Reproduction 

Normally faster than sexual reproduction – allows rapid population growth into new or vacated habitats; early populations have a competitive advantage. 

Beneficial in stable, unchanging habitats – no new selective pressures, new adaptations are not beneficial; genetic stability if favored. 

Beneficial for sparse populations or where individuals do not move

Sexual Reproduction

the creation of genetically unique offspring by the fusion (fertilization) of two haploid gametes (sperm & eggs) resulting in the formation of a diploid (2n) zygote. 

Male gametes vs. Female gametes

Male gametes (i.e. sperm) are relatively small & move by means of a flagellum. 

Female gametes (i.e. eggs) are much larger cells & are not self-propelled. 

Hermaphrodite

an organism that has both male and female reproductive organs

Dioecious plants vs. Monoecious plants

Dioecious plants – two types of flowers: male flowers on “male plants”; female flowers on female plants.

Monoecious plants - two types of flowers (male & female) occurring on the same plant.

Describe evolutionary trends in vertebrate sexual reproduction between three animals

Fish & Amphibians: External Fertilization & Reproduction.  Dependent on water.

Reptiles: Internal (cloacal) fertilization. Amniotic egg; Not dependent on water. 

Mammals: Internal fertilization. Not dependent on water. Develops completely within the female body; Development linked to placenta; Live birth

Gonads

reproductive organs specialized for the production of gametes.

Germ Cells

specialized diploid somatic stem cells that divide by mitosis, and then by meiosis to produce gametes. 

Spermatogonia

produce spermatocytes. 

Gametogenesis

the process of gamete formation. 

Oogenesis

the development of a mature egg (or ovum). 

Spermatogenesis

the production of mature sperm cells.

Hormonal Control of the Testes:

Hypothalamus secretes gonadotropin-releasing hormone → anterior pituitary releases follicle-stimulating hormone (FSH) & luteinizing hormone (LH). 

FSH and LH purposes

FSH stimulates sperm production by testes. 

LH promotes release of androgens (testosterone).

Testes

male gonads, produce sperm cells, secretes testosterone and other androgens. 

Epididymis

 a duct emerging from testes where sperm cells are collected, matured and stored prior to ejaculation.

Vas Deferens

thick-walled tubes (or ducts) that serve in the rapid transport of sperm cells from the epididymis to the ejaculatory duct via the seminal vesicle. 

Seminal Vesicles

secrete fluids containing fructose and prostaglandins that are mixed with sperm to become part of seminal fluid.

Ejaculatory Ducts

mix seminal fluids and forcefully eject them into and through the penis. 

Prostate Gland

secretes fluids that become part of the semen to help neutralize the acidic pH of the female reproductive tract.

Bulbourethral Glands

(aka Cowper’s glands) secrete a cleansing and lubricating mucus into urethra prior to the entrance of the semen. 

Penis

the male organ of sexual intercourse that delivers semen into the female reproductive tract.

Spermatic cord

(tissue that encloses vas deferens & vascular tissue, and attaches them to the body) will contract & relax accordingly to bring testes closer/farther away from body mass for warming/cooling.

• Optimum temperature for human sperm production is 3-5c cooler than core temperature

Spermatogenesis Timeline

including sperm maturation takes about 2.5 – 3 months; however, the process of meiosis takes about 3 days.

Sperm cells can survive for 3 – 5 days in the cervical fluids; it takes about 5-30 minutes for sperm to reach the upper end of the fallopian tubes where conception usually takes place.

Meiosis Process for babies:

1. Meiosis starts during fetal development by about 3 months, but stops at Prophase I. 

2. At 4 months development, ovaries contain 7 million primary oocytes. 

3. Baby is 3 inches long, with most organs & tissues (including gonads) developed. 

4. Meiosis I resumes at puberty – one oocyte per 28 day fertility cycle. 

5. Meiosis II continues, but stops again in metaphase II (at ovulation). 

6. Meiosis II is completed only if fertilization occurs.

Ovaries

female gonads, produce and mature oocytes, with cyclical release during ovulation.  Ovaries also produce estrogens and progesterone.

Infundibula & Oviducts

Funnel-shaped opening & channels (fallopian tubes) that capture & conduct or direct the oocyte from the ovary to the uterus.  Fertilization of the egg normally occurs in the oviduct. 

Uterus

chamber where the embryo fully develops into a new individual.  Integral parts of the uterus include the endometrium, myometrium and cervix. 

Endometrium

inner lining of the uterus where the blastocyst becomes implanted; becomes thickened and vascularized to become the maternal portion of the placenta during pregnancy. 

Myometrium

thick muscular layer of the uterus that stretches during pregnancy and contracts during the birth process. 

Cervix

a narrowed opening of the uterus into the vagina; secretes mucus that enhances sperm movement into the uterus and forms a plug to minimize the risk of bacterial infection of the fetus. 

Vagina

the female organ of sexual intercourse; also serves as the birth canal. 

Bartholin’s Glands

secrete mucus to lubricate the vagina.

Ovarian Cycle (6 steps)

1. Epithelial cells become organized around a primary oocyte to become a primary follicle. 

2. Meiosis I is completed to form a secondary follicle containing a secondary oocyte & 1st polar body 

3. Meiosis II follows; Follicle swells with follicular fluid to form a vesicular follicle. 

4a. Follicle continues to swell & distend the surface of the ovary. 

4b. Ovulation –  the Vesicular follicle bursts to release secondary oocyte (“egg”).  Meiosis stops at Metaphase II. 

5. Follicular remains develop into the corpus luteum. 

6. Corpus luteum degenerates.

The Human Female Reproductive Cycle:

A complex, hormonally controlled physiological process in the human female involving the maturation and release of an oocyte and the preparation of the uterus for pregnancy, and then the return to the “unprepared” condition if pregnancy does not occur. Involves two coordinated cycles in one.

Two systems of the female reproductive cycle

The ovarian cycle – the development of an oocyte into a matured ovulated egg.

The uterine cycle – aka the menstrual cycle – the preparation of the uterine lining to receive a developing zygote.

Releasing hormone

Secreted by: Hypothalamus

Function: Regulates secretion of LH and FSH by piturarity

FSH

Secreted by: Pituitary

Function: Stimulates growth of ovarian follicle

LH

Secreted by: Pituitary

Function: Stimulates growth of ovarian follicle and production of secondary oocyte. Promotes ovulation; promotes development of corpus luteum and secretion of other hormones

Estrogen

Secreted by: Ovarian follicle

Function: Low levels inhibit pituitary, high levels stimulate hypothalamus; promotes growth of endometrium

Estrogen and Progesterone

Secreted by: Corpus leteum

Function: Maintain endometrium; high levels inhibit hypothalamus and pituitary; sharp drops promote menstruation

9 steps of The Human Female Reproductive Cycle 

1. Releasing hormone from hypo-thalamus stimulates anterior pituitary to release FSH & LH. 

2. FSH stimulates the growth & maturation of a follicle.  LH triggers ovulation and supports the endometrium in the uterus. 

3. Follicle starts to produce & release more & more estrogen as it grows. 

4. Increasing levels of estrogen stimulate increased output of FSH & LH, until a surge in both occurs. 

5. Surge in FSH & LH triggers ovulation & formation of corpus luteum.

6. Corpus luteum starts to produce & secrete estrogen & progesterone 

7. Estrogen & progesterone Feed back to inhibit release of FSH & LH by anterior pituitary. 

8. Decrease in LH causes corpus luteum to degenerate & gradually quit producing estrogen & progesterone.

9. Low levels of estrogen & progesterone 

Trigger the disintegration & shedding of endometrium (i.e. menstruation).

fertilization

the union of sperm and egg to form a diploid zygote.

how are Sperm adapted to reach and fertilize an egg. 

Flagellated tail, streamlined shape. 

Many mitochondria for ATP production. 

A “head” containing the haploid nucleus. 

An acrosome containing enzymes that help it penetrate the egg.

7 steps for fertilization in sea urchins

1. Sperm cell contacts the jelly coating of the egg & the acrosome releases enzymes. 

2. Acrosomal enzymes digest a hole in the jelly coating, allowing penetration of sperm cells. 

3. Species-specific proteins on sperm cells bind to species-specific receptor proteins in the egg. 

4. Sperm & egg cell plasma membranes fuse, allowing the sperm nucleus to enter the egg cell (time zero). 

5. Egg cell plasma membrane becomes impenetrable to other sperm cells (< 2 sec.). 

6. Vitelline layer hardens & separates from the plasma membrane, becoming impenetrable to sperm (20 sec); meiosis II finishes; nuclei fuse (20 min); 1st cell division (90 min). 

7. Sperm & egg nuclei fuse.

Cleavage

the first major phase of embryo development characterized as a series of rapid synchronous cell divisions without cell enlargement that transforms the zygote into a hollow ball of cells called a blastula (in frogs) or a blastocyst (in humans). (INCREASE IN CELLS BUT NOT SIZE). Takes a few hours in frogs, few days in humans

Very little gene expression occurs during cleavage stage.

Gastrulation

Produces a Three-Layered Embryo. Organs develop from the three embryonic layers

Involves:

• Differential cell division & growth. 

• Infolding, cell migration & specialization of cells into three layers. 

• Formation of the blastopore, followed by the formation of the gastrula. 

Varies from one species to another.

Animal Pole

a region of small, rapidly dividing cells; thought to become the three germ layers. 

Vegetal Pole

a region of large slowly dividing cells; thought to become the yolk & placenta.

Blastopore

becomes the embryonic anus.

Tissue & organ development involves 

Changes in cell shape, 

Cell migration, 

Programmed cell death (apoptosis).

Embryonic Induction

via cell surface interactions and/or diffusible signal molecules. 

Cell Migration

where cells move by amoeboid motion along chemical signal trail. 

Apoptosis

i.e. Programed Cell Death – where unnecessary cells or tissues are induced to “self destruct” by nearby cells that also remove cellular debris.

Embryonic Induction

individual cells (or groups of cells) induce other nearby cells (groups) to switch on/off certain genes that cause the receiving cells to differentiate –

e.g. embryonic formation of the notochord from the mesoderm induces the adjacent ectoderm cells to fold inwards to start forming the neural tube.

Pattern Formation

the emergence of a body form with specialized tissues, organs & limbs forming at the correct time & in the correct locations. Involves Waves of gene expression in response to:

• Spatial variations of signal chemicals (e.g., “morphogens”) in the embryo 

• Under “master control” by homeotic genes

Homeotic Gene

aka HOX gene – determine how, when, & where different body parts (or segments) develop during embryo development. Genes that regulate the expression of other genes.

Pregnancy

(or gestation) – the state or condition of carrying the developing young of a species within the female reproductive tract.

Generally, smaller animals have shorter gestation periods; larger animals have longer gestation periods.

Overview of Early Developmental Events Development 

1. begins with fertilization – normally occurring in the oviduct. 

2. Cleavage results in the formation of a hollow blastocyst (6 days). Inner cell mass trophoblast & cavity. 

3. Blastocyst attaches to uterine wall, and “hatches”. 

4. Trophoblast secretes enzymes that degrade endometrium, facilitate implantation & help form placenta.

Implantation:

Trophoblast secretes enzymes that degrade endometrium, facilitates implantation; Cells multiply & spread into endometrium; Becomes the Chorion & forms the embryo’s contribution to the placenta.

7 days

Early Gastrulation

Migration of embryonic cells & formation of the three distinct embryonic cell layers (Ectoderm, Endoderm, Mesoderm); Amnion & Yolk sac membranes also become evident. 

9 days

Late Gastrulation

Adjacent layers of ectoderm, mesoderm & endoderm have distinctly formed into the embryonic disc.

16 days

Placenta

a composite organ comprised of fetal chorionic villi & maternal blood vessels of the endometrium.  Functions in exchange of nutrients, gases & elimination of fetal wastes; Doe not allow mixing of maternal & fetal blood supplies; Some undesirable substances can still cross

Forms at 31 days

The First Trimester

the period of greatest change. 

The embryo forms but looks like other vertebrate embryos. 

Extra-embryonic membranes form. 

All major organ systems (except circulatory & respiratory systems) are established. 

Called a fetus after 8-9 weeks – starts to look distinctly human; can move its arms & legs.

he Second Trimester  (13 – 27 weeks) 

Fetus greatly increases in size (from  5 to 19 cm) & weighs 0.5 kg ( 1 lb.); assumes the “fetal position”. 

Human features become well-defined, including facial, hand & feet features; eyes are open at the end of 2nd trimester. 

The Third Trimester

(28 weeks – birth) is also a time of rapid growth 

The circulatory and respiratory systems mature. 

Muscles thicken, and the skeleton hardens. 

Average length = 50 cm (20 inches). 

Average weight = 3 – 4 kg (6 – 8 lbs.)

Describe labor and birth of human baby

Childbirth Is Induced by Hormones & Other Chemical Signals. 

Highest levels of estrogen are reached late in pregnancy. 

Estrogen stimulates the formation of oxytocin receptors in uterus. 

Fetal cells & mother’s pituitary gland secrete increasing amounts of oxytocin. 

What does Oxytocin stimulate

Uterine contractions. 

Placental synthesis & release of prostaglandins. 

What does prostaglandin do

Prostaglandins stimulate more contractions of the uterus. Amnion ruptures, labor proceeds.