Biology 212Anatomy & Physiology

Human Development

One of the best ways to understand the structure of the adult human body is to understand how it developed.

Similarly: abnormalities during development produce abnormalities in adult structure.

Assuming:

You paired up with an appropriate partner

You elicited normal sexual responses from each other.

A few milliliters of semen, containing 200- to 600-million sperm, were deposited high in the vagina, just below the cervix of the uterus

Scenario I:

The woman is not within a few days before or after ovulation

Her cervix will be plugged with thick mucous, so few if any sperm will enter the uterus and subsequently the Fallopian tubes.

The few sperm which might make it into the Fallopian tubes are not pushed distally (toward the ampulla)

The sperm do not meet an oocyte and die within 48 hours

No fertilization occurs

Scenario 2:

The woman has just ovulated, or she will ovulate within 1 - 2 days.

The mucous plug in her cervix will be thin and watery; sperm can easily penetrate it

Most sperm will die in her vagina, but a few million (1 - 2%) are pulled through the cervix, into the uterus, and into the Fallopian tubes where they are propelled towardthe ampulla (+100,000 make it this far) Sperm meet the oocyte, and fertilization can occur

At ovulation, oocyte is surrounded by a thick membrane called the

, and by follicular cells called the

surrounding the opening of the Fallopian tube sweep this structure into the infundibulum

Oocyte is viable for , so fertilization must occur within this time.

Fertilization occurs within ampulla of Fallopian tube

If fertilization does not occur, oocyte will degenerate and die

A few thousand sperm reach the ampulla of the Fallopian tube within 30 to 60 minutes after ejaculation

But

They are not capable of fertilizing oocyte

Must undergo processes called

and

which will take 8 to 10 hours after ejaculation

Prior to ejaculation, membrane around head of sperm is strongSemen contains chemicals to prevent weakeningTail is moderately active

After ejaculation, Chemicals in semen diluted / washed away.

Secretions from uterus & oviducts cause membrane to weaken.

Movement of tail increases.

Occurs as sperm make contact with corona radiata and zona pellucida

Requires 15 to 60 minutes

Acrosome swells, becomes leaky, releases digestive enzymes

Enzymes digest path through corona radiata and zona pellucida

Hundreds of sperm usually involved in this digestion

(Acrosome swells, becomes leaky, releases digestive enzymes)

Head of one sperm makes contact with outside of oocyte, binds onto it, and is quickly taken into it.

: Plasma membrane of oocyte changes its electrical charge, which prevents other sperm from binding onto it.

Two systems prevent entry of more than one sperm:

: Zona pellucida swells, pushing other sperm away from the oocyte.

Once head of sperm enters oocyte, its nucleus is released into the cytoplasm.

Nucleus of sperm (23 chromosomes) and nucleus of oocyte (23 chromosomes) meet and fuse together.

Cell now called , with 46 chromosomes

Human Development: 3 Periods

Fertilization 2 weeks

2 weeks 8 weeks

8 weeks 38 weeks (birth)

Fertilization: Lateral part of oviduct (Fallopian tube) One to two days after ovulationResulting cell =

Zygote undergoes repeated divisions (mitosis), called , as it is transported through oviduct toward uterus

Eventually forms solid mass of 20 – 30 cells called , same size as the zygote. This reaches uterus 3 to 4 days after fertilization

Morula does not embed in uterus immediately.

Stays loosely attached for 4 to 5 days, nourished by secretions of endometrium.

Continues dividing into +100 cells, forming hollow sphere called

Fluid-filled cavity in center =

Blastocyst develops mass of cells at one end, called which will develop into the embryo.

Cells surrounding cavity, called will develop into the placenta.

Cells of trophoblast secrete digestive enzymes, forming finger-like projections into endometrium of uterus.

Blastocyst burrows into endometrium, which will heal over. This process is called

As trophoblast is causing implantation and developing into placenta,

Embryoblast is enlarging and developing into embryo

Fluid-filledDevelops betwee

Embryoblast =

Now called

(12 – 14 days after fertilization)

Pre-embryonic period now over, Embryonic period begins.

Fertilization – 2 wks = Pre-embryo

During next six weeks of period, cells of embryoblast will differentiate into beginnings of all organs.

2 wks – 8 wks = Embryonic period

8 wks – birth = Fetal period

First: Cells of embryonic disk separate into two, and then three germ layers:

Endoderm - nearest the blastocyst cavity (changes name to yolk sac)

Mesoderm - in middle

Ectoderm - nearest amniotic cavity

View From Top, Showing Ectoderm

View From Edge (Cross-section) Showing All Three Layers

As embryo develops:

Endoderm will form epithelial linings of digestive and respiratory systems.

Mesoderm will form skeletal, muscular, urinary, reproductive, circulatory systems, as well as muscle and connective tissue layers of digestive and respiratory systems .

Ectoderm will form skin and nervous system

The first system to begin developing is the This appears as a groove along the ectoderm, which closes over to form a

The neural tube enlarges at one end to form remains cylindrical at the other end to form

~ Day 22 – 23, sides of flat embryonic disk begin to fold toward yolk sac.

By day 28: Embryo has “normal” body shape, amniotic cavity pulled around to surround it.

Placenta continues to develop from trophoblast; many blood vessels develop within it to form villi which

Embryo floating in amniotic fluid; stays attached to placenta by containing

~ Day 28: and begin developing.

Bones, muscles, and blood vessels develop from mesoderm as limbs get bigger, nerves grow out from spinal cord.

Other organs continue developing internally.

Length ~ 4mm (width of pencil)

Late embryonic period (4 – 8 weeks):

Nervous system continues to develop; brain rapidly enlarges and folds

Eyes and ears begin to develop

Digestive system forms

Heart folds, divides into chambers, begins contractions

Limbs continue developing, fingers and toes separate

Lungs bud off from digestive system and grow

Kidneys, bladder, gonads develop

Face develops as two halves on side of head; move to front and fuse

End of embryonic period

• Body has human shape

• Almost all organs have begun developing

• Head growth still most pronounced

• Limbs complete

• Length ~ 3 centimeters

• Mass ~ 1 – 2 grams

Beginning of fetal period (8 weeks):

Mass of quarter: 5.7 grams

Placenta well developed:

Fetal heart pumps blood from embryo to placenta through umbilical arteries:

In placenta:

Carbon dioxide & wastes

Oxygen & nutrients

Blood flows from placenta back to embryo through umbilical veins:

High in oxygen & nutrientsLow in carbon dioxide & other wastes

Ninth week:

(Organs and systems continue development)

Fetus = 5 cm long, 4 - 5 grams

External genitalia develop, but male and female still very similar: penis / clitoris scrotum / labia

Tenth week:

(Organs and systems continue development)

Fetus = 7 cm long, 8 - 10 grams

External genitalia easily determined to be male or female

After tenth week:

(Organs and systems continue to develop)

Fetus increases rapidly in size

After 17 weeks (four months)

Skin formed, but thin. Blood vessels easily seen through it

Hair starting to develop. Fingernails and toenails forming.

Eyelids still fused shut.

Ovaries form primordial follicles.

May suck thumb.

Moves arms, legs, head

Mother can feel movement.

After 26 weeks (six months):

Hair present over entire body. Will become longer on scalp

Eyebrows and eyelashes forming

Eyelids almost open

Lungs producing surfactant

Testes begin descent through inguinal canal

Swallowing amniotic fluid

Movement of hands, feet, face

After 34 weeks (eight months):

Skin pink, smooth

Longer hair on head

Eyes open, respond to light

Fingernails and toenails have reached ends of fingers and toes

Testes have reached scrotum

Fingers can grasp objects, fine movement of face and eyes

Nine months:

Ready for birth