Biology 212 Anatomy & 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 toward the 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 zona pellucida, and by follicular cells called the corona radiata Fimbria surrounding the opening of the Fallopian tube sweep this structure into the infundibulum

Oocyte is viable for 12-24 hours, 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 Capacitation and Acrosome Reaction which will take 8 to 10 hours after ejaculation

Capacitation: Prior to ejaculation, membrane around head of sperm is strong Semen contains chemicals to prevent weakening Tail is moderately active After ejaculation, Chemicals in semen diluted / washed away. Secretions from uterus & oviducts cause membrane to weaken. Movement of tail increases.

Acrosome Reaction: Occurs as sperm make contact with corona radiata and zona pellucida Requires 15 to 60 minutes Acrosome swells, becomes leaky, releases digestive enzymes

(Acrosome swells, becomes leaky, releases digestive enzymes) Enzymes digest path through corona radiata and zona pellucida Hundreds of sperm usually involved in this digestion

Head of one sperm makes contact with outside of oocyte, binds onto it, and is quickly taken into it. Two systems prevent entry of more than one sperm: Fast Block: Plasma membrane of oocyte changes its electrical charge, which prevents other sperm from binding onto it. Slow Block: 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 Zygote with 46 chromosomes

Human Development: 3 Periods Pre-embryonic: Fertilization 2 weeks Embryonic: 2 weeks 8 weeks Fetal: 8 weeks 38 weeks (birth)

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

Zygote undergoes repeated divisions (mitosis), called cleavage, as it is transported through oviduct toward uterus Eventually forms solid mass of 20 – 30 cells called morula, 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 blastocyst. Fluid-filled cavity in center = blastocyst cavity.

Blastocyst develops mass of cells at one end, called embryoblast or inner cell mass, which will develop into the embryo. Cells surrounding cavity, called trophoblast, 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 implantation.

As trophoblast is causing implantation and developing into placenta, Embryoblast is enlarging and developing into embryo Fluid-filled amniotic cavity develops between embryoblast and trophoblast Embryoblast = flat disk between amniotic cavity and blastocyst cavity. Now called embryonic disk or simply embryo. (12 – 14 days after fertilization)

Pre-embryonic period now over, Embryonic period begins. During next six weeks of embryonic period, cells of embryoblast will differentiate into beginnings of all organs. Fertilization – 2 wks = Pre-embryo 2 wks – 8 wks = Embryonic period 8 wks – birth = Fetal period

Endoderm - nearest the blastocyst cavity (changes name to yolk sac) 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 nervous system The first system to begin developing is the nervous system. This appears as a groove along the ectoderm, which closes over to form a neural tube. The neural tube enlarges at one end to form brain; remains cylindrical at the other end to form spinal cord

~ 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 and form villi which project into pools of the mother’s blood in her endometrium. Embryo floating in amniotic fluid; stays attached to placenta by umbilical cord containing umbilical vessels which carry blood between placenta and embryo.

~ Day 28: Arm buds and leg buds 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 Limbs continue developing, fingers and toes separate Digestive system forms Lungs bud off from digestive system and grow Heart folds, divides into chambers, begins contractions Kidneys, bladder, gonads develop Face develops as two halves on side of head; move to front and fuse

End of embryonic period (8 weeks): Beginning of fetal 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 Mass of quarter: 5.7 grams

Placenta well developed: Fetal heart pumps blood from embryo to placenta through umbilical arteries: Low in oxygen High in carbon dioxide & other wastes

In placenta: Carbon dioxide & wastes diffuse out of fetal blood into maternal blood Oxygen & nutrients diffuse from maternal blood into fetal blood Blood flows from placenta back to embryo through umbilical veins: High in oxygen & nutrients Low 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

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

Fetus: Surrounded by amniotic fluid ~ 1 liter/quart All organs mature, but: Lungs not inflated, but fetus inhaling amniotic fluid No food in digestive system; Fetus swallowing amniotic fluid Temperature regulation not active Blood shunted away from lungs through two openings

Adjustments after Birth: As body cools, brain stimulates heat production. As infant nurses, digestive system begins to produce and secrete enzymes for digestion. Starts breathing, lungs inflate (many minutes). Blood starts flowing to lungs as shunts close (days).