Biology 212 Anatomy & Physiology Human Development Dr. Thompson

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 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

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. Capacitation :

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

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. Fast block: 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: 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 Fertilization 2 weeks 2 weeks 8 weeks 8 weeks 38 weeks (birth)

Zygote undergoes repeated divisions (mitosis), called cleavage, 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 enzyme s, forming finger-like projections into of uterus. Blastocyst

As trophoblast is causing implantation and developing into placenta, Embryoblast is enlarging and developing into embryo Fluid-filled Embryoblast = (12 – 14 days after fertilization)

Pre-embryonic period now over, Embryonic period begins. Fertilization – 2 wks = Pre-embryo During next six weeks of embryonic 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 nervous system. This appears as a groove along the ectoderm, which closes over to form a neural tube. The neural tube

~ 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 Embryo floating in amniotic fluid; stays attached to placenta

~ 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 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: Low in oxygen High in carbon dioxide & other wastes

In placenta: Carbon dioxide & wastes Oxygen & nutrients 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, 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, 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

Congratulations! You have just given vaginal birth to a healthy baby. She has been inspected: all parts are where they are supposed to be.

Plus, she is hungry! Your pituitary gland has been producing a lot of the hormone prolactin, which has caused your breasts to enlarge and develop many glands to produce milk. Your baby will naturally try to find your breast and begin to suck on it. This stimulates your pituitary gland to secrete the hormone oxytocin, which causes muscle cells in the ducts of the milk glands to force the milk toward the nipple, where it is expressed through openings.

First few days: Milk is thick, high in fat, protein, minerals, antibodies. Called colostrum Later: ~90% water ~ 7% sugar Also includes proteins, fats, vitamins, minerals

Your uterus continues to contract for a few days to expel functional layer of endometrium. Assisted when nursing stimulates oxytocin. These contractions also cause your uterus to begin returning to its normal, nonpregnant shape and size; will take a few weeks Joint between your pubic bones tightens up again; will take couple of weeks.

Baby, of course, started breathing as soon as she was born. With each breath, more microscopic air sacs (“alveoli”) in her lungs open up; Within few minutes: adequate to support her need for oxygen. Complete opening of all alveoli: couple of days.

Two openings in / near heart had allowed blood to bypass lungs. These two openings now close, so blood flows through lungs to pick up oxygen and get rid of carbon dioxide. Will take couple of days. (Adjustments of the newborn)

Right after birth: Her temperature dropped by a couple of degrees. Within few minutes: able to generate heat to regulate body temperature. (Adjustments of the newborn) Few days: Stub of umbilical cord will dry up and fall off, leaving the umbilicus (“belly button”) as a scar.

First 12 to 15 months called Infancy. Period of rapid growth – notably nervous, circulatory, skeletal, muscle systems Learning gross motor coordination Legs become strong enough to bear weight; walking begins to develop Visual acuity develops Learns to recognize patterns of vison, hearing Verbal expression and comprehension increase rapidly; speech begins to develop Lungs & heart strong enough to support active movement

Childhood: end of infancy until puberty begins. Less rapid growth Continued improvement in function of most systems, particularly nervous and muscular Significant increase in cognitive function; verbal expression and comprehension; fine motor skills Interestingly: very few other species have extended childhoods; Evolved to allow for this cognitive development

Pubescent period lasts through puberty. Caused when pituitary begins secretion of follicle stimulating hormone and luteinizing hormone. Rapid skeletal and muscular growth again. Hips and shoulders widen proportionately more Ovaries, uterus, oviducts, vagina, testicles, prostate, seminal vesicles begin adult functions

Pubescent period lasts through puberty. Caused when pituitary begins secretion of follicle stimulating hormone and luteinizing hormone. Secondary sexual characteristics develop: Enlargement of clitoris/penis, labia/scrotum Growth of pubic, anal, and axillary (armpit) hair; Specialized sweat glands begin functioning Breasts develop in both girls and boys (gynecomastia) Voice changes occur Hair coursens over most of body Cognitive function and verbal skills increase quickly Emotional changes common. Rapid skeletal and muscular growth again. Hips and shoulders widen proportionately more Ovaries, uterus, oviducts, vagina, testicles, prostate, seminal vesicles begin adult functions

AgeFemaleMale 10 Increase in height and fat deposition. Breast development begins. Spermatogenesis begins. Interstitial cells appear & secrete testosterone 11 Breast development continues. Pelvis widens. Pubic hair appears. Growth spurt begins. Increased fat deposition. Testes begin to enlarge 12 Ovaries, oviducts, uterus, vagina, clitorus, & labia mature. Pubic hair more apparent. Menstruation begins. Epididymis, prostate, vas deferens, seminal vesicles, penis, & scrotum mature. Pubic hair appears. 13 Breasts fuller. Axillary hair appears. Regular ovulation and menstruation. Pubic hair more apparent. Frequent erections and nocturnal emissions. 14 Skeletal growth slows. Breasts complete. Sebaceous (oil) and sweat glands increase. Acne common. Growth of larynx deepens voice. Axillary and facial hair appear. Shoulders begin to widen. 15 Voice deepens slightly. Regular menstrual periods. Ejaculations fully fertile. Moderate breast development. 16 Adult stature reached.Chest hair appears. Continued muscle & skeletal growth. Sebaceous (oil) and sweat glands increase. Acne common. 17 Adult stature reached. “Typical” sequence of events in puberty

Average age of puberty has decreased in past century 1900: 14.2 years in girls 1920: 13.8 years 1940: 13.4 years 1960: 12.8 years 1980: 12.6 years 2000: 11.6 years 2011: 10.8 years Strangely: Few people seems to be following this type of information in boys, but it also appears to be declining.

Factors influencing decreasing age of puberty: a) Better general health and growth b) Better nutrition: More fat and protein in diet c) Increase periods of light Interestingly: Body weight at beginning of puberty has remained relatively constant in girls at about 47 kg (104 pounds), assuming body fat of at least 18%. No such correlation between weight and puberty appears to exist in boys.

Adolescent period lasts three to four years after puberty. Bone and muscle mass continue to increase in both sexes, but more pronounced in males. Fat deposition continues to increase in both sexes, but more pronounced in women. Mental maturity increases; Emotions typically stabilize.

Adult period: Very little growth or neural development occur. Fat deposition continues in both sexes. Reproduction and child-rearing typically occur in this period. Decreased function and diseases of many organs (e.g. cancers, heart disease, respiratory disease) common. Gradual loss of verbal skills, visual and auditory acuity

Senescence (“old age”) involves deterioration and loss of function of most organs. Bone and muscle mass lost. Mental and verbal skills lost.