Chapter 17

Development and Aging Mader, Sylvia S. Human Biology. 13 th Edition. McGraw-Hill, 2014.

Points to Ponder

• What is fertilization? • Describe the steps in fertilization.

• What is cleavage? Growth? Morphogenesis? Differentiation?

• What are the 4 extraembryonic membranes?

• Be familiar with what happens during pre-embryonic development, fetal development and development after birth.

• Follow the path of fetal circulation.

• What determines the sex of an individual? Be sure to understand the 3 hormones involved and the SRY gene.

• What are two conditions in which sex determination is ambiguous and 2 conditions in which there are abnormal development of the sex organs?

• What are the 3 stages of birth?

• What can you do to help prevent birth defects?

• What are the 3 hypotheses of aging?

• What is the effect of aging on the body?

17.1 Fertilization

Fertilization

• Fertilization is the union of the sperm and egg to form zygote • Egg is surrounded by an outer matrix called the

zona of pellucida

• Outside this matrix has a few layers of follicular cells collectively called the

corona radiata

– These cells had nourished oocyte within the ovarian follicle

Steps of Fertilization

• Steps of fertilization: 1. Several sperm penetrate the corona radiata 2. Acrosomal enzymes digests a portion of the zona pellucida 3. The sperm binds to and fuses with the egg’s plasma membrane 4. Sperm nucleus enters the egg 5. Sperm and egg nucleus will fuse

17.1 Fertilization

Fertilization

17.1 Fertilization

What prevents more than one sperm from entering?

• The egg’s plasma membrane changes to prevent other sperm from binding – depolarizes • Vesicles within the egg – release enzymes that cause the zona pellucida to become impenetrable and sperm cannot bind

17.2 Pre-embryonic and embryonic development What are the main processes of development?

• Cleavage – cells undergo division (2, 4, 8 etc) without the embryo increasing in size • • • Growth – cells undergo division as well as increase in size Morphogenesis – the embryo begins to take shape as cells migrate Differentiation – when cells take on specific structure and function (the nervous system is the first visible system)

• • • •

Functions of the extraembryonic membranes (Outside of the embryo)

Chorion: Gas Exchange – fetal half of the placenta, the organ that provides the embryo with nourishment and gets rid of wastes Allantois: Collects Nitrogenous Wastes – gives rise to the bladder – blood vessels of umbilical cord carry blood to/from the fetus Yolk sac: Provides Nourishment – contains many blood vessels – blood cells first form Amnion: Provides Protection – contains amniotic fluid that cushions and protects the embryo

17.2 Pre-embryonic and embryonic development

Extraembryonic membranes

What are the stages of development?

1.

Pre-embryonic development 1 st week of development after fertilization 2.

Embryonic development 2 nd 2 nd week after fertilization until the end of the month 3.

Fetal development the 3 rd through the 9 th months of development 4.

Development after birth stages of life including infancy, childhood, adolescence and adulthood

1. Pre-embryonic development

• • • • Cleavage – cell division that increases the number of cells Morula – compact ball of embryonic cells that becomes a blastocyte Early blastocyst – inner cell mass  becomes the embryo • All inner cells have capabilty to become any cell type – Inner mass covered by layer of cells  becomes chorion Implantation – embryo embeds into the uterus around day 6

• • • •

2. Embryonic development: week 2

Pregnancy begins after implantation Human chorionic gonadotropin (HCG) secreted – maintain corpus luteum and endometrium HCG is the basis for a pregnancy test The inner cell mass detaches itself and becomes the embryonic disk – goes through gastrulation to become 3 primary germ layers

3 Primary Germ Layers

• Ectoderm: outer layer – Epidermis of skin – Epithelial lining of oral cavity and rectum – Nervous system • Mesoderm: middle layer – Skeleton, muscular system, dermis of skin – Cardiovascular system, urinary system – Reproductive system, out layers of respiratory and digestive system • Endoderm: inner layer – Epithelial lining of digestive tract and respiratory tract and associated glands – Epithelial lining of urinary bladder

17.2 Pre-embryonic and embryonic development

2. Embryonic development: week 3

• • • Nervous system begins to develop The posterior neural tube will become the spinal cord and brain Development of the heart begins

17.2 Pre-embryonic and embryonic development

2. Embryonic development: weeks 4 & 5

• • 4th week : – Chorionic villi form – Umbilical cord forms – Limb buds form (later develop into legs and arms) – Hands and feet are apparent 5 th week : – Head enlarges – Eyes, ears and nose become prominent

2. Embryonic development: weeks 4 & 5

17.2 Pre-embryonic and embryonic development

The embryo at week 5

17.2 Pre-embryonic and embryonic development

2. Embryonic development: weeks 6-8

• Embryo begins to look like a human being • Reflex actions occur – Startle response to touch • All organ systems have been established • 38mm by the end of this period and weight ~ the same as an aspirin tablet

3. Fetal development

• Placenta – Source of progesterone and estrogen during pregnancy – Functions of hormones: • Negative feedback on hypothalamus and anterior pituitary prevent new follicles from maturing • Maintain endometrium so menstruation does not occur – Gasses and wastes are exchanged between the maternal and fetal sides of the placenta via diffusion

Fetal circulation

• Umbilical artery – Carries O2-poor blood to the placenta towards the maternal side • Umbilical vein – Carries O2-rich blood, rich in nutrients from the placenta to the fetus • O2-rich and O2-poor blood mix in the heart of the fetus • Path of O2-rich blood – Umbilical vein vena cava   Liver  join venous duct  return blood to right atrium  shunted to left atrium via oval opening  merge with inferior enter the heart  left ventricle pump blood into the aorta • Path of O2-poor blood – Enter right atrium  aorta via arterial duct pumped into pulmonary trunk  joins

Fetal circulation

Note: Fetal and maternal blood do not mix because exchange of materials between the two occurs at the chorionic villi.

17.3 Fetal development • • • • • •

3. Fetal development: months 3 & 4

Hair develops The head slows in growth so that the body size can catch up – Cartilage begins to be replaced by bone Fontanels “soft spots” remain in the skull for birthing and brain growth Able to distinguish female from male (month 3) – Presence of form SRY gene on Y chromosome leads to the development of testes and male genital – Lack of SYR gene allows ovaries and female genital to Can hear heartbeat with the stethoscope (month 4) By the end of this period the fetus is ~6 inches and ~6 ounces

17.3 Fetal development

3. Fetal development: months 5-7

• • • • Fetal movement can be felt by the mother Fetus is in fetal position Eyelids are fully open Fetus size has increased to ~12 inches and ~3 pounds

17.3 Fetal development

3. Fetal development: months 8 & 9

• Weight gain is about a pound per week • Fetus rotates so that the head is pointed towards the cervix • At the end of fetal development the fetus weighs ~7.5 pounds and ~20.5 inches

Development of the sex organs

Sex of an individual is determined at conception (XX is female and XY is male) - Six weeks: both male and females have the same types of ducts If the SRY (the sex determining region on the Y chromosome) gene is present – at ~ week 6 testes develop and testosterone is produced 1. Testosterone stimulates Wolffian ducts to become male genital ducts 2. Wolffian ducts enter the urethra, which belongs to both the urinary and reproductive systems in male 3. Anti-mullerian hormone causes Mullerian ducts to regress

Development of the sex organs

If the SRY (the sex determining region on the Y chromosome) gene is absent 1. Ovaries develop instead of testes 2. Wolffian ducts regress 3. Estrogen from ovaries causes Mullerian ducts to develop into the uterus and oviducts

Development of the sex organs

17.3 Fetal development

Development of the sex organs

• At 14 weeks primitive testes and ovaries with gametes are already developing • The development of the external organs is dependent on the presence or absence of dihydrotestosterone (DHT) produced by the testes

Hormones produce by the Testes

• Testosterone: – Stimulate development of epididymides, vasa deferentia, seminal vesicles, etc • • Anti-Mullerian hormone: – prevents development of female structures Dihydrotestosterone: – directs development of the urethra, prostate gland, penis and scrotum

Abnormal development of the sex organs • • XY female syndrome – an individual develops into a female – piece of Y chromosome containing the SRY gene is missing XX male syndrome – an individual develops into a male – same small piece of the Y containing the SRY gene is present on an X chromosome – SRY gene allows formation of the testes – Testes produce 3 important hormones involved in the development of the male genital

17.3 Fetal development

Ambiguous sex determination

• • • Results from the absence of testosterone, Anti-Mullerian hormone and/or DHT Androgen insensitivity syndrome : – all hormones are made – testosterone receptors on cells are ineffective – individual has testes that do not descend and outwardly appear to be female Male pseudo-hermaphroditism : – an individual appears female until puberty when anti-Mullerian hormone is produced but the testes never produce testosterone or DTH

What are the good and bad changes in the mother’s body?

• • • • • • • Nausea and vomiting are common symptoms early on (morning sickness) Some mothers report an overall increase in energy levels and sense of well-being Acid reflux and constipation are common problems There is an increase in vital capacity Incontinence is not uncommon The placenta produces peptide hormones that makes cells resistant to insulin so diabetes can result Melanocyte activity increases in some areas

Pregnancy: Uterus Relaxes

• Placenta produce progesterone which: – Decrease uterine motility by relaxing smooth muscle • Arteries Dilate  leads to low blood pressure – Stimulate renin-angiotension-aldosterone results in sodium and water retention – Increase in blood cells • Gastrointestinal effects of pregnancy • Heartburn: relaxation of esophageal sphincter • Constipation: caused by a decrease in intestinal tract motility

Pregnancy: Pulmonary Values Increase

• Bronchial tubes relax • Increase vital capacity and tidal volume • Maternal oxygen level changes little, but blood carbon dioxide levels fall by 20% – creates concentration gradient favorable to flow of carbon dioxide from fetal blood to maternal blood at the placenta

• •

Birth

True labor is characterized by uterine contraction that occur every 15-20 minutes and last for at least 40 seconds – 3 stages: 1 st : Effacement occurs; Cervical Dilation • cervical canal slowly disappears • babies head acts as a wedge to cause cervical dilation – 2 nd : Uterine contractions; Birthing • uterine contractions every 1-2 minutes lasting for ~1 minute e • an incision is made to the opening to help the baby as its head reaches the exterior • once the baby is born the umbilical cord is cut and tied – 3 rd : Afterbirth; release of placenta • Afterbirth is delivered about 15 minutes after birth of baby

17.4 Pregnancy and birth

Birth

17.4 Pregnancy and birth

Health focus: preventing birth defects

• Get physical exams by a trained doctor • Have good health habits: – proper nutrition and adequate sleep and exercise • Avoid smoking, alcohol and drug abuse • Avoid having x-rays • Avoid certain medications and supplements • Avoid sexually transmitted diseases or know if you have one

17.5 Development after birth

Aging

• Stages of life: infancy, childhood, adolescence and adulthood • 3 hypotheses of aging: – Genetic origin : • suggests mitochondrial activity is involved with aging – Whole body processes • suggests aging may be a result from body systems or type of tissue – such as loss of hormonal activities or cross-linking in support tissues – Extrinsic factors : • suggests that aging may be due to years of poor health habits such as a poor diet and lack of exercise

17.5 Development after birth What are the effects of age on body systems?

• Skin becomes thinner, less elastic and dry • Less adipose in the skin so one feels colder more easily • Decrease in melanocytes leading to gray hair while some of the remaining cells are larger leaving “age spots” (dark spots on the skin) • Heart shrinks and arteries become more rigid • Reaction time slows and senses are muted

What are the effects of age on body systems?

• Lens in the eye loses ability to accommodate • Blood pressure usually increases • Bone density declines • Muscle mass decreases • Weight gain results from a decrease in metabolism and an increase in inactivity • Females undergo menopause and males andropause