Development and Aging

Chapter 17

Stages of Development

Gamete formation

Fertilization

Cleavage

Gastrulation

Organ formation

Growth, tissue specialization

Cleavage

• Divides up cytoplasm

• Each blastomere ends

up with a portion

• Creates variation

among blastomeres

Three Primary Tissues

• Gastrulation rearranges blastomeres

and creates:

Endoderm

Mesoderm

Ectoderm

Organogenesis

• Process by which organs form

Cell determination

Cell differentiation

Morphogenesis

Fertilization

• Sperm penetrates to egg cytoplasm

• Secondary oocyte undergoes meiosis II;

forms mature egg

• Egg nucleus and sperm nucleus fuse to

form diploid zygote

Pregnancy

• Averages 38 weeks from fertilization

• Takes 2 weeks for blastocyst to form

• Weeks 3 to 8 are embryonic period

• Weeks 9 to birth are fetal period

Early Divisions

• Cleavage begins within 24 hours of fertilization

Day 1 Day 2 Day 3 Day 4(morula)

Day 5 - Blastocyst Forms

• Cell secretions produce a fluid-filled

cavity in center of ball of cells

• Layers of blastocyst

– Inner cell mass

– Trophoblast

– Blastocoel

inner cell mass

Implantation Begins

• Blastocyst attaches to endometrium; begins to burrow into maternal tissues

blastocoel

inner cell mass

trophoblast

Uterine cavity

Human Chorionic Gonadotropin (HCG)

• Hormone secreted by the blastocyst

• Stimulates corpus luteum to keep making progesterone and estrogens

• This maintains endometrium, prevents menstruation

• Can be detected by week 3 with a home pregnancy test

Extraembryonic Membranes

DAY 14

yolk sac

chorionic cavitychorionic

villi

chorion

amniotic cavity

connecting stalk

• The amniotic

membrane will

enclose embryo

• Yolk sac forms

• Chorion begins to

form fingerlike villi

The Placenta• Interlocking fetal and

maternal tissues

• Performs digestive, respiratory, and urinary functions for the fetus

• Materials exchanged across membrane that separates bloodstreams

Gastrulation - Day 15

• Primitive streak

forms along one

axis of the inner

cell mass

• Cells migrate

inward here to form

endoderm and

mesoderm

Vertebrate Body Plan Emerges

paired neural folds

somites

pharyngealarches

Morphogenesis

• Orderly changes result in specialized tissues and early organs

• Cells migrate

• Whole sheets of cells expand and fold

• Programmed cell death sculpts body parts

Cell migration

Embryonic Period

• Weeks 3 to 8

• By the close of embryo period

– Appears human

– Primordial tissues of all internal and external structures have formed

Week 4

Week 8

Fetal Period

• Weeks 9 to birth

• Fetus is initially about 1 inch long

• Fetus born before 22 weeks cannot survive

• Survival is poor before 28 weeks because lungs are not fully formed

• By 36 weeks, survival is 95 percent

Fetal Circulation

• Fetal hemoglobin

• Temporary bypass vessels form and

function until birth

• Umbilical arteries

• Foramen ovale and the arterial duct

• Venous duct

Birth (Labor)

• Cervical canal dilates

• Amniotic sac ruptures

• Uterine contractions

drive fetus from uterus

• Placenta is expelled as

afterbirth

Lactation

• During pregnancy, progesterone and estrogen stimulate gland development

• After birth, prolactin induces synthesis of enzymes for milk production

• Oxytocin triggers contractions

Cloning Embryos

• Biological carbon copy of an organism

• Use chemicals to spur unfertilized, donated eggs to divide

OR

• Fuse enucleated egg with an adult cell

Fetal Nutrition

• All nutrients for fetal growth and

development must be delivered via

the placenta

• Mother’s diet affects fetal health

• Smoking may affect ability to absorb

nutrients and to pass them to fetus

Teratogens

• Drugs and environmental factors that may induce deformities during development

• Effect depends upon time of exposure

• Most have no effect until after second week

• Thalidomide

Stages of Human Development - Prenatal

• Zygote - Single cell

• Morula - Solid ball of cells

• Blastocyst - Ball with fluid-filled cavity

• Embryo - 2 weeks to 8 weeks

• Fetus - 9 weeks to birth

Stages of Human Development - Postnatal

• Newborn - First 2 weeks after birth

• Infant - 2 weeks to 15 months

• Child – To 10-12 years

• Pubescent - At puberty

• Adolescent - Puberty to maturation

• Adult

• Old age

Aging

• Programmed life-span hypothesis

• Cumulative-assaults hypothesis

Aging Skin and Hair

• Number of fibroblasts in the dermis starts to decrease

• Elastin replaced by collagen

• Loss of sweat glands

• Hair follicles die or become less active

• Pigment-producing cells die

Aging Muscle and Bone

• Fibers in skeletal muscle atrophy

• Lost muscle tends to be replaced by fat

and, with time, collagen

• Bones become weaker, more porous,

and brittle

• Cartilage in joints deteriorate

Aging Cardiovascular and Respiratory Systems

• Walls of alveoli break down

• Heart muscle becomes slightly smaller

• Less blood and oxygen are delivered to

muscles and other tissues

• Blood vessels become narrowed and

less elastic

Aging Nervous System

• Brain neurons die steadily throughout life

• Neurofibrillary tangles

• Beta amyloid plaques

• Memory loss

• Slowed reflexes

• Farsightedness

Aging Reproductive Systems

• Reduced secretion of estrogens and progesterone triggers menopause

• Falling levels of testosterone reduce male fertility

• Vascular changes impair ability to achieve erection

• Prostate-gland enlargement

Aging Immunity, Nutrition, and Urinary System

• Number of T cells falls and B cells become less active

• Mucus-secreting glands of GI tract degenerate

• Basal metabolic rate declines

• Weakening of muscles causes urinary incontinence