Development and Aging Chapter 17. Stages of Development Gamete formation Fertilization Cleavage...

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