3 - Development

Development

gradual modification of anatomical structures and physiological characteristics from fertilization to maturity

Development is how anatomical structures ______

change

Why to cells differentiate?

to create the diverse tissues of the adult

Cells differentiate due to changes in gene expression

some genes are “turned off” while others are “turned on”

Developmental History of a Human

Pre-embryonic period

Embryonic period

Fetal period

Pre-embryonic period

First two weeks after fertilization of egg

Embryonic period

Third through eighth weeks

The embryonic period is where all major organ systems:

begin to develop

Fetal period

Ninth through thirty-eighth weeks

The fetal period is where growth and development:

continues and ends at birth

Fertilization

fusion of two haploid gametes, secondary oocyte and sperm, 23 chromosomes

zygote

diploid cell with 46 chromosomes

Oocyte is _______ times larger than sperm

2000

What provides organelles and nourishment for ~1 week?

oocyte

Fertilization occurs:

in uterine tube within a day of ovulation

Spermatozoon Head

contains nucleus and chromosomes

Acrosome

a compartment at tip of spermatozoon head

Acrosome contains

enzymes essential to fertilization

Spermatozoon Neck

contains centrioles

Spermatozoon Middle piece

mitochondria and microtubules

Spermatozoon Tail

only flagellum in the human body

The secondary oocyte leaving the follicle is in

metaphase of meiosis II

Cells of the corona radiata

surround the oocyte

For fertilization to occur,

sperm must penetrate the corona radiata

The sperm binds receptors in

the zona pellucida beneath corona radiata

enzymes from the acrosome

digest a path to surface of oocyte

when the sperm contacts the surface,

the sperm and oocyte membranes fuse

Oocyte activation

Fusion between sperm and oocyte membranes

Enzymes inactivate sperm receptors and harden the zona pellucida

• only one sperm fertilizes

• Dad’s contribution = 23 chromosomes, 1 centriole, and

• determines sex

The oocyte completes

meiosis II

Cellular metabolism increases as

many proteins necessary for development are rapidly produced

Amphimixis

the male and female pronuclei (1n+1n) fuse

2n Cell =

zygote has 46 chromosomes

The zygote begins

cleavage, a series of cell divisions

A single cell ultimately develops into

a mature adult

Differentiation

changes in genetic activity of specific cells

As the zygote divides, cells have

differing cytoplasmic compositions

differing cytoplasmic compositions

alters the genetic activity of each cell

facilitate the development of diverse cell lines

The time spent in prenatal development

gestation

Gestation is divided into

three trimesters (each 3 months long)

First Trimester

Period of embryological and early fetal development

Rudiments of all major organ systems appear

Second Trimester

Development of organs and organ systems

Body shape and proportions change

Third Trimester

Rapid fetal growth and deposition of adipose tissue

Most major organ systems are fully functional

First Trimester includes four major stages

Cleavage

Implantation

Placentation

Embryogenesis

Cleavage

a sequence of cell divisions that transforms a zygote into a multicellular blastocyst

Implantation

begins when the blastocyst attaches to the endometrium and continues as the blastocyst invades maternal tissues

Placentation

occurs as blood vessels form around the blastocyst and the placenta develops

Embryogenesis

the formation of a viable embryo with foundations for all the major organ systems

The First Trimester

Most dangerous period in prenatal life

~40% of conceptions survive first trimester

Pregnant women should avoid drugs and other disruptive stresses

The cells produced by cleavage divisions

blastomeres

3 days of cleavage, the pre-embryo is a solid ball of cells

morula

The cells of the morula reorganize into a hollow ball

blastocyst

blastocoele

inner cavity

blastocoele

blastomeres then form

trophoblast

inner cell mass

trophoblast

outer layer facilitates nutrient exchange

inner cell mass

cells that will form the embryo

Implantation occurs

~7 days after fertilization

Blastocyst reaches

uterine lining

Trophoblast

Erodes into uterine epithelium

Divides into 2 layers

secretes human Chorionic gonadotropin

Formation of the Bilaminar Germinal Disc and Extraembryonic Membranes

~ 8 days, bilaminar germinal disc

bilaminar germinal disc

two layers of differentiated cells

bilaminar germinal disc layers

Hypoblast

Epiblast

3 extraembryonic membranes:

1. Yolk sac

2. Amnion

3. Chorion

1. Yolk sac:

early blood cell and vessel formation

2. Amnion:

contains fluid-filled amniotic cavity

3. Chorion:

Outermost membrane forms part of placenta

Main functions of the placenta:

Metabolic

Immune

Endocrine

Metabolic function of the placenta:

share resources with mom

Immune function of the placenta:

isolate baby from mom

Endocrine function of the placenta:

produces hormones itself; takes over the ovaries

connecting stalk

develops into the umbilical cord

contains umbilical arteries and vein

Placenta is both embryo and mother

Embryo – chorion/chorionic villi contain branches of umbilical vessels

Mom - functional layer of the endometrium

Gas, waste, nutrient, and some immunity exchanged

the placenta also acts as

an endocrine organ

Human chorionic gonadotropin (hCG), human placental lactogen (hPL)

hormones produced by the placenta

placental prolactin

helps produce milk

relaxin

targets connective tissue

softens collagen in CT

• collagen connections between bone

Land R colaxal bone

hips widen

• - opens up birth canal

progesterone, estrogens

sex hormone production

The Endocrine Placenta maintains

uterine lining

The Endocrine Placenta prepares

mom for delivery and childcare

At ~12 days, what stage is development at?

gastrulation

~12 days, Gastrulation

Form all three primary germ layers from epiblast cells

Ectoderm, mesoderm, endoderm

Once all three primary germ layers are present =

embryo

Starts with epiblast cells migrating through

primitive streak

All three germ layers are derived from

the epiblast

Cells between the epiblast and hypoblast

mesoderm

Some displace the hypoblast cells

endoderm

Cells remaining in the epiblast

ectoderm

deuterostome

animals with a second mouth and anus

Embryonic Period begins with

establishment of three primary germ layers

By end of embryonic period,

main organ systems are developed

At 1 month, the embryo is

about 0.2 in long and weighs 0.02 g

At 2 months, the embryo is

about 1 inch long and weighs about 2.7 g

At 3 months, the embryo is

about 3 inches long and weighs about 26 g

During Second Trimester

Fetus grows faster than surrounding placenta

Growth in length

Mom’s survival probability lowers as

baby’s weight increases

During Third Trimester

Most of the organ systems become ready

Growth rate starts to slow

Largest weight gain

Fetus and enlarged uterus displace many of the mother’s abdominal organs

At 4 months, the embryo is

about 5 inches long and weighs about 150 g

Pregnancy and Maternal Systems

Developing fetus is dependent on maternal organ systems

Maternal adaptations include increases in:

Respiratory rate and tidal volume

Blood volume

Nutrient and vitamin intake

Glomerular filtration rate

Size of uterus and mammary glands

Third Trimester involves

Structural and Functional Changes in the Uterus