Chapter 3

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Chapter 3 Outline

• Overview of Embryology

• Gametogenesis

• Pre-embryonic Period

• Embryonic Period

• Fetal Period

Stages of Prenatal Development

Pre-embryonic periodFirst 2 weeks after fertilization of egg/formation of zygote

Embryonic periodThird through eighth weeks when all major organ systems begin to develop

Fetal periodNinth through thirty-eighth weeks when growth dominates; fetal period ends at birth

Human Development

• From fertilized egg through adult maturation– Fertilization to birth = embryogenesis– After birth

• maturation of body and reproductive organs

• production of sex cells (gametes), eggs or sperm = gametogenesis

Life Cycle of Humans

Figure 3.1

Gametogenesis

• Gamete (sperm or egg) production– gametes are haploid (contain 23 chromosomes)– all other body cells are diploid (have 23 pairs of

chromosomes)• in mitosis a diploid cell produces two

genetically identical diploid “daughter” cells

• Reproductive organs produce haploid cells by meiosis

Meiosis

• Division of a diploid cell producing two haploid “daughter” cells– resulting cells are not identical to each other– crossing over may occur allowing exchange

of genetic material between paired homologous chromosomes• producing gametes that contain a

combination of genes from both parents = genetic diversity

Meiosis

• Occurs in diploid cells of testes and ovaries

– each contain 46 chromosomes (23 from each parent)

– results in 4 haploid cells

– involves two division cycles

• meiosis I and meiosis II

Meiosis I

Figure 3.2

Meiosis I: Prophase I

1. Nuclear envelope breaks down.2. Homologous double-stranded

chromosomes pair up in a process called synapsis to form a tetrad.

3. Crossing over ensures genetic diversity in future generations.

Meiosis I: Metaphase I

1. Pairs of homologous tetrads form two rows in the center of the cell.

2. Each row is a mix of tetrads from mother and father.

3. Spindle fibers from centrioles attach to the paired chromosomes.

Meiosis I: Anaphase I

1. Pairs of homologous (double-stranded) chromosomes are pulled to opposite ends of cell.

2. Daughter cells will receive a random combination of maternal and paternal sister chromatids.

3. This random separation of maternal and paternal sister chromatids is called reduction division.

Meiosis I: Telophase I

1. Chromosomes arrive at far ends of the cell.

2. Nuclear membranes form around the two sets of chromosomes.

3. A cleavage furrow forms and cytoplasm is divided by cytokinesis into two daughter cells.

4. Each daughter cell has 23 double-stranded chromosomes (each chromosome has two sister chromatids).

Meiosis I

Figure 3.2

Meiosis II

Figure 3.2

Meiosis II: Prophase II

1. Resembles prophase of mitosis

2. Nuclear envelope breaks down in daughter cells from meiosis I

3. Double-stranded chromosomes collect near center of cell

4. Crossing over occurs in first meiotic prophase only

Meiosis II: Metaphase II

1. Double-stranded chromosomes form a single line at the equator of each daughter cell.

2. Spindle fibers extend from the centrioles and attach to the centromeres of the double-stranded chromosomes.

Meiosis II: Anaphase II

1. Sister chromatids of each double-stranded chromosome are pulled apart at the centromere.

2. Each chromatid is now a single-stranded chromosome.

3. The single-stranded chromosomes migrate to opposite poles of the cell.

Meiosis II: Telophase II

1. Nuclear envelopes form around each set of single-stranded chromosomes at opposite ends of the cell.

2. A cleavage furrow forms and the cell’s cytoplasm divides by cytokinesis.

3. The daughter cells are now haploid containing only 23 single-stranded chromosomes.

Summary of Meiosis

1. Starts with one diploid cell

2. Meiosis I produces two diploid daughter cells

3. Meiosis II turns two diploid cells into four haploid cells

4. Crossing over only occurs in prophase I

Oogenesis

• Parent cells that produce haploid oocytes (eggs) through meiosis are oogonia.

• Oogonia are located in the ovaries and enter prophase I during fetal development.

• Oogenesis stops in females until puberty.

• The cells in prophase I are primary oocytes.

Oogenesis─Continued

• Monthly, after puberty, a number of primary oocytes begin to mature by resuming meiosis I.

• Meiosis I produces two daughter cells but cytokinesis divides the cells unequally.– The smaller cell is a polar body and will die.

– The larger cell is the secondary oocyte, which stops developing at metaphase II and will be ovulated.

Oogenesis─Concluded

• If fertilized, the secondary oocyte completes meiosis II.– Meiosis II produces two daughter cells with

uneven division of cytoplasm.• The larger cell is the ovum, containing 23

chromosomes that will combine with the 23 provided by the sperm that fertilized it.

• The smaller cell is a polar body that dies.• If the secondary oocyte is not fertilized, it

degenerates in about 24 hours.

Ovulation

• The ovum is expelled from the ovary with two surrounding structures:– the corona radiata─several layers of

cuboidal cells– the zona pellucida─a clear layer of

proteins on the ovum under the corona radiata

• Sperm must penetrate both structures in order to fertilize the ovum

Spermatogenesis

• Parent cells that produce haploid sperm through meiosis are spermatogonia– only live in the testes of the male– each spermatogonium divides by mitosis to

produce two genetically identical cells called primary spermatocytes

Spermatogenesis

• Each primary spermatocyte undergoes meiosis producing four haploid spermatids containing 23 chromosomes.

– Spermatids must undergo further changes called spermiogenesis to become mature sperm.

Structure of Mature Sperm

Figure 3.3

Mature Sperm

• Sperm deposited in the female reproductive tract are unable to fertilize a secondary oocyte.– They must undergo capacitation or

conditioning in the vagina to change the membrane of the acrosome, a membranous cap at the head of the sperm.

– The acrosome contains digestive enzymes that will be released upon contact with the cells of the corona radiata and facilitate the penetration of the sperm’s nucleus into the cytoplasm of the egg.

1. Pre-embryonic Period

• Fusion of sperm and secondary oocyte is fertilization– usually occurs in upper 1/3 of uterine tube– nucleus of ovum fuses with nucleus of sperm

(properly called pronuclei prior to fusion)– resulting single diploid cell is the zygote

• on rare occasions, two or more sperm may penetrate the egg’s cytoplasm, a condition called polyspermy that is immediately fatal

Figure 3.3

Week 1 (Early)

• After the zygote is formed, it undergoes a series of mitotic divisions called cleavage.– The number of cells increase, but total size remains

the same.– This process, called compaction, results in increased

contact between the cells.

• A 16-cell stage organism is called a morula.– The morula arrives in the uterine cavity about day 3 or

4.

Week 1 (Late)

• One to 2 days after the morula enters the uterine cavity, it develops a fluid-filled cavity in its center.

• This cavity is the blastocyst cavity and the organism is now a blastocyst.

Figure 3.4

Week 1 (Late)

• Shortly after blastocyst formation, differentiation forms two regions:– trophoblast─outer ring of cells that will

develop into the chorion– embryoblast (inner cell mass)─ cluster

of tightly packed cells inside one portion of the trophoblast• cells of the inner cell mass are

pluripotent (able to differentiate into any cell type found in the human body)

Week 1 (Late)

• At the end of the first week after fertilization, the zona pellucida has degraded.– The trophoblast can make direct contact

with cells that line the inside of the uterus.• The cells that line the inside of the

uterus form a layer called the endometrium.

Week 1 (Late)

• The endometrium consists of two layers:– deep basal layer and superficial functional layer

• blastocyst invades the functional layer• its trophoblast turns into two layers:

– inner cellular layer─cytotrophoblast– outer thick layer─ syncytiotrophoblast,

which continues to invade the endometrium and pulls the blastocyst deeper into the endometrium

– by end of week 2, the blastocyst has disappeared from the surface of the endometrium

Figure 3.6

Week 2 (Early)

• By day 8, the cells of the embryoblast differentiate into two distinct types:– hypoblast─layer of small cuboidal cells

facing the blastocyst cavity– epiblast─layer of columnar cells deep to

the hypoblast

• Together, these two layers form a flat disc called the bilaminar germinal disc

Week 2 (Early)

• The bilaminar germinal disc and trophoblast produce three extraembryonic membranes:– yolk sac– amnion– chorion

Week 2 (Early)

• The yolk sac is formed from and is continuous with the hypoblast layer.

• It does not store yolk in humans but does serve as a site for early blood cell and vessel formation.

Week 2 (Early)

• The amnion is a thin layer of cells that forms above and is derived from the epiblast.

• A fluid-filled amniotic cavity appears between the amnion and epiblast layer.– The fluid is produced by the cells of the

amnion and will protect the embryo from “drying out.”

Figure 3.6

Week 2 (Early)

• The chorion is the outermost membrane and is formed by the rapidly expanding syncytiotrophoblast and cytotrophoblast.

• A major function of the chorion is the formation of the placenta.

Figure 3.7

Week 2 (Late)

• The placenta is a highly vascularized organ that serves as a physical and biochemical interface between embryo and mother.

• The main functions of the placenta are– exchange of nutrients, waste products,

and blood gases between embryo and mother.

– transmission of maternal antibodies to the embryo.

– production of many hormones, predominantly estrogen and progesterone.

Week 2 (Late)

• The placenta is comprised of tissues from both embryo and mother.– The embryonic portion of the placenta is the

chorion.– The maternal portion is from the functional

layer of the endometrium.

Week 2 (Late)

• The early embryo is attached to the placenta by a structure called the connecting stalk.

• Eventually, the connecting stalk will develop into the umbilical cord through which the umbilical arteries and veins will be transmitted.

Week 2 (Late)

• Fingerlike projections called chorionic villi appear at the leading edge of the chorion.– The villi project into the functional layer of the

endometrium.– Inside the villi are branches from umbilical

blood vessels (embryonic source).– Outside the villi is maternal blood.– Metabolic exchange in the placenta occurs

across the wall of the villi.

Figure 3.7

2. Embryonic Period

• Weeks 3–8

• One of the earliest events to occur during week 3 is the establishment of three primary germ layers from which all adult human structures are derived (except the embryonic part of the placenta)

• By the end of week 8, the main organ systems have developed

Gastrulation

• The process by which cells from the epiblast migrate to form all three primary germ layers– starts during week 3 with formation of

the primitive streak• Once all three germ layers are present,

the trilaminar structure can be called an embryo

Primitive Streak

Figure 3.9

Primitive Streak

• A thin depression on the surface of the epiblast– the cephalic end of the streak is raised and

thickened forming the primitive node– a depression in the node is the primitive pit

• Cells from the epiblast layer move through the primitive streak to locate themselves between the epiblast and hypoblast layers

Primitive Streak

Figure 3.8

Primary Germ Layers

• The cells between the epiblast and hypoblast layers become the primary germ layer known as mesoderm.

• Other migrating cells displace the hypoblast cells and become endoderm.

• Cells remaining in the epiblast will become ectoderm.

• All three germ layers are derived from the epiblast.

Folding of the Embryonic Disc

• Early in week 3, the embryo is a flattened disc-shaped structure.

• During late week 3, the embryo begins growing faster than the space in which it resides.

• In order to continue growing, the embryo must begin a series of folds.

Folding of the Embryonic Disc

• Three folds of the embryo occur during weeks 3−4:

– cephalocaudal (cephalic = head; caudal = tail) fold

– transverse (lateral) fold

Folding of the Embryonic Disc

Figure 3.10

Ectoderm

• Ectoderm is located on the external surface of the embryo

• Ectoderm cells will eventually develop into the following structures:

– epidermis of the skin

– derivatives of epidermis, including hair and nails

– nervous system

Neurulation

• The formation of the neural tube from overlying ectoderm cells is called neurulation.– The neural tube will develop into the nervous

system.• The formation of the neural tube begins with the

appearance of the notochord, which is derived from mesoderm.– The notochord is a rod-shaped structure

internal and parallel to the primitive streak.

Notochord

Insert Figure 3.11a

Figure 3.11

Neurulation

• The notochord induces the overlying ectoderm to begin the formation of the neural tube.– Thickening of the overlying ectoderm forms a neural

plate.– The lateral edges of the neural plate form neural

folds.– The depression between the folds is the neural

groove.– The neural folds approach midline and fuse to form

the neural tube.

Figure 3.11

Mesoderm

• The middle germ layer forms five regions:– notochord─tightly packed midline cells– paraxial─beside notochord, develops into units

called somites that form axial skeleton, muscle, dermis of the skin, and most connective tissues

– intermediate─lateral to paraxial, develops into most of the urinary and reproductive systems

– lateral plate─lateral to intermediate, forms most components of cardiovascular system, lining of all body cavities, and connective tissue of the limbs

– head mesenchyme─forms the connective tissue and musculature of the face

Figure 3.11

Endoderm

• Will develop into many internal structures following the foldings of the embryo– linings of the digestive, respiratory, and

urinary systems

Derivatives of the Germ Layers

Figure 3.12

Organogenesis

• The process constructing the organs of the body– rudimentary forms of most organ systems are

complete by the end of the embryonic period (week 8)

– during this period normal development of organs can be interfered with by agents called teratogens• a teratogen is any agent that can cause

congenital malformations (birth defects)

3. Fetal Period

• Begins at week 9 and ends at birth (usually week 38)

• Characterized by maturation and growth of tissues and organs

Fetal Period