INTRODUCTION TO HUMAN DEVELOPMENT - · PDF fileWhat is Embryology Embryology is ......

INTRODUCTION TO

HUMAN DEVELOPMENT

Dr. Nabil Khouri

Objectives

To provide a general description of human embryological

processes as follows:

Germ cell formation & fertilization

Prenatal development

Induction, competence, and differentiation

Formation of three-layerd embryo

Formation of neural crest

Formation of human tissues

What is Embryology

Embryology is literally “the study of the embryo”

More generally it refers to “the study of prenatal development”

Embryo = 2-8 weeks (our focus)

Foetus = 9 weeks to birth

Embryology: From Egg to Embryo Terms

1. Pregnancy – events occurring from the time of fertilization (conception) until the infant is born2. Gestation period – extends from the last menstrualperiod until birth (280 days)3. Conceptus – refers to the developing offspring at any time during the pregnancy

A) Pre-embryo – first two weeks following fertilizationB) Embryo – from the third through the eighth weeks after fertilizationC) Fetus – ninth week through birthD) Infant – at birth

Development involves:

1) Cell division and differentiation

Differentiation is the creation of different types of cells

2) changes in anatomical structures

Anatomical changes include gradual modification of physical and physiological characteristics

Development begins at fertilization

Differentiation and development

Development can be divided into: Prenatal and postnatal

development

Prenatal development begins at fertilization and ends with birth

Prenatal development includes:

Embryological development

Changes occurring during the first two months after

fertilization

Fetal development

Begins at the start of the ninth week and continues until

birth

Postnatal development

Commences at birth and continues to maturity

Stages of development

Fertilization is fusion of two haploid gametes (egg and

sperm) each with 23 chromosome to produce a zygote

that contains 46 chromosomes

Fertilization occurs in the uterine tubes

Within a day of ovulation

Spermatozoa cannot fertilize an ovum until after

capacitation

Fertilization (conception) “Definition”

Embryology: Fertilization “fusion of sperm and egg”

1. Copulation must occur accordingly

A) Egg is viable for 12-24 hrs after ovulation

B) Sperm is viable for 24-72 hrs after ejaculation

C) Copulation must occur no more than three days before ovulation and no later than 24 hours after

2. Sperm must reach the egg

A) Only a few hundred thousand sperm in a males ejaculate actually make it to the uterine tubes

1) Millions leak immediately from the vagina

2) Millions are destroyed by acidity of the vagina

3) Only about 2,000-3,000 sperm actually make it to the egg

Acrosomal reaction

3. The process of Sperm penetration requires 1. Capacitation, 2.

Acrosomal reaction, 3. Fertilization membrane formation

1) Capacitation – as the sperm swims towards the oocyte, the cholesterol that keeps the acrosome “tough” degrades causing the membrane to weaken and the enzymes within to be released Takes about 6-8 hours

2) Acrosomal reaction – release of the acrosomal enzymes triggered by the sperm coming into contact with oocyte membranes

a) First-arriving sperm create holes in the corona radiata it often takes hundreds of sperm to create the holes.

b) A later-arriving sperm reaches the zona pellucida and releases enzymes creating a hole in the zona pellucida

c) Once the sperm reaches the oocyte membrane, its nucleus is pulled into the oocyte cytoplasm

Spermatozoa release hyaluronidase and acrosin

Enzymes required to penetrate corona radiate

Single spermatozoan contacts oocyte, fertilization begins

Oocyte activation

Polyspermy prevented by membrane depolarization and cortical reaction

3). Upon entry of the nucleus, the oocyte creates a Fertilization membrane just beneath the zona pellucida

forces out other entering sperm & prevents entry of

future sperm

Ovulation occurs before the oocyte is completely mature

i.e. oocyte is ovulated before the process of meiosis is

completed

Ovulated oocyte is in metaphase of meiosis II

If the egg is fertilized the process of meiosis will

complete

The released oocyte is surrounded by a thick

specialised extracellular matrix, the ZONA PELLUCIDA,

that in turn is covered in layers of cells,

the GRANULOSA LAYER.

If fertilization does not occur, the oocyte disintegrate

without completing the process of meiosis

The Oocyte at Ovulation

https://embryology.med.unsw.edu.au/embryology/index.php/Zona_pellucida

https://embryology.med.unsw.edu.au/embryology/index.php/Granulosa_cell

Fertilization

https://youtu.be/7G2rL5Cutd4

Fertilization Steps

• Step 1 Ovulation

• At ovulation the oocyte is in metaphase of meiosis II

• Both the occyte and the polar body is surrounded by corona radiate

• Step 2 Fertilization

Oocyte is surrounded by the sperms

Acrosomal enzyme from several

sperms creates gaps in corona radiata

One sperm makes contact with the

oocyte membrane

• Sperm and oocyte fused

• The process of meiosis will complete

The first trimester is the period of embryological and

early fetal development

Four processes occur during the first trimeter

1) Cleavage

2) Implantation

3) Placentation

4) Embryogenesis

The First Trimester

Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.

Discussed

Pre-embryonic Development

1. CLEAVAGE occurs as the zygote travels through the

uterine tube and into the uterus

A) Period of rapid mitotic divisions

1) 2-cell stage – 36 hours after fertilization

2) 4-cell stage – about 48 hours

3) 8-cell stage – about 72 hours

4) Morula – solid ball of cells that is 16 or more

cells big

Cleavage and

Formation of

Morula and

Blastocyst

2. The morula hollows out and fills with fluid (now known as a blastocyst)

A) Blastocoel – hollowed-out region of the blastocyst

3. The zona pellucida disintegrates and releases the blastocyst

A) The blastocyst is composed of 2 cell layers

1) Trophoblasts –the large flattened cells of the outer layer

a) will take part in placenta formation

b) b) secrete hCG to prompt the corpus luteum to continue secreting progesterone in order to maintain the endometrium

2) Inner Cell Mass (embryoblast) – a cluster of small

rounded cells of the inner layer becomes the actual embryo

Division of

the zygote Zygote. Formation of

the morula

Morula

Blastula Or

Blastocyt

Structure of the blastocyst (blastula):

Implantation

A) When the blastocyst reaches the uterus, it initially floats

freely, receiving nourishment from the endometrial secretions

(uterine milk)

B) Six or seven days after ovulation, the trophoblast cells embed

into the endometrium and begin secreting digestive enzymes

that degrade the endometrial surface

C) As the endometrium is eroded, the blastocyst burrows into the

lining

D) The endometrial lining reacts by growing over the blastocyst

E) The chorion develops from the trophoblast cells starting to give

rise to the placenta

Copyright © 2006/07 The

University of Sharjahslide 24

Implantation of the blastocyte:

Uterine wall

Endometrium

Digested

endometrium

Blastocyte

Relation of

Blastocyst to

Endometrium –

Time of

Implantation

Implantation

Decidua

As the blastocyst implants, usually on the posterior wall of the fundus or body of the uterus, it is oriented so that the inner cell mass faces the endometrium

Different regions of the decidua are named based on their positions relative to the site of the implanted blastocyst

1. Decidua basalis

2. Decidua capsularis

3. Decidua parietalis

Relation of Blastocyst to Endometrium – Time

of Implantation

Implantationhttps://youtu.be/pO84bn6IhGU?list=PLG4nig

1r_0xyqML6OvgCX0QXAq8cRkQsp

Embryonic Development

A. The blastocyst is converted into the gastrula in which the

embryonic membranes develop and three primary germ

layers form

1. Gastrulation – process by which the embryonic

tissues are formed

2. The embryonic membranes form as the inner cell

mass splits to form upper and lower cell layers

During the Second Week of Development

Development of the trophoblast: the trophoblast

develops two layers:

1. outer syncytiotrophoblast

2. inner cytotrophoblast

Trophoblast produces human chorionic gonadotropin

(hCG)

Events of Second Week of Development

Second Week of Development

Development of the bilaminar embryonic disc

hypoblast (primitive endoderm)

epiblast (primitive ectoderm)

bilaminar embryonic disc

Amniotic cavity

Amniotic fluid

Events of Second Week of Development



Development of the yolk sac

Exocoelomic membrane

in humans, the yolk sac is relatively empty, small, and

progressively decreases in size

it has several important functions

Embryology

A) Amnion (amniotic sac) – forms from the upper cell layer

This sac fills with amniotic fluid that provides a buoyant

environment that protects the developing embryo

B) Yolk sac – forms from the lower cell layer It serves to form part

of the digestive tube, produces the earliest blood cells and blood

vessels, and is the source of primordial germ cells of the embryo’s

gonads

C) Allantois – forms as a small out-pocketing of the yolk sac

1) Acts as the structural base of the umbilical cord and becomes

part of the urinary bladder

D) Chorion – develops from proliferating trophoblast cells giving

rise to the placenta


Development of sinusoids: small spaces called

lacunae

Lacunar networks

Sinusoids

maternal blood and endometrial secretions enter the

lacunar networks to provide embryonic nutrition and to

serve as a disposal site for embryonic wastes


Development of the chorion

it surrounds the embryo and, later, the fetus

it eventually becomes the major embryonic component of the

placenta

Gonadotropin (hCG)

Chorionic cavity

Connecting (body) stalk, the future umbilical cord


Third Week of Development

Gastrulation

the bilaminar embryonic disc is transformed into a trilaminar embryonic disc consisting of three primary germ layers:

ectoderm, mesoderm, and endoderm

Gastrulation

primitive streak

primitive node

Invagination results in formation of the three primary germ layers


Formation of the tri-laminar disc:

Primitive streak

Bilaminar disc

Epiblast layer

Hypoblast layer

Third week

Formation of primitive streak at the midline causing the disc to have right

and left half, each half will be as mirror to the other

Ectoderm layer

Endoderm layer

Mesoderm layer

Formation of the mesodermal layer:

Movement of some cells of epiblast toward hypoblast at the

primitive streak forms a third embryonic layer called mesoderm.

During the third week

The primary germ layers form along the embryo

A) Ectoderm – gives rise to skin and nervous system

B) Endoderm – gives rise to the functional linings of the

digestive, respiratory, and urogenital systems

C) Mesoderm – gives rise to muscle, bone, blood vessels,

kidneys and all the other components of organs (except

linings)


Gastrulation

Third Week of Development

Notochordal process develops

notochord

The (1) oropharyngeal membrane and the (2)

cloacal membranes develop

Gastrulation:https://www.youtube.com/watch?v=3AOoikTEfeo&list=PLG

4nig1r_0xyqML6OvgCX0QXAq8cRkQsp&index=3

Development of Notochordal Process

The embryonic layers are distinct from each other and

give rise to specific tissues such as:

Ectoderm

Nervous system.

Sensory epi. of eye , ear & nose.

Epidermis, hair and nails.

Mammary & cutaneous glands.

Epi.of sinuses,oral &

nasal cavities.

Intra-oral glands.

Tooth enamel.

Formation of the tissues:

Formation of the tissues:

Mesoderm

Muscle tissue

C.T

Bone .

Cartilage.

Pulp.

Dentin .

Cementum.

PL.

Ecto-mesoderm

Alveolar Bone .

By the end of the Third Week of

Development

Allantois develops

Neurulation

The head end of the neural tube develops into the

brain

Formation of Neural

Crest

Neural plate: will fold to

form neural groove

Neural groove: fuses to

form neural tube

Neural tube: Gives rise to

brain and spinal cord

Neurulation:

https://www.youtube.com/w

atch?v=lGLexQR9xGs&list=

PLG4nig1r_0xyqML6OvgCX

0QXAq8cRkQsp&index=4

Neurulation and Development of Somites

Fourth Week of Development

Organogenesis

Embryonic folding

head fold, tail fold, and two lateral folds

Formation of the primitive gut

Pharyngeal (throat)

Future ears and eyes occurs

Upper limb buds, lower limb buds, and a heart

prominence

Embryonic

Folding

https://www.yout

ube.com/watch?v

=yXUv4MPuNTA&

list=PLG4nig1r_0

xyqML6OvgCX0Q

XAq8cRkQsp&ind

ex=5

Development of

Pharyngeal

Arches, Clefts,

and Pouches

Fifth Through Eighth Weeks of Development

Rapid development of the brain and head

Development of distinct limb regions

Further development of the eyes and ears

Tail disappears

External genitals begin to differentiate

End of the eighth week, the embryo has clearly

human characteristics


Placenta Formation

A. Functions to exchange waste products and blood gases

B. The chorion develops chorionic villi which extend

into the endometrium where they come into

contact with maternal blood supply

C. Placenta takes over the role of secreting hCG

and also secretes relaxin: causes the pubic

symphysis to soften and become more flexible

Placentation

The process of forming the placenta

Shaped like a pancake, it consists of a fetal portion

formed by chorionic villi and a maternal portion

formed by the decidua basalis


Development of the Chorionic Villi and

Placenta

Development of lacunae

Chorionic villi grow and blood capillaries develop

within them

Maternal and fetal blood do not normally mix


Development of Chorionic Villi


Placenta

Umbilical cord consists of:

two umbilical arteries that carry deoxygenated fetal

blood and wastes from the fetus to the placenta

one umbilical vein that carries oxygenated blood and

nutrients from the placenta into the fetus

Wharton’s jelly


Placenta and Umbilical Cord


Embryology

4. Circulation in fetus versus newborn

A) Fetal circulation has several adaptations so that the

lungs and liver are largely bypassed because they are

non-functional

1) The umbilical vein carries oxygen- and nutrient-

rich blood from the placenta to the fetus

2) The umbilical arteries carry deoxygenated,

waste-laden blood from the fetus to the placenta

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