Module 7

defined as the time from fertilization

to the earliest (primordial) stages of organ development

Embryonic Period

the time between the embryonic period and

parturition (the end of gestation), during which organs grow

and begin to function.

Fetal Period

each daughter cell of the cleavage process is termed a

blastomere

inner blastomeres form gap junctions to maximize intercellular

communication; they are destined to become

Inner cell mass

outer blastomeres become flattened and form tight junctions; they develop the capacity to secrete fluid; They are destined to become

Trophoblast

To form ________ _______, one fertilized egg cell (ovum) splits and develops two embryo with exactly the same genetic information.

identical twins

where two egg cells (ova) are fertilized by two sperm and produce two genetically unique embryo, who are no more alike than individual neonate born at different times.

Fraternal twins

There are two types of twins

• – Monozygotic - Identical

• -- Dizygotic - Fraternal

(united ventrally at the chest);

Thoracopagus

(united ventrally at the abdomen);

Omphalopagus

(united ventrally at head and chest);

Cephalopagus

(united ventrally at the pelvis);

Ischiopagus

United laterally

Parapagus

one body, two heads

Dicephalus

one body, two faces

Diprosopus

(united at the heads);

Craniopagus

(united caudally);

Pygopagus

(united dorsally at the spine)

Rachipagus

How many flexures does the tube-shaped embryo undergo?

Three flexures

What shape does the embryo become after undergoing flexures?

C-shaped

Formation of blood vessels

Angiogenesis

Where does angiogenesis begin?

In the splanchnic mesoderm of the yolk sac

What are blood islands composed of?

Mesenchyme and hemocytoblasts

Where are blood islands found in the early embryo?

Area opaca (around the area pellucida)

What type of cells form blood islands?

Mesenchyme cells

What do blood islands give rise to?

Embryonic blood vessels

What is the function of blood islands?

Initiate formation of the embryonic circulatory system

What type of placenta involves almost the entire surface of the allanto-chorion?

Diffuse

Which animals have a diffuse placenta?

Horses and pigs

What type of placenta has multiple discrete areas of attachment called cotyledons?

Cotyledonary

In cotyledonary placentation, what are the maternal contact sites called?

Caruncles

What is the cotyledon-caruncle complex called?

Placentome

Which animals have cotyledonary placenta?

Ruminants

What type of placenta forms a complete or incomplete band around the fetus?

Zonary

Which animals have zonary placentas?

Carnivores (dogs, cats, seals, bears, elephants)

What type of placenta is single and _______ in shape?

Discoid

Which animals have a discoid placenta?

Primates and rodents

Chorion touches endometrial epithelium

Epitheliochorial placenta

Chorion touches maternal endothelium

Endotheliochorial placenta

Chorion touches maternal blood

Hemochorial placenta

Which placenta type retains all maternal layers?

Epitheliochorial

Fetal chorion contacts endometrial epithelium — what placenta type?

Epitheliochorial

Animals with epitheliochorial placenta?

Horses, pigs, ruminants

Which placenta type retains only uterine endothelium?

Endotheliochorial

Fetal chorion contacts maternal blood vessel endothelium — what placenta type?

Endotheliochorial

Animals with endotheliochorial placenta?

Dogs, cats

Which placenta type loses all maternal layers?

Hemochorial

Fetal chorion directly contacts maternal blood — what placenta type?

Hemochorial

Animals with hemochorial placenta?

Humans, rodents

region(s) of apposition between uterine lining and fetal

membranes where metabolites are exchanged for sustaining

pregnancy.

Placenta

forms the surface fetal membrane.

Chorion

is a discrete area of interdigitation between a maternal

caruncle and a fetal cotyledon.

Placentone

are microcotyledonary (microplacentomes are distributed

diffusely).

Equine placentas

consist of rows of relatively large placentomes.

Ruminant placentas

Forms the outer boundary of the conceptus

Chorion

Encloses the embryo in a fluid-filled cavity

Amnion

Membrane that provides umbilical vessels

Allantois

Membrane most important in egg-laying vertebrates

Yolk sac

Origin of chorion

Trophoblast

Origin of allantois

Hindgut splanchnopleure

Function of yolk sac in horses and dogs

Temporary placenta

Membrane that binds to inner surface of chorion

Allantois