Gestation
The period of
fetal development
beginning with
fertilization and ending
with parturition
Leonardo da Vinci, "The Foetus in Utero," 1489
Fertilization
• Zygote: diploid cell resulting from the fusion of
the male and female pronuclei (syngamy)
• Embryo: organism in the early stages of
development; not recognizable as a member of a
specific species
Cleavage
• Following fertilization the zygote undergoes
several mitotic divisions inside the zona pellucida
(overall size does not change).
• 1st cleavage yields a 2 celled embryo,
– each cell is called a blastomere and is totipotent
• Divisions continue rapidly until the 32 cell stage
Preattachment development of the embryo
8-cell embryo
morula
Senger, pg 286
early
blastocyst
hatched
blastocyst
Maternal Recognition of Pregnancy
• Necessary to prevent luteolysis
• Occurs prior to implantation/placentation
• Differs among species
Pregnancy Recognition Signals
• Cow – bovine interferon tau (bIFNt); produced by
conceptus, days 15-16 of gestation are critical
• Sheep – ovine interferon tau (IFNt); produced by
conceptus, days 13-14 of gestation
• Sow – Estradiol; produced by conceptus and
causes a reroute of PGF2α; must occur days 11-12
• Mare – Protein/estrogen complex and embryo
migration; beginning day 12 embyro must migrate
12-15 times per day
Senger, pgs 291-295
Differentiation
• Formation of three germ layers
• Formation of the extraembryonic
membranes (placenta)
• Formation of organs
• Rapid changes in relative size
Three Embryonic Germ Layers
• Inner cell mass (ICM) = embryo
Mesoderm
Ectoderm
Muscle, skeleton,
Cardiovascular system,
Reproductive system
Nervous system, skin, hair
ICM
Endoderm
Digestive system, lungs,
endocrine system
Blastocoele
• Trophoblast = chorion
Senger, pg 81-82
Extraembryonic Membranes
Senger, pg 290
Function of Fetal Membranes and Fluids
Yolk Sac – Originates from fetal midgut.
Nutrient supply for early embryo.
Absorbs uterine sectretions from endometrium to stimulate early embryonic
development.
Amnion – Innermost membrane directly surrounding the fetus.
Protects from
injury and provides lubrication for parturition. Prevents lung collapse and opens
digestive tract.
Allantois – Originates from the gut and forms the umbilicus. Supports blood
vessels. Reservoir of nutrients and wastes.
Chorioallantois: product of fusion between the allantoic and chorionic membranes
Chorion –Outermost membrane in direct contact with uterine tissue.
Becomes
vascularized by allantoic vessels. Site of hormone production, nutrient and gas
exchange.
Placenta
PLACENTA: organ of nutrient and waste exchange between
the fetal blood and maternal blood
PLACENTAL SHAPE: defines the proportion of surface
area shared between fetal membranes and maternal uterine
tissue where exchange occurs
PLACENTAL TYPE: defines the structure of cell layers
separating fetal blood from maternal blood
Placental Shapes
• Diffuse – noninvasive
• Cotyledonary –
placentomes are the point
of high throughput
maternal/fetal contact
• Zonary/Discoid –
invasive; most direct
contact between fetal
and maternal blood
Senger pgs 308-310
Diffuse Placenta
Uterine
Fetal
Endometrium
Chorion
Senger, pg 308
Cotyledonary Placenta
Senger, pg 310
Cotyledonary Placenta
Placentomes
Umbilicus
Fetus
Types of Placentas
Maternal Blood
Maternal endothelium
Maternal connective tissue
Type = degree of
invasiveness, based on
layers separating maternal
blood from the fetal
epithelium
Maternal epithelium
Least Invasive
epitheliochorial
Fetal epithelium
synepitheliochorial
Fetal connective tissue
endotheliochorial
Fetal endothelium
Fetal Blood
Most Invasive
hemochorial
Placentation by Species
SPECIES
Cow
Ewe
Sow
Mare
Dog, Cat
Primates
Rodents
SHAPE
Cotyledonary
Cotyledonary
Diffuse
Diffuse
Zonary
Discoid
Discoid
TYPE
Epitheliochorial
Synepitheliochorial
Epitheliochorial
Epitheliochorial
Endotheliochorial
Hemochorial
Hemochorial
Hormones Produced by the Placenta
• equine Chorionic Gonadotropin: maintains primary CL,
responsible for formation and maintenance of accessory CL.
• human Chorionic Gonadotropin: maintain CL
• Progesterone: in some species (ewe, mare, woman) the placenta
takes over progesterone production later in gestation.
~”progesterone block” – inhibits myometrial contractions
• Estrogen: peak of E2 signals preparturient period in some species
• Placental Lactogen: stimulates growth of fetus and mammary glands
• Relaxin: softens connective tissue in the cervix and relaxes pelvic
ligaments
Fetal Growth
Growth: period of development from embryo to fully developed
fetus, prior to parturition
Example of development relative to time for the bovine
Calcification of Bone Matrix
Extensive Bone Formation
Tooth Formation
Hair, eyes, muzzle
Hair over entire body
70 days
180 days
110 days
150 days
230 days
Relative Development
parturition
Shift from
classification as
embryo to fetus
ZP Disintegration
fertilization
placentation
Cleavage
Differentiation
Growth
More than 50% of the total weight of the fetus at parturition
is gained during the last two months of gestation
Species
Cleavage
Differentiation
Growth
Cow
Days 0-12
Days 13-45
Days 45-283
Sheep
Days 0-10
Days 11-45
Days 45-147
Mare
Days 0-14
Days 15-68
Days 68-336
Sow
Days 0-6
Days 7-28
Days 28-113
Parturition
The process by which the uterus
expels the products of conception
Stages of Parturition
• Stage I – preparatory stage involving cervical
dilation and positioning of the fetus in the birth
canal via myometrial contractions.
• Stage II – time of hard labor and expulsion of
the fetus.
• Stage III – expulsion of the placental
membranes and subsequent uterine involution.
Brief Definitions
• Dystocia – difficult birth
– Usually refers to calving difficulty
• i.e., absolute or relative
• Presentation – direction of fetal delivery
– Anterior, posterior, or transverse
• i.e., anterior = head first
• Position – orientation of the fetus
– Dorsal or ventral side up or lateral
• Posture – location of the legs, head, and neck
– relates to normal/abnormal posture
Cattle
• Dystocia is the major cause of calf loss
– Relative Dystocia – normal sized calf and a small
birth canal
– Absolute Dystocia – abnormally large calf and a
normally sized birth canal
• Most common cause of dystocia is from an oversized calf
• Hiplock – shoulders of the calf “lock” onto the
bones of the pelvis during delivery
– Results in the calf getting stuck in the birth canal
• Stages:
Cattle
– Stage I – lasts from 1-4 hours (seldom noticed)
– Stage II – begins with the rupture of the allantochorion
• Should allow for 2-3 hours for a normal birth to occur without
interfering
– Stage III – begins after expulsion of the fetus
• Lasts up to 4 – 5 hours after fetal expulsion
• Presentation:
– Anterior
• Position:
– Dorsal side up
• Posture:
– Front legs first (slightly offset), bottom of hooves facing
down, and nose between front legs
Sheep
• The process of
parturition in the
sheep is very similar
to that in cattle
• Twins are present if a
front and rear leg
appear together
Swine
• Duration of farrowing
(parturition):
– 4 – 48 minutes per piglet
• Presentation:
– Anterior or posterior
• Position:
– Dorsal or ventral side up
• Posture:
– Not specific
Horses
Foaling time is difficult to predict
– Relaxation of the sacrosciatic ligament
– Distended mammary glands - days
• Waxing – within 48 hours
Foaling
•
Stages:
– Stage I – lasts ~ 1 hour
• Restlessness, sweating
in flanks
– Stage II – very rapid, lasts
~ 10-20 minutes
– Stage III – lasts from 1-12
hours
Horses
• Presentation:
– Anterior
• Position:
– Dorsal side up
• Posture:
– Front legs first (slightly offset), bottom of hooves
facing down, and nose between front legs
• It is crucial that the foal is born within 30 minutes
of the beginning of stage II. Failure to do so, will
most likely lead to the death of the foal.
– Due to the “crushing” of the umbilical cord between the
foal and the dam
• deprives the foal of oxygen
Hormones of Parturition
• ACTH – induces parturition in response to fetal
stress
• Fetal Cortisol – induces the release of PGF 2alpha
and produces the enzymes needed to convert
placental progesterone to estradiol
• Placental Progesterone – maintains pregnancy and
will be converted to estradiol during parturition
• Estradiol – enhances secretions of female tract
(lubrication) and myometrial contractions, as well as
enhance the receptors for PGF 2alpha and oxytocin
Hormones of Parturition
• PGF 2alpha – enhances myometrial contractions,
induces luteolysis and the release/secretion of
relaxin
• Relaxin – relaxes the pelvic ligaments, allowing
them to stretch for expulsion of the fetus
• Oxytocin – enhances myometrial contractions
(produces the most forceful contractions)
Induction of Parturition
**Not Recommended Unless the Life of the Dam or the Fetus is Threatened**
• Inducing parturition can enhance reproductive
management by controlling the time of
parturition
– Dangerous unless parturition is impending (within
one day of occurring)
– Retained placenta often occurs
• Forcing expulsion of the fetus and the fetal membranes
(i.e., the placenta) before the placenta is ready to detach
from the uterus
Agents Used to Induce Parturition
• Cattle
– Glucocorticoids
– Prostaglandins
– Estrogens
• Sheep
– Glucocorticoids
• If given after day 140,
response within 48hrs
• Swine
– Glucocorticoids
– Prostaglandins
• If given after day 110,
response within 30hrs
• Horses
– Glucocorticoids
– Prostaglandins
• All or none response
Oxytocin: Works well, but the dose and timing are extremely
important. All impending signs of parturition must be present.
(general rule: give ~ 1hr before parturition)
Senger, pg 321
Species Gestation
Length
Pregnancy
recognition
signal
Placental
Shape
Placental Type
Cow
283 days
IFNtau
Cotyledonary
epitheliochorial
Sheep
147 days
IFNtau
Cotyledonary
synepitheliochorial
Mare
336 days
Migrationestrogen/protein
Diffuse
epitheliochorial
Sow
113 days
Estrogen
Diffuse
epitheliochorial
Lactation
Neuroendocrine Reflex Arc
1.
2.
3
3.
4.
4
2
1
5
5.
5
6
7
8
6.
7.
8.
Teat stimulation
(suckling)
Stimulation of nerves in
the spinal cord
Nerve signals to the
brain
Release of oxytocin from
posterior pituitary into
blood
Oxytocin received by the
mammary tissue
Contraction of alveoli
Contraction of ducts
Milk ejection
Mammary Gland Anatomy
Tissue of Mammary Gland:
Secretory cells
Alveoli
Myoepithelial cells
Ducts
Blood Vessels
Mammary Gland Anatomy
Initiation of Lactation: (lactogenesis)
1. Increased formation and development of duct system
2. Increased formation and development of secretory cells
3. Development of capacity for milk synthesis
Hormones Required for Lactation
1. Estrogen: stimulates duct growth
2. Progesterone: stimulates alveolar development
3. Growth Hormone (somatotropin): stimulates milk production
4. Thyroid Hormones: enhance development and function of mammary
tissue
5. Corticoids: enhance synthesis of enzymes necessary for milk
biosynthesis
6. Prolactin: initiation of lactogenesis
7. Oxytocin: stimulate myometrial contractions to release milk from
alveoli to ducts
8. Insulin: maintain function and survival of secretory cells
9. Placental Lactogen: general mammary tissue growth
Adrenalin: prevents milk ejection; blocks secretion of oxytocin, causes vasoconstriction
in mammary gland
Administration of bST to dairy cattle increases milk production 15-40%. Suggested that bST
stimulates insulin-like growth factor-1 (IGF-1) which stimulates the secretory cells.