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

Senger, pg 286

8-cell embryo

morula

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

• Trophoblast = chorion

EctodermNervous system, skin, hair

MesodermMuscle, skeleton, Cardiovascular system, Reproductive system

EndodermDigestive system, lungs, endocrine system

Senger, pg 81-82

ICM

Blastocoele

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

Endometrium

Fetal

Chorion

Senger, pg 308

Cotyledonary Placenta

Senger, pg 310

Cotyledonary Placenta

Fetus

Umbilicus

Placentomes

Types of Placentas

Maternal Blood

Maternal endothelium

Maternal connective tissue

Maternal epithelium

Fetal epithelium

Fetal connective tissue

Fetal endothelium

Fetal Blood

Type = degree of invasiveness, based on

layers separating maternal blood from the fetal

epithelium

Least Invasive epitheliochorial

synepitheliochorial

endotheliochorial

Most Invasive

hemochorial

SPECIES SHAPE TYPE

Cow Cotyledonary Epitheliochorial

Ewe Cotyledonary Synepitheliochorial

Sow Diffuse Epitheliochorial

Mare Diffuse Epitheliochorial

Dog, Cat Zonary Endotheliochorial

Primates Discoid Hemochorial

Rodents Discoid Hemochorial

Placentation by Species

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 70 daysExtensive Bone Formation 180 daysTooth Formation 110 daysHair, eyes, muzzle 150 daysHair over entire body 230 days   

Relative Development

Cleavage Differentiation Growth

More than 50% of the total weight of the fetus at parturition is gained during the last two months of gestation

fertilization

ZP Disintegration

parturition

placentation

Shift from classification as embryo to fetus

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

Cattle• Stages:

– 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 Migration-estrogen/protein

Diffuse epitheliochorial

Sow 113 days Estrogen Diffuse epitheliochorial

Lactation

1. Teat stimulation (suckling)

2. Stimulation of nerves in the spinal cord

3. Nerve signals to the brain

4. Release of oxytocin from posterior pituitary into blood

5. Oxytocin received by the mammary tissue

6. Contraction of alveoli7. Contraction of ducts8. Milk ejection

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3

2

4

56

7

8

5

Neuroendocrine Reflex Arc

Tissue of Mammary Gland:Secretory cellsAlveoliMyoepithelial cellsDuctsBlood Vessels

Mammary Gland Anatomy

Mammary Gland Anatomy

Initiation of Lactation: (lactogenesis)1. Increased formation and development of duct system2. Increased formation and development of secretory cells3. Development of capacity for milk synthesis

  

Hormones Required for Lactation 

1. Estrogen: stimulates duct growth2. Progesterone: stimulates alveolar development3. Growth Hormone (somatotropin): stimulates milk production4. Thyroid Hormones: enhance development and function of mammary

tissue5. Corticoids: enhance synthesis of enzymes necessary for milk biosynthesis6. Prolactin: initiation of lactogenesis7. Oxytocin: stimulate myometrial contractions to release milk from alveoli to ducts8. Insulin: maintain function and survival of secretory cells9. 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.