Early Embryogenesis: Key Stages

Early Embryogenesis

• Attachment to the uterus requires development within the zona pellucida, blastocyst hatching, maternal recognition of pregnancy, and formation of extraembryonic membranes
• Ootid definition: the stage where male and female pronuclei are visible in the oocyte's cytoplasm
• Syngamy is the fusion of male and female pronuclei, resulting in a zygote

Embryo vs. Fetus vs. Conceptus

• Embryo: an organism in early development that doesn't resemble a specific species
• Fetus: a potential offspring in the uterus, generally recognizable as a specific species
• Conceptus: the product of conception, encompassing the embryo in early stages, the embryo with extraembryonic membranes, and the fetus with the placenta

Preattachment Embryonic Development

• Cleavage divisions: mitotic divisions the zygote undergoes
• First cleavage division product: a two-celled embryo
• Morula definition: >32 individual blastomeres
• Blastocyst formation:
  • Outer cells flatten with pressure
  • Fluid accumulates
  • Gap junctions polarize inner cells
• Outer cells flatten during blastocyst formation
• Gap junctions allow Na+ pumping into the embryo, enabling H2O accumulation

Blastocyst Composition

• Inner cell mass (ICM) forms the embryo proper
• Trophoblast forms the fetal placenta
• Blastocoele is fluid filled

Blastocyst Layers

• Endoderm develops into the digestive, lung, and endocrine systems; it is the inside layer
• Mesoderm develops into muscle, skeleton, cardiovascular, and reproductive systems; it is the middle layer
• Ectoderm develops into the nervous system, skin, and hair; it is the outside layer

Regulation of Hatching

• Hatching is regulated by blastocyst growth, fluid accumulation, trophoblast enzymes weakening the zona pellucida, and intermittent pressure pulses from blastocyst contraction

Requirements of Hatched Blastocyst

• Relies on nutrients secreted into the uterine lumen
• Requires adequate progesterone production by the corpus luteum (CL)
• Requires appropriate uterine responsiveness to progesterone

Post-Hatching Morphology

• In pigs, cattle, and sheep, embryo morphology changes from spherical to tubular to filamentous after hatching

Preattachment Period

• Many domestic species have a prolonged preattachment period lasting weeks
• Primates and rodents experience blastocyst implantation very quickly after entering the uterus

Maternal Recognition of Pregnancy Definition

• Biochemical signal produced by the conceptus in sufficient amounts, at the correct time, to prevent pregnancy termination

Goal of Maternal Recognition

• To produce sufficient progesterone by the CL
• Necessary for embryogenesis and attachment to the endometrium to occur
• Must occur before luteolysis

Maternal Recognition Mechanisms

• Cow & Ewe (Interferon Tau - IFNT):
  • Secreted by trophoblast cells
  • Acts on endometrial cells to prevent OTR expression and PGF2 alpha pulses
  • Stops oxytocin binding
  • Induces secretion of proteins from uterine glands for fetal use
• Mare (3 proteins/estrogens):
  • Doesn't change shape
  • Requires conceptus contact, moving 12-14 times/day via uterine contractions
  • Prevents PGF2 alpha production
  • Uterine Migration Phenomenon
• Sow (Estradiol):
  • Conceptus secretes estradiol (PGF2 alpha still produced)
  • E2 reroutes PGF2 alpha into the uterine lumen to be destroyed
  • Endocrine-Exocrine Theory
• Woman (Human chorionic gonadotropin - hCG):
  • LH-like protein supporting CL
  • Promotes trophoblast cells of the chorion

PGF2A Secretion

• Large luteal cells secrete oxytocin, which binds to oxytocin receptors in the endometrium, stimulating secretion of PGF2A during the late luteal phase

IFNT Secretion

• Secreted by trophoblast cells within the conceptus

IFNT's Effect on Endometrial Cells

• It Prevents OTR expression, stops oxytocin binding, and prevents PGF2 alpha pulses

Use of IFNT-Induced Secretions

• Used by the conceptus

Detrimental Action of IFNT

• IFNT can travel to the ovary, causing luteolysis

MRP in Pigs

• Estradiol and the endocrine-exocrine theory

Oxytocin Production (Cyclic Sow)

• Produced by the endometrium, posterior pituitary, and corpus luteum

Non-Pregnant Sow

• Oxytocin binds to OTR
• PGF2 alpha synthesis is promoted
• PGF2 alpha enters the uterine vein
• Transported to the ovary, causing luteolysis

Pregnant Sow

• Conceptus secretes estradiol (E2)
• E2 reroutes PGF2 alpha into the uterine lumen for destruction
• No PGF2 alpha enters the uterine vein, preventing transport to the ovary and luteolysis

Number of Embryos in Pigs

• Requires two fetuses present in each uterine horn, for a total of four

Uterine Migration

• In mares, the presence of conceptus prevents luteolysis by moving around via uterine contractions (12-14 times per day)
• Allows distribution of pregnancy recognition factors to endometrial cells (proteins and estrogen)

hCG Role in Humans

• Supports the growth of the CL, trophoblast cells, and placental development

Term Definitions

• Yolk Sac: Primordial germ cell and blood stem cell source that regresses in the first trimester
• Amnion: Protective sac filled with amniotic fluid allowing symmetrical fetal development
• Chorion: Outermost placental layer derived from trophectoderm, taking up nutrients
• Allantois: Fills with nutrient-rich fluid, fuses with chorion, and provides blood supply
• Allantochorion: Final fusion of allantois and chorion, contributing to the placenta

Placenta Functions

• A transient metabolic interface between mother and fetus(s)
• Functions as an endocrine gland to maintain pregnancy
• Promotes fetal and mammary growth
• Induces parturition

Ways to Classify a Placenta

• Chorionic villus distribution
• Number of layers between fetal and maternal blood

Chorionic Villus Function

• Small, finger-like projections on the chorion surface
• Increases surface area for nutrient exchange
• Functional unit of the fetal placenta

Placentation Types (Villus Distribution)

• Diffuse: uniform villi distribution (pig, horse)
• Cotyledonary: villi on button-like cotyledons (sheep, cattle)
• Zonary: villi zone around the conceptus middle (dog, cat)
• Discoid: villi in regionalized disc (humans, primates, rodents)

Epitheliochorial Placenta

• 6 layers
  1. Endometrial capillaries
  2. Endometrial interstitium
  3. Endometrial epithelium
  4. Chorionic epithelium
  5. Chorionic interstitium
  6. Chorionic capillaries

Synepitheliochorial Placenta

• 6 layers like epitheliochorial, uterine epithelium temporarily erodes, then regrows

Endotheliochorial Placenta

1. Endometrial capillaries
2. Endometrial interstitium
3. Chorionic epithelium
4. Chorionic interstitium
5. Chorionic capillaries

Hemochorial Placenta

1. Chorionic epithelium
2. Chorionic interstitium
3. Chorionic capillaries

Placentas by Invasiveness

• Least to Most Invasive: Epitheliochorial -> Synepitheliochorial -> Endotheliochorial -> Hemochorial

Placenta Types by Species

• Epitheliochorial: sows, mares
• Synepitheliochorial: cattle, sheep
• Endotheliochorial: cat, dog
• Hemochorial: primates, rodents

Nutrient Transport Across Placenta

• Occurs via vascular system, simple/facilitated diffusion, and active transport

Transport Across Placenta

• DO NOT cross: Lipids, Large peptide hormones, antibodies (except in humans/rodents)
• DO cross: glucose, amino acids, minerals, oxygen, water

Placental Hormones

• Progesterone and Estrogens maintain pregnancy, support ovarian function and fetal growth, aid mammary development, and induce parturition

Unique Factor of Mare Placenta

• Endometrial cups secrete eCG (pregnant mare serum gonadotropin-PMSG)

Endometrial Cups

• Maintains the CL and therefore progesterone production
• Initiates accessory CL formation to boost hormone levels

hCG Function

• hCG is from trophoblast cells of the chorion and provides a luteotropic stimulus for the CL

Placental Lactogen Function

• This is found in many species
• Stimulates fetal growth (somatotrophic effects) and mammary gland development/function (lactogenic effects)

Progesterone Function(s)

• Blocks myometrial contractions
• Stimulates endometrial gland secretion of nutrients in early pregnancy

Signs of Parturition

• Behavioral changes: restlessness, solitary behavior, anxiety, pawing/stamping, digging a bed or nest
• Mammary glands development
• Vulva swelling
• Pelvic ligaments relax

Parturition Events

• Parturition involves a complex cascade of physiological events triggered by the fetus
• Labor involves distinct stages (0-3)
• Myometrium transforms from quiescent to highly contractile
• Cervix remodels through softening and dilation
• Fetal membranes rupture
• Uterine contents expel
• The Uterus repairs and returns to its non-pregnant state (involution)
• Uterine space becomes a limiting factor releasing adrenal corticotropin (ACTH) by the fetal pituitary
• Stimulates fetal adrenals to release cortisol

Initiating Parturition

• Fetal cortisol increases reproductive tract secretions, especially in the cervix
• The Progesterone block is removed allowing myometrial contractions

Progesterone Block Removal

• Fetal cortisol causes the placenta to synthesize PGF2A
• PGF2A facilitates the required progesterone decline
• PGF2A release stimulates myometrial contractions

Relaxin Role

• Glycoprotein from the CL or placenta
• PGF2A stimulates its release
• Prepares birth canal by loosening supportive tissues

End of Pregnancy

• Relaxin softens the cervix and promotes pelvic ligament elasticity
• Increased estradiol, decreased progesterone, varying by species, depending on which part produces the progesterone

Progesterone Block Removal

• Progesterone conversion to estrogens decreases progesterone
• Estrogens stimulate myometrial contractions and increase mucus production in the cervix and vagina

Ferguson Reflex

• Progesterone block removal leads to estrogen and myometrial contractions
• Pushes fetus against the cervix
• This Initiates a neuroendocrine reflex as the posterior pituitary releases oxytocin
• Oxytocin travels through the bloodstream interacting with the myometrium to increase contractions

Myometrial Contractions & Cervical Dilation (Stage 1)

• Behavioral changes: restlessness, solitary behavior, anxiety
• Progesterone block removed
• Estrogen increases cervical and uterine mucous for lubrication and contraction
• Relaxin allows stretching and relaxing of the pelvic ligaments
• Usually the longest stage of parturition
• Fetus enters the birth canal which puts pressure on the cervix
• Signals to the pituitary to release oxytocin - Ferguson Reflex
• Repeated myometrial contractions

Fetal Expulsion (Stage 2)

• Rapid phase
• Ideal fetus position
  • Head and front feet first: cattle, sheep, horses
  • Both forwards and backward normal: pigs, cats, dogs
• Strong myometrial and abdominal contractions
  • Significant changes in placental structure and function causes fetal hypoxia(stress) induced movement
• Placental membranes rupture causing fluid discharge with the amnion that must be removed
• Umbilical cords break or are bitten off

Placental Expulsion (Stage 3)

• Usually a rapid process after parturition
• Requires release of chorionic villi through vasoconstriction
• Occasionally there some complications like retained placental membranes occur

Dystocia

• Common parturition complication
• One of the major causes of offspring loss from Fetus position/ presentation issues, fetal size imbalances, birth canal capacity issues, and multiple births
• Can lead to delays in placental expulsion

Puerperium

• The four key events during that point of recovery are currently undisclosed/unwritten

Myometrial Contractions Importance

• They facilitate discharge of fluids and tissue debris from the uterus
• Compresses the uterine vasculature to minimize hemorrhage risk
• Reduces the uterus size

Lactational Anestrus

• Suppression of ovulation and estrous cycle caused by the suckling stimulus impact on GnRH and LH

Metritis (Uterine Infection)

• Bacterial infection of all uterine layers, associated with systemic infection, ranging from mild to severe

Endometritis (Endometrium Infection)

• Infection of only the endometrium, impacting postpartum fertility in cattle
• Indicators: absence of heat behavior, delayed ovulations onset, increased time between ovulations

Pyometra

• Accumulation of pus in the uterus

Lactation

• The capacity of animals to produce milk through the mammary gland is used to support newborn development

Fetal Mammogenesis

• Mammogenesis is the variation by location and number of species
  1. Fetal Mammary Ridge Develops
  2. ) Primary Mammary Bud Develops as Epidermal Tissues grow inward into the dermis also named mesenchyme
  3. Secondary Mammary Bud Develops as Primary Bud Branches out as it grows into the dermis
  4. Canalization (allows passage of milk. Duct system and myoepithelial cell formation from milk producing units to the teat)

Mammogenesis:

• Stages of Growth and Development of the Mammary Gland
• Birth to puberty: Isometric mammary gland growth
• Puberty to pregnancy: Allometric Mammary Gland Growth
  • Repeated estrogen/progesterone exposure (estrous cycles)
  • E2- duct size and number, P4- alveoli development
  • Increase duct and alveolar structure development
  • Growth slows after several estrous cycles until pregnancy
  • Overfeeding during prepubertal period (cattle) causes more fat deposition and lesss secretory tissue, decreasing future milk production
• Pregnancy to parturition: 90% mammary tissue in late gestation

Lactation:

• Colostrum: First milk containing high # of immunoglobulins (2-3 days) with passive immunologic protection
• Decreased antibody transfer causes negative long term health consequences

Milk Let Down

1. Tactile Stimulation of Teat
2. Sensory impulses travel through afferent nerves to the hypothalamus
3. Oxytocin released from posterior pituitary travels in circulation to mammary gland
4. Causes contraction of myoepithelial cells and milk squeezed from alveolus into small ducts and then larger ducts.

Milk Fever

• Known as Hypocalcemia.
• Calcium is depleted faster than replenished inside blood and bones.

Mastitis

• Inflammation of the udder system
• Varies in degrees of severity and in causation