Fetal Growth and Development
Overview of Postnatal and Prenatal Growth in Mammals
Introduction to Pre and Post Natal Growth
Objective of Upcoming Lectures (Weeks 8-12):
In-depth exploration of:
Pre and postnatal growth
Fetal growth in model species
Development and function of fetal organs
Hormonal control of fetal growth
Maternal influences on fetal development
Adaptations to pregnancy
Conditions of the placental environment
Terminology:
Prenatal: Before birth
Perinatal: Around the time of birth
Neonatal: Newborn phase
Postnatal: After birth
Pubertal: Transition to reproductive maturity.
Importance of Prenatal and Postnatal Stages
Impact on Lifetime Reproductive Success:
Affects future reproductive capabilities in both genders.
Growth and Development of Fetus:
Critical for ensuring the fetus develops properly.
Survival and Health Outcomes:
Influences neonatal survival rates, health post-birth, and growth trajectories.
Human Impact:
Discussion on whether human actions can affect these outcomes.
Model Species in Reproduction and Growth Studies
Use of Model Species for Research:
Selection based on unique female reproduction characteristics.
Types of Model Species:
Livestock:
Historical data from extensive observational studies.
Control over genetic, environmental, management, and dietary factors.
Essential for global food and resource production.
Humans:
Extensive knowledge but considerable variability, difficult to replicate.
Companion Animals:
Dogs and cats: Limited control but significant human relevance.
Research Species:
Rodents like rats and mice: Economically viable models for human relevance.
Wild and exotic species: Useful but limited in detailed observations.
Stages of Gestation and Development in Mammals
Gestation Trimesters:
First Trimester:
Major activities: Axis orientation, embryo stage, and placental attachment.
Second Trimester:
Focus on fetal stage: Organ and limb formation.
Third Trimester:
Final organ development and functional growth.
Influence of Stress/Disease:
External stress or disease can induce developmental abnormalities by altering hormone levels, impacting rapidly dividing cells at critical stages.
Growth vs Development Definitions
Growth:
Defined as the increase in length, height, or weight of an organism, organ (e.g., brain), or tissue (e.g., fat).
Development:
A systematic process of cell growth and differentiation that modifies organ and body structure and function until maturity.
Hormonal Regulation of Growth
Allometric and Isometric Growth Patterns:
Allometric Growth:
Body parts develop at different rates.
Isometric Growth:
Body proportions remain constant throughout growth except in limited species.
Species Variability:
Reference to findings from Hossner (2005) regarding hormonal regulation in farm animals.
Influences on Fetal Growth Rates
Differential Growth Changes:
Growth phases lead to shifts in body proportions (head, trunk, limbs).
In early fetal stages, the head/brain represents a larger body length percentage before diminishing in proportion.
Impact Factors on Growth Rates:
Maternal nutrition extremes.
External stressors affecting maternal hormonal levels.
Pre and Postnatal Growth Phases
Timeline of Growth:
Initiates at fertilization and concludes with puberty.
Growth Characteristics:
Most evident in fetal and postnatal phases:
Long bones continue to develop until puberty.
Estrogen plays a role in growth plate closure, terminating height increase.
Post-Puberty Changes:
Following puberty, further increases in size and weight of the body and organs, driven by muscle, fat, and bone growth.
Growth Curves and Patterns
Animal Weight Growth Patterns:
A: Sigmoid Curve (Body weight × Age):
Common in mammals; weight increases sharply during fetal and postnatal periods until maturity.
B: Curvilinear Growth Curve (Weight gain/day × Age):
Shows peak daily weight gain postnatally, slowing down before puberty.
Critical Thinking Question:
What are the potential implications of maximized weight gain during all growth phases?
Comparative Fetal Development among Model Species
Relationship between Maternal Size and Fetal Growth:
Heavier maternal body size corresponds to a greater total litter birth weight ($r = 0.99$) and placenta weight ($r = 0.90$).
Fetal weight gain rate also increases ($g/d$) with maternal size ($r = 0.95$).
Data Overview from Various Species:
Gestation Length and Growth Rates:
The table below summarizes growth data for different species:
Cow:
Maternal weight: 545 kg
Individual birth weight: 35 kg
Gestation length: 280 days
Fetal growth rate: 125 g/d
Pig:
Maternal weight: 180 kg
Individual birth weight: 1.3 kg
Gestation length: 115 days
Fetal growth rate: 11.3 g/d
Sheep:
Maternal weight: 70 kg
Individual birth weight: 4.5 kg
Gestation length: 150 days
Fetal growth rate: 30 g/d
Human:
Maternal weight: 56 kg
Individual birth weight: 3.5 kg
Gestation length: 280 days
Fetal growth rate: 12.5 g/d
Cat:
Maternal weight: 5 kg
Individual birth weight: 0.1 kg
Gestation length: 63 days
Fetal growth rate: 1.59 g/d
Rat:
Maternal weight: 0.15 kg
Individual birth weight: 0.005 kg
Gestation length: 21 days
Fetal growth rate: 0.24 g/d
Considerations for Reproductive Methods
Factors Affecting Reproductive Outcomes:
Newborn maturity impacts the amount of maternal resources needed.
Availability of maternal space and size relative to fetal size is critical.
Determination of gestation length based on maternal nutritional support capacity.
Maternal potential for lactation and growth in offspring.
Adaptations for climate and nutrient availability impacting reproductive strategies.
Species Variation in Maturity at Birth
Categories of Development at Birth:
Altricial Species:
Immature at birth, reliant on maternal care (e.g., rodents, humans, dogs, cats).
Precocial Species:
Well-developed at birth, able to fend for themselves quickly (e.g., most livestock, many wild animals).
Factors Influencing Fetal Growth Within a Species
Key Drivers of Fetal Growth Rates:
Uterine development, profoundly influenced by:
Size of the dam and gene expression pre and postnatally.
Maternal and fetal hormones and development factors.
Nutritional status of the mother.
External stressors, particularly in the climate.
Organ and Tissue Growth Stages
Hormonal Influence on Growth:
Largest mass changes occur in vital tissues (organs, muscle, bone) during growth stages.
Primary hormones involved include:
Growth Hormone (GH)
Insulin-like Growth Factor 1 (IGF-1)
Insulin
Thyroid hormones
Sex steroids
Glucocorticoids .
Comparative Body and Brain Development
Observations on Development Across Species:
Precocial Species:
Higher initial brain-to-body weight ratios at birth.
Altricial Species:
Develop less and have lower ratios.
General Phases of Organ/Tissue Growth
Distinct Growth Phases by Organ Type:
Pre and Postnatal Phases:
Rapid development phases organized as follows:
Head and brain tissues grow fastest during the fetal and neonatal phases.
Bone and muscle exhibit growth efficiency in neonatal to postnatal phases.
Adipose tissues develop last, with intramuscular fat being the final tissue to reach maturity.
Mediated through Hormones:
• GH
• IGF 1
• Insulin
• Thyroid hormones
• Sex Steroids
• Glucocorticoids
Uterine Development
• the key driver of fetal growth rate
• Mediated by
• Dam size (pre and post-natal gene expression effects)
• Maternal and Fetal Hormones and growth factors
• Maternal nutrition
• External stressors (environment-climate)
Endocrine, paracrine and autocrine hormones
and growth factors regulate Cell growth and cell
division
Fetal Organ Development Stages
Organ Identification:
By the 2nd trimester, organs are formed yet still developing.
Neural System:
Rapid growth seen in 2nd and 3rd trimesters, sensory systems become functional.
Circulatory System Development:
Heart and vascular systems are formed by the end of the 2nd trimester.
Specific Organ Developments in the Fetus
Fetal Skeleton:
Formation progresses from cartilage to primary and secondary bone. Growth plates remain active until puberty.
Lungs:
Develop structurally in the 2nd trimester, becoming identifiable by the 3rd trimester but not functionally capable until birth.
Kidneys:
Form in the 2nd trimester leading to functional nephrons in the 3rd trimester.
Liver and G.I. System Development
Liver:
Formation begins in the 2nd trimester and continues developing functionality (e.g., glycogen storage, amino acid breakdown, and part of blood function).
Gastrointestinal System:
Remains non-functional during fetal life, not digesting nutrients but relying on placental supply.
Reproductive System Development Stages
Gonadal Differentiation:
Variability in timeline across species (25-80% completion during the 2nd trimester).
Endocrine Influences:
Gonadotropins stimulate gonadal production of hormones (e.g., testosterone and estrogen).
Reproductive system
• variation in gestation stage of gonad differentiation (25-80%,
Table/image cat)
• By 2nd trimester
• Gonadotropins from fetal pituitary bind gonads
• Testes produce Testosterone and Ovary produces some
testosterone and can aromatize to Estrogen (up to birth)
• Endocrine system
• Thyroid Active (Thyroid) in early 2nd trimester and impacts
metabolism and liver glucose
• Adrenal Active in 3rd TRI with cortisol increase
• Mammary glands
• Develop in lateral, caudal or pectoral lines in fetus by specie
Canine Fetal Development Highlights
Gestation Duration:
Approximately 64 days, with key developmental milestones noted by specific days (e.g., structures visible at day 20, eyes and body hair distinguishing features noted afterward).