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VOCABULARY LIST
AIDS - Acquired Immuno-Deficiency Syndrome
Antibiotic - A chemical substance produced by a microorganism which has the capacity
to inhibit the growth of or kill another microorganism.
Antibodies - Also known as immunoglobulins, they are a collection of proteins produced
by the body in response to antigens.
Antigen - In the classic sense of the word, an antigen is a foreign protein capable of
inducing an immune response. However, you may see the word used
to describe various proteins found on the surface of certain cells within
the body (example - MHC Class I & II antigens).
Antitoxin - Antibody produced by the body against a toxin.
Autoimmune disorder - A condition in which constituents of the body's immune system
turn against it's own cells or tissues.
Attenuate - To render less virulent (pathogenic).
B lymphocyte - Cellular component of the immune system capable of producing antibodies
in response to antigens.
Basophil - Cellular component of the immune system. Following stimulation, basophils
release histamine that causes the symptoms of allergies.
CD markers - (Cluster designation markers) Cell surface markers (proteins) which are used
to differentiate different populations of cells.
CD4+ T cells - The CD4 marker is found on T helper cells. CD4+ cells recognize antigens
in association with MHC Class II molecules.
CD8+ T cells - The CD8 marker is found on T cytotoxic/supressor cells. CD8+ cells
recognize antigens in association with MHC class I molecules.
Carrier animal - An animal which no longer displays symptoms of disease but is capable
of spreading the disease to other animals.
Colostrum - By definition, colostrum is NOT milk. It is the first secretion of the mammary gland
after parturition. For milk marketing purposes, the first six milkings (3 days) are considered
to be colostrum, and therefore not sold.
Disease - A disturbance in the structure or function of an organ or body part.
Eosinophil - Cellular component of the immune system which is important in the
destruction of parasites.
Febrile - An overall increase in body temperature ("Fever").
Health - A state in which all parts of the body are functionally normal.
Immunity - Protection against disease
Active Immunity - Immunity acquired by the production of antibody or lymphoid
cells by the body in response to antigenic stimulus. Can develop in response to
vaccination or infection by a pathogenic organism.
Passive Immunity - Temporary immunity acquired by the transfer of antibody or
lymphoid cells from an immune donor. Examples: Ingestion of colostrum by
the newborn.
Infectious disease - Disease caused by or capable of being communicated by disease
producing organisms.
Interleukins (IL) - A group of molecules involved in signaling between cells of the immune
system.
Leukocytes - White blood cells. Leukocytes develop from pluripotent stem cells originating from
the bone marrow. Leukocytes include T lymphocytes, B lymphocytes,
Monocytes/Macrophages, neutrophils, basophils, and eosinophils.
Macrophage - Cellular component of the immune system capable of ingesting and destroying
foreign particles. Following ingestion, macrophages break down the foreign particle and
reexpress parts of the particle on it's cell surface to present to T cells & B cells.
Major Histocompatability Complex (MHC) - A genetic region whose products are responsible
for graft rejection between individuals and function in signalling between lymphocytes and
antigen presenting cells.
Morbidity - A diseased condition or state. Animal is not producing to the best of it's
capabilities.
Mortality - Incidence of death.
Neutrophil - Cellular component of the immune system which posesses the ability to ingest
and destroy foreign particles.
Parasite - An organism that lives upon or within another, causing harm to the host.
Pathogen - Disease producing organism.
Pyrogen - A fever producing substance. Can be produced by bacteria or leukocytes.
T lymphocyte (T cell) - Cellular component of the immune system:
T helper (TH) cell - A subclass of T cells that help generate T cytotoxic cells and
cooperate with B cells in antibody production.
T cytotoxic (TC) cell - A subclass of T cells that have the ability to kill cells.
Toxin - A poison.
Vaccine - A suspension of attenuated or killed microorganisms (bacteria, virus)
administered for the prevention of disease.
Vector - A living carrier that transfers and infectious agent from one host to another.
Examples: Mosquitoes, flies.
Vehicle - A non-living carrier of infectious agents. Examples: Contaminated boots,
transporting vehicles, non-sterile surgical instruments & needles.
Zoonotic disease - Disease that is transmittable from animal to human.
DEVELOPMENT OF THE IMMUNE SYSTEM
Neonatal Adaptive Immune System
Gut mucosal immunity
The majority of the development of intestinal muscosal immunity occurs during the first postnatal
months. In human infants, IgA producing B cells are not present in the gut until 10d of age. A few
IgG and IgM producing cells are present in the fetus – these numbers significantly increase after
birth, and the percentage of IgM producing cells is actually higher in infancy than later in life. In
intestinal epithelium, HLA class I molecules are expressed throughout gestation, while HLA class II
molecules are not expressed until birth, and increase in numbers until 2 weeks of age.
T Cell and Thymus Development
T lymphocyte (T cell) - Cellular component of the immune system:
T helper (TH) cell - A subclass of T cells which help generate T cytotoxic cells and
cooperate with B cells in antibody production. CD4+ cells.
T cytotoxic (TC) cell - A subclass of T cells which have the ability to kill cells. CD8+ cells.
CD markers - (Cluster designation markers) Cell surface markers (proteins) which are used
to differentiate different populations of cells.
Within the thymus, T cells are "educated" to recognize self vs. nonself. The thymus is a
lymphoepithelial organ (stromal background is epithelial tissue). It is a bi-lobed structure in man,
but can have a series of nodules in other species. There is a connective tissue capsule which
extends down into the organ, dividing it into septa or lobules. The cortex contains a population of
interdigitating cells, while the medulla contains a layer of epithelial cells and Hassall's corpusles
(degenerating interdigiting cells). Blood vessels typically enter the cortex - medulla and branch out.
Four layers of cells on blood vessels control the influx of cells in the adult (blood-thymus barrier).
However, fetal & neonatal thymic blood vessels are more permeable to the passage of molecules
and cells. Most lymphocyte precursors arrive in thymus prior to vascularization. Incoming
progenitor cells settle in the cortex and divide, filling the cortex with small cells called thymocytes.
More than 90% of the cells generated within the cortex die there – a very inefficient process.
Those cells which survive mature into T cells. The maturation process starts at cortex and travels to
medulla, where the T cells develop the ability to recognize self vs. nonself. What is responsible
for T cell education?
1. Interdigitating cells associated with epithelial cells are rich in MHC class II molecules
2. Epithelial cells generate thymopoietin and thymosterin to stimulate differentiation
Following maturation, T cells leave the thymus via veins at the cortex-medullary junction. No
lymphatics enter the thymus, however a number of lymphatic capillaries begin at the medulla and
the connective tissue of the septa. These lymphatics drain into lymph nodes of the mediastinum.
The thymus is not fully developed at time of birth (pigs still show marked development at 2-3
weeks of age).
There are four defined steps of intrathymic maturation of T cells:
Irreversible commitment of hemopoietic cells towards T cell differrentiation
Selection of T cell repertoire which provides a bias towards self MHC antigens
Selection of populations for export
Functional diversification into subsets
The T cell receptor (TCR) is a glycoprotein on the surface of T cells that is capable of binding
antigens and MHC class II molecules specifically. In the adult, the majority of peripheral T cells
express the alpha beta form of TCR and are either CD4+ or CD8+. Only 3% are gamma delta TCR
(CD4- & CD8-). The TCR function is unknown. These CD4-/CD8- are able to kill non-specifically,
are not restricted by either MHC class I or II molecules, and are capable of both antibody dependant
cell mediated cytotoxicity (ADCC) or cell destruction in the absence of antibody.
There is a high frequency of gamma delta TCRs on neonatal ruminant T cells; this could provide an
early cover of non-MHC restricted cellular immunity until a more mature alpha beta TCR T cell
system becomes established.
The CD3 molecule is a 5 chain protein in mouse (3 (so far) in human) believed to function by
transmitting the activation signal from the TCR to other molecules within the cell. When a T cell
leaves the thymus, it must have TCR, CD3, CD4 or CD8.
Cell mediated immunity (CMI) matures prior to the end of gestation, evident by the capacity of the
newborn to reject graft tissue, however, intrauterine blood transfusions rarely elicit graft vs. host
reactions.
Cytokines are important for communication between the cells of the immune system. IL-7 may
mediate expansion of the earliest detectable thymocyte precursors during embryonic development.
The majority of d. 13-17 murine fetal thymocytes constitutively express IL-1R. Populations of fetal
thymic epithelial and bone marrow derived stromal cells constitutively produce IL-1alpha and IL-1ß
mRNA, respectively. The IL-2 & IL-2R genes are constitutively expressed by d. 15 on murine fetal
thymocytes. Fetal thymocytes proliferate in response to exogenous IL-2, and IL-2 promotes the
selective outgrowth of gamma delta TCR T cells. Day 15 murine fetal thymocytes constitutively
produce IL-4 mRNA and express IL- 4R. This IL-4 plays a role in development of mature
peripheral T cells. IL-1, IL-2, IL-4 and IL-7 all play a central role in precursor T cell proliferation
and maturation.
B cell development
Pluripotent stem cells within blood islands of yolk sac give rise to progenitor cells that migrate to
the fetal liver where B cell development begins. Later in fetal development the bone marrow
assumes this function. In human infants, pre B-cells can be detected in the fetal liver at 7-8 weeks
of gestation, and are characterized by surface IgM expression by 10 (20) weeks.
Properties of Ig classes and subclasses:
1. IgM
MW 970,000
Petameric structure in serum
Does not readily move out of vascular system
Does not cross the placenta
First Ig produced during an immune response
Potent complement activator
2. IgG
MW 146,000
Monomeric structure in serum
Predominant Ig in serum
4 subclasses:
IgG1
IgG2
IgG3
IgG4
All subclasses cross the human placenta except IgG2
IgG3 is the most effective activator of complement
IgG1 and IgG3 have Fc regions that effectively bind to macrophage Fc receptors
3. IgA
MW 160,000
Limited quantity of IgA in serum
Serum IgA is predominantly in the monomeric form
Most of IgA produced is associated with mucosal surfaces and is in dimer form, called secretory
IgA (SIgA)
IgA present in various body fluids - saliva, intestinal and bronchial mucus, nasal secretions, sweat,
colostrum and milk
4. IgD
5. IgE
Antibody response
In the adult, following antigenic stimulation, the initial antibody response consists mainly of IgM.
Maturation of the humoral immune response involves "class switching" in which a rearrangement
of genes occurs within DNA to produce IgG - these rearrangements are delayed during fetal
life.
Stimulation of cord blood leukocytes with pokeweed mitogen (PWM), a T-cell dependant antigen,
caused normal proliferation of lymphocytes, however, there was a delay in differentiation.
Therefore, there was a decrease in production of plasma cells and a diminished antibody production
that was limited to IgM. This is primarily because a deficiency in T-helper cells. In the presence of
adult T cells, neonatal B-cells are able to trigger the production of IgM, IgG and IgA.
When a T-independant antigen was used to stimulate neonatal T cells, there was a difference in
the ability to synthesize different subclasses of IgG: IgG1 and IgG3 were produced, however IgG2
and IgG4 were not produced until 24 months of age. There is a possibility that abnormalities in
interleukin receptors on B cells may play a role in deficient responses in neonatal B cells.
T-suppressor cells bearing Fc receptors for IgM occur in cord blood as early as 26 weeks and are
capable of inhibiting proliferation of Ab secretion by maternal lymphocytes. These T-suppressor
cells are found in peripheral blood of neonates and older infants and decrease to near adult levels by
3 months of age.
In summary, normal numbers of B and T cells are present at birth, yet humoral immune responses
are functionally immature. This is due to a regulatory imbalance between T cell mediated help and
suppression, as well as B cell immaturity. Newborns are able to respond better to protein antigens
than to capsular polysaccharide antigens. They demonstrate a delayed ability to switch from IgM to
IgG, which results in a developmental lag prior to attainment of adult levels of serum IgM (1 year),
IgG (5-6 years), and IgA (10-14 years). While serum IgA appears slowly, SIgA evolves earlier, by
3 weeks of age (SIgA does not cross placenta, however maternal IgA dimers and IgM independant
of secretory component (SC) could bind to SC in the fetus).
The predominant B cell population in the neonate expresses CD5, while CD5+ B cells represent a
minor B cell subset in adults. CD5+ B cells may be required for normal establishment of both T
cell and B cell repertoire. The fetal antibody repertoire is highly restricted - 105 antibody
specificities vs a normal adult repetoire of 107 antibody specificities. In addition, while the human
neonate has adult levels of B cells at birth, most domestic species only show 1/3 adult levels of B
cells at birth. They reach adult levels at 20 days in the foal & calf, 30 days in the pig.
Conclusions
Regulatory imbalance between T cell mediated help and supression, as well as B cell immaturity
Gap between development of lymphocytes and their ability to cooperate effectively to generate a
normal antibody response
Neonates Have a Dysfunctional Adaptive Immune System
Neonatal Innate Immune System
In the absence of adaptive immunity, the neonate must rely on complement and effector cells of the
innate immune system to remove invasive organisms. The phagocytic ability of these cells is
normal/increased, and they appear to kill with equal efficiency as adults; however, they are less
responsive to activation by lymphokines. There is a decreased/normal IL-1 production by neonatal
macrophages, and lower expression of Class II molecules, thus decreased antigen presentation, at
least in mice. This is possibly due to decreased IFN production by neonatal lymphoid cells.
Neonatal neutrophils have normal phagocytic and killing ability, but are defective in their ability to
move towards a chemotactic stimulus. The rigid cytoskeleton is incapable of deforming and
redistributing adhesion sites. The cells fail to undergo critical changes in cell membrane potential,
and do not increase cAMP levels.
Complement, the other component of the innate immune system, does not cross the placenta, but is
present in colostrum. In the neonatae, lower serum complement levels (due to decreased hepatic
synthesis rates) leads to decreased ability to activate the complement cascade. Cord blood
monocytes fail to produce complement in response to lipopolysaccharide (LPS) in vitro.
LEUKOCYTE ADHESION DEFICIENCY
The importance of neutrophil function in immunological defense is dramatically illustrated by the
genetic disorder known as leukocyte adhesion deficiency (LAD). LAD is an autosomal recessive
disease characterized by drastically reduced expression of the alpha 2 integrins, which results in
significantly impaired neutrophil adhesion and extravasation. The extreme importance of the alpha
2 integrins in immunity is demonstrated by the diversity of the clinical symptoms of LAD. A
primary symptom of leukocyte adhesion deficiency is a persistent leukocytosis due to profound
neutrophilia, believed to be caused by the inability of neutrophils to migrate into peripheral tissue.
As a result of the weakening of the host immunological defense system, LAD animals are
predisposed to poor wound healing and lethal bacterial infections. Animals suffering from LAD
have also been observed to have chronic pneumonia, ulcerative and granulomatous inflammation of
oral mucosa, gingivitis, chronic or recurrent diarrhea, generalized dermatomycosis, peripheral
lymphadenopathy, stunted growth, and anorexia.
NEONATAL CALF IMMUNE FUNCTION
Lymphocyte Function
At birth, the calf is born essentially agammaglobulinemic, with a limited ability to respond to
antigenic challenge (Hubbert, 1974). Ingestion of immunoglobulins, as well as other humoral and
cellular factors, via colostrum represents the singular mode of passive immunization. De novo
synthesis of antibodies by calves is negatively correlated with the peak concentration of maternally
derived antibodies. Hypogammaglobulinemic calves initiate de novo antibody synthesis earlier
that calves with high concentrations of maternal antibodies. However, all newborn calves are
capable of producing a restricted antibody repertoire following antigenic challenge, consisting
mainly of a primary IgM response.
Antibody formation is dependent on the cooperative interaction of antibody-producing B
lymphocytes and T lymphocytes. Human newborn T lymphocytes are functionally immature, and
markedly limited in their ability to support Ig production. Deficient production of T
lymphocyte-derived interleukin (IL)-2, IL-4, and IL-6 in human infants may contribute to the
decreased capacity to produce antibody, as well as the limited antibody repertoire. Blastogenic
responsiveness of colostrum-fed newborn calf lymphocytes to T cell-dependent mitogens is
depressed relative to that of 10-d old calf lymphocytes. Suppression of lymphocyte proliferation
can be attributed to factor present newborn serum factors; lymphocytes isolated from older calves
responded in a fashion similar to that of neonatal lymphocytes when incubated in neonatal serum.
In addition, lymphocytes from colostrum-fed calves respond to a lesser extent to mitogenic stimuli
than colostrum- deprived calves, suggesting colostrum ingestion may directly contribute to
suppression of lymphocyte function.
The importance of colostral cellular factors is exemplified by the lack of relationship between
concentrations of colostrum-derived serum opsonins (complement and Ig) and effective clearing of
infection. Normal concentrations of complement and Ig have been associated with defective
opsinophagocytosis, whereas adequate opsinophagocytosis can occur at low serum concentrations
of opsonins. These results suggest passive absorption of opsonic factors alone does not guarantee
adequate host defense against infection. Colostrum ingestion directly effects neonatal circulating
leukocyte numbers and leukocyte functional ability. Total leukocyte counts and neutrophil numbers
are higher in colostrum-fed calves than in colostrum-deprived calves 6-24 h postpartum. Calves
fed pooled colostrum supplemented with colostral leukocytes have higher concentrations of IgA
and IgM antigen specific antibodies against Escherichia coli than calves fed cell-deprived pooled
colostrum. Calves fed colostrum supplemented with colostral leukocytes shed significantly less
enteropathogenic Escherichia coli bacteria in their feces than calves fed cell-deprived colostrum.
Research suggests leukocytes may directly be absorbed prior to gut closure in newborn lambs.
Fluorescein isothiocyanate (FITC)-labelled colostral or maternal peripheral blood leukocytes appear
in the blood of newborn lambs following oral administration. Peak appearance of FITC-labelled
cells occurs 6-12 h post ingestion, although labelled cells are absorbed to some extent when
administered at 3 d postpartum. Therefore, leukocytes present in colostrum may directly contribute
to passive immunity and resistance to disease in newborn calves.
Neutrophil Function
Due to the functional immaturity of the lymphocyte-mediated adaptive immune response at birth,
constituents of the innate immune system play a decisive role in host immunologic defense during
this critical time period. Within the innate immune system, neutrophils are the primary mediators
of an inflammatory response, and thus are major contributors to host defense. Neutrophil
functional deficiencies have been documented in both human infants and newborn calves,
suggesting neutrophil defects may contribute to the overwhelming susceptibility to infection in the
neonate. Initial activation of neutrophils requires adequate binding of soluble stimulatory factors
(chemoattractants) with cell surface receptors. N- Formyl-methionyl-leucyl-phenylalanine receptor
concentration and binding kinetics in human neonates are equivalent to adult values, suggesting that
in this case, any abnormalities occur subsequent to chemotactic factor binding. Phagocytic ability
of both human and bovine newborns is comparable to that of adults, however low opsonic activity
of neonatal serum has been related to impaired phagocytosis by neonatal neutrophils of both
species. Discrepancies exist within the literature as to killing ability of neonatal calf neutrophils;
however this may be due to use of various stimuli to activate cells. Phorbol myristate
acetate-stimulated neonatal neutrophils generate significantly less superoxide anion than adult
neutrophils. Conversely, opsonized zymosan-stimulated neutrophils from newborn calves generate
significantly more superoxide anion than stimulated adult neutrophils, suggesting a
stimulus-specific effect rather than a general deficiency.
Both neonatal human and calf neutrophils are able to form a characteristic bipolar shape in response
to stimulus, however, further research in human infants has illustrated these cells are unable to
redistribute adhesion sites from the head of the cell to the uropod. Additionally, human neonatal
neutrophils are unable to decrease cellular adhesion following sequential exposure to increasing
concentrations of chemotactic factor. Failure of neonatal neutrophils to redistribute adhesion site
and downregulate adhesion ultimately results in a markedly diminished ability to migrate when
compared to adult neutrophils.
Neonatal neutrophil LFA-1-dependent adhesion to endothelial ICAM-1 is comparable to that of
adult neutrophils. However, chemotactic factors that induce Mac-1-dependent adhesion to
endothelial cells, purified ICAM-1, and protein-coated glass in adult neutrophils do not affect
neonatal neutrophil adhesion in the same manner. The ability of chemotactic factor to mobilize
Mac-1 from intracellular store was diminished in neonatal neutrophils, and functional alterations of
Mac-1 necessary for binding to ICAM-1 did not proceed normally. These author s attributed the
reduced modulation of neonatal neutrophil adhesiveness and defective migration to a combination
of quantitative and qualitative Mac-1 abnormalities. Previous research suggests phagocytic cells
must arrive at a site of infection, ingest the pathogen, and kill it within a 2-4 h period following
initial challenge to prevent establishment of an infection. Therefore, quantitative Mac-1
deficiencies and abnormal interactions of Mac-1 with ICAM-1 may significantly impair neonatal
neutrophil migration in vivo, contributing to the increased susceptibility to disease.
Efficiency of signal transduction and contractile protein assembly in neonatal neutrophils is
impaired, and may contribute to adhesion and migratory deficits. Stimulus-induced increases in
human neonatal neutrophil [Ca2+]i are lower than those of adult cells; however, influx of
extracellular calcium is comparable to that of adult neutrophil. Newborn calves (<24 h of age)
mobilize Ca2+ as efficiently as adult cows. Although microtubule and microfilament formation
have yet to be investigated in the neonatal calf, Ca2+-dependent contractile protein assembly
appears to be defective in human neonates, and this has been attributed to a failure to form F-actin
in response to stimuli.
Relative deficiencies of neonatal defense mechanisms has been attributed in part to increased
immunosuppressive serum factors present following parturition. Plasma concentrations of cortisol
are elevated at birth in calves, and decrease significantly during the first 48 h of life. It has been
well established that cortisol has an immunosuppressive effect on lymphocyte and neutrophil
function in cattle, and may similarly limit functional ability of these cells in the neonatal calf.
Additionally, plasma opioid concentrations in neonatal calves are elevated at parturition, and may
contribute to modulation of chemotactic migration. Calves born to cows experiencing dystocia
have higher plasma cortisol and a-endorphin concentrations than calves requiring no assistance.
Significant elevations in these immunosuppressive factors may contribute to the increased perinatal
mortality associated with difficulties at calving.
Neonatal Passive Immunity: Transport of Macromolecules
Colostral transfer has an important role in calf management: failure of acquisition of passive
immunity is the leading cause of morbidity and mortality in neonatal calves more than 3 days old.
There is a correlation between mobidity and mortality and low serum Ig concentration. Conversely,
low serum Ig concentration can produce productive neonates; the influence of cellular and nonspecific immune factors in colostrum are poorly understood but may be more important than
previously recognized.
Ig concentrations in colostrum are influenced by maternal age, parity, breed, and nutritional status.
There is an inverse relationship between colostral volume and Ig concentration. Other factors
influencing colostral quality include premature parturition, premature lactation or premilking, time
elapsed since parturition, handling of colostrum, storage, pooling, neonatal vigor, and maternal
stimulation.
Intestinal absorption and transport of immunoglobulins by the newborn can be either selective or
non-selective. Selective absorption occurs in those species that absorb antibodies throughout the
suckling period, e.g., rats and mice, while non-selective absorption occurs in those species where
closure occurs prenatally or within the first few days after birth.
Non-selective transport is the primary means of macromolecular transmission in ungulates. Both
heterologous and homologous antibodies are transmitted, and proportions of serum
immunoglobulins post-closure are the same as in ingested colostrum. There may be some selective
transport by coated vesicles in proximal small intestine, although quantitatively this is considered to
be of minor importance. The jejunum appears to be the most efficient area of the small intestine in
terms of macromolecular transmission. Distal small intestine has been reported to take up a greater
portion of ingested immunoglobulins, but high levels of hydrolytic enzyme activity in ileal vesicles
make them inefficient in terms of transport. Non-protein macromolecules of similar molecular
weight to immunoglobulins are transported in a kinetically similar manner Polyethyleneglycol
(PEG) in solution (20% w/v) with albumin (2% w/v) and ovalbumin (2% w/v), although of similar
molecular weight to these molecules, appears to be transported by a mechanism separate from that
for albumins.
The morphology of neonatal small intestine is especially adapted to immune transmission. The
glycocalyx is sparse on the microvillus membrane. A preformed organelle, termed the apical
canalicular system (ACS), becomes apparent shortly after feeding. Intestinal enterocytes take up
colostrum through intermicrovillous pores. The ACS acts to concentrate colostral material into
subapical vacuoles. These gradually fill with enough material to be recognized as eosinophilic
droplets. Elevation of intracellular pressure caused by growing droplets may limit further uptake of
colostrum. The nucleus at this point may be pressed to the base of the cell. The inversion process
involves translocation of the nucleus and the eosinophilic droplet. For the first 6 to 8 h after
feeding, eosinophilic droplets maintain a supranuclear position, transposing subnuclearly by 16 to
18 h. Subnuclear vacuoles show high levels of alkaline phosphatase activity not present prior to
translocation, which led Healy and Dinsdale (1979) to propose that merger with the Golgi apparatus
may occur during this period. During transport, the vacuolar membrane fuses with the basolateral
membrane and vacuolar contents are exocytosed into intercellular spaces. Levels of enzymes
associated with the brush border membrane rise in the serum of the neonate concomitantly with
macromolecules and are apparently exocytosed along with other vacuolar contents. These enzymes
show a high activity in intestinal tissue at birth and are depleted within 2 d.
Spaces between and below intestinal enterocytes are especially dilated immediately after birth,
extending up to the terminal bar. The lamina propria is poorly developed with few lymphoid cells.
Macromolecules are taken up by lymph capillaries which are highly fenestrated at birth. No
basement membrane is apparent around the lymphatic endothelium at this time. Lymph flow
increases dramatically after colostrum ingestion. Absorption and transport to lymph from the
duodenum takes 1 to 2 h, and the maximum concentration is reached by 3 to 4 h after ingestion.
Since uptake occurs within 15 min, accumulation and transport must be rate-limiting. Rate of
transport decreases with increasing age at first feeding in the calf. Lymphatics access the general
circulation via the ductus thoracicus, and access may be promoted by the higher flow rate of blood.
Immunoglobulins first appear in plasma 3 h after feeding, with IgG appearing prior to IgM or IgA.
Peak levels occur 6 to 12 h after feeding, and feedback inhibition by high serum antibody titers
apparently does not occur.
Reported differences in absorptive efficiencies among immunoglobulin classes can be attributed to
differential rates of equilibration between intravascular and extravascular compartments and slower
uptake of IgM and IgA by lymphatics. Time of closure may vary between different classes of
immunoglobulins, although there is not complete agreement on this point. Loss of IgG and IgM
from serum occurs at a gradually accelerating rate once threshold levels are attained. These
complicating factors make comparisons such as relative absorptive efficiencies difficult to calculate
with any degree of accuracy. Linear correlations existing between concentrations of IgG and IgM
in colostral secretions and in the serum of the newborn serve to discount any claims of major
differences in absorptive efficiencies among Ig classes.
FACTORS AFFECTING MACROMOLECULAR TRANSPORT
In addition to the presence of immunoglobulins in solution other factors are necessary for
transmission to proceed efficiently. Balfour and Comline (1962) tentatively identified accelerating
factors in colostrum. A low-molecular-weight protein found in the whey fraction, when combined
with glucose 6-phosphate and inorganic phosphate, accounts for most of the accelerating ability of
colostrum. These factors were not independently effective. The protein increases the propensity for
immunoglobulins to enter solution, and may be analogous to surface-active agents in enhancing
absorption. Indirect intracellular functions are also possible. These factors may be colostrumspecific, inasmuch as globulins added to milk are poorly absorbed in comparison to colostral
globulins; however, the presence of milk proteins or even non-protein macromolecules like
polyvinylpyrrolidone (PVP) enhance the transport of IgG to some extent. Fermentation of
colostrum diminishes transport capacity, with pH-buffering partially restoring this ability. The high
osmolality of colostrum may be important in immunoglobulin transport, since intraluminal
hyperosmolality appears to stimulate pinocytosis. Formation of a curd in the abomasum after
colostrum ingestion is also necessary for optimum absorption. The high level of vitamin A in
colostrum and high serum corticosteroid levels in the neonate at birth) both affect lysosomal
membranes, albeit by different mechanisms, and thus interfere with normal intracellular digestive
processes. In this manner they enhance the non-specific transport function of neonatal intestine.
Immunoglobulins dissolved in ionically-balanced salt solutions to match colostrum are capable only
of minimal transport. Addition of glucose or lactose has no effect, but the addition of short-chain
fatty acids, lactate or pyruvate to such solutions accelerated transport despite diminished lymph
flow. Potassium isobutyrate appears to be especially effective in this regard, but, when added to
colostrum, has a deleterious effect on both efficiency of absorption and total Ig absorbed. This may
be caused by a shift in ionic concentration.
Smith demonstrated that increasing concentrations of potassium or decreasing concentrations of
sodium in a protein solution inhibits uptake of protein molecules. Lecce found that IgG would not
bind to the brush border of newborn pig enterocytes in the absence of sodium. Brown, Smith and
Witty (1968) postulated that the acceleration of metabolism caused by an increased intracellular
concentration of sodium or movement of sodium down a concentration gradient may provide
energy required for protein transport. Calcium is also required for pinocytotic activity, and
absorptive capacity is lower in calves with low levels of calcium in their blood at birth.
Smith and Pierce (1967) studied the effects of various amino acids on gamma-globulin absorption
and transport. Alanine, which is absorbed via an active transport mechanism, stimulates
immunoglobulin transport in newborn pig intestine. Conversely, leucine, absorbed via facilitated
diffusion, inhibits not only the transport of gamma-globulin but also that of glucose and fluid.
Inhibition of globulin transport may well be a secondary response to diminished availability of
glucose or water to the enterocyte. Polycations, however, stimulate transport of IgG while
simultaneously inhibiting glucose and fluid transfer. Concentrations used in these studies may
affect brush border membrane structure, or, alternatively, accelerating factors, especially
polycations, may have a direct effect on membrane charge in the brush border. Membrane charge
affects macromolecular adsorption to cellular surfaces, and uptake selectivity in neonates appears
related to net charge on the immunoglobulin.
CESSATION OF MACROMOLECULE UPTAKE
Uptake of macromolecules by the small intestine continues as long as vacuolization is present.
Vacuolated enterocytes disappear following definite patterns, with proximal segments of small
intestine losing their uptake ability long before the distal portions, and cells nearer the crypts before
those at the tips of the villi. At birth, uptake occurs along the entire length of the intestinal villus,
but never in the crypts. Both IgA and IgM are strongly adsorbed to the lumenal surface of the crypt
epithelium but not to the villus epithelium. These are not absorbed, but instead coat cell surfaces
and provide local protection. El-Nageh (1967) reported absorption along the entire length of jejunal
villi at 6 h post-partum, while only the apical third of the villi are capable of uptake in the 2-day-old
calf. Additionally, cessation of uptake proceeds caudally in the small intestine. In piglets, the
duodenum ceases uptake shortly after birth, with the jejunum following at approximately d 4 to 11,
and the ileum terminating 2 to 3 wk later. Duration of uptake is decreased by the presence of
digesta. However, transposition of an ileal segment to a duodenal position will not affect duration
of uptake in the transposed segment, suggesting that duration of uptake is either genetically or
humorally determined, with the influence of digesta being uniform throughout the intestine. Murata
and Namioka (1977) and Moog and Yeh (1979) noted certain distinctive histological changes as
cessation of uptake proceeds. The terminal web appears to develop as pinocytosis ceases. Golgi
complexes, mitochondria and rough endoplasmic reticulum become more prominent.
Intermicrovillus pores disappear along with vacuoles, and density of the glycocalyx increases. New
cells formed in the crypts after birth may develop vacuoles, but require at least 4 d after the last
DNA synthesis. Therefore, slow cell turnover in the neonate helps prolong the period of uptake,
and cessation is due to a combination of increasing cell turnover rate coupled with redifferentiation
of intestinal enterocytes.
CESSATION OF MAROMOLECULE TRANSPORT
Conversely, cessation of transport (closure) is not necessarily related to cell replacement. The
closure process is the loss of the ability of intestinal enterocytes to exocytose vacuolar contents,
which is a gradual phenomenon, with efficiency of transport slowly diminishing prior to complete
cessation. Despite potential continued uptake of macromolecules, most transport ceases, although
residual, size-dependent transport may continue throughout the first week. Material taken up by the
enterocyte is not released into intercellular spaces and is simply shed along with the cell during
normal cell turnover. Staley et al. (1969) postulated that a shift in the position of the Golgi
apparatus causes a change in cell polarity which favors lumenal rather than basal secretion. Jordan
and Morgan (1968) suggested that a progressive change in the net charge of cell membranes,
presumably basolateral membranes, either by gradual loss of the inherent positive charge or by
development of a negative charge, may be responsible for the gradual loss of transport capability.
Cell migration from crypts to villus tips define the cell turnover period. At birth, and through the
first 16 d in rat pups, this is a relatively slow process, requiring 6 to 7 d. Starting at d 16, the
process rapidly accelerates, with cell turnover on d 18 estimated at 2 to 3 d. Height of villi
increases by 40% between d 15 and 23, with size of the crypts increasing 300% during the same
period. The number of crypt cells increases dramatically starting at d 19, indicating cell
proliferation is occurring in addition to cell hypertrophy. Sympathectomy diminishes mitotic rate
somewhat, but acceleration of proliferation still occurs within the same time frame, suggesting a
humoral trigger. The correlation between cell turnover and cessation of uptake does not carry over
to other species, e.g., rabbits, hamsters and guinea pigs. Cell replacement in the neonatal intestine
of ungulates is quite slow, well in excess of 48 h in calves and lambs. In the newborn pig, values
between 5 and 19 d are published, with variation due more to experimental method than actual
individual differences. In rats and mice, cessation of uptake and closure coincide.
Estimates of the time of closure in calves range from 8 to 48 h postpartum, with the consensus near
24 h. Estimates vary due to procedure utilized for determining closure, feeding regimen and other
variables. Estimates that take into account the large increase in plasma volume associated with
feeding extend the period of absorption several h; however, techniques necessary to estimate
plasma volume introduce additional sources of error. Changes in plasma volume are fairly uniform
among calves, even with different feeding regimens. Shannon and Lascelles (1968) reported that
transport of gamma-globulin ceases within 24 h of first colostrum feeding. The work of Stott et al.
(1979a) tends to verify these findings. They estimate time of closure of calves fed at birth near 22
h while for those calves first fed at 24 h closure does not occur until 33 h. These data, however, are
biased in that calves experiencing spontaneous closure prior to feeding were censored. This
encompassed 50% of calves fasted for 24 h, indicating that differences in closure times between
calves fed at birth and those fed at 24 h may actually be much less.
In rats and mice, closure accelerates over time starting on d 16. In rats weaned at 21 d, cell
replacement with a mature type cell coincides with closure. Although gastric development appears
to hold a primary role in the closure process, cell replacement is equally important, especially in
weaned rats.
Gastric and Pancreatic Development
Many hypotheses have been put forth attempting to explain the process of closure as a function of
increasing digestive capability of the neonate. Hill (1956) postulated that closure was a function of
gastric development and increasing proteolytic activity. In the newborn guinea pig (no significant
postnatal transmission of antibodies), parietal cells are abundant and gastric pH is 1.0 to 2.0.
Similar findings have been reported in human infants within 2 h of birth. In species with postnatal
transmission, gastric development parallels closure. Inhibition of gastric function and(or) the use of
trypsin inhibitor enhances macromolecular absorption in mature animals. Development of
digestive function appears to influence closure in rats and mice. Significant proteolysis occurs in
the gut of suckling rat pups. Jordan and Morgan (1968) postulated that development of selective
proteolytic activity could explain the progressive increase in selectivity of protein transmission
through the suckling period. Potency of gastric secretions continues to develop with parietal cells
appearing by the end of the second week and cell numbers increasing rapidly until d 25. Gastric pH
drops from 4.4 to 2.7 during this period. By the end of the third week, no antibodies reach the
lower gut intact. Normal 24-day-old rats show a fivefold increase in peptic activity and a 10-fold
increase in tryptic activity over levels seen in 12-day-old rats. Halliday (1956) reported closure at
21 d despite continued suckling and continued presence of antibodies in milk. Morris and Begley
(1970) demonstrated that IgG was still transported in 29-day-old unweaned rats if infused directly
in the small intestine; however, oral administration was ineffective. Duodenal infusion was
ineffective in weaned 29-day-old rats. Some aspect of weaning or the change in diet affects closure,
although premature weaning or diet changes will not induce closure. However, premature weaning
diminishes absorptive capacity.
Closure in ungulates is independent of gastric and pancreatic development, despite nearly identical
time frames. Balfour and Comline (1962) and Kruse (1983) reported minimal hydrolysis in the
gastrointestinal tract of calves over the first 2 d, although a somewhat higher rate of proteolysis
exists in piglets. Abomasal pH is relatively high at birth (near 7.0) and steadily decreases to a pH of
3.0 at 36 h. Accordingly, there is a rapid, progressive increase in the number of parietal cells during
the first 48 h postpartum. Although the cellular basis for mucus and pepsin secretion is present at
birth, gastric proteolysis is due primarily to rennin. Some excretion of Ig(Fab) fragments occurs in
the newborn period presumably due to this action. Despite inhibiting gastric activity in calves,
Deutsch and Smith (1957) could demonstrate no transmission of globulins at 40 h. Non-protein
macromolecules that are resistant to gastric proteolysis cease transport at the same time as
immunoglobulins. Antibodies introduced directly into the duodenum, thus bypassing gastric
proteolysis, are absorbed only during the first day. Uninhibited tryptic digestion of
immunoglobulins occurs at closure and is hypothesized as being important in initiating closure.
High levels of trypsin inhibitor are present in colostrum and prevent tryptic digestion of susceptible
immunoglobulins. Chamberlain et al. (1965) added trypsin inhibitor to immunoglobulin solutions
and fed this to 3-day-old piglets. Trypsin inhibitor proved ineffective in stimulating transmission.
Deoxyribonuclease activity in pancreatic secretions was also inhibited postpartum. This treatment
was similarly without effect.
Levels of alkaline phosphatase in intestinal tissue rise at the time of closure in rats. To determine if
changes in alkaline phosphatase trigger intestinal maturation, Clarke and Hardy (1969) added
alkaline phosphatase to a gamma-globulin solution and fed this to suckling rats. The lack of
response led to the conclusion that closure and changes in alkaline phosphatase are both
independent consequences of the same process. The effect of amniotic fluid in the gut was studied
by Deutsch and Smith (1957) by feeding amniotic fluid with milk over the first 36 h. No absorption
of immunoglobulins was noted when colostrum was fed at 40 h.
Dietary Factors
Although mechanisms involved in cessation of uptake are similar among all ungulates, there are
marked differences among species in cessation of transport. In pigs, closure is a diet-induced
phenomena. Fasted pigs continue to take up and transport macromolecules until death (at about 4
d), however, spontaneous closure has been documented in fasted pigs during the second day,
suggesting that fasting does not halt the closure process, but greatly delays it. Closure in lambs also
appears to be diet-dependent. Fasted calves, on the other hand, differ very little from fed calves
with regard to period of absorption. All ungulates, if fed near birth, will cease macromolecular
transport in a similar fashion. Colostrum intake accelerates closure in all ungulates to varying
degrees.
The difference between a fasted piglet or lamb and a fasted calf may be related to the postnatal
blood glucose pattern. Newborn pigs and lambs are susceptible to fasting hypoglycemia. Glycogen
reserves at birth are relatively limited in these species compared to calves. Blood glucose levels
decrease shortly after birth and do not recover without feeding. In unstressed fed animals, glucose
levels gradually rise over a 2- to 3-wk period. Neonatal calves and foals present a different picture.
Glucose levels are lower at birth, but they rise to twice adult levels within the first 24 h. This rise
is independent of nutritional status. Levels then gradually decline over the next 6 wk. Thus, the
availability of glucose to the neonatal small intestine may be one factor influencing the closure
process.
Dietary induction of closure in the pig supports this scenario. In searching for nutritional factors
that initiate closure in pigs, Lecce et al. (1964) found that colostral proteins, fats, vitamins and
minerals are without effect, whereas a fat- and protein-free colostral whey induces closure in a
normal manner. Later studies show that pure solutions of various sugars induce closure and that at
least 300 milliequivalents of glucose are required. Solutions of glycine or inorganic salts are
ineffective. Direct contact with sugar solutions may not be necessary. Leary and Lecce (1978)
reported that feeding induces closure even in isolated intestinal segments, suggesting that induction
of closure is humorally-regulated and not dependent on lumenal exposure to glucose.
Insulin-induced hypoglycemia in the newborn calf will significantly delay closure, although
fructose-induced hypoglycemia is ineffective, possibly due to fructose utilization as an energy
source under these conditions. These results would tend to confirm the energy dependence of this
process and underscore the similarities in the basic mechanism of closure across species despite the
differences in postnatal expression.
Endocrine Factors
The unique composition of colostrum suggests that some of its constituents may prolong the
absorptive period. Pope and Ray (1953) note that estrogenic activity in colostrum is similar to that
in the serum of the dam but calves maintained on three (250 ml) transfusions of maternal blood
were unable to absorb antibodies at 40 h. Neither calves, lambs nor rats treated with injections of
estrogenic compounds appear to experience any delay in closure. The high content of histamine in
colostrum prompted Patt et al. (1972) to add histamine to gamma-globulin-enriched milk. This
combination is detrimental to absorptive capacity and appears to induce premature closure.
Other hormone treatments studied include effects of progesterone, progesterone in combination
with estrogen, testosterone, ACTH, aldosterone and somatotropin. None of these treatments are
effective in extending the absorptive period prior to closure.
The effects of thyroxine on closure are well-documented. Thyroxine can be characterized as a nonspecific metabolic enhancer that increases cardiac output and ventilation rate, which in turn leads to
enhanced tissue metabolism and oxygen consumption. Thyroxine is trophic to small intestinal
tissue and therefore increases its oxygen consumption directly. Activity at the gut level leads to
increased motility. Thyroxine also potentiates the stimulatory effects of corticosteroids,
epinephrine, glucagon and growth hormone. Thyroxine levels rise in rat pups from birth through
weaning. Chan et al. (1973) and Malinowska et al. (1974) administered high levels of thyroxine
to suckling rats and reported precocious cessation of immunoglobulin absorption. However, effects
of thyroxine were indistinguishable from those expected from corticosteroids stimulated by this
treatment. Moog and Yeh (1979) report that changes in the terminal ileum of the suckling rat are
abolished by hypophysectomy, and increases in mitotic index associated with closure in rats are not
seen in hypophysectomized animals. They theorize that inhibition of normal developmental
changes is due to prevention of maturation of the pituitary-adrenal response system. Normal
ultrastructural changes can be restored in such surgically-altered animals by daily injections of
either cortisone acetate or thyroxine, although enzymatic changes associated with each treatment
are different. Since thyroxine does not alter corticosteroid levels under these conditions, effects
either are direct tissue effects or are mediated through a separate, undetermined mechanism.
Investigations utilizing thyroidectomized or adrenalectomized rat pups have also demonstrated that
cortisone and thyroxine are independently capable of inducing normal maturational changes in the
small intestine. Repeated injections of thyroxine to either fetal rat pups or 14-day-old rats induce a
decrease in IgG receptors and maturation of intestinal enzyme profile. Microvillus membranes also
mature as evidenced by a decreased lipid:protein ratio. The effect on closure in rats was not
examined, but these changes are consistent with those expected during this process.
Plasma triiodothyronine and thyroxine levels in ungulates are elevated at birth and decline through
the first week. Thyroxine is the predominant form at birth. Boyd and Hogg (1981) observe that
endogenous concentrations of thyroxine at birth bear no discernable relationship to subsequent
immunoglobulin absorption. Cabello and Levieux report on a series of experiments on the effect of
thyroxine on passive immunization in lambs. Three of these studies showed a decrease in
absorptive capacity in response to exogenous thyroxine, one an enhancement, and one had no
effect. Two showed precocious closure, while one resulted in a delay in closure. Triiodothyronine,
which is more potent biologically than the tetraiodothyronine utilized in these studies, has no effect
on either absorption of immunoglobulins or time of closure while an increase in levels of thyroidstimulating hormone are associated with a shortened period of absorption. Thyroxine has also
been reported to induce adrenal maturation, which might well be its most important effect in the
perinatal period with regard to closure. In searching for a humoral trigger for closure, most
attention has focused on the role of glucocorticoids, especially in rats. Evidence of several
endocrine interrelationships have evolved from this work, but no conclusive data have been
produced to link corticosteroids directly to closure.
Daniels et al. (1973b) has related changes in plasma corticosterone to changes in the small intestine
that accompany closure. Levels remain near 1 ug/dl until d 18 to 21, then rapidly rise to 5 to 7 ug/dl
and continue to gradually increase to 15 ug/dl on d 28. Concentration of cortisol remains
unchanged throughout this period. Patt (1977) saw no changes in corticosterone in the same time
frame, but his technique was less likely to detect differences in the low levels present at this point.
The ability of pharmacological doses of various corticosteroids (especially cortisone acetate) to
induce precocious closure after d 10 in the rat has been thoroughly studied. Morris and Morris
(1976) showed that exogenous corticosteroids at levels high enough to induce precocious closure
also initiate rapid cell turnover and maturation of intestinal epithelium in distal small intestine. IgG
receptor levels are diminished in proximal intestine, although the possibility of cytological changes
remains. Enzyme activities are also precociously altered by this treatment. Transmission of IgG
began to decrease on the first day of treatment. In contrast, bilateral adrenalectomy delays onset of
closure by 4 d, but does not abolish it. When closure does occur, it proceeds at a normal rate. The
fact that closure proceeds with only a transient delay suggests that although corticosteroids may
have a permissive role in closure, they are not essential. Malinowska et al. (1972), in support of
this conclusion, noted that while corticosteroid levels are extremely high at birth in rat pups and
rabbits, and despite an increase at d 14 in rabbits, closure does not occur until the final
corticosteroid surge at the end of the third week. Effects due to exogenous corticosteroids in rats
are more accurately categorized as pharmacological responses than as physiological effects.
Attempts to reproduce the effects of corticosteroids in ungulates have been universally unsuccessful
in terms of reproducible effects on closure. The prepartum surge in cortisol acts in a regulatory
capacity on small intestinal maturation and proliferation. In fed calves, corticosteroid levels
decreased rapidly during the first 12 h postpartum and gradually during the next 12 h. Fasted calves
show the same initial decline, but levels rise during the second 12 h if fasting continues. Feeding
induces a transient hyperadrenalemia. Lambs and piglets present a similar picture, but the relative
magnitude of change is less dramatic. The effect of exogenous corticosteroids imposed on this
picture serves to diminish absorptive capacity of macromolecules without affecting the time of
closure. In contrast to these studies, various studies have reported increased absorptive capacity as
a result of exogenous corticosteroid treatment. Administration of drugs at birth to decrease cortisol
levels in lambs induced a precocious closure. Studies relating endogenous cortisol concentrations
at birth to closure or to absorptive capacity also produce conflicting results. Stott and Reinhard
(1978), looking at dystocial and eutocial calves, found no variation due to differences in cortisol
levels at birth. Cabello and Levieux (1978, 1980) and Cabello et al. (1983) confirm these findings.
Boyd and Hogg (1981) report a negative correlation between absorptive capacity and endogenous
cortisol concentration at birth with no effect on closure. As long as body core temperature is
unaffected, temperature stresses on newborn animals have little effect on absorptive capacity and
time of closure. Extreme cold decreases the rate of antibody transport without affecting absorptive
capacity, as would be expected based on reports of transport inhibition in isolated intestinal loops
exposed to hypothermic conditions. Stott (1980) stated that heat stress also diminishes absorptive
capacity, and suggested this is a secondary response to concurrent hyperadrenalemia. Cold stress
induces a significant increase in concentration of cortisol, which may be responsible for any adverse
absorptive effects.
The effects of corticosteroids on closure may be due as much to increasing tissue metabolism in
general as to any direct action on the small intestine. They induce hyperglycemia and, additionally,
may increase the hyperglycemia induced by the high growth hormone levels present at birth in
ungulates. Corticosteroids are important in mobilization and oxidation of lipids and stimulate
tissue glycolysis. They decrease the oxygen affinity of hemoglobin, leading to increases in oxygen
delivery at the tissue level.
A potential effect of corticosteroids in relation to closure may result from their ability to induce
gastrin receptors. This phenomenon closely parallels the apparent effects of corticosteroids on
closure previously discussed. A single injection of corticosterone acetate into 7-day-old rat pups
results in the premature appearance of gastrin receptors by d 10. The same treatment has been
shown to induce closure by d 11. Receptors normally appear in the rat pup between 18 and 20 d of
age. Adrenalectomy delays the normal appearance of gastrin receptors until d 25. Adrenalectomy
has been shown to delay closure similarly. Therefore, the reported effects of corticosteroids on
closure may possibly be mediated by gastrin.
Gastrin is secreted from G cells. Although there are G cells scattered throughout the intestinal tract,
the highest concentration of these cells is in the antral portion of the stomach or abomasum. The
actions of gastrin are gut-specific, with the exception of an apparent mildly trophic effect on the
pancreas. In suckling rats, serum gastrin levels are high from birth through weaning. Antral levels
are low, indicative of the lack of receptors. Antral levels rise on d 20, reaching adult levels by d 22.
After d 25, antral and serum levels decline to normal basal levels. Early weaning does not affect
the timing of these changes, but does diminish their magnitude. High gastrin concentrations in the
perinatal period have been reported in other mammals. The development of gastric acidity in the
neonatal period of all species is due in part to the interaction of gastrin and its receptor. This
process, as previously discussed, also parallels closure.
The gastrin/receptor interaction regulates differentiation and proliferation of epithelial cells in the
small intestine. Gastrin activity increases ratios of RNA:body weight, gut weight:body weight, and
protein:body weight as well as DNA synthesis in intestinal tissue. There is a 50% increase in
mRNA synthesis within an hour after injection of exogenous pentagastrin, with an increase in
protein synthesis following 2 h later and peaking within 6 h. By 16 h, DNA synthesis is
maximized.
The action of gastrin on gastrointestinal tissue is accompanied by a sharp increase in oxygen
consumption at the cellular level. Oxygen, then, may act as a limiting factor in tissue response to
stimulation by gastrin. This suggests a scenario where initiation of closure is prevented prenatally
by lack of oxygen availability to gastrointestinal tissue. This could be mediated directly or via
formation of gastrin receptors.
Oxygen
Although the placenta functions as the organ of nutrient transport, fetal intestinal oxygen
-1
-1
consumption is fairly high (0.4 ml O2
). This is primarily due to the rapid growth of
this tissue in late gestation. Despite rapid fetal growth, intestinal tissue as a percentage of fetal
weight increases from 6.2 to 7.2% during late gestation. During postnatal development, however,
-1
intestinal tissue at rest consumes 1.4 ml O2 umin-1
despite extracting only 28% of delivered
oxygen. Oxygen consumption increases 65 to 72% during digestion. Energy requirements also
increase due to increased gastrointestinal motility and the increase in energy expended for transport
functions. This is accomplished via increased oxygen extraction and increased blood flow to the
mucosal-submucosal layer. Oxygen consumption in suckling rat intestine has been shown to
increase in the presence of gamma-globulin. Uptake of gamma-globulin is an active, energycoupled process that can be reversibly inhibited by various metabolic antagonists (iodoacetate,
arsenate, fluoride, 4,6-dinitro-o-cresol, phlorhizin, cold and anaerobiosis).
If the change in oxygen availability at birth initiates closure in the calf, either directly or through
some secondary mechanism or mechanisms, then maintaining the arterial Po2 of the newborn calf at
fetal levels should delay closure. Inconclusive results were obtained in a study examining the
effects of hypoxia in the immediate postnatal period on time of closure. Arterial Po2 was
maintained near 25 mm Hg for a period of 24 h. Time of closure was significantly delayed in
hypoxic calves fed colostrum from birth; however, no differences were noted when colostrum
feeding was delayed until 24 h. This may have been due to other (oxygen-independent) changes
occurring at birth that influence intestinal development.
Gestational Factors
If time of closure is determined by prepartum changes in the fetal serum profile, changes in
gestation length and subsequent maturity of the newborn might be expected to alter absorptive
capacity and time of closure. Extended gestation in lambs diminishes absorptive capacity and
induces precocious closure. No such relationship seems to exist in calves. If extending gestation
induces precocious closure, there should be some response to prematurity. Calves removed by
cesarian section 2 to 3 wk prior to due date are able to absorb immunoglobulins from colostrum fed
at birth but not at 38 h. Surprisingly, fetal calves are reportedly unable to absorb high levels of
gamma-globulin introduced into the amniotic fluid during the final trimester of gestation.