Gastrointestinal Development
Structural Changes and Colostrum
Relative weight and length of the digestive tract is greater at birth than in the adult, as caloric
intake per unit of body weight is higher. Tremendous growth is stimulated immediately after
birth by growth factors in colostrum.
Intestinal villi first appear at about 60 days after conception in the human fetus, with a welldefined brush border apparent by 17 weeks. The total number of villi increase throughout
gestation in both the small and large intestine, but disappear from the large intestine prior to
birth. Microvilli develop 25 weeks prior to birth in human infants, however, they are not
apparent in the rabbit until the final week prior to parturition.
At birth, the gut shows a high level of structural development. Intestinal villi are lined with a
single layer of columnar epithelial cells with well-defined microvilli at the apical surface of the
cells. The crypts of Lieberkuhn appear during the first trimester. The primary site of cell
proliferation is in the lower regions of the crypts, with generation time for crypt cells much
longer than that of adult crypt cells; the migration rate from the base of the crypts to the tips of
the villi is also proportionately longer in perinatal gut (5 days vs. 2-3 days for adults). More
rapid proliferation and cell migration occur shortly after birth, although the timing of this change
is species-dependent. Precocial species show this shift within the first week after parturition,
while in more altricial species the shift occurs near the time of weaning (typically ~3 weeks).
Increased production of gastrin-stimulated polyamines is highly correlated to this change in cell
proliferation.
As with other tissues, protein deposition and density is a function of increasing fetal maturity.
With increasing maturity, the cholesterol and phospholipid content of the microvillus membranes
decrease and the protein:lipid ratio increases. Phosphatidylcholine content increases while
phosphatidylinositol levels decrease. Palmitate:stearate and oleate:linoleate ratios increase.
Mucus glycoproteins have a decreased carbohydrate:protein ratio.
Stomach glands develop during mid-gestation and the presence of acid and pepsin are apparent
immediately in human fetuses, although the ability to produce pepsin increases with increasing
maturity; premature infants therefore have a decreased ability to produce pepsin when compared
to full-term infants. By contrast, there are no differentiated cells in the stomach of the 1 day-old
rat and no gastric glands.
At birth, stomach pH is near neutral, reflecting the pH of the swallowed amniotic fluid. After
birth, the gastrin-induced acid secretion increases rapidly in human infants; the pH drops below
3.0 by 8 hours. In domestic species or those dependent on postnatal transmission of colostral
immunoglobulins, the change in pH does not occur until the second day of life.
The exocrine pancreas functions through acinar cells; most of these cells are well-developed by
10 days postnatally. Cell renewal in the pancreas decreases five-fold by weaning, although
pancreatic weight increases 10-fold.
Importance of Colostrum in Gut Development
Rate of gain over the first 6 months is influenced by the level of passive immunity aquired the first
day. Milk production during the first lactation has been positively correlated with immunoglobulin
concentration in the newborn period. The incidence of both neonatal mortality and disease is
strongly correlated with efficiency of immune transmission. While many of these effects have been
correlated to concentrations of immunoglobulins obtained, the same correlation would be valid for
any colostral-borne factor. Since immunoglobulins are "passive" proteins in terms of their ability to
alter tissue growth and development parameters, it is far more likely that "active" factors, such as
growth factors, are responsible for extended benefits of colostral ingestion.
The presence of high concentrations of several bioactive peptides in the colostrum or milk of
different species is well established. Recent work has shown that concentrations of IGF-I are nearly
10-fold higher in the colostrum of multiparous cows than in mature milk. The role of these factors
in regulating neonatal development has only recently begun to be elucidated. Mammary secretions
have been shown to stimulate intestinal cell proliferation and growth in vitro and in vivo, and
although identification of the primary growth factor involved varies, IGF-I and EGF have both been
shown to be involved in this response. Epidermal growth factor has been shown to be effective in
stimulating proliferation of neonatal intestine in mice, rats, rabbits and piglets. These may be
receptor-mediated events as the presence of receptors have been demonstrated in rats and mice as
well as in human fetal small intestine. The other primary growth factor identified in colostrum,
IGF-I, has also been shown to stimulate small intestinal epithelial cell proliferation. The presence of
receptors have been demonstrated in crypt cells of the rat small intestine.
While colostral growth factor effects at the gut level have been investigated to some extent, studies
demonstrating direct in vivo effects have yet to be conducted. In addition, the larger issue of
potential growth factor influences on development of systems beyond the gut is largely unexplored.
A variety of biologically active peptides have been shown to be absorbed intact across the neonatal
intestine which suggests that these peptides may affect other systems beyond the gut if they reach
physiologically active concentrations. Recent reports have reported that peptides absorbed from
milk or colostrum may play regulatory roles in several systems, although definitive research in this
area is lacking. The tremendous diversity of biological effects exerted by these peptides throughout
virtually all organ systems of an animal, coupled with the dynamic changes occurring in these
systems during the perinatal period, suggests that growth factors obtained by the calf from the
colostrum of the dam may be a critically important mediator of neonatal development.
Furthermore, the developmental changes initiated during this period may extend throughout the
lifetime of the animal, altering productivity and, ultimately, profitability. Thus, colostral growth
factors may well be responsible for the well-documented "long-term" benefits of colostrum
ingestion for the calf.
Transport of Macromolecules
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 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 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 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.
Digestive Functions
Digestive enzymes may either appear as a function of age or be induced by increasing
concentrations of substrate. Disaccharidase activities are demonstrable by the end of the first
trimester. Lactase usually appears earlier in the jejunum than the ileum, although it is not
apparent until the latter one-half of gestation, with activity increasing prior to birth. In suckling
animals, lactose digestion occurs along the entire length of the digestive tract; after weaning,
activity decreases and becomes restricted to the upper portion of the small intestine. Other
disaccharidases, although present during fetal life, have very low (or functionally absent)
activities until induced by substrate postnatally following weaning. Amylase is present during
fetal development in the pancreas of the horse, cow, and sheep, but not consistently present until
6-12 months of age in infants. In dogs and rats, the activity increases dramatically following
weaning. Induction of many of these enzymes can also be induced by administration of
glucocorticoids or, in some cases, thyroxine. Endogenous concentrations of glucocorticoids
increase in rats during the third postnatal week; administration of exogenous glucocorticoids to
suckling rats will induce precocious increases in many disaccharidases, aminopeptidases, and
alkaline phosphatase; it also results in precocious decreases in lactase and lysosomal hydrolases
which normally characterize digestive activity during this period. Conversely, adrenalectomy or
hypophysectomy will delay (but not prevent) these normlow in altricial species until after the
first few weeks of life. The production of intrinsic factor is correspondingly low until the third
week of life. Absorption of monosaccharides can be demonstrated in early prenatal life.
Glucose transport occurs as early as the tenth week of gestation in human fetuses and jejunal
transport ability increases with increasing age. Active transport of glucose has been theorized to
play a role in the regulation of amniotic fluid volume. Galactose is absorbed by newborn infants
as readily as glucose, although prenatal absorption has not been demonstrated. Fructose is
absorbed by diffusion and accumulates in the intestinal epithelium.
Secretion of proteolytic enzymes begins during the second half of gestation, but dramatic
increases in proteolytic activity are not apparent until after the first week of life in calves, and the
fifth week in pigs and sheep. Trypsin activity increases throughout the latter parts of gestation in
sheep, calves, and human fetuses, and disappears after weaning. By contrast, chymotrypsin
activity increases up to weaning. Absorption of amino acids is demonstrable as early as the 12th
week of gestation in human fetuses; this may be important in recycling the amino acids from
amniotic fluid. Concentrations of amino acids are higher in amniotic fluid are much higher than
in either adult or neonatal plasma.
Pancreatic lipase is present towards the end of the first half of gestation, although duodenal fluid
has a lower lipase activity shortly after birth than later in life. In puppies and rats, activity
increases during the suckling period and nears adult values by weaning. In pigs, no change in
lipase activity is apparent during the first weeks of life. In calves, high concentrations of a
pregastric esterase is secreted in saliva until weaning. Bile acids are at low concentrations until
after birth; the high ratio of cholic:glycodeoxycholic acids present at birth in human infants is not
reversed until nearly a year and a half of age. Although active transport of sodium is present in
fetal life, absorption of calcium is low until after the first few postnatal weeks.
Meconium
Meconium first appears at the end of the first trimester, and becomes increasingly firm, solid,
and dark with increasing gestational age. It is comprised primarily of swallowed amniotic fluid
components, mucus secretions, saliva, cells from skin and the upper alimentary tract, calcium
soaps, cholesterol crystals, lanugo hair, with some bilirubin and biliverdin thrown in for color.