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The jejunal absorptive cell of the newborn pigAn electron microscopic study.

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The Jejunal Absorptive Cell of the Newborn Pig:
An Electron Microscopic Study '
College of Veterinary Medicine, Oklahoma State University,
Stillwater, OkEahoma
The morphology of the intestinal absorptive cell in the newborn pig
is strikingly different from that of the three-week-old pig. The former's unusual characteristics are: a poorly developed hirsute layer over the microvilli, an abundance of
pinocytotic vacuoles and apical tubules, and a subnuclear location of the Golgi apparatus. In unfed newborn pigs the morphology does not change appreciably by 42 hours.
Spinous processes line the apical tubules which may be associated with protein absorption. The retention of the tubular system in the absence of feeding (for 42 hours) and
its disappearance with feeding, lends support to the concept that this system is important in the absorption of protein macromolecules in the newborn pig. The position of
the Golgi may be instrumental in regulating the ability of the cell to absorb large
molecular weight proteins.
The absorptive intestinal cell of the newborn pig absorbs unaltered colostral protein
during the first 36 hours of postnatal life
(Brambell, '58) : thereafter this function
is lost. The mechanism by which the intestinal epithelium loses this capacity to
absorb macromolecular protein is poorly
understood. Absorption from the intestine
of the newborn occurs mainly by pinocytosis (Clark, '59). Decrease or loss of pinocytotic activity, with consequent expenditure of the luminal surface membrane, may
account for the failure of the intestinal
epithelial cell to absorb protein macromolecule (closure) (Lecce, '66). Such closure
of the intestinal epithelial cell occurs concurrently with the development of mature
morphologic characteristics (Mattisson
and Karlsson, '64).
It has been postulated that ontogeny of
enzymatic activity in the cells migrating
up the intestinal villi signifies maturation
(Wilson, '62) and decreased absorption of
proteins (Payne and Marsh, '62). Administration of cortisone derivatives accelerates both the appearance of alkaline phosphatase in the intestinal epithelium (Moog,
'53) and the loss of protein absorption
(Halliday, '58; Clark, '59; Payne and Marsh,
'62). Cortisones may therefore be implicated as a factor in closure of the intestinal
Several efforts have been made to investigate the structural changes in the neonatal jejunal cell during absorption. Clark
ANAT. REC., 161: 497-516.
('59) observed an apical tubular system in
the neonatal jejunal cell of mice and rats
which transported undigested proteins into
the cell. This tubular system atrophied as
this mode of protein absorption ceases.
Epithelial cells, both of the mammalian
yolk sac (Dempsey, '53) and of the proximal convoluted tubules of the kidney
(Maunsback, '66a), have a tubular system
which has been implicated in protein absorption. The apical tubular system has
also been demonstrated to be the pathway
by which ferritin is absorbed from the intestinal wall of rats and rabbits (Graney,
'65; and Kraehenbuhl et al., '67). Kraehenbuhl further demonstrated the pathway of
ferritin into the intracellular space, then
through the basement membrane and into
the vessels. Anderson ('65) fed milk proteins to day-old puppies and observed vacuoles containing lipid and protein in the
apical cytoplasm of the intestinal cells. He
speculated that as lipid was released from
the cytoplasmic vacuoles into the intracellular space, proteins passed along with it.
In the newborn pig, Mattisson and Karlsson ('64, '65, '66) indicate that the jejunum
is the most active area of the intestine for
protein absorption. Like other neonates,
the absorptive cells in this region were observed to have a nongranular membrane
system of tubules and vacuoles in the api1This investigation was supported in part by Public Health S e m c e Research Grants NB05350 and
A1 06461.
at pH 7.2-7.4. After initial fixation for
six hours at 4"C., the jejunal segment was
washed in four to five changes of the respective buffer, the final wash lasting overnight. The following day, the jejunum was
removed from the dental wax and cut into
pie-shaped segments. Secondary fixation
was by (4°C) 1% osmium tetroxide in veronal acetate buffer and, after two hours, was
washed in several changes of the same
buffer. The specimen was dehydrated rapidly in a graded series of cold ethanol-water
mixture and embedded in Epon (812)Araldite. The pie-shaped segments were
oriented in Epon-Araldite in gelatin capMATERIALS AND METHODS
sules by the methods described by Pitmann
and Pitmann ('66). The Epon-Araldite was
Three groups were examined by electron cured at 40°C and 29-30 inches of vacuum
microscopic techniques. One group of overnight and transferred the following day
newborn pigs was delivered by cesarean to an environment of 60°C at atmospheric
section at 110-112 days of gestation, and pressure for 20 hours. Gold sections were
were killed prior to feeding one to two cut on a Porter-Blum M-2 microtome, were
hours after birth. A second group of pigs mounted on unsupported 120 mesh copper
was delivered naturally, were not permitted grids, and were stained in lead citrate for
to nurse and were placed in separate clean five to ten minutes (Venable and Coggedry boxes. Within four hours after birth shall, '65). A Philips EM-200 was used
the pylorus was transected via a midline for examination and photography of the
incision. The pyloric stump was exteri- sections.
orized and the anterior end of the duodeRESULTS
num was ligated, replaced in the abdomen,
and the incision closed. Intestinal contact
Light microscopy of the newborn pig
with orally ingested material was thereby jejunum. The villi of the newborn pig
prevented. These animals received 15-20 jejunum were well developed, tall, slender,
ml of 5% glucose in 0.85% saline by sub- and did not branch (fig. 1). The intestinal
cutaneous injection every four to eight epithelium was of the simple columnar
hours, and were killed 42 hours after birth. type with few interspersed goblet cells.
A third group of pigs was delivered by Throughout the entire villus, the columnar
cesarean section under germ-free condi- cells had a prominent striated border betions and maintained in plastic germfree neath which the apical cytoplasm was clear
isolators for three weeks. They were fed and did not contain visible cytoplasmic orsterile condensed canned milk (Veramel). ganelles. Although the nuclei in the columAll animals were sacrificed by stunning nar cells at the villus tips were round to
with a blow on the head and exsanquinated. oval and occupied the apical protein of the
The abdominal cavity was opened within cytoplasm, toward the base of the villus,
two to three minutes of stunning and the the nuclei appeared oblong and were lojejunum removed immediately.
cated more basally. In the cells lining the
upper two-thirds of the villus, vacuoles
Tissue processing
were prominent, generally small in the
A section of the jejunum was placed supranuclear position and larger below the
immediately in (4°C) normal saline nucleus. In the crypts, however, the cyto(0.85% ), opened along its mesenteric plasm of the lining cells was basophilic
border, pinned flat on dental wax, and im- and differentiation of the nucleus was
mersed quickly in cold (4°C) 5% glu- difficult. The cytoplasmic basophilia detaraldehyde containing either Palade's creased progressively in cells situated at
('52) or Pitmann's ('66) phosphate buffer higher levels on the villus.
cal cytoplasm. Lipid and granular electron dense material, suggestive of protein,
was present in the pinocytotic tubules and
vacuoles after exposure to colostrum. Since
the apical pinocytotic system of tubules
and vacuoles seems to be responsible
for the neonatal absorption of protein
and lipid, a more detailed examination of
this apparatus is indicated. This investigation confirms the presence of an apical
system of tubules and vacuoles in the jejunal cell of the unfed newborn pig and
elaborates on its substructural arrangement.
Electron microscopy of the absorptive
cell. Cells at three positions on the villuscrypt structure were examined by electron
microscopy (fig. 1). Observations were
made primarily on the structure of the
microvilli, the apical tubules, the apical
and basal vacuoles, and the Golgi apparatus.
All absorptive cells of the neonatal porcine jejunum had well developed microvilli over the entire villus (fig. 2). The
length and number of the microvilli between the tip, the midportion of the villus
and crypt varied. On the tip and in the
crypt, the microvilli were short and clubshaped. In the mid-villus, the microvilli
were longer and closely packed. Adjacent
microvilli in all regions occasionally had
a common base. Irregularities of the microvilli were more common in the crypts where
individual microvilli may be connected by
The trilaminar membrane of the cell was
continuous over the microvilli (fig. 3 ) . A
hirsute layer was not well developed over
the microvilli of the neonate (fig. 2), but
was well formed at three weeks of age
(fig. 4). A central core of filaments ran
from the tip of the microvilli into the apical cytoplasm. These intracellular filaments condensed into well defined rootlets
in those cells in the upper two-thirds of the
villus, but were ill-defined in the cells of
the crypts. Rootlets adjacent to intracellular junctional complexes appeared to enter into these structures.
Junctional complexes which united adjacent cells at their apical ends were similar
to those described by Farquhar and Palade
('63). Basal intracellular spaces were always present between the cells in the
crypts, but variable and inconsistent between the cells on the upper two-thirds of
the villus.
The terminal web (Palay and Karlin,
'59) in the intestinal cell of the mouse,
was not prominent in the absorptive cell
of the newborn pig. The cytoplasmic zone
beneath the microvillous border was free
of cellular organelles and had a faint granular appearance. An apical tubular network in this organelle free zone could only
be seen at magnification of 2,000 or more
(fig. 5). Apical tubules were confined primarily to the cells of the upper villus and
were less numerous in the cells adjacent
to the crypts. Although the tubules were
usually parallel with the long axis of the
cell, they did branch and anastomose to
form a complex network, and appeared to
terminate after they penetrated the cell
€or 2 to 3 p (fig. 2 ) . These tubules were
slender (30-60 mp) near the luminal
plasmalemma, but along the tubules, there
were dilated portions which, in the deeper
apical cytoplasm, appeared to be associated with the formation of vacuoles (fig. 6).
The tubules had a trilaminar membrane,
which was approximately 95-100A in
thickness. Both electron dense layers of
the trilaminar membrane were of comparable thickness. Fuzzy extensions of the
inner electron dense layer projected into
the lumen of the tubules and in some
tubules formed dense spines. These spines
were most obvious at the opening of the
tubules into the intestinal lumen but occurred anywhere along the tubules (fig. 7).
At the base of some microvilli the Iuminal
electron dense component of the trilaminar
membrane contained small blebs (fig. 3).
In the three-week-old pig, the apical tubules were absent in the absorptive epithelial cells (fig. 4).
Numerous vacuoles (0.1-5.0
in diameter) occurred in the absorptive cells
of the jejunum. The smaller vacuoles were
most prominent in the apical cytoplasm,
and appeared to originate either by pinocytosis or by dilatation of the apical tubular
system. The coalescence of small vacuoles
to form large supranuclear vacuoles created deep indentations on the apical surface of the nuclear membrane (fig. 8).
Initially, the membranes of the vacuoles
were prominent but, as the vacuoles coalesced, they became less distinct. Often
the membranes appeared to have delaminated into the vacuoles (fig. 5). Within
some vacuoles the delaminated membranes
formed prominent myelin-like whorls
(fig. 9). The small apical vacuoles generally contained little material except
free membranes. The larger basal vacuoles
contained a flocculent material which was
either dense and homogenous or clumped
in bizarre arrangements. The Golgi apparatus was usually closely associated with
the basal vacuoles and consisted of a profuse flattened stack of trilaminar mem-
neonatal cell were: a poorly developed
hirsute layer over the microvilli, abundant
pinocytotic vacuoles and apical tubules, the
apical position of the nucleus, and the subnuclear location of the Golgi apparatus.
The maturation of this cell presumably
is hastened, if not initiated, by contact with
non-digested protein. Thirty-six hours
after birth, the maternal colostrum is no
longer absorbed (Payne and Marsh, '62)
and the apical tubular system disappears
(Mattisson and Karlsson, '65). The hirsute
mucopolysaccharide coating over the microvilli is apparently synthesized after whole
protein absorption ceases, for although absent in the neonate it is readily apparent
in the absorptive cell of the three week
old piglet. The terminal web of filaments
supporting the rootlets of the microvilli
(McNabb and Sandborn, '64) was neither
apparent in the newborn, nor in the threeweek-old pig. The failure to observe the
terminal web may be a consequence of glutaraldehyde fixation since after osmium
fixation it is visible as a thin web.
Vodovar ('64) concluded that the pig
intestinal epithelial cell must elongate during maturation, since the newborn procine
epithelial cell is 6-9
long and that
of the adult 40 p. The lateral infoldings
of the newborn jejunal cell supports the
concept that elongation occurs during development. It is uncertain, however,
whether this initial cell persists on the
villus long enough to undergo elongation.
Although the rate of cellular turnover in
pigs is undetermined, the villi reach apparently mature length before 42 hours
when degenerating cells were evident at
the tip.
Existence of an apical tubular system
and of numerous vacuoles is perhaps the
most obvious characteristic of the neonatal
intestinal absorptive cell. The passage of
a nondigested protein through this tubular
system has been demonstrated by electron
microscopy, but the role of the tubules in
absorption has not been ascertained.
Maunsbach ('66b) speculated that the apical tubules in the epithelial cell of the
proximal convoluted tubule of the kidney
The morphology of the intestinal epi- may be extensions from the surface plasthelial cell in the newborn pig was strik- malemma to. the apical vacuoles. The simiingly different from that of the adult. larity in membrane structure between the
Unusual characteristics displayed by the apical tubules, the vacuoles, and the plas-
brane-bound vacuoles which were surrounded by numerous tiny vacuoles (fig.
10). The Golgi apparatus of the neonatal
absorptive cell in the upper two-thirds of
the villus was subnuclear, while that of
epithelial cells near the crypt area was
around the midportion of the cell. Many
large vacuoles and inclusions were associated with the subnuclear Golgi apparatus
of the neonatal absorptive cell. At three
weeks of age, the Golgi apparatus of the
absorptive cell was supranuclear (fig. 11).
In the absorptive cell of the newborn
pig, the mitochondria and granular endoplasmic reticulum are present in the basal
end of the cell (figs. 5, 8) whereas in
the three-week-old pig they have become
dispersed throughout the entire cell (fig.
Electron microscopy of the absorptive
cells in 42-hour-old unfed pigs. Fortytwo hours after birth the villi had long
columns of degenerating cells stacked on
their tips (fig. 12). The microvilli of these
cells were swollen and fragmented. The
mitochondria were swollen, vacuolated,
and some cristae were separated or absent.
Throughout the cytoplasm of the degenerating cells, there were many small membrane enclosed vacuoles which appeared
to have originated from degenerating apical tubules and mitochondria. Despite
apparent degeneration and instability of
the cell membrane, the desmosomal attachments were intact and held the cells together. There was no evidence of nuclear
degeneration as indicated by shrinkage,
serration of the nuclear membrane, or abnormal clumping of the chromatin (fig. 13).
Below the degenerating cells the absorptive cells were morphologically similar to
those observed in other newborn pigs (fig.
14). The apical tubular system was very
prominent and was not degenerating into
myelin-like membranes. The mitochondria
and granular endoplasmic reticulum were
interspersed with the tubular system, a feature which was not seen in the newborn
cell. The Golgi apparatus was subnuclear.
malemma of the intestinal absorptive cell
supports such a thesis. Tubular dilatations
observed during this investigation, indicate that some vacuoles arise in this manner. The concept of membrane flow as
proposed by Bennett ('56) may account for
the tubule-like appearance of the invaginated surface plasmalemma before vacuoles are formed. The mechanism by which
small spines form within the tubules is
uncertain, they may begin as blebs on the
trilaminar membrane at the base of the
microvilli (fig. 3). These blebs condensed
into spine-like structures during the formation of the apical tubules. Roth and Porter
('62) reported a pinocytotic vacuole lined
with spinous processes in the sinusoidal
cell of the liver. They associated this vacuole with protein absorption. These spinous
structures observed in the intestinal absorptive cell may indicate receptor sites or
sites of enzyme activity. The presence of
phosphatases in the plasmalemma (Goldfischer et al., '64) and their association
with the formation of pinocytotic vesicles
has been reported by Marchesi ('65).
The disappearance of the vacuole from
the cell of the unfed newborn pig apparently occurs, by the delamination of its
membrane into the lumen of the vacuole
and by digestion of the resulting myelinlike structure in the region of the Golgi
apparatus. The presence of myelin-like
structures within the cell have been shown
by Swift and Hruban ('64) to be a site of
focal cytoplasmic degradation. In the intestinal epithelial cell of the newborn pig,
the cytoplasmic degradation involved the
vacuole walls. Behnke ( ' 6 3 ) observed cytoplasmic inclusions, including myelin-like
figures, in the differentiating duodenal cells
of rat fetuses. These myelin-like figures
were prominent when the squamous cells
were degenerating and the epithelium was
reorganized into the villi with columnar
epithelial cells.
The subnuclear Golgi apparatus is a
unique feature of the newborn intestinal
absorptive cell which, to our knowledge,
has not been previously reported. The
Golgi is located in a supranuclear position
in the three-week-old piglet, indicating a
chance in polarity of the cell as the cell
matures. The position of the Golgi apparatus may be instrumental in regulating
the ability of the cell to absorb large molecular weight proteins. In the unfed surgically altered animals some change in
polarity was observed by 42 hours, with
the appearance of mitochondria and granular endoplasmic reticulum in the apical
The maturation and reorganization of
the cell is apparently accelerated by contact with colostrum and/or protein; pinocytotic activity declines, the tubular system
disappears, an inverse polarity of the cell
occurs, and the absorption of intact proteins ceases. If there is no exposure to
colostrum or any other nutrient, pinocytotic activity and the tubular system persist
at least up to 42 hours and perhaps longer.
The prolonged pinocytosis and tubule formation casts some doubt on the theory
that expenditure of the luminal surface
membrane impairs absorption. Persistence
of this tubular system up to 42 hours is
compatible with the observations of Payne
and Marsh. They reported that piglets
maintained on a protein free diet up to
106 hours still actively absorb immunoglobulins. Despite the incrimination of
protein in intestinal closure, a heat stable
low molecular weight component dialyzed
from colostrum or skim milk will also initiate closure (Lecce et al., '64). Whether
or not this component and other substances
which hasten closure act by accelerating
the change in polarity of the cell or disappearance of the tubular system warrants
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at, apical tubules
av, apical vacuoles
b, blebs
bv, basal vacuoles
d, desmosomes
dat, dilated apical tubules
dm, delaminating membrane
Dm, vacuolated mitochondria
er, granular endoplasmic reticiulum
FM, fragmented microvilli
g, goblet cell
G, Golgi
gc, glycocalyx
if, infoldings
is, intracellular space
m, mitochondria
M, microvilli
N, nucleus
r, rootlets
s, spines
tlm, trilaminar membrane
v, vacuoles
A 6 P paraffin section from the jejunum of a newborn, unfed pig.
Three locations on the villus crypt structure, examined by electron
microscopy, are indicated (arrows) ; 10% formalin fixation. Hematoxylin and eosin staining. X 140.
All tissues for electron microscopy were fixed initially in 5% glutaraldehyde, postfixed in 1% osmium tetroxide and stained with lead
The apical surface of a n absorptive epithelial cell from the jejunum of
a newborn, unfed pig. The microvilli ( M ) , apical vacuoles ( a v ) , and
apical tubules ( a t ) are well developed. X 29,000.
3 Cells from the midportion of the villus of a newborn unfed pig, sometimes have small blebs ( b ) on the trilaminar membrane ( t l m ) at the
base of the microvilli (M). Rootlets ( r ) extend from the microvilli into
the apical cytoplasm. X 70,000.
Theodore E. Staley, E. Wynn Jones and Arvle E. Marshall
The apical surface of an absorptive epithelial cell from the jejunum of
a three-week-old pig. The microvilli ( M ) with a covering of glycocalyx
(gc) are well developed. No apical tubules or vacuoles can be seen i n
the apical cytoplasm. X 40,000.
The absorrtive cells from a newborn, unfed pig i n midportion of an
intestinal villus. The apical tubules ( a t ) are profuse and the apical
cytoplasm contains vacuoles ( a v ) of various sizes. The larger ones
have membranes delaminating ( d m ) into the interior of the vacuole.
The Golgi ( G ) apparatus is located beneath the nucleus. Goblet cells
( g ) . X 5,000.
Theodore E. Staley, E. W y n n Jones and Arvle E. Marshall
In the newborn unfed absorptive cell the apical tubules ( a t ) have a
trilaminar membrane wall, very similar to the membrane over the surface of the microvilli. Dilated portions of the tubules ( d a t ) can be
seen forming apical vacuoles (av). X 60,000.
The apical end of newborn unfed absorptive cell showing the formation
of spine-like structures ( s ) on a n invagination site between microvilli.
The apical tubules ( a t ) also show spines ( s ) on the inner component
of the trilaminar membrane. X 70,000.
Theodore E. Staley, E. Wynn Jones and Arvle E. Marshall
A n absorgtive erithelial cell from the midportion of an intestinal villus
from a newborn, unfed pig. The apical vacuoles ( a v ) during their formation produce indentations in the surface of the nuclei. These cells
have extensive infoldings of the lateral cell membrane ( i f ) . The Golgi
(G) in these cells are located i n a subnuclear position. X 4,800.
Myelin-like structures in the absorptive epithelial cell of the newborn
unfed pig. These structures begin as membranes which delaminate into
the vacuole ( 9 a ) (fig. 5) and the vacuole closes around the membranes
(9b). The membranes condense into a myelin-ilke structure (9c).
X 23,000.
10 The Golgi ( G ) apparatus in the intestinal epithelial cell of the newborn unfed pig is situated below the nucleus ( N ) . In this position
it comes into contact with large basal vacuoles (bv) filled with a
clumped material. The basal intracellular space ( i s ) is quite large
between adjacent cells. X 14,000.
Theodore E. Staley, E. Wynn Jones and Arvle E. Marshall
11 The absorptive epithelial cells from the jejunum of a three-week-old
germfree piglet. The cell a t this age assumes a more conventional
appearance. The apical tubules and vacuoles are absent, the nuclei
are basally situated, and the Golgi (G) are above the nuclei. X 5,000.
12 The jejunal villi of a 42-hour unfed newborn pig. Columns of degenerating cells (arrows) have piled on the tips of the villi. These
degenerating cells are more cuboidal in shape than the normal columnar cells ( * ) lower on the villus. Toluidine blue stain. X 280.
Theodore E. Staley, E. Wynn Jones and Arvle E. Marshall
13 Degenerating cells from the tip of a villus of a 42-hour-old unfed newborn pig. Evidence of degeneration includes fragmented microvilli
(FM), vacuolated mitochondria (Dm) and many membrane enclosed
vacuoles (v). The desmosomes ( d ) between cells are intact. X 5,000.
The absorptive cell of a 42-hour-old unfed pig. Prominent apical tubules ( a t ) , mitochondria ( m ) , and the beginning formation of a
granular endoplasmic reticulum (er), are evident i n the apical cytoplasm. X 15,000.
Theodore E. Staley, E. Wynn Jones and Arvle E. Marshall
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