Development of the lymphatic system 222 Development of lymph nodes 222 Development of spleen and tonsils 223 Clinically oriented questions 223. Embryology is concerned with the origin and development of a human being from a zygote to birth.
IMPORTANCE OF AND ADVANCES IN EMBRYOLOGY
An ovarian follicle containing an oocyte, ovulation and phases of the menstrual cycle is shown. The isolation and cultivation of human embryonic and other stem cells may hold great promise for the development of molecular therapies.
DESCRIPTIVE TERMS
For example, the spine develops in the dorsal part of the embryo and the sternum is in the ventral part of the embryo. Doctors date a pregnancy from the first day of the last normal period, but the embryo.
REPRODUCTIVE ORGANS
The body of the uterus tapers from the fundus to the isthmus, the narrowed area between the body and the cervix (see Figure 2-2A). The fallopian tubes, measuring 10 cm in length and 1 cm in diameter, extend laterally from the horns of the uterus (see Figure 2-2A).
GAMETOGENESIS
Each primary spermatocyte subsequently undergoes a reduction division—the first meiotic division—to form two haploid secondary spermatocytes, which are approximately half the size of primary spermatocytes (see Figs. 2-5). The secondary oocyte released during ovulation is surrounded by a covering of amorphous material—the zona pellucida—and a layer of follicular cells—the corona radiata—(see Fig. 2-4C).
FEMALE REPRODUCTIVE CYCLES
At birth, all primary oocytes have completed prophase (the first stage of mitosis) of the first meiotic division (see Figure 2-6). At ovulation (release of the oocyte), the nucleus of the secondary oocyte begins a second meiotic division, but only progresses to metaphase.
ABNORMAL GAMETOGENESIS
The primary oocyte is surrounded by follicular cells - the cumulus oophorus - which protrude into the enlarged antrum. Before ovulation, the secondary oocyte and some cells of the cumulus oophorus detach from the interior of the distended follicle (see Figure 2-10B).
MITTELSCHMERZ AND OVULATION
This increase, induced by a high blood estrogen level (Fig. 2. 11), appears to cause the stigma to rupture and expel the secondary oocyte along with the follicular fluid (see Fig. 2-10D). Subsequently, fluid-filled spaces appear around the follicle cells, which merge to form a single cavity, the antrum, containing follicular fluid (see Fig. 2-9B).
ANOVULATION AND HORMONES
The ejected secondary oocyte is surrounded by the zona pellucida, an acellular glycoprotein layer, and one or more layers of follicular cells, arranged radially to form the corona radiata and cumulus oophorus (see Figure 2-4C). When the stigma ruptures, the secondary oocyte is expelled from the ovarian follicle with the follicular fluid.
ANOVULATORY MENSTRUAL CYCLES
Degeneration of the corpus luteum is prevented by human chorionic gonadotropin (hCG) (see Chapter 4). Ischemia (reduced blood supply) of the spiral arteries occurs from narrowing as a result of the decrease in the secretion of progesterone (see Fig. 2-2C).
TRANSPORTATION OF GAMETES Oocyte Transport
Withdrawal of hormones also results in cessation of glandular secretions, a loss of interstitial fluid, and marked shrinkage of the endometrium. As small pieces of endometrium break off and pass into the uterine cavity, the torn ends of the spiral arteries bleed into the uterine cavity, resulting in an accumulated loss of 20 to 80 ml of blood.
SPERM COUNTS
As the spiral arteries contract for prolonged periods, stasis (stagnation of blood and other fluids) and patchy ischemic necrosis (death) occur in the superficial tissue. Small pools of blood form and break through the endometrial surface, resulting in bleeding into the uterus and vagina.
VASECTOMY
At termination of pregnancy, ovarian and menstrual cycles resume after a variable time.
MATURATION OF SPERMS
VIABILITY OF OOCYTES AND SPERMS
FERTILIZATION
The proteolytic enzyme acrosin, as well as esterases and neuraminidase, appear to cause the lysis of which is released from the sperm acrosome. Furthermore, the tail movements of the sperm are important during the penetration of the corona radiata.
CLEAVAGE OF ZYGOTE
The finger-like outgrowths of the syncytiotrophoblast extend through the endometrial epithelium and invade the connective tissue of the endometrium. Figure 3–4 Attachment of the blastocyst to the endometrial epithelium during the early stages of its implantation.
IN VITRO FERTILIZATION AND EMBRYO TRANSFER
The process of in vitro fertilization (IVF) of egg cells and transfer of dividing zygotes or blastocysts into the uterus has offered an opportunity to many infertile couples. The intracytoplasmic sperm injection technique involves injecting sperm directly into the cytoplasm of a mature oocyte.
PREIMPLANTATION DIAGNOSIS OF GENETIC DISORDERS
Using currently available techniques, a cleaving zygote known to be at risk for a specific genetic disorder can be diagnosed before implantation during IVF. Early spontaneous abortions occur for a variety of reasons, an important one being the presence of chromosomal abnormalities.
ABNORMAL EMBRYOS AND SPONTANEOUS ABORTIONS
The 10-day conceptus (embryo and extraembryonic membrane) is completely embedded in the endometrium
When the conceptus implants, the connective tissue cells of the endometrium undergo remodeling – the decidual reaction – as a result of cyclic adenosine monophosphate and progesterone signaling. Note the reduction in relative size of the primary umbilical vesicle and the appearance of primary chorionic villi.
DEVELOPMENT OF CHORIONIC SAC
The embryo, amniotic sac and umbilical vesicle are suspended by the connecting stem in the chorionic cavity (see fig.). Transvaginal ultrasound (endovaginal ultrasound) is used to measure the diameter of the chorionic sac.
EXTRAUTERINE IMPLANTATION SITES
The usual site in the posterior wall of the uterine body is indicated by an X. The approximate order of frequency of ectopic implantations is indicated alphabetically (A, most common, H, least common).
IMPLANTATION SITES OF BLASTOCYSTS
Although cervical pregnancy is properly included among the sites of uterine pregnancy, it is often considered an ectopic pregnancy.
INHIBITION OF IMPLANTATION
Can a drug taken during the first 2 weeks of pregnancy cause an abortion? Figure 4–6 A, Coronal section of the uterus and fallopian tube illustrating an ectopic pregnancy in the ampulla of the fallopian tube.
GASTRULATION: FORMATION OF GERM LAYERS
Caudal to the primitive streak there is a circular area—the cloacal membrane—that marks the future site of the anus (see Fig. 5-5A and D). At the beginning of the third week, the primitive streak appears on the dorsal aspect of the embryonic disc (see Fig. 5-2B).
NEURULATION: FORMATION OF THE NEURAL TUBE
As the notochord elongates, the neural plate expands and eventually extends cranially to the oropharyngeal membrane (see Figure 5-4C). The neural folds are particularly pronounced at the cranial end of the embryo and are the first signs of brain development (Fig. 5-7C).
DEVELOPMENT OF SOMITES
Somatic mesoderm and upper embryonic ectoderm form the embryonic body wall (see Fig. 5-7F), while splanchnic mesoderm and lower embryonic endoderm form the gut wall.
EARLY DEVELOPMENT OF CARDIOVASCULAR SYSTEM
DEVELOPMENT OF INTRAEMBRYONIC COELOM
At the beginning of the third week, the formation of blood vessels or vasculogenesis begins in the extraembryonic mesoderm of the umbilical cord, connecting stalk and chorion. By the end of the third week, the blood is flowing and the heart begins to beat on day 21 or 22.
DEVELOPMENT OF CHORIONIC VILLI
The villi that grow on the sides of the stem villi are branched chorionic villi (terminal villi). It is through the walls of the branching villi that the main exchange of material between the blood of the mother and the embryo takes place.
SACROCOCCYGEAL TERATOMA
Branchial villi are bathed in constantly changing maternal blood in the intervillous space (see Fig. 5-11C). The tubular heart fuses in the embryo with blood vessels connecting the stalk, chorion, and umbilical vesicle to form the primordial cardiovascular system (Fig. 5-11C).
ABNORMAL NEURULATION
Figure 5–11 Illustrations of the development of the secondary chorionic villi into the tertiary chorionic villi. Figure 5-12 A female infant with a large sacrococcygeal teratoma that developed from remnants of the primitive streak.
ABNORMAL GROWTH OF TROPHOBLAST
These moles exhibit variable degrees of trophoblastic proliferation and produce excessive amounts of human chorionic gonadotropin. These tumors invariably metastasize (spread) through the blood to various sites, such as the lungs, vagina, liver, bones, intestines, and brain.
FOLDING OF EMBRYO
Simultaneously, the primordial heart and the oropharyngeal membrane move to the ventral surface of the embryo (Fig. 6-2). During folding, part of the endodermal germ layer is incorporated into the embryo as the hindgut (see Fig. 6-3C).
GERM LAYER DERIVATIVES
During lateral (longitudinal) coiling, part of the endoderm of the umbilical sac becomes embedded in the embryo as the foregut, the beginning of the pharynx (see Fig. 6-2C). As the abdominal wall is formed by the fusion of the lateral folds, part of the germinal layer of the endoderm is incorporated into the embryo as the midgut.
CONTROL OF EMBRYONIC DEVELOPMENT
As the embryo grows, the tail region protrudes beyond the cloacal membrane, the future site of the anus (Fig. 6-3B). Note that the septum transversum, heart, pericardial coelom and oropharyngeal membrane have moved to the ventral surface of the embryo.
HIGHLIGHTS OF THE FOURTH TO EIGHTH WEEKS
Seated height, or crown-hip length, is used to estimate the age of older embryos (see Fig. 6-5B and C). Standing height, or crown-to-heel length, is sometimes measured at 14 to 18 weeks (see Fig. 6-5D).
ESTIMATION OF EMBRYONIC AGE
ULTRASONOGRAPHIC EXAMINATION OF EMBRYOS
By the end of the fourth week, the caudal neuropore is normally closed (see Fig. 6-10). The primordia of the digits (fingers)—the digital rays—begin to develop in the hand plates (Fig. 6-13A and B).
HIGHLIGHTS OF FETAL PERIOD
At the beginning of the 16th week, the developing bones are clearly visible on ultrasound images. Usually, by the end of this period, the skin is pink and smooth, and the upper and lower extremities appear plump.
FACTORS INFLUENCING FETAL GROWTH
Fetuses of this age are unable to survive if born prematurely, primarily because the airways are immature. As term approaches (37-38 weeks), the nervous system is sufficiently mature to perform some integrative functions.
PROCEDURES FOR ASSESSING FETAL STATUS
The umbilical cord usually falls off 7 to 8 days after birth, at the end of the early neonatal period. Continuous fetal heart rate monitoring in high-risk pregnancies is routine and provides information on fetal oxygenation.
NEONATAL PERIOD
A gentle stroke on the baby's cheek causes the baby to turn towards the touch with its mouth open. Down syndrome (trisomy 21) is the most commonly known chromosomal disorder, and children born with this condition have varying degrees of intellectual disability.
PLACENTA
Placental septa divide the fetal part of the placenta into irregular convex areas - cotyledons (see Fig. 8-3). Decidua basalis - the part of the decidua deep to the conceptus (embryo and membranes) that forms the maternal part of the placenta.
ULTRASONOGRAPHY OF CHORIONIC SAC
The villous chorion is where chorionic villi persist and form the fetal part of the placenta. The large surface area of the placental membrane facilitates the transport of substances in both directions between the placenta and the maternal blood.
PARTURITION
The placenta also plays a major role in the production of steroid hormones (ie progesterone and estrogen). During the first trimester, the uterus expands from the pelvic cavity, and by 20 weeks it usually reaches the level of the navel.
HEMOLYTIC DISEASE OF THE NEONATE
Separation of the placenta results in bleeding and formation of a large hematoma (mass of blood). The umbilical cord is attached to the edge of the fetal surface of the placenta.
PLACENTAL ABNORMALITIES
In placenta accreta, there is abnormal attachment of the placenta to the myometrium (the muscle layer). The umbilical cord usually attaches near the middle of the fetal surface, and its epithelium is continuous with the amnion, which adheres to the chorionic plate of the placenta (see Figs. 8-9B), giving the fetal surface a smooth texture.
ABSENCE OF UMBILICAL ARTERY
In placenta previa, the placenta overlies the internal os of the uterus, blocking the cervical canal. The attachment of the cord to the placenta is usually near the center of the fetal surface of the placenta (see Fig. 8-9B), but it may attach in other places (see Fig. 8-10).
AMNION AND AMNIOTIC FLUID
As the amnion enlarges, it gradually obliterates the chorionic cavity and forms the epithelial covering of the umbilical cord (see Fig. 8-13A and B). The formation of the fetal part of the placenta and the degeneration of the chorionic villi are also shown.
UMBILICAL VESICLE
ALLANTOIS
DISORDERS OF AMNIOTIC FLUID VOLUME
PREMATURE RUPTURE OF FETAL MEMBRANES
Because they result from the fertilization of an oocyte and develop from a zygote (see Fig. 8–16), MZ twins are of the same sex, are genetically identical, and are similar in physical appearance. Physical differences between MZ twins are environmentally induced, e.g. at anastomosis of the placental vessels, resulting in differences in the blood supply from the placenta (Fig. 8-17).
MULTIPLE PREGNANCIES
DZ twins always have two amnions and two chorions (see Fig. 8-15A), but the chorions and placenta may be fused (see Fig. 8-15B). The intraembryonic part of the allantois runs from the umbilicus to the urinary bladder, with which it is continuous (see Chapter 13, fig.
TWIN TRANSFUSION SYNDROME
The risk of recurrence in families with a set of DZ twins is approximately three times that of the general population. After birth, the urachus becomes a fibrous cord, the median umbilical ligament, which extends from the top of the bladder to the navel.
ESTABLISHING ZYGOSITY OF TWINS
Figure 8–16 Illustrations of how about 65% of monozygotic twins develop from one zygote by division of the inner cell mass. Note the wide difference in size due to an uncompensated arteriovenous anastomosis of the placental vessels.
CONJOINED TWINS
Separation of the blastomeres can occur at any time from the two-cell stage to the morula stage, producing two identical blastocysts. What is the scientific basis for the home pregnancy tests sold in pharmacies.
EMBRYONIC BODY CAVITY
These membranes fuse with the dorsal mesentery of the esophagus and septum transversum (see Fig. 9-6C). The septum transversum and pleuroperitoneal membranes fuse with the dorsal mesentery of the esophagus.
DEVELOPMENT OF DIAPHRAGM
By the sixth week, the developing diaphragm is at the level of the thoracic somites. Arrows indicate the development of the pleural cavities as they expand into the body wall.
POSTEROLATERAL DEFECT OF DIAPHRAGM
A posterolateral diaphragmatic defect is the only relatively common congenital anomaly involving the diaphragm (Fig. As a result, the abdominal organs are displaced upward into the pocket bulge of the diaphragm.
EVENTRATION OF DIAPHRAGM
Therefore, the phrenic nerves then lie on the fibrous pericardium of the heart arising from the pleuropericardial membranes (see Fig. 9. 5C and D). The costal border of the diaphragm receives sensory fibers from the lower intercostal area. nerves due to the origin of the peripheral portion of the diaphragm from the lateral body walls (see Fig. Figure 9–7 A and B, Extension of the pleural cavities into the body walls to form the peripheral portions of the diaphragm, the costodiaphragmatic recesses, and the characteristic domed configuration of the diaphragm.
RETROSTERNAL (PARASTERNAL) HERNIA
The phrenic nerves in the fetus enter the diaphragm by passing through the pleuropericardial membranes. A man had a routine chest x-ray about 1 year ago and was told that a small part of his small intestine was in his chest.
PHARYNGEAL ARCHES
The facial skin is supplied by the fifth cranial nerve (CN V or trigeminal nerve); however, only the two caudal branches (maxillary and mandibular) supply derivatives of the first pharyngeal arch (see Fig. 10-6B). These muscles are supplied by the facial nerve (cranial nerve VII), the nerve of the second pharyngeal arch.
PHARYNGEAL POUCHES
The dorsal part of every fourth pouch develops into a superior parathyroid gland, which lies on the dorsal surface of the thyroid gland (see Fig. 10-7B). If the fifth pharyngeal pouch develops, it is rudimentary and becomes part of the fourth pharyngeal pouch.
PHARYNGEAL GROOVES
The elongated ventral portion of every fourth pouch develops into the ultimopharyngeal body, which fuses with the thyroid gland, giving rise to the parafollicular cells (C cells) of the thyroid gland. C cells differentiate from neural crest cells that migrate from the pharyngeal arches into the fourth pair of pharyngeal pouches.
AURICULAR SINUSES AND CYSTS
The parathyroid glands emerging from the third pouches descend with the thymus and are lowered to a lower position than the parathyroid glands emerging from the fourth pouch (see Figure 10-8).
CERVICAL (BRANCHIAL) SINUSES
CERVICAL (BRANCHIAL) FISTULA
CERVICAL (BRANCHIAL) CYSTS
CERVICAL (BRANCHIAL) VESTIGES
Figure 10–9 A, The adult pharynx and neck regions indicating the former sites of openings of the cervical sinus and pharyngeal pouches (2 and 3). Treacher Collins syndrome (mandibulofacial dysostosis), is most often caused by an autosomal dominant gene defect (TCOF1), and results in underdevelopment of the zygomatic bones in the face - malar hypoplasia.
FIRST PHARYNGEAL ARCH SYNDROME
Figure 10-12 A large cervical cyst (B) shown on computed tomography of the neck region of a woman who had a "lump." Birth defects of the second pharyngeal groove are the most common of such defects.
PHARYNGEAL MEMBRANES
An X-ray was taken after the injection of a contrast agent showing the course of the fistula through the neck. Figure 10–13 Infant with first arch syndrome, a pattern of birth defects resulting from insufficient migration of neural crest cells into the first pharyngeal arch.
DEVELOPMENT OF THYROID GLAND
Note the deformed auricle of the external ear, preauricular appendage, defect in the cheek between the auricle and the mouth, hypoplasia of the mandible and macrostomia (large mouth). Only one pair of membranes contributes to the formation of mature structures; the first membrane becomes the tympanic membrane (see Fig. 10-7C).
ECTOPIC PARATHYROID GLANDS
ABNORMAL NUMBER OF PARATHYROID GLANDS
DEVELOPMENT OF TONGUE
A and B, Schematic sagittal sections of the head and neck regions at 5 and 6 weeks showing successive stages in thyroid development. If the thyroid is not recognized, the person may become permanently dependent on thyroid medication.
ECTOPIC THYROID GLAND
Rarely, an ectopic thyroid gland is located along the normal route of its descent from the tongue (see Fig. 10-14B). Incomplete descent of the thyroid gland results in a sublingual thyroid gland that appears high in the neck, at or just below the hyoid bone (Fig.
THYROGLOSSAL DUCT CYSTS AND SINUSES
Most tongue muscles are derived from myoblasts (myogenic precursors) that migrate from the occipital somites (see Figure 10-5A). Cysts in the tongue may originate from remnants of the thyroglossal duct (see Figures 10-14A).
CONGENITAL LINGUAL CYSTS AND FISTULAS
The copula is formed by fusion of the ventromedial parts of the second pair of pharyngeal arches. The line of fusion of the anterior and posterior parts of the tongue is roughly indicated by a V-shaped groove - the terminal sulcus (see Figure 10-21C).
ANKYLOGLOSSIA
The posterior third of the tongue is mainly innervated by the glossopharyngeal nerve (CN IX) of the third pharyngeal arch. All the muscles of the tongue are supplied by the hypoglossal nerve (CN XII), except the tongue muscle.
DEVELOPMENT OF FACE
DEVELOPMENT OF SALIVARY GLANDS
The mesenchyme in the margins of the placodes proliferates, producing horseshoe-shaped elevations - the medial and lateral nasal prominences (see Fig. Also note the nasal pits (NP) in the ventrolateral regions of the frontonasal prominence.
DEVELOPMENT OF NASAL CAVITIES
This membrane ruptures by the end of the sixth week, bringing the nasal and oral cavities into communication (see Fig. 10-27B and C). Proliferating epithelial cells (epithelial plug) fill the anterior lumen of the nasal cavity by 7 to 8 weeks.
POSTNATAL DEVELOPMENT OF PARANASAL SINUSES
After the secondary palate develops, the choanae are located at the junction of the nasal cavity and pharynx (see Fig. 10-27D). They also fuse with the nasal septum and the posterior part of the primary palate.
DEVELOPMENT OF PALATE
The nasal septum develops in a descending growth pattern from the inner portions of the fused medial nasal prominences (see Fig. 10-29C, E, and G). Anterior cleft defects include cleft lip, with or without cleft of the alveolar part of the maxilla.
CLEFT LIP AND CLEFT PALATE
A complete cleft anomaly is one in which the cleft extends through the lip and the alveolar part of the maxilla to the incisive fossa, which separates the anterior and posterior parts of the palate (see Fig. 10-33E and F). Oblique facial clefts associated with cleft lip result from the failure of the maxillary prominences to fuse with the lateral and medial nasal prominences.
FACIAL CLEFTS
This septum divides the cranial part of the foregut into a ventral part, the laryngotracheal tube (primordium of the larynx, trachea, bronchi and lungs), and a dorsal part (primordium of the oropharynx and esophagus) (see Fig. 11- 2F) ). The opening of the laryngotracheal tube into the pharynx becomes the primordial laryngeal inlet (see Fig. 11-2F and Fig. 11-3C).
DEVELOPMENT OF TRACHEA
DEVELOPMENT OF LARYNX
LARYNGEAL ATRESIA
DEVELOPMENT OF BRONCHI AND LUNGS
TRACHEAL STENOSIS AND ATRESIA
TRACHEOESOPHAGEAL FISTULA
Figure 11-8 Sequential stages in the development of the bronchi, bronchus, and lungs. This period overlaps with the pseudoglandular period because cranial segments of the lungs mature faster than caudal segments.
OLIGOHYDRAMNIOS AND LUNG DEVELOPMENT
Approximately 150 million primordial alveoli, half the number in adults, are present in the lungs of full-term neonates. Fetal breathing movements occur before birth, exerting sufficient force to cause aspiration of some of the amniotic fluid into the lungs.
NEONATAL RESPIRATORY DISTRESS SYNDROME
LUNGS OF NEONATES
LUNG HYPOPLASIA
This site of rapid growth defines the greater curvature of the stomach (see Fig. 12-2D). Before rotation, the cranial and caudal edges of the stomach are in the median plane (see Fig. 12-2B).
ESOPHAGEAL ATRESIA
It first appears as a fusiform enlargement of the caudal part of the foregut that is oriented in the median plane (see Fig. 12-2B). After rotation, the abdomen assumes its final position, with its long axis nearly transverse to the longitudinal axis of the body (see Fig. 12-2E).
ESOPHAGEAL STENOSIS
During rotation and growth of the abdomen, its cranial region moves to the left and slightly inferior, and its caudal region moves to the right and superior (see Fig. 12-2C to E). The smooth muscle, mainly in the lower third of the esophagus, develops from the surrounding splanchnic mesenchyme.
HYPERTROPHIC PYLORIC STENOSIS
Rotation of the stomach pulls the dorsal mesogastrium to the left, thereby enlarging the bursa. The omental bursa communicates with the greater part of the peritoneal cavity through a small opening - the omental foramen (see Fig.
DUODENAL STENOSIS
The small caudal portion of the hepatic diverticulum becomes the gallbladder and the peduncle forms the cystic duct (see Figure 12-4B and C). As the duodenum grows and rotates, the entrance of the bile duct is carried to the dorsal aspect of the duodenum (see Figure 12-4C and D).
DUODENAL ATRESIA
The diverticulum enlarges and bifurcates as it grows between the layers of the ventral mesogastric (see Fig. 12-4A). The stomach is attached to the dorsal wall of the abdominal cavity with the primordial dorsal mesogastrium (see fig.
BIRTH DEFECTS OF LIVER
Note that the entrance of the bile duct into the duodenum gradually shifts from its initial position to a posterior position. This explains why the bile duct passes behind the duodenum and the head of the pancreas in adults.
EXTRAHEPATIC BILIARY ATRESIA
The lesser omentum that runs from the liver to the lesser curvature of the stomach (hepatogastric ligament) and from the liver to the duodenum (hepatoduodenal ligament). Formation of the dorsal pancreatic bud depends on signals from the notochord (activin and fibroblast growth factor 2) that block the expression of sonic hedgehog (Shh) in the endoderm.
DEVELOPMENT OF SPLEEN
The ventral mesentery, arising from the mesogastrium, also forms the visceral peritoneum of the liver. Most of the pancreas arises from the larger dorsal bud of the pancreas, which appears first.
ANNULAR PANCREAS
The ventral bud of the pancreas forms the uncinate process and part of the head of the pancreas. The connective tissue sheath and interlobular septa of the pancreas develop from the surrounding splanchnic mesenchyme.
ACCESSORY SPLEENS
Notches on the upper border of the adult spleen are remnants of the grooves that separated the fetal lobules. In one of the peritoneal folds, usually near the hilum of the spleen or the tail of the pancreas, there may be one or more small splenic masses (about 1 cm in diameter) of fully functioning splenic tissue.
MIDGUT
