Embryology and Prenatal Development of Human Spina Bifida Manifesta

Chester A. Swinyard, M.D., Ph.D. Hideo Nishimura, M.D. Shakuntala Chaube, Ph.D.


This paper presents basic embryological data on the development of myelomeningocele in human embryos and fetuses, makes some pertinent observations on the development of myeloschitic fetuses, and refers to some of the practical problems presented in postnatal life by newborns with this developmental defect (Some of the data in this paper was presented by one of us (CAS) in a talk at the first conference on bracing of the child with spina bifida sponsored by the Committee on Prosthetics Research and Development of the Division of Engineering, National Research Council, and held at the University of Virginia, Charlottesville, Va., Oct. 2-4, 1969).

Myelomeningocele associated with spina bifida is but one of a variety of defects which result from developmental aberrations of the dorsal midline ectoderm which ultimately gives rise to the brain and spinal cord.

Lumbosacral Defect

If the defect occurs in the severe form in the cephalic aspect of the neuraxis, it is incompatible with life and presents at birth most commonly as a stillborn anencephalic specimen; or the child may be liveborn and survive a few hours. The specific defect about which we are writing occurs (in 90 per cent of the cases) in that portion of the neuraxis which becomes the lumbosacral spinal cord. This localized lumbosacral spinal cord defect is compatible with live birth. In these newborns, the spinal cord defect presents as a flat plate on the surface of the lumbosacral area where it bulges as an egg- to grapefruit-sized mass in the lower back. It may be skin covered, covered only by meninges, or present as an uncovered, open neural plate which is bathed by the cerebrospinal fluid which issues from the central canal of the normal (closed) spinal cord immediately above the defective area.

Terms

Before dealing with the embryology of the defect, it is necessary to clarify a number of terms which are used carelessly and frequently lead to misunderstanding. It has become traditional to refer to these patients as children with spina bifida and, indeed, they do have several lumbosacral vertebrae in which the laminar plates fail to fuse in the midline to form a single dorsal spinous process. The effect of this failure of bony development is evidenced by two stubby osteal spines, one on each side of the midline; hence, the term "spina bifida." Although all newborns with the spinal cord defect of myelomeningocele also have spina bifida, the latter defect is not primary either with respect to time of occurrence or clinical importance. For example, the spinal cord is formed into a tubular structure about eight weeks before it is enclosed by vertebral arches1. We should also bear in mind that vertebral laminar plates may be defective even though the spinal cord is normal. In this event, we are dealing with a spina bifida which is not externally manifest, a condition referred to as spina bifida occulta (Table 1 ).

Spina bifida occulta occurs in nearly 50 per cent of all newborn babies4 and may be found in 20 to 40 per cent of the adult population11. This defect is generally believed to be of little clinical importance. However, it is common to ascribe significance to it when it is known to be present and the patient has low back pain for which no other causative factor can be found.

Another source of confusion originates in a type of spina bifida manifesta in which the spinal cord is normally developed but, because of its failure to develop a bony roof over the spinal cord (deficient lamina), the skin, meninges, subarachnoid space and cerebrospinal fluid bulge as a sac protruding between the bifid spines. This condition is properly called meningocele. The patient has little or no paralysis because the spinal cord is normally developed. The muscle weakness which may develop is related to involvement of spinal nerves in the sac wall with resultant compression and stretching of the nerves. It is generally possible to distinguish between myelocele and meningocele by transillumination of the mass with a strong light. On occasions, however, the true situation cannot be definitively established until the time of neurosurgical dissection of the sac.

Source of Study Specimens

Before describing and illustrating myelocele development in human embryos, we wish to make some observations concerning the source of the study specimens upon which this paper is based. Revision of the Japanese Eugenic Protection Law in 1952 allowed abortion for psychosocial and medical reasons. This program enabled one of us (HN) to collect more than 25,000 human embryos and fetuses accompanied by maternal reproductive and medical data15. The observations made in this paper are derived from study of 62 embryos which presented myeloschisis with or without defects of the developing brain. A comprehensive account of these specimens will be published elsewhere. However, it is pertinent to note that, in a population of human embryos and fetuses, the incidence of severe spinal cord and/or brain defects during the fourth to tenth week of gestation is many times higher than it is at birth14. This means that many of the specimens studied would have been spontaneously aborted if not removed by dilatation and curettage. The importance and magnitude of the problems presented by this birth defect can be appreciated if it is realized that it occurs approximately once in every 750 live births23. On this basis, there may be annually about 8,000 newborns and their families, in the United States, who are confronted by this birth defect. The incidence is several times higher in Ireland, Scotland, and Wales12.

Etiology

The etiology of the defect is as yet unknown. It is generally believed, however, that it represents an interaction of an unknown environmental factor with a susceptible gene pool. The genetic influence behaves as a multifactorial genetic trait. It is possible to produce animal models of this birth defect in a number of species (rat, guinea pig, hamster) by administration, to the pregnant animal at the strategic time in gestation, of one of a number of teratogenic compounds. The literature on experimentally induced myelocele has been reviewed by Kalter10. None of the compounds which produce the birth defect in animals has been shown to produce myelocele in man and the responsible environmental factor remains unknown.

Circadean and Morphological Aspects of Normal and Myeloschitic Spinal Cord Development

The time of development processes is an important but exceedingly difficult factor to measure accurately. However, it is a necessary consideration in any study of the rapidly changing dynamic processes of embryological development. Time or duration of development, commonly referred to as the gestational or ovulatory age of a developing embryo or fetus, is based on a presumed time of fertilization. The presumed time is generally calculated as being 14 days after the first day of the last menstrual period. This calculation of gestational age assumes that every woman has a regular 28-day menstrual cycle. Another variable not taken into consideration, and one which cannot be eliminated, is the time required for the spermatozoon to reach the ovum (usually in the middle one third of the uterine tube) and effect fertilization. This variable may involve a time span as long as 72 hours. The foregoing variables related to time of fertilization result in marked variation in the stage of development reached by embryos and fetuses which are presumed to have the same gestational or ovulatory age.

For many years, embryologists have used the remarkable collection developed by Dr. George Streeter at the Carnegie Laboratory of Embryology19-22 as a basis of comparison for their obtained human embryos. The specimens in this collection were derived mostly from spontaneous abortions, and gestational age was based upon menstrual history. Dr. Streeter recognized the difficulties of accurate timing of gestational age with respect to body length (crown-rump length in millimeters) and weight. He therefore tried to verify some stages of development and supplement the collection with time-mated pregnancies of nonhuman primates. He also realized that, in view of the time variable and the marked rapidity with which externally visible changes occurred within a single day during the period of the embryo (up to eight weeks) and young fetus (after eight weeks), recognition of developmental stages would be enhanced by the use of multiple criteria of morphological development in describing the human embryos and fetuses of his collection. He referred to identifiable stages of development as "Horizons" and described twenty-three horizons which range from the zygotic (Horizon I) to a fetus about 48 days old (Horizon XXIII).

In 1956, Witschi27 compared weight and length of a collection of embryos with Streeter's Horizons. One of us15 has compared with Streeter's collection the estimated ovulation age, crown-rump length in millimeters, and body weight in milligrams of 672 embryos and fetuses derived from mothers with a regular cycle. It was concluded that "crown-rump length and body weight are generally more reliable indicators of developmental state than clinically estimated age." Fig. 1 derived from this study shows the estimated ovulation age and developmental horizon in these three collections.

The brain and spinal cord are the first organs to be developed in the embryo. In Fig. 2 , the sequence of events which occur in the formation of a tubular brain and spinal cord is illustrated. The most dynamic part of the process occurs during the 21st to 28th day of gestation. A 21-day human embryo has a total length of about 1.5 mm. The spinal cord is developing rapidly; the cephalic end of the neuraxis has precociously developed and shows evidence of three enlargements which represent the three primary brain vesicles. At this stage of development, there are no cartilaginous formations which are precursors of the axial skeleton. There are, however, seven pair of condensed masses of mesoderm on either side of the spinal cord which are known as mesodermal somites. Note that, at the level of the second and third mesodermal somite, the free edges of the neural plate, which represent the thickened and depressed midline skin of the back, have started to fuse, forming a neural tube with a central canal. This initial area of neural tube closure lies approximately in the midthoracic region.

During the next seven days, the neural tube will literally close as though it were being zippered, both cephalically and caudally. During this time, 24 additional pair of mesodermal somites appear on each side of the tube, the embryo has more than doubled in length (3.6 mm), and the neuraxis is a tube except for the open anterior and posterior neuropore. The anterior neuropore closes about the 28th day and the posterior neuropore closes about gestational day 30. As previously mentioned, failure of closure in the brain area results in serious brain malformations (exencephaly, anencephaly, etc.). Our concern is with the lumbosacral area of the spinal cord where failure to close, or reopening after closure, results in myelomeningocele associated with spina bifida. A photograph of a human embryo with a lumbosacral myelocele is shown as Fig. 3 .

It is apparent from the diagrammatic representations of the normal formation of the spinal cord (Fig. 2 ) and of the myelocele (Fig. 3 ) that the basic lesion in myelomeningocele is a localized open segment or segments of the spinal cord.

Pathogenesis

Shortly after Tulp25 provided the first medical description of this birth defect, speculation regarding the pathogenesis of the lesion had been advanced. In 1769, Morgagni13 postulated that the open area was produced as a result of localized rupture of a closed neural tube caused by the pressure of cerebrospinal fluid from a hydrocephalic head. This theory aroused little comment for more than one hundred years until Von Recklinghausen26 challenged it in 1886 and suggested that the tube was open in a localized area because it had failed to close. This theory became widely accepted3, and when Patten18 described "overgrowth" of the open area, he suggested that the overgrowth had prevented closure. In 1960, Gardner5 revived the Morgagni pressure-rupture theory largely on the basis of enlargement of the central canal of the spinal cord above the open area. Recently, Padget17 attempted to support the rupture theory by study of a 7 mm monkey embryo in which there was a subcutaneous bleb over the mesencephalon extending to the lower limb buds where the bleb appeared to be ruptured. It is not the purpose of the present paper to add fuel to this controversy. Our only desire is to outline the present state of understanding and/or misunderstanding of the pathogenesis of the open part of the spinal cord which is responsible either directly or indirectly for the problems which confront these children.

"Overgrowth"

In Fig. 3 it is evident that the myelocele is much larger in volume than the normal spinal cord at the same neurosegmental level. This is the phenomenon of so-called myelocele "overgrowth" which was first described by Patten18 in 1952. Following this initial observation, others have described localized neural tube overgrowth in animals following surgical procedures16,8 and in human exencephaly6. In 1962, Källen9 marshaled abundant evidence to show that this phenomenon of overgrowth is a true hyperplasia of the neural tissue. We shall point out later that this occurrence of marked hyperplasia of neural tissue, which is localized to the area of the myelocele, may have importance in connection with some of the neural defects presented by these patients.

Källen called attention to the increased frequency of mitotic figures in the unorganized hyperplastic, myeloschitic tissue of animals. We have observed more mitotic figures in overgrown myeloschitic tissue of human embryos than in normal spinal cord of the same segment24. We have histological evidence that neuroblasts derived from this mitotic activity are capable of sprouting axons and have observed branches of the sciatic nerve reaching and innervating the primitive muscles of the thigh. It is possible that myeloschitic overgrowth which continues into later fetal life plus fibrous tissue formation around the anläge of developing vertebrae precipitate nerve tension and compression which contribute to muscle denervation.

Denervation

The electromyographic studies of Ingberg and Johnson7 and our studies2 indicate that in the newborn or infant with myelomeningocele there is evidence of denervation. The distorted relationships between the dorsal and ventral roots which enter and leave the myelocele could also explain the loss of kinesthetic sensation and the bowel and bladder dysautonomia which result in incontinence characteristic of these children. Even though most of the muscle denervation appears to occur prenatally, there is little doubt that infection of the myelocele and postnatal nerve root tension related to increased cerebrospinal fluid pressure contribute to further denervation and provide abundant reason for early neurosurgical treatment of the myelocele.

Orthopaedic and Orthotic Aspects

It is fitting that the Committee on Prosthetics Research and Development of the National Academy of Sciences-National Research Council encourage greater interest in the orthopaedic and orthotic aspects of the care of these children. Advances in the control of infection, hydrocephalus, and renal destruction are significantly increasing the number of children with this birth defect who are surviving into adulthood. This increased population poses a significant public health problem.

Chester Swinyard and Shakuntala Chaube are from Children's Division, Institute of Rehabilitation Medicine, New York University Medical Center, New York, N. Y.

The Department of Anatomy, Kyoto University Faculty of Medicine, Kyoto, Japan

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