BLASTOCYST

The human begins as the fertilized ovum, which cleaves itself into smaller and smaller embryonic cells known as blastomeres, each carrying all the genetic information needed to build any tissue or organ in the body (Williams et al., 1995). These cells at first form a solid sphere, but rearrangement and continued cell division produce a hollow sphere known as the blastocyst (Patten, 1964). The blastocyst is a single cell layer thick except at one pole, where the cells form into a disc that is two cell layers thick (Brash, 1951b) (Fig. 12-2, A). This disc is called the inner cell mass to distinguish it from all the other cells, which are referred to collectively as the trophoblast (Mossman, 1987). The hole in the center of the blastocyst is known as the blastocyst cavity.

FIG. 12-2 Cross section of the blastocyst. A, At the beginning of the second embryonic week. B, In the middle of the second embryonic week.

The inner cell mass and the trophoblast have different fates. The former goes on to form the embryo proper. The latter can be likened to embryonic scaffolding that eventually is dismantled or discarded. Before that happens, trophoblast cells proliferate by mitosis until they are several cell layers thick. The innermost layer of the trophoblast still lines the blastocyst cavity. It remains cellular and is called the cytotrophoblast (Baxter, 1953). The cells in the outermost layer begin to lose their cell membranes, thus forming a syncytium known as the syncytiotrophoblast (Williams et al., 1995) (Fig. 12-2, B). This syncytium releases proteolytic enzymes that enable the blastocyst to digest its way into the lining of the uterus, called the endometrium. This process is known as implantation and usually is completed by the end of the second embryonic week. The implanted blastocyst does not leave the uterine wall until it bursts out in the form of a newborn (Beck et al., 1985).

Also during the second week of development, the inner cell mass has been changing (Williams et al., 1995). A space, the first sign of the amnionic (amniotic) cavity, has appeared within it (see Fig. 12-2, B). The amnionic cavity can be thought of as being enclosed by walls, a roof, and a floor (Langebartel, 1977). The roof and walls become the amnion. The floor becomes the dorsal surface of the embryo and later transforms into the outer layer of the skin on the newborn’s back (Sadler, 2004). By this time the inner cell mass is attached to the cytotrophoblast by only a narrow bridge of cells that is called the connecting stalk (Fig. 12-3). The connecting stalk marks the tail or caudal end of the embryo just as the amnion marks its dorsal surface. A close observer would note that the cells in the inner cell mass have grouped themselves into two layers (see Fig. 12-2, A). One layer floors the amnionic cavity and is called the epiblast. The other layer roofs the blastocyst cavity and is called the hypoblast (Moore and Persaud, 2003). The hypoblast gives rise to a layer of cells that comes to line the blastocyst cavity. The blastocyst cavity then is known as the primitive yolk sac even though it contains a negligible amount of yolk (Scammon, 1953) (see Fig. 12-3).

FIG. 12-3 Longitudinal section of the blastocyst at the end of the second embryonic week.

The yolk sac cells next bud off other cells that deposit themselves on the outside of the yolk sac, where they make up a layer called extraembryonic mesoderm (Moore and Persaud, 2003) (Fig. 12-4). The triple layer, consisting of the cytotrophoblast sandwiched between the syncytiotrophoblast and extraembryonic mesoderm, is called the chorion and is the embryonic portion of the placenta (Goss, 1966). That means the cavity inside the chorion actually corresponds to the previous blastocyst cavity or primitive yolk sac (Sadler, 2004). To recognize the changes that have occurred (e.g., the imminent appearance of the secondary or definitive yolk sac and extraembryonic mesoderm), this cavity is renamed the extraembryonic coelom and is also called the chorionic cavity (Goss, 1966) (see Fig. 12-4).

FIG. 12-4 Longitudinal section through bilaminar embryo at the end of the second embryonic week before appearance of mesoderm.

GASTRULATION AND DEVELOPMENT OF THE NOTOCHORD

At the beginning of the third week of development, a narrow groove forms on the caudal portion of the dorsal surface of the epiblast known as the primitive streak (Hamilton and Mossman, 1972) (Fig. 12-5). Epiblast cells begin to migrate toward the primitive streak. Once there, the cells detach from the epiblast and invaginate deep to it through the primitive streak. Once invaginated, the cells move in all directions. Some of these cells displace the hypoblast and form the endoderm (Sadler, 2004). The endoderm eventually will form the lining of the respiratory and gastrointestinal tracts. Other invaginating cells form a new layer of cells between the epiblast and the newly formed endoderm called the intraembryonic mesoderm. The cells remaining in the epiblast become the ectoderm. The ectoderm, mesoderm, and endoderm are the three germ layers from which the entire embryo will differentiate. The process of forming these germ layers and creating a trilaminar embryonic disc is known as gastrulation.

FIG. 12-5 Frontal section through chorion and amnion during third embryonic week revealing a view of the floor of the amnionic cavity as would be seen by an observer positioned on the roof of that cavity.

Some of the invaginated epiblast cells moving superiorly in the midline toward the future head turn into a structure known as the notochord (Fig. 12-6; see also Fig. 12-1). The notochord is a relatively rigid rod of cells that marks the longitudinal axis of the embryonic body and induces the nervous system to develop. The vertebral column later occupies this site, and by then the notochord has turned into the part of the intervertebral disc known as the nucleus pulposus.

FIG. 12-6 Midsagittal section through embryo during the third embryonic week showing how folding has converted the proximal part of the yolk sac into a foregut (at the head end), a hindgut (at the tail end), and a midgut (which still lacks a floor and is confluent with the yolk sac cavity).

NEURULATION

Also during the third week of development, the notochord induces the ectoderm to form a thickening along its length in the midline called the neural plate, which is the primordium of the entire nervous system (Arey, 1965). The cells of the neural plate are known as the neuroectoderm. A groove forms, which runs the length of the neural plate, with two folds flanking it. This neural groove deepens and sinks below the surface. The neural folds close over it, thus forming a hollow tube of neuroectoderm called the neural tube (Scothorne, 1976) (Fig. 12-7). The neural tube is the primordium of the brain and spinal cord, and the process of its formation is called neurulation. Other neuroectoderm cells invaginate along with the neural tube but remain apart from it (Romanes, 1972a,b). They are collectively known as the neural crest (see Fig. 12-7) and give rise, among other things, to many important components of the nervous system, including all sensory neurons, all postganglionic autonomic neurons, and all ganglia, both sensory and motor (Scothorne, 1976). The rest of the ectodermal cells, which are not part of the neural plate, are destined mostly to form the epidermis (the surface layer of the skin), and are appropriately named surface ectoderm.

FIG. 12-7 Cross section through the amnionic (amniotic) cavity during the third embryonic week showing the formation of the neural tube and neural crest from its floor.

SOMITE DEVELOPMENT

The mesoderm lying just lateral to the notochord parallels the long axis of the embryo and is called the paraxial mesoderm. Beginning during the third embryonic week, the paraxial mesoderm subdivides into blocks of cells known as somites, which are major contributors to the muscles, bones, and the dermis of the body. Just lateral to the somites on both sides of the embryo, other mesodermal cells have formed into a structure called intermediate mesoderm. The intermediate mesoderm eventually will develop into the majority of the genitourinary system (Fig. 12-8; see also Fig. 12-1). Lateral to the intermediate mesoderm is the lateral mesoderm, which will, along with the overlying ectoderm, form the lateral and ventral parts of the body wall.

FIG. 12-8 Cross section through the two concentric tubes that run the length of the embryonic body at the end of the third embryonic week: the inner primitive gut tube (splanchnopleure) and the outer body wall tube (somatopleure). The space between them becomes the body cavity.

The somites are staging areas from which mesoderm cells deploy to new locations (Sadler, 2004). The cells of the ventromedial portion of the somite will migrate toward the notochord and neural tube. These cells eventually produce hard tissues, such as bone, cartilage, and ligaments, so the part of the somite from which they are derived is appropriately called sclerotome (Harrison, 1972) (Fig. 12-9).

FIG. 12-9 Cross section through two stages in somite differentiation: before and after dermis and sclerotome cells have migrated to the destinations shown by the arrows.

The rest of the somite is called the dermomyotome or epithelial plate of the somite. Traditionally, once myoblasts could be identified in the somite, termed the myotome, the remainder of the somite was called the dermatome, which went on to form the dermis of the skin (Beck et al., 1985) (see Fig. 12-9). It is now believed that the superficial portion of the somite, the region of the traditionally defined dermatome, contributes a significant amount of muscle precursor cells as it elongates through the body wall (Williams et al., 1995). Also, the only dermis that appears to be derived exclusively from somites may be that covering the epaxial musculature, which is a much more restricted distribution than implied by the term dermatome. The remainder of the dermomyotome forms myoblasts that will eventually form skeletal muscles (Fischman, 1972) (see the following).

During the next several weeks, the amnion undergoes a series of folds creating the embryo’s body wall and defining the lumen of the embryonic gut (Fig. 12-10). The embryo’s body cut in cross section at this time appears as two concentric rings. The inner ring is the splanchnopleure; the outer is the somatopleure (Goss, 1966) (see Fig. 12-8). The outer ring actually has the shape of a jeweler’s signet ring. Tucked close together inside the signet part are the neural tube, neural crest, notochord, somites, intermediate mesoderm, and mesenchyme (see Fig. 12-8). The signet part corresponds to what is variously called the back, the posterior body wall, and the posterior abdominal or posterior thoracic wall (Callander, 1939; Grant, 1952; Davenport, 1966).

FIG. 12-10 Cross section through the amnionic cavity and neural tube showing how the original amnion transforms itself into two concentric tubes during the third embryonic week. The inner tube is the embryo’s body wall; the outer is the definitive amnion.

VERTEBRAL DEVELOPMENT

Typical Vertebrae

Vertebral development can be conveniently divided into four stages. The notochordal stage is first. The notochord not only forms the original basis for the development of the vertebrae by defining the axis of the embryo, it also forms some of the cells of the intervertebral disc. The second stage is the mesenchymal, or blastemal, stage. Mesenchyme is the connective tissue of the embryo that is, at least in the case of the vertebrae, derived from paraxial mesoderm. Each vertebra is formed from a mesenchymal model. The third stage is the cartilage stage, in which the vertebral mesenchyme is replaced by hyaline cartilage. The osseous stage is last, in which the hyaline cartilage is replaced by bone. Therefore each vertebra is formed by the process of endochondral ossification.

Cells of the sclerotome migrate medially from the somite to surround the neural tube and notochord (Breathnach, 1958). Instead of forming a continuous tunnel of cells, they form a series of discrete blocks of mesenchyme (Williams et al., 1995). Those cells ventral to the neural tube that specifically surround the notochord are known as the centrum. The cells dorsolateral to the neural tube are called the neural arch (O’Rahilly, 1986) (Fig. 12-11). Each neural arch forms a pedicle, which projects ventromedially toward the centrum, and a lamina, which projects dorsomedially toward the corresponding lamina from the other side. The laminae eventually fuse and expand to form the spinous process. Cranial and caudal projections of the neural arch form articular processes, which will develop into the zygapophysial joints with adjacent vertebrae. A lateral projection of the neural arch forms the true transverse process. Projecting anterolaterally from the neural arch is the costal element, which expands to meet the tip of the transverse process. The patterning of the mesenchyme into presumptive vertebrae appears to be regulated by HOX genes (Sadler, 2004).

FIG. 12-11 Four cross-sectional views and a lateral view of a vertebra composed at first almost entirely of sclerotome cells (tan), which are gradually replaced by cartilage (blue), which is almost entirely replaced by bone (purple).

For years the accepted view has been that the sclerotome cells forming a centrum derived from two contiguous somites (Beck et al., 1985). This dual origin accounts for the staggered position of myotomes and centra (Sadler, 2004). This accepted view was challenged by Verbout (1985). However, experimental support for the accepted view also has been adduced (Bagnall et al., 1987). Most evidence still suggests that approximately the cranial half of a centrum is derived from the caudal portion of the somite above, whereas the caudal half of a centrum is derived from the cranial portion of the somite below (Williams, 1995) (Fig. 12-12).

FIG. 12-12 Frontal section through notochord and somites depicting three stages in the formation of intervertebral discs and vertebrae. Cells migrate from the sclerotome to surround the notochord, but cell density is not the same everywhere along the notochordal length. Cell condensations mark the sites where intervertebral discs will appear. The vertebral centra will develop in the spaces between cell condensations. Other sclerotome cells located dorsolaterally form condensations that mark the sites where neural arches will appear. The spinal nerve and intervertebral blood vessels appear in the spaces between the developing neural arches. Other sclerotome cells located laterally condense next to the disc at the site where the costal element will appear.

Regardless of their origin, by the end of the second embryonic month, some of the mesenchymal cells of the vertebrae change into chondroblasts and begin laying down an extracellular cartilaginous matrix (Hall, 1978). There are typically two chondrification centers in each centrum, located on either side of the notochord. These rapidly replace the notochord in the region of the centrum with cartilage to form a single mass of cartilage (Williams et al., 1995) (see Fig. 12-11). Also, each side of the neural arch develops at least one chondrification center.

By the end of the embryonic period, primary ossification centers appear in the cartilaginous vertebra. These usually appear in a cranial to caudal pattern, beginning in the upper cervical region by approximately week 8 and reaching the sacral area by week 22 (Williams, 1995). Typically two primary ossifications centers form in the centrum (ventral and dorsal), which rapidly fuse into one center (Moore and Persaud, 2003). There is also a primary ossification center that develops in each side of the neural arch (O’Rahilly & Muller, 1992) (see Fig. 12-11). By birth, each vertebra consists of three bony segments connected by hyaline cartilage: the centrum and each half of the neural arch. The cartilages between the centrum and each neural arch actually are located within the region that will become the adult vertebral body. This region is known as the neurocentral synchondrosis (or junction) and usually fuses together into compact bone by age six (Maat et al., 1996). Therefore the terms vertebral body and centrum are not equivalent. Likewise the terms neural arch and vertebral arch are not synonymous. The vertebral body encompasses all of the centrum and a portion of each neural arch. The vertebral arch is only a portion of each neural arch (Trotter and Peterson, 1966).

At approximately the time of puberty, secondary ossification centers appear in the remaining cartilage (Sinclair, 1972) (see Fig. 12-11). Typically each vertebra develops five secondary ossification centers: one for the tip of each transverse process, one for the tip of the spinous process, and two anular apophyses (outdated term: anular epophyses) one on the superior and one on the inferior rim of the vertebral body. A vertebra can increase in height and diameter before and after birth by employing intrinsic bone-depositing mechanisms around the circumference, or periosteum, and at the ends (Bogduk and Twomey, 1987). Also, there can be some variation in the distribution, and sometimes even number, of ossification centers from vertebra to vertebra, accounting for some of their individual variation (Inkster, 1951).

Although each vertebra has costal elements, the eventual fates of these elements vary by region of the spine. In the cervical spine, the costal elements eventually form most of the adult transverse process, including the portions that form the ventral and dorsolateral boundaries of the foramen of the transverse process. Only the portion of the adult transverse process that forms the dorsomedial border of the foramen of the transverse process develops from the embryonic transverse process. In the thoracic region, the costal elements reach their maximal length and form the ribs. In the lumbar spine, the costal elements form the anterior and lateral portions of the adult transverse processes. The embryonic transverse process forms only the dorsomedial portion of the adult lumbar transverse process, which includes the accessory process (Robertson, 1966).

Intervertebral Discs and Other Joints

Although the mesenchyme of the sclerotome replaces the notochordal tissue to form the centra of the vertebrae, the notochordal tissue located between the developing vertebrae actually expands to form the presumptive nucleus pulposus of the intervertebral discs (see Figs. 12-9 and 12-12

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