Overview of Embryology

Prenatal development consists of four sequential stages:

1 Gametogenesis. Gametogenesis is the process of formation and development of specialized generative cells called gametes, which unite at fertilization to form a single cell called a zygote.

2 Early embryonic period (weeks 1 and 2). The early embryonic phase encompasses the 2-week period from fertilization to implantation of the embryo, during which the zygote repeatedly divides. During week 2, the amniotic cavity and trilaminar embryonic disk are formed. The early embryo usually is aborted if a lethal or serious genetic defect is present, although at this time, the early embryo is less susceptible to teratogens than during the remainder of the embryonic period.

3 Embryo (weeks 3 to 8). Week 3 is the first week of organogenesis. The trilaminar embryonic disk develops, somites begin to form, and the neuroplate closes to form a neural tube (Fig. 84-1).³⁷ At week 4, the limb buds become recognizable, and the somites differentiate into three segments. The dermatome becomes skin, the myotome becomes muscle, and the sclerotome becomes cartilage and bone. Serious defects in limb development may originate at this time (Fig. 84-2).³⁶ By week 8, the basic organ systems are complete.

4 Fetus (week 8 to term). The first half of the fetal period is characterized by rapid growth and changes in body proportions. The lower limbs become proportionate, and most bones start to ossify. During the second half of gestation, growth continues, and body proportions become more infant-like (see Fig. 84-2).

Figure 84-1 A-D, Trilaminar disk. A, The neural tube closes. The mesoderm differentiates into dermatome (B), myotome (C), and sclerotome (D).

(From Staheli L: Growth. In Staheli L: Practice of pediatric orthopedics, Philadelphia, 2001, Lippincott Williams & Wilkins.)

Figure 84-2 Prenatal development. This chart summarizes musculoskeletal development during embryonic and fetal life.

(From Staheli L: Growth. In Staheli L: Practice of pediatric orthopedics, Philadelphia, 2001, Lippincott Williams & Wilkins.)

Timing and Staging of Development

Gestational age based on the date of the mother’s last menstrual period overestimates the actual gestational age by more than 2 weeks. To estimate age more accurately, embryos are staged according to the method of Streeter.³⁹ This system, a derivative of the Carnegie Embryonic Staging System, divides the embryonic period into 23 stages based on clearly defined details of either external form or the development of structures.^26,²⁷ The maturity of older embryos and fetuses is based on measurement of the crown-rump length.²⁸

Normal Sequential Embryologic Development of the Knee *

Week 6

Figure 84-3 shows an embryo at 6 weeks old, Streeter stage 17. The cartilaginous anlagen of femur and tibia are separated by cells of uniform density, the future femorotibial joint. Early evidence of cavitation, a sign of beginning joint formation, in the otherwise homogeneous, uniform interzone is easily recognizable.

Figure 84-3 Embryo 117N, 6 weeks old, Streeter stage 17, sagittal section (see text). C, Cavitation; F, femur; T, tibia (Goldner, ×100).

Week 7

Figure 84-4 shows an embryo at 7 weeks old, Streeter stage 19. Already at this stage, the lateral femoral condyle and medial femoral condyle are well formed. The lateral collateral ligament spans from the femur to the fibular head, and the medial collateral ligament connects the femur to the tibia.

Figure 84-4 Embryo 123N, 7 weeks old, Streeter stage 19, frontal section (see text). FH, Fibular head; LC, lateral femoral condyle; LCL, lateral collateral ligament; MC, medial femoral condyle; MCL, medial collateral ligament; T, tibia (Azan, ×100).

Week 8

Figure 84-5 shows an embryo at 8 weeks old. Not only is the posterior cruciate ligament seen, but also the multiple sites of beginning cavitation. Persistence of some of these intra-articular strands may lead to the development of plicae.

Figure 84-5 Embryo 50N, 8 weeks old, Streeter stage 23, sagittal section (see text). C, Area of cavitation; F, femur; PCL, posterior cruciate ligament; T, tibia (Azan, ×100).

Week 10

Figure 84-6 shows a fetus 10 at weeks old. Between the medial femoral condyle and tibia, the medial meniscus can be seen. A small plica connects the midpart of the medial meniscus with the medial femoral condyle.

Figure 84-6 Fetus HHF2, 10 weeks old, frontal, slightly oblique section going through the posterior part of the medial compartment (see text). MC, Medial femoral condyle; MCL, medial collateral ligament; MM, medial meniscus; T, tibia (Azan, ×100).

Figure 84-7 also shows a fetus at 10 weeks old. Not only the femur, tibia, and fibular head but also the anterior cruciate ligament are seen in the figure. At this stage, no vascular channels are present in the epiphyses.

Figure 84-7 Fetus HKSAG3, 10 weeks old, sagittal section (see text). ACL, Anterior cruciate ligament; F, femur; FH, fibular head; T, tibia; P, patella (Goldner, ×20).

Week 12.5

Figure 84-8 shows a fetus at 12.5 weeks old. A dense layer of cells covers the articular surfaces of the femur and tibia. Lateral to the lateral femoral condyle, the popliteus muscle is seen. Both menisci are well formed. Vessels are present at their periphery. Vascular channels are present in the femoral epiphysis. The posterior cruciate ligament inserts into the tibia.

Figure 84-8 Fetus HK24F, 12.5 weeks old, frontal section (see text). F, Femur; LM, lateral meniscus; MM, medial meniscus; PCL, posterior cruciate ligament; PO, popliteus muscle; T, tibia (Goldner, ×20).

Week 15.5

Figure 84-9 shows a fetus at 15.5 weeks old. The much longer lateral facet of the patella helps distinguish it from the medial facet. The lateral retinaculum is much denser than the medial retinaculum.

Figure 84-9 Fetus 130N, HKFr, 15.5 weeks old, frontal section going through the patellofemoral joint (see text). F, Femur; LFP, lateral facet of the patella; LR, lateral retinaculum; MR, medial retinaculum (Azan, ×20).

Week 16.5

Figure 84-10 shows a fetus at 16.5 weeks old. The ossification processes that started in the diaphyses of the femur and tibia are progressing toward the metaphyses. At this stage, blood vessels still cross the growth plate. The suprapatellar bursa extends under the quadriceps muscle. The posterior cruciate ligament originates in the intercondylar fossa and infrapatellar fat pad.