The lungs are divided into various lobes by a layer of in-folded reflections of visceral pleura referred to as pulmonary fissures [1]. There are two types of fissures: lobar or Major Fissure (MF) and Accessory Fissure (AF) [1]. The MF includes the oblique and horizontal fissure.

The oblique fissure divides the left lung into two lobes: superior and inferior lobes. This fissure runs from the costal to the mediastinal surface of the lung both above and below the hilum. Superficially, this fissure begins at the posterosuperior part of the hilum then ascends obliquely backwards to cross the posterior border. It further descends forward across the costal surface to reach the lower border almost at its anterior end. The left horizontal fissure is a normal variant found in 10% of patients [2] [3].

The oblique and horizontal fissure divides the right lung into 3 lobes: superior, middle and inferior lobe. The oblique fissure separates the inferior lobe from the middle and upper lobes, and corresponds closely to the left oblique fissure. The short horizontal fissure separates the superior and middle lobes. It passes from the oblique fissure, near the midaxillary line, horizontally forwards to the anterior border of the lung, level with the sternal end of the fourth costal cartilage, and passes further backwards to the hilum on the mediastinal surface. The oblique fissure is usually visible on a lateral radiograph and on a high-resolution CT scan as a curvilinear line from the lateral aspect to the hilum while the horizontal fissure is usually visible on a frontal chest radiograph [3].

An accessory fissure in the lung is a congenital variation, which occurs due to failed obliteration of the fissure separating individual bronchopulmonary segments during development [4]. The commonly found accessory fissures are the Superior Accessory Fissure (SAF), the Inferior Accessory Fissure (IAF), the Left Minor Fissure (LMF) and the azygous fissure. The SAF is seen in the territory of the lower lobe which partially or completely separates the superior segment of the lower lobe from the basal segments. The IAF is seen around the medial basal segment of the lower lobe and the fissure extends laterally and upwards towards the major fissure. An LMF separates the lingula from the rest of the left upper lobe [5] [6].

The MF is considered “complete” when the lobes remain held together only at the hilum by the bronchi and pulmonary vessels; and “incomplete” when there are areas of parenchymal fusion between the lobes [7] [8]. As major fissures form the boundaries of the pulmonary lobes, the knowledge of their normal and variant configuration is essential for recognizing their variable imaging appearances [9]. Furthermore, incomplete fissures may alter the usual patterns of the collapse of the lung seen in lobar lesions. They may also give rise to the atypical appearance of pathologic conditions, for instance, pneumonia and carcinoma in a particular lobe is often limited to that lobe by the fissures and may spread to adjacent lobes through the incomplete fissures [10]. In addition, knowledge on variations in the completeness of fissures is the key in patients with emphysema who undergo one-way valve bronchoscopic lung volume reduction. The most important factor that prevents the achievement of desired atelectasis after endobronchial one-way valve treatment is interlobar collateral ventilation through an incomplete interlobar fissure [11] [12].

Variations in grading of major fissures and the presence of accessory fissures have been described and their frequencies observed to differ between populations [5] [13] [14] [15] [16]. Data from the African population are scarce and altogether absent from the Kenyan population [15]. The purpose of this study was to describe the variations in human pulmonary fissuration in a cadaveric Kenyan population

The present study was carried out in the department of Human anatomy, University of Nairobi, Kenya. Data were collected from 48 formalin-fixed Kenyan cadavers during routine undergraduate dissection. Dissection of the thorax was carried out following instructions given in the Cunningham’s Manual of Practical Anatomy.

The 96 lungs were examined by two authors (V.M and V.K). Only those lungs with intact visceral pleura covering all over except at hilum were included. The lungs that were damaged during dissection, and/or with pathological lesions e.g. trauma and necrosis were excluded. Thirty eight right and 32 left lungs which met the inclusion criteria and data on variations of MF and presence of AF were recorded irrespective of the gender of the deceased. The lungs were observed for 1) presence or absence of major fissures; 2) variations in major fissures, that is, complete versus incomplete; and 3) presence and type of accessory fissure, if any. The accessory fissures that were considered in this study as illustrated in Figure 2 were: azygous fissure, Superior Accessory Fissure (SAF), Inferior Accessory Fissure (IAF), and Left Minor Fissure (LMF). The anatomical classification proposed by [7] was used to determine the presence and completeness of the major fissures (Table 1). Figures 1-3 illustrate major fissure, accessory fissure, and grading of major fissure respectively.

Seventy lungs, 32 left and 38 right-sided lungs met the inclusion criteria, and were therefore included in the study. Out of the 38 right lungs examined, the horizontal fissure was absent in 4 lungs (10.5%), incomplete in 16 lungs (42.1%) (Figure 4) and complete in 18 lungs (47.3%). The Oblique fissure was absent in none of the lungs (0%), incomplete in 14 lungs (36.8%), and complete in 24 lungs (63.1%) (Figure 5).

Out of the 32 left lungs, the complete oblique fissure was noted in 21 lungs (65.6%) (Figure 6), and incomplete in 11 lungs (34.4%) (Figure 7). The observations regarding variation in the oblique, horizontal, and accessory fissures in both the right and left lungs are summarized in Table 2.

Accessory fissures were observed in 3 right lungs; out of which only the inferior accessory fissure observed in 2 (5.3%) (Figure 8). Accessory fissures were observed in 12 left lungs (37.5%) with all of them being left minor fissures (LMF) (Figure 9 and Figure 10). Observations of accessory fissures are summarized in Table 3.

Our findings using the Craig and Walker’s classification (Table 3) demonstrate that 18 (56.25%) left and 20 (56.25%) right lung oblique fissures, together with 13 (34.21%) of the horizontal fissure was classified as grade I. Three left lung (9.38%) and 4 right lungs (10.53%) oblique fissures together with 5 right lung (13.16%) horizontal fissures were categorized as grade II. Grade III fissures were observed in 11 left lung (34.37%) and 14 right lung (36.84%) oblique fissures, together with 16 right lung (42.11%) horizontal fissures. Only 4 (10.53%) grade IV horizontal fissures were found in the right lungs.

The lungs develop from the lung bud of the cranial foregut which subsequently differentiates into the various components of the respiratory tree and parenchyma. With progressive development, the lung bud grows caudally into the surrounding mesenchyme and bifurcates into right and left bronchial buds. The right bronchial bud then divides into three secondary bronchi while the left bud into two secondary bronchi. Each lung then develops by a process of repeated dichotomous branching of the secondary bronchi. After several generations of

branching, bronchopulmonary segments are formed [17] [18]. In the early fetal period, bronchopulmonary segments are separated by spaces which later on get obliterated except along the division of principal bronchi giving rise to major fissures; that is, the oblique and horizontal in the right lung and the oblique fissure in the left [17] [18]. Variations in major fissures; that is, absence of or incomplete oblique and horizontal fissures are due to a complete or partial defect in the obliteration of these spaces along the division of principal bronchus [19]. An accessory fissure is the result of non-obliteration of spaces along bronchopulmonary segments that are normally obliterated [20].

Several authors have reported the anomalous fissures and lobes following the Craig and Walker [7] proposed fissural classification. Studies on the subject have been performed on different cadaveric populations and their findings are compared with the present study as shown in Table 4. Variation of the lung fissures is well reported with papers on cadaveric populations from Turkey [21], Ethiopia [15], Napalese [5], The United States [22] and India [1] [3] [14] [23] - [44].

The prevalence of the absence of horizontal fissures was comparatively more than the absence of oblique fissures in both the right and the left lung in all previous studies including the present study, with the exception of study reported by Bhardwaj et al. [44]. Similar to our study, the oblique fissure of right lung was reported to be absent in 14 out of the 24 studies, with the remaining researchers not finding any cases of missing oblique fissures. Absent oblique fissures of the left lung were reported in 13 out of 24 studies, including the present study. As observed from the different studies, an absent horizontal fissure is a commonly occurring variation (Table 4). The only study not to report a single case of an absent horizontal fissure was that by Mamatha et al. [43].

Other than variations in major (oblique and horizontal) fissures, radiologists and clinicians have to be aware of possibilities of having accessory fissures. In our study, LMF was found to be the most frequently occurring accessory fissure. This was consistent with findings by Sudikshya et al. and Çimen et al. [5] [34]. Grading of fissures is important in planning of pulmonary surgeries for easy approach in surgical procedure and to anticipate post-operative complications. For example, in cases of incomplete fissuration with resultant parenchymal fusion, more dissections may be required in order to reach the bronchi and pulmonary arteries during surgical resections leading to more hemorrhage and air leak [5] [45]. Therefore, knowledge of anatomical variations alerts the surgeons to potential complications that may be encountered during surgical intervention.

Patients with incomplete/lower pulmonary fissure development have a higher chance of tumor invasion, which would therefore be related to lung cancer prognosis and that fissures limit the spread of lung cancer. The degree of fissure development was found to be a prognostic factor in patients with resected stage I adenocarcinoma [46].

Accessory fissures, like the major fissures, not only serve as natural barriers against infection but are also useful in localizing focal pulmonary parenchymal disease and in differentiating pleural from parenchymal disease [47] [48]. Thus, identifying AF is important for precise localization of lesions and characterization of diseases [1].

The limitation of this study is failure of the study to investigate broncho-vascular abnormalities or variations associated with incomplete or accessory fissures. As authors of this study highlighted, these variations (incomplete fissure, or accessory fissure) may have a clinical implication, especially because of associated variations in pulmonary vessel (pulmonary artery, pulmonary vein), bronchial tree or even lymphatic drainage, which are critical in lung cancer surgery.

Knowledge of the anatomy and normal variants of the MF and AF is necessary for appreciation of gross lobar anatomy, provide useful information for recognizing their variable appearances on imaging, as well as the related abnormalities. In addition, being aware of these variations before pulmonary segmentectomy and lobectomy may inform and adjust the preoperative plans.

The authors sincerely thank those who donated their bodies to science so that anatomical research could be performed. Results from this research can potentially increase mankind’s overall knowledge. Therefore, these donors and their families deserve our highest gratitude.

The authors declare no conflicts of interest.

Mutua, V., Cheruiyot, I., Bundi, B., Mong’are, N., Kipkorir, V. and Othieno, E. (2021) Variations in the Human Pulmonary Fissures and Lobes: A Cadaveric Study. Open Access Library Journal, 8: e7787. https://doi.org/10.4236/oalib.1107787